Chapter 6 Solutions — Control and Coordination

A reflex action is an involuntary, rapid, and automatic response to a stimulus that does not involve conscious thinking. It is controlled by the spinal cord (reflex arc) and occurs without the involvement of the brain's higher centres.

Example: Pulling the hand away from a hot object.

Walking, on the other hand, is a voluntary action that involves conscious thought and coordination by the brain (cerebellum and cerebrum). It requires deliberate decisions about when to start, stop, change speed, or change direction.

Key differences:

- Reflex actions are involuntary; walking is voluntary.

- Reflex actions are controlled by the spinal cord; walking is controlled by the brain.

- Reflex actions are very fast (milliseconds); walking involves sustained coordinated movements.

- Reflex actions do not require prior learning; walking is a learned skill that improves with practice.

In a true reflex action, the brain does not play a direct role. The reflex arc — consisting of receptor, sensory (afferent) nerve, spinal cord, motor (efferent) nerve, and effector — handles the response entirely at the spinal cord level without waiting for brain processing.

However, the brain does play an indirect role:

- The sensory information is also relayed to the brain simultaneously. So while the reflex action is already being carried out by the spinal cord, the brain receives the information and becomes aware of it.

- For example, when you touch a hot plate, your hand pulls away immediately (reflex via spinal cord), but a moment later you consciously feel the pain and think about what happened (brain involvement).

Reflex arcs have evolved in animals because the thinking process of the brain is too slow for certain emergency situations. Nerves from all over the body converge in the brain, creating a complex network. To avoid delays, reflex arcs bypass the brain and use the shorter spinal cord pathway for rapid responses to harmful stimuli.

Auxin is a plant hormone that promotes growth.

Auxin is synthesised at the shoot tips (apical meristem) and promotes cell elongation. When light falls on one side of a plant shoot, auxin diffuses towards the shaded side. This causes the cells on the shaded side to elongate more than those on the lit side, making the shoot bend towards the light (phototropism).

Other growth-promoting plant hormones include:

- Gibberellins — promote stem elongation, seed germination, and flowering.

- Cytokinins — promote cell division, especially in the presence of auxin.