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Living organisms must respond to changes in their surroundings (stimuli) such as light, heat, sound, smell and touch. In animals this is done by the nervous system and hormones. The organs that receive stimuli are receptors; the organs that respond are effectors (muscles and glands). Control and coordination make the different organs work together as one body.
The neuron is the functional unit of the nervous system.
The gap between two neurons is a synapse. When the electrical impulse reaches the nerve ending, it triggers the release of a chemical. The chemical crosses the gap, starts a similar electrical impulse in the dendrite of the next neuron. The same process transmits the signal to a muscle or gland.
A sudden, automatic response to a stimulus, which does not involve thinking, is a reflex action, for example withdrawing the hand on touching a hot object or blinking when something enters the eye. The pathway of the impulse is the reflex arc.
A reflex is involuntary and instantaneous, and is controlled by the spinal cord. Walking is a voluntary action and involves thought and decision, controlled by the brain. (Involuntary actions such as the heartbeat and digestion are controlled by the medulla; every reflex is involuntary, but not every involuntary action is a reflex.)
The brain is in a bony box, the skull (cranium), and surrounded by cerebrospinal fluid which acts as a shock absorber. The spinal cord is enclosed in the vertebral column. Both together form the central nervous system; the nerves going out from them form the peripheral nervous system.
Muscle cells have special proteins. When an electrical impulse arrives, these proteins change their shape and arrangement, so that the cells become shorter. This is how the muscle contracts and the body moves.
Signals between the brain and the parts of the body below the injury are disrupted, so there is loss of sensation and voluntary movement in those parts (paralysis). Reflexes below the injury may still work, since the arc is in the cord.
Plants respond to stimuli but have no nerves or muscles. They use electrical-chemical signals and plant hormones, with two kinds of movement.
The leaves of the sensitive plant (Mimosa pudica, touch-me-not) droop and fold when touched. The change is not due to growth. The cells at the base of the leaf change their shape by changing the amount of water in them: they lose water and shrink, so the leaf folds. The signal is carried from cell to cell by electrical-chemical means.
Directional movements caused by growth are tropisms.
In the sensitive plant, the movement is brought about by changes in the water content of the cells, with no specialised proteins or nerves. In our legs, the movement is by muscle proteins responding to nerve impulses. The plant movement is generally slow except in this case, and the animal movement is fast.
Chemical substances made in one part of the plant and moving to another to control and coordinate growth and responses are plant hormones.
When light comes from one side, auxin diffuses to the shady side of the shoot. This stimulates the cells there to grow longer, so the shoot bends towards the light. In the same way, a tendril grows around a support because the auxin is concentrated on the side away from the support, making those cells grow faster.
Put some soaked seeds in two sets of pots. In one, place a small porous clay pot with water in the soil at one side, and in the other pot leave dry soil. After some days, the roots in the first pot bend towards the water source.
Hormones are chemical messengers secreted by endocrine glands directly into the blood, which carries them to the target organ.
The timing and amount of hormone release are controlled by a feedback mechanism. For example, when blood sugar rises, the pancreas secretes more insulin; when the sugar level falls, the secretion is reduced. The pituitary is called the master gland because it regulates other glands.
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A reflex action is sudden, automatic and involuntary, needs no thinking, and is controlled by the spinal cord (for example pulling the hand from a hot object). Walking is a voluntary action, which involves thought and is controlled by the brain (the cerebrum, with the cerebellum for balance).
When the electrical impulse reaches the nerve ending of one neuron, it makes the nerve ending release a chemical. The chemical crosses the gap (synapse) and starts a similar electrical impulse in the dendrite of the next neuron.
The cerebellum, in the hindbrain.
The smell molecules reach the olfactory receptors in the nose. These receptors send an electrical impulse via the sensory neurons to the forebrain (the olfactory area), where the information is interpreted as a smell.
In most reflex actions (such as withdrawing the hand from a hot object) the reflex arc is completed in the spinal cord, so the brain is not needed for the response and time is saved. The brain is informed at the same time, through other neurons, and so we become aware of the event afterwards. In reflexes of the head region, such as blinking, the arc is completed in the brain itself.
Chemical substances made in one part of the plant that move to another part to control growth and responses, for example auxin, gibberellins, cytokinins and abscisic acid.
In the sensitive plant, the leaf folds quickly on touch because cells at the base change shape by losing water (a change in turgor); it is not growth. Movement of a shoot towards light is a growth movement, in which auxin makes the cells on the shaded side grow longer, so it is slow.
Auxin (also gibberellins and cytokinins).
The part of the tendril in contact with the support grows less, while the auxin collects on the other side, away from the support, making those cells grow longer and faster. The tendril therefore curls round the support.
Take two small plastic pots with soil and sow some seeds (for example, pea or gram) in each; water both until the seeds sprout.
In one pot, bury a small porous clay pot or a tube filled with water in the soil on one side, and water no more. In the other, leave the soil dry and water it evenly all around.
After a few days, carefully take out the plants and observe the roots.
In the first pot the roots bend towards the water source: hydrotropism; in the control pot the roots grow downward.
By hormones, which are chemicals secreted by endocrine glands directly into the blood. The blood carries them to the target organs, where they bring about changes; the secretion is regulated by feedback mechanisms.
Iodine is necessary for the thyroid gland to make thyroxin, which regulates the metabolism of carbohydrates, proteins and fats. Deficiency of iodine causes goitre, so iodised salt is recommended.
Adrenaline makes the heart beat faster, so more oxygen is supplied to the muscles; the breathing rate increases; the blood supply to the digestive system and skin is reduced (their vessels narrow), and more blood goes to the skeletal muscles. This prepares the body to deal with a emergency (fight or flight).
In diabetes, the pancreas does not produce enough insulin, so the sugar level in the blood rises. Insulin injections bring the blood sugar down to a normal level.
Answer: (d) Cytokinin
Answer: (b) synapse
Answer: (d) all of the above
Receptors detect changes in the environment (stimuli) and convert them into nerve impulses. If they fail (for example, a damaged eye, ear, or loss of sensation in the skin) the body cannot get information; we may not see or hear, or we could burn ourselves or get injured without feeling pain.
See the diagram in the notes. A neuron has dendrites (receive information), a cell body (passes it as an electrical impulse), an axon (carries it away over a long distance) and nerve endings (release a chemical to pass it to the next neuron or muscle). Its function is to receive, conduct and transmit information as electrical impulses.
When light falls on a shoot from one side, auxin made at the tip diffuses to the shaded side. It makes the cells there grow longer than on the lighted side, and so the shoot bends towards the light.
The signals passing between the brain and the parts of the body below the injury: sensory signals going up to the brain and motor commands from the brain going down. This can cause loss of sensation and paralysis below the injury.
By plant hormones. Auxin (growth, phototropism), gibberellins (stem growth), cytokinins (cell division) and abscisic acid (inhibits growth, wilting). They are made in one part and move to others to bring about growth and responses.
To detect changes in the surroundings, to respond properly, to make the different organs work together in an integrated manner and keep the body conditions stable, so that the organism can survive.
Involuntary actions (for example heartbeat, digestion, salivation) are not under the control of our will and are usually controlled by the medulla and the autonomic system, without the path of a reflex arc; they may be slow. Reflex actions are sudden, rapid responses to a stimulus, with a short reflex arc through the spinal cord, and are also without thought. Every reflex is involuntary, but not every involuntary action is a reflex.
In the sensitive plant there are no special cells for conduction or movement; the cells at the base of the leaf change their water content (and hence shape) in response to a touch signal, so the leaf folds. In our legs, nerve impulses from the brain or spinal cord make the muscle cells contract by changing the shape of special proteins in them, and so the bones move.