Control and Coordination

In a neuron, where is information acquired, along which part does it travel as an electrical impulse, and where is it turned into a chemical signal?

It is acquired at the dendritic tip, where a chemical reaction creates an electrical impulse. The impulse passes through the cell body and then along the axon; at the end of the axon it sets off the release of chemicals that carry the signal onward.

Synapse

The gap between the end of one neuron and the next cell. Chemicals released at the end of the axon cross it and start a similar electrical impulse in a dendrite of the next neuron; a similar junction passes impulses on to muscle cells or glands.

What is the difference between a reflex action and walking?

A reflex action, such as pulling a hand back from a flame, is a sudden response made without thinking, wired through the spinal cord. Walking is a voluntary action decided by the brain, with the cerebellum keeping it precise and balanced.

Reflex arc

The connection between the nerve that detects a stimulus and the nerve that moves a muscle, made in the spinal cord where they first meet. It turns input into output action quickly without waiting for the brain, although the information still travels on to the brain.

Why have reflex arcs evolved in animals?

Thinking involves complicated interactions of many nerve impulses and takes time, so touching a hot object we could be burnt before deciding to move. Many animals have little or no complex thinking network, and even where one exists, reflex arcs remain more efficient for quick responses.

Central nervous system

The brain and spinal cord, which collect information from every part of the body and integrate it. They communicate with the rest of the body through the peripheral nervous system: cranial nerves leaving the brain and spinal nerves leaving the spinal cord.

Fore-brain

The brain's chief thinking region. It contains areas receiving sensory impulses for hearing, smell and sight, association areas that interpret them with stored information, and motor areas controlling voluntary muscles; a separate centre in it signals when we have eaten enough.

Cerebellum

A part of the hind-brain responsible for the precision of voluntary actions and for maintaining posture and balance. It is what lets us walk straight, ride a bicycle or pick up a pencil.

Medulla

The part of the hind-brain that controls involuntary actions, which we cannot easily control by thinking, including blood pressure, salivation and vomiting.

How is the movement of the leaves of a sensitive plant different from the movement of our legs?

The sensitive plant has no nerves or muscles and no growth is involved: the touch is signalled electrically-chemically from cell to cell, and cells change shape by gaining or losing water. Leg muscle cells move because special proteins in them change shape and arrangement in response to nerve impulses.

How does the tendril of a pea plant coil around a support?

Tendrils are sensitive to touch. The side touching the support grows more slowly than the side facing away, so the tendril curls round the support and holds on.

Tropism

A directional growth movement of a plant part towards or away from a stimulus. Shoots bend towards light and roots away from it; roots grow down and shoots up in response to gravity; pollen tubes grow towards ovules, an example of chemotropism.

How does auxin make a shoot bend towards light?

Auxin is made at the shoot tip and helps cells grow longer. When light comes from one side, auxin diffuses towards the shady side, so cells on that side grow longer than those facing the light, and the shoot bends towards the light.

Abscisic acid

A plant hormone that inhibits growth, with effects that include wilting of leaves. It contrasts with growth-promoting hormones: auxins and gibberellins help stems grow, and cytokinins promote cell division in fruits and seeds.

Why do multicellular organisms use chemical communication as well as electrical impulses?

Electrical impulses reach only cells connected by nervous tissue, and a cell needs time to reset before sending another impulse. A released chemical diffuses to all cells regardless of nerve connections and acts steadily and persistently, though more slowly.

How does adrenaline prepare the body to fight or run away?

Secreted by the adrenal glands directly into the blood, it makes the heart beat faster, supplying more oxygen to the muscles. Less blood goes to the skin and digestive system and more to the skeletal muscles, and breathing speeds up.

Thyroxin

A hormone made by the thyroid gland that regulates carbohydrate, protein and fat metabolism to give the best balance for growth. Iodine is essential for making it, so iodised salt is advised; iodine deficiency can cause goitre, marked by a swollen neck.

Growth hormone

A hormone secreted by the pituitary gland that regulates growth and development of the body; its deficiency in childhood leads to dwarfism. When its level is low, the hypothalamus releases a releasing factor that stimulates the pituitary to release more.

Insulin

A hormone produced by the pancreas that regulates the blood sugar level. If it is not secreted in proper amounts, blood sugar rises and causes harmful effects, which is why some people with diabetes are given insulin injections.

How is the amount of a hormone released kept precise?

By feedback mechanisms. A rise in blood sugar is detected by pancreatic cells, which then make more insulin; as the sugar level drops, they release less.

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