Neuromuscular Junction and Neuromuscular Transmission
Anatomy of Neuromuscular Junction
Neuromuscular junction is a junction between a motor neuron and a muscle.
The cell body of a motor neuron is located in the ventral horn of the spinal cord
↓
Near the muscle, its axon branches into multiple processes
↓
Each axon process innervates a separate muscle fiber midway along the length of the fiber
↓
The junction between the axon process and a muscle fiber is called the neuromuscular junction or motor endplate.
↓
Here the axon process further branches like a tree into multiple bulb-shaped presynaptic terminals called buttons
↓
The buttons finally contact the muscle.
Motor Unit: The assembly of all muscle fibers innervated by one motor neuron is called the motor unit.
Presynaptic Terminal
is the nerve terminal of the motor neuron.
Contains large number of synaptic vesicles → store neurotransmitter acetylcholine.
Also contains abundant mitochondria → supply ATP required for synthesis of acetylcholine.
The membrane contains voltage-gated Ca channels.
Postsynaptic Membrane
is the membrane of muscle fiber.
Contains acetylcholine receptors which are basically ligand-gated Na channels.
have extensive foldings called postjunctional folds → increase surface area of postsynaptic region → more receptors can be accommodated in small area.
Synaptic Cleft
is the space between the presynaptic nerve terminal and the postsynaptic membrane.
is filled with meshwork of proteins and proteoglycans.
contains an enzyme acetylcholinesterase → destroy acetylcholine.
Physiology of Neuromuscular Transmission
Synthesis of Acetylcholine
Substrates for synthesis of acetylcholine are: acetyl CoA and choline
↓
An enzyme choline acetyltransferase catalyzes transfers of acetyl group from CoA to choline
↓
acetylcholine is formed in the cytosol
↓
Acetylcholine-Hydrogen Exchanger transports the acetylcholine from cytosol into the synaptic vesicle
↓
Acetylcholine is stored inside the vesicle till nerve terminal receives impulse.
Neurotransmission
Impulse opens the voltage-gated Ca channel on presynaptic membrane
↓
Ca ions diffuse into the nerve terminal
↓
Causes exocytosis of synaptic vesicles
↓
Acetylcholine is released into the synaptic cleft
↓
It binds with the acetylcholine receptor (a type of Na channel)
↓
Opens the Na channel
↓
Na ions diffuse into the muscle fiber
↓ Generation of action potential on postsynaptic membrane
↓
↓
↓
Muscle contraction.
Termination of Acetylcholine Action
This can occur through two mechanisms:
1. Acetylcholinesterase enzyme breaks acetylcholine into acetate and choline → choline is transported back into the nerve terminal → reutilized for synthesis of acetylcholine.
2. Diffusion of acetylcholine out of the synaptic cleft
↓
Termination of acetylcholine action
↓
Prevents continued stimulation of the receptors
↓
Muscle relaxes.