Nervous System Ex

1. For each of the following, state precisely where it is situated and how its functions are performed:
(i) a sensory neurone;
(ii) the iris;
(iii) a synapse. [10]

ANSWERS
(i) Sensory neurone is found in a nerve, and is found between a receptor and the central nervous system (brain/spinal cord);
Its function is to transmit nervous impulses (electrical signals) from the receptor to the central nervous system;

(ii) Coloured region around the pupil of the eye;
Controls amount of light entering the eye by controlling the pupil’s size;
This is to prevent damage to the retina, and is brought about by a reflex action;
An increase in light intensity would cause the circular muscles of the iris to contract and the radial muscles to relax, causing the pupil to become smaller and reduces light entering the eye;
An decrease in light intensity would cause the circular muscles of the iris to relax and the radial muscles to contract, causing the pupil to become larger and increase light entering the eye;

(iii) The synapse is the gap between 2 neurones;
Chemicals (neurotransmitters) secreted diffuse across;
To transmit nerve impulses from one neurone to another;

2. (a) What is a reflex action? [3]

(b) Nerves and hormones are both used to control processes within body. Using a reflex action illustrated by your eyes and the regulation of blood sugar by insulin as examples, show how nervous and hormonal controls
(i) resemble; and
(ii) differ from one another. [7]

ANSWERS
(a) An immediate response; to an external stimulus; without conscious control;
(b) (i) Resemblance -
Both require receptors to detect stimuli;
Both involve conduction of messages/signals;
Both bring about response that is beneficial for the body (protection / homeostasis);

*For example:
When bright light (stimulus) reaches the eye, photoreceptors on retina (receptors) sense it, impulses are sent to the brain via the sensory neurones of the optic nerve. Impulses are then sent to the iris muscles, causing a reduction in the size of the pupil (response); (need not describe the whole process of an reflex action here.)

After an intake of sugary food, digestion and absorption, there is an increase of blood glucose level (stimulus), cells in the pancreas (receptor) senses an increase in the concentration of blood, the Islet of Langerhans then secretes insulin which brings down the sugar content in the blood (response);

(b)(ii) Differences -
Aspects: Nervous Control / Hormonal Control
Message carried by: Neurones / Blood plasma
Message/signal: Electrical impulses / Chemical (Hormones)
Effector: Muscles (Iris muscles) / Liver & muscles
Response: Fast / Slower
Region: Localised (Eye only) / Wide area (Affects liver, muscles, and body cells)

3. Explain how a reflex arc helps to protect the body from unnecessary damage.
ANSWER
The reflex arc is a rapid pathway between receptor and effector. It enables a particular response to result rapidly from a particular stimulus (see examples below), without the need to involve the decision-making processes of the brain (which takes time). This is particularly valuable from protecting the body from damage, Examples are:
• cranial reflex – blinking as an object approaches the eye;
• spinal reflex – removing a hand from a hot surface.

Additional information (NOT necessary for answering the above question):
Intermediate / relaying neurones do not just complete the reflex arc, they also help to transmit the nerve impulse to the brain. The brain may therefore be ‘aware’ of the activity of the reflex arc as the response happens or slightly after it (depending on the distance involved).

The brain can modify the activity of the reflex arc by inhibiting the motor neurone so that it does not respond so easily to the incoming impulses from the sensory neurone. The brain can also ‘call’ for other responses / activities to complement that reflex action (e.g. turning the head to the hot surface after the hand have been withdrawn from it).