Skeletal muscles connect one bone to another. Usually they attach to bone in antagonistic pairs such as tricep and bicep muscles. The attachment occurs via bundles of collagen fibers called tendons.
Skeletal muscles can only generate action potentials if stimulated by the somatic nervous system. They are non-myogenic and are under voluntary control.
Skeletal Muscles have a striated appearance due to their many sarcomeres.
Skeletal muscles have more than one nucleus per cell, which permits the simultaneous contraction of all the actin-myosin contractile units along the length of a muscle fiber.
The heart is composed of cardiac muscle, which has certain features allowing for automatic pumping of blood and timed contractions of various sections of the heart.
The autonomic nervous system regulates the rate of contraction of cardiac muscles.
Cardiac muscle has myogenic activity much like smooth muscle. Cardiac muscle can automatically generate action potentials without nervous stimulation.
When an action potential is initiated in one cardiac muscle cell, it is passed along to its neighbors through gap junctions. This is why the heart contracts in unison. Gap junctions are channels to connect one cytoplasm to the next, allowing flow of ions and transmission of electrical signal. Both cardiac muscle and smooth muscle have gap junctions.
Cardiac muscle cells and smooth muscle cells have only one nucleus per cell, unlike skeletal muscle cells which are multi-nucleated. Muscle fiber cells have multiple nuclei because each muscle fiber arises from the fusion of many myoblasts.
Smooth muscle is found in organs and vessels such as the digestive tract, bladder, uterus and blood vessel walls. Smooth muscle cells exhibit myogenic activity, are non-striated, and form a complicated contraction mechanism with intermediate filaments and dense bodies. Some smooth muscle cells have gap junctions and can maintain contractions after stimulation.
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