Locomotion & Movement

Movement is a defining property of living organisms, manifested at both cellular and organismal levels. Three fundamental types of movement exist: ciliary movement (as in ciliated epithelium of the respiratory tract and in fallopian tubes that propel the ovum), flagellar movement (as in spermatozoa), and muscular movement (the most prominent, enabling locomotion and organ function). Muscular movement depends on the coordinated action of the skeletal and muscular systems working together as the musculoskeletal system.

Three types of muscle tissue exist in the human body, each with distinctive properties. Skeletal muscle is striated (showing alternating light and dark bands under a microscope), voluntary (under conscious control), and multinucleated (formed by fusion of myoblasts). Smooth muscle is non-striated, involuntary, and uninucleate — found in the walls of visceral organs like the intestine, blood vessels, and urinary bladder. Cardiac muscle is uniquely striated yet involuntary, uninucleate (occasionally binucleate), and features intercalated discs that allow rapid electrical conduction between cells, enabling the heart to contract as a synchronized unit.

The ultrastructure of skeletal muscle reveals the sarcomere as the functional contractile unit, bounded between two Z-lines (Z-discs). Within the sarcomere: the A band (anisotropic/dark band) contains the entire length of thick myosin filaments and also overlapping portions of thin actin filaments; the I band (isotropic/light band) contains only actin filaments and is bisected by the Z-line; the H zone is the central region of the A band containing only myosin (no actin overlap); and the M-line is the midpoint of the H zone where myosin filaments are anchored.

The sliding filament theory explains muscle contraction. When a motor nerve impulse arrives at the neuromuscular junction, acetylcholine (ACh) is released, generating an action potential along the sarcolemma (muscle cell membrane) that travels into T-tubules. This triggers calcium ion (Ca₂+) release from the sarcoplasmic reticulum (SR). Ca₂+ binds to troponin-C on the actin filament, causing a conformational change that shifts tropomyosin away from the myosin-binding sites on actin. The myosin head (already energized by ATP hydrolysis) attaches to the exposed binding site, forming a cross-bridge. The power stroke pulls the actin filament toward the centre of the sarcomere. A fresh ATP molecule then binds to the myosin head, detaching it, and the cycle repeats. During contraction, the I band and H zone shorten (because actin slides over myosin), but the A band remains constant in length — a critical NEET fact.

The human skeletal system comprises 206 bones in adults, organized into the axial skeleton (80 bones: skull 22, vertebral column 26, ribs 24, sternum 1, hyoid 1 — with the hyoid being the only bone not articulating with any other bone) and the appendicular skeleton (126 bones: pectoral girdle, upper limbs, pelvic girdle, lower limbs). Joints are classified as: fibrous (immovable, e.g., sutures of skull), cartilaginous (slightly movable, e.g., pubic symphysis, intervertebral discs), and synovial (freely movable). Six subtypes of synovial joints exist — hinge (knee, elbow), pivot (atlas-axis), ball-and-socket (shoulder, hip), gliding (intercarpal), saddle (carpometacarpal of thumb), and ellipsoid/condyloid (wrist).

Common disorders include: myasthenia gravis (autoimmune destruction of ACh receptors at neuromuscular junctions, causing progressive muscle weakness), muscular dystrophy (genetic progressive degeneration of skeletal muscles), tetany (sustained muscle contraction due to low blood calcium), arthritis (joint inflammation — osteoarthritis from wear, rheumatoid from autoimmunity), osteoporosis (decreased bone density, common in post-menopausal women), and gout (uric acid crystal deposition in joints).

The key testable concept is that during muscle contraction the I band and H zone decrease in width while the A band remains constant, because actin filaments slide over myosin without the filaments themselves changing length.