Control and Coordination — Notes

A stimulus is a change in the surroundings (light, heat, sound, touch, chemicals). Organisms respond to stimuli in a controlled way; control and coordination make the right parts act together, at the right time and in the right amount. In animals this is done by the nervous system and the endocrine (hormonal) system.

Receptors are specialised cells, usually in sense organs, that detect stimuli: photoreceptors (eye), phonoreceptors (ear), olfactory receptors (nose), gustatory receptors (tongue), and touch, heat and pain receptors (skin). Effectors are the muscles and glands that respond.

A neuron is the structural and functional unit of the nervous system: dendrites acquire information → cell body (with nucleus) → axon carries the electrical impulse → nerve endings. At the nerve ending the impulse causes the release of chemicals, which cross a small gap, the synapse, and start a new electrical impulse in the dendrite of the next neuron. A similar junction passes impulses to muscles and glands. Because chemicals are released only at the axon end, impulses cross a synapse in one direction only.

Actions are voluntary (by choice, controlled by the forebrain, e.g. writing), involuntary (automatic, controlled by the midbrain and hindbrain, e.g. heartbeat, salivation) or reflex (a sudden, automatic response to a stimulus without conscious thought, e.g. pulling the hand away from a flame).

The path of a reflex is the reflex arc: receptor → sensory neuron → relay neuron in the spinal cord → motor neuron → effector. Reflex arcs are formed in the spinal cord, close to where the nerves enter, so the response is very quick and protects the body. The impulse also travels to the brain, which becomes aware of the event after the response has begun — that is why pain is felt a moment after the hand moves.

Reflex arcs are thought to have evolved because they are efficient where real thinking is not fast enough; the brain still handles the complex decisions.

The central nervous system (CNS) is the brain and the spinal cord; the peripheral nervous system (PNS) is the cranial nerves (from the brain) and spinal nerves (from the spinal cord).

• Forebrain (cerebrum): the main thinking part. Sensory areas receive impulses (sight, hearing, smell, touch); association areas link them with memory; motor areas control voluntary muscles.
• Midbrain: some involuntary actions, including reflex movements of the head and eyes.
• Hindbrain: medulla — blood pressure, salivation and vomiting; pons — helps regulate breathing; cerebellum — posture, balance and the precision of voluntary actions (walking straight, riding a bicycle, picking up a pencil).

The brain lies in a bony box, the skull (cranium), and is cushioned by fluid that absorbs shocks. The spinal cord is protected by the vertebral column.

When an impulse reaches a muscle, special proteins in the muscle cells change their shape and arrangement, the cells shorten, and the part moves.

Electrical impulses have limits: they reach only cells connected by nervous tissue, and a cell needs time to reset after each impulse before it can send another. So multicellular organisms also use chemical communication — hormones carried by the blood, which reach all cells (only cells with the right receptors respond). Plants, which have no nervous tissue, rely on chemical and electrical-chemical signals.

Plants have no nervous system, yet they respond to stimuli in two ways.

Immediate response (no growth): the leaflets of the touch-me-not (Mimosa pudica) fold and droop within seconds of a touch. Information passes from cell to cell by electrical-chemical means, and certain cells change shape by losing water. The movement is not directional and involves no growth (a nastic movement).

Growth movements (tropisms): directional growth towards (positive) or away from (negative) a stimulus, caused by unequal growth on two sides:

• Phototropism — shoots grow towards light; roots away from it.
• Geotropism — roots grow towards gravity (positive); shoots away (negative).
• Hydrotropism — roots grow towards water.
• Chemotropism — the pollen tube grows towards the ovule.
• Thigmotropism — tendrils coil around a support because the side touching it grows more slowly than the side away from it.

Plant hormones (phytohormones) coordinate growth and responses.

• Auxin — made at the shoot tip; makes cells elongate. When light comes from one side, auxin diffuses to the shaded side, cells there grow longer, and the shoot bends towards the light.
• Gibberellins — help the growth of the stem.
• Cytokinins — promote cell division; found in high concentration where cells divide rapidly, such as fruits and seeds.
• Abscisic acid — inhibits growth; its effects include the wilting of leaves; it is linked with responses to water stress.

Endocrine glands are ductless: they release hormones directly into the blood, which carries them to target cells.

Adrenaline is released into the blood in an emergency: the heart beats faster, sending more oxygen to the muscles; blood supply to the digestive system and skin is reduced as small arteries there contract; breathing becomes faster. Iodised salt prevents iodine deficiency and goitre (swollen neck).

Feedback mechanism: the timing and amount of hormone release are regulated by feedback. For example, when blood sugar rises, cells of the pancreas detect it and release more insulin; as the level falls, insulin secretion is reduced.