Structure & Function of Reflex Arcs

The sequence of a reflex arc always follows this specific order:

1. Stimulus: A change in the environment (e.g., the sharp point of a pin).
2. Receptor: Cells that detect the stimulus (e.g., pain receptors in the skin).
3. Sensory neurone: A long neurone that carries electrical impulses from the receptor to the central nervous system (CNS).
4. Relay neurone: A short neurone located entirely within the CNS (specifically the spinal cord or the unconscious part of the brain) that connects sensory and motor neurones.
5. Motor neurone: A long neurone that carries electrical impulses from the CNS out to the rest of the body.
6. Effector: The organ (muscle or gland) that brings about the response.
7. Response: The physical action taken (e.g., the bicep muscle contracts to pull the hand away).

Why are Reflexes so Fast?

• Why does a reflex happen faster than a normal reaction? The secret is a biological shortcut.
• Reflexes are fast because they bypass the conscious part of the brain (the cerebral cortex).
• This creates a shorter pathway with fewer junctions, reducing the total response time.
• We can calculate how fast these impulses travel using the standard speed equation: $\text{Speed (m/s)} = \frac{\text{Distance (m)}}{\text{Time (s)}}$.
• For example, if an impulse travels $1.6$ m at a speed of $100$ m/s, the time taken (excluding junction delays) is $1.6 \div 100 = 0.016$ seconds.

The primary purpose of this rapid response is protective. By reacting instantly, the body minimises or prevents severe tissue damage. While the reflex is automatic, an electrical impulse is simultaneously sent to the conscious brain, which is why you only feel the pain after you have already pulled your hand away.

Crossing the Gap: Synaptic Transmission

• Nerve cells in your body never actually touch each other.
• A synapse is a tiny physical gap between two neurones.
• Because electrical impulses cannot jump across empty space, the signal must be converted into a chemical messenger called a neurotransmitter.

This chemical mechanism is highly specific and happens in six steps:

1. An electrical impulse reaches the end of the first (presynaptic) neurone.
2. This triggers the release of neurotransmitters from small storage sacs called vesicles.
3. The neurotransmitters are released into the synaptic gap.
4. The neurotransmitters move across the gap by diffusion (moving from a high concentration to a low concentration).
5. The neurotransmitters bind to specific receptor molecules on the membrane of the second (postsynaptic) neurone, fitting together like a lock and key.
6. This binding triggers a new electrical impulse to fire in the second neurone.

Synapses ensure impulses only flow in one direction because vesicles are only found at the end of the first neurone, and receptor molecules are only found on the start of the second neurone. However, chemical diffusion is the slowest part of the reflex arc, which is why reflex pathways have as few synapses as possible.

Effectors and Responses

• An effector is the final destination in the reflex arc that carries out the action.
• In the human body, effectors are strictly limited to two categories: muscles or glands.
• If the effector is a muscle, the response is always contraction (e.g., circular muscles in the iris contracting to shrink the pupil in bright light).
• If the effector is a gland, the response is always secretion (e.g., salivary glands secreting saliva when you smell food, or the adrenal gland secreting hormones).