The SA Node in the right atrium
When a muscle cell responds to a stimulus, calcium is released from the sarcoplasmic reticulum. This calcium binds to troponin, triggering a conformational change that allows myosin to bind to actin and initiate muscle contraction.
Nerve impulses, or action potentials, are sent through motor neurons from the central nervous system to the muscles in response to a stimulus. These impulses trigger the release of neurotransmitters at the neuromuscular junction, which bind to receptors on muscle fibers. This binding initiates a series of biochemical events that lead to muscle contraction, enabling the muscles to respond to the stimulus.
The property of automaticity allows smooth and cardiac muscles to spontaneously initiate their own contractions without needing an external stimulus. This is due to the presence of pacemaker cells in these muscle tissues that can generate action potentials independently.
When a muscle fiber is subjected to a stimulus and contracts completely, this phenomenon is known as the "all-or-nothing response." In this response, if the stimulus reaches a certain threshold, the muscle fiber will contract fully; if it does not reach this threshold, there will be no contraction. This principle applies to individual muscle fibers, while whole muscles can exhibit varying degrees of contraction based on the number of fibers activated.
Anerobic contraction of muscles.
Muscle excitability is the ability to respond to a stimulus, such as a nerve signal or hormone, by generating an electrical impulse that leads to muscle contraction. This trait allows muscles to react to signals from the nervous or endocrine systems to produce movement.
The general opposite of contraction is expansion, or possibly inflation. For muscles, the opposite of contraction is relaxation.
A hereditary disease of the muscles characterized by prolonged contraction of the muscles whenever voluntary motion is attempted, contraction and relaxation being slow and the muscles stiff.
The nervous system is what will initiate contractions as well as determine the strength of a contraction.
An involuntary muscle contraction is called a spasm.
prolonged contraction of the muscles
extension, contraction