Locomotion and Movement — NEET Notes

Types of movement

All locomotion is movement, but not all movement is locomotion. NCERT draws the line carefully: movement is the displacement of any part of the body, while locomotion is the voluntary movement that takes the entire organism from one place to another — walking, running, climbing, flying, swimming. Animals locomote in search of food, shelter, mate, breeding grounds, favourable climate, or to escape predators. Crucially, the same structures often serve both. Paramoecium uses cilia to drive food into its cytopharynx and to swim. Hydra uses its tentacles to capture prey and to somersault. We use limbs for posture and for walking.

The cells of the human body show three distinct types of movement, and NEET wants all three named in this exact order: amoeboid, ciliary, and muscular.

Specialised cells like macrophages and leucocytes in blood use amoeboid movement to crawl through tissues and engulf pathogens — the same mechanism Amoeba uses, redeployed for immunity. Sperm cells use flagellar movement, which NCERT treats alongside ciliary movement since cilia and flagella share the same 9+2 microtubule architecture. The difference is sheer count and length: cilia are short and numerous; flagella are long and few (typically one per cell).

Three muscle types — skeletal, smooth, cardiac

Muscle is a specialised mesodermal tissue. Four properties define it: excitability (response to stimulus), contractility (ability to shorten and develop tension), extensibility (ability to be stretched), and elasticity (ability to recoil). NCERT classifies muscle by three criteria — location, microscopic appearance, and regulation. Three categories result, and you must be able to map each to its location, striation pattern, and voluntary status.

Skeletal muscle — structure

A skeletal muscle is a hierarchy. The whole organ is wrapped in a connective-tissue sheath called the fascia and contains parallel bundles called fascicles (muscle bundles). Each fascicle contains many muscle fibres — the individual cells. Each muscle fibre is bounded by a plasma membrane, the sarcolemma, enclosing the cytoplasm called the sarcoplasm. The muscle fibre is a syncytium: the sarcoplasm holds many nuclei because it forms from the fusion of myoblasts during development. The endoplasmic reticulum of the muscle fibre is the sarcoplasmic reticulum, which is the store house of calcium ions — a fact NEET 2023 tested directly.

Inside each muscle fibre run hundreds to thousands of parallel myofibrils, also called myofilaments. Under the microscope, each myofibril shows alternating dark and light bands — the source of the "striated" appearance. The dark band is the A-band (anisotropic); the light band is the I-band (isotropic). The striped look comes from the regular distribution of two contractile proteins: actin in the I-band and myosin in the A-band.

The sarcomere — functional unit

Zoom in on a single myofibril and you see a repeating module — the sarcomere. NCERT's definition is exact: a sarcomere is the portion of the myofibril between two successive Z-lines. It is the functional unit of contraction. Every component of the sarcomere has a name worth memorising, because NEET writes statement-based questions where one wrong label is the trap.

Thick & thin filaments — the contractile proteins

The two rod-like filaments are built from polymerised proteins. They run parallel to each other and parallel to the long axis of the myofibril. Thin filaments are actin-based and anchored to Z-lines. Thick filaments are myosin-based and held in the centre by the M-line. In the resting state, the free ends of thin filaments partially overlap the free ends of thick filaments — the overlap is what gets longer during contraction.

Four proteins make up the thin filament: F-actin (×2 strands of polymerised G-actin), tropomyosin (long fibrous protein running along the F-actin groove), and troponin (a three-subunit complex spaced at regular intervals along tropomyosin). The thick filament is essentially one protein — myosin — polymerised into a bundle with its globular heads sticking out as cross arms.

Sliding filament theory of muscle contraction

The mechanism is captured in a single sentence: contraction of a muscle fibre takes place by the sliding of thin filaments over thick filaments. Neither filament shortens. Neither filament changes length. The thin filaments simply walk inward along the thick filaments, pulling the Z-lines closer together. This is the sliding filament theory — proposed in 1954 independently by Huxley & Hanson and Huxley & Niedergerke.

The motor neuron carries the signal. Where it meets the sarcolemma is the neuromuscular junction (motor-end plate). A neural impulse triggers release of the neurotransmitter acetylcholine, which generates an action potential in the sarcolemma. This action potential races through the muscle fibre and causes the sarcoplasmic reticulum to dump calcium ions into the sarcoplasm. Everything that follows hinges on calcium.

Repeated cycles drive the muscle through ever-shorter sarcomere lengths. Relaxation begins when Ca²⁺ is pumped back into the sarcoplasmic cisternae — troponin re-masks the actin binding sites, cross-bridges no longer re-form, and the Z-lines return to their resting positions. Without ATP, however, the cycle cannot restart and the myosin heads remain locked on actin — this is the molecular basis of rigor mortis.

Repeated activation depletes ATP and accumulates lactic acid from anaerobic glycolysis — the muscle fatigues. Muscle stores a backup energy reserve called creatine phosphate, which can be quickly converted to ATP. Muscles also contain myoglobin, an oxygen-storing pigment. Muscles rich in myoglobin and mitochondria appear red (red fibres, aerobic). Muscles poor in myoglobin look pale (white fibres, anaerobic, with abundant sarcoplasmic reticulum).

Contraction — what shortens, what stays the same

This is one of the most-asked patterns in NEET. The trick is to remember what the bands represent. The A-band is the length of the thick filament — it cannot change because the thick filament does not shorten. The I-band is the part of the thin filament outside the overlap zone — as actin slides inward, this strip shrinks. The H-zone is the part of the A-band without overlap — as actin slides into it, this zone narrows and can disappear at maximum contraction.

Skeletal system — the 206-bone framework

Bones and cartilages form the skeletal system. Bone has a hard matrix reinforced with calcium salts; cartilage has a pliable matrix with chondroitin salts. Both are specialised connective tissues. The adult human skeleton has 206 bones, split into the axial skeleton (along the body's central axis) and the appendicular skeleton (limbs and girdles).

Axial skeleton in detail

The skull has 22 bones, in two sets. Eight cranial bones (frontal, parietal ×2, temporal ×2, occipital, sphenoid, ethmoid) form the hard cranium that protects the brain. Fourteen facial bones form the face. A separate U-shaped hyoid bone sits at the base of the buccal cavity (not articulating with any other bone). Each middle ear contains three tiny ear ossicles — malleus, incus, and stapes — for a total of six. The skull articulates with the vertebral column through two occipital condyles, making the human skull dicondylic.

The vertebral column has 26 serially arranged vertebrae, dorsally placed, extending from the base of the skull. Each vertebra has a central neural canal through which the spinal cord passes. The first vertebra (atlas) articulates with the occipital condyles. Five regions, from skull downward:

• Cervical — 7 (true for almost all mammals, including the giraffe)
• Thoracic — 12
• Lumbar — 5
• Sacral — 1 (fused from 5)
• Coccygeal — 1 (fused from 4)

Total: 7 + 12 + 5 + 1 + 1 = 26. The vertebral column protects the spinal cord, supports the head, and anchors the back muscles and ribs.

The sternum is a single flat bone on the ventral midline of the thorax. There are 12 pairs of ribs, each a thin flat bicephalic bone (two articulation surfaces dorsally — hence "two-headed"). Each rib attaches dorsally to a thoracic vertebra. The ventral attachment splits the ribs into three groups, a NEET-tested classification:

• True ribs (vertebrosternal) — pairs 1–7: attach ventrally to the sternum directly via hyaline cartilage.
• False ribs (vertebrochondral) — pairs 8, 9, 10: do not reach the sternum directly; instead they join the cartilage of the 7th rib.
• Floating ribs (vertebral) — pairs 11, 12: no ventral attachment at all.

Together, thoracic vertebrae + ribs + sternum form the rib cage, protecting the heart and lungs.

Appendicular skeleton — limbs and girdles

Each limb has 30 bones. The fore limb (arm) contains:

• Humerus (1) — upper arm
• Radius and ulna (2) — forearm
• Carpals (8) — wrist bones
• Metacarpals (5) — palm bones
• Phalanges (14) — finger bones

Total: 1 + 2 + 8 + 5 + 14 = 30.

The hind limb (leg) contains:

• Femur (1) — the thigh bone, longest bone in the body
• Tibia and fibula (2) — lower leg
• Tarsals (7) — ankle bones
• Metatarsals (5) — sole bones
• Phalanges (14) — toe bones

Total: 1 + 2 + 7 + 5 + 14 = 29 (the cup-shaped patella covers the knee ventrally — included to make 30).

The pectoral girdle connects the upper limb to the axial skeleton. Each half contains a clavicle (collar bone, slender with two curvatures) and a scapula (a large triangular flat bone lying dorsally between the 2nd and 7th ribs). The scapula has a dorsal ridge — the spine — which extends as a flat acromion process. The acromion articulates with the clavicle. Below it sits the glenoid cavity, which articulates with the head of the humerus to form the shoulder joint.

The pelvic girdle consists of two coxal bones. Each coxal bone is formed from three fused bones — ilium, ischium, and pubis. At their fusion point sits a cup-shaped cavity called the acetabulum, which receives the head of the femur to form the hip joint. The two coxal bones meet ventrally at the pubic symphysis, joined by fibrous cartilage.

Joints — fibrous, cartilaginous, synovial

A joint is the point of contact between two bones, or between a bone and a cartilage. Muscles generate force; joints transmit it. NCERT classifies joints by structure into three groups, and within the synovial group, by movement, into five subtypes. NEET tests both layers — a single question can ask which structural class corresponds to vertebrae, which subtype lives between atlas and axis, or which type sits between carpals.

• Fibrous joints — immovable. Bones fused by dense fibrous connective tissue. Example: the sutures between flat skull bones forming the cranium.
• Cartilaginous joints — slightly movable. Bones joined by cartilage. Example: the joints between adjacent vertebrae in the vertebral column (intervertebral discs of fibrocartilage). NEET 2022 tested exactly this.
• Synovial joints — freely movable. Characterised by a fluid-filled synovial cavity between articulating surfaces. These are the joints of locomotion.

Disorders of muscular & skeletal system

NCERT names a tight set of disorders and NEET asks about each of them in turn. Group them by what they affect — neuromuscular junction, muscle fibres themselves, bone density, or joints — and the answer patterns become obvious.

• Myasthenia gravis — an autoimmune disorder affecting the neuromuscular junction. Antibodies block acetylcholine receptors on the sarcolemma, leading to fatigue, weakening, and paralysis of skeletal muscle. In severe cases, even jaw muscles fail.
• Muscular dystrophy — a progressive degeneration of skeletal muscle, mostly due to genetic disorder. NEET 2019 tested this directly: muscular dystrophy is the inherited muscular disorder among tetany, myasthenia gravis, and botulism.
• Tetany — rapid spasms (wild contractions) in muscle due to low Ca²⁺ in body fluid. Calcium is needed to terminate nerve signals; without it, muscles cannot relax properly.
• Arthritis — inflammation of joints. Joint pain, stiffness, restricted movement. Rheumatoid arthritis is the crippling autoimmune variant.
• Osteoporosis — age-related disorder characterised by decreased bone mass and increased chance of fractures. Decreased levels of estrogen is a common cause (post-menopausal women are highly prone). NEET 2022 set the trap: the reason said "increased levels of estrogen" — which is wrong. Decreased estrogen, not increased.
• Gout — inflammation of joints due to accumulation of uric acid crystals. Uric acid is a product of protein metabolism; in gout it precipitates as needle-like crystals in joints (especially the big toe), causing painful inflammation.