(a) Fig. 7.1 shows the reflex arc that is involved when someone steps on a pin.
(i) The skin is a sense organ and can detect the pin.
State the type of stimulus that the cells in the skin are responding to.
(ii) Describe the reflex action shown in Fig. 7.1.
(b) The response of the pupil to changes in light intensity is also an example of a reflex action.
(i) Fig. 7.2 shows the size of the pupil in dim light.
On Fig. 7.3, draw the expected appearance of the pupil in bright light.
(ii) State the name of the structure labelled A in Fig. 7.2.
(iii) State the name of the tissue in the eye that detects light.
▶️ Answer/Explanation
(a)(i) Answer: Touch (pressure/mechanical stimulus)
The skin’s sensory receptors detect mechanical pressure from the pin, converting it into an electrical impulse for the nervous system.
(a)(ii) Answer:
The reflex arc involves:
1. Receptors in the skin detect the pin.
2. Sensory neuron (X) carries the impulse to the spinal cord.
3. Relay neuron (Z) processes the signal.
4. Motor neuron (Y) sends the impulse to the leg muscles (effectors), causing them to contract and withdraw the foot.
(b)(i) Answer: Draw a smaller pupil in Fig. 7.3
In bright light, the pupil constricts (becomes smaller) to reduce light entry and protect the retina.
(b)(ii) Answer: Iris
Structure A is the iris, which controls pupil size via smooth muscles.
(b)(iii) Answer: Retina
The retina contains photoreceptors (rods and cones) that detect light and convert it into neural signals.
(a) Fig. 8.1 is a diagram representing a reflex action.
When the knee is tapped with a small rubber hammer, the leg will immediately straighten.
(i) Identify the parts labelled X and Y in Fig. 8.1.
(ii) State the name of the effector in the example shown in Fig. 8.1.
(iii) Describe the stimulus in the example shown in Fig. 8.1.
(iv) State two features of reflex actions.
(b) The shortest neurones in the human body are 0.0004mm.
The longest neurones are 1.5m.
Calculate how many times longer the longest neurones are than the shortest.
(c) State the name given to the junction between neurones.
▶️ Answer/Explanation
(a)(i) X – spinal cord
Y – motor neurone
(a)(ii) (leg) muscle
(a)(iii) hammer tapping (knee / leg)
(a)(iv) It is rapid / immediate
It doesn’t require conscious thought / is automatic
e.g. protective / may not involve the brain
(b) \(3.75 \times 10^6\)
Solution: To find how many times longer the longest neurones (1.5 m) are than the shortest (0.0004 mm), first convert both to the same unit (meters). 0.0004 mm = \(4 \times 10^{-7}\) m. Now, divide the longest by the shortest: \(\frac{1.5}{4 \times 10^{-7}} = 3.75 \times 10^6\).
(c) synapse / synaptic gap / synaptic cleft