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movements include smooth muscles (Fig. 3.1, which are found in organs like the stomach and intestines. Their cells arespindle-shaped,haveasinglenucleusandlackstriations. Skeletal muscle They help in slow, continuous movements like digestion. The cardiac muscles (Fig. 3.13are found only in the heart. Their fibres are cylindrical and branched with a single nucleus, and have faint striations. Cardiac muscles work tirelessly and rhythmically, enabling the heart to beat Smooth muscle throughout life without fatigue. 3.3.4 How does the body sense, communicate and respond? Have you noticed how quickly you pull your hand away from something hot, or how you remember the lyrics of Cardiac muscle a song learned long ago? These actions are controlled Fig. 3.13: Different types of musby nervous tissue, which forms the body’s control and coordination network. The brain acts as the control centre; coordinating activities, memory and responses across the Cell body body. Muscles, both voluntary and involuntary, cannot Nucleus function independently. They receive instructions from the nervous tissue. For example, during exercise, the brain Axon signals the heart to beat faster to meet the body’s increased terminals Axo oxygen demand. n The cells of nervous tissue are called neurons (Fig. 3.14) or nerve cells, which are specialised to receive, process and Dendrites transmit messages. Each neuron has three main parts — the cell body, which contains the nucleus and controls cell Fig. 3.14: Structure of a neuronctivities; dendrites, which receive signals from other neurons; and an axon, a long fibre that carries messages fromthecellandendsataxonterminals.Theaxonterminals transmit the messages to other cells. Cartilage 3.4 The Musculoskeletal System The musculoskeletal system (Fig. 3.15) is made up of Bones Muscles bones, muscles, joints, cartilage, tendons and ligaments. Joint This system helps us stand upright, move, maintain posture and protect delicate organs. The musculoskeletal system functions under the control of the nervous system. Muscles pull on bones to produce movement. They are attached to bones by strong, flexible bands called tendons (Fig. 3.15). When a muscle contracts, the tendon transmits this force Tendon to the bone, resulting in movement at a joint. Have you wondered how much of your body weight comes from bones? On an average, the adult human skeleton makes up Ligament about 12 – 15 per cent of body weight, though this can vary with age, gender and body composition. Fig. 3.15: Musculoskeletal system in human body 38 Exploration|Grade 9