Learning Objectives
Explain the process involved with initiating muscle contraction and relaxation
By the end of this section, you will be able to:
- Describe the connection between motor neurons and muscles
- Explain the mechanism of neurotransmitter signaling generating a post synaptic electrical signal
- Explain the process of excitation-contraction coupling
- Explain how muscle contraction and relaxation is related to calcium handling at the sarcoplasmic reticulum
- Diagram the process of cross-bridge cycling
The Neuromuscular Junction
The process of muscle contraction begins at the site where a motor neuron’s terminal meets the muscle fiber—called the neuromuscular junction (NMJ). Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron at a NMJ. Excitation signals from the motor neuron are the only way to functionally activate skeletal muscle fibers to contract.
External Website
Every skeletal muscle fiber is supplied by a motor neuron at the NMJ. Watch this video to learn more about what happens at the NMJ. (a) What is the definition of a motor unit? (b) What is the structural and functional difference between a large motor unit and a small motor unit? (c) Can you give an example of each? (d) Why is the neurotransmitter acetylcholine degraded after binding to its receptor?
Excitation-Contraction Coupling
All living cells have membrane potentials, or electrical gradients across their membranes based on the distribution of positively and negatively charged ions. The inside of the membrane is usually around -60 to -90 mV, relative to the outside. Neurons and muscle cells can use their membrane potentials to generate and conduct electrical signals by controlling the movement of charged ions across their membranes to create electrical currents. This movement is controlled by selective opening and closing of specialized proteins in the membrane called ion channels. Although the currents generated by ions moving through these channel proteins are very small, they form the basis of both neural signaling and muscle contraction.
Both neurons and skeletal muscle cells are electrically excitable, meaning that they are able to generate action potentials. An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. This allows a signal to be transmitted quickly over long distances.
In skeletal muscle, cross-bridge formation and contraction requires the presence of calcium (Ca++) inside the muscle cell. Excitation signalling of action potentials from the motor neuron are coupled with calcium release. Thus, the excitation-contraction coupling process begins with signaling from the nervous system at the neuromuscular junction (Figure 10.3.1) and ends with calcium release for muscle contraction.
Most motor neurons that tell the skeletal muscle fibers to contract originate in the spinal cord. A smaller number of motor neurons are located in the brainstem for activation of skeletal mus