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

– input sensory information to central nervous system.

Interneurons

– functions of central nervous system such as processing information and sending out orders.

Efferent

– output information to motor nervous system (effector organs).

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 transmits information to other cells with only one direction.

Myelin sheath

– wrapped axon by schwann cell becomes myelin.

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

** Synapse

: transmission of messages from a synaptic terminal to another cell with only one direction.

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

Keeping up the resting potential of membrane and closing Na⁺ and K⁺ channels.
Strong negative inside of neuron with [Na⁺] outside and [K⁺] inside (polarization).

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 reaches action potential.

d. Falling phase of action potential

Closing Na⁺channels and opening K⁺channels.
Decreasing action potential and efflux K⁺ ion into outside of neuron.

e. Undershoot

More negative membranes than resting state with all closing Na⁺channels and opening K⁺channels.
Refractory potential with a reduction in excitability.

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.
The more abundant the myelinated axon the faster the travel of action potential.

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 Releasing of neurotransmitters into synaptic cleft by concentrated Ca²⁺
⑤ Binding neurotransmitter to ligand-gated ion channel and then generating action potential by diffusing Na⁺ into neuron.
⑥ Degradation of neurotransmitter by acetylcholinerases or re-uptaking into presynaptic cell.

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.

Synapse cleft

– narrow gap between presynaptic membrane and postsynaptic membrane.

– current of neurotransmitter to another 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 • 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 – Covering of central nerve 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
Grey matter on 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

Made up of motor neurons and muscle fibers.
One neuron controls many muscle fibers.

2. Autonomic neurotransmitters and receptors

	Preganglionic neuron		Postganglionic neuron
	Neurotransmitter	Receptor	Neurotransmitter	Receptor
Parasympathetic system	ACh (Acetylcholin)	Nicotinic cholinergic	ACh	Muscarinic cholinergic
Sympathetic system	ACh	Nicotinic cholinergic	NE (Norepinephrine)	Adrenergic