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NEUROMUSCULAR JUNCTION |FINDYOURSELF
Neuromuscular
junction
A NEUROMUSCULAR JUNCTION is an area of contact between
a muscle fibre and a neuron.
Fig. An
electron micrographic sketch of the junction between a single axon terminal and
the muscle fiber membrane.
MOTOR
END-PLATE
Definition:
It is the specialized portion of a muscle fibre
immediately under a terminal nerve fibre. The nerve fibre invaginates a muscle
fibre but lies outside the muscle fibre plasma membrane. The entire structure
is called the motor end-plate.
NEUROMUSCULAR JUNCTION
A neuromuscular junction thus consists of:
•
Presynaptic terminal (Nerve
fibre) with vesicles containing the NT
•
A synaptic cleft (20-30 nm
wide)
•
A synaptic trough or gutter (Muscle
fibre) which has numerous folds called subneural clefts.
•
Neuroreceptors for the NT.
•
The NT at an NMJ is
ACETYLCHOLINE
(Ach). TheThe
presynaptic membrane of the neuron contains linear dense bars. To each side of
the dense bars are protein particles penetrating the neural membrane. These are
the voltage-gated calcium channels.
•
When
an action potential spreads over the terminal, these channels open and allow
calcium ions to diffuse from the synaptic space to the interior of the nerve
terminal.
•
The
vesicles then fuse with the neural membrane and empty their acetylcholine into
the synaptic space by the process of exocytosis.
•
Each Ach receptor complex has a total
molecular weight of 275,000.
•
Each
receptor complex is composed of 5 subunits:
- 2 alpha
- 1 beta
- 1 gamma
- 1 delta.
•
The
channels remains closed unless 2 Ach molecules attach to the 2 alpha subunits
which open the gate.
•
The
opened acetylcholine channel has a diameter of about 0.65 nanometer,
which is large enough to allow the important positive ions— Na+, K+ and Ca++
—to move easily through the opening.
The
Steps in Neuromuscular Junction
1.
An
AP reaches the presynaptic terminal of the NMJ.
2.
The
change in voltage causes the opening of the voltage-gated calcium channels
which cause exocytosis of the Ach containing secretory vesicles.
3.
The
NT Ach is secreted into the synaptic cleft.
4.
Ach
crosses the synaptic cleft to reach the subneural clefts which contains the
Ligand-gated Ach channel.
5.
The
channels are activated and open allowing the Na+ to move to the inside of the
muscle fiber. As long as the Ach is present in the synaptic cleft, it keeps
activating the Ach channels which remain open.
6.
The
influx of Na+ into the muscle lead to the initiation of the END PLATE POTENTIAL
(EPP).
END-PLATE
POTENTIAL
•
At
the motor end-plate, the large influx of the Sodium ions leads to a large
number of positive charges pouring into the muscle.
•
This
creates a local positive potential change inside the muscle fiber membrane,
called the end plate potential. It is usually about 50-75 mv.
•
In
turn, this end plate potential initiates an action potential that spreads along
the muscle membrane and thus causes muscle contraction.
Acetylcholine
Receptor:
•
Each
Ach receptor complex has a total molecular weight of 275,000.
•
Each
receptor complex is composed of 5 subunits:
- 2 alpha
- 1 beta
- 1 gamma
- 1 delta.
•
The
channels remains closed unless 2 Ach molecules attach to the 2 alpha subunits
which open the gate.
•
The
opened acetylcholine channel has a diameter of about 0.65 nanometer,
which is large enough to allow the important positive ions— Na+, K+ and Ca++
—to move easily through the opening.
