Anatomy: Peripheral Nervous System

Nerves

Groups of axons in the PNS

Ganglia

Groups of neural cell bodies in the PNS

Reminder

Most nerves carry both afferent + efferent axons
The exceptions are all in your head! (cranial nerves)

Spinal nerves and associated structures

Dorsal root ganglion (cell bodies of sensory neurons)
Dorsal root- sensory
Ventral root- motor
Spinal nerve- short
Dorsal ramus- sensory and motor
Ventral ramus- sensory and motor

Diagram of spinal nerve rami

Review of peripheral nervous system

Afferent (sensory)- Carries impulses to CNS
Efferent (motor) carry impulses to efferent organs
Somatic motor nervous system
Visceral motor (autonomic) nervous system
Sympathetic- fight or flight
Parasympathetic- rest and digest

Somatic motor

1 synapses between CNS and effector
Effectors- skeletal muscle
Voluntary? Usually

Visceral motor (autonomic)

2 synapses between CNS and effector
Effectors- cardiac and smooth muscle, glands
Voluntary? Never

Sympathetic nervous system general features

Fight or flight- arises from thoracic and lumbar spinal nerves
Preganglionic axon/fiber- short
Ganglion- near the spinal cord
Postganglionic axon/fiber- long
Axons highly branched

Sympathetic nervous system: Neurotransmitters

Preganglionic neurotransmitter: acetylcholine (Ach)
Postganglionic neurotransmitter: norepinephrine (“NE”)= noradrenaline -mostly (with exceptions)

Sympathetic nervous system: Physiological responses

Triggered by exercise, emergencies, usually a coordinated response
Increased heart rate
Increased blood flow to skeletal muscle
Decrease blood flow to many organs
Dilate bronchioles
Release of glucose reserves by liver
Digestion/peristalsis shuts down
Sweaty skin

Adrenal medulla

The largest sympathetic ganglion
Modified ganglionic neurons secrete into blood- I.e. hormones- mainly epinephrine (adrenaline)

Diagram of sympathetic NS

Diagram of adrenal medulla and it’s innervation

Parasympathetic nervous system

Rest and digest
Arises from cranial nerves and sacral spinal nerves
90% is via the vagus nerves (cranial nerve X)

Parasympathetic general features

Preganglionic axon/fiber- long
Ganglion- within or near effector organ
Postganglionic axon/ fiber- short
Axons have few branches

Parasympathetic nervous system: neurotransmitters

Preganglionic neurotransmitter: acetylcholine (“ACh”)
Postganglionic neurotransmitter: Acetylcholine (“ACh”)

Parasympathetic nervous system: physiological responses

Triggered at rest
Each organ controlled separately
Decrease heart rate
Constrict bronchioles
Increase motility (peristalsis), and secretions, of GI tract
ONLY innervates visceral organs and other organs shown here (not general body periphery)

Diagram of parasympathetic NS

Sensory system

Sensory receptors- convert stimuli into electrical signal (=transduction)
Consist of:
Receptive ending of sensory neuron
Or
Receptor cell (which then signals sensory neuron)

Classification of sensory receptors by stimulus type

Mechanoreceptors- touch, pressure, vibration, stretch
Thermoreceptors- temperature
Chemoreceptors- chemicals
Photoreceptors- light
Nociceptors- pain

Somatic vs visceral senses

Somatic senses- monitor external environmental, skin, body wall, limbs
Visceral senses- monitor visceral organs

General sense

Receptors are distributed widely

Special senses

Have compact, specialized sense organs
Special visceral senses for chemoreception:
Olfaction (smell)
Gustation (taste)
Special somatic senses-
Mechanoreception:
Hearing, equilibrium (balance)
Photoreception:
Vision