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Muscle Tissue and the Mode of Contraction Help (page 3)

By — McGraw-Hill Professional
Updated on Aug 18, 2011

Muscle Twitch, Summation, and Tetanus

A single action potential to the muscle fibers of a motor unit produces a muscle twitch, a rapid, unsustained contraction (Figure 7-5). If the impulses are applied to a muscle in rapid succession through several motor units, one twitch will not have completely ended before the next begins. Since the muscle is already in a partially contracted state when the second twitch begins, the degree of muscle shortening in the second contraction will be slightly greater than the shortening that occurs with a single twitch. The additional shortening due to the rapid succession of two or more action potentials is termed summation. At high stimulation frequencies, the overlapping twitches sum to one strong, steady contraction call tetanus.

Macroscopic Structure of Muscle

Skeletal muscle tissue, in association with connective tissue, is organized into muscle bundles. This muscle architecture determines the force and direction of the contracting muscle fibers. Muscle fibers may be organized with parallel fibers, convergent fibers, pennate (feathershaped) fibers, or as a sphincter (circular) muscle.

Loose fibrous connective tissues bind muscles at various levels to unify the force of contraction. Surrounding each muscle fiber is a connective tissue called the endomysium. A group of individual muscle fibers is bound together by another connective tissue, the perimysium, to form a fasciculus. Many fasciculi make up an individual muscle. Each muscle is surrounded by a third connective tissue, the epimysium. These three connective tissues are continuous with the tendon that secures the muscle to bone.

Muscles are attached to the skeleton at two locations. The origin of a muscle is the more stationary attachment of the muscle; the insertion the more movable attachment. In the appendages, the origin is generally proximal in position and the insertion is distal in position.

Practice problems for these concepts can be found at:

Muscle Tissue and the Mode of Contraction Practice Problems

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