Reflexes, receptors & neural responses

The big idea: A reflex is a fast, automatic response to a stimulus that happens without conscious thought.

When you touch something sharp, your hand pulls back before you even feel the pain.

This is possible because the signal travels along a fixed nerve pathway called a reflex arc, which routes the message through the spinal cord instead of waiting for the brain to decide.

Reflexes protect the body from harm and keep it working smoothly.

Reflex

A fast, automatic response to a stimulus that does not need conscious thought.

Reflex arc

The fixed nerve pathway a reflex follows: stimulus -> receptor -> sensory neuron -> relay neuron (CNS) -> motor neuron -> effector -> response.

Stimulus

A change in the environment that is detected by the body (e.g. a pin prick, bright light, heat).

Receptor

A sensory cell or sensory nerve ending that detects a stimulus and starts a nerve impulse.

Effector

A muscle or gland that carries out the response (e.g. a muscle that contracts to move the body).

Relay neuron (interneuron)

A short neuron inside the CNS that connects the sensory neuron to the motor neuron.

Why reflexes are fast: A reflex is quick because the signal takes a short cut through the spinal cord and does not have to travel up to the brain and back.

Skipping the brain means the response happens in a fraction of a second — exactly what you need to pull away from danger.

Every reflex follows the same ordered pathway. Learn the seven steps in order and you can answer almost any reflex-arc question.

The trick is to remember that the signal goes IN to the CNS along a sensory neuron and OUT from the CNS along a motor neuron — the relay neuron is the bridge between them, inside the spinal cord.

Where the synapses are: The neurons of a reflex arc do not touch — they meet at synapses.

The synapse between the sensory neuron and the relay neuron, and the one between the relay neuron and the motor neuron, both sit inside the CNS (in the grey matter of the spinal cord).

This is a favourite 1-mark exam point: synapses in a reflex arc are located in the central nervous system, not out in the skin or the muscle.

Receptors are not all the same. Different sensory receptor types are tuned to detect different kinds of stimulus — which is why the mouth can sense the temperature, texture and taste of food all at once.

Sensory receptor types: Each receptor type responds to one kind of stimulus:

Mechanoreceptors detect touch, pressure and texture.

Thermoreceptors detect temperature (hot and cold).

Chemoreceptors detect chemicals — this is how taste and smell work.

Photoreceptors detect light.

Because the mouth and lips contain several of these types together, you can feel that food is warm, soft and sweet all at the same moment.

A memory hook: Sensory = Sending in; Motor = Moving out.

And for the order of the arc: 'Some Run Slowly, Running Mostly Empty, Resting' — Stimulus, Receptor, Sensory, Relay, Motor, Effector, Response.

How this is tested: On Paper 3 a 3-mark Outline question can ask how a named reflex (such as the pupil reflex to bright light) produces a fast response — give the pathway in order, naming the receptor, the CNS and the effector.

Also on Paper 3, a 4-mark Explain question asks how different sensory receptor types detect different features of food (mechanoreceptors, thermoreceptors, chemoreceptors).

On Paper 1A, single-mark questions ask you to identify the receptor in a pain reflex (a sensory nerve ending), the effector (a muscle), or where the synapses between neurons sit (in the CNS / spinal cord).

✓ Why this scores full marks: It names a receptor (photoreceptor), routes the signal through the CNS, and names the effector and what it does (iris muscle -> smaller pupil).

A 3-mark 'outline' needs three distinct stages of the pathway, not the word 'reflex' repeated.