Muscle

Muscle is tissue specialized for force generation through contractile machinery. Skeletal muscle, smooth muscle and cardiac muscle differ in cellular organization and control. Shortening is one possible outcome: a muscle can also generate force while its overall length remains nearly constant.^[ref]

The piriformis muscle is a deep skeletal muscle connecting the anterior sacrum to the upper femur. It passes through the greater sciatic foramen and helps rotate the extended thigh outward and abduct the flexed thigh. Its position makes it a useful landmark: the sciatic nerve usually emerges below it, although the relationship varies between people.^[ref]

An anatomical variant alone does not establish a cause of pain. Adam L. Bartret and colleagues classified sciatic nerve routes on 783 hip magnetic resonance imaging examinations; variants occurred in about 19%, without a statistically significant association with a recorded diagnosis of piriformis syndrome. This observation separates a visible anatomical difference from evidence that the difference explains symptoms.^[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]

Cardiac muscle is the specialized striated muscle of the heart. Its contractile cells are mechanically connected and electrically coupled at intercalated discs. Mechanical connections transmit force, while gap junctions help electrical excitation spread between cells.^[ref]

Excitation and contraction are related but distinct stages. Changes in membrane potential regulate calcium handling, and calcium enables the actin-myosin machinery to generate force. A sinoatrial node normally sets the pace for the coordinated sequence, rather than an independent motor neuron commanding each beat.

This separation explains why an electrical recording and a measurement of pumping answer different questions. An electrocardiogram reports summed electrical activity; contraction also depends on cellular mechanics, filling and the loads against which the heart pumps.

A spasm is an involuntary episode of muscular contraction. It describes the observed motor event rather than a unique neural or muscular cause. Its location, duration and relation to other findings are needed to interpret the mechanism.

The stylopharyngeus is a muscle running from the styloid region into the pharyngeal wall. It elevates the pharynx during swallowing and is supplied by the glossopharyngeal nerve. Its innervation distinguishes it from several neighboring pharyngeal muscles.

The serratus anterior links the ribs to the medial border of the scapula. It protracts and assists upward rotation of the scapula while holding it against the thoracic wall. Its function therefore supports arm elevation without directly crossing the glenohumeral joint.

The pectoralis minor runs from upper ribs to the coracoid process. It stabilizes and moves the scapula, rather than attaching directly to the humerus. Its deep position also makes it a landmark for nearby axillary vessels and nerves.

The popliteus is a posterior knee muscle connecting the proximal tibia with the lateral femoral region through its tendon. It contributes to controlling rotation and initiating flexion from extension. Its action depends on which skeletal segment is relatively fixed.

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 semimembranosus is a medial hamstring muscle contributing to hip extension, knee flexion and medial rotation of the flexed leg. Its tendon also gives an expansion associated with the oblique popliteal ligament. One muscle can therefore contribute to both active motion and passive support.

The quadriceps femoris is the anterior thigh muscle group transmitting force through the quadriceps tendon, patella and patellar ligament to extend the knee. Its components do not all cross the same joints; rectus femoris also crosses the hip.

The external urethral sphincter is striated muscular tissue contributing to control of the urethral outlet. It works with bladder and other outlet mechanisms rather than determining continence alone. Its arrangement differs between male and female anatomy.

The superficial transverse perineal muscle crosses toward the perineal body from the lateral perineal region. It contributes to stabilization of this central attachment. Its small size does not make it equivalent to the deeper pelvic diaphragm.

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.

The bulbospongiosus is a superficial perineal muscle associated with the bulb and nearby erectile structures, with sex-specific arrangements. Its contractions contribute to local support and expulsion of fluid in relevant anatomy. It is distinct from the deeper urethral sphincter complex.

The obturator internus arises from the internal pelvic surface and turns through the lesser sciatic region to reach the proximal femur. It contributes to hip rotation and stabilization. Its pelvic fascia also forms the pudendal canal, linking muscular and neurovascular anatomy.

The adductor magnus is a large medial thigh muscle with regions differing in attachment, action and innervation. It contributes to hip adduction, while its posterior component also supports extension. Regional anatomy explains why one named muscle need not behave as a single uniform actuator.

The obturator externus runs from the external obturator region toward the proximal femur. It contributes to lateral rotation and stabilization of the hip. Its side of origin distinguishes it from obturator internus despite the similar names.

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 adductor longus is a medial thigh muscle extending from the pubic region to the femur. It contributes to hip adduction and has position-dependent effects on flexion. Its relationship to nearby muscles defines an important part of the medial thigh architecture.

The adductor brevis is a short medial thigh muscle deep to adductor longus. It contributes to hip adduction and helps stabilize the femur relative to the pelvis. Its deep location is an anatomical distinction rather than evidence of a wholly separate movement type.

The psoas major extends from the lumbar region toward the proximal femur and joins the iliacus contribution to the iliopsoas apparatus. It is a major hip flexor. Its connection across the lumbar-pelvic region also links spinal posture with lower-limb movement.

The anconeus is a small muscle on the posterolateral elbow. It assists elbow extension and stabilization and is supplied by the radial nerve. Its action complements the much larger triceps rather than producing an unrelated elbow movement.

The supinator muscle wraps around the proximal radius and contributes to forearm supination. A deep branch of the radial nerve passes through its region. Its rotational action differs from elbow flexion even though both can accompany turning the palm upward.

The brachioradialis spans the lateral elbow to the distal radius and flexes the elbow, particularly with the forearm near a neutral rotational position. It is supplied by the radial nerve. Its flexor action therefore does not imply the innervation pattern typical of every other elbow flexor.

The buccinator muscle forms an important muscular part of the cheek. It keeps food positioned against the teeth during mastication and helps control oral pressure. Its role differs from the jaw-closing muscles even though they cooperate during eating.

The temporalis muscle extends from the temporal region to the mandibular coronoid process. It elevates the mandible, with posterior fibers contributing to retraction. Its broad origin and converging insertion explain how different fiber regions can have different pulling directions.

The lateral pterygoid muscle has attachments involving the sphenoid region and mandibular joint apparatus. It contributes to protrusion and coordinated mandibular movements. Its roles differ from those of a simple jaw-closing elevator and depend on bilateral coordination.

An adductor muscle moves a body part toward a defined reference axis or plane. Hip adductors move the thigh toward the body midline. The movement name must be interpreted relative to the specified joint and reference, rather than inferred from muscle position alone.

The levator veli palatini muscle elevates the soft palate. Its action contributes to closing the nasal route during swallowing and appropriate speech sounds. Elevation is distinct from the tension-related contribution of tensor veli palatini muscle.

A diaphragm is a partition spanning an opening or separating regions; in muscular anatomy, the name commonly refers to the respiratory diaphragm. The broader structural term also appears in pelvic diaphragm. Location and composition should therefore be specified rather than assuming every anatomical diaphragm is the same sheet.

The attachment conventionally designated as the insertion of a muscle, commonly the more mobile or distal attachment. The convention must be distinguished from observations of a particular movement.

The attachment conventionally designated as the origin of a muscle, commonly the more fixed or proximal attachment. It is distinct from the muscle insertion.

Muscle contraction is active force generation by muscular contractile machinery. A muscle can shorten, lengthen under load or maintain nearly constant length while active. Force production and visible shortening are therefore different observations, linked by the load and mechanical constraints.

A force exerted by an active or passively stretched muscle. Its contribution to rotation about an anatomical joint depends on the muscle moment arm, rather than its magnitude alone.

The perpendicular distance from a muscle force's line of action to the relevant axis of an anatomical joint. It can change with posture; multiplying it by the muscle force gives the corresponding torque.

An intercostal muscle spans adjacent ribs and contributes to thoracic mechanics. The layered arrangement includes external intercostal muscles and internal intercostal muscles. Fiber direction and regional attachments affect how each layer influences rib movement.

An internal intercostal muscle is part of the deeper intercostal layer. Its action depends on the portion considered: interosseous and interchondral regions can contribute differently to expiration and inspiration. Treating the entire layer as one uniform expiratory actuator loses that anatomical distinction.

An external intercostal muscle spans neighboring ribs in the superficial intercostal layer. It supports the intercostal spaces and can assist rib elevation during inspiration. Its fiber orientation differs from the deeper internal layer.

An abdominal muscle contributes to movement or stabilization of the trunk and to control of abdominal pressure. Different layers can cooperate during forceful expiration and postural tasks. Pressure generation and movement depend on coordinated activity rather than isolated shortening of one layer.

A scalene muscle connects cervical vertebrae with upper ribs. The group contributes to neck movement and can assist inspiration by acting on the upper thorax. Spaces between its members also form important routes for vessels and the brachial plexus.

The sternocleidomastoid muscle connects the sternum and clavicle with the mastoid region. Unilateral activity contributes to side-bending and rotation of the head, while bilateral actions depend on the head and neck position. It can also act as an accessory respiratory muscle.

The tensor veli palatini muscle contributes to tension of the soft palate and opening of the auditory tube. Its tendon changes direction around a bony pulley region. Its trigeminal motor supply distinguishes it from several neighboring muscles of the palate and pharynx.

The stylopharyngeus muscle is the stylopharyngeus, a pharyngeal elevator supplied by the glossopharyngeal nerve. It links the styloid region to the pharyngeal wall. The expanded name identifies the same anatomical muscle rather than an additional neighboring structure.

A contractile assembly in a skeletal-muscle fibre consists of serial sarcomeres. Their coordinated shortening contributes to shortening of the fibre; each sarcomere contains overlapping thin actin and thick myosin filaments.

One of a repeated series of axial muscle units, useful in comparing early chordate organization.