Chapter 12 - Nervous Tissue

Front 1

- contains about 100 billion neurons (nerve cells) and is housed in the skull.

Back 1

Brain

Front 2

- are 12 pairs and emerge from the base of the brain.

Back 2

Cranial nerves

Front 3

- is a bundle of hundreds to thousand of axons of neurons, also has C.T. and blood vessels. Occurs outside of the brain and spinal cord

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Nerve

Front 4

- connects to the brain and passes down the vertebral column.

Back 4

Spinal Cord

Front 5

- are 31 pairs and emerge from the spinal cord.

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Spinal Nerves

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- small masses of nervous tissue located outside of the brain and spinal cord. They primarily contain cell bodies of neurons.

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Ganglia

Front 7

- networks of neurons in walls of the GI tract.

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Enteric plexuses

Front 8

- specialized cells, or dendrites of sensory neurons, that monitor internal or external changes.

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Sensory receptors

Front 9

- the nervous system detects changes (stimuli) within or outside of the body.

Back 9

Sensory function

Front 10

carry out sensory function. They carry this information to the brain and spinal cord, or from a lower to higher level in the spinal cord and brain.

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Sensory (afferent) neurons

Front 11

- the nervous system analyzes information, stores it, and decides on the appropriate response.

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Integrative function

Front 12

integrative function neurons and are the most common neurons in the body.

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Interneurons

Front 13

- it responds to stimuli by initiating muscle contraction or glandular secretions.

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Motor Function

Front 14

carry information from the brain to the spinal cord or out of these to effectors (E.g. muscle cells or gland cells) to respond.

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Motor (efferent) neurons (have an effect)

Front 15

- consists of the brain and spinal cord, which integrate information and determine the response.

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Central nervous system (CNS)

Front 16

- consists of cranial nerves, spinal nerves, ganglia, and sensory receptors. These innervate (supply neurons to) the rest of the body (periphery). Consists of three subsystems: somatic, autonomic, and enteric.

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Peripheral nervous system (PNS)

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Somatic Nervous System (SNS)

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a) Consists of sensory neurons that take information from the body wall and sense receptors (Esp. in the head and limbs) to the CNS.
b) Also motor neurons conduct information to effectors that are the skeletal muscles (this is voluntary).

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Autonomic Nervous System (ANS)

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a) Sensory neurons that take information primarily from the viscera (or organs) to CNS.
b) Motor neurons take information from the CNS to effectors that are smooth and cardiac muscle or glands. It is usually involuntary. This motor part consists of two divisions.
(1) Sympathetic Division (2) Parasympathetic Division

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Sympathetic Division

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-produces the fight-or-flight response
- often involves expenditure of energy (E.g. increased heart rate)

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Parasympathetic Division

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-rest and digest
- often restores or conserves energy (E.g. decreased heart rate).

Front 21

Enteric Nervous System

Back 21

- consists of enteric plexuses of the GI tract and can work independently of ANS.
a) Its sensory neurons monitor chemical changes in the GI tract and stretching of its walls.
b) Its motor neurons control contraction of the GI tract, its secretions, and activity of the GI tract endocrine (hormone releasing) cells.

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Cell Body:

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1. Contains the nucleus and cell organelles.
2. Also contains nissl bodies which are the rough E.R. that produces proteins that are involved in growth and, in the PNS, to regenerate damage.
3. Axon hillock

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Axon hillock

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- a cone-shaped elevation of the cell body where the axon joins the cell body.

Front 24

Axon:

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1. There is typically just one, but it may branch into axon collaterals.
2. It carries nerve impulses to another neuron, muscle fiber, or gland cell. (Carries the output)
3. Initial segment
4. Trigger zone
5. Axoplasm
6. Axolemma
7. Synaptic end bulbs or varicosities

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- the part of the axon closest to the cell body (axon hillock)

Back 25

Axon: Initial segment

Front 26

- it is at the junction of the axon hillock and initial segment. Is where action potentials arise.

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Axon: Trigger zone

Front 27

-is the axon cytoplasm.

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Axon: Axoplasm

Front 28

- is the axon plasma membrane.

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Axon: Axolemma

Front 29

- occur at ends of axons and contain synaptic vesicles. These store neurotransmitters which affect other neurons, muscles, etc. (A synapse is a junction between two neurons, or a neuron and an effector cell.)

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Axon: Synaptic end bulbs or varicosities

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- typically has several dendrites and one axon connected to the cell body.

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Multipolar neuron

Front 31

- one main axon and one main dendrite connected to the cell body.

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Bipolar neuron

Front 32

- there is only one process from the cell body and this fiber connects to the main fiber. Their trigger zone is at the junction of the axon and dendrite.

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Unipolar Neuron

Front 33

- are more common than neurons and serve to assist the neurons.

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Neuroglial (glial) cells

Front 34

- any neuronal process emerging from the cell body (e.g. dendrite or axon).

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Nerve fiber

Front 35

- are processes from the cell body that are the receiving (input), part of the neuron and are usually short and highly branched.

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Dendrites

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Myelination

Back 36

- a myelin sheath is formed by neuroglial cells around most axons. It is a multi-layered lipid and protein covering axons. It prevents impulses from jumping across neurons and helps increase impulse speed.

Front 37

Myelination:
Nodes of Ranvier

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- are gaps along the myelin sheath, which speed up the rate of impulse transmission.

Front 38

Myelination:
Neurolemma

Back 38

- is the outer, nucleated cytoplasmic layer of the myelin sheath in PNS axons. It helps regenerate an axon when it is injured. CNS axons do not have this.

Front 39

Myelination:
In the PNS, each Schwann cell

Back 39

forms one sheath between two nodes.

Front 40

Myelination:
In the CNS, each oligodendrocyte

Back 40

forms sheaths around several adjacent axons

Front 41

- areas of nervous tissue that consist of myelinated processes of neurons.

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White matter

Front 42

- areas of nervous tissue that consist of neuron cell bodies, dendrites, axon terminals, unmyelinated axons, and neuroglia. A nucleus is a cluster of cell bodies in the CNS.

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Gray matter

Front 43

-is a difference in the electrical charge on one side of the plasma membrane vs. the other side.

Back 43

membrane potential

Front 44

-, which is a slightly negative charge on the inside of a neuron that is at rest (not conducting an impulse). It typically is about -70 millivolts. We say that this area of the neuron is polarized.

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Resting membrane potential

Front 45

- when these are stimulated to open, they allow ions to move through the plasma membrane to change the electrical charges (inside and outside the neuron) during nerve impulse transmission.

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Gated Ion Channels

Front 46

Describe the factors that maintain a resting membrane potential.

Back 46

A. There is a greater concentration of negatively charged phosphates and proteins on the inside of the plasma membrane.
B. There is a greater concentration of positively charged sodium ions (Na+) on the outside.
C. The membrane is practically impermeable to these but it is moderately permeable to potassium (K+) because there are many more K+ leakage channels than other leakage channels.
D. K+ is at a higher concentration on the inside of the neurons so it tends to leak out.
E. The more K+ that leaks out, the more negative the inside becomes. This draws some K+ back in. But the fact that K+ is diffusing (leaking) out contributes to the overall negative charge on the inside.

Front 47

Describe graded potentials

Back 47

A. Graded potentials- are generated by the opening of ligand-gated channels or mechanically gated channels.
B. They usually occur at dendrites or cell bodies and they only travel as far as the axon trigger zone. Here they may be summed to produce an action potential.
C. There is a gradation (variation) in amplitude of the electrical signal depending on the strength of the stimulus.

Front 48

make the inside of a neuron less negative

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Depolarizing graded potentials

Front 49

make the inside of a neuron more negative

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Hyperpolarizing graded potentials

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Nerve action potential

Back 50

is a temporary depolarization (neuron becomes more positive inside), and then repolarization (inside becomes negative again), of a part of a neuron. It last 1/1000 of a second (1 millisecond) and involves voltage-gated channels.
Action potentials work by the all-or-none principle. Once they are initiated at the trigger zone, they reach a constant amplitude (+ 30 mV on the inside) and they proceed to the end of the axon

Front 51

1. At the trigger zone, sufficient incoming depolarizing graded potentials open voltage-gated sodium channels enough so that the movement of Na+ in brings the membrane potential to a threshold level.
2. This stimulates activation gates on many nearby voltage-gated Na+ channels to open suddenly and Na+ rushes in by simple diffusion to make the inside positive. This is called depolarization.
This has also stimulated gates on the K+ channels to open but these K+ gates are slower to move (they open at the end of deplorization) the inside remains positive for only a fraction of a second until the opening of the delayed K+ channels (which returns the inside of the membrane to negative voltage)

Back 51

Action Potential:
Depolarization phase

Front 52

1. Inactivation gates on Na+ channels close so that Na+ stops rushing in.
2. The one gate on the K+ channels opens so that K+ flows out by simple diffusion to return the inside to a negative (polarized) voltage again. These K+ gates eventually close.

Back 52

Action Potential:
Repolarization phase

Front 53

- a time period when a recently stimulated neuron cannot initiate a second action potential. Larger diameter axons have a shorter _________ period and thus can transmit more impulses per second.

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Action Potential:
Refractory period

Front 54

The Na+/K+ pump works more or less between action potentials to pump ____ back out to keep _____ concentration higher on the outside.

Back 54

Action Potential:
Na+
Na+

Front 55

Explain the events of signal transmission at a chemical synapse.

Back 55

Presynaptic neuron sends the signal across the synaptic cleft (a gap between the neurons) and postsynaptic neuron receives the signal.
A. When the nerve impulse (action potential) arrives at the end of the presynaptic neuron it opens voltage-gated Ca2+ channels.
B. Ca2+ rushes into the neuron (because it is at higher concentration on the outside).
C. This triggers the release of a neurotransmitter from synaptic vesicles, by exocytosis, into the synaptic cleft.
D. The neurotransmitter diffuses across this gap and bonds with neurotransmitter receptors on the postsynaptic neuron.
E. This stimulates their channels to open and allow certain ions in or out, causing a change in electrical charges called the postsynaptic potential

Front 56

- depolarization of one area of the plasma membrane opens the voltage gated Na+ channels in adjacent areas as the impulse travels along the membrane. (E.g. C fibers which are the smallest diameter axons and are unmyelinated)

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Continuous conduction

Front 57

- occurs in myelinated fibers and depolarization of one node of Ranvier causes Na+ channels only at the next node to open. This is faster and more efficient. (E.g. A and B fibers which are larger diameter axons and myelinated).

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Saltatory conduction

Front 58

- the neurotransmitter opens cation channels and mainly Na+ flows in to make the inside more positive (it has a depolarizing effect). If this happens enough, an action potential will be generated at the trigger zone.

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Excitatory Postsynaptic Potential (EPSP)

Front 59

- the neurotransmitter typically opens Cl- channels to allow Cl- in (or it opens K+ channels to allow K+ out) so that the inside of the postsynaptic neuron becomes more negative (it has a hyperpolarizing effect). This will decrease the chances of an action potential being generated.

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Inhibitory Postsynaptic Potential (IPSP)

Front 60

- once the message has been sent across a synapse, the neurotransmitter is removed from the synapse by diffusion, enzymatic degradation, reuptake by presynaptic neurons, or uptake by other cells. This prevents the neurotransmitter from further influencing the postsynaptic neuron. Prozac® interferes with reuptake of serotonin.

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Removal of neurotransmitters

Front 61

- several presynaptic end bulbs are firing which increases neurotransmitter build up at the postsynaptic neuron.

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Spatial summation

Front 62

- one presynaptic end bulb fires many times to increase neurotransmitter build-up at the postsynaptic neuron.

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Temporal summation

Front 63

Neurotransmitters are divided into two classes,

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-small molecule neurotransmitters
- neuropeptides (3-40 amino acids).

Front 64

Acetylcholine

Back 64

- is excitatory at neuromuscular junctions. It can be excitatory or inhibitory at other synapses.

Front 65

Glutamate and Aspartate

Back 65

- are excitatory in the CNS. Nearly all excitatory neurons in the CNS communicate via glutamate.

Front 66

Gamma Aminobutyric Acid (GABA) and Glycine

Back 66

- are inhibitory in the CNS.

Front 67

Norepinephrine (NE), Dopamine, and Serotonin

Back 67

- play important roles especially in emotions, mood control, and regulating sleep. They can be excitatory or inhibitory.

Front 68

ATP and other purines

Back 68

- are excitatory in the CNS and PNS.

Front 69

Nitric oxide

Back 69

- is a gas and is not stored in vesicles but it is produced on demand. It may play a role in memory and learning.

Front 70

Enkephalins, Endorphins, and Dynorphins

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- neuropeptides that are natural painkillers. They are also linked to improved learning, pleasure, and regulation of puberty onset, sexual drive, and mental illness.

Front 71

Substance P

Back 71

- involved in transmitting pain input into the CNS.

Front 72

- one neuron only stimulates one neuron, etc.

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Simple series circuit

Front 73

- a single input neuron stimulates increasing numbers of output neurons.

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Diverging circuit

Front 74

- several independent input neurons stimulate a single postsynaptic output neuron.

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Converging circuit

Front 75

- output neuron signals is sent back through the circuit again resulting in a series of repeated output signals. It can be use for breathing control, waking and short-term memory.

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Reverberating circuit

Front 76

- various input neurons synapse and interact with each other but also synapse with one output neuron. May be helpful in precise activities such as math calculations

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Parallel-after-discharge circuit

Front 77

- the nervous system’s capability to change based on experience (E.g. learning due to changes in synapses, new dendrites).

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Plasticity

Front 78

- differentiation of stem cells into new neurons (new evidence indicates this happens in the hippocampus, which is involved in learning, but not elsewhere in the CNS).

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Neurogenesis

Front 79

Describe the events involved in damage and repair of peripheral neurons.
PNS:

Back 79

1. Damage to axons and dendrites may be repaired if the cell body and Schwann cells survive and stay active.
2. Nissl bodies react to damage by breaking up into fine granular masses (chromatolysis).
3. The axon and myelin sheaths, distal to the damage, are broken down (Wallerian degeneration). But the neurolemma remains to form a regeneration tube.
4. New axon grows from the area proximal to the damage site slowly through the regeneration tube and follow the old neurolemma outward.

Front 80

Describe the events involved in damage and repair of peripheral neurons.
CNS:

Back 80

Damage to the axons is not repaired. Instead of Schwann cells, it has oligodendrocytes which lack a neurolemma.

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Multiple sclerosis (MS):
Causes:

Back 81

Causes:
unclear but there may be a genetic susceptibility and exposure to an environmental factor (perhaps a herpesvirus) which causes destruction of myelin sheaths in the CNS.

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Multiple sclerosis (MS):
Symptoms:

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Symptoms:
in the most common form, the first attack includes feelings of heaviness or weakness in muscles, abnormal sensations, and double vision. Subsequent attacks produce progressive loss of
function interspersed with remission periods.

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Epilepsy:
Causes:

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Causes:
brain damage at birth, metabolic disturbances, head injuries, and tumors and abscesses of
the brain.

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Epilepsy:
Symptoms:

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Symptoms:
sensation occurs when receptors haven’t been stimulated, skeletal muscles may contract
involuntarily.

Front 85

An acute demyelinating disorder in which macrophages strip myelin from axions in the PNS. It is the most common cause of acute paralysis in North America and Europe and may result from the immune systems' response to a bacterial infection. Most patients recover completely or partially, but about 15% remain paralized

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Guillian-Barre Syndrome

Front 86

A malignant tumor tha consists of immature nerve cells (neuroblasts); occurs most commonly in the abdomen and most frequently in the adrenal glands. Although rare, it is the most common tumor in infants

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Neuroblastoma

Front 87

any disorder that affects the nervous system but particularly a disorder of a cranal or spinal nerve.

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neuropathy

Front 88

a fatal disease caused by a virus that reaches the CNS via fast axonal transport. It is usually transmitted by the bite of an animal. The symptoms are excitement, aggressiveness, and madness, followed by paralysis and death

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rabies