Chapter 17: Locomotion and Movement | Summary

Muscle is a specialised tissue of mesodermal origin that contributes about 40–50% of the body weight of a human adult. Muscles possess four special properties: excitability, contractility, extensibility, and elasticity. Based on location, muscles are classified into three types — skeletal, visceral, and cardiac. Skeletal muscles are closely associated with skeletal components, appear striated under a microscope, are under voluntary nervous control, and are primarily involved in locomotion and changes of body posture.

Visceral (smooth) muscles are located in the inner walls of hollow visceral organs like the alimentary canal and reproductive tract. They are non-striated, involuntary, and help in the transportation of food and gametes. Cardiac muscles are the muscles of the heart. They are striated and branched like skeletal muscles, but involuntary like smooth muscles. They are not directly controlled by the nervous system.

Each organised skeletal muscle is made of a number of muscle bundles or fascicles held together by a common collagenous connective tissue layer called fascia. Each fascicle contains numerous muscle fibres. Each muscle fibre is lined by a plasma membrane called the sarcolemma, which encloses the sarcoplasm. A muscle fibre is a syncitium, meaning its sarcoplasm contains many nuclei. The sarcoplasmic reticulum (the endoplasmic reticulum of muscle fibres) is the storehouse of calcium ions.

The sarcoplasm contains a large number of parallelly arranged filaments called myofibrils (myofilaments). Each myofibril shows alternate dark (A band / Anisotropic) and light (I band / Isotropic) bands. The striated appearance results from the distribution of two proteins — actin (thin filaments in I bands) and myosin (thick filaments in A bands). The Z line bisects each I band and anchors thin filaments, while the M line holds thick filaments in the centre of the A band. The functional unit of contraction between two successive Z lines is called the sarcomere. The central part of the thick filament not overlapped by thin filaments at rest is the H zone.

Each actin (thin) filament is composed of two F (filamentous) actins helically wound together. Each F actin is a polymer of monomeric G (Globular) actins. Two filaments of tropomyosin run close to the F actins along their entire length. A complex protein, troponin, is distributed at regular intervals on tropomyosin. In the resting state, a subunit of troponin masks the active binding sites for myosin on the actin filaments, preventing contraction.

Each myosin (thick) filament is also a polymerised protein, made of many monomeric units called meromyosins. Each meromyosin has two parts: a globular head with a short arm (heavy meromyosin, HMM) and a tail (light meromyosin, LMM). The HMM portion (head and short arm) projects outward at regular distances from the surface of the polymerised myosin and forms the cross arm. The globular head acts as an active ATPase enzyme and contains binding sites for both ATP and actin.

Muscle contraction is best explained by the sliding filament theory, which states that contraction of a muscle fibre occurs by the sliding of thin filaments over thick filaments. Contraction is initiated by a signal from the CNS via a motor neuron. The junction between a motor neuron and the sarcolemma is called the neuromuscular junction or motor-end plate. The neural signal triggers release of acetylcholine, which generates an action potential in the sarcolemma. This causes Ca++ ions to be released from the sarcoplasmic reticulum.

The rise in Ca++ levels causes calcium to bind to a subunit of troponin on the actin filaments, removing the masking of active sites for myosin. The myosin head, using energy from ATP hydrolysis, binds to exposed actin sites forming a cross bridge. This pulls the actin filaments towards the centre of the A band, and the attached Z lines move inward — shortening the sarcomere. During contraction, the I bands shorten but the A band retains its length. Repeated activation leads to fatigue through lactic acid accumulation from anaerobic glycogen breakdown. When Ca++ is pumped back into the sarcoplasmic reticulum, actin sites are masked again and relaxation occurs.

Muscles can be classified as red fibres or white fibres based on the amount of myoglobin they contain. Red fibres contain high amounts of myoglobin — the red-coloured oxygen-storing pigment — and are rich in mitochondria. They rely on aerobic respiration for energy and are therefore also called aerobic muscles. They are adapted for sustained, continuous activity.

White fibres contain very little myoglobin and therefore appear pale or whitish. They have fewer mitochondria but possess a high amount of sarcoplasmic reticulum. They depend on anaerobic processes for energy and are suited for rapid, short-burst contractions. The reaction time of muscle fibres varies across different muscle types.

The skeletal system consists of a framework of 206 bones and a few cartilages. Bone has a very hard matrix due to calcium salts, while cartilage has a slightly pliable matrix due to chondroitin salts. The skeletal system is divided into two principal divisions: the axial skeleton (80 bones along the main body axis — skull, vertebral column, sternum, and ribs) and the appendicular skeleton (limb bones and girdles). The skull has 22 bones: 8 cranial and 14 facial. The hyoid bone is a single U-shaped bone at the base of the buccal cavity. Each middle ear contains three ear ossicles (Malleus, Incus, Stapes).

The vertebral column consists of 26 vertebrae in five regions: cervical (7), thoracic (12), lumbar (5), sacral (1 fused), and coccygeal (1 fused). Of the 12 pairs of ribs, the first 7 are true ribs (attached directly to the sternum), pairs 8–10 are false/vertebrochondral ribs (joining the 7th rib via cartilage), and pairs 11–12 are floating ribs (not ventrally connected). The appendicular skeleton includes the pectoral girdle (clavicle + scapula), pelvic girdle (two coxal bones, each a fusion of ilium, ischium, and pubis), and the bones of the upper and lower limbs (30 bones each).

Joints are points of contact between bones, or between bones and cartilages. Force generated by muscles is used to carry out movement through joints, where the joint acts as a fulcrum. Joints are classified into three structural types: fibrous, cartilaginous, and synovial. Fibrous joints allow no movement and are seen in flat skull bones that fuse end-to-end using dense fibrous connective tissues in the form of sutures. Cartilaginous joints join bones with cartilage and permit limited movement, as seen between adjacent vertebrae.

Synovial joints are characterised by a fluid-filled synovial cavity between the articulating surfaces of two bones, allowing considerable movement. They are critical for locomotion. Examples include: ball and socket joint (humerus and pectoral girdle), hinge joint (knee), pivot joint (atlas and axis), gliding joint (between carpals), and saddle joint (carpal and metacarpal of thumb).

Several disorders affect the muscular and skeletal systems. Myasthenia gravis is an auto-immune disorder that affects the neuromuscular junction, leading to fatigue, weakening, and paralysis of skeletal muscles. Muscular dystrophy involves progressive degeneration of skeletal muscle, mostly due to a genetic disorder. Tetany is characterised by rapid spasms and wild contractions in muscles, caused by low Ca++ levels in body fluids.

Arthritis refers to inflammation of joints. Osteoporosis is an age-related disorder characterised by decreased bone mass and increased susceptibility to fractures; decreased levels of estrogen are a common cause. Gout is the inflammation of joints due to the accumulation of uric acid crystals. These disorders highlight the importance of maintaining musculoskeletal health.

Chapter 17 establishes that movement and locomotion are fundamental to life, with the human body employing a sophisticated interplay of muscular, skeletal, and neural systems. The sliding filament theory elegantly explains how actin and myosin filaments interact — driven by ATP and regulated by calcium ions and troponin — to produce contraction at the level of the sarcomere. The skeletal system, comprising 206 bones organised into axial and appendicular divisions, provides the structural framework for these movements, with joints acting as fulcrums for muscle-generated forces.

From a board examination perspective, this chapter is highly important and regularly tested. Key areas include: the structure of the sarcomere and contractile proteins, the step-by-step mechanism of muscle contraction, the classification of bones and joints (with specific examples), the distinction between red and white muscle fibres, and the clinical disorders of the musculoskeletal system. Students should be able to draw and label diagrams of a sarcomere, the human skull, vertebral column, rib cage, and pectoral/pelvic girdles, as these are frequently asked in CBSE board exams.