Chapter 12 - Neural Tissue (B)

Front 1

Action Potentials

Back 1

changes in transmembrane potential that, once initiated, propogate along the entire length of an axon, ultimately reaching the synaptic terminal

Front 2

Threshold

Back 2

when depolarization is large enough to initiate an action potential and the voltage-gated channels open.

Threshold for an axon is typically around -60mV corresponding to a depolarization of 10mV.

Front 3

All-or-None Principal

Back 3

if the depolarization is large enough to reach the threshold then an action potential occurs. There is either an action potential or no action potential. No in between.

Front 4

Generation of action potential (4 steps)

Back 4

1. Depolarization to threshold

2. Activation of soidum channels & rapid depolarization

3. Inactivation of Sodium Channels & Activation of Potassium Channels

4. The return to normal permeability

Front 5

Depolarization to threshold

Back 5

- a graded potential at the axon hillock bring flow of Na+ ions that causes depolarization of the membrane beyond the 'threshold' level of that membrane. Hence, the axon hillock is the site of initiation of an action potential.

Front 6

Activation of soidum channels & rapid depolarization

Back 6

at threshold, numerous voltage-gated sodium channels open and sodium ions rush into the cytoplasm.

- rapid diffusion of Na+ ions is driven by the electrochemical gradient, which causes a rapid depolarization

-the transmembrane potential has changed from -60mV to positive values

Front 7

Inactivation of sodium channels & activation of potassium channels

Back 7

when the transmembrane potential reaches +30mV, soidum channels close and potassium channels open.

- this rapid diffusion of Na+ ions out of the cell, causes rapid repolarization (due to loss of positive ions from inside of cell)

- transmembrane potential then shifts back towards resting levels

Front 8

The return to normal permeability

Back 8

potassium channels close at about -70mV

- because the closing of the potassium channels takes at least 1 msec, the movement of some extra potassium ions while the channels are closing may cause a brief hyperpolarization (around -90mV)

- the membrane potential quickly returns to the resting level of -70mV

Front 9

Refactory Period

Back 9

period during which another action potential cannot be generated.

- from the time an action potential begins until the normal resting potential is restored, the membrane will not respond to additional stimuli

- can last between 0.4-4msec depending on neuron. a typical neuron can therefore transmit about 10-1,000 action potentials per second

Front 10

Absolute Refractory Period

Back 10

sodium channels opening at threshold until end of sodium channel inactivation - cell membrane cannot respond to another stimulus

Front 11

Relative Refractory Period

Back 11

from the time sodium channels regain resting conditions until transmembrane potential stable and at resting level - another AP can occur if membrane sufficiently depolarized (enough positive ions present to reach threshold)

Front 12

Propagation

Back 12

communication of 'information' between neurons by sending AP's along their lengths to axon terminals, which are located wither close by or in many cases are often many cm's away.

Front 13

Continuous propogation

Back 13

occurs along un-myelinated axons

-as depolarization occurs in the intial axon segment, the spreading of sodium ions causes the adjacent portion of the membrane to reach threshold

- an action potential develops at this location, and the initial segment enters the refractory period

- this process continues as the sodium continues to leak down the axon

Front 14

Saltatory Propagation

Back 14

fast propagation that occurs along myelinated axons

- the action potential jumps from one node of the axon to the next.

- insulation myelin sheath does not conduct electrical impulses, so only the 'bare' parts of the axon respond

Front 15

The propogation of nerve impulses occur _____ and require ____ energy in myelinated compared with un-myelinated neurons

Back 15

faster, less

Front 16

Propagation: continuous vs saltatory

Back 16

Continuous: unmyelinated, type C fibers (slowest 1m/sec), uses most ATP

Saltatory: myelinated, type A fibers type B fibers

Front 17

Type A fibers

Back 17

fastest 120m/sec - 4-20Mm (widest) -> effector is skeletal muscles

Front 18

Type B fibers

Back 18

intermediate 18m/sec - 2-4Mm (narrower) -> effector is everything other than skeletal muscles

Front 19

Factors affecting speed of propagation (2)

Back 19

myelination, diameter of fibers

Front 20

How do local anesthetis work?

Back 20

they act by blocking the opening of ion channels. Nerve impulses cannot propogate past the obstructed region, and pain signal do not reach the CNS.

Front 21

Localized Cooling... Does it work as local anesthetic?

Back 21

it produces an anesthetic effect by slowing the propogation of nerve impulses but does not stop pain. The application of ice to an injured area reduces pain sensations.

Front 22

Poison arrow or dart frog toxins

Back 22

Dart poison found in South America have very toxic poisons, many of which appear to alter normal function of the ion channels involved in cell membrane depolarization. Some of these toxins are strong cardiotoxins, affecting ion permeability of heart tissue specifically.

Front 23

Synapse

Back 23

the site of communication between two neurons or between a neuron and an effector cell (e.g. muscle gland)

Front 24

Presynaptic vs Post Synaptic Neurons

Back 24

the neuron whose axon terminals are at the synaptic cleft is the presynaptic neuron, while the neuron whose dendrites or cell body are at the synaptic cleft is the post synaptic neuron.

Front 25

Two types of Synapses

Back 25

Electrical Synapses and Chemical Synapses

Front 26

Electrical Synapses (5)

Back 26

- no neurotransmitter

- neurons at the synapses are locked together with gap junctions and their membranes are joined with connexons that allow the cells to exchanges ions from one to the other

-these synapses provide faster communication and better synchronization

- they are very rare and are found in the eye and some areas of the brain and heart

- the membranes of the two neurons are separated by a very small space of only 2nm

Front 27

Chemical Synapses (4)

Back 27

- use neurotransmitters (the NMJ is an example of this type)

- neurons get close to, but do not actually touch, other neurons or effecto cells, which leaves a space between cells called a synaptic cleft (about 40nm)

- the presynaptic neuron releases a neurotransmitter in the synaptic cleft, which then binds to receptors on the postsynaptic neuron (transmission sequence: electrical signal (AP) -> chemical signal (neurotransmitter) -> electrical signal (action potential)

- these are by far the most common synapses

Front 28

Cholinergic synapses

Back 28

synapes that release acetycholine (ACh) and are the most widespread synapses (e.g. NMJ, all neuron to neuron synapses in PNS)

- however, note that over 50 substances are known or suspected neurotransmitters.

Front 29

A typical cholinergic synapse transmits a signal as follow: (7)

Back 29

1. Arriving AP depolarizes axon of presynaptic neuron - voltage-gated Ca2+ channels open on presynaptic neuron

2. Ca2+ enter the cytosol of the axon terminal which triggers the release of ACh. Since calcium ions are more concentration in the extracellular fluid, they rush inward through channels

3. ACh is release via exocytosis. The release of ACh stops soon because calcium ions are pumped quickly from the cytosol back to the ecf

4. ACh binds to receptors on post-synaptic cell

5. Chemically gated Na+ channels open -> sodim rushes into post-sytnaptic cell which causes depolarization

- this depolarization is a graded potential; the great the amount of ACh release, the larger the depolarization.

- if the depolarization reaches threshold, an action potential will be triggered

6. ACh is quickly removed from the synaptic cleft by AChe -> turned into Acetate & Choline

7. ACh is broken down, reabsorbed and recycled by the resynaptic neuron

Front 30

Synaptic Knob is also ___ ____

Back 30

synaptic terminal

Front 31

Reuptake inhibitors

Back 31

drugs that work at the synaptic cleft by inhibiting neurotransmitter recycling.

- Prozac & Zoloft are selective reuptake inhibitors. They slow the reabsorbtion of serotonin, which stays in the synaptic cleft longer than it normally would. This seems to have a positive effect on mood. It has been noted that some depressed individuals have low serotonin levels.

- Cocaine inhibits the removal of dopamine from synapses in the specific areas of the brain. The resulting rise in dopamine concentrations has an excitatory effect responsible, at least in part, for the 'high' experienced by cocaine users.

Front 32

Postsynaptic Potentials (2 types)

Back 32

AP's that develop in the postsynaptic membrane in response to a neurotransmitter.

- Excitatory postsynaptic potential (EPSP)

- Inhibitory postsynaptic ptoential (IPSP)

Front 33

Excitatory postsynaptic potential (EPSP)

Back 33

- neurotransmitter causes a graded depolarization

- usually result from the opening of chemically regulated sodium channels that lead to depolarization of the cell membrane

Front 34

Inhibitory postsynaptic ptoential (IPSP)

Back 34

the neurotransmitter causes a graded hyperpolarization

- IPSPs usually result from the opening of chemically-regulated potassium (K+) channels or chloride (Cl-) channels that lead to hyperpolarization of the cell membrane (make inside of cell more negative)

Front 35

Summation (2 types)

Back 35

integration of many impulses received by neurons that causes postsynaptic neuron to reach threshold.

- Temporal summation

- Spatial summation

Front 36

Temporal Summation

Back 36

a sub-threshold impulse is delivered by a neuron, and then another quickly follows. ('temporal' means time-related)

- the two impulses come from the same source and add together to reach threshold

Front 37

Spatial Summation

Back 37

a sub-threshold impulse is delivered by one neuron, and a second sub-threshold impulse comes from another neuron.

- the two impulses come from different sources and add together to reach threshold

Front 38

Grand post-synaptic potential

Back 38

the net effect of the sum of all the EPSPs and IPSPs that occur at any given point in time in one postsynaptic neuron determines the ultimate effect these inputs have on the postsynaptic neuron.

Front 39

Neurons can transmit APs at rates of up to ____ to ___ times per second

Back 39

500-1000 times per second

Front 40

Multiple Sclerosis

Back 40

an autoimmune disease (thought to be (pesticides, organic compounds possible causes) that causes a progressive destruction of myelin sheaths of neurons in the CNS. Nerve impulses are slowed and short-circuited. It usually appears between the ages of 20 and 40, it affects females twice as much as males, and it is most prevalent among white people. An MRI reveals numerous plaques in the white matter the brain and spinal cord.

Front 41

Epilepsy

Back 41

characterized by short, recurrent, periodic attacks of motor, sensory or psychological malfunction. They are initiated by abnormal, synchronous electrical discharges from millions of neurons in the brain. Epileptic seizures afflicts about 1% of the world's population. There are many factors that may lead to epilepsy, including brain damage, infections, tumors, and trauma. Many forms of epilepsy are controlled quite effectively with medication such that people with epilepsy can live rather "normal" lives. In such cases, acquaintances may never know they have the condition unless they are told.