6.1.5 Reflex Arcs

Key Features of Reflex Arcs

• Reflexes are fast and involuntary, providing a protective function.
• They involve a specific pathway of neurones, avoiding the brain to save time.

Pathway of a Reflex Arc

1. Stimulus: A change in the environment (e.g., a sharp object touching the skin).
2. Receptor: Detects the stimulus and generates a nerve impulse.
3. Sensory Neurone: Transmits the impulse from the receptor to the spinal cord.
4. Intermediate (Relay) Neurone: Found in the spinal cord, relays the impulse from the sensory neurone to the motor neurone.
5. Motor Neurone: Transmits the impulse from the spinal cord to the effector.
6. Effector: A muscle or gland that produces a response (e.g., moving a hand away from the sharp object).
7. Response: The action taken to avoid harm.

Example of a Reflex Arc:

• Stimulus: Heat from a hot surface.
• Receptor: Temperature receptors in the skin.
• Sensory Neurone: Carries the signal to the spinal cord.
• Relay Neurone: Connects the sensory and motor neurones in the spinal cord.
• Motor Neurone: Sends the signal to the muscles in the hand.
• Effector: Muscles in the hand contract.
• Response: Hand is withdrawn from the hot surface.

Key Terms

• Stimulus: A detectable change in the environment.
• Receptor: A specialised cell or organ that detects stimuli.
• Effector: A muscle or gland that produces a response.