A motor neuron stimulates a muscle fiber by firing a nerve impulse, which causes the muscle fiber to contract. Cardiac and smooth muscles contract without nervous stimulation, while skeletal muscles require excitation by motor neurons to contract.
The human body contains more than 600 muscles that function for movement, heat production and posture maintenance. Muscles are characterized by their excitability, contractility, extensibility and elasticity.
Muscles consist of fascicles that contain myofibrils, which are the driving forces of muscle contraction. Directly connected to the muscles are nerves, which are carriers of motor neurons. The neuromuscular junction, also known as the synapse, is the intersection between a motor neuron and a muscle fiber. When a motor neuron transmits a nerve impulse to the synapse, the neurotransmitter acetylcholine is produced. This triggers an action potential that stimulates the myofibrils, which in turn cause the muscle fibers to contract.