Given alongside is a partial diagrammatic representation of a certain phenomenon pertaining to the nervous system.

(i) Name the parts numbered 1-7

(ii) What phenomenon does the diagram represent? ____________

(iii) Name the parts not shown in the diagram that should be included to complete the pathway of the phenomenon. ______________

(i) 1. White matter

Explanation: In the spinal cord, white matter is present on the outer side. In the diagram of reflex arc via the spinal cord, the outer portion is the white matter.

2. Spinal cord

Explanation: The diagram depicts the route of reflex action which takes place in the spinal cord. In the diagram, the organ contains a central canal which confirms this to be the spinal cord.

3. Dorsal root

Explanation: The nerve impulse passes through an axon terminal branch of the sensory neuron present in the dorsal root of the spinal cord.

4. Association neuron

Explanation: The are neurons that connect the sensory and motor neurons. Hence, also known as connecting neurons.

5. Ventral root

Explanation: Nerve impulse generated by association neuron is carried to effector organ via motor neuron present in the ventral root of the spinal cord.

6. Gray matter

Explanation: The gray matte containing cell bodies of motor and association neurons lies on the inner side of the spinal cord.

7. Central canal

Explanation: The small central canal is present in the center of the spinal cord which runs the entire length and is continuous with the cavities of the brain.

(ii) The diagram represents the spinal reflex action.

Explanation: As seen in the diagram, when the sensory nerve ending is stimulated, a nerve impulse travels along a sensory (afferent) neuron to the spinal cord. The association neuron transfers the impulse to a motor (efferent) neuron. The motor neuron carries the impulse to a muscle, which contracts and moves a body part.

(iii) Parts that can be included:

1. Sensory neuron

Explanation: Contain sensory fibres that carry impulses from the receptor to the spinal cord.

2. Motor neuron

Explanation: Contain motor fibres that carry impulses from spinal cord to the effector.

3. Dorsal root ganglion

Explanation: The cell bodies of sensory nerves that are outside the spinal cord form a swollen mass of cells called a dorsal root ganglion.

4. Receptor

Explanation: A sense organ in the skin, muscle, or another organ that receives that signal. Hence, it is also known as the stimulus.

5. Effector

Explanation: This structure by which animal respond, such as muscles, glands, etc. These express the signal that was interpreted by the central nervous system.