Synapses and Reflex Arcs

This lesson covers how nerve impulses cross the gap between neurones (synapses) and how reflex arcs produce fast, automatic responses. This is essential content for Edexcel GCSE Biology (1BI0) Topic 2: Cells and Control.

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Synapses

A synapse is the tiny gap (also called the synaptic cleft) between two neurones, or between a neurone and an effector (such as a muscle). Neurones do not physically touch each other — there is always a synapse between them.

How Impulses Cross a Synapse

Electrical impulses cannot jump across the synaptic gap. Instead, the signal is transmitted by chemical messengers called neurotransmitters. The process works as follows:

1. An electrical impulse arrives at the end of the first neurone (the pre-synaptic neurone).
2. This triggers the release of neurotransmitter molecules from tiny vesicles (sacs) at the axon terminal into the synaptic cleft (the gap).
3. The neurotransmitter molecules diffuse across the synaptic cleft.
4. The neurotransmitter molecules bind to specific receptors on the membrane of the second neurone (the post-synaptic neurone).
5. This binding triggers a new electrical impulse in the post-synaptic neurone.
6. The neurotransmitter is then broken down or reabsorbed by the pre-synaptic neurone, so the signal does not continue indefinitely.

Exam Tip: The most common neurotransmitter you need to know is acetylcholine. Remember that transmission across a synapse is chemical (using neurotransmitter molecules), not electrical. This is a key distinction — electrical impulses travel along neurones, but chemical transmission occurs across synapses.

Why Synapses Are Important

• Synapses ensure impulses travel in one direction only — from the pre-synaptic neurone to the post-synaptic neurone. This is because neurotransmitter vesicles are only found on one side and receptors only on the other.
• Synapses allow connections between multiple neurones, enabling complex processing of information.
• Synapses can be affected by drugs and toxins. For example:
  • Some drugs mimic the shape of neurotransmitters and bind to receptors (agonists).
  • Some drugs block receptors so neurotransmitters cannot bind (antagonists).
  • Some drugs prevent the reabsorption or breakdown of neurotransmitters, prolonging their effect.

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Reflex Arcs

A reflex arc is the nerve pathway involved in a reflex action. Reflex actions are rapid, automatic, involuntary responses that do not require conscious thought.

The Reflex Arc Pathway

The pathway of a reflex arc is:

Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector

Component	Role
Receptor	Detects the stimulus (e.g., heat receptors in the skin).
Sensory neurone	Carries the impulse from the receptor to the spinal cord.
Relay neurone	Located in the spinal cord. Connects the sensory neurone to the motor neurone.
Motor neurone	Carries the impulse from the spinal cord to the effector.
Effector	A muscle (which contracts) or a gland (which secretes).

Key Features of Reflex Actions

• They are fast — because the impulse takes the shortest possible route through the spinal cord, bypassing the brain.
• They are automatic — they happen without conscious thought or decision.
• They are involuntary — you cannot choose to stop them.
• They are protective — they help prevent injury.
• They bypass the brain — the impulse is processed in the spinal cord, not the brain. (The brain may be informed afterwards, but it does not control the initial response.)

Exam Tip: A very common exam question is: "Explain why reflex actions are important." The key answer is that they are fast and automatic, which helps protect the body from harm. The impulse is processed in the spinal cord, not the brain, which is why the response is so rapid.

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Examples of Reflex Actions

1. The Withdrawal Reflex (Hand on a Hot Object)

1. Stimulus: Heat from a hot pan.
2. Receptor: Thermoreceptors (heat receptors) in the skin of the hand detect the high temperature.
3. Sensory neurone: Carries the impulse from the thermoreceptors in the hand to the spinal cord.
4. Relay neurone: In the spinal cord, the impulse passes across a synapse from the sensory neurone to the relay neurone, and then across another synapse to the motor neurone.
5. Motor neurone: Carries the impulse from the spinal cord to the muscles in the arm.
6. Effector: The bicep muscle contracts.
7. Response: The hand is pulled away from the hot pan.

This entire process happens in a fraction of a second, before you are consciously aware of the pain.

2. The Pupil Reflex

• Stimulus: Bright light shining into the eye.
• Receptor: Photoreceptors in the retina detect the bright light.
• Response: The circular muscles of the iris contract and the radial muscles relax, making the pupil smaller (constrict). This reduces the amount of light entering the eye, protecting the retina.

In dim light, the reverse happens: the radial muscles contract and circular muscles relax, making the pupil larger (dilate) to let in more light.

Light Condition	Circular Muscles	Radial Muscles	Pupil Size
Bright light	Contract	Relax	Smaller (constricts)
Dim light	Relax	Contract	Larger (dilates)

3. The Knee-Jerk Reflex

• Stimulus: A tap just below the kneecap stretches a tendon.
• Receptor: Stretch receptors in the tendon detect the sudden stretch.
• Response: The quadriceps muscle in the thigh contracts, causing the lower leg to kick forward.
• This reflex helps maintain posture and balance. Doctors test it to check that the nervous system is functioning properly.

Exam Tip: In the pupil reflex, remember there are TWO sets of muscles in the iris — circular and radial. They work antagonistically (when one contracts, the other relaxes). Be precise about which muscle does what in bright vs dim conditions.

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Reflex Arc Diagram

The following diagram shows the components of a simple reflex arc:

graph LR
    A["Stimulus"] --> B["Receptor detects change"]
    B --> C["Sensory neurone"]
    C -->|"SYNAPSE"| D["Relay neurone in spinal cord"]
    D -->|"SYNAPSE"| E["Motor neurone"]
    E --> F["Effector - muscle or gland"]
    F --> G["Response"]

Note that there are two synapses in this pathway:

1. Between the sensory neurone and the relay neurone.
2. Between the relay neurone and the motor neurone.

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Voluntary vs Involuntary Responses