The Neuromuscular Junction

Graphics are used with permission of A.D.A.M. Software, Inc. and Benjamin/Cummings Publishing Co.

 

Page 1.  Introduction

Motor neurons stimulate muscle cells to contract at the neuromuscular junction.

 

Page 2.  Goals

To examine the structure of a neuromuscular junction.

To understand the sequence of events occurring at the neuromuscular junction following a stimulus.

 

Page 3.  Role of Motor Neuron

Axons of motor neurons innervate skeletal muscle cells at the neuromuscular junction.

 

Page 4.  Anatomy of a Neuromuscular Junction

The following parts of a neuromuscular junction and skeletal muscle cell are described:

Axon terminal                      Synaptic Vesicles                         Synaptic Cleft

Motor End Plate                    T Tubule                                       Sarcolemma

Terminal Cisternae & Sarcoplasmic Reticulum                     Sarcomere

Label this diagram:

 

Page 5.  Overview of Neuromuscular Junction Activity

The muscle cell, including the T Tubules are polarized.  Stimulation of the motor end plate on a muscle cell by acetylcholine triggers depolarization resulting in contraction of the sarcomeres.

 

Page 6.  Arrival of Action Potential at Axon Terminal

When the action potential arrives at the axon terminal voltage-regulated calcium channels open allowing calcium ions to enter the axon terminal.

 

** Note that when the action potential moves down the axon, there is a reversal of charge from positive out, negative in, to positive in, negative out.  This process is called depolarization.  The charge then reverses in a process called repolarization.   The action potential moves down the axon in a wave-like fashion.

 

Page 7.  Fusion of Synaptic Vesicles

Presence of calcium ions in the axon terminal cause synaptic vesicles to fuse with the membrane.

 

Page 8.  Release of Acetylcholine

Acetylcholine is released into the synaptic cleft & calcium ions are pumped out of the axon terminal.

** During the animation note that in addition to the acetylcholine going into the synaptic cleft (blue), calcium ions also move out of the axon terminal (red).

 

Page 9.  Acetylcholine Binds to Receptor Sites

Acetylcholine binds to receptor sites on the motor end plate, causing an influx of sodium ions and a small efflux of potassium ions which results in a local depolarization of the motor end plate.

** Carefully note the following steps that occur during this animation:

1. Acetylcholine (light blue ball) binds to the acetyl choline receptor (green).

2. The chemically regulated ion channel (purple) opens.

3. Sodium ions, Na+ (gold balls) ,diffuse from their higher concentration (in the synaptic cleft) to their lower concentration (inside the muscle cell). Potassium ions, K+, (bright blue balls) diffuse from their higher concentration (inside the muscle cell) to their lower concentration (in the synaptic cleft).

4. Depolarization of the membrane within the motor end plate.

** What is not shown in this animation is what happens to the positive and negative charge across the membrane during depolarization.  Before the membrane is depolarized, there is more positive charge on the synaptic cleft side of the membrane of the motor end plate and more negative charge inside the muscle cell.  When the chemically regulated ion channel opens, this charge reverses so now there is more negative charge in the synaptic cleft and more positive charge inside the cell.  This occurs because much more sodium (which is positive) enters the cell, than potassium which leaves the cell.  In fact, there is so little potassium that leaves the cell through this chemically regulated ion channel that your textbook or your instructor may not even mention it.

 

Page 10.  Breakdown of Acetylcholine

Acetylcholine diffuses away from its receptor site, the ion channel closes, and acetylcholine is then broken down by acetylcholinesterase.

** Carefully note the following steps that occur during this animation:

1. Acetylcholine (light blue ball) diffuses away from the acetylcholine receptor (green) which is a part of the chemically regulated ion channel (purple).  Note that as the acetylcholine falls off the receptor, the ion channel on the receptor closes, preventing further flow of sodium and potassium ions.

2. The acetylcholine binds to the enzyme acetylcholinesterase (aqua-colored).

3. The acetylcholinesterase breaks down the acetylcholine into two pieces, inactivating it.

** Note that after the acetylcholine has broken down, its parts are taken back up into the axon terminal where they can be reassembled into acetylcholine again.  This is not shown on the animation.

 

Page 11.  Action Potential Propagation

An action potential is generated which propagates along the sarcolemma in all directions and down the T Tubules.

 

Page 12.  Calcium Release from Terminal Cisternae

The action potential causes the release of calcium ions from the terminal cisternae into the cytosol.

 

Page 13.  Contraction of the Muscle Cell

Calcium ions trigger a contraction of the muscle cell.

 

Page 14.  Neuromuscular Animation

The sequence of events in the neuromuscular animation are given.

 

Page 15.  Summary

Each skeletal muscle cell is individually stimulated by a motor neuron.

The neuromuscular junction is the place where the terminal portion of a motor neuron axon meets a muscle cell membrane, separated by a synaptic cleft.

An action potential arriving at the axon terminal brings about the release of acetylcholine, which leads to depolarization of the motor end plate.

Depolarization of the motor end plate triggers an action potential that propagates along the sarcolemma and down the T Tubules.

This action potential causes the release of calcium ions from the terminal cisternae into the cytosol, triggering contraction of the muscle cell.

 

 

 

 

Study Questions