Systems Neuroscience Midterm 1 Flashcards

Front 1

How are action potentials generated in a cell?

Back 1

The action potentials generated by a cell are all similar in size and duration,
and they do not diminish as they are conducted down the axon.

Front 2

How is Vm determined in an action potential?

Back 2

Vm, can be determined by inserting a microelectrode in the cell. A voltmeter is used to measure the electrical potential difference between the tip of this intracellular microelectrode and another placed outside the cell. When the neuronal membrane
is at rest, the voltmeter reads a steady potential difference of about
–65 mV. During the action potential, however, the membrane potential briefly becomes positive. Because this occurs so rapidly—100 times faster than the blink of an eye—a special type of voltmeter, called an oscilloscope,
is used to study action potentials. The oscilloscope records the voltage as it
changes over time.

Front 3

What are some identifiable parts of the action potential?

Back 3

Rising phase: a rapid depolarization of the membrane thatcontinues until Vm reaches a peak value of about 40 mV. The part of the action potential where the inside of the neuron is positively charged with respect to the outside is called the overshoot. The falling phase of the action potential is a rapid repolarization until the membrane is actually more negative
than the resting potential. The last part of the falling phase is called the undershoot, or after-hyperpolarization. Finally, there is a gradual restoration of the resting potential. From beginning to end, the action potential lasts about 2 milliseconds (msec).

Front 4

Chain of events to stimulate an action potential by pain

Back 4

(1) the thumbtack enters the skin, (2) the membrane of the nerve fibers in the skin is stretched, (3) Na-permeable channels open. Because of the large concentration gradient and the negative charge of the cytosol, Na
ions enter the fiber through these channels. The entry of Na depolarizes the membrane; that is, the cytoplasmic (inside) surface of the membrane becomes less negative.

Front 5

Threshold

Back 5

The critical level of depolarization that must be crossed in order to trigger an action potential is called threshold. Action potentials are caused by depolarization of the membrane beyond threshold.

Front 6

Different ways depolarization can be caused

Back 6

-Entry of Na+ through specialized ion channels

-In interneurons, depolarization is usually caused by Na entry through channels that are sensitive to neurotransmitters released by
other neurons.

-neurons can be depolarized by injecting electrical current through a microelectrode

Front 7

Intracellular v.s. extracellular recording

Back 7

Intracellular: The goal of intracellular recording is simple: to measure the potential difference between the tip of the intracellular electrode and another electrode placed in the solution bathing the neuron (continuous with the earth,
and thus called ground). The intracellular electrode is filled with a concentrated salt solution (often KCl) having a high electrical conductivity.

Extracellular: we measure the potential difference between the tip of the recording electrode and ground. The electrode can be a fine glass capillary filled with a salt solution, but it is often simply a thin insulated metal wire. Normally, in the absence of neural activity, the
potential difference between the extracellular recording
electrode and ground is zero.

Front 8

Firing Frequency

Back 8

The cell generates action potentials at a rate of something like one per second, or 1 hertz (Hz). If we crank up the current a little bit more, however, we will find that the rate of action potential generation increases, say, to 50 impulses per second (50 Hz). Thus, the firing frequency of action potentials reflects the magnitude of the depolarizing current.

Front 9

What is the maximum firing frequency?

Back 9

Although firing frequency increases with the amount of depolarizing current, there is a limit to the rate at which a neuron can generate action
potentials. The maximum firing frequency is about 1000 Hz.

Front 10

Absolute Refractory Period

Back 10

Once an action potential is initiated, it is impossible to initiate another for 1 msec. This period of time is called the absolute refractory period. In addition, it can be relatively difficult to initiate another action potential for several
milliseconds after the end of the absolute refractory period.

Front 11

Relative Refractory Period

Back 11

During this relative refractory period, the amount of current required to depolarize
the neuron to action potential threshold is elevated above normal

Front 12

Three types of protein molecules in the ideal cell

Back 12

-Sodium-potassium pumps

-potassium pumps

-sodium channels

Front 13

What is the Nernst equation and how does it work?

Back 13

IK =gK(Vm-EK).

1. The net movement of K ions across the membrane is an electrical current. We can represent this current using the symbol IK.
2. The number of open potassium channels is proportional to an electrical conductance. We can represent this conductance by the symbol gK. 3. Membrane potassium current, IK, will flow only as long as Vm doesn't equal EK. The

driving force on K is defined as the difference between the real membrane potential and the equilibrium potential, and it can be written as Vm -EK.

Front 14

How could we account for the falling phase of the action potential?

Back 14

Simply assume that sodium channels quickly close and the potassium channels remain open, so the dominant membrane ion permeability switches back from Na to K. Then K would flow out of the cell until the membrane potential again equals EK. Notice that if gK increased during the falling
phase, the action potential would be even briefer.

Front 15

Voltage clamp

Back 15

The voltage clamp enabled Hodgkin and
Huxley to “clamp” the membrane potential of an axon at any value they chose. They could then deduce the changes in membrane conductance that occur at different membrane potentials by measuring the currents that
flowed across the membrane. In an elegant series of experiments, Hodgkin and Huxley showed that the rising phase of the action potential was indeed caused by a transient increase in gNa and an influx of Na ions, and
that the falling phase was associated with an increase in gK and an efflux of K ions.

Front 16

How did Hodgkin and Huxley determine transient changes in gNA?

Back 16

To account for the transient changes in gNa, Hodgkin and Huxley proposed the existence of sodium “gates” in the axonal membrane. They hypothesized that these gates are “activated”—opened—by depolarization above threshold and “inactivated”—closed and locked—when the membrane acquires a positive membrane potential. These gates are “deinactivated”—
unlocked and enabled to be opened again—only after the membrane potential returns to a negative value.

Front 17

Voltage-gated sodium channel

Back 17

The protein forms a pore in the membrane that is highly selective to Na ions, and the pore is opened and closed by changes in the electrical potential of the membrane.

Front 18

Patch Clamp

Back 18

The patch-clamp method entails sealing the tip of an electrode to a very small patch of neuronal membrane. This patch then can
be torn away from the neuron, and the ionic currents across it can be measured as the membrane potential is clamped at any value the experimenter selects. With luck, the patch will contain only a single channel, and the behavior of this channel can be studied. Changing the membrane potential of a patch of axonal membrane from 80 to 65 mV has little effect on the voltage-gated sodium channels. Changing the membrane potential from 65 to 40 mV, however, causes these channels to pop open.

Front 19

Characteristic Pattern of Behaviour for Voltage-Gated Sodium Channels

Back 19

1. They open with little delay.
2. They stay open for about 1 msec and then close (inactivate).
3. They cannot be opened again by depolarization until the membrane
potential returns to a negative value near threshold.

A single channel does not an action potential make. The membrane of an axon may contain thousands of sodium channels per square micrometer (μm2), and the concerted action of all these channels is required to generate
what we measure as an action potential.

Front 20

Steps of Patch Clamping

Back 20

Lower the fire-polished tip of a glass recording electrode, 1–5 μm in diameter, onto the membrane of the neuron (part a), and then apply suction through the electrode tip (part b). A tight seal forms between the walls of the electrode and the underlying patch of membrane. This “gigaohm” seal leaves the ions in the electrode only one path to take, through the channels in the underlying patch of membrane. If the electrode is then withdrawn from the cell, the membrane patch can be torn
away (part c), and ionic currents can be measured as steady voltages are applied across the membrane (part d). One can resolve currents flowing through single channels. If the patch contains a voltage-gated sodium channel, for example, then changing the membrane potential from 65 to 40 mV will cause the channel to open, and current (I) will flow through it (part e).

Front 21

Generalized epilepsy with febrile seizures

Back 21

-This is a channelopathy due to single amino acid mutations in the extracellular regions of one
sodium channel

-The seizures in this disorder occur in response to fever. They are usually confined to early childhood, between 3 months and 5 years of age. Although precisely how the seizures are triggered by an increase in brain temperature is not clear, among other effects, the mutations
slow the inactivation of the sodium channel, prolonging the action potential.

Front 22

Tetrodotoxin (TTX)

Back 22

-Pufferfish toxin, which selectively blocks the sodium channel by binding to a specific site on the channel, and blocks all sodium-dependent action potentials

-Fatal if ingested

Front 23

Saxitoxin

Back 23

Saxitoxin is produced produced by dinoflagellates of the genus Gonyaulax and concentrated in clams, mussels, and other shellfish that feed on these marine protozoa. Occasionally, the dinoflagellates bloom, causing what is known as a “red tide.” Eating shellfish at these times can be fatal, because of the unusually high concentration of the toxin.

-This is a sodium channel toxin

Front 24

Batrachotoxin

Back 24

-Instead of blocking sodium channel toxins, it causes them to open inappropriately

-Isolated from the skin of species of Colombian frog

-Batrachotoxin causes the channels to open at
more negative potentials and to stay open much longer than usual, thus scrambling the information encoded by the action potentials. Toxins produced by lilies (veratridine) and buttercups (aconitine) have a similar mechanism of action. Sodium channel inactivation is also disrupted by toxins
from scorpions and sea anemones.

Front 25

Voltage-Gated Potassium Channels

Back 25

-Unlike sodium gates, potassium gates do not open immediately upon depolarization; it
takes about 1 msec for them to open. Because of this delay, and because this potassium conductance serves to rectify, or reset, the membrane potential, they called this conductance the delayed rectifier

-The channel proteins consist of four separate polypeptide subunits that come together to form a pore between them.

Front 26

Orthodromic Conduction

Back 26

Normally, action potentials conduct only in one direction; this is called orthodromic conduction.

Front 27

Antidromic Conduction

Back 27

Backward propagation, sometimes elicited experimentally, is called antidromic conduction.) Note that because the axonal membrane is excitable (capable of generating action potentials) along its entire length,
the impulse will propagate without decrement. The fuse works the same way, because it is combustible along its entire length. Unlike the fuse, however, the axon can regenerate its firing ability.

Front 28

Lidocaine

Back 28

Lidocaine and other local anesthetics prevent action potentials by binding to the voltage-gated sodium channels. The binding site for lidocaine has been identified as the S6
alpha helix of domain IV of the protein (Figure A). Lidocaine cannot gain access to this site from the outside.The anesthetic first must cross the axonal membrane and then
pass through the open gate of the channel to find its binding site inside the pore.This explains why active nerves are blocked faster (the sodium channel gates are open more
often). The bound lidocaine interferes with the flow of Na that normally results from depolarizing the channel.

Front 29

Saltatory conduction

Back 29

Myelin allows current to spread farther and faster between nodes, thus speeding action potential conduction. In myelinated axons, action
potentials skip from node to node.

Front 30

Spike-Initiation Zone

Back 30

Another word for the spike initiation zone is "Axon Hillock" Its the area where the soma ends and the axon starts. Here, all EPSPs (Excitatory post-synaptic potentials) and IPSPs (inhibitory post-synaptic potentials) sum up in order to make an action potential either more or less likely to occur. Think of the axon hillock as a funnel where all the neuronal input gathers.

Front 31

Chemical Synapses

Back 31

-Comprises of most of the synapses in the body

-The presynaptic and postsynaptic membranes at chemical synapses are separated by a synaptic cleft that is 20–50 nm wide, 10 times the width of
the separation at gap junctions.

-The axon terminal typically contains dozens of small membrane-enclosed spheres, each about 50 nm in diameter, called synaptic vesicles

-

Front 32

Secretory Granules

Back 32

Many axon terminals also contain larger vesicles, each about 100 nm in diameter, called secretory granules. Secretory granules contain soluble protein that appears dark in the electron microscope, so they are sometimes called large, dense-core vesicles.

Front 33

Membrane Differentiations

Back 33

Dense accumulations of protein adjacent to and within the membranes on either side of the synaptic cleft are collectively called membrane differentiations. On the presynaptic side, proteins jutting into the cytoplasm of the terminal along the intracellular face of the membrane sometimes look like a field of tiny pyramids, called active zones.

Front 34

Active zones

Back 34

The pyramids, and the membrane associated
with them, are the actual sites of neurotransmitter release, called active
zones. Synaptic vesicles are clustered in the cytoplasm adjacent to the
active zones. These pyramids are on the presynaptic side of the synapse.

Front 35

Postsynaptic Density

Back 35

The protein thickly accumulated in and just under the postsynaptic membrane is called the postsynaptic density. The postsynaptic density contains the neurotransmitter receptors, which convert the intercellular chemical signal (i.e., neurotransmitter) into an intracellular signal (i.e., a change in membrane potential, or a chemical change) in the postsynaptic cell.

Front 36

CNS Synapses

Back 36

In the CNS, different types of synapse may be distinguished by which part of the neuron is postsynaptic to the axon terminal. If the postsynaptic membrane is on a dendrite, the synapse is said to be axodendritic.If the postsynaptic membrane is on the cell body, the synapse is said to be axosomatic. In some cases, the postsynaptic membrane is on another axon, and these synapses are called axoaxonic. When dendrites form synapses with each other, they are called dendrodendrites.

Front 37

Gray's type I versus gray's type II synapses

Back 37

Synapses in which the membrane differentiation on the postsynaptic side is thicker than that on the presynaptic side are called asymmetrical synapses, or Gray’s type I synapses; those in which the membrane differentiations are of similar thickness are called symmetrical synapses, or Gray’s type II synapses. Gray’s type I synapses are usually excitatory, while Gray’s type II synapses are
usually inhibitory.

Front 38

Neuromuscular Junction

Back 38

Synaptic junctions also exist outside the
central nervous system. For example, axons of the autonomic nervous system innervate glands, smooth muscle, and the heart. Chemical synapses also occur between the axons of motor neurons of the spinal cord and
skeletal muscle. Such a synapse is called a neuromuscular junction, and it has many of the structural features of chemical synapses in the CNS.

Front 39

Three chemical categories of neurotransmitters

Back 39

(1) amino acids, (2) amines, and (3) peptides

Front 40

Amino acids examples

Back 40

GABA, Glutamate, Glycine

Front 41

Amines

Back 41

Ach, dopamine, norepinephrine, histamine, epinephrine, 5-HT

Front 42

Peptides

Back 42

Cholecystokinin (CCK), Dynorphin, Enkephalins, N-acetylaspartylglutamate (NAAG), neuropeptide Y, somatostatin, substance P, thyrotropin-releasing hormone, vasoactive intestinal peptide (VIP)

Front 43

The amine acetylcholine (ACh)

Back 43

The amine acetylcholine (ACh) mediates fast synaptic transmission
at all neuromuscular junctions. Slower forms of synaptic transmission
in the CNS and in the periphery are mediated by transmitters from all three
chemical categories. The amino acids mediate synaptic transmission at all CNS synapses.

Front 44

Voltage-Gated Calcium Channels

Back 44

-Similar to sodium channels, except they are for calcium

-There is a large inward driving force on Ca2. Remember that the internal calcium ion concentration—[Ca2]i—at rest is very low, only 0.0002 mM; therefore, Ca2 will flood the cytoplasm of the axon terminal as long as the
calcium channels are open. The resulting elevation in [Ca2]i is the signal that causes neurotransmitter to be released from synaptic vesicles.

Front 45

Transmitter-Gated Ion Channels

Back 45

Receptors known as transmittergated
ion channels are membrane-spanning proteins consisting of four or
five subunits that come together to form a pore between them (Figure 5.13).
In the absence of neurotransmitter, the pore is usually closed. When neurotransmitter
binds to specific sites on the extracellular region of the channel,
it induces a conformational change—just a slight twist of the subunits—which within microseconds causes the pore to open. The functional consequence
of this depends on which ions can pass through the pore.

Front 46

T and V SNARES

Back 46

Vesicles have “v-SNAREs,” and the outer membrane has “t-SNAREs” (for “target”
membrane). The cytosolic ends of these two complementary types
of SNAREs can bind tightly to one another, allowing a vesicle to “dock”
very close to a presynaptic membrane and nowhere else (Figure A).

Front 47

excitatory postsynaptic
potential (EPSP)

Back 47

A transient postsynaptic membrane depolarization caused by the presynaptic release of neurotransmitter is called an excitatory postsynaptic potential (EPSP). Synaptic activation of AChgated and glutamate-gated ion channels causes EPSPs.

Front 48

inhibitory
postsynaptic potential (IPSP)

Back 48

Because it tends to bring the membrane potential away from threshold
for generating action potentials, this effect is said to be inhibitory. A
transient hyperpolarization of the postsynaptic membrane potential caused
by the presynaptic release of neurotransmitter is called an inhibitory
postsynaptic potential (IPSP)

Front 49

G-protein-coupled receptors steps

Back 49

1. Neurotransmitter molecules bind to receptor proteins embedded in the
postsynaptic membrane.
2. The receptor proteins activate small proteins, called G-proteins, that are
free to move along the intracellular face of the postsynaptic membrane.
3. The activated G-proteins activate “effector” proteins.

Front 50

second messengers

Back 50

Effector proteins can be G-protein-gated ion channels in the membrane
(Figure 5.16a), or they can be enzymes that synthesize molecules called
second messengers that diffuse away in the cytosol (Figure 5.16b). Second
messengers can activate additional enzymes in the cytosol that can
regulate ion channel function and alter cellular metabolism.

Front 51

metabotropic receptors

Back 51

Because Gprotein- coupled receptors can trigger widespread metabolic effects, they are
often referred to as metabotropic receptors.

Front 52

Autoreceptors

Back 52

Besides being a part of the postsynaptic density, neurotransmitter receptors are also commonly found in the membrane of the
presynaptic axon terminal. Presynaptic receptors that are sensitive to the
neurotransmitter released by the presynaptic terminal are called autoreceptors. Autoreceptors appear to function as a sort of
safety valve to reduce release when the concentration of neurotransmitter
in the synaptic cleft gets too high.

Front 53

Cholinergic

Back 53

A term to describe cells that produce and release acetylcholine

Front 54

Noradrenergic

Back 54

The neurons that use the amine neurotransmitter norepinephrine (NE) are called noradrenergic.

Front 55

Glutamatergic and GABAergic synapses

Back 55

They use glutamate and GABA as synapses

Front 56

Peptidergic

Back 56

Peptidergic synapses use peptides

Front 57

Cholinergic system

Back 57

ACh and all the molecular machinery associated with it are collectively called the cholinergic system.

Front 58

Three criteria of a neurotransmitter

Back 58

1. The molecule must be synthesized and stored in the presynaptic neuron.
2. The molecule must be released by the presynaptic axon terminal upon
stimulation.
3. The molecule, when experimentally applied, must produce a response in the postsynaptic cell that mimics the response produced by the release of neurotransmitter from the presynaptic neuron.

Front 59

Immunocytochemistry

Back 59

-One of the two methods along with in situ hybridization in the localization of transmitters and transmitter-synthesizing enzymes

-Once the neurotransmitter candidate has been chemically purified, it is injected into the bloodstream of an animal, where it stimulates an immune response. Antibodies bind to specific sites on the foreign molecule. The best antibodies for immunocytochemistry bind very tightly to the transmitter of interest, and
bind very little or not at all to other chemicals in the brain. When these labeled antibodies are applied to a section of brain tissue, they will color just those cells that contain the transmitter candidate

Front 60

In situ hybridization

Back 60

-One of the two methods along with immunocytochemistry in the localization of transmitters and transmitter-synthesizing enzymes.

-If the sequence of nucleic acids in a strand of mRNA is known, it is possible to construct in the lab a complementary strand that will stick, like a strip of Velcro, to the mRNA molecule. The complementary strand is called a probe,
and the process by which the probe bonds to the mRNA molecule is called hybridization

-In this method the probes are usually labelled by making them hyperactive

Front 61

Autoradiography

Back 61

Hybridized probes are detected by laying the brain tissue on a sheet of special film that is sensitive to radioactive emissions. After exposure to the tissue, the film is developed
like a photograph, and negative images of the radioactive cells are visible as clusters of small dots. This technique for viewing the distribution of radioactivity is called autoradiography.

Front 62

How do in vitro brain slices work?

Back 62

-Since most of the synapses in the CNS (unlike in the PNS) are intermingled, they are difficult to study, so it's hard to stimulate a single population of synapses with a single neurotransmitter

-To stimulate release, the slices are bathed in a solution containing a high K concentration. This treatment causes a large membrane depolarization, thereby stimulating transmitter release from the axon terminals in the tissue. Because transmitter release requires the entry of Ca2 into the axon terminal, it must also be shown that the release of the neurotransmitter candidate from the tissue slice after depolarization occurs only when Ca2 ions are present in the bathing solution.

Front 63

Microionophoresis

Back 63

-A method used to study synaptic mimicry in postsynaptic cells

-This method enables a researcher to apply drugs or neurotransmitter candidates in very small amounts to the surface of neurons. The responses generated by the drug can be compared to those generated by synaptic stimulation.

Front 64

Receptor Subtype

Back 64

As a rule, no two neurotransmitters bind to the same receptor; however, one neurotransmitter can bind to many different receptors. Each
of the different receptors a neurotransmitter binds to is called a receptor subtype. For example, acetylcholine binds to different receptors and has two subtypes.

Front 65

Two ACh receptor subtypes

Back 65

Nicotinic ACh receptors in skeletal muscle and muscarinic ACh receptors in the heart. Nicotinic and muscarinic receptors also exist in the brain.

Front 66

Three receptor subtypes of glutamate receptors

Back 66

AMPA, NMDA, kainate receptors.

The neurotransmitter glutamate activates all
three receptor subtypes, but AMPA acts only at the AMPA receptor, NMDA acts only at the NMDA receptor, and so on.

Front 67

Opiates

Back 67

Opiates are a broad class of drugs that are both medically important and commonly abused. Their effects include pain relief, euphoria,
depressed breathing, and constipation.

Front 68

Endorphins

Back 68

Endorphins are endogenous opioid inhibitory neuropeptides. They are produced by the central nervous system and pituitary gland. The term implies a pharmacological activity (analogous to the activity of the corticosteroid category of biochemicals) as opposed to a specific chemical formulation.

Front 69

Enkephalins

Back 69

-A type of opiate neurotransmitters

-An enkephalin is a pentapeptide involved in regulating nociception in the body. The enkephalins are termed endogenous ligands, as they are internally derived and bind to the body's opioid receptors.

Front 70

Ligand-binding method

Back 70

Any chemical compound that binds to a specific site on a receptor is called a ligand for that receptor. The technique of studying receptors using radioactively labeled ligands is called the ligand-binding method. Ligand-binding methods have been enormously
important for mapping the anatomical distribution of different neurotransmitter
receptors in the brain.

Front 71

Dale’s principle

Back 71

The idea that a neuron has only one neurotransmitter is often called Dale’s principle. Many peptide-containing neurons violate Dale’s principle because these cells usually release more than one neurotransmitter: an amino acid or amine and a peptide.

Front 72

Co-transmitters

Back 72

When two or more transmitters are released from one nerve terminal, they are called co-transmitters. Co-transmitters violate Dale's principle

Front 73

Choline acetyltransferase (ChAT)

Back 73

The enzyme involved in synthesis of ACh. Only cholinergic neurons contain ChAT, so this enzyme is a good marker for cells that use ACh as a neurotransmitter.

Front 74

ACh Transporter

Back 74

ChAT synthesizes ACh in the cytosol of the axon terminal, and the neurotransmitter is concentrated in synaptic vesicles by the actions of an ACh transporter.

Front 75

Rate limited step of ACh synthesis

Back 75

Choline is taken up by the cholinergic axon terminals via a specific transporter. Because the availability of choline limits how much ACh can be synthesized in the axon terminal, the transport of choline into the neuron is said to be the rate-limiting step in ACh synthesis.

Front 76

Acetylcholinesterase (AChE)

Back 76

AChE degrades ACh into choline and acetic acid. AChE is secreted into the synaptic cleft and is associated with cholinergic axon terminal membranes. However, AChE is also manufactured by some noncholinergic neurons, so this enzyme is not as useful a marker for cholinergic synapses as ChAT. Much of the resulting choline is taken up by
the cholinergic axon terminal and reused for ACh synthesis.

Front 77

Catecholamines

Back 77

-The amino acid tyrosine is the precursor for three different amine neurotransmitters that contain a chemical structure called a catechol. The catecholamine neurotransmitters are dopamine (DA), norepinephrine (NE), and epinephrine, also called adrenaline. Catecholaminergic neurons are found in regions of the nervous system involved
in the regulation of movement, mood, attention, and visceral function.

Front 78

Tyrosine hydroxylase

Back 78

This catalyzes the first step in catecholamine synthesis, the conversion of tyrosine to a compound called dopa. All catecholamines contain this.

Front 79

Two general types of neurotransmitter transporter

Back 79

One type, the neuronal membrane transporter, shuttles transmitter from the extracellular fluid, including the synaptic cleft, and concentrates it up to 10,000 times higher within the cytosol of the presynaptic terminal. A second type, the vesicular transporter,
then crams transmitter into vesicles at concentrations that may be 100,000 times higher than in the cytosol.

Front 80

Example of end-product inhibition in catecholamines

Back 80

Decreased catecholamine release by the
axon terminal causes the catecholamine concentration in the cytosol to rise, thereby inhibiting TH (tyrosine hydroxylase).

Front 81

A strategy to treat Parkinson's disease using catecholamines or precursors

Back 81

One strategy for treating Parkinson’s disease is the administration of dopa, which causes an increase in DA synthesis in the surviving neurons, increasing the amount of DA available for release.

Front 82

Dopamine-hydroxylase (DBH),

Back 82

An enzyme that converts dopamine into norepinephrine. It is interesting to note
that DBH is not found in the cytosol, but instead is located within the synaptic vesicles. Thus, in noradrenergic axon terminals, DA is transported
from the cytosol to the synaptic vesicles, and there it is made into NE.

Front 83

Phentolamine N-methyltransferase
(PNMT)

Back 83

-Adrenergic neurons contain this enzyme

-It converts norepinephrine into epinephrine

-Curiously, PNMT is in the cytosol of adrenergic axon terminals. Thus, NE must first be synthesized in the vesicles, released into the cytosol for conversion into epinephrine, and then the epinephrine must again be transported into vesicles for release.

Front 84

Monoamine oxidase (MAO)

Back 84

-This helps to degrade catecholamines

-Once inside, the axon terminal, the catecholamines may be reloaded into synaptic vesicles for reuse, or they may be enzymatically destroyed by the action of monoamine oxidase (MAO), an enzyme found on the outer membrane of mitochondria.

Front 85

Synthesis of serotonin

Back 85

Tryptophan is converted first into an intermediary called 5-HTP (5-hydroxytryptophan) by the enzyme tryptophan hydroxylase. The 5-HTP is then converted to 5-HT by the enzyme 5-HTP decarboxylase. Serotonin synthesis appears to be limited by the availability of tryptophan
in the extracellular fluid bathing neurons.

Front 86

Serotonin Reuptake

Back 86

Following release from the axon terminal, 5-HT is removed from the synaptic cleft by the action of a specific transporter. The process of serotonin reuptake, like catecholamine reuptake, is sensitive to a number of different
drugs. For example, several clinically useful antidepressant drugs, including fluoxetine (trade name Prozac), are selective inhibitors of serotonin reuptake. Once it is back in the cytosol of the serotonergic axon terminal, the
transmitter is either reloaded into synaptic vesicles or degraded by MAO.

Front 87

How are glutamate and GABA synthesized and broken down?

Back 87

Glutamatic acid decarboxylase (GAD) is used to turn glutamate into GABA. The synaptic actions of the amino acid neurotransmitters are terminated by selective uptake into the presynaptic terminals and glia, once again via
specific Na-dependent transporters. Inside the terminal or glial cell, GABA is metabolized by the enzyme GABA transaminase.

Front 88

Retrograde messengers

Back 88

-Endocannabinoids (endogenous cannabinoids) are an example of this

-Retrograde signalling is communication from the "post" to "pre" direction

-Retrograde messengers serve as a kind of feedback system to regulate the conventional forms of synaptic transmission, which of
course go from “pre” to “post."

Front 89

Two types of cannabinoid receptors

Back 89

Two types of cannabinoid receptors are now known: CB1 receptors are in the brain, and CB2 receptors are mainly in immune tissues elsewhere in the body.

Front 90

Several unusual qualities about endocannabinoids

Back 90

1. They are not packaged in vesicles like most other neurotransmitters; instead, they are manufactured rapidly and on-demand.
2. They are small and membrane permeable; once synthesized, they can diffuse rapidly across the membrane of their cell of origin to contact neighboring cells.
3. They bind selectively to the CB1 type of cannabinoid receptor, which is mainly located on certain presynaptic terminals.

Front 91

Nitric Oxide

Back 91

NO may be another example of a retrograde messenger. Because NO is small and membrane permeable, similar to endocannabinoids, it can diffuse much more freely than most other transmitter molecules, even penetrating through one cell to affect another beyond it. Its influence may spread throughout a small region of local tissue, rather than being confined to the site of the cells that released them. On the other
hand, NO is evanescent and breaks down very rapidly.

Front 92

Transmitter-Gated Channels

Back 92

-11 nm long

-It is a pentamer, an amalgam of five protein subunits arranged like the staves of a barrel to form a single pore through the membrane

Front 93

Properties of amino acid-gated channels

Back 93

-The pharmacology of their binding sites describes which transmitters affect them and how drugs interact with them.
-The kinetics of the transmitter binding process and channel gating determine the duration of their effect.
-The selectivity of the ion channels determines whether they produce excitation
or inhibition and whether Ca2 enters the cell in significant amounts.
-The conductance of open channels helps determine the magnitude of their effects.

Front 94

Glutamate-Gated Channels

Back 94

Three glutamate receptor subtypes bear the names of their selective agonists: AMPA, NMDA, and kainate. Each of these is a glutamate-gated ion channel. The AMPAgated
and NMDA-gated channels mediate the bulk of fast excitatory synaptic transmission in the brain. Kainate receptors also exist.

AMPA-gated channels are permeable to both Na and K, and most of them are not permeable to Ca2.

NMDA-gated channels are permeable to Ca2, and it is voltage dependent.

Front 95

The Ubiquitous G-Proteins

Back 95

G-proteins all have the same basic mode of operation:
1. Each G-protein has three subunits, termed alpha, beta, and gamma. In the resting state, a guanosine diphosphate (GDP) molecule is bound to the G-alpha subunit, and the whole complex floats around on the inner surface of
the membrane.
2. If this GDP-bound G-protein bumps into the proper type of receptor and if
that receptor has a transmitter molecule bound to it, then the G-protein releases
its GDP and exchanges it for a GTP that it picks up from the cytosol.
3. The activated GTP-bound G-protein splits into two parts: the G-alpha subunit
plus GTP, and the G-beta-gamma complex. Both can then move on to influence various
effector proteins.
4. The G-alpha subunit is itself an enzyme that eventually breaks down GTP into GDP. Therefore, G-alpha eventually terminates its own activity by converting the bound GTP to GDP.

5. The G-alpha and G-beta-gamma subunits come back together, allowing the cycle to begin
again.

Front 96

The Shortcut Pathway of G-protein coupled effector systems

Back 96

(a) G-proteins in heart muscle are activated by ACh binding to muscarinic receptors. (b) The activated G subunit directly gates a potassium channel.

Front 97

Second Messenger Cascade

Back 97

The whole process that couples the neurotransmitter, via multiple steps, to activation
of a downstream enzyme is called a second messenger cascade.

Front 98

Divergence

Back 98

The ability of one transmitter to activate more than one subtype of receptor, and cause more than one type of postsynaptic response, is called divergence.

Front 99

Convergence

Back 99

Neurotransmitters can also exhibit convergence of effects. Multiple transmitters,
each activating their own receptor type, can converge to affect the same effector systems. Convergence in a single cell can occur at the level of the G-protein, the second messenger cascade, or the type of ion channel.

Front 100

Anatomical references in the brain

Back 100

The anatomical reference, pointing toward the rat’s nose is known as anterior or rostral. The direction pointing toward the rat’s tail is posterior or caudal. The direction pointing up is dorsal, and the direction pointing down is ventral.

Front 101

Bilateral symmetry

Back 101

A characteristic of the brain which means it can be divided into two equal halves. The invisible line running down the middle is called the midline, and structures near the midline are called medial. Structures farther away from the midline are called lateral. If two structures are on the same side it is called ipsilateral and structures on opposites sides are called contralateral.

Front 102

Three anatomical planes of section

Back 102

Midsagittal plane: splits the brain in left and right halves. Sections parallel to the midsagittal plane are in the sagittal plane. The horizontal plane is parallel to the ground and splits the brain in dorsal and ventral sections. The coronal plane is perpendicular to the ground and it splits the anterior and posterior of the brain.

Front 103

The Cerebrum

Back 103

-Largest and rostal-most part of the brain

-There are two cerebral hemispheres down the middle, separated by the deep sagittal fissure

-The right hemisphere receives sensations and movements from the left side of the body and the left hemisphere receives sensations and movements from the left side of the body

Front 104

The Cerebellum

Back 104

-Lies behind the cerebrum, like a little brain

-Contains as many neurons as both cerebral hemispheres combined

-It is a primarily motor control center with extentions in the cerebrum and spinal cord

-The left side of the cerebellum corresponds to the left side of the body and the right side corresponds to the right side of the body

Front 105

The Brain Stem

Back 105

-Forms a stalk from which the cerebrum and cerebellum sprout

-Serves to relay information from cerebrum to spinal cord and cerebellum, and vice versa

-The the brain stem is also the site where
vital functions are regulated, such as breathing, consciousness, and the control of body temperature

Front 106

The Spinal Cord

Back 106

-The spinal cord is encased in the bony vertebral column and is attached to the brain stem

-It is the major conduit of information from the skin, joints, and muscles of the body to the brain, and vice versa

-A transection of the spinal cord results in anesthesia (lack of feeling) and paralysis of muscles in parts of the body caudal to the cut. This paralysis does not mean that they cannot function, but that they cannot be controlled by the brain

-The spinal cord communicates with the body via spinal nerves, which are part of the PNS

-Each spinal nerve attaches to the spinal cord by means of two branches, the dorsal root and the ventral root

Front 107

The Somatic PNS

Back 107

-All the spinal nerves that innervate the skin, the joints, and the muscles that are under voluntary control are part of the somatic PNS.

-The somatic sensory axons, which innervate and collect information from the skin, muscles, and joints, enter the spinal cord via the dorsal roots. The cell bodies of these neurons lie outside the spinal cord in clusters called dorsal root ganglia. There is a dorsal root ganglion for each spinal nerve

Front 108

The Visceral PNS (also called Autonomic Nervous System)

Back 108

-The visceral PNS consists of the neurons that
innervate the internal organs, blood vessels, and glands.

Front 109

Afferent and Efferent Neurons

Back 109

Afferent neurons carry information towards a point and efferent neurons carry information away from a point.

Front 110

Cranial nerves

Back 110

-There are 12 cranial nerves that arise from the brain stem and innervate (mostly) the head

-Many cranial nerves contain a complex mixture of axons that perform different functions

-They are located in the CNS, and both divisions of the PNS

Front 111

The Meninges

Back 111

-The three membranes protecting the brain from the bones in the skull

-The outer most membrane is the dura mater, has a leather-like consistency and forms a tough inelastic bag

-The arachnoid membrane is right under the dura mater, and it has an appearance and consistency like a spider web. If the blood vessels passing through the dura are ruptured, blood can collect here and form what is called a subdural hematoma

-The pia mater is under the arachnoid membrane, and it is a thin membrane that adheres closely to the brain

-Along the pia run many blood vessels that ultimately dive into the substance of the underlying brain. The pia is separated from the arachnoid by a fluid-filled space. This subarachnoid space is filled with salty clear liquid called cerebrospinal fluid (CSF). Thus, in
a sense, the brain floats inside the head in this thin layer of CSF.

Front 112

The Ventricular System

Back 112

-The fluid-filled caverns and canals inside the brain constitute the ventricular system

-The fluid in this system is the cerebrospinal fluid, which is the same as the fluid in the subarachnoid space, which is produced by choroid plexus in the ventricles

-The CSF flows from the paired ventricles of the
cerebrum to a series of connected, unpaired cavities at the core of the brain stem. CSF exits the ventricular system and enters the subarachnoid space by way of small openings, or apertures, located near where the
cerebellum attaches to the brain stem.

-In the subarachnoid space, CSF is absorbed by the blood vessels at special structures called arachnoid villi. If the normal flow of CSF is disrupted, brain damage can result

Front 113

Computed Tomography

Back 113

The goal of CT is to generate an image of a slice of brain. To accomplish this, an X-ray source is rotated around the head within the plane of the desired cross section. On the other side of the head, in the trajectory of the X-ray beam, are
sensitive electronic sensors of X-irradiation.

Front 114

Magnetic Resonance Imaging (MRI)

Back 114

-Yields a more detailed map of the brain than CT

-it doesn't require X-irradiation, and images of brain slices can be made in any plane desired

-MRI uses information about how hydrogen atoms in the brain respond to perturbations of a strong magnetic field. The electromagnetic signals emitted by the atoms are detected by an array of sensors around the head and fed to a powerful computer that constructs a map of the brain.

Front 115

Functional Brain Imaging

Back 115

-Two types are PET (Positron emission tomography) and functional magnetic resonance imaging (fMRI)

-The basic principle is simple. Neurons that are active demand more glucose and oxygen. The brain vasculature responds to neural activity by directing more blood to the active regions. Thus, by detecting changes in blood flow, PET
and fMRI reveal the regions of brain that are most active under different circumstances.

Front 116

Gray Matter

Back 116

A generic term for a collection of neuronal cell bodies in the CNS. When a freshly dissected brain is cut open, neurons appear gray.

Front 117

Cortex

Back 117

Any collection of neurons that form a thin sheet, usually at the brain’s surface. Cortex is Latin for
“bark.” Example: cerebral cortex, the sheet of neurons found just under the surface of the cerebrum.

Front 118

Nucleus

Back 118

A clearly distinguishable mass of neurons, usually deep in the brain (not to be confused with the
nucleus of a cell). Nucleus is from the Latin word for “nut.” Example: lateral geniculate nucleus, a cell group in the brain stem that relays information from the eye to the cerebral cortex.

Front 119

Substantia

Back 119

A group of related neurons deep within the brain, but usually with less distinct borders than those of nuclei. Example: substantia nigra (from the Latin for “black substance”), a brain stem cell group involved in the control of voluntary movement.

Front 120

Locus

Back 120

A small, well-defined group of cells. Example: locus coeruleus (Latin for “blue spot”), a brain stem cell group involved in the control of wakefulness and behavioral arousal.

Front 121

Ganglion

Back 121

A collection of neurons in the PNS. Ganglion is from the Greek for “knot.” Example: the dorsal root ganglia, which contain the cells bodies of sensory axons entering the spinal cord via the dorsal roots. Only one cell group in the CNS goes by this name: the basal ganglia, which are structures lying deep within the cerebrum that control movement.

Front 122

Endoderm

Back 122

The endoderm ultimately gives rise to the lining of many of the internal organs (viscera)

Front 123

Mesoderm

Back 123

From the mesoderm arise the bones of the skeleton and the muscles.

Front 124

Ectoderm

Back 124

The nervous system and the skin derive entirely from the ectoderm. The ectoderm also gives rise to the neural plate.

Front 125

Nerve

Back 125

A bundle of axons in the PNS. Only one collection of CNS axons is called a nerve: the optic nerve.

Front 126

White matter

Back 126

A generic term for a collection of CNS axons. When a freshly dissected brain is cut open, axons
appear white.

Front 127

Tract

Back 127

A collection of CNS axons having a common site of origin and a common destination. Example:
corticospinal tract, which originates in the cerebral cortex and ends in the spinal cord.

Front 128

Bundle

Back 128

A collection of axons that run together but do not necessarily have the same origin and destination.
Example: medial forebrain bundle, which connects cells scattered within the cerebrum and brain stem.

Front 129

Capsule

Back 129

A collection of axons that connect the cerebrum with the brain stem. Example: internal capsule, which connects the brain stem with the cerebral cortex.

Front 130

Commissure

Back 130

Any collection of axons that connect one side of the brain with the other side.

Front 131

Lemniscus

Back 131

A tract that meanders through the brain like a ribbon. Example: medial lemniscus, which brings touch information from the spinal cord through the brain stem.

Front 132

Formation of the neural tube and neural crest

Back 132

Formation of the neural tube and neural crest. These schematic illustrations follow
the early development of the nervous system in the embryo. The drawings above are dorsal
views of the embryo; those below are cross sections. (a) The primitive embryonic CNS
begins as a thin sheet of ectoderm. (b) The first important step in the development of the
nervous system is the formation of the neural groove. (c) The walls of the groove, called
neural folds, come together and fuse, forming the neural tube. (d) The bits of neural ectoderm
that are pinched off when the tube rolls up is called the neural crest, from which the
PNS will develop. The somites are mesoderm that will give rise to much of the skeletal
system and the muscles.

Front 133

Neurulation

Back 133

The process by which the neural plate becomes the neural tube is called neurulation. Neurulation occurs very early in embryonic development,
about 22 days after conception in humans.

Front 134

Differentiation

Back 134

The process by which structures become more complex and functionally specialized during development is called differentiation.

Front 135

Three Primary Brain Vesicles

Back 135

-The entire brain is comprised of three primary vesicles of the neural tube

-The prosencephalon is also called the forebrain. Behind the prosencephalon lies another vesicle called the mesencephalon, or midbrain. Caudal to this is the third primary vesicle, the rhombencephalon, or hindbrain. The rhombencephalon connects with the caudal neural tube, which gives rise to the
spinal cord.

Front 136

Differentiation of the forebrain

Back 136

-Secondary vesicles sprout off both sides of the proencephalon

-The unpaired structure after vesicles have sprouted off is called diencephalon

-Optic stalks are formed, and they become the optic nerves and the two retinas in adults

Front 137

Differentiation of the Telencephalon

Back 137

The telencephalon continues to develop
in four ways: (1) The telencephalic vesicles grow posteriorly so that they lie over and lateral to the diencephalon. (2) Another
pair of vesicles sprout off the ventral surfaces of the cerebral hemispheres, giving rise to the olfactory bulbs and related structures that participate in the sense of smell. (3) The cells of the walls of the telencephalon divide and differentiate into various structures. (4) White
matter systems develop, carrying axons to and from the neurons of the telencephalon.

Front 138

Fluid-filled spaces within the cerebral hemispheres

Back 138

The fluid-filled spaces within the cerebral hemispheres are called the lateral ventricles, and the space at the center of the diencephalon is
called the third ventricle. The paired lateral ventricles are a key landmark in the adult brain.

Front 139

Two types of gray matter in the telencephalon

Back 139

These neurons form two different types of gray matter in the telencephalon: the cerebral cortex and the basal telencephalon.

Front 140

What does the diencephalon differentiate into?

Back 140

The diencephalon differentiates into
two structures: the thalamus and the hypothalamus

Front 141

Three major white matter systems in the developing forebrain

Back 141

The cortical white matter contains all the axons that run to and from the neurons in the
cerebral cortex. The corpus callosum is continuous with the cortical white matter and forms an axonal bridge that links cortical neurons of the two cerebral hemispheres. The cortical white matter is also continuous with the internal capsule, which links the cortex with the brain stem, particularly
the thalamus.

Front 142

Cerebral Cortex

Back 142

The cortex is the brain structure that has expanded the most over the course of human evolution. Cortical neurons receive sensory information, form perceptions of the outside world, and command voluntary movements.

Front 143

The thalamus: gateway to the cerebral
cortex

Back 143

The sensory pathways from the eye, ear, and skin all relay in the thalamus before terminating in the cerebral cortex. The arrows indicate the direction of information flow.

Front 144

Optic nerve

Back 144

The optic nerve connects the eye to the brain. The optic nerve carries the impulses formed by the retina, the nerve layer that lines the back of the eye and senses light and creates impulses. These impulses are dispatched through the optic nerve to the brain, which interprets them as images.

Front 145

Differentiation of the midbrain

Back 145

-It differentiates little during subsequent brain development

-The dorsal surface of the mesencephalic vesicle becomes a structure called the tectum. The floor of the midbrain becomes the tegmentum. The CSF-filled space in between constricts into a narrow channel called the cerebral aqueduct. The aqueduct connects rostrally with the third ventricle of the
diencephalon.

Front 146

Two structures of the tectum in the midbrain

Back 146

-The superior colliculus receives direct input from the eye, so it is also called the optic tectum. One function of the optic tectum is to control eye movements, which it does via synaptic connections with the motor neurons that innervate the eye muscles.

-The inferior colliculus also receives sensory information, but from the ear instead of the eye. The inferior colliculus serves as an important relay station for auditory information en route to the thalamus.

Front 147

The Tegmentum In the Midbrain

Back 147

The tegmentum is one of the most colorful regions of the brain because it contains both the substantia nigra (the black substance) and the red nucleus. These two cell groups are involved in the control of voluntary movement. Other cell groups scattered in the midbrain have axons that project widely throughout much of the CNS and function to regulate consciousness, mood, pleasure, and pain.

Front 148

Differentiation of the Hindbrain

Back 148

The hindbrain differentiates into three important structures: the cerebellum,
the pons, and the medulla oblongata—also called, simply, the medulla. The pons contains nuclei that relay signals from the forebrain to the cerebellum, along with nuclei that deal primarily with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture.

The medulla contains the cardiac, respiratory, vomiting and vasomotorcenters and therefore deals with the autonomic (involuntary) functions of breathing, heart rate and blood pressure.

Front 149

Differentiation of the spinal cord

Back 149

The spinal canal (or vertebral canal or spinal cavity) is the space in vertebrae through which the spinal cord passes.Cut in cross section, the gray matter of the spinal cord (where the neurons
are) has the appearance of a butterfly. The upper part of the butterfly’s wing
is the dorsal horn, and the lower part is the ventral horn. The gray
matter between the dorsal and ventral horns is called the intermediate zone. Everything else is white matter, consisting of columns of axons that run up
and down the spinal cord.Lateral ventricles

Front 150

Lateral ventricles

Back 150

Related brain structures:

Cerebral cortex
Basal telencephalon

Front 151

Third ventricle

Back 151

Related brain structures:

Thalamus
Hypothalamus

Front 152

Cerebral aqueduct

Back 152

Related brain structures:

Tectum
Midbrain tegmentum

Front 153

Fourth Ventricle

Back 153

Related brain areas:

Cerebellum
Pons
Medulla

Front 154

Sulci and gyri

Back 154

The many convolutions on the surface of the human cerebrum: The grooves in the surface of the cerebrum are called sulci (singular: sulcus), and the bumps are called gyri (singular: gyrus). Remember, the thin sheet of neurons that lies just under the surface of the cerebrum is the cerebral cortex.

Front 155

Temporal lobe

Back 155

The tip of the “horn” lies right under
the temporal bone (temple) of the skull, so this portion of the brain is called
the temporal lobe.

Front 156

Frontal lobe

Back 156

The portion of the cerebrum lying just
under the frontal bone of the forehead is called the frontal lobe.

Front 157

Central Sulcus

Back 157

The deep central sulcus marks the posterior border of the frontal lobe, caudal to which lies the parietal lobe, under the parietal bone. Caudal to that, at the back of the cerebrum under the occipital bone, lies the occipital lobe

Front 158

Types of Cerebral Cortex

Back 158

First, the cell bodies of cortical neurons
are always arranged in layers, or sheets, that usually lie parallel to the surface of the brain. Second, the layer of neurons closest to the surface (themost superficial cell layer) is separated from the pia mater by a zone that
lacks neurons; it is called the molecular layer, or simply layer I. Third, at least one cell layer contains pyramidal cells that emit large dendrites, called apical dendrites, that extend up to layer I, where they form multiple branches.

Front 159

Cytoarchitectural map of the neocortex

Back 159

-Constructed by German neuroanatomist Korbinian Brodmann

-In this map, each area of cortex having
a common cytoarchitecture is given a number. Thus, we have “area 17” at the tip of the occipital lobe, “area 4” just anterior to the central sulcus in the frontal lobe, and so on.
What Brodmann guessed, but could not show, was that cortical areas that look different perform different functions.

Front 160

Three types of cortex that the neocortex exists in

Back 160

The first type consists of primary sensory areas, which are first to receive signals from the ascending sensory pathways. For example, area 17 is designated as primary visual cortex,
or V1, because it receives input from the eyes via a direct path: retina to thalamus to cortex. The second type of neocortex consists of secondary sensory areas, so designated because of their heavy interconnections with the primary sensory areas. The third type of cortex consists of motor areas, which are intimately involved with the control of voluntary movement

Front 161

Somatic Sensation

Back 161

Somatic sensation enables our body to feel, to ache, to chill, and to know what its parts are doing. It is sensitive to many kinds of stimuli: the pressure of objects against the skin, the position of joints and muscles, distension of the bladder, and the temperature of the limbs and of the brain itself. When stimuli become so
strong that they may be damaging, somatic sensation is also responsible for the feeling that is most offensive, but vitally important—pain.

Front 162

Two ways somatic sensory system is different from other sensory systems

Back 162

1. Its receptors are distributed throughout the body instead of being concentrated at small, specialized locations.

2. It responds to many different type of stimuli instead of just one.

Front 163

Two major types of skin

Back 163

The two major types of skin are called hairy and glabrous (hairless), as exemplified by the backs and palms of your hands.

Front 164

How sensitive is skin?

Back 164

Skin is sensitive enough that a raised dot
measuring only 0.006 mm high and 0.04 mm wide can be felt when stroked by a fingertip. For comparison, a Braille dot is 167 times higher.

Front 165

Mechanoreceptors

Back 165

-They are sensory receptors in the somatic sensory system which are sensitive to physical distortion such as bending or stretching.

-Present throughout the body, they monitor contact with the skin, as well as pressure in the heart and blood vessels, stretching of the digestive organs and urinary bladder, and force against the teeth. At the heart of each mechanoreceptor are unmyelinated axon branches. These axons have mechanosensitive ion channels; their gating depends on stretching, or changes in tension, of the surrounding membrane.

Front 166

Pacinian corpuscle

Back 166

-A mechanoreceptor that is in the skin, the largest and best studied receptor

-It can be as long as 2 mm and almost 1 mm in diameter—big enough to be seen with your
naked eye.

-functioning as a sensory receptor of pressure and vibration

-rapidly adapting

Front 167

Ruffini's endings

Back 167

Found in both hairy and glabrous skin, are slightly smaller than Pacinian corpuscles.

Slowly adapting

Front 168

Meissner's corpuscles

Back 168

Meissner’s corpuscles are about one tenth
the size of Pacinian corpuscles and are located in the ridges of glabrous skin (the raised parts of your fingerprints, for example)

-Rapidly adapting

Front 169

Merkel's disks

Back 169

Located within the epidermis, Merkel’s disks each consist of a nerve terminal and a flattened,
non-neural epithelial cell. In this case, it may be that the epithelial cell is the mechanically sensitive part, because it makes a synapse-like junction with the nerve terminal.

-Slowly adapting

Front 170

Krause end bulbs

Back 170

In Krause end bulbs, which lie in the border regions of dry skin and mucous membrane (around the lips and genitals, for example), the nerve terminals look like knotted balls of string.

Front 171

What vibrations are pacinian and meissner corpuscles sensitive to?

Back 171

Pacinian corpuscles are most sensitive to vibrations of about 200–300 Hz, while Meissner’s corpuscles respond best around 50 Hz.

Front 172

Two Point Discrimination

Back 172

Two-point discrimination varies at least twentyfold across the body. Fingertips have the highest resolution. The dots of Braille are 1 mm high and 2.5 mm apart; up to six dots make a letter. An experienced Braille reader can scan an index finger across a page of raised dots and read about 600 letters per minute, which is roughly as fast as someone reading aloud.

Front 173

Primary afferent neuron

Back 173

Axons bringing information from the somatic sensory receptors to the spinal cord or brain stem are the primary afferent axons of the somatic sensory system. The primary afferent axons enter the spinal cord through the dorsal roots; their cell bodies lie in the dorsal root ganglia. Primary afferent axons have widely varying diameters, and their size correlates with the type of sensory receptor to which they are attached.

Front 174

How are primary afferent neurons named according to their sizes?

Back 174

A-alpha: Group 1, have a diameter of 13-20 um, speed of 80-120 m/sec, their sensory receptors are proprioceptors of skeletal muscle

A-beta: Group 2, have diameter of 6-12 um, speed of 35-75, and their sensory receptors are mechanoreceptors of skin.=

A-gamma: Group 3, diameter 1-5 um, speed 5-30 m/sec, sensory receptors pain and temperature

C: Group 4, diameter 0.2-1.5, speed 0.5-2 m/sec, sensory receptors temperature, pain, itch

Front 175

Spinal segments

Back 175

-There are 30 of them in the spine

-They are divided into 4 groups, namely cervical which begin at the neck (and comprise of 8 sections), thoracic around mid body (next 12), lumbar cord (next 5, towards the end of the spinal cord), and sacral cord (end of spinal cord, last 5)

-Spinal nerves are named for the level of the spinal cord from which they exit and are numbered in order from rostral to caudal.

Front 176

Dermatosomes

Back 176

The segmental organization of spinal nerves and the sensory innervation of the skin are related. The area of skin innervated by the right and left dorsal roots of a single spinal segment is called a dermatome; thus, there is a
one-to-one correspondence between dermatomes and spinal segments. When
mapped, the dermatomes delineate a set of bands on the body surface.

Front 177

How do you lose sensation in one dermatosome?

Back 177

When a dorsal root is cut, the corresponding dermatome on that side of the body does not lose all sensation. The residual somatic sensation is explained by the fact that the adjacent dorsal roots innervate overlapping areas. To lose all sensation in one dermatome, therefore, three adjacent dorsal roots must be cut.

Front 178

Cauda equina

Back 178

The bundles of spinal nerves streaming
down within the lumbar and sacral vertebral column are called the cauda equina, Latin for “horse’s tail.” The cauda equina courses down the spinal column within a sack of dura, filled with cerebral spinal fluid (CSF).

Front 179

Lumbar puncture

Back 179

The cauda equina courses down the spinal
column within a sack of dura, filled with cerebral spinal fluid (CSF). In a method called lumbar puncture, used to collect CSF for medical diagnostic tests, a needle is inserted into this CSF-filled cistern at the midline (this procedure is also called a spinal tap). If the needle is inserted a little off center, however, a nerve can be touched. Not surprisingly, this causes a sensation of sharp pain in the dermatome supplied by that nerve.

Front 180

What is each half of the gray matter in the spinal cord divided into?

Back 180

Front 181

Second-order sensory neurons

Back 181

The neurons that receive sensory input from primary afferents are called second-order sensory neurons. Most of the second-order sensory neurons of the spinal cord lie within the dorsal horns.

Front 182

The dorsal column–medial lemniscal pathway

Back 182

The ascending branch of the large sensory axons (A-beta) enters the ipsilateral dorsal column of the spinal cord, the white matter tract medial to the dorsal horn. The dorsal columns carry information about tactile sensation and limb position toward the brain. The axons of the dorsal column terminate in the dorsal column nuclei, which lie at the junction of the spinal cord and medulla. At this point in the pathway, information is still represented ipsilaterally;
that is, touch information from the left side of the body is represented in the activity of cells in the left dorsal column nuclei. From this point onward, the somatic sensory system of one side of the brain is concerned with sensations deriving from the other side of the body. The medial lemniscus rises through the medulla, pons, and midbrain, and its axons synapse upon neurons of the ventral posterior (VP) nucleus of the thalamus. neurons of the VP nucleus then project to specific regions of primary somatosensory cortex, or S1.

Front 183

Contrast enhancement

Back 183

The amplification of differences in
the activity of neighboring neurons.

Front 184

Lateral inhibition

Back 184

Where neighboring cells inhibit one another.

Front 185

Trigeminal nerves

Back 185

The trigeminal nerve (the fifth cranial nerve, or simply CN V) is a nerve responsible for sensation in the face and motor functions such as biting and chewing. There are twin
trigeminal nerves, one on each side, and each breaks up into three peripheral nerves that innervate the face, mouth areas, the outer two-thirds of the tongue, and the dura mater covering the brain. Additional sensation from
the skin around the ears, nasal areas, and pharynx is provided by other cranial nerves: the facial (VII), glossopharyngeal (IX), and vagus (X).

Front 186

Area 3b

Back 186

Area 3b is the primary somatic sensory cortex because (1) it receives dense inputs from the VP nucleus of the thalamus; (2) its neurons are
very responsive to somatosensory stimuli (but not to other sensory stimuli); (3) lesions here impair somatic sensation; and (4) electrically stimulated, it evokes somatic sensory experiences. Area 3a also receives a dense
input from the thalamus; however, this region is concerned with the sense of body position rather than touch.

Front 187

Somatotopy

Back 187

The mapping of the body’s surface sensations onto a structure in the brain is called somatotopy.

Front 188

Retinotopy

Back 188

We have seen previously that the brain has maps of other sensory surfaces, such as the light-sensitive retina in the eye (retinotopy)

Front 189

Tonotopy

Back 189

The brain has maps of sensory surfaces, such as the frequency sensitive cochlea in the inner ear (tonotopy).

Front 190

Homunculus

Back 190

A somatotopic map is sometimes called a homunculus (from the Latin diminutive of “man”; the little man in the brain).