The Neuromuscular Junction



Study Questions on the Neuromuscular Junction:

1. What causes skeletal muscle cells to contract?


     Stimulation (depolarization) by the motor neuron that innervates the skeletal muscle fiber (cell).



2. What is a motor neuron?


     Single nerve cell extending from the brain or spinal cord to a muscle or gland.



3. What part of the motor neuron carries impulses to the muscle? 


     The axon is the process that carries the impulse (action potential) from the nerve cell body to the muscle that it innervates.



4. Match the following terms to their description:



Axon terminal                                                             Synaptic Vesicles            Synaptic Cleft

Motor End Plate                                                         T Tubule                            Sarcolemma

Terminal Cisternae & Sarcoplasmic Reticulum      Sarcomere


 ___T-tubules__________ a. Invaginations of the sarcolemma penetrating deep into the  interior of the muscle cell.


______Synaptic cleft______  b. The space between the axon terminal and the motor end   plate.



______Axon terminal______ c. The swollen distal end of the motor neuron axon.



______Sarcolemma_______ d. The muscle cell membrane.



______Synaptic vesicles___ e. Structures within the axon terminal that contain the neurotransmitter acetylcholine.


______Sarcomere_________ f. The contractile unit of a muscle cell that extends from one Z line to the next.


______Terminal Cist. & SR__g. Structures within skeletal muscle cells that serve as reservoirs of calcium ions.


______Motor End Plate_____h. A folded region of the sarcolemma at the neuromuscular junction.



5. List the following events in the order they occur:


__3___ a. The motor end plate is depolarized.


__5___ b. The sarcomeres contract.


__2___ c. Acetylcholine is released from the axon terminal into the synaptic cleft.


__4___ d. The depolarization triggers an action potential, which propagates along the sarcolemma and the T tubules.


__1___ e. An action potential arrives at the axon terminal



6.  What happens at the neuromuscular junction when the action potential arrives at the axon terminal.


     Voltage-regulated Ca2+ channels open and allow Ca2+ to flow into the axon, causing synaptic vesicles to fuse with the plasma membrane of the axon terminal.  Acetylcholine (ACh), released from the motor neuron's synaptic vesicles by exocytosis, then diffuses across the synaptic cleft and binds to receptors on the motor end plate of the muscle cell causing depolarization of the muscle cell (and ultimately release of Ca2+ from the sarcoplasmic reticulum).



7.     What happens to the acetylcholine after it diffuses away from its receptor on the motor end plate?


     The acetylcholine is broken down (into acetate and choline) and inactivated by the enzyme, acetylcholinesterase.



8.  What happens as the action potential moves down the T Tubules?


     Action potentials originating from depolarization at the motor end plate propagate along the skeletal muscle membrane (sarcolemma) and down the T-tubules.  The depolarization activates certain channels in the T-tubule (Ca2+ channels) that are connected to channels in the terminal cisternae of the sarcoplasmic reticulum (SR).  This causes Ca2+ to be released from the SR and diffuse into the sarcoplasm.


9.     What happens when calcium ion is present in the cytosol of the muscle cell?


     When Ca2+ is present in the sarcoplasm of the muscle cell, the contraction cycle begins.  Ca2+ binds to troponin, moves tropomyosin away from the myosin binding sites on actin, and the myosin heads attach to actin beginning the cycle.



10.  Place the following events in their proper sequence:  (NOTE:  You need to know, and understand, this sequence!!!)


__4___ a. Acetylcholine is released into the synaptic cleft.


__8___ b. Action potential propagates along the sarcolemma and down the T Tubules.


__3___ c. Synaptic vesicles fuse to membrane of axon terminal.


__6___ d. Motor end plate becomes depolarized.


__7___ e. Action potential is initiated on the sarcolemma.


__1___ f. Action potential arrives at the axon terminal.


__9___ g. Calcium ions are released from the terminal cisternae.


__5___ h. Acetylcholine binds to receptor sites on the motor end plate.


__10__ i. The muscle cell contracts.


__2___ j. Calcium ions enter the axon terminal.