Nmj anatomy by shaweta Khosa

1. Shaweta Khosa

2. Neuromuscular Junction
-Neuromuscular Junction
A neuromuscular junction exists
between a motor neuron and a skeletal
muscle.
- Synapse
A junction between two neurons

5. INNERVATION OF SKELETAL MUSCLE
FIBERS
•Large, myelinated nerve fibers
•Originate from large motor neurons in the
anterior horns of the spinal cord
•Each nerve fiber, branches and stimulates from
three to several hundred skeletal muscle fibers
•The action potential initiated in the muscle
fiber by the nerve signal travels in both
directions toward the muscle fiber ends

7. MOTOR END PLATE
•The nerve fiber forms a complex of branching nerve
terminals that invaginate into the surface of the
muscle fiber but lie outside the muscle fiber plasma
membrane
• Entire structure - motor endplate.
•Covered by one or more Schwann cells that insulate
it from the surrounding fluids.

10. AXON TERMINAL
•SYNAPTIC VESICLES
• Size 40 nanometers
• Formed by the Golgi apparatus in the cell body of the
motor neuron in the spinal cord.
• Transported by axoplasm to the neuromuscular junction
at the tips of the peripheral nerve fibers.
• About 300,000 of these small vesicles collect in the nerve
terminals of a single skeletal muscle end plate.

12. •MITOCHONDRIA
•Numerous
•Supply ATP
•Energy source for synthesis of excitatory
neurotransmitter, acetylcholine
•DENSE BARS
•Present on the inside surface of neural
membrane

14. •VOLTAGE GATED CALCIUM CHANNELS
• Protein particles that penetrate the neural
membrane on each side of dense bar
•When an action potential spreads over the
terminal, these channels open and calcium
ions diffuse to the interior of the nerve
terminal.
•The calcium ions, exert an attractive influence
on the acetylcholine vesicles, drawing them to
the neural membrane adjacent to the dense
bars.

15. •The vesicles then fuse with the neural
membrane and empty their acetylcholine into
the synaptic space by the process of
exocytosis
•Calcium acts as an effective stimulus for
causing acetylcholine release from the
vesicles
•Acetylcholine is then emptied through the
neural membrane adjacent to the dense bars
and binds with acetylcholine receptors in the
muscle fiber membrane

19. MUSCLE FIBER MEMBRANE
•SYNAPTIC TROUGH
•The muscle fiber membrane where it is
invaginated by a nerve terminal and a depression
is formed
•SYNAPTIC CLEFT
•The space between the nerve terminal and the
fiber membrane is called the synaptic space or
synaptic cleft

21. •SUBNEURAL CLEFT
•Numerous smaller folds of the muscle membrane at
the bottom of the gutter
• Greatly increase the surface area.
•ACETYLCHOLINE RECEPTORS
• Acetylcholine-gated ion channels
•Located almost entirely near the mouths of the sub
neural clefts lying immediately below the dense bar
areas

23. ACETYLCHOLINE RECEPTORS
• Acetylcholine-gated ion channels
• Molecular weight -275,000
•SUBUNITS
•Two alpha, one each of beta, delta, and gamma
•Penetrate all the way through the membrane
• Lie side by side in a circle- form a tubular channel
•Two acetylcholine molecules attach to the two alpha
subunits, opens the channel
•RESTING STATE
•2 Ach molecules not attached to the alpha subunit
•Channel remains constricted

25. •OPENED Ach CHANNEL
•2 Ach molecules attached to the alpha subunit of
receptor
•Diameter- 0.65 nanometer
•Allows important positive ions—SODIUM,
potassium, and calcium to move easily through the
opening.
•Disallows negative ions, such as chloride to pass
through because of strong negative charges in the
mouth of the channel that repel these negative
ions.

28. •SODIUM IONS
•Far more sodium ions flow through the
acetylcholine channels to the inside than any other
ions
•The very negative potential on the inside of the
muscle membrane, –80 to –90 mili volts, pulls the
positively charged sodium ions to the inside of the
fiber
•Simultaneously prevents efflux of the positively
charged potassium ions when they attempt to pass
outward

29. •END PLATE POTENTIAL
•Opening the acetylcholine-gated channels allows
large numbers of sodium ions to pour to the inside
of the fiber
• Sodium ions carry with them large numbers of
positive charges
• Creates a local positive potential change inside the
muscle fiber membrane, called the end plate
potential.
•End plate potential initiates an action potential that
spreads along the muscle membrane
• Causes muscle contraction

31. Events of Neuromuscular Junction
1. Propagation of an action potential to a terminal
button of motor neuron.
2. Opening of voltage-gated Ca2+
channels.
3. Entry of Calcium into the terminal button.
4. Release of acetylcholine (by exocytosis).
5. Diffusion of Ach across the space.
6. Binding of Ach to a receptor on motor end plate.

32. Examples of Chemical Agents and
Diseases that Affect the Neuromuscular
Junction
Mechanism Chemical Agent or Disease
Alters Release of Acetylcholine
* Cases explosive release of acetylcholine * Black widow spider venom
* Blocks release of acetylcholine * Clostridium botulinum toxin
Block acetylcholine Receptor
* Bind reversibly * Curare
* Auto antibodies inactivate acetylcholine * Myasthenia gravis
receptors
Prevents inactivation of acetylcholine
* Irreversibly inhibits acetylcholinesterase * Organophosphates
* Temporary inhibits acetylcholinesterase * Neostigmine

34. THANKS