Biology · Lesson 13

Nervous coordination & sense organs

JAMBWAECNECO

What the syllabus asks

JAMB: Nervous coordination: CNS and PNS, neurone, reflex action; sense organs: eye and ear structure and functions, defects of vision

WAEC: Co-ordination: nervous system (central and peripheral, neurones, synapse, reflex and voluntary actions, reflex arc), brain and spinal cord; sense organs: eye (structure, image formation, accommodation, defects and correction), ear (hearing and balance), skin, nose and tongue

NECO: Co-ordination: nervous system (central and peripheral, neurones, synapse, reflex and voluntary actions, reflex arc), brain and spinal cord; sense organs: eye (structure, image formation, accommodation, defects and correction), ear (hearing and balance), skin, nose and tongue

What you will learn

Describe the organisation of the nervous system.
Describe the structure and function of sensory, relay and motor neurones and of the synapse.
Explain reflex actions with a labelled reflex arc and compare them with voluntary actions.
State the functions of the main parts of the brain.
Describe the structure of the eye, image formation, accommodation and control of pupil size.
Explain eye defects and their correction.
Describe the ear and explain hearing and balance; outline skin, nose and tongue as sense organs.

Key points

Coordination: stimulus → receptor → coordinator → effector → response.
CNS = brain + spinal cord; PNS = cranial (12 pairs) and spinal (31 pairs) nerves.
Sensory neurones to CNS, relay within CNS, motor neurones to effectors.
Synapses pass impulses in one direction by neurotransmitter diffusion.
Reflex: fast, automatic, inborn; reflex arc through the spinal cord. Voluntary actions are controlled by the cerebrum.
Cerebrum: thought; cerebellum: balance; medulla: heartbeat and breathing; hypothalamus: temperature.
Accommodation: near — ciliary contract, ligaments slack, lens thick; far — the reverse.
Myopia: concave lens; long sight: convex lens.
Ear: cochlea hears; semicircular canals balance; Eustachian tube equalises pressure.

Start here: Why organisms need coordination

Imagine a danfo driver in Lagos traffic. His eyes see a child step into the road, his brain decides, and within a fraction of a second his foot hits the brake. Seeing, deciding and acting must be linked quickly. This linking of body activities is coordination.

Animals have two coordinating systems:

The nervous system: fast electrical impulses along nerves, short-lived, precise, aimed at particular muscles or glands.
The endocrine system: chemical hormones carried in blood, slower, longer-lasting and widespread (the lesson “Hormones”).

Every response follows the same chain: stimulus → receptor → coordinator → effector → response. A stimulus is any change, such as light, sound, heat, pressure or a chemical. Receptors detect it. The coordinator (brain or spinal cord) processes the information. Effectors (muscles or glands) carry out the response.

Worked example 13.1

Your mouth waters when you smell suya. Identify the stimulus, receptor, coordinator and effector.

Stimulus: chemicals in the smell of suya.
Receptor: smell receptors in the nose.
Coordinator: brain.
Effector: salivary glands, which secrete saliva (the response).

Smell → nose → brain → salivary glands

Try it yourself. Give two differences between nervous and hormonal coordination.

Show solution

Nervous: electrical, fast, short-lived, localised. Hormonal: chemical, slow, long-lasting, widespread.

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Practice questions

1. The part of the brain that controls balance is the

cerebellum
cerebrum
medulla oblongata
hypothalamus

Show answer

A: cerebellum
Cerebellum = balance and coordination; medulla = breathing and heartbeat.

2. A reflex action is controlled by the

cerebrum
cerebellum
thalamus
spinal cord

Show answer

D: spinal cord
Simple reflexes (like pulling your hand off a hot pot) pass through the spinal cord.

3. The part of the eye that controls the amount of light entering is the

lens
cornea
iris
retina

Show answer

C: iris
The iris changes pupil size.

4. Short-sightedness is corrected using a

cylindrical lens
diverging (concave) lens
converging lens
bifocal lens

Show answer

B: diverging (concave) lens
In myopia the image forms in front of the retina; a diverging lens moves it back.

5. The basic unit of the nervous system is the

neurone
nephron
alveolus
synapse

Show answer

A: neurone
A nerve cell.

Key words

Stimulus: A change in the environment that an organism detects.
Receptor: A cell or organ that detects a stimulus.
Effector: A muscle or gland that carries out a response.
Neurone: A nerve cell that carries impulses.
Axon: Long fibre carrying impulses away from the cell body.
Dendrite: Short branch carrying impulses towards the cell body.
Myelin sheath: Fatty insulating layer around many axons that speeds impulses.
Synapse: The junction between two neurones.
Neurotransmitter: Chemical released at a synapse, e.g. acetylcholine.
Reflex action: A rapid automatic response to a stimulus.
Conditioned reflex: A reflex learned through experience.
Accommodation: Adjustment of the lens to focus near or distant objects.

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