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Chapter 12 Neuromuscular Junction #2

Neuromuscular Junction
What is the place called where a motor neuron stimulates a muscle cell?
How are skeletal muscle cells electrically insulated from each other?
Motor Neuron
A single nerve cell that extends from the Brain or Spinal Cord to a muscle or gland.
Structures to create Muscles Contractions
T-Tubule -Invaginations of the sarcolemma penetrating deep into the interior of the muscle cell.
Synaptic Cleft -the space between the axon terminal and the motor end plate.
Axon Terminal -the swollen distal end of the motor neuron axon.
Sarcolemma -the muscle cell membrane.
Synaptic Vesicles -structures within the axon terminal that contain the neurotransmitter acetylcholine (ACh).
Terminal Cisternae & Sarcoplasmic Reticulum (SR) -structures within skeletal muscle cells that serve as reservoirs of calcium ions.
Motor End Plate -a folded region of the sarcolemma at the neuromuscular junction.
Neuromuscular Junction & ACh
1.) An ACTION potential arrives at the axon terminal.
2.) Acetylcholine (ACh) is released from the axon terminal into the synaptic cleft.
3.) The motor end plate is DEPOLARIZED.
4.) The Depolarization triggers an ACTION potential which propagates along the sarcolemma and the T-Tubules.
5.) The Sarcomeres CONTRACT !!!
What happens at the NM Junciton when the AP arrives at the axon terminal?
The VOLTAGE change of the membrane OPENS VOLTAGE-GATED CALCIUM CHANNELS, allowing Calcium ions to enter the axon terminal.
What is the effect of the presence of calcium ions inside the axon terminal?
Calcium ions inside the axion terminal cause several Synaptic Vesicles to FUSE w/ the membrane of the axon terminal.
What two events happen after the Synaptic Vesicles Fuse with the membrane of the axon terminal?
1.) The neurotransmitter ACh contained with the Vesicles is liberated by EXOCYTOSIS into the Synaptic Cleft.
2.) Calcium ions are PUMPED OUT of the AXON TERMINAL.
What is Exocytosis?
Bulk transport of material OUT of cell by 1st FUSEing a membrane-bound vesicle, ( 0 ), w/ plasma membrane . When membrane ruptures , the contents of the vesicle are RELEASED into the extracellular space (or synaptic cleft)
What is a Action Potential?
The ELECTRICAL SIGNAL consisting of the DEPOLARIZATION & subsequent REPOLARIZATION of a nerve or muscle cell membrane.
What is Depolarization?
DECREASE in the negative Resting Membrane Potential ( the VOLTAGE that exists across the plasma membrane ).
What happens to ACh after it's RELEASED into the synaptic cleft?
ACh binds to ACh RECEPTOR sites of CHEMICALLY-GATED ion channels on the "MOTOR END PLATE"
What happens after the ACh BINDS to the ACh receptor on the motor end plate?
This causes the channels to OPEN permitting an INflux of Sodium Na+ ions and a small EFlux (out) of Potassium K+ ions.
This ion EXCHANGE causes a local Depolarization of the "MOTOR END PLATE"
When does the CHEMICALLY-GATED ion channel on the motor end plate close?
After a brief period, ACh DIFFUSES away fm its Receptor site (ion channel Closes).
What happens to the ACh after it DIFFUSES away from its receptor on the motor end plate?
ACh is then BROKEN DOWN by the enzyme: Acetyl-cholines-ter-ase
The movement of the sodium ions throughout the Chemically gated ion channel initiates a Depolarization of the motor end plate. What happens after this depolarization is generated?
An ACTION POTENTIAL which propagates along the sarcolemma in all directions and down the T-tubules.
What happens as the action potential moves down the T-tubules?
Calcium ions are released from the TERMINAL-CISTERNAE into the CYTOSOL.
What happens when the calcium ion is present in the cytosol of the muscle cell?
CONTRACTION of the muscle cell / fiber (only one cell)
10 Steps to a Contraction of a Motor neuron (Muscle Cell / Fiber)
1.) ACTION POTENTIAL arrives at the axon terminal.
2.) Calcium ions ENTER the axon terminal.
3.) Synaptic vesicles fuse to membrane of axon terminal.
4.) ACh is released, by EXOCYTOSIS, into the synaptic cleft; then calcium is pumped OUT of axon terminal.
5.) ACh binds to receptor sites on the motor end plate.
6.) Motor end plate becomes depolarized.
7.) Action Potential is initiated on the sarcolemma.
8.) Action Potential propagates along the sarcolemma and down the T-tubules.
9.) Calcium ions are released from the terminal cisternae.
10.) The muscle cell contracts.