Muscle tissue

Muscle tissue is tissue specialized for force generation through contractile proteins. Skeletal, cardiac and smooth muscle differ in organization and control. Tissue-level output also depends on supporting matrix and transmission of force between cells and attachments.

A muscle cell is a cell specialized for contractile force generation. Its shape, internal contractile organization and control differ among muscle types. A skeletal muscle fiber is an unusually large multinucleate example, while a smooth muscle cell has a different cellular arrangement.

A smooth muscle cell is a contractile cell without the regular visible striations of skeletal and cardiac muscle. It contributes to structures such as vessel walls and the digestive tract. Its activity can change both local force and the geometry of a hollow organ.

Muscle atrophy is reduction in muscle mass, often involving decreased size of individual muscle fibers. Altered use, neural supply, nutrition or disease can contribute. The reduction is an outcome to explain rather than a single mechanism shared by every case.

Muscle tone is the background resistance or tension present in a muscle under specified conditions. Neural activation and passive tissue properties contribute differently depending on the measurement. Resistance during an examination is therefore not simply a direct readout of one motor-neuron firing rate.

A contractile muscle class whose cells lack the conspicuous sarcomeric striations of skeletal and cardiac muscle. Differentiated smooth-muscle layers can also mechanically constrain neighbouring growing tissues.

The internal anal sphincter is a ring of smooth muscle at the lower end of the rectum and around the upper anal canal. It contributes involuntary resting closure. The surrounding external anal sphincter is a different muscle system with voluntary control, so the two should not be treated as one anatomical ring.^[ref]

Rectal distension can trigger relaxation of the internal anal sphincter, the rectoanal inhibitory reflex. Researchers can inflate a rectal balloon while recording anal pressure, separating this relaxation from other contractile responses and from reported sensation. A study of 23 subjects observed inhibitory and contractile reflexes during graded balloon distension.^[ref] A continuous pressure trace therefore reveals coordinated components of continence that are difficult to infer from static anatomy alone.

The rectoanal inhibitory reflex is relaxation of the internal anal sphincter in response to distension of the rectum. Because the internal anal sphincter is smooth muscle, this relaxation is distinct from voluntary control of the external anal sphincter. In manometry, a rectal balloon supplies a controlled distension while anal pressure sensors record the response over time.^[ref]

A fall in pressure can coexist with a contractile response elsewhere or at another time. The same study measured inhibitory and contractile reflexes and asked subjects about sensation; a relaxation trace therefore cannot simply be equated with the conscious urge to empty the rectum.^[ref]

A complementary experiment recorded tension in isolated human internal anal sphincter strips. A donor of nitric oxide relaxed the strips; blocking its synthesis suppressed relaxation evoked by electrical nerve stimulation, and arginine reversed the inhibition in a stereospecific control. These interventions support a role for nitric oxide in nerve-mediated relaxation. An isolated strip identifies a mechanism in the effector muscle; it does not reproduce the whole intact reflex circuit.^[ref]