Control and Coordination Class 10 Notes

All living organisms detect changes in their surroundings and respond to them. The change itself is called a stimulus, and the reaction shown by the body is called a response.Animals respond using a nervous system and, in a few cases, hormones. Plants respond only through slow growth movements, since they have neither nerves nor muscles.

• Stimulus: any change in the environment, such as light, heat, sound, touch, smell or a chemical
• Response: the reaction shown by an organism to a stimulus
• Receptor: the specialised cell or sense organ that detects a particular stimulus
• Effector: a muscle or gland that carries out the response once the message arrives
• Animals respond quickly because nerve impulses travel at very high speed
• Plants respond slowly because their response involves growth

The neuron is the structural and functional unit of the nervous system. It carries an electrical impulse from one part of the body to another at remarkable speed.A motor neuron has a cell body with dendrites, a long axon covered by a myelin sheath, and nerve endings that pass the impulse on to a muscle or a gland.

• Dendrites: short branched extensions that receive the incoming impulse
• Cell body or cyton: contains the nucleus and cytoplasm and carries out metabolic activity
• Axon: a long fibre that conducts the impulse away from the cell body
• Myelin sheath: an insulating covering that speeds up conduction
• Nerve endings or axon terminals: release the impulse at the next cell
• Synapse: the tiny gap between two neurons, crossed by a chemical

The human nervous system has two main divisions. The central nervous system consists of the brain and the spinal cord, and it receives information and sends out orders.The peripheral nervous system is the network of cranial and spinal nerves that connects the central nervous system to every other part of the body.

• Brain: the master control centre, protected by the skull and the meninges
• Spinal cord: runs inside the vertebral column and controls reflex actions
• Cranial nerves: twelve pairs arising from the brain, mostly serving the head and neck
• Spinal nerves: thirty-one pairs arising from the spinal cord and supplying the body and limbs
• Receptors in the skin, eye, ear, nose and tongue detect the stimuli
• Voluntary actions are planned in the cerebrum and sent through motor nerves

A reflex action is a rapid, automatic and involuntary response to a stimulus, controlled by the spinal cord rather than by the brain. The pathway taken by the impulse is called the reflex arc.Because the brain is not consulted first, the response is much faster than a voluntary action, which is exactly what the body needs when it faces sudden danger.

• A receptor in the skin detects the heat
• A sensory neuron carries the impulse to the spinal cord
• A relay neuron in the spinal cord passes the message forward
• The brain is informed a fraction of a second later, which is when pain is felt
• A motor neuron carries the command back to the muscle
• The effector muscle contracts and the hand is pulled away

Actions are grouped by the way they are produced. A voluntary action is under conscious control, an involuntary action happens without any intention, and a reflex action is a special involuntary response to a stimulus.A condition reflex is a learned response that was once voluntary but has become automatic with practice, and that is how habits and fears are formed.

• Voluntary: writing, walking, speaking, dancing
• Involuntary: heartbeat, breathing, peristalsis, blinking
• Reflex: withdrawing from a hot object, sneezing, blinking at dust
• Condition reflex: saliving at the smell of food, braking at a red light
• Voluntary actions are guided by the cerebrum of the brain
• Involuntary and reflex actions are largely guided by the medulla and the spinal cord

Plants have no muscles, yet they do move. Their movements are slow, directional growth movements called tropic movements, and they happen in response to an external stimulus such as light, water, gravity or touch.Growth in plants happens only at the growing tip, which is why a plant bends instead of moving from one place to another.

• Phototropism: the shoot bends towards light and the root bends away from it
• Geotropism or gravitropism: the root grows downwards and the shoot grows upwards
• Hydrotropism: the root grows towards water
• Chemotropism: the growth of a pollen tube towards the ovule
• Thigmotropism or touch response: the tendril of a pea plant coils around a support
• Nastic movements: non-directional, such as the opening of a flower or the drooping of a bean leaf

Plant hormones are chemical messengers made in one part of the plant and transported to another, where they act in very small amounts. Auxin is the main growth hormone and it promotes cell elongation.Darwin's experiment on the coleoptiles of oat seedlings showed that the tip is the light-sensitive part and that the bending happens in the region below the tip, which is where auxin collects on the shaded side.

• Auxin: promotes cell elongation and causes bending towards light
• Gibberellin: promotes growth and elongation of the stem
• Cytokinin: promotes cell division and delays the ageing of leaves
• Abscisic acid: inhibits growth, promotes wilting of leaves and closes the stomata
• Hormones are produced in trace quantities and act only on their target tissues
• A hormone moves by diffusion and is not used up, unlike food and water

Not every response in animals is fast. Hormones are chemical messengers made by endocrine glands, carried in the blood to a target organ, and they act slowly but over a long duration.The pancreas makes insulin, which lowers blood sugar, and a shortage of it causes diabetes. The pituitary, thyroid, adrenal and the testes or ovaries also work through hormones.

• Endocrine glands are ductless and pour hormones directly into the blood
• Hormones travel at the speed of the blood, so the response is slow but long lasting
• Insulin from the pancreas controls blood sugar levels
• Testosterone from the testes controls the male secondary sexual characters
• Oestrogen from the ovaries controls the female secondary sexual characters
• Too little or too much thyroxine changes body weight and temperament