7 min readPathway sheetPart II (2 marks) definition; Part III (4 marks) or Part IV (7 marks) with the reflex arc diagram

Reflex action and the reflex arc

A reflex action is a quick, automatic response to a stimulus, controlled by the spinal cord without thinking. Its path is the reflex arc.

Reflex actionSudden, automatic response to a stimulus, mostly controlled by the spinal cord

Reflex arcReceptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector

ExamplesPulling the hand away from a hot object; knee jerk

The reflex arc

A message also goes up to the brain, so you feel the pain a moment after the hand moves.

What happens when you touch a hot tava

1
Receptors in the skin of your finger detect the heat.
2
A sensory neuron carries the impulse to the spinal cord.
3★
In the spinal cord, a relay (inter) neuron passes it to a motor neuron.
4
The motor neuron carries the order to the arm muscle.
5
The muscle, the effector, contracts and you pull your hand away.
6
Later the message reaches the brain, and you feel the pain.

★ key step

Voluntary vs reflex (involuntary) actions

Voluntary actionDone by choice, after thinking. Controlled by the brain (cerebrum). Slower. Examples: writing, walking, picking up a pen.

Reflex actionAutomatic, without thinking. Mostly controlled by the spinal cord; a few, like blinking and sneezing, by the brain. Very fast. Examples: knee jerk, pulling the hand from a flame.

Why reflexes matter

They protect the body from harm, like burns and cuts.
They are very fast because the spinal cord acts before the brain has time to think.
Simple reflexes need no learning; we are born with them.
Doctors tap just below the knee (knee jerk) to check that the spinal cord and nerves work.

How to draw the reflex arc

1
Draw the T.S. of the spinal cord: an oval with an H-shaped grey matter inside.
2
Draw a patch of skin on one side. Label receptor (skin).
3
From the skin, draw a neuron to the back (dorsal) side of the cord, with a swelling on the way. Label sensory neuron and dorsal root ganglion.
4
Inside the grey matter, draw a short neuron. Label relay neuron.
5
From the front (ventral) side, draw a neuron to a muscle. Label motor neuron and effector (muscle).
6
Put arrows on all the neurons to show the direction of the impulse.

In real life

When a drop of hot oil splashes from the kadai while frying vadai, your hand jerks back before you even think. In a health check, doctors tap your knee to test the knee-jerk reflex.

Exam angle

2 marks: “Define reflex action. Give an example.” A quick, automatic response to a stimulus, controlled by the spinal cord without thinking; e.g. pulling the hand away from a hot object.

2 marks: “Distinguish between voluntary and reflex actions.” Voluntary: by choice, controlled by the brain, slower, e.g. writing. Reflex: automatic, mostly controlled by the spinal cord, fast, e.g. knee jerk.

7 marks: “What is a reflex arc? Explain it with a labelled diagram, using an example.” Definition; diagram with receptor, sensory neuron, dorsal root, relay neuron, motor neuron, ventral root, effector; steps for touching a hot object; importance of reflexes.

Remember it

Receptor, Sensory, Relay, Motor, Effector: 'Really Smart Reflexes Move Early'.