Muscle

Muscle is one of the four tissue types. The other three types are: epithelium, connective tissue and nervous tissue. The primary purpose of muscle tissue is to contract. Muscle contraction is used to move parts of the body, as well as to move substances within the body.

There are three general types of muscle. The first two are "striated", they contain sarcomeres; the third type is "smooth":
· cardiac muscle - found within the heart

· skeletal muscle - attached to the skeleton and used to facilitate movement

· smooth muscle - found within the intestines and blood vessels.

Contraction and relaxation

The three types of muscles have significant differences, but all use the movement of actin, against myosin, to produce contraction and relaxation. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, and by the motor nerves and motoneurons in particlular. Muscles and muscular activity account for a lot of the body's energy consumption. Muscles stores energy for their own use in the form of glycogen which represents about 1% of their mass.

Muscular composition

Muscle is composed of muscle cells (sometimes known as "muscle fibers"). Within the cells are myofibrils; myofibrils contain sarcomere, which is composed of actin and myosin. Muscle cells are grouped together into bundles of fascicle; the bundles are then grouped together to form muscle. Muscle spindles are distributed throughout the muscles and provide sensory information to the central nervous system.

Motor system

Vertebrates move muscles in response to voluntary and autonomic signals from the brain. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with unique regions in the brain. Muscles react to reflexive nerve stimuli that do not always send signals all the way to the brain, but most muscle activity is the result of complex interactions between various areas of the brain.

Nerves that control muscles in mammals correspond with neuron groups along the primary motor area of the brain's cerebral cortex. Commands are routed to basal ganglia and to the cerebellum before being relayed to connections in the pons and medulla for transmission to synapses at the muscles. Along the way, feedback loops such as that of the extrapyramidal system contribute signals to influence muscle tone and response.

Deeper muscles such as those involved in posture often are inervated from origins in the brain stem.

Muscle Sense

Sometimes known as muscle memory, the sense of where our bodies are in space is called proprioception, the perception of body awareness. More easily demonstrated than explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. (This can be demonstrated by anyone closing their eyes and waving their hand around. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses).

 

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