Neuromuscular Junction – Tutorial

Introduction to Neuromuscular Junctions

A skeletal muscle cell (fiber) contracts when stimulated by a motor neuron at a special synapse known as a neuromuscular junction.  Although the axon of a single motor neuron can branch and stimulate the contraction of a group of many skeletal muscle cells simultaneously (known as a motor unit), each skeletal muscle cell has only one neuromuscular junction.  A neuromuscular junction consists of the end of a motor neuron known as the  synaptic terminal or axon terminal, a region of the muscle cell membrane (sarcolemma) known as the motor end plate, and a narrow space that separates the synaptic terminal from the motor end plate known as the synaptic cleft.

The signal for a skeletal muscle cell to contract travels as an action potential along a motor neuron.  When an action potential reaches the synaptic terminal (axon terminal) it stimulates the exocytosis of the neurotransmitter acetylcholine (ACh) from the synaptic terminal into the synaptic cleft.  The acetylcholine molecules then diffuse across the synaptic cleft and bind to receptors on the motor end plate (region of the sarcolemma with ACh receptors).  The binding of acetylcholine to ACh receptors on the sarcolemma opens Na+ gates which allows Na⁺ to diffuse into the cytoplasm of the muscle cell (sarcoplasm).  If enough Na⁺ enter the sarcoplasm the membrane potential in the muscle cell will reach threshold resulting in the generation of an action potential that spreads along the sarcolemma of the muscle cell causing the muscle cell to contract.

Tutorial:  Use the image slider below to learn more about the structure and function of a neuromuscular junction. 

Microscopy:  Use the image slider below to learn how to use a microscope to study neuromuscular junctions.

Tutorial:  Use the hotspot image below to learn more about the structure and function of a neuromuscular junction.

Tutorial:  Use the image slider below to study numerous examples of neuromuscular junctions.