VII. Nervous system

Key focus of this chapter: neuron and nervous system

This chapter focuses on neuron and nervous system and gives concise summaries of the important things about structure of neuron, transmission, central nervous system, and peripheral nervous system.

A. Organization of the nervous system

Afferent

– carry sensory information toward the central nervous system (CNS).

Interneurons

– integrate and process information within the CNS.

Efferent

– carry motor signals from the CNS to effectors such as muscles and glands.

B. Structure of neuron

Dendrites

– information receptors.

– highly branched shapes.

Cell body

– containing nucleus and functions of cellular metabolism and protein synthesis.

Axon

– nerve fiber that normally conducts action potentials away from the cell body toward other cells.

Myelin sheath

– In the PNS, Schwann cells form myelin around axons; in the CNS, oligodendrocytes form myelin.

– myelin insulated layer helps the faster rates of conducting action potential.

** Synapse

: junction for communication between cells; chemical synapses normally transmit signals from the presynaptic cell to the postsynaptic cell.

C. Transmission

1. Transmission of action potential

① Influx Na⁺ ions into inside of neuron and opening Na⁺ channels.
② Efflux K⁺ ions to outside of neuron and opening K⁺ channels.
③ Propagation of action potential.

2. Action potential with voltage-gated ion channels

a. Resting state

The resting membrane potential is maintained by ion gradients, selective leak channels (especially K⁺), and the Na⁺/K⁺ ATPase; most voltage-gated Na⁺ and K⁺ channels are closed.
Strong negative inside of neuron with [Na⁺] outside and [K⁺] inside (polarization).

b. Depolarization

Depolarization toward threshold triggers the opening of voltage-gated 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.
The membrane potential becomes positive and reaches the peak of the action potential.

d. Falling phase of action potential

Voltage-gated Na⁺ channels inactivate while voltage-gated K⁺ channels open, causing repolarization.
Decreasing action potential and efflux K⁺ ion into outside of neuron.

e. Undershoot

The membrane becomes temporarily more negative than at rest because voltage-gated K⁺ channels close slowly.
During the relative refractory period, a stronger-than-normal stimulus is needed to trigger another action potential.

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 action-potential conduction velocity.

4. Chemical synapse

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

a. Processing

① Arrival of action potential to the synapse.
② Neurotransmitters in synaptic vesicles.
③ Opening of Voltage-gated Ca²⁺ channel and influx Ca²⁺ into neuron.
④ Ca²⁺ influx triggers synaptic vesicle fusion with the presynaptic membrane and neurotransmitter release into the synaptic cleft.
⑤ Neurotransmitters bind postsynaptic receptors and can produce excitatory or inhibitory postsynaptic potentials; an action potential occurs only if depolarization reaches threshold.
⑥ Neurotransmitter signaling ends through enzymatic breakdown (e.g., acetylcholinesterase for acetylcholine), reuptake, or diffusion, depending on the transmitter.

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 receptors on the postsynaptic cell.

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 • Cerebral cortex (gray matter) on the surface– cell bodies, dendrites, and synapses; basal nuclei are deeper gray matter • White matter beneath the cerebral cortex– predominantly myelinated axons • Corpus callosum – communication between left brain and right brain
	Diencephalon	Thalamus	• Relays most sensory information (except olfaction) and motor-related signals to the cerebral cortex • 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 breathing and cardiovascular functions (heart rate and blood vessel tone), swallowing, and digestive reflexes
	Cerebellum		• Coordination, equilibrium (motor balance)

Brainstem – midbrain, pons, medulla oblongata
Meninges – protective membranes surrounding the brain and spinal cord

2. Spinal cord

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

White matter on outside – transmission of neural information
Grey 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 control smooth muscle, cardiac muscle, and glands Visceral afferent (sensory) pathways provide information for autonomic reflexes.		• Control of skeletal muscles, including voluntary movements and involuntary somatic reflexes • Efferent (motor) nerves - response from external stimuli - reflex activity by spinal cord
Parasympathetic division	Sympathetic division
• Promotes rest-and-digest functions and energy conservation • Generally decreases heart rate, constricts bronchi, and promotes digestion • Increasing digestion • Vagus nerve (cranial nerve X) arises from the medulla and supplies parasympathetic fibers to the heart, lungs, and much of the digestive tract, including the proximal colon	• Mobilizes energy for the fight-or-flight response • Generally increases heart rate, dilates bronchi, and promotes glycogen breakdown; sympathetic preganglionic fibers stimulate adrenal medulla release of epinephrine and norepinephrine • Decreasing digestion

1. Motor unit

Made up of one motor neuron and the skeletal muscle fibers it innervates.
A motor unit consists of one motor neuron and all the muscle fibers it innervates; the number of fibers varies.

2. Autonomic neurotransmitters and receptors

	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)	Adrenergic

Sympathetic exceptions: most eccrine sweat glands receive postganglionic ACh acting on muscarinic receptors. The adrenal medulla receives preganglionic ACh acting on nicotinic receptors and releases epinephrine and norepinephrine into the blood.