VII. Nervous system

Key focus of this chapter: neurons and the nervous system

This chapter reviews neuron structure, neural signaling, and the central and peripheral nervous systems.

A. Organization of the nervous system

Afferent

– carry sensory information to the central nervous system.

Interneurons

– integrate information within the central nervous system.

Efferent

– carry motor commands from the central nervous system to effectors, such as muscles and glands.

B. Structure of a neuron

Dendrites

– receive incoming signals.

– typically highly branched.

Cell body

– contains the nucleus and carries out cellular metabolism and protein synthesis.

Axon

– nerve fiber that usually conducts signals away from the cell body toward other cells.

Myelin sheath

– Schwann cells form myelin around axons in the peripheral nervous system; oligodendrocytes form myelin in the central nervous system.

– the insulating myelin sheath increases the speed of action potential conduction.

** Synapse

: a specialized junction where a neuron communicates with another cell. Chemical synapses usually transmit signals in one direction; electrical synapses use gap junctions and are often bidirectional.

C. Transmission

1. Transmission of action potential

① Opening of voltage-gated Na⁺ channels allows Na⁺ influx and rapid depolarization.
② Na⁺ channel inactivation and opening of voltage-gated K⁺ channels allow K⁺ efflux and repolarization.
③ Local currents depolarize adjacent membrane to threshold, regenerating the action potential along the axon without loss of amplitude.

2. Action potential with voltage-gated ion channels

a. Resting state

At rest, potassium leak channels help establish the membrane potential; most voltage-gated Na⁺ and K⁺ channels are closed.
The interior is negative relative to the outside (often about −70 mV). Na⁺ concentration is higher outside, and K⁺ concentration is higher inside; the Na⁺/K⁺-ATPase maintains these concentration gradients.

b. Depolarization

A depolarizing stimulus moves the membrane potential toward threshold.
At threshold, rapid opening of voltage-gated Na⁺ channels initiates an all-or-none action potential.

c. Rising phase of action potential

Na⁺ influx causes further depolarization and opens additional voltage-gated Na⁺ channels by positive feedback.
The membrane potential rises above 0 mV and reaches its peak.

d. Falling phase of action potential

Voltage-gated Na⁺ channels inactivate while delayed voltage-gated K⁺ channels open.
K⁺ efflux repolarizes the membrane.

e. Undershoot

Slow closure of voltage-gated K⁺ channels briefly makes the membrane more negative than its resting potential.
During the absolute refractory period, Na⁺ channel inactivation prevents another action potential. During the relative refractory period, a stronger stimulus is required.

3. Saltatory conduction

: rapid propagation of action potentials along a myelinated axon.

Action potentials are regenerated at successive nodes of Ranvier, appearing to jump between nodes along the myelinated axon.
For comparable axons, myelination and a larger axon diameter increase conduction speed.

4. Chemical synapse

: place for transmission of messages between two neurons or between a neuron and a target cell.

a. Process

① An action potential arrives at the presynaptic axon terminal.
② The terminal membrane depolarizes.
③ Voltage-gated Ca²⁺ channels open, allowing Ca²⁺ to enter the terminal.
④ Increased cytosolic Ca²⁺ triggers fusion of neurotransmitter-containing synaptic vesicles with the presynaptic membrane and release by exocytosis.
⑤ Neurotransmitter binds postsynaptic receptors, producing excitatory or inhibitory responses. An action potential occurs only if depolarization reaches threshold at the postsynaptic neuron’s trigger zone.
⑥ Neurotransmitter is cleared by reuptake, diffusion or enzymatic breakdown; acetylcholinesterase specifically breaks down acetylcholine.

b. Function of each part

Neurotransmitter

– intercellular chemical messengers.

Synaptic vesicles

– store neurotransmitters in the presynaptic terminal.

Voltage-gated Ca²⁺ channel

– open when an action potential depolarizes the presynaptic terminal, allowing Ca²⁺ to enter.

Synaptic cleft

– narrow gap between the presynaptic and postsynaptic membranes.

– neurotransmitter molecules diffuse across the cleft to the postsynaptic cell.

Ligand-gated ion channels

– postsynaptic receptors that open or close ion channels when a neurotransmitter binds.

D. Central nervous system

1. Brain

Regions	Classification		Functions and features
Forebrain	Cerebrum		• Integrating and analyzing sensory information, communication, language, learning, remembering, creative thought, motor response. • Consists of cerebral cortex (gray matter), white matter, and basal nuclei • Gray matter on the outside– dendrite, cell body • Whiter matter on the inside-myelinated axon • Corpus callosum – communication between left brain and right brain
	Diencephalon	Thalamus	• Sensory input and output for cerebrum • Control of sleeping and wakefulness
		Hypothalamus	• Regulation of homeostasis such as blood pressure, temperature, or water balance • Sexual drive, thirst, hunger
Midbrain	Midbrain		• Part of brainstem
Hindbrain	Pons		• Regulation of breathing rates
	Medulla oblongata		• Regulation of respiratory system (heart, breathing, blood vessels), swallowing, digestion
	Cerebellum		• Coordination, equilibrium (motor balance)

Brainstem – midbrain, pons, medulla oblongata
Meninges – protective membranes surrounding the central nervous system

2. Spinal cord

: carries signals between the brain and body and integrates spinal reflexes.

White matter on the outside – transmission of neural information
Gray matter on the inside – processing of information

E. Peripheral nervous system

Peripheral nervous system
Autonomic nervous system		Somatic nervous systems
• Involuntary control • Afferent (sensor) and efferent (motor) nerves response from internal stimuli such as smooth muscle, cardiac muscles, cardiovascular, digestive organs, excretory, and endocrine.		• Voluntary control (conscious control) by skeletal muscles • Efferent (motor) nerves - response from external stimuli - reflex activity by spinal cord
Parasympathetic division	Sympathetic division
• Decreasing energy generation to rest and digest states • Respond with decreasing heart rate, respiration, rate of converting glycogen to glucose, and secretion of epinephrine • Increasing digestion • Vagus nerve – one of cranial nerve (starting from the medulla oblongata) to innervate viscera such as stomach, liver, large and small intestines, lung, and heart	• Increasing energy generation from stress • Respond with increasing heart rate, respiration, rate of converting glycogen to glucose, and secretion of epinephrine • Decreasing digestion

1. Motor unit

A motor unit consists of one motor neuron and all the skeletal muscle fibers it innervates.
One neuron controls many muscle fibers.

2. Autonomic neurotransmitters and receptors

The adrenal medulla receives preganglionic sympathetic ACh at nicotinic receptors and releases epinephrine and norepinephrine into the bloodstream.

	Preganglionic neuron		Postganglionic neuron
	Neurotransmitter	Receptor	Neurotransmitter	Receptor
Parasympathetic system	ACh (acetylcholine)	Nicotinic cholinergic	ACh	Muscarinic cholinergic
Sympathetic system	ACh	Nicotinic cholinergic	Usually NE (norepinephrine) Exception: ACh to eccrine sweat glands	Usually adrenergic Eccrine sweat glands: muscarinic cholinergic