CHAPTER 7: The Nervous System

The Nervous System

From Kandel et al., Principles of Neural Science, 4th Ed.

Two divisions of the nervous system Central nervous system (CNS) Peripheral nervous system (PNS)

Overall structure of the CNS 7 major parts of the CNS

1. Cerebrum (also cerebral hemisphere)

2. Diencephalon 3. Midbrain 4. Cerebellum 5. Pons 6. Medulla oblongata 7. Spinal cord

3 broad regions of the brain 1. Forebrain 2. Midbrain 3. Hindbrain

Two cell types of the nervous system Glial Neurons

Terminology Functional classes of neurons:

Afferent neurons --> sensory neurons (cell body outside spinal cord, nerving endings inside)

Efferent neurons --> motor neurons (cell body inside spinal cord, nerve endings outside) Somatic motor neurons (skeletal muscle) Autonomic motor neurons (cardiac muscle, smooth muscle, glands)

Interneurons (Association neuron) Cell body and nerve endings both inside spinal cord

Groups of cell bodies: Nuclei (CNS) vs. ganglia (PNS) Groups of axons: tracts (CNS) vs. nerves (PNS)

Morphological Classes of Neurons

FIGURE 7.4

PseudounipolarOne axon split into two ends

BipolarTwo distinct axons

Multipolar One axon, multiple processes emanate from body

Astrocytes (CNS) Most abundant type of glial cell Multiple functions via perivascular (end-) feet on capillaries

& neurons Oligodendrocytes (CNS)

Function: insulation of central axons One oligodendrocyte per ~15 internodes Myelin sheath (white matter; gray matter is dendrites and

cell bodies) Schwann cells (PNS)

Function: insulation of peripheral axons One Schwann cell per internode Neurilemma

Predominant Glia of Vertebrates

Astrocytic Functions

FIGURE 7.10

Glucose uptake from blood; lactate delivery to neurons

K+ re-uptake Neurotransmitter

(glutamate) re-uptake Glutamate/Glutamine

shuttle to re-supply neurons with neurotransmitter

Synaptogenesis & neurogenesis

Establish & maintain the blood-brain barrier

Neurotransmitter release -- role in electrical excitability?

Ion Channels Ion channels are transmembrane proteins that conduct ions Ion channels are specific Ion channels are gated (regulated permeability):

1. Voltage-gated 2. Ligand-gated 3. Mechanically-gated

From Kandel et al., Principles of Neural Science, 4th Ed.

The Action Potential (AP) A propagated electrical wave running the length of the axon Stereotypical All-or-none event Fixed amplitude, ~ 100 mV (from -70 mV to +30 mV) Threshold, ~ -55 mV Shape & duration of the waveform reflects changes in

membrane permeability to Na+ and K+

From Kandel et al., Principles of Neural Science, 4th Ed.

Ion Channels of the AP Fast Na channel

Exists in three distinct states Closed at Vr Open during depolarization Inactivated during refractory

Voltage-gated (two gates) Main gate Inactivation gate

Slow K channel Exists in two distinct states

Closed at Vr Open when depol.

Voltage-gated (one gate) Main gate

Absolute refractory period is due to the properties of the sodium channel Ensures unidirectionality of nerve impulse Places limits on A.P. frequency

Relative refractory = slow K+ channel

Events in the presynaptic cell

FIGURE 7.23

Catecholamines: neurotransmission & cleft clearance

FIGURE 7.30

Neurotransmission and cleft clearance: a novel

1. Dopamine produced and stored in synaptic vesicles 2. Action potentials open gated Ca2+ channels, leading to release of neurotransmitter to post synaptic cell 3. Inactivation of all the neurotransmitters by COMT in post synaptic cell; a successful neurotransmitter binds to the ligand-gated channel instead of permeating the postsynaptic cell 4. Reuptake of most neurotransmitter from synaptic cleft by post synaptic 5. Inactivation of most neurotransmitter by MAO in post synaptic

Cable Properties of the axon

Cable Properties Passive electrical

spread Depolarizations

below threshold Hyperpolarizations

Attenuation of electrical signal over long distances

Characteristic of synaptic or receptor potentials

FIGURE 7.18