The Nervous System and Sense Organs — ICSE Class 10 Biology Important Questions

Answer: (b) Neuron.

The neuron (nerve cell) is the structural and functional unit of the nervous system; it receives and transmits nerve impulses.

Answer: (b) Cerebellum.

The cerebellum maintains body balance, posture and the coordination of voluntary muscular movements.

Answer: (c) Retina.

The lens focuses light to form a real, inverted image on the retina, which contains the light-sensitive rods and cones that send impulses to the brain via the optic nerve.

Answer: (b) The reflex arc through the spinal cord produces the response before the impulse reaches the brain.

The withdrawal is a spinal reflex: the impulse travels a short arc (receptor to spinal cord to muscle) causing an instant response, while the sensation of pain reaches the brain a moment later.

Answer: (a) At a synapse the chemical transmitter is released only from the axon ending and received only by the next neuron's dendrite, so transmission is one-way. R correctly explains A.

A reflex action is a rapid, automatic and involuntary response to a stimulus, controlled mainly by the spinal cord without the involvement of the will.

Example: the immediate withdrawal of the hand on touching a hot object (or blinking of the eye when an object suddenly approaches it).

(i) Sensory (afferent) neuron: carries nerve impulses from the receptor (sense organ) towards the central nervous system (brain or spinal cord).

(ii) Motor (efferent) neuron: carries nerve impulses from the central nervous system to the effector (a muscle or gland), which then responds.

Three main parts of a neuron:

1. Dendrites (with the cell body / cyton): the branched processes and the cell body receive impulses; the cell body contains the nucleus and controls the cell's activities.

2. Axon: a long fibre that carries the nerve impulse away from the cell body towards the next neuron or the effector.

3. Myelin sheath and nerve endings (axon terminals): the myelin sheath insulates the axon and speeds up conduction, while the terminals pass the impulse on across a synapse to a muscle or the next neuron.

1. Cerebrum: the largest part; the centre of intelligence, memory, will, reasoning and voluntary actions, and it interprets sensations.

2. Cerebellum: maintains balance and posture and coordinates voluntary muscular movements.

3. Medulla oblongata: controls involuntary activities such as heartbeat, breathing, swallowing and blood pressure.

A synapse is the minute gap or junction between the axon terminal of one neuron and the dendrite of the next neuron (or an effector).

Transmission:

1. When the impulse reaches the axon terminal, it triggers the release of a chemical neurotransmitter into the synaptic gap.
2. The neurotransmitter diffuses across the gap and stimulates the dendrite of the next neuron, starting a fresh impulse in it.
3. Because the chemical is released only from the axon side, transmission across a synapse is one-way.

A reflex arc is the shortest nervous pathway that produces a reflex action. When a pin pricks the finger, the impulse follows this path:

1. Receptor: the pain receptors in the skin of the finger detect the prick (stimulus).

2. Sensory (afferent) neuron: carries the impulse from the receptor to the spinal cord, entering through the dorsal root.

3. Relay neuron (in the spinal cord): connects the sensory neuron to the motor neuron within the grey matter of the spinal cord.

4. Motor (efferent) neuron: carries the impulse out of the spinal cord (ventral root) to the effector.

5. Effector: the muscle of the arm contracts and the hand is withdrawn instantly.

Because the response is completed at the level of the spinal cord, it is very fast and involuntary; the impulse also travels up to the brain, so the pain is felt a moment later.

Functions of the parts of the eye:

1. Cornea: the transparent front part that allows light to enter and does most of the bending (refraction) of light.

2. Iris: the coloured, muscular ring that controls the size of the pupil, thus regulating the amount of light entering the eye (small pupil in bright light, large pupil in dim light).

3. Lens: a transparent, biconvex, elastic structure that fine-focuses light to form a sharp, real, inverted image on the retina; its curvature can be changed by the ciliary muscles.

4. Retina: the innermost light-sensitive layer containing rods (dim-light and black-and-white vision) and cones (bright-light and colour vision); it converts light into nerve impulses carried to the brain by the optic nerve.

Accommodation: the ability of the eye to adjust the focal length of its lens so that objects at different distances are focused sharply on the retina. For a near object the ciliary muscles contract and the lens becomes more convex (thicker); for a distant object they relax and the lens becomes thinner.

(i) The iris alters the size of the pupil. On entering the dark hall the radial muscles of the iris contract and the pupil dilates (widens) to let in more light.

(ii) The two cells are rods and cones. Rods function in dim light (giving black-and-white vision), while cones work in bright light and give colour vision. As the rods become sensitive, the boy begins to see in the dark.

(iii) The change in pupil size is involuntary — it is a reflex action (the pupillary or light reflex).

(iv) The image must fall on the yellow spot (fovea centralis / macula lutea), the region of the retina with the greatest density of cones, for the sharpest and clearest vision.