Locomotion and MovementNEET Zoology · Class 11 · NCERT Chapter 17

Introduction

Animals need to move to find food, escape predators and find mates. Movement is any change in position of a body part. Locomotion is the movement of the whole body from one place to another. All locomotion is movement, but not all movement is locomotion. For example, the beating of your heart is movement, not locomotion.

Expect 1 question from this chapter in NEET 2027. The most tested areas are: the sarcomere bands and which ones change during contraction, the sliding filament theory sequence, muscle type properties, the 206-bone count and axial vs appendicular split, and the named disorders.

Types of Movement

Living cells and organisms show three main types of movement:

• Amoeboid movement: cells extend pseudopodia (cytoplasmic projections) by reorganising the cytoskeleton. Examples: Amoeba, macrophages and leucocytes in the human body. Amoeboid movement is how your white blood cells migrate to sites of infection.
• Ciliary movement: tiny hair-like cilia beat in a coordinated stroke. Examples: the cilia lining the respiratory tract (sweep debris out), the fallopian tube (move the ovum towards the uterus), the Eustachian tube and the flagella of sperm cells.
• Muscular movement: contraction of muscle fibres. This is used for locomotion in most animals, including all movements of the human body, including the heart, gut and limbs.

Types of Muscles

The human body has three types of muscle. Each type suits a different function.

• Skeletal muscle (striated/voluntary): attached to bones. Has alternating light and dark bands (striations) when viewed under a microscope. Controlled by the somatic nervous system (voluntary). Each fibre is a very long cell with multiple nuclei at the periphery. Also called red muscle (when myoglobin-rich) or white muscle (when glycolytic). Fatigues with sustained effort.
• Smooth muscle (non-striated/involuntary): found in the walls of hollow internal organs (viscera) such as the gut, uterus, blood vessels and urinary bladder. Also called visceral or unstriated muscle. No visible striations. Controlled by the autonomic nervous system (involuntary). Each cell has a single central nucleus. Spindle-shaped cells. Does not fatigue easily.
• Cardiac muscle (striated/involuntary): found only in the wall of the heart. Striated like skeletal muscle, but involuntary like smooth muscle. Fibres are branched and joined end to end by intercalated discs, which allow the electrical signal to spread rapidly so the whole heart contracts as one unit. Single nucleus (occasionally two). Never fatigues under normal conditions.

Properties of Muscles

All muscle has four special properties:

• Excitability: ability to respond to a stimulus (nerve impulse or hormone).
• Contractility: ability to shorten when stimulated.
• Extensibility: ability to be stretched beyond the resting length.
• Elasticity: ability to return to the original (resting) length after stretching.

Structure of Skeletal Muscle

Look at a cross-section of skeletal muscle from the outside inward:

1. Muscle (the whole organ): enclosed in a sheath of connective tissue. A whole skeletal muscle is made of many muscle bundles (fascicles).
2. Fascicle: a bundle of muscle fibres wrapped together.
3. Muscle fibre: a single elongated multinucleated cell. The cell membrane is called the sarcolemma. The cytoplasm is called the sarcoplasm. The endoplasmic reticulum is called the sarcoplasmic reticulum (SR). The SR stores Ca2+ and releases it when the muscle is stimulated.
4. Myofibril: rod-like structures that run the length of the muscle fibre, packed tightly inside the sarcoplasm. Each myofibril is made of repeating units called sarcomeres.

The sarcolemma has invaginations called T-tubules (transverse tubules) that run deep into the fibre. When an action potential spreads along the sarcolemma, it travels down the T-tubules to reach the sarcoplasmic reticulum deep inside the fibre, triggering Ca2+ release.

The Sarcomere

The sarcomere is the structural and functional unit of the myofibril. One sarcomere runs from one Z line to the next Z line. The bands you see in a sarcomere are caused by the arrangement of thick and thin filaments:

• A band (dark band): spans the full length of the thick myosin filament. Contains both the thick myosin filaments and (in the overlap region) the thin actin filaments. The A band does NOT change length during contraction.
• I band (light band): region between two A bands of adjacent sarcomeres. Contains only thin actin filaments. The I band shortens during contraction as actin slides into the A band.
• H zone: the central part of the A band that has only thick myosin filaments (no actin overlap). The H zone shortens or disappears during contraction as the actin slides in and covers it.
• M line: a line of protein at the very centre of the A band (midpoint of the sarcomere) that holds the thick filaments in register.
• Z line (Z disc): the boundary of each sarcomere. Thin actin filaments attach here. The Z line moves closer to the M line when the muscle contracts and the sarcomere shortens.

Thin and Thick Filaments

Each myofibril contains two types of protein filaments:

• Thin filament (actin filament): made of three proteins. Actinforms a double helical strand and has the binding sites where myosin heads attach. Tropomyosin wraps around the actin strand and, at rest, physically blocks the myosin-binding sites on actin. Troponin is a complex of proteins attached to both actin and tropomyosin. When Ca2+ binds to troponin, it shifts tropomyosin away from the binding sites, exposing them.
• Thick filament (myosin filament): made of many myosin molecules. Each myosin molecule has a long tail and a globular head. The heads, also called cross bridges, are the motor units. When energised by ATP, they swing and pull the actin filament.

Mechanism of Muscle Contraction

Muscle contraction works by the sliding filament theory, proposed by Hugh Huxley and Andrew Huxley in 1954. The thin actin filaments slide over the thick myosin filaments, pulling the Z lines closer together. The filaments themselves do not get shorter. The sarcomere shortens because the filaments slide.

Cross-Bridge Cycle

The contraction sequence in order:

1. A nerve impulse arrives at the neuromuscular junction. The motor neuron releases acetylcholine (ACh) into the synaptic cleft.
2. ACh binds to receptors on the sarcolemma, generating an action potential. The action potential spreads along the sarcolemma and into the T-tubules.
3. T-tubule depolarisation triggers the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm.
4. Ca2+ binds to troponin on the thin filament. Troponin shifts tropomyosin away from the myosin-binding sites on actin. The binding sites are exposed.
5. Energised myosin heads (already cocked in the high-energy position after ATP hydrolysis) attach to the exposed actin sites, forming cross bridges.
6. The myosin head bends, pulling the actin filament towards the M line (the power stroke). ADP and Pi are released.
7. A new ATP molecule binds to the myosin head, causing it to detach from actin. ATP is hydrolysed (ADP + Pi), re-cocking the head in the high-energy position. The cycle repeats as long as Ca2+ and ATP are present.
8. When the nerve impulse stops, ACh is broken down by acetylcholinesterase. The sarcoplasmic reticulum pumps Ca2+ back in (using ATP). Troponin releases Ca2+, tropomyosin slides back and blocks actin sites. The muscle relaxes.

Band Changes During Contraction

• A band: does NOT change length (myosin filament length stays the same).
• I band: shortens (less actin visible outside the A band).
• H zone: shortens or disappears (actin slides in and covers the central myosin-only region).
• Sarcomere: shortens overall (Z lines move closer together).

Remember: I band and H zone shorten. A band stays the same. This is the single most tested fact about the sarcomere in NEET.

The Skeletal System

The human adult skeleton has 206 bones. It is divided into two major parts: axial skeleton (80 bones) and appendicular skeleton (126 bones).

The skeleton has five main functions: support, protection, movement, mineral storage (calcium and phosphorus) and blood cell production (in the red bone marrow).

Axial Skeleton (80 bones)

The axial skeleton forms the central axis of the body:

• Skull (22 bones): 8 cranial bones (protect the brain) and 14 facial bones. The lower jaw (mandible) is the only movable bone of the skull.
• Vertebral column (26 bones): 7 cervical + 12 thoracic + 5 lumbar + 1 sacrum (5 fused vertebrae) + 1 coccyx (4 fused vertebrae). Total 26 functional units.
• Ribs (24 bones, 12 pairs): True ribs (1 to 7) attach directly to the sternum. False ribs (8 to 10) attach via cartilage. Floating ribs (11 and 12) have no sternal attachment.
• Sternum (1 bone): the breastbone. Three parts: manubrium, body and xiphoid process.
• Hyoid and ear ossicles (7): hyoid (1, supports the tongue) and 3 ear ossicles in each ear (malleus, incus and stapes). The stapes is the smallest bone in the body.

Appendicular Skeleton (126 bones)

• Pectoral girdle (4 bones): 2 clavicles (collar bones) and 2 scapulae (shoulder blades). Connects the upper limbs to the axial skeleton.
• Upper limbs (60 bones total, 30 per arm): humerus (1) + radius and ulna (2) + carpals (8) + metacarpals (5) + phalanges (14).
• Pelvic girdle (2 bones): 2 coxal (hip) bones, each formed by the fusion of the ilium, ischium and pubis.
• Lower limbs (60 bones total, 30 per leg): femur (1, the longest bone in the body) + patella (1, kneecap) + tibia and fibula (2) + tarsals (7) + metatarsals (5) + phalanges (14).

Joints

A joint is the point where two bones meet. There are three main categories:

• Fibrous joints: bones are joined by fibrous tissue. No movement possible. Examples: sutures of the skull, tooth in its socket (gomphosis).
• Cartilaginous joints: bones are joined by cartilage. Allow slight movement. Examples: joints between vertebrae (intervertebral discs), the joint between the ribs and the sternum.
• Synovial joints: the most common type. Have a fluid-filled joint cavity lined by a synovial membrane that secretes synovial fluid. The synovial fluid lubricates the joint. Allow the widest range of movement. NEET tests several types:
  • Ball and socket: maximum range of movement in all directions. Shoulder, hip.
  • Hinge: flexion and extension only (like a door hinge). Knee, elbow, ankle.
  • Pivot: rotation only. Atlas and axis of the vertebral column (lets you turn your head), proximal radio-ulnar joint.
  • Gliding (plane): small gliding movements. Between carpal bones (wrist), between tarsal bones (ankle).
  • Saddle: two-plane movement. Carpometacarpal joint of the thumb.

Disorders of the Muscular and Skeletal System

• Myasthenia gravis: an autoimmune disease. The immune system makes antibodies that attack and destroy the acetylcholine receptors at the neuromuscular junction. With fewer receptors, muscle stimulation is weak. Result: progressive muscle weakness, especially in the face and limbs. The muscles tire very quickly.
• Muscular dystrophy: a group of genetic diseases that cause progressive degeneration of skeletal muscle fibres. The most common form (Duchenne muscular dystrophy) is caused by the absence of the protein dystrophin, which normally stabilises the sarcolemma. Boys are mainly affected (X-linked).
• Tetany: rapid, uncontrolled contractions (cramps and spasms) of muscle. Caused by low calcium (hypocalcaemia) in the blood. Low Ca2+ makes nerve and muscle membranes excessively excitable. Can also be triggered by low magnesium or alkalosis.
• Arthritis: inflammation of joints. Osteoarthritis is the most common form: slow wearing away of cartilage at the joint surface, usually in older adults.Rheumatoid arthritis is an autoimmune disease where the immune system attacks the synovial membrane of joints.
• Osteoporosis: bone density and bone mass decrease over time, making bones porous, weak and prone to fracture. Most common in post-menopausal women because oestrogen normally protects bone density. Calcium and vitamin D deficiency are risk factors.
• Gout: uric acid crystals deposit in the joints (especially the big toe), causing extreme pain, swelling and inflammation. Caused by excess uric acid in the blood (hyperuricaemia), often from a diet high in purines (meat, shellfish) or from reduced excretion by the kidneys.

Worked NEET Problems

Summary Cheat Sheet

• Three movement types: amoeboid (Amoeba, leucocytes), ciliary (respiratory tract, fallopian tube), muscular (most animal locomotion).
• Skeletal muscle: striated, voluntary, multi-nucleate, fatigable. Smooth: non-striated, involuntary, single central nucleus, no fatigue. Cardiac: striated, involuntary, single nucleus, branched with intercalated discs.
• 4 muscle properties: excitability, contractility, extensibility, elasticity.
• Sarcolemma = plasma membrane of muscle fibre. Sarcoplasm = cytoplasm. Sarcoplasmic reticulum = ER; stores and releases Ca2+.
• Sarcomere = Z line to Z line. A band (dark, full myosin length, no change). I band (light, actin only, shortens). H zone (only myosin, centre of A band, shortens/disappears). M line (centre, myosin anchor). Z line (actin anchor, moves closer).
• Thin filament proteins: actin (binding sites for myosin), tropomyosin (blocks sites at rest), troponin (binds Ca2+, unblocks sites).
• Thick filament: myosin with globular heads (cross bridges). Energised by ATP hydrolysis.
• Sliding filament theory: nerve impulse → ACh → action potential → T-tubules → SR releases Ca2+ → Ca2+ binds troponin → tropomyosin moves → actin exposed → myosin attaches → power stroke → I band and H zone shorten → ATP detaches head → relaxation when Ca2+ pumped back.
• Total bones: 206. Axial: 80. Appendicular: 126.
• Axial: skull (22), vertebral column (26), ribs (24), sternum (1), hyoid + ear ossicles (7).
• Appendicular: pectoral girdle (4), upper limbs (60), pelvic girdle (2), lower limbs (60).
• Joints: fibrous (immovable, skull sutures), cartilaginous (slight movement, vertebral joints), synovial (fluid-filled cavity, most movable).
• Synovial types: ball and socket (shoulder, hip), hinge (knee, elbow), pivot (atlas-axis), gliding (carpals, tarsals), saddle (thumb base).
• Disorders: myasthenia gravis (ACh receptors destroyed), muscular dystrophy (no dystrophin), tetany (low Ca2+), arthritis (joint inflammation), osteoporosis (low bone density), gout (uric acid crystals in joints).

Next: explore the interactive learning widgets to compare muscle types side by side, walk through the sliding filament theory, and explore the 206-bone skeletal system. Work through the 14+ NEET PYQs with full solutions. To time yourself on this chapter, take the free 10-question mock test.