The Nervous System and Reflexes
What Is Homeostasis?
Homeostasis is the regulation of the internal conditions of the body to maintain a stable environment, despite changes in the external environment. This is essential for cells to function properly (especially enzymes).
Conditions that must be controlled include:
- Body temperature (around 37°C)
- Blood glucose concentration
- Water and ion levels
- CO₂ levels (removed by the lungs)
- Urea levels (removed by the kidneys)
All homeostatic control systems involve:
1. Receptors — detect stimuli (changes in the environment)
2. Coordination centres — receive and process information (brain, spinal cord, or glands)
3. A stimulus triggers a response via receptors → coordination centre → effectors
4. Effectors — carry out the response (muscles or glands)
The Nervous System
The nervous system allows the body to react to its surroundings and coordinate behaviour. It uses electrical impulses for fast, short-lived responses.
Structure of the Nervous System
| Component | Function |
|---|---|
| Central Nervous System (CNS) | The brain and spinal cord — processes information and coordinates responses |
| Sensory neurones | Carry impulses from receptors to the CNS |
| Relay neurones | Found in the CNS; connect sensory and motor neurones |
| Motor neurones | Carry impulses from the CNS to effectors |
| Effectors | Muscles (contract) or glands (secrete hormones/chemicals) — carry out the response |
Pathway of a Nervous Response
Stimulus → Receptor → Sensory neurone → CNS (relay neurone) → Motor neurone → Effector → Response
Example: Touching a hot plate
1. Stimulus — high temperature
2. Receptor — temperature receptors in the skin detect heat
3. Sensory neurone — carries impulse to the spinal cord
4. Relay neurone (in CNS) — passes impulse to motor neurone
5. Motor neurone — carries impulse to the effector
6. Effector — muscle in the arm
7. Response — arm pulls away
Synapses
A synapse is the tiny gap between two neurones. Electrical impulses cannot cross this gap, so the signal is transmitted chemically:
1. An electrical impulse arrives at the end of the pre-synaptic neurone
2. This triggers the release of neurotransmitter chemicals (e.g. acetylcholine) from vesicles into the synaptic cleft
3. The neurotransmitters diffuse across the gap
4. They bind to receptors on the post-synaptic neurone
5. This triggers a new electrical impulse in the next neurone
6. The neurotransmitter is then broken down or reabsorbed
Synapses ensure impulses travel in one direction only (neurotransmitters are only released from one side).
Reflex Actions
A reflex action is a rapid, automatic response to a stimulus that does not involve conscious thought. Reflexes protect the body from harm.
The pathway for a reflex is called a reflex arc:
Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response
The key difference from a normal response is that the reflex arc passes through the spinal cord, not the brain. The brain may be informed afterwards, but it does not control the initial response.
Examples of reflexes:
- Pulling your hand away from a hot object
- Pupil constriction in bright light
- Knee-jerk reflex
- Blinking when something approaches the eye
- Dropping a sharp object
Required Practical: Reaction Time
Aim: Investigate the effect of a factor on human reaction time.
Method (ruler drop test):
1. Person A holds a ruler vertically; Person B places their thumb and finger at the 0 cm mark (without touching)
2. Person A drops the ruler without warning
3. Person B catches the ruler as quickly as possible
4. Record the distance the ruler fell (in cm)
5. Use a conversion table to convert distance to reaction time
6. Repeat and calculate a mean
7. Test different conditions (e.g. effect of caffeine, distraction, practice, dominant vs non-dominant hand)
Computer-based method: An online reaction time test (clicking when a screen changes colour) is more accurate and eliminates human error in dropping the ruler.
Comparing Nervous and Hormonal Systems
| Feature | Nervous system | Hormonal (endocrine) system |
|---|---|---|
| Signal type | Electrical impulses | Chemical (hormones) |
| Carried by | Neurones | Blood |
| Speed | Very fast | Slower |
| Duration | Short-lived | Longer-lasting |
| Target | Specific (one effector) | Widespread (multiple target organs) |
Exam Tips
- The reflex arc bypasses the brain — state this clearly in answers
- Synapses transmit signals chemically using neurotransmitters — this is a common 4-mark question
- Receptors detect stimuli; effectors carry out the response — do not confuse these