Structure Of Skeletal Muscle And Motor Units
A. A motor unit consists of a single motor neuron and the muscle fibers innervated by it.
B. Epimysium is the same as investing fascia. Actin (thin) and myosin (thick) filaments are contractile elements in the muscle fibers.
Whereas the structural unit of a muscle is a skeletal striated muscle fiber, the functional unit of a muscle is a motor unit , consisting of a motor neuron and the muscle fibers it controls. When a motor neuron in the spinal cord is stimulated, it initiates an impulse that causes all the muscle fibers supplied by that motor unit to contract simultaneously. The number of muscle fibers in a motor unit varies from one to several hundred. The number of fibers varies according to the size and function of the muscle. Large motor units, in which one neuron supplies several hundred muscle fibers, are in the large trunk and thigh muscles. In smaller eye and hand muscles, where precision movements are required, the motor units include only a few muscle fibers. Movement (phasic contraction) results from the activation of an increasing number of motor units, above the level required to maintain muscle tone.
FUNCTIONS OF MUSCLES
Muscles serve specific functions in moving and positioning the body:
- A prime mover ( agonist ) is the main muscle responsible for producing a specific movement of the body. It contracts concentrically to produce the desired movement, doing most of the work (expending most of the energy) required. In most movements, there is a single prime mover, but some movements involve two prime movers working in equal measure.
- A fixator steadies the proximal parts of a limb through isometric contraction while movements are occurring in distal parts.
- A synergist complements the action of a prime mover. It may directly assist a prime mover, providing a weaker or less mechanically advantaged component of the same movement, or it may assist indirectly, by serving as a fixator of an intervening joint when a prime mover passes over more than one joint, for example. It is not unusual to have several synergists assisting a prime mover in a particular movement.
- An antagonist is a muscle that opposes the action of another muscle. A primary antagonist directly opposes the prime mover, but synergists may also be opposed by secondary antagonists. As the active movers concentrically contract to produce a movement, antagonists eccentrically contract, relaxing progressively in coordination to produce a smooth movement.
Source: Clinically Oriented Anatomy
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