VII. Nervous system

Key focus of this chapter: neuron and nervous system

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

A. Organization of the nervous system

Afferent

– carry sensory information toward the central nervous system.

Interneurons

– process and integrate information within the central nervous system.

Efferent

– carry motor commands away from the central nervous system to effector organs.

B. Structure of neuron

Dendrites

– receive incoming signals.

– usually highly branched.

Cell body

– contains the nucleus and supports cellular metabolism and protein synthesis.

Axon

– conducts action potentials, normally from the initial segment toward the axon terminals.

Myelin sheath

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

– myelin insulates axons and increases action-potential conduction speed.

** Synapse

: chemical synapses normally transmit from the presynaptic terminal to the postsynaptic cell; electrical synapses may transmit in both directions.

C. Transmission

1. Transmission of action potential

① Voltage-gated Na⁺ channels open, allowing Na⁺ influx and depolarization.
② Na⁺ channels inactivate and voltage-gated K⁺ channels open; K⁺ efflux repolarizes the membrane.
③ Propagation of action potential.

2. Action potential with voltage-gated ion channels

Scientific schematic — Membrane potential is measured in millivolts (mV). During repolarization, Na⁺ channels are inactivated; during undershoot they recover while K⁺ channels close slowly. These states supplement the simplified open/closed channel symbols above.

a. Resting state

At rest, most voltage-gated Na⁺ and K⁺ channels are closed; leak channels remain open and the Na⁺/K⁺ pump maintains ion gradients.
The cell interior is negative relative to the exterior; Na⁺ concentration is higher outside and K⁺ concentration is higher inside.

b. Depolarization

Beginning depolarization and opening Na⁺ channels.
Membrane depolarization reaches threshold.

c. Rising phase of action potential

Increasing Na⁺ influx into inside of neuron with more opening of Na⁺ channels.
Membrane potential rises to the action-potential peak.

d. Falling phase of action potential

Voltage-gated Na⁺ channels inactivate while voltage-gated K⁺ channels open.
K⁺ efflux makes the membrane potential more negative (repolarization).

e. Undershoot

The membrane briefly becomes more negative than its resting potential because voltage-gated K⁺ channels close slowly; Na⁺ channels recover from inactivation.
During the absolute refractory period, inactivated Na⁺ channels prevent another action potential; during the relative refractory period, a stronger stimulus is needed.

3. Saltatory conduction

: fast propagation of action potential along with a myelinated axon.

Action potential jumps from node of Ranvier to next node of Ranvier along insulated myelin sheath.
Myelination and larger axon diameter generally increase conduction speed; the number of myelinated axons does not determine speed in an individual axon.

4. Chemical synapse

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

a. Processing

① An action potential reaches the presynaptic terminal.
② Neurotransmitters in synaptic vesicles.
③ Voltage-gated Ca²⁺ channels open and Ca²⁺ enters the presynaptic terminal.
④ The rise in presynaptic Ca²⁺ triggers vesicle fusion and neurotransmitter release into the synaptic cleft.
⑤ Neurotransmitter binds postsynaptic receptors and produces an excitatory or inhibitory postsynaptic response. An action potential occurs only if excitation brings the axon initial segment to threshold.
⑥ Neurotransmitter is removed by reuptake, diffusion, or enzymatic breakdown; acetylcholinesterase specifically breaks down acetylcholine.

b. Function of each part

Neurotransmitter

– intercellular chemical messengers.

Synaptic vesicles

– packaging of neurotransmitter in presynaptic cell.

Voltage-gated Ca²⁺ channel

– opening of Ca²⁺ channel when action potential reaches a synapse.

Synaptic cleft

– narrow gap between presynaptic membrane and postsynaptic membrane.

– neurotransmitters diffuse across the cleft to postsynaptic receptors.

Ligand-gated ion channels

– receptor of neurotransmitter at postsynaptic membrane.

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 • White 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 cardiovascular and respiratory functions, swallowing, and digestive reflexes
	Cerebellum		• Coordination, equilibrium (motor balance)

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

2. Spinal cord

: communication of neural information between brain and body parts and regulation of reflexes.

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

E. Peripheral nervous system

Peripheral nervous system
Autonomic nervous system		Somatic nervous system
• Involuntary control • Autonomic efferent (motor) pathways respond to visceral sensory input response from internal stimuli such as smooth muscle, cardiac muscles, cardiovascular, digestive organs, excretory, and endocrine.		• Motor control of skeletal muscles, including voluntary movement and reflexes • Efferent (motor) nerves - response from external stimuli - reflex activity by spinal cord
Parasympathetic division	Sympathetic division
• Conserving energy and supporting rest-and-digest functions • Slows heart rate and promotes digestive activity; effects vary by target organ • Increasing digestion • Vagus nerve (cranial nerve X) supplies parasympathetic fibers to thoracic and many abdominal organs; pelvic splanchnic nerves supply the distal colon and pelvic organs	• 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

Consists of one motor neuron and all skeletal muscle fibers it innervates.
One neuron controls many muscle fibers.

2. Autonomic neurotransmitters and receptors

Exceptions: sympathetic postganglionic fibers to most sweat glands release acetylcholine onto muscarinic receptors. Preganglionic sympathetic fibers stimulate adrenal-medulla chromaffin cells through nicotinic receptors, releasing 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	NE (Norepinephrine)	Adrenergic