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Erectile tissue is vascular tissue whose filling and outflow properties permit a substantial change in volume or stiffness. Its behavior depends on blood flow and smooth-muscle regulation, linking vascular physiology to organ mechanics. It is not simply ordinary muscle that becomes rigid by contracting.

Researcher in membrane electrophysiology. Neher and collaborators recorded currents from individual ion channels in muscle membranes, connecting discrete molecular openings with electrical signals measurable at the cell surface.^[ref]

The external anal sphincter is the ring of skeletal muscle surrounding the distal anal canal. Its contraction helps maintain faecal continence; its motor neurons allow voluntary squeezing as well as reflex control. It works alongside the smooth muscle of the internal anal sphincter, so continence depends on coordinated structures rather than one isolated ring.^[ref]

Branches of the pudendal nerve supply the external anal sphincter. Dissection of 28 cadavers found a local nerve plexus near the sphincters in five, illustrating why a schematic nerve route need not capture every individual's branching. Preserving muscle alone therefore does not guarantee preservation of its motor supply.^[ref]

An extraocular muscle rotates the eye through its attachment to the sclera. The six globe-moving muscles have position-dependent horizontal, vertical and torsional actions.

An eyelid is a movable protective fold covering the exposed eye. Blinking distributes the tear film and helps clear the surface. The tarsal plate provides support, while associated muscles control opening and closure.

Recording electrical activity over facial muscle regions, such as the cheek and brow. It can detect small facial responses that are not obvious to an observer. A response compatible with a smile or frown does not by itself establish the corresponding subjective emotion.

The facial motor nucleus is a pontine motor nucleus whose axons travel in the facial nerve to facial-expression muscles. Its fibers curve around the abducens nucleus, contributing to the facial colliculus. The surface landmark therefore reflects more than one nerve system.

Movement of facial muscle groups, described without assuming its psychological cause. Calling a movement an expression of an emotion adds an interpretation that requires independent evidence.

The glenoid labrum is a rim of fibrocartilaginous tissue around the glenoid cavity. It deepens the socket and contributes attachment and stability at the glenohumeral joint. Stability also depends on the capsule and muscles, so the labrum is one component of a coupled system.

The gracilis is a long superficial medial thigh muscle crossing both hip and knee. It contributes to hip adduction and to flexion and rotation at the knee. Its multi-joint course makes its action depend on the positions of both joints.

The hamstrings are posterior thigh muscles with important actions at the hip and knee. In the usual human grouping they include semitendinosus, semimembranosus and the long head of biceps femoris. Their multi-joint geometry links hip position to knee-flexion mechanics.

The hip is the joint region connecting the femur to the pelvis. The femoral head articulates with the acetabulum, while capsule, ligaments and muscles stabilize movement. Its deeper socket favors a different mobility-stability balance from the shallower shoulder joint.

The human skeleton is the framework of bones that supports the body, protects organs and supplies attachment points for muscles. The axial skeleton includes the skull, vertebral column and rib cage; the appendicular skeleton includes the limbs and their girdles. Movement depends on this framework interacting with joints and tendons.

The hypoglossal nucleus contains motor neurons whose axons travel through the hypoglossal nerve to most muscles of the tongue. It lies near the floor of the fourth ventricle, beneath the hypoglossal trigone. Surface position and motor connections describe different aspects of the same nucleus.

An abnormally low potassium concentration in blood. Loss from the body and redistribution into cells are different causes, while effects on membrane excitability connect the chemical change with muscle or cardiac dysfunction.

The inferior oblique muscle is an extraocular muscle contributing to extorsion, elevation and abduction of the eye. Its effect depends on gaze position; elevation is especially informative when the eye is adducted. Its course differs from the rectus muscles arising posteriorly in the orbit.

The infratemporal fossa is a deep space below the temporal region and medial to the mandibular ramus. It contains muscles, vessels and nerves associated with mastication, including parts of the pterygoid musculature. Its importance lies in the relations among these crowded structures rather than a visible external boundary.

The iris is the adjustable diaphragm around the pupil. Its muscles alter the opening through which light enters the eye, while its pigment contributes to visible eye color. Changing pupil aperture and changing lens curvature are distinct optical controls.

The ischiocavernosus is a superficial perineal muscle associated with the crus of erectile structures. Contraction influences the mechanical and vascular conditions of those structures. It is related to the bulbospongiosus region but has different attachments.

An oral apparatus involved in acquiring and processing food. Its bony articulations and associated muscles can change function during evolution.