The Muscular System

Functions and Properties of the Muscular System

The muscular system consists of all the muscles of the body and is responsible for virtually every movement of the organism. According to Dr. L. Dintwa, the system serves three primary functions. The first is the movement of the body and its parts, which includes macroscopic actions such as walking and running, as well as necessary physiological actions like chewing and swallowing. The second function is the maintenance of posture; specific postural muscles constantly oppose the action of gravity, such as the neck muscles supporting the weight of the head. The third function is heat production. Muscle tissue is highly metabolically active and generates a significant amount of heat during activity and through the process of shivering to maintain homeostasis.

To fulfill these functions and contribute to bodily homeostasis, skeletal muscle tissue possesses four specialized properties. Electrical excitability is the ability of the tissue to respond to specific stimuli by producing electrical signals known as action potentials. Contractility refers to the ability of the muscular tissue to contract forcefully when it is stimulated by an action potential. Extensibility is defined as the ability of muscular tissue to stretch without sustaining damage. Finally, elasticity is the ability of muscular tissue to return to its original length and shape after a period of contraction or extension.

Classification and Characteristics of Muscle Tissues

Muscle tissue is categorized into three distinct types: skeletal, cardiac, and smooth muscle. Skeletal muscle is primarily found in the limbs and is attached to bones. Its cells are characterized as being elongated, striated (having a striped or banded appearance), and multinucleated (containing more than one nucleus). Skeletal muscle is classified as voluntary because it is subject to conscious control. Cardiac muscle is located exclusively in the heart, forming the pumping mass of the organ. Its cells are branched, striated, and typically contains a single nucleus, though some may have more. Cardiac muscle functions as a single unit and is involuntary, meaning it is not under conscious control; notably, the heart always contracts to its full extent. Smooth muscle is found in the walls of hollow internal organs (viscera) and is characterized by spindle-shaped cells that lack striations and contain a single nucleus. It is involuntary and often arranged in longitudinal or circular layers; the alternate contraction of these layers in the intestine results in peristalsis.

In general muscle terminology, the prefixes "myo" and "mys" refer to muscle, while the prefix "sarco" refers to flesh. Muscle cells are synonymously referred to as muscle fibers due to their elongated shape. The contraction of any muscle type is fundamentally caused by the movement of microfilaments, which are protein fibers within the cell.

Gross Anatomy and Connective Tissue of Skeletal Muscle

A skeletal muscle is considered a separate organ composed of hundreds to thousands of muscle fibers (cells). Macroscopically, these fibers are bundled together into groups called fascicles. The entire structure is supported and protected by various layers of connective tissue and fascia. Fascia is a dense sheet or broad band of irregular connective tissue that lines the body wall and limbs, supporting and surrounding muscles and other organs. Three specific layers of connective tissue extend from the fascia to strengthen the muscle. The epimysium is the outermost layer and encircles the entire muscle. The perimysium surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles known as fascicles. The endomysium is a thin sheath of areolar connective tissue that penetrates the interior of each fascicle to separate individual muscle fibers from one another.

These three connective tissue layers (epimysium, perimysium, and endomysium) extend beyond the muscle fibers to form a tendon, which is mostly composed of collagen fibers and attaches the muscle to bone. When these connective tissue elements extend as a broad, flat layer, the structure is referred to as an aponeurosis. In terms of vascularization and innervation, skeletal muscles are supplied with blood vessels and nerves.

Microscopic Anatomy of Skeletal Muscle Fibers

A muscle fiber is a single, long, cylindrical, multinucleated cell wrapped in a plasma membrane called the sarcolemma. The cytoplasm, known as the sarcoplasm, contains numerous mitochondria, stores of glycogen, and myoglobin. Within the fiber are myofibrils, which are long, rod-like structures made of repeating contractile units called sarcomeres. Sarcomeres are the functional units of myofibrils, extending from one Z-line to the next, and their arrangement gives the muscle its striated appearance. Within the sarcomere, thick filaments are primarily composed of the protein myosin, while thin filaments are primarily composed of actin. Z-lines (or Z-discs) anchor the actin filaments, and the M-line anchors the myosin filaments in the center of the sarcomere.

The sarcomere is further divided into specific regions: the A band is the dark, middle part extending the length of the thick filaments (including the overlap with thin filaments); the I band is the lighter area containing only thin filaments; the H zone is a narrow region in the center of the A band containing only thick filaments; and the M line is the center of the H zone containing proteins that hold the thick filaments together. Additionally, titin filaments assist in the structural integrity of the sarcomere.

Cellular support for contraction involves the sarcoplasmic reticulum ($SR$) and Transverse ($T$)-tubules. The sarcoplasmic reticulum is a specialized smooth endoplasmic reticulum that surrounds myofibrils and stores $Ca^{2+}$. Dilated end sacs of the $SR$ are called terminal cisterns. T-tubules are invaginations of the sarcolemma that penetrate deep into the fiber to transmit action potentials, triggering the release of calcium from the $SR$. A triad consists of one transverse tubule and the two terminal cisterns located on either side of it. Skeletal muscles also contain satellite cells, which are quiescent stem cells that become active to repair or regenerate muscle tissue following injury or exercise.

Muscle Attachments, Functional Groups, and Lever Systems

Muscles produce movement by exerting force on tendons, which in turn pull on bones. The site of attachment to the stationary or more fixed bone is called the origin, while the site of attachment to the more movable bone is called the insertion. The fleshy portion of the muscle between the tendons is the muscle belly. In the limbs, the origin is typically proximal and the insertion is distal. Muscles often function in groups: the prime mover (agonist) is the main muscle responsible for a movement (e.g., biceps brachii for elbow flexion); the antagonist performs the opposite action and must relax to allow the movement (e.g., triceps brachii during elbow flexion); synergists assist the prime mover by adding force or stabilizing joints; and fixators are a type of synergist that stabilize the origin of the prime mover.

In producing movement, bones act as levers and joints act as fulcrums ($F$). A lever is acted upon by two forces: the effort ($E$), which is the force of muscular contraction, and the load ($L$) or resistance, which is the weight of the body part being moved. Motion occurs when the effort exceeds the load. Levers are categorized into three classes based on the relative positions of the components: first-class levers ($EFL$), second-class levers ($FLE$), and third-class levers ($FEL$). Third-class levers are the most common in the human body.

Naming Criteria and Fascicle Arrangement

Skeletal muscles are named based on several features. Direction refers to the orientation of fascicles relative to the midline (e.g., rectus means parallel, transverse means perpendicular, oblique means diagonal). Size categories include maximus (largest), minimus (smallest), longus (long), brevis (short), latissimus (widest), magnus (large), major (larger), minor (smaller), and vastus (huge). Shape names include deltoid (triangular), trapezius (trapezoid), serratus (saw-toothed), rhomboid (diamond-shaped), and orbicularis (circular). Action names indicate functions مثل flexor, extensor, abductor, adductor, levator, depressor, supinator, pronator, and sphincter. Other naming criteria include the number of origins (biceps, triceps, quadriceps), location (e.g., temporalis near the temporal bone), and sites of origin/insertion (e.g., sternocleidomastoid).

Fascicle arrangement affects the power and range of motion of a muscle. Parallel arrangements have fascicles parallel to the longitudinal axis. Fusiform muscles are nearly parallel but taper toward tendons (e.g., stylohyoid). Circular arrangements form sphincters (e.g., orbicularis oculi). Triangular arrangements have fascicles spreading over a broad area that converge at a central tendon (e.g., pectoralis major). Pennate muscles have short fascicles relative to muscle length and can be unipennate (one side of tendon), bipennate (both sides), or multipennate (attaching from many directions, e.g., deltoid).

Regional Musculature: Internal Anatomy and Innervations

Muscles of Facial Expression, Mouth, and Scalp

These muscles generally move the skin rather than joints. All facial muscles listed below are innervated by the Facial (VII) nerve. The occipitofrontalis consists of a frontal belly (raises eyebrows) and an occipital belly (draws scalp posteriorly), connected by the epicranial aponeurosis. The orbicularis oris closes and protrudes the lips (kissing). The zygomaticus major and minor draw the angle of the mouth superiorly and laterally (smiling). The levator labii superioris raises the upper lip, while the depressor labii inferioris and depressor anguli oris lower the lip and mouth corners. The buccinator (originating from alveolar processes) presses cheeks against teeth and assists in chewing and whistling. The risorius draws the corner of the mouth laterally (grimacing). The mentalis elevates and protrudes the lower lip (pouting). The platysma is a flat neck muscle that draws the lower lip inferiorly.

Muscles of the Orbit, Eyebrows, and Eye Movement

The orbicularis oculi (innervated by Facial nerve VII) closes the eye. The corrugator supercilii draws the eyebrow inferiorly (frowning). Extrinsic eye muscles control eyeball movement: the Superior, Inferior, and Medial recti, and the Inferior oblique are innervated by the Oculomotor (III) nerve. The Lateral rectus is innervated by the Abducens (VI) nerve, and the Superior oblique is innervated by the Trochlear (IV) nerve. These muscles allow for elevation, depression, abduction, and adduction of the eye. The levator palpebrae superioris (innervated by III) opens the eye.

Muscles of Mastication and the Tongue

The masseter and temporalis both elevate the mandible to close the mouth; the temporalis also retracts the mandible. The medial and lateral pterygoids move the mandible from side to side and protract it. These four muscles are all innervated by the mandibular division of the Trigeminal (V) nerve. Extrinsic tongue muscles (innervated by Hypoglossal XII nerve, except Palatoglossus which is innervated by Vagus X) include the genioglossus (protrudes tongue), styloglossus (retracts tongue), palatoglossus (elevates posterior tongue), and hyoglossus (depresses tongue).

Muscles of the Neck and Swallowing

Suprahyoid muscles (digastric, stylohyoid, mylohyoid, and geniohyoid) elevate the hyoid and assist in opening the mouth. The digastric has two bellies: the anterior (innervated by V) and posterior (innervated by VII). Infrahyoid muscles (omohyoid, sternohyoid, sternothyroid, and thyrohyoid) generally depress the hyoid bone and larynx; these are largely innervated by branches of cervical spinal nerves C1-C3. The sternocleidomastoid (innervated by Accessory XI nerve) flexes the head when acting bilaterally and rotates the head when acting unilaterally. Other head-moving muscles include the semispinalis capitis, splenius capitis, and longissimus capitis.

Muscles of the Trunk and Breathing

Abdominal Wall and Thorax

The abdominal muscles include the rectus abdominis (flexes vertebral column), external oblique, internal oblique, and transversus abdominis (compresses abdomen). These are innervated by thoracic spinal nerves T7-T12 and some lumbar nerves. The quadratus lumborum (origin: iliac crest; insertion: 12th rib and L1-L4) flexes the vertebral column laterally. For breathing, the diaphragm is the primary muscle (innervated by phrenic nerve C3-C5); its contraction causes inhalation. External intercostals elevate ribs for inhalation, while internal intercostals draw ribs together for forced exhalation. These are innervated by thoracic spinal nerves T2-T12.

Pelvic Floor and Pectoral Girdle

The pelvic floor includes the levator ani (pubococcygeus and iliococcygeus) and ischiococcygeus, which support pelvic viscera. The external urethral sphincter and external anal sphincter control excretion. Muscles moving the pectoral girdle include the pectoralis minor (abducts scapula), serratus anterior (boxer's muscle; abducts scapula), trapezius (upward rotates/adducts scapula), and levator scapulae (elevates scapula).

Muscles of the Upper Limbs

Arm and Forearm

Muscles moving the humerus include the axial muscles: pectoralis major (adduction/medial rotation) and latissimus dorsi (extension/adduction). Scapular muscles moving the humerus include the deltoid (abduction), subscapularis (medial rotation), supraspinatus (abduction), infraspinatus (lateral rotation), teres major and minor, and coracobrachialis. Forearm flexors include the biceps brachii (also supinates), brachialis, and brachioradialis. The triceps brachii and anconeus act as forearm extensors. Forearm pronators are the pronator teres and pronator quadratus, while the supinator performs the opposite action.

Wrist, Hand, and Fingers

Anterior compartment muscles (flexors) include the flexor carpi radialis, palmaris longus, flexor carpi ulnaris (innervated by Ulnar nerve), and flexor digitorum superficialis and profundus. Posterior compartment muscles (extensors) include the extensor carpi radialis longus/brevis, extensor digitorum, extensor digiti minimi, and extensor carpi ulnaris. Deep muscles such as the abductor pollicis longus and extensor pollicis longus/brevis move the thumb (pollicis). The intrinsic palm muscles include the thenar muscles (moving the thumb, like opponens pollicis), hypothenar muscles (moving the little finger), and intermediate muscles like lumbricals and interossei (palmar for adduction, dorsal for abduction).

Muscles of the Lower Limbs

Gluteal Region and Thigh

The iliopsoas (psoas major and iliacus) flexes the thigh at the hip. The gluteus maximus (innervated by inferior gluteal nerve) extends the thigh, while the gluteus medius and minimus (innervated by superior gluteal nerve) abduct it. Lateral rotators include the piriformis, obturator internus/externus, and quadratus femoris. The adductor compartment includes the adductor magnus, longus, and brevis, along with the pectineus and gracilis. The anterior compartment contains the quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius), which extend the leg at the knee. The sartorius is the longest muscle in the body. The posterior compartment (hamstrings) includes the biceps femoris, semitendinosus, and semimembranosus, which flex the leg and extend the thigh.

Leg and Foot

The anterior compartment of the leg includes the tibialis anterior (dorsiflexion/inversion), extensor hallucis longus (great toe extension), and extensor digitorum longus. The lateral compartment contains the fibularis (peroneus) longus and brevis (plantar flexion/eversion). The superficial posterior compartment consists of the gastrocnemius (flexes leg/plantar flexes foot), soleus, and plantaris, which all insert on the calcaneus via the Achilles tendon. Deep posterior muscles include the popliteus (unlocks knee), tibialis posterior (inversion), flexor digitorum longus, and flexor hallucis longus. Intrinsic foot muscles like the flexor digitorum brevis and the interossei facilitate fine movement of the toes.