Chapter 8: Neurons and Communication

Central Nervous System (CNS)

Brain and spinal cord

Peripheral Nervous System (PNS)

• Sensory (afferent) and efferent pathways, including somatic, autonomic (sympathetic, parasympathetic), and enteric nervous systems

cell body (soma)

contains the nucleus

dendrites

receive input

axon

transmits signals

Synapse

site of communication

Myelin sheath

insulates axon

Structural classification of neurons

Multipolar, bipolar, pseudounipolar, anaxonic

Functional classification of neurons

Sensory, motor (efferent), interneurons

Schwann cells

glial cells in the PNS

form myelin

satellite cells

glial cells in the PNS

support ganglia

Oligodendrocytes

glial cells in the CNS

myelinate CNS neurons

Astrocytes

Name glial cells in the CNS

maintain BBB, chemical balance

Microglia

glial cells in the CNS

immune defense

Ependymal cells

glial cells in the PNS

line ventricles, stem cell source

Slow axonal transport

0.2–2.5 mm/day, moves soluble proteins

fast axonal transport

~400 mm/day, moves organelles

Anterograde

to axon terminal

retrograde

back to cell body

resting membrane potential (RMP)

determined by:

• Primarily K⁺ concentration gradient

• Resting permeability to K⁺, Na⁺, Cl⁻

• Typical RMP ≈ -70 mV

phases of action potential

1. Resting

2. Depolarization (Na⁺ influx)

3. Repolarization (K⁺ efflux)

4. Hyperpolarization

5. Return to RMP

absolute refractory periods

Na⁺ channels are inactive → no new AP

Relative refractory period

K⁺ channels open → stronger stimulus needed

saltatory conduction

• APs jump between nodes of Ranvier in myelinated axons

• Increases conduction speed

neurotransmitter release

• AP reaches axon terminal

• Voltage-gated Ca²⁺ channels open

• Ca²⁺ triggers vesicle fusion

• Neurotransmitter released into synaptic cleft

Electrical synapses

gap junctions, fast, bidirectional

Chemical synapses

use neurotransmitters, slower, unidirectional

major neurotransmitter classes

• Acetylcholine

• Amines (dopamine, serotonin, norepinephrine)

• Amino acids (GABA, glutamate)

• Peptides, purines, gases, lipids

erceptors that neurotransmitters bind to

• Ionotropic: Ligand-gated ion channels (fast)

• Metabotropic: GPCRs (slow, modulatory)

Glutamate

main excitatory neurotransmitter in the brain

GABA

the main inhibitory neurotransmitter in the brain

acetylcholine receptors

• Nicotinic: Skeletal muscle, autonomic neurons

• Muscarinic: CNS and parasympathetic responses

divergence in neural networks

One neuron → many targets

convergence in neural networks

Many inputs → one target neuron

long-term potentiation (LTP)

• Synaptic strength increases with activity

• Involves AMPA and NMDA glutamate receptors

• Thought to underlie learning and memory

tetrodotoxin

• Found in pufferfish

• Blocks voltage-gated Na⁺ channels

• Causes paralysis and can be fatal