Contractions of skeletal muscles are graded in force and in duration. For example, many of the same muscles are used to pick up a pencil as to pick up this book, but much more force will have to be developed to lift the book. The ways in which muscle contractions can be graded have been presented. However, not all the mechanisms are used by all muscles. Most skeletal muscles in the body are innervated by somatic nerves and are under voluntary control. In each nerve are many axons of a-motor neurons. Each of these axons branches to innervate a number of skeletal muscle cells. An a-motor neuron and the muscle cells it innervates comprise a motor unit. Motor units are small in those muscles over which fine motor control exists, and large in muscles without such fine control. Because the force generated by a whole muscle will depend on the number of its motor units that are active at any one time, the central nervous system can regulate muscle contractions by regulating the number of motor units activated and by the frequency of stimulation of each motor unit.
Length-tension relationships are not important as determinants of skeletal muscle force in vivo. Muscles are attached near fulcrum points of the skeleton at lengths near Lo (Fig. 12). Such attachment limits length changes such that muscle cells seldom are at more than ±15% of Lo. Attachment of muscle near fulcrum points
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