NCERT Summary: Summary of Biology- 2

Muscular and Skeletal System

Movement is a defining characteristic of animals. Movement is produced by muscle contraction and transmitted and supported by a skeleton. Animal skeletons occur in three broad types: hydrostatic skeletons, exoskeletons, and endoskeletons (internal skeletons).

Types of Skeletons

• Hydrostatic skeletons are fluid-filled closed chambers. Internal pressure generated by muscles acting on the fluid causes movement and maintains body shape. Examples include many worms and the sea anemone. In a sea anemone, longitudinal muscles in one layer and circular muscles in another layer alternately contract to elongate or shorten the body.

• Exoskeletons are external, hardened coverings that protect the body and provide attachment sites for muscles. They are characteristic of the Phylum Arthropoda. Muscles attach to the inner surface of the exoskeleton.

Exoskeletons

• Because an exoskeleton restricts growth, animals with exoskeletons must periodically moult (shed the old exoskeleton) and form a larger one. The bulk and mechanical constraints of exoskeletons limit maximum body size. Some animals, such as spiders, combine exoskeletal rigidity for protection with internal fluid pressure mechanisms for movement.
• Endoskeletons are internal supporting frameworks. Vertebrates possess mineralised endoskeletons composed mainly of bone and cartilage. Muscles lie external to the endoskeleton and attach to bones.

Endoskeletons

• In sharks and rays the skeleton remains composed entirely of cartilage. In most other vertebrates an embryonic cartilaginous skeleton is progressively replaced by bone during development. Adult vertebrates retain cartilage where flexibility is required, for example in some joints, the ribs, trachea, nose, and external ear.

The Skeleton and Muscles (Musculoskeletal System)

• The musculoskeletal system comprises the skeleton and associated muscles. It provides support, enables movement, and contributes to homeostasis by allowing animals to move to favourable environments and by producing heat through muscular activity.
• Bone marrow produces blood cells and immune cells; bone also functions as a calcium sink, helping to regulate blood calcium levels.

Musculoskeletal system

Major Divisions of the Skeleton

• The axial skeleton includes the skull, vertebral column, and rib cage.

Axial skeleton

• The appendicular skeleton consists of the limbs (or wings, fins) and their supporting girdles - the pectoral girdle and pelvic girdle.

• The human skull (cranium) is formed by many bones joined at largely immovable joints called sutures. At birth some of these sutures are incomplete, producing soft spots or fontanels that close during infancy (usually by 14-18 months).

Skull bones

• The vertebral column is composed of about 33 individual vertebrae separated by intervertebral cartilage (disks) that permit flexibility and absorb shock. With age the disks can deteriorate, producing back pain. The sternum articulates with most ribs; cartilage between ribs and sternum allows the rib cage to change shape during breathing.

Vertebral column

• Each limb has characteristic bones: the upper limb has a single proximal bone, the humerus; the lower limb has the femur.
• Distal to an elbow or knee, limbs have two parallel bones: the radius and ulna in the forearm, and the tibia and fibula in the lower leg. The carpals form the wrist and the tarsals the ankle.
• Hands and feet end in five digits (fingers or toes). The hand bones include the metacarpals; the foot bones include the metatarsals.

Foot and Hand Bones 

• The pectoral girdle (clavicle and scapula) connects forelimbs to the trunk. The humerus articulates with the scapula at the shoulder joint and is stabilised by muscles and ligaments; a dislocated shoulder results when the humeral head slips from the scapular socket.
• The pelvic girdle is formed by two hip bones that articulate with the vertebral column and with each femur to form the pelvis. In land animals the pelvic girdle transfers body weight to the legs; in fish it is relatively small because buoyancy supports body weight. Pelvic girdles in bipeds differ in shape from those of quadrupeds, reflecting differences in locomotion.

Bone Structure

• Bones vary in shape and size but share a common organisation: a mineralised matrix (largely calcium salts) reinforced by collagen fibres, with living bone cells embedded in this matrix.
• Compact bone forms dense outer layers and the shafts of long bones; it contains concentric systems around vascular channels called Haversian canals. These canals carry blood vessels and nerves that nourish bone cells.
• Spongy (cancellous) bone forms the inner regions near joint ends and contains a porous network. Spongy bone at the ends of long bones and in the sternum, pelvis, and vertebrae houses red marrow, where haematopoietic stem cells generate blood and immune cells. The central cavity of long bones often contains yellow marrow that stores fat.
• The outer surface of bone is covered by the periosteum, a vascular connective tissue layer. The inner layer of the periosteum contains cells that can form new bone or remodel existing bone in response to stress or repair following fractures; the periosteum is richly innervated, and damage to it causes pain.

Skeletal Muscle Systems

• Skeletal muscles act on bones to produce movement. Tendons attach muscles to bone across joints so that muscle contraction moves bones relative to one another.
• Muscles typically work in antagonistic pairs: when one muscle contracts (flexor) the opposing muscle (extensor) relaxes, producing controlled movement.
• Muscle cells exhibit both electrical and chemical activity. An electrical stimulus across the muscle cell membrane triggers contraction, converting chemical energy into mechanical work; repeated and coordinated contractions produce locomotion.

Skeletal Muscle Structure and Mechanism of Contraction

• Muscle fibres (muscle cells) are long, multinucleated cells whose cytoplasm is filled by contractile elements called myofibrils. Myofibrils are organised into repeating units called sarcomeres, which extend from one Z line to the next. Sarcomeres contain alternating thick and thin filaments.
• Thick filaments are primarily composed of myosin, and thin filaments are composed mainly of actin anchored at the Z line. The overlapping arrangement of filaments gives striated muscle its banded appearance.
• Muscle contraction occurs when sarcomeres shorten by the sliding filament mechanism: myosin heads bind to actin, swivel toward the sarcomere centre, detach and reattach cyclically, pulling thin filaments inward and shortening the sarcomere. Each cross-bridge cycle shortens the sarcomere by a small fraction; many cycles per second produce measurable shortening and force.
• The energy for contraction comes from ATP. ATP binding to the myosin head and its hydrolysis power the cross-bridge cycles. Because muscles hold only small amounts of ATP, they rapidly regenerate ATP from ADP. Creatine phosphate serves as a rapid phosphate donor to regenerate ATP during short, intense activity.
• Calcium ions (Ca2+) are essential for contraction. When a muscle fibre is stimulated, Ca2+ is released into the sarcomere, exposing binding sites on actin for myosin heads to attach. When stimulation ends, Ca2+ is pumped back into storage, covering the binding sites and ending contraction.

Contraction in Non-Muscle Cells and Specialised Muscles

• Actin and myosin interactions also occur in many non-muscle cells; cytoplasmic myosin interacting with cortical actin can change cell shape and produce movements used in processes such as intracellular transport and intestinal peristalsis for nutrient absorption.
• Certain fish have highly modified muscle cells called electroplates that generate electric discharges. For example, the South American electric eel possesses thousands of electroplates organised in columns and can produce large electrical outputs for defence and predation.

Mechanical Principles: Levers

• Vertebrate bones and muscles use the mechanical principles of levers to amplify force or velocity of motion. The mechanical advantage depends on lever arm lengths and the relative positions of fulcrum, load, and effort.
• There are three classes of levers commonly identified in anatomy, distinguished by the order of fulcrum, effort, and load; different lever classes provide different trade-offs between force and speed of movement.

The Nervous System

Divisions of the Nervous System

• The nervous system monitors internal and external conditions and coordinates responses via complex feedback loops.
• The Central Nervous System (CNS) comprises the brain and spinal cord.
• The Peripheral Nervous System (PNS) consists of nerves that connect the CNS to receptors, muscles and glands.
• Not all animals possess highly specialised nervous systems. Simpler nervous organisations are found in small or sessile animals, whereas large, mobile animals tend to have more complex nervous systems. The evolution of advanced nervous systems is closely linked with increases in body size, mobility and behavioural complexity.

Nervous Systems in Different Animal Groups

• Radially symmetric animals such as coelenterates (cnidarians) and many echinoderms have a diffuse nerve net without a central brain; these nerve nets can nevertheless coordinate complex behaviours.
• Bilateral animals exhibit cephalization, the concentration of sensory organs and neural processing in a head region, and often possess paired nerve cords and ganglia. For example, flatworms show clusters of nerve cells (ganglia) that form a simple brain.
• In chordates the nervous system is dorsal; evolutionary trends include the development of a spinal cord, progressive cephalization with increasingly complex brains, and elaboration of nervous circuits for sensory processing, motor control and higher functions.

The Neuron: Cellular Unit of the Nervous System

• Nervous tissue consists primarily of two cell types: neurons and glial cells. Neurons transmit signals; glial cells support, nourish and insulate neurons.
• The neuron is the functional unit of the nervous system. Human brains contain on the order of 100 billion neurons, varying greatly in form and size.

Parts of a Neuron

All neurons share three principal regions:

• Dendrites receive incoming signals and convey them to the cell body.
• Cell body (soma) contains the nucleus and the metabolic machinery of the cell.
• Axon conducts electrical impulses away from the cell body toward other neurons, muscles or glands. Many axons are ensheathed by a myelin layer formed by specialised glial cells (in the PNS these are Schwann cells), which increases conduction speed. The gaps between myelinated segments are the Nodes of Ranvier, where action potentials are generated and from which impulses effectively 'jump' along the axon, greatly increasing conduction velocity.

Types of Neurons

• Sensory neurons transmit information from sensory receptors toward the CNS; they typically have long dendrites and short axons.
• Motor neurons carry commands from the CNS to muscles or glands; they usually have long axons and shorter dendrites.
• Interneurons lie entirely within the CNS and connect neurons to other neurons, forming the networks that integrate information and generate responses.

The Nerve Message: Membrane Potentials and Action Potentials

• Neuronal membranes maintain an electrical potential difference between the inside and the outside; the outside is relatively positive and the inside relatively negative when the cell is at rest.
• The resting potential arises primarily from unequal distributions of Na+ and K+ ions across the membrane and from selective membrane permeability; this ionic distribution is actively maintained by the sodium-potassium pump (Na+/K+-ATPase).
• An action potential is a rapid, temporary reversal of membrane polarity that propagates along the axon. After an action potential passes a region of membrane, a brief refractory period prevents immediate re-excitation and ensures unidirectional propagation of the impulse.

Steps in an Action Potential

• At rest the interior of the neuron is negative relative to the outside.
• A stimulus opens voltage-gated Na+ channels so that Na+ flows into the cell, depolarising the membrane and producing the action potential (the inside becomes positive relative to the outside).
• Subsequently, voltage-gated K+ channels open and K+ flows out, beginning repolarisation of the membrane toward the resting potential.
• The Na+/K+ pump and other ion transport mechanisms restore the original ionic distributions by pumping Na+ out and K+ in, re-establishing the resting potential.

Synapses and Chemical Transmission

• A synapse is the junction between a neuron and another cell (neuron, muscle or gland). Within a neuron, signals travel electrically; across the synaptic cleft, communication is usually chemical.

CNS Synapse

• When an action potential reaches the axon terminal, it triggers fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft. Neurotransmitters bind to receptors on the postsynaptic membrane and alter its electrical state, initiating or inhibiting an action potential in the next cell.
• Neurotransmitters are typically small molecules; some are also hormones. The precise action depends on transmitter type and receptor subtype. Disorders of neurotransmission can cause serious disease: for example, Parkinson's disease involves a deficiency of the neurotransmitter dopamine in specific brain regions, and progressive loss of those neurons produces tremor, rigidity and postural instability.