The Nervous System and Reflexes

Homeostasis

Keeping internal conditions (temperature, blood glucose, water/ion levels) constant so cells and enzymes work properly. Uses receptors, a coordination centre (brain/spinal cord), and effectors (muscles/glands).

The nervous system pathway

stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response

The CNS = brain + spinal cord.

Synapses

Junctions between neurones. The signal crosses as chemicals (neurotransmitters) that diffuse across the gap and trigger a new electrical impulse in the next neurone.

Reflex actions

Rapid, automatic responses that protect the body (e.g. pulling your hand off something hot).

• They bypass the conscious brain, so they're faster.
• Pathway = the reflex arc: receptor → sensory → relay (in spinal cord) → motor → effector.

Exam tip

A reflex is fast because it does not involve the conscious brain. Learn the reflex arc order and that synapses transmit signals using chemicals.