Reflex arc

Where it happens

A reflex arc operates in the nervous system whenever the body must react to a stimulus too quickly for conscious thought, such as pulling a hand away from a sharp pin or blinking when an object approaches the eye.

The spinal cord is the coordinating centre for most reflexes tested at SPM level. In cross-section it shows an H-shaped core of grey matter, made of cell bodies, surrounded by white matter, made of myelinated axons.

The sensory neurone enters through the dorsal root, and its cell body sits in the dorsal root ganglion outside the cord. The motor neurone leaves through the ventral root, with its cell body inside the grey matter.

Reflexes of the head, such as blinking or pupil constriction, use the brain stem instead of the spinal cord but follow the same five-part plan.

Inputs and outputs

• Input: a stimulus, a change in the environment such as heat, pressure, a sharp object or a bright light.
• Input: a receptor that converts the stimulus into a nerve impulse, for example a pain receptor in the skin.
• Input: neurotransmitter released at each synapse to carry the signal across the gap.
• Output: a nerve impulse delivered by the motor neurone to the effector.
• Output: a response by the effector, a muscle contracting or a gland secreting.
• Output: a separate impulse sent up the spinal cord to the brain, so that the pain is felt shortly after the hand has moved.

The steps

1. A stimulus, such as the heat of a hot pan, is detected by receptors in the skin of the finger.
2. The receptor generates a nerve impulse in the sensory neurone.
3. The sensory neurone carries the impulse along its axon into the spinal cord through the dorsal root.
4. At a synapse in the grey matter, neurotransmitter passes the impulse to a relay neurone.
5. The relay neurone passes the impulse across a second synapse to a motor neurone.
6. The motor neurone carries the impulse out of the spinal cord through the ventral root to the effector.
7. The effector, the biceps muscle, contracts and pulls the hand away; at the same time the relay neurone sends an impulse up the spinal cord to the brain, so the person becomes aware of the pain afterwards.

Why it matters and how it is controlled

Because the relay neurone lies in the spinal cord, the impulse does not need to travel to the brain before a response happens. This makes a reflex much faster than a voluntary action and protects the body from harm, such as heat or a sharp object, before the pain is even consciously felt.

Reflexes are involuntary, but the brain can still influence some of them. A person who knows a hot plate is coming may hold on to it briefly because impulses from the brain travel down the spinal cord and inhibit the motor neurone.

Control also depends on the synapses: each synapse adds a short delay, so the simplest reflexes with the fewest synapses are the fastest. Speed is further raised by the myelin sheath on the sensory and motor neurones, which lets the impulse jump between nodes of Ranvier instead of travelling the whole length of the membrane.

How it is examined

You may be asked to name the five components of a reflex arc in the correct order, to explain why a reflex is faster than a voluntary response, or to label a diagram of the spinal cord showing the pathway of a spinal reflex.

A typical structured question gives a cross-section of the spinal cord with three neurones drawn in. You may be asked to identify which is sensory and which is motor from the position of the cell body, to draw arrows showing the direction of impulses, and to mark where the synapses lie.

Essay questions compare a reflex with a voluntary action or ask how the response protects the body. Expect the term sequence receptor, sensory neurone, relay neurone, motor neurone, effector to appear in the mark scheme in that order.

Common misconceptions

Worked exam-style question

Question. A boy steps barefoot on a sharp stone and lifts his foot at once. A moment later he feels the pain.

(a) Name the type of response shown when the foot is lifted. (b) Describe the pathway taken by the nerve impulse from the sole of the foot to the muscle that lifts the foot.

(c) Explain why the boy feels the pain only after the foot has moved. (d) Give one reason why this type of response is useful to the body.

Model answer. (a) A reflex action. (b) Pain receptors in the skin of the sole detect the stimulus and generate an impulse in the sensory neurone, which enters the spinal cord through the dorsal root.

The impulse crosses a synapse to a relay neurone in the grey matter, then crosses a second synapse to a motor neurone, which leaves through the ventral root and carries the impulse to the effector, the leg muscle, which contracts and lifts the foot. (c) The reflex is completed in the spinal cord without involving the brain; a separate impulse travels up the spinal cord to the brain, and this longer journey means the feeling of pain arrives after the muscle has already responded.

(d) It is rapid and automatic, so the body is protected from injury before conscious thought would allow.

Source:SRC-DSKP-EN

Frequently asked questions