Control &
Coordination

The neuron is the unit of the nervous system. Dendrites receive the impulse, the cell body and axon carry it onward, and it crosses to the next neuron over a tiny gap — the synapse — usually via chemicals. Nerves run from the brain and spinal cord to the whole body.

A reflex action is a rapid, automatic response. To save time, it is handled by the spinal cord rather than the brain. The pathway it follows is the reflex arc. Walk through it:

Try the interactive at the top of the page — step through the reflex arc from stimulus to response.

• Cerebrum (forebrain) — thinking, memory, voluntary actions, the senses.
• Cerebellum (hindbrain) — balance, posture and precise movement.
• Medulla (hindbrain) — involuntary actions: heartbeat, breathing, blood pressure.

The brain and spinal cord (the central nervous system) are protected by bone (skull, vertebral column) and cushioning cerebrospinal fluid.

Plants have no nerves, but they respond with directional growth called tropisms, controlled by plant hormones (auxin, gibberellin, cytokinin, abscisic acid). Pick a stimulus:

Endocrine glands release hormones into the blood for slower, longer-lasting control. Key ones:

• Pituitary — the "master gland" (e.g. growth hormone).
• Thyroid — thyroxine (needs iodine; deficiency → goitre).
• Pancreas — insulin (controls blood sugar; deficiency → diabetes).
• Adrenal — adrenaline (the "fight or flight" hormone).