When muscle cells are stimulated, they contract and exert a force in one direction. Stimulation of muscle cells is caused by nerve impulses carried from the central nervous system to the muscles. When the nerve impulse reaches the end of the neuron, it causes a transfer of the action potential in muscles, which leads to a contraction.
There are three types of muscle cells in the body, which are cardiac, smooth and skeletal. Cardiac muscle is only found within the heart and has its own intrinsic method for contracting, although nerves can stimulate it to speed up or slow down as necessary. Smooth muscle is found in layers surrounding the organs, and it is stimulated by the autonomic, or involuntary, nervous system. Skeletal muscle is made up of fibers and causes movement. The action potential in muscles of the skeleton is carried by the somatic, or voluntary, nervous system.
Muscle cells will not contract on their own, but must be stimulated first by a nerve impulse. The axons of neurons meet muscle cells at the neuromuscular junction. To ensure that muscle contraction is simultaneous and fast, there are many neuromuscular junctions found across a muscle. All of these neurons send impulses at the same time to initiate an action potential in muscles. Having many neuromuscular junctions for each muscle allows the body to control the force of the contraction by varying the number of units that send the impulse to the muscle.
When the action potential reaches the terminal end of the axon at a neuromuscular junction, vesicles fuse with the cell membrane to allow the release of a neurotransmitter — acetylcholine. The neurotransmitter spreads across the gap between the neuron and the muscle cell, until it reaches the sarcolemma, which is the membrane surrounding a muscle cell. Acetylcholine causes the permeability of the sarcolemma to change, so that sodium ions can enter and leave the membrane. This change in ions depolarizes the membrane and causes an action potential in muscles to be fired.
When a muscle is at rest, tropomyosin blocks the myosin binding sites found on the actin filaments. During a contraction, myosin attaches to actin and performs a type of rowing action along the actin filaments. This causes the muscle to contract. For this to occur, myosin must be able to bind to actin, so the tropomyosin must be moved.
The depolarization caused by the nerve impulse spreads across the sarcolemma and the T system — a system of tubes connected to the sarcoplasmic reticulum. Both the T system and the sarcoplasmic reticulum contain calcium ions, which are released when there is an action potential in muscles. The calcium ions diffuse throughout the muscle cell and attach to a protein called troponin, which is attached to the tropomyosin filaments found on the actin fibers. The troponin changes shape when calcium ions attach to it, which moves the tropomyosin filaments and frees the myosin binding sites along actin fibers. Myosin can now come in contact with actin and cause a muscle contraction.