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NMJ (Neuromuscular Junction ) Physiology Reference Book : Guyton د. طه صادق أحمد

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NMJ (Neuromuscular Junction ) Physiology Reference Book : Guyton د. طه صادق أحمد. Objectives. At the end of this lecture the student should be able to : Describe the anterior horn cell and motor unit . Describe and define the structure and function of the neuromuscular junctuion .

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objectives
Objectives
  • At the end of this lecture the student should be able to :
  • Describe the anterior horn cell and motor unit .
  • Describe and define the structure and function of the neuromuscular junctuion .
  • Describe acetylcholine vesicles , exocytosis , EPP ( end-plate potential ), acetylcholinesterase , drugs that inactivate cholinesterase( e.g., neostigmine ), Myasthenia Gravis .
slide3

Q:What is a Motor Unit ?

  • الوحدة الحركية ؟
  • It is the motor

neuron (Anterior

Horn Cell) and all

the muscle fibers

it supplies

the neuromuscular junction consists of
The Neuromuscular junction consists of

A/ Axon Terminal : contains

around 300,000 vesicles which

contain the neurotransmitter

acetylcholine (ACh).

B/ Synaptic Cleft :

20 – 30 nm ( nanometer ) space

between the axon terminal & the

muscle cell membrane. It contains

the enzyme cholinesterase which

can destroy Ach .

C/ Synaptic Gutter ( Synaptic Trough)

It is the muscle cell membrane

which is in contact with the

nerve terminal . It has many folds

called Subneural Clefts , which

greatly increase the surface area ,

allowing for accomodation of large

numbers of ACh receptors . ACh

receptors are located here .

slide6

The entire structure of axon terminal , synaptic cleft and synaptic gutter is called “ Motor End-Plate ” .

  • ACh is synthesized locally in the cytoplasm of the nerve terminal , from active acetate (acetylcoenzyme A) and choline.
  • The enzyme responsible is Choline Acetyl Transferase.
  • This enzyme is a marker of Cholinergic Neurons
  • After being synthesized , ACh is rapidly absorbed into the synaptic vesicles and stored there.
  • The synaptic vesicles themselves are made in the nerve soma (celol-body by Golgi apparatus ) .
  • Then they are carried by Axoplasmic Transport to the nerve terminal , which contains around 300,000 vesicles .
slide7
When a nerve impulse

reaches the nerve

terminal , it opens calcium

channels 

calcium diffuses from

the ECF into the axon

terminal  Ca++ releases

ACh from vesicles by

a process called

Exocytosis .

ACh combines with its

receptors in the subneural

clefts.

This opens sodium Na+

channels  & Na+ diffuses

into the muscle causing a

local,non-propagated

potential called the

“ End-Plate Potential (EPP)”,

whose value is 50 – 75 mV.

slide8
This EPP triggers a

muscle AP which spreads

inside the muscle to make

it cntract

One nerve impulse can

release 125 ACh vesicles ,

which is more than enough

to generate produce one

End-Plate Potential (EPP) .

slide9
After ACh acts on the receptors , it is destroyed (hydrolyzed) by the enzyme Acetylcholinesterase ( also called cholinesterase ) into Acetate & Choline . The Choline is actively reabsorbed into the nerve terminal to be used again to form ACh. This whole process of Ach release, action & destruction takes about 5-10 ms .
examples of drugs acting on the nmj
Examples of Drugs Acting on the NMJ
  • Drugs that inactivate Cholinesterase , called

Anticholinesterase drugs , preventing it from destroying ACh

& hence sparing ACh and allowing Ach to acccunulate &

stimulate the muscle .

Example of this drug category is Tensilon ( Edrophonium ) ,

which is used in to test for Myasthenia Gravis