Exam 2; B-1: The Neuromuscular Junction

Define: Motor Unit

1 neuron innervates multiple muscle fiber

What is the smallest motor unit ratio?

1:6

Example of 1:6 motor unit ratio

Oculomotor nerve; control muscles of the eyes

What is the largest motor unit innervation ratio?

1:2000

What are muscle fibers innervated by the largest motor unit ratio?

Anti-gravity m.

eg. m. Of back leg

What is neuromuscular junction?

Where motor nerve meets muscle fiber

Which potential does not reach threshold?

mEPP

What is mEPP?

A random release of 1 vesicle of ACh

Not related to Ca**

What mechanism remove ACh from synaptic cleft?

Enzymatic Degradation

What is End Plate potential?

EPSP at NMJ

Always reaches threshold

Action of EPP:

EEPs→Theshold→Ca**channel open→release of Ach→blinds to nAch receptors

Depolarize

what happen to nAch receptors during Werian-Degeration?

nAch Receptors migrates to other parts of the muscle

Define: Denervation Hypersensitivity

nAch receptors migrates to other portion of m. Fiber

Small amount of ACh can trigger m. Contraction

What happen to the nAch receptors when nerves are reattached to muscle fiber (heal)?

Disappear

Back to 1 site ACh sensitivity

2 weeks after nerve injury, muscle fibers become hypersensitive. Why?

Denervation Hypersensitivity

What happen when muscle fibers become hypersensitive?

Fibrillation

Define: Fibrillation

Regular twitching of single m. Fiber

Non visible on skin; detected by electrophysiological testing(EMG)

How does Curare affect NMJ?

Blinds to recognition portion of nAch receptors

Leads to: Fail to open channel

What are Curare?

M. Paralyzing agent

nAch receptors antagonist

How does Botulism reacts on pre-synaptic SMN (spinal motor nerve)?

Blocks Ca2** entry—> reduces ACh release

How are Botulism reacts on muscle?

Weakness/paralysis

Eg of Botulism toxin

Botox

What is infant Botulism?

10% of honey contain Clostridium Botulinum

Children under 1; GI tract is not mature enough

Clostridium Botulinum Colonize large intestine and produce of Botulinum toxin

Botulinum toxin got absorbed

Why can’t children under 1 intake honey?

GI tract not mature enough to keep Clostridium botulinum from colonizing GI

Define: Pre-Synaptic Disorder

Dysregulaation of Ca** channel

What is affect in Post-Synaptic Disorder?

nAch receptor

What happen to EEP in Pre-Synaptic disorder?

Lower

What happen to mEPP in Pre-synaptic disorder?

NO change

What happen to EEP and in Post-synaptic Disorder?

Lower

Is Mysthenia Gravis (MG) pre-synaptic or post-synaptic disorder

Post

Is LEMs pre-synaptic or post synaptic receptor disorder?

Pre

Cause of MG:

Antibodies attack nAch receptors→decrease amount of nAch receptors

widening of synaptic cleft

What are the function of anti-cholinesterase drug?

Prevents the break down of Ach

Inhibiting acetylcholinesterase

Allows nAch receptors open longer→depolarization at NMJ→increase muscle strength

what is Physostigmine and which disorder use it as treatment?

• An antiCholinesterase drug

• Myasthenia Gravis (MG)

Which disorder:

Worst in periods of activity

Better after periods of rest

Myasthenia Gravis (MG)

Which disorder:

Muscles improve after activity

LEMs

Associated tumor with MG

Thymoma

Associated Tumor with LEMs

Oat-cell Carcinoma of lung ( lung cancer)

Conditions of LEMs

Muscle weakness (begins in legs), loss of tendon reflexes, autonomic dysfunction

Why are dry mouth seen in LEMs?

Automatic system dysfunction

Automatic system: parasympathetic and sympathetic

parasympathetic controls salivation

Any cure for LEMs?

NO cure

What mechanism could strength improve with activity in LEMs?

Post-tetanic Potentiation

Define: Post-tetanic potentiation

Increase in NT release after high frequency of action potentials