Peripheral Nerve Disorders

Question 0

Components of the Motor Unit

Answer 0

• Each anterior horn cell innervates many muscle fibers.
• Each muscle fiber is innervated by one anterior horn cell
• Anterior horn cell & innervated muscle fibers act as a single unit.
• Each anterior horn cell discharge leads to discharge of all innervated muscle fibers.

Question 1

Mother Nature’s Rules of the PNS

Answer 1

1. Nerves never cross the midline!
   
   • One exception occurs in the midbrain
2. Nuclei of motor components are in the anterior horn of the spinal cord or analogous cranial nerve nuclei
3. Nuclei of sensory elements are in the dorsal root ganglia or analogous cranial nerve nuclei
   
   • One exception occurs in the brain stem
4. Longer axons are more sensitive to disease than are shorter ones

Question 2

Motor Unit Physiology

Answer 2

1. Excitation
   - Anterior horn cells (AHC): Innervated by committee (CNS)
   - Nerve muscle junction:
   
   • One stimulus from AHC: one contraction
   • Multiple stimuli: further contraction
     - Fasciculation: Sign of a sick anterior horn cell
2. Dependence of muscle cells on innervation
   - Atrophy
   - Fibrillation
   - Search for re-innervation
3. Synaptic failure = myasthenia
   - Role of myelin in conduction

Question 3

Somatic Sensory Modalities

Answer 3

• Three Neuron Chain
• Pain
• Temperature
• Vibration
• Position
• Fine touch

Question 4

Pain fibers and Position sense fibers

Answer 4

• have cell bodies in the DRG
• Pain fibers synapse in the spinal cord
  
  • cross, and ascend to the thalamus
  • Enter the spinal cord at the Zone of Lissauer
• Position fibers ascend in the ipsilateral spinal cord
  
  • synapse in the Nuc. Gracilus
  • Crosses and ascends to the thalamus

Question 5

Findings that indicate a PNS neuropathy

Answer 5

1. Pattern
   - Bilateral abnormalities
   - Distal (longer nerves fail first)
2. Weakness and atrophy (Denervated muscles atrophy)
3. Loss of sensation and “hyperpathia”
4. Loss of reflexes and absence of “abnormal” reflexes
   - May be caused by loss of muscle innervation
   - May be caused by loss of sensory limb or reflex arc.

Question 6

PNS Nerve Disorders: Focal vs. Diffuse

Answer 6

1. Focal
   - Acute: Traumatic nerve or root
   - Chronic: Compression Syndrome
2. Diffuse/Bilateral:
   - Acute: Guillain Barre Syndrome
   - Chronic: Peripheral neuropathy syndrome

Question 7

Polyneuropathies: Fast

Answer 7

• Guillain-Barre:
• Autoimmune disorder of myelin
• Mixed motor/sensory/autonomic
• Cause of death:
  
  • Respiratory
  • Cardiac
• Management
  
  • IVIG
  • Plasmapheresis

Question 8

Polyneuropathies: Slow

Answer 8

1. Disorders primarily of axons
2. The big three
   - Diabetes
   - Alcohol
   - Thiamine Deficiency
3. Less common
   - Renal failure
   - Familial
   - Toxic (heavy metals, common anti-cancer agents)
   - Lupus and other autoimmune disorders
   - Remote effects of carcinoma
   - Leprosy

Question 9

Myelin disorders:

Answer 9

1. Chronic Idiopathic Distal Polyneuropathy

2. B12 Deficiency

Question 10

Fast Focal Neuropathies

Answer 10

• Trauma
• Herpes (Shingles)
• Bells palsy, brachial plexitis and other focal demyelinating disorders
• Diabetes

Question 11

Slow Focal Neuropathies

Answer 11

• Compression
  
  • Nerve Roots
  • Plexi (Cervical, Lumbar)
  • Named nerves
• Neoplasm (schwannoma)

Question 12

Synaptic Disorders

Answer 12

• Myasthenic disorders: slow, diffuse, and inconsistent
• Botulism: rapid diffuse
• “Nerve Agents” Fast, diffuse, autonomic

Question 13

Muscle disorders

Answer 13

1. Congenital dystrophies:
   - Duchenne’s
   - Limb Girdle
   - Myotonic
2. Acquired
   - Autoimmune
   - Endocrine