Muscular System Anatomy and Physiology

Muscle physiology shows up at the bedside constantly: contractures in the immobile patient, fall risk from age-related muscle loss, the acetylcholine pathway that neuromuscular blockers shut down, the shivering that generates heat in a cold patient. Skeletal muscle moves the body, holds posture, stabilizes joints, and produces heat. Know the structure and the contraction mechanism, because both drive assessment and care.

Functions of the Muscular System

Producing movement is common to all muscle types, but skeletal muscle does three other jobs.

1. Producing movement. Skeletal muscles drive all locomotion and let us respond quickly to the external environment.
2. Maintaining posture. Postural muscles work almost continuously, making constant small adjustments to hold an erect or seated posture against gravity.
3. Stabilizing joints. As muscles pull on bones, they also stabilize joints. Tendons are critical for reinforcing joints with poorly fitting articulating surfaces.
4. Generating heat. Heat is a by-product of muscle activity. As ATP powers contraction, nearly three-quarters of its energy escapes as heat, which is vital for maintaining normal body temperature.

Anatomy of the Muscular System

Microscopic Anatomy of Skeletal Muscle

Skeletal muscle cells are multinucleate.

• Sarcolemma. Many oval nuclei sit just beneath the plasma membrane, called the sarcolemma in muscle cells.
• Myofibrils. Long ribbonlike organelles, the myofibrils, push the nuclei aside and nearly fill the cytoplasm.
• Light and dark bands. Alternating dark and light bands along the aligned myofibrils give the cell its striped appearance.
• Sarcomeres. Myofibrils are chains of contractile units called sarcomeres, aligned end to end like boxcars in a train.
• Myofilaments. Each sarcomere holds two types of threadlike protein myofilaments.
• Thick filaments. The larger thick filaments (myosin filaments) are mostly bundled myosin, plus ATPase enzymes that split ATP to power contraction.
• Cross bridges. The midparts of the thick filaments are smooth, but the ends are studded with projections (myosin beads), called cross bridges when they link thick and thin filaments during contraction.
• Thin filaments. The thin filaments (actin filaments) are made of the contractile protein actin plus regulatory proteins that allow or prevent myosin-bead binding. They anchor to the Z disc.
• Sarcoplasmic reticulum. A specialized smooth endoplasmic reticulum whose tubules and sacs surround every myofibril. Its major role is to store calcium and release it on demand.

Muscle Movements, Types, and Names

Types of Body Movements

Every one of our 600-odd skeletal muscles attaches to bone or other connective tissue at no fewer than two points.

• Origin. Attached to the immovable or less movable bone.
• Insertion. Attached to the movable bone. When the muscle contracts, the insertion moves toward the origin.
• Flexion. A movement, usually in the sagittal plane, that decreases the joint angle and brings two bones closer. Common at hinge joints and at ball-and-socket joints.
• Extension. The opposite of flexion: a movement that increases the angle or distance between two bones or body parts.
• Rotation. Movement of a bone around its longitudinal axis, common at ball-and-socket joints.
• Abduction. Moving a limb away from the body's midline.
• Adduction. The opposite of abduction: moving a limb toward the midline.
• Circumduction. A combination of flexion, extension, abduction, and adduction at ball-and-socket joints. The proximal end stays put while the distal end moves in a circle.

Special Movements

A few movements occur at only a few joints.

• Dorsiflexion and plantar flexion. Lifting the foot so its superior surface approaches the shin is dorsiflexion; depressing the foot is plantar flexion.
• Inversion and eversion. Turn the sole medially to invert the foot; turn the sole laterally to evert it.
• Supination and pronation. Supination is the forearm rotating laterally so the palm faces anteriorly and the radius and ulna are parallel; pronation is the forearm rotating medially so the palm faces posteriorly.
• Opposition. The saddle joint between metacarpal 1 and the carpals allows opposition of the thumb.

Interactions of Skeletal Muscles in the Body

Muscles are arranged so that whatever one does, another can reverse, which produces an immense range of movement.

• Prime mover. The muscle with major responsibility for a particular movement.
• Antagonists. Muscles that oppose or reverse a movement. When the prime mover is active, its antagonist is stretched and relaxed.
• Synergists. Help prime movers by producing the same movement or reducing undesirable movements.
• Fixators. Specialized synergists that hold a bone still or stabilize the origin of a prime mover, so all tension moves the insertion bone.

Naming Skeletal Muscles

Muscles come in many shapes and sizes. Their names encode useful information:

• Direction of fibers. Rectus (straight) means fibers run parallel to an imaginary line; oblique means they run at a slant to it.
• Relative size. Maximus (largest), minimus (smallest), and longus (long) often appear in muscle names.
• Location. Some muscles are named for an associated bone, such as the temporalis and frontalis over the temporal and frontal bones.
• Number of origins. Biceps, triceps, and quadriceps indicate two, three, or four origins.
• Origin and insertion. Some muscles are named for their attachment sites.
• Shape. Some have a distinctive identifying shape.
• Action. Action-based names use terms like flexor, extensor, and adductor.

Arrangement of Fascicles

Skeletal muscles consist of fascicles, and the arrangement sets a muscle's structure and function.

• Circular. Fascicles in concentric rings, typically surrounding external body openings, which they close by contracting.
• Convergent. Fascicles converge toward a single insertion tendon, giving a triangular or fan shape.
• Parallel. Fascicles run parallel to the long axis, producing straplike muscles. A modification, fusiform, gives a spindle shape with an expanded belly.
• Pennate. Short fascicles attach obliquely to a central tendon. In the extensor digitorum of the leg, fascicles insert on one side (unipennate); on opposite sides or several sides, the muscle is bipennate or multipennate.

Gross Anatomy of Skeletal Muscles

Only the most important muscles are covered here.

Head and Neck Muscles

Head muscles fall into two groups: facial muscles and chewing muscles.

Facial Muscles

There are five facial muscles:

• Frontalis. Covers the frontal bone, running from the cranial aponeurosis to the skin of the eyebrows. Raises the eyebrows and wrinkles the forehead. The small occipitalis sits at the posterior end of the cranial aponeurosis.
• Orbicularis oculi. Fibers run in circles around the eyes. Closes the eyes and lets you squint, blink, and wink.
• Orbicularis oris. The circular muscle of the lips. Closes the mouth and protrudes the lips, the "kissing" muscle.
• Buccinator. Runs horizontally across the cheek and inserts into the orbicularis oris.
• Zygomaticus. Runs from the corner of the mouth to the cheekbone, the "smiling" muscle, raising the corners of the mouth.

Chewing Muscles

The buccinator is described with the facial muscles.

• Masseter. Runs from the zygomatic process of the temporal bone to the mandible, covering the angle of the lower jaw. Closes the jaw by elevating the mandible.
• Temporalis. A fan-shaped muscle over the temporal bone, inserting into the mandible and acting as a synergist of the masseter in closing the jaw.

Neck Muscles

Neck muscles move the head and shoulder girdle and are mostly small and straplike. Two are covered here:

• Platysma. A sheetlike muscle covering the anterolateral neck. Pulls the corners of the mouth inferiorly, producing a downward sag.
• Sternocleidomastoid. Paired two-headed muscles, one on each side of the neck. When both contract together, they flex the neck.

Trunk Muscles

Trunk muscles include those that move the vertebral column, the anterior thorax muscles that move the ribs, head, and arms, and the abdominal wall muscles that move the vertebral column and form the "natural girdle" of the abdominal wall.

Anterior Muscles

• Pectoralis major. A large fan-shaped muscle covering the upper chest. Forms the anterior wall of the axilla and adducts and flexes the arm.
• Intercostal muscles. Deep muscles between the ribs. External intercostals raise the rib cage to inhale; internal intercostals, deep to them, depress the rib cage to force air out during forceful exhalation.
• Muscles of the abdominal girdle. The rectus abdominis, external and internal obliques, and transversus abdominis form a natural girdle reinforcing the trunk. The paired straplike rectus abdominis are the most superficial; the external obliques are paired superficial muscles of the lateral walls; the internal obliques lie deep to them; and the transversus abdominis is the deepest, with fibers running horizontally across the abdomen.

Posterior Muscles

• Trapezius. The most superficial muscles of the posterior neck and upper trunk. Extend the head and can elevate, depress, adduct, and stabilize the scapula.
• Latissimus dorsi. Two large flat muscles covering the lower back, important for bringing the arm down in a power stroke.
• Erector spinae. The prime mover of back extension, also providing resistance that controls bending at the waist.
• Quadratus lumborum. Forms part of the posterior abdominal wall. Acting separately, each flexes the spine laterally; together, they extend the lumbar spine.
• Deltoid. Triangle-shaped muscles forming the rounded shoulders, the prime movers of arm abduction.

Muscles of the Upper Limb

Upper limb muscles fall into three groups: those that arise from the shoulder girdle and cross the shoulder joint into the humerus, those that move the elbow joint, and the muscles of the forearm.

Muscles of the Humerus that Act on the Forearm

All anterior arm muscles flex the elbow. In order of decreasing strength: brachialis, biceps brachii, brachioradialis.

• Biceps brachii. The familiar arm muscle that bulges when the elbow flexes. Powerful prime mover for forearm flexion and supinates the forearm.
• Brachialis. Lies deep to the biceps and is as important in elbow flexion. It lifts the ulna as the biceps lifts the radius.
• Brachioradialis. A fairly weak muscle arising on the humerus and inserting into the distal forearm.
• Triceps brachii. The only muscle on the posterior humerus. Powerful prime mover of elbow extension and antagonist of the biceps brachii.

Muscles of the Lower Limb

Lower limb muscles move the hip, knee, and foot joints. They are among the largest and strongest in the body, specialized for walking and balance.

Muscles Causing Movement at the Hip Joint

• Gluteus maximus. A superficial hip muscle forming most of the buttock. A powerful hip extensor that brings the thigh in line with the pelvis.
• Gluteus medius. Runs from the ilium to the femur, beneath the gluteus maximus for most of its length. A hip abductor important in steadying the pelvis during walking.
• Iliopsoas. A fused muscle of the iliacus and psoas major. A prime mover of hip flexion that also keeps the upper body from falling backward when standing erect.
• Adductor muscles. Form the muscle mass on the medial thigh and adduct the thighs together.

Muscles Causing Movement at the Knee Joint

• Hamstring group. The posterior thigh mass, three muscles (biceps femoris, semimembranosus, semitendinosus) that originate on the ischial tuberosity and insert on both sides of the proximal tibia.
• Sartorius. The most superficial thigh muscle, a synergist for the cross-legged position.
• Quadriceps group. Four muscles (the rectus femoris and three vastus muscles) that flesh out the anterior thigh and powerfully extend the knee.

Muscles Causing Movement at the Ankle and Foot

Five muscles move the ankle and foot:

• Tibialis anterior. A superficial muscle on the anterior leg, arising from the upper tibia and running along the anterior crest to the tarsal bones.
• Extensor digitorum longus. Lateral to the tibialis anterior, arising from the lateral tibial condyle and proximal fibula. A prime mover of toe extension and a dorsiflexor of the foot.
• Fibularis muscles. Three muscles (longus, brevis, tertius) on the lateral leg that together plantar flex and evert the foot.
• Gastrocnemius. A two-bellied muscle forming the curved half of the posterior leg, a prime mover for plantar flexion.
• Soleus. Deep to the gastrocnemius. Because it arises from the tibia and fibula, it does not affect knee movement.

Physiology of the Muscular System

Skeletal Muscle Activity

Nerve Stimulus and the Action Potential

Skeletal muscle cells must be stimulated by a nerve impulse to contract.

• Neurotransmitter. When a nerve impulse reaches the axon terminals, a neurotransmitter is released. The one that stimulates skeletal muscle is acetylcholine (ACh).
• Temporary permeability. If enough acetylcholine is released, the sarcolemma at that point becomes temporarily more permeable to sodium ions, which rush in, and to potassium ions, which diffuse out.
• Action potential. More sarcolemma channels open to let only sodium enter, generating an electrical current called an action potential. Once started, it is unstoppable: it travels over the entire sarcolemma, carrying the impulse end to end, and the result is contraction.
• Enzyme breakdown. Acetylcholine is broken down to acetic acid and choline by enzymes on the sarcolemma. So a single nerve impulse produces only one contraction, and the cell relaxes until the next round of acetylcholine release.

Mechanism of Muscle Contraction: The Sliding Filament Theory

When the nervous system activates a muscle fiber, the myosin heads attach to binding sites on the thin filaments and the sliding begins.

• Relaxed muscle cell. At rest, regulatory proteins on the actin myofilaments block myosin binding. When an action potential sweeps the sarcolemma and excites the cell, calcium ions are released from intracellular storage.
• Contraction trigger. The flood of calcium is the final trigger. As calcium binds the regulatory proteins on the actin filaments, those proteins change shape and position.
• Attachment. Myosin physically attaching to actin springs the trap, snapping the myosin heads toward the center of the sarcomere. Because actin and myosin are firmly bound, the thin filaments are pulled toward the sarcomere center.

Age-Related Physiological Changes in the Musculoskeletal System

Speed and power of contraction decline with age. Exercise can strengthen muscle, but muscle fibers steadily decrease by age 50, a condition called sarcopenia. Overall stature is lost, making kyphosis, osteoporosis, and pathologic fractures common. Reaction time slows, worsened by decreased muscle tone, which itself follows from diminished physical activity.

These changes threaten elder safety. Assess for factors that raise fall risk and reduce the ability to perform activities of daily living (ADLs). Emphasize the importance of calcium supplements and vitamin D.