Lecture 63: Peripheral Nervous System

What is the difference between myelinated and unmyelinated axons?

myelinated: rapid signal transmission

unmyelinated: slow, continuous signaling (autonomic/sensory)

What is the function of the PNS?

connect the CNS to the rest of the body, enabling both sensory input (afferent) and motor output (efferent)

What is the difference between the somatic and autonomic nervous sytems?

somatic: controls voluntary skeletal muscle movement via spinal and cranial nerves

autonomic: controls involuntary functions

How is the autonomic nervous system further divided?

PSNS: rest and digest

SNS: fight or flight

enteric: regulates GI motility and secretion via myenteric and submucosal plexuses

What is axoplasmic transport and what are the two different types?

bidirectional movement of nutrients and molecules:

• anterograde: cell body → axon terminal

• retrograde: axon terminal → cell body

What allows rapid signal propagation through depolarization waves?

action potentials

Myelination by ________ provides insulation and trophic support to the PNS?

schwann cells

What cranial nerve arises from the rostral olfactory bulb?

CN 1 (olfactory)

What cranial nerve arises from the forebrain?

CN II (optic)

What cranial nerve arises from the brainstem?

CN III-XII:

• CN III and IV – arise from the midbrain

• CN V-XII – emerge from the pons and medulla oblongata

• Myelinated by Schwann cells (unlike CN I and II)

What cranial nerves are sensory, motor, and mixed?

• Sensory: I, II, VIII

• Motor: III, IV, VI, XI, XII

• Mixed: V, VII, IX, X

How is the enteric system divided?

• Myenteric plexuses: located in muscle wall – helps with motility

• Submucosal plexuses: Submucosal – aids digestion fluid secretion and absorption, blood flow

What cells in the ganglia provide metabolic and structural support?

satellite glial cells

What is the endoneurium, perineurium, and epineurium?

• Endoneurium – surrounds individual nerve fibers

• Perineurium – encloses nerve fascicles

• Epineurium – encases groups of fascicles

The dorsal rot relays _______ information, while the ventral root relays ______ information.

sensory, motor

What is dysautonomia and what are some examples?

failure of the autonomic nervous system:

• equine grass sickness in horses

• Key-Gaskell syndrome in cats

What are the causes and clinical signs of dysautonomia?

causes: autoimmune or environmental toxins

clinical signs: GI stasis, urinary incontinence, bradycardia, prolapsed third eyelid, mydriasis

What is peritonitis-induced autonomic dysfunction?

cytokine-mediated degeneration of enteric neurons in the myenteric and submucosal plexuses → neuronal chromatolysis, edema of nerve fiber bundles, and supporting cell hyperplasia

What is congenital hypomyelinating polyneuropathy?

a mutation in the MPZ gene in golden retrievers → thin myelin sheaths, excess (pos. defected) schwann cells → hopping gait, decreased reflexes, limb circumduction

What is coyotillo toxicosis in ruminants?

toxicity from karwinol A from karwinskia humboldtiana fruit → primary demyelination targeting schwann cells → hindlimb weakness incoordination

What are some endocrine related neuropathies?

• Diabetes mellitus: Causes diabetic neuropathy (rear limb weakness, plantigrade stance (flat hock) in cats).

• Hypothyroidism: Can produce sensory and motor deficits.

• Pathogenesis involves metabolic and vascular changes to nerves.

What are some nutritional neuropathies?

• Vitamin A deficiency: Optic nerve compression → blindness (calves, pigs).

• Riboflavin (B2) deficiency: “Curled-toe paralysis” in poultry—demyelination and endoneurial edema

What are some toxic neuropathies?

• Lead: Neuronal necrosis (CNS) + demyelination (PNS).

• Organomercurials: Damage dorsal root ganglia neurons.

• Vincristine: Chemotherapy agent disrupting microtubules → distal axonal degeneration.

• Ionophore toxicosis (salinomycin in cats): Polyneuropathy with axonal degeneration and demyelination

How does botulism cause PNS disease?

clostridium botulinum neurotoxin (types A-G) in horses, cattle and birds → blocks ACh release at neuromuscular junction → flaccid paralysis → death via respiratory vailure

What are the types of traumatic nerve injury?

• Neurapraxia: Temporary conduction block (recoverable).

• Axonotmesis: Axon destroyed but connective tissue intact (good prognosis)

What is neuroma?

bulbous regrowth after nerve severance (ex. tail dock neuroma)

How does neurogenic shock occur?

secondary to CNS trauma → autonomic dysfunction → vasodilation → hypotension → shock

What are the three types of nerve sheath neoplasms?

1. schwannoma: most common in dogs

2. neurofibroma

3. perineurioma

What is this?

schwannoma

What is this? note the thickening of spinal nerve roots and fibers with tan brown tissue.

spinal nerve schwannoma

How does equine grass sickness affect horses?

loss of enteric and autonomic neurons → gut stasis and dsyphagia → distended GI tract, esophagitis from reflux

How does recurrent laryngeal paralysis (“roaring”) occur in horses?

axonal injury of the recurrent laryngeal nerve (left side more common in tall male horses), leading to neurogenic atrophy of the cricoarytenoid dorsalis muscle → inspiratory noise, exercise intolerance

How does polyneuritis equi (cauda equina neuritis) affect horses?

suspected autoimmune disorder targeting sacrococcygeal nerves → granulomatous inflammation and fibrosis around affected nerves → tail and perineal paralysis, urinary/fecal incontinence, pelvic muscle atrophy

How does acute idiopathic polyneuritis (coonhound paralysis) affect dogs?

autoimmune demyelination of ventral spinal roots and peripheral nerves → ascending flaccid paralysis (hindlimbs to forelimbs), hyperesthesia, weakness

How does sensory neuropathy of english pointers occur?

hereditary sensory neuropathy → degeneration of dorsal root ganglia and spinal nerves → insensitivity to pain, self-mutilation of paws