A ring like band of muscle that surrounds a bodily opening, constricting and relaxing to control flow.
Skeletal muscle can be categorised into four groups based on its anatomical arrangement.
Parallel muscles are characterized by fascicles that run parallel to one another, and contraction of these muscle groups acts as an extension of the contraction of a single muscle fiber. Most skeletal muscles in the body are parallel muscles; although they can be seen in a variety of shapes such as flat bands, spindle shaped, and some can have large protrusions in their middle known as the belly of the muscle.
Parallel muscles can be divided into fusiform and non-fusiform types based on their shape. Fusiform muscles are more spindle shaped (their diameter at the center is greater than at either end), whereas, non-fusiform muscles are more rectangular with a constant diameter.
The biceps brachii is an example of a fusiform parallel muscle, and is responsible for flexing the forearm.
Convergent muscles have a common point of attachment, from which the muscle fascicles extend outward, not necessarily in a specific spatial pattern, allowing the muscle to cover a broad surface. These muscles do not tend to exert as much force on their tendons. Muscle fibers can often exert opposing effects during contraction, such as not pulling in the same direction depending on the location of the muscle fiber. Covering a broad surface these fibers allow for more versatile types of movement. Because the fascicles pull on the tendons at an angle, they do not move the tendon as far as their parallel muscle counterparts. Despite this they generate greater tension because they possess a greater amount of muscle fibers than similarly sized parallel muscles.
The pectoralis major found in the chest is an example of a convergent muscle, and is responsible for flexing the upper arm.
In Pennate muscles, the tendon runs through the length of the muscle. Fascicles pull on the tendon at an angle, thus not moving as far at the parallel muscles during a contraction. However, these muscles tend to have relatively more muscle fibers than similarly sized parallel muscles, and thus carry more tension.
If all the fascicles of a pennate muscle are on the same side of the tendon, the pennate muscle is called unipennate. If the fascicles lie to either side of the tendon the muscle is called bipennate. If the central tendon branches within a pennate muscle, the muscle is called multipennate.
The rectus femoris found in the thigh, and responsible for its flexion, is an example of a bipennate muscle.
The fibers of the circular or sphincter muscles are arranged concentrically around an opening or recess. As the muscle contracts, the opening it circumvents gets smaller. For this reason, these muscles are often found at the entrances and exits of external and internal passageways. Skeletal circular muscles are different from smooth muscle equivalents due to their structure and because they are under voluntary control
The orbicularis oris which controls the opening of the mouth is an example of a circular muscle.