HP LEC 7: The Peripheral Nervous System

PNS is composed of:

1. Nerves: Cranial and Spinal Nerves

2. Ganglia

Nerves

bundles of axons in the PNS are referred to as nerves, these structures in the periphery are different than the central counterpart called a tract

1. Cranial Nerves

2. Spinal Nerves

Ganglia

• group of neuron cell bodies in the periphery

• can be categorized: 1) Sensory Ganglia or 2) autonomic ganglia referring to their primary functions

Cranial Nerves

• Nerves that arise directly from the nuclei in the brain

• 12 pairs of Cranial Nerves

• most are mixed nerves: with both sensory and motor fibers (ex: vagus nerve)

• nerves associated with vision, olfactory, and hearing are sensory only

Spinal Nerves

• nerves that airse directly from the spinal cord

• 31 Pairs: 8 Cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

• mixed nerves (can carry both motor and sensory info)

How do spinal nerves enter and motor fibers exit in the PNS?

• sensory fibers enter the cord via the posterior/dorsal root, and the motor fibers exit by way of the anterior/ventral root

Ganglia

• group of neuron cell bodies in the periphery

• work as a relay station for nerve signals: one nerve enters (preganglionic) and another nerve exists (postganglionic) from each ganglion

Pre and Post ganglionic Neurons- where are the cell bodies of their neurons?

• the cell bodies of the preganglionic neurons are in the brainstem or spinal cord of the CNS

• the cell bodies of the postganglionic neurons are in the ganglia

Different Parts of the PNS

1. Somatic Nervous System

2. Autonomic Nervous System

Somatic Nervous System (function, what kind of nerves does it include?)

• serves the skin, skeletal muscles, and tendons

• sensory & motor nerves that innervate the limbs & body

• includes nerves that take sensory information from external sensory receptors to the CNS and motor commands away from the CNS to the skeletal muscles

• sensory nerve fibers in the peripheral nerves are the peripheral axonal process of neurons in the dorsal root ganglion

Autonomic Nervous System

• innervates all effector organs and tissues except for skeletal muscles

• it is autonomic bc it functions subconsciously and involuntarily

Autonomic Nervous System Regulates:

1. activities of glands

2. smooth muscle function

3. function of heart & circulatory system

4. function of digestive system

Visceral organs innervated by the autonomic nervous system

• unlike somatic motor neurons (always simulatory), autonomic motor neurons can stimulate OR inhibit

Autonomic Nervous System Responses

1. Sympathetic: fight or flight

2. Parasympathetic: rest & digest

Sympathetic

• fight or flight

• involved in responses that would be associated with flighting or fleeing, such as increasing heart rate and blood pressure as well as constricting blood vessels in the skin & dilating them in muscles

• the sympathetic division is most active during times of excitement and physcial activity

Parasympathetic

• rest and digest

• involved in energy conservation functions and increases gastrointestinal motility and secretion

• increases bladder contractility

• parasympathetic division is most active during rest and stimulates digestive activities

Sympathetic Divison: 3 steps

1. the axons of the preganglionic neurons come from the thoracic and lumbar regions of the spinal cord

2. Preganglionic neurons synapse in sympathetic ganglia that run parallel to the spinal cord (not all tho)

• called the paravertebral ganglia

• paravertebral ganglia are connected forming a sympathetic chain of ganglia

3. Many of the sympathetic neurons that exit the spinal cord below the diaphragm do NOT synapse in the sympathetic chain of ganglia

• Collateral Ganglia: includes celiac, superior mesenteric, and inferior mesenteric ganglia

Sympathetic Division: chain of ganglia- function

• it allows nerve fibers to travel to spinal nerve that are superior & inferior to the one in which they originated

Sympathetic Division: Convergence & Divergence

bc preganglionic neurons can branch & synpase in ganglia at any level there is:

1. Divergence: one preganglionic neuron synapses on several postganglionic neurons at different levels

2. Convergence: Several preganglionic neurons at different levels synpase on one postganglionic neuron 

• allows the sympathetic division to act as a SINGLE unit through mass activation & to be tonically active

Parasympathetic Division

• preganlionic neurons come from the brain (brainstem) and sacral region of the spinal cord

• also called the craniosacral division, some preganglionic neurons synapse on small terminal ganglia or intramural ganglia (interamural- live near or within the organs they innervate)

• exception are the 4 parasympathetic ganglia of the head & neck

Parasympathetic Divison: Vagus Nerve

vagus nerve (X) represents the main component of the parasympathetic nervous system: oversees a vast array of crucial bodily functions including:

1. control of mood

2. immune response

3. digestion

4. heart rate

• establishes one of the connections between the brain & the gastrointestinal tract & sends the information about the state of the inner organs to the brain via afferent sensory fibers

ANS: Cholinergic Synaptic Transmission

a) acetylcholine (ACh) is the neurotransmitter used by all preganglionic neurons (sympathetic & parasympathetic

b) always excitatory- nicotinic receptors

c) it is the neurotransmitter released from most parasympathetic post ganglionic neurons

d) can be excitatory or inhibitory- muscarinic receptors

Adrenergic Synaptic Transmission

a) norepinephrine is the neurotransmitter released by most sympathetic postganglionic neurons

b) can be excitatory or inhibitory- nonadrenergic alpha or beta receptors

Other Autonomic Nuerotransmitters

Some postganglionic autonomic neurons do not release ACh or norepinephrine:

• called: “nonadrenergic, noncholinergic fibers”

• proposed neurotransmitters include ATP, vasoactive intestinal peptide (VIP), and nitric oxide (NO)

Organs with Dual Innervation

1. Most visceral organs are innervated by both sympathetic & parasympathetic neurons

2. The activity of the two divisions of the autonomic system can be:

• antagonistic

• complementary

• cooperative

Antagonistic action of sympathetic & parasympathetic autonomic system

• occurs when both divisions produce OPPOSITE effects on the same target

Ex:

• Heart rate: sym increases, para decreases

• Digestive functions: sym decreases, para increases

• Pupil diameter: sym dilates, para contricts

Complementary action of sympathetic & parasympathetic autonomic system

• occur when both divisions produce similar effects on the same target

Ex: 

Salivary Gland Secretion: parasympathetic division stimulates secretion of saliva; sympathetic constricts blood vessels so the secretion is thicker

Cooperative Action action of sympathetic & parasympathetic autonomic system

• occur when both divisions produce different effects that work together to promote a single action

Ex:

Urination: parasympathetic division aids in urinary bladder contraction; sympathetic helps bladder muscle tone to control urination reflex (triggered when the bladder fills with urine)

Organs without Dual Innervation

Organs that are innervated by sympathetic division ONLY:

1. Adrenal medulla

2. Arrector pili muscles in skin

3. Sweat glands in skin

4. Most blood vessels

• regulated by increases & decrease in sympathetic nerve activity

• important for body temperature regulation through blood vessels & sweat glands

Sensory Physiology (sensory system, modality)

a sensory system is a part of the nervous system responsible for processing sensory information, it consists of:

• sensory receptors

• neural pathways

• parts of the brain involved in sensory perception

In Each sensory modality:

• a specific type of stimulus energy is transformed into electrical signals by specialized receptors

• the sensory information is transmitted to the central nervous system by trains of action potentials (along the neural pathway) that represents aspects of the stimulus