Reflex arcs and reflex actions – Response and Coordination (Senior 3 Biology) | Ganzaa

Response and Coordination

Lessons

Senior 3 · Biology

Response and Coordination

4 lessons

Lesson 3 of 4

Reflex arcs and reflex actions

25 min

Faster than thinking

You reach for the handle of a saucepan on the imbabura (charcoal stove) and touch the hot rim by mistake. Your hand jumps away before you have had time to think "that is hot"; the pain arrives a fraction of a second later. The movement was not decided by your cerebrum at all. It was a reflex action, wired through your spinal cord, and it is the reason the burn is small.

Definition

Reflex action: a rapid, automatic response to a stimulus that happens without conscious thought. It is involuntary: you do not choose it. Example: pulling your hand away from a hot object.

Reflex arc: the pathway of neurones that an impulse follows in a reflex action: receptor → sensory neurone → relay neurone → motor neurone → effector.

Voluntary action: an action that you decide to make, controlled by the cerebrum, such as picking up a pen.

Conditioned reflex: a reflex that has been learnt, because a new stimulus has repeatedly come together with the stimulus that normally causes the reflex. Example: a dog that salivates at the sound of a bell that has often rung just before it was fed.

Following the impulse round the arc

Follow Figure 1 step by step:

Stimulus: heat from the pan.
Receptor: pain and temperature receptors in the skin of the fingertip start an impulse.
Sensory neurone: carries the impulse to the spinal cord. It enters through the dorsal root; its cell body lies in the swelling on that root, the dorsal root ganglion.
Relay neurone: in the grey matter of the spinal cord, the impulse crosses a synapse to a relay neurone, and then a second synapse to a motor neurone.
Motor neurone: its cell body is in the grey matter; its axon leaves through the ventral root and runs to the arm.
Effector: the biceps muscle contracts.
Response: the arm bends at the elbow and the hand is pulled away.

At the same time, the relay neurone passes impulses up the spinal cord to the brain, which is why you feel the pain and know what happened, just after the hand has moved. The brain can also override some reflexes: you can stop yourself dropping a hot plate of food if you know there is no safe place to drop it.

Figure 1: the reflex arc for pulling a hand away from a hot pan

Figure 1: not to scale. Arrowheads on the neurones show the direction of the impulse. In the body the dorsal and ventral roots join to form one spinal nerve; they are drawn apart here for clarity.

Grey matter and white matter

In the spinal cord, the grey matter, shaped like a butterfly in cross-section, contains the cell bodies of relay and motor neurones and the synapses between them. The white matter round it is made of myelinated fibres running up and down the cord; myelin is fatty, which makes it look white. In the cerebrum the arrangement is the other way round: grey matter on the outside, white matter inside.

Examples of reflexes

Reflex	Stimulus	Response	Coordinated by	How it protects or helps you
Withdrawal reflex	Heat or a sharp point on the skin	Limb pulled away	Spinal cord	Limits burns and cuts
Knee-jerk reflex	A tap on the tendon below the kneecap	Lower leg kicks forward	Spinal cord	Part of keeping the body upright; doctors use it to test the nerves
Blinking	An object approaching or touching the eye	Eyelids close	Brain (cranial reflex)	Protects the cornea
Pupil reflex	Bright light entering the eye	Pupil becomes smaller	Brain (cranial reflex)	Protects the retina (Unit 10)
Coughing and sneezing	Dust or irritants in the airways or nose	Sudden blast of air out	Medulla (cranial reflex)	Clears the airways

The knee-jerk is the simplest reflex arc of all: it has no relay neurone, so the sensory neurone synapses directly with the motor neurone. Reflexes coordinated by the spinal cord are called spinal reflexes; those coordinated by the brain are cranial reflexes.

Reflex and voluntary actions compared

Feature	Reflex action	Voluntary action
Conscious thought	Not needed; automatic	Needed; you decide
Coordinated by	Spinal cord or lower parts of the brain	Cerebrum
Speed	Very fast	Slower
Response to the same stimulus	The same every time	Can vary; you may choose differently
Learnt?	Inborn (except conditioned reflexes)	Often learnt, e.g. writing
Example	Blinking when dust blows into the eye	Catching a ball in a game

Practical: test your knee-jerk reflex and your reaction time

Knee-jerk. Work in pairs. Your partner sits on a high table or crosses one leg over the other so the lower leg hangs freely. Tap gently just below the kneecap with the side of your hand. Record what happens, and whether your partner can stop it.

Reaction time. Hold a 30 cm ruler vertically, with its zero mark level with your partner's open thumb and finger. Drop it without warning; your partner catches it as soon as possible. The distance it falls, $d$ , gives the reaction time $t$ , because a falling object obeys $d = \frac{1}{2} g t^2$ , so $t = \sqrt{\frac{2d}{g}}$ , with $g = 9.8\ \text{m s}^{-2}$ . Repeat five times and take the mean. Catching a ruler is a voluntary action (you see it fall and decide to grip), so it is slower than a reflex.

Warning

Safety. Tap the knee gently, with the side of the hand or a soft rubber hammer, never with a ruler's edge or anything hard, and never on the kneecap itself. Do not test anyone who has a knee injury or pain. In the ruler test, drop the ruler from a small height only, keep faces well away from it, and never use a metal ruler with a sharp edge. Never test reflexes with heat, a flame or a sharp object.

Worked example 1: reaction time from a ruler

In five trials the ruler fell 0.19 m, 0.21 m, 0.20 m, 0.22 m and 0.18 m before it was caught. Find the reaction time.

Mean distance: $\frac{0.19 + 0.21 + 0.20 + 0.22 + 0.18}{5} = \frac{1.00}{5} = 0.20\ \text{m}$ .
Reaction time: $t = \sqrt{\frac{2 \times 0.20}{9.8}} = \sqrt{0.0408} \approx 0.20\ \text{s}$ .
Interpret: about a fifth of a second. This is a voluntary response: the impulse must go from the eyes to the brain, be processed by the cerebrum, and then go down to the hand. A spinal reflex such as the knee-jerk takes typically only a few hundredths of a second.

Worked example 2: where is the damage?

After an accident, a patient can feel a pin prick on her right hand, but cannot pull the hand away, and she has no withdrawal reflex in that hand. Which part of the reflex arc is most likely to be damaged, and why?

She feels the pin: so the receptors, the sensory neurone and the route up to the brain are working.
She cannot move the hand, either voluntarily or by reflex: both kinds of action use the same final pathway, the motor neurone to the muscle.
Conclusion: the motor neurones to the hand (in the ventral root or the nerve to the arm), or the muscle itself, are damaged. If instead the sensory neurone were damaged, she would feel nothing and have no reflex, but could still move the hand when she chose to.

Warning

Common mistakes. (1) Writing that the brain decides a reflex: a spinal reflex is coordinated in the spinal cord; the brain is only informed. (2) Drawing the arrows on a reflex arc the wrong way: the impulse goes in through the dorsal root and out through the ventral root. (3) Putting the sensory neurone's cell body inside the spinal cord: it is in the dorsal root ganglion, outside the grey matter. (4) Forgetting the response itself: an answer must name the effector and what it does, e.g. "the biceps contracts and the hand is pulled away".

Reflex action (and reflex arc), Khan Academy India. Watch for the path from receptor to effector and why the spinal cord, not the brain, coordinates it; compare it with Figure 1.

Tip

Memory hook: "Dorsal in, Ventral out" (DIVO). Sensory impulses come in through the dorsal root (the side nearer your back); motor impulses go out through the ventral root (the side nearer your front).

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Practice: reflex actions

Define a reflex action and give two examples.
Put these in order for a withdrawal reflex: motor neurone, receptor, effector, relay neurone, sensory neurone.
Where is the cell body of (a) a sensory neurone, (b) a motor neurone, in a spinal reflex?
Why do you feel pain only after your hand has already moved away from a hot object?
A ruler falls 0.15 m before it is caught. Calculate the reaction time ( $g = 9.8\ \text{m s}^{-2}$ ).
Give three differences between a reflex action and a voluntary action.
Explain why the knee-jerk reflex is faster than the withdrawal reflex.
A man has a damaged dorsal root in the nerve supplying his left foot. Predict whether he can (a) feel a pin prick on the foot, (b) show a withdrawal reflex, (c) move the foot when he chooses. Explain.

Show answersHide answers

A rapid, automatic response to a stimulus without conscious thought; e.g. withdrawal from a hot object, blinking, the knee-jerk, coughing.
Receptor → sensory neurone → relay neurone → motor neurone → effector.
(a) In the dorsal root ganglion, just outside the spinal cord; (b) in the grey matter of the spinal cord.
The reflex is coordinated in the spinal cord by the shortest pathway; the impulses that reach the brain and give the feeling of pain take a longer route.
$t = \sqrt{\frac{2 \times 0.15}{9.8}} = \sqrt{0.0306} \approx 0.17\ \text{s}$ .
Any three: a reflex needs no conscious thought, a voluntary action does; a reflex is coordinated by the spinal cord or lower brain, a voluntary action by the cerebrum; a reflex is faster; a reflex is the same each time; most reflexes are inborn.
It has no relay neurone, so there is only one synapse; each synapse adds a short delay, so fewer synapses make a faster response.
(a) No: sensory impulses from the foot cannot reach the CNS. (b) No: the arc is broken at the sensory neurone. (c) Yes: the motor neurones in the ventral root still work, so the brain can still send impulses to the muscles.

Key points

A reflex is rapid, automatic and involuntary; it protects the body.
Reflex arc: receptor → sensory neurone (in through the dorsal root) → relay neurone (grey matter) → motor neurone (out through the ventral root) → effector.
Spinal reflexes are coordinated by the spinal cord; the brain is informed afterwards. Cranial reflexes, such as blinking, by the brain.
Reaction time from a dropped ruler: $t = \sqrt{\frac{2d}{g}}$ .

Sources and credits

Image: The knee jerk, Wellcome Collection, Wikimedia Commons, CC BY 4.0.
Video: Reflex action (and reflex arc), Khan Academy India, embedded from YouTube.

Written by Ganzaa: the text, Figure 1, the tables, the practical, the worked examples and the practice questions.

1Faster than thinking

2Following the impulse round the arc

3Figure 1: the reflex arc for pulling a hand away from a hot pan

4Grey matter and white matter