Peripheral Nervous System

PNS

Consists of 12 pairs of cranial nerves* and 30 pairs of spinal nerve

Can regenerate if injured

Motor neurons

1. Cell bodies found in the ventral horn of the spinal cord

2. Called lower motor neurons

Somatosensory Neurons

• Cell bodies located in the small ganglia adjacent to the spinal cord

Autonomic neurons

• All 30 pair of spinal nerve contain sympathetic autonomic fibers that innervate smooth muscle in blood vessels and sweat glands

Cervical Plexus

Consists of nerves form C1-C5

Phrenic nerve C3- C5 is especially important for respiration

Brachial Plexus

Formed from spinal nerves from C5- T1

Brachial Plexus is divided into Roots Trunks Divisions Cords and Branches

5 main branches are : Musculocutaneous, Axillary, Radial, Median and Ulnar

Erb’s Palsy or Erb-Duchenne Palsy

Obstetric brachial plexus injury

Results in upper extremity weakness and sensation loss

• Suprascapular nerve, musculocutaneous and axillary nerves are most frequently damaged

Recovery is dependent upon severity of injury. May include:

• Spontaneous recovery

• Rehabilitation

• Surgery

Martin Sheen

Damage from forceps resulted in Erb’s palsy. His left arm is 3 inches shorter than his right.

Thoracic Spinal Nerves

Does not form a plexus

Dorsal rami innervate structure on the back (muscles) and the skin

Ventral rami run laterally beneath the ribs to form the intercostal nerves

Herpes Zoster

Viral infection of the sensory ganglia of spinal and cranial nerves

Caused by varicella virus

Usually appears along a single dermatomal pathway

Causes a painful (nerve pain), blistering rash

Lumbar Plexus

Nerve fibers of L1 – L4

Sensory only: Genitofemoral, Lateral femoral cutaneous, Saphenous

Motor and Sensory: Iliohypogastric, Ilioinguinal, Obturator, Femoral

Sacral Plexus

Contains nerves from L4-L5, S1-4

Sciatic Nerve is the largest branch off the sacral plexus (and largest nerve in body)

Divides into tibial nerve and common fibular nerve

Epineurium

outermost covering; continuous with the dura mater around the spinal cord

Perineurium

groups nerve fibers into bundles (fascicles) . Protects nerves from the blood

Endoneurium

innermost connective tissue sheath

Nerve Fiber Classifications

Fastest nerves - large diameter axons with the thickest myelin sheaths

2 Types of Lower Motor Neurons

Alpha Motor Neurons – innervate muscles to create muscle tension and produce movement. Fast conducting with a thick myelin sheath

Gamma Motor Neurons – Innervate muscle spindles to maintain and alter muscle tension. Do not cause movement. Slower conducting

Peripheral Nerve Injury and Healing

Neuropathy – Damage to the peripheral nerves that can impair motor and sensory functions

1. Ex: carpal tunnel syndrome, diabetic neuropathy,

2. Motor impairments may result in loss of sweating, orthostatic hypotension (sympathetic neurons), bowel and bladder incontinence, sexual impotence, blurred vision, etc.

Regeneration typically occurs at a rate of 1 mm/day.

Motor end plates will stay intact for about 1 year, so for the best chance of restoration of motor function, the motor neuron must reconnect to the muscle within about 1 year.

When the axon, myelin sheath and all connective tissues are severed, the chance of recovery is less likely.

Muscle Atrophy due to Denervation

Denervated skeletal muscle will lose about 70% of cross-sectional area within 2 months

Diabetic Neuropathy

Affects peripheral nerves

Symptoms

1. Paresthesias

2. Paresis

3. Hypotension

4. Tachycardia

5. Digestive problems

6. Bowel and bladder incontinence

7. Sexual dysfunction

8. Thickening of blood vessels leading to ischemia