The Nervous System and Reflexes

The human nervous system consists of two main parts:

Central Nervous System (CNS)

• Made up of the brain and spinal cord
• Acts as the coordination centre — receives information, processes it, and sends out responses

Peripheral Nervous System (PNS)

• All the nerves that connect the CNS to the rest of the body
• Includes sensory neurons (carry signals TO the CNS) and motor neurons (carry signals FROM the CNS)

The nervous system follows a pathway:

$\text{Stimulus} \rightarrow \text{Receptor} \rightarrow \text{Coordinator (CNS)} \rightarrow \text{Effector} \rightarrow \text{Response}$

There are three types of neurons:

Sensory Neurons

• Carry electrical impulses from receptors to the CNS
• Have a long axon and a cell body in the middle of the neuron
• Connect receptors in sense organs to relay neurons in the CNS

Relay Neurons (Interneurons)

• Found within the CNS (brain and spinal cord)
• Connect sensory neurons to motor neurons
• Short neurons with many connections

Motor Neurons

• Carry electrical impulses from the CNS to effectors (muscles or glands)
• Have a long axon, a cell body at one end, and branched endings (motor end plates) that connect to muscle fibres

Neuron Structure (Common Features)

• Cell body — contains the nucleus and most of the cytoplasm
• Dendrites — short branching fibres that receive impulses from other neurons
• Axon — a long fibre that carries the electrical impulse along the neuron
• Myelin sheath — a fatty layer insulating the axon; speeds up impulse transmission; has gaps called Nodes of Ranvier where the impulse "jumps" between
• Axon terminals — branched endings that form connections (synapses) with other neurons or effectors

A synapse is the tiny gap between two neurons (or between a neuron and an effector).

Electrical impulses cannot cross the gap directly. Instead:

1. An electrical impulse arrives at the axon terminal of the first neuron (pre-synaptic neuron)
2. This triggers the release of chemical messengers called neurotransmitters from vesicles into the synaptic gap
3. Neurotransmitters diffuse across the gap and bind to receptors on the membrane of the next neuron (post-synaptic neuron)
4. This triggers a new electrical impulse in the second neuron
5. The neurotransmitters are then broken down or reabsorbed to prevent continuous stimulation

Synapses ensure impulses travel in one direction only (from pre-synaptic to post-synaptic neuron), because neurotransmitters are only released from one side and receptors are only on the other side.

A reflex action is a rapid, automatic (involuntary) response to a stimulus. Reflexes protect the body from harm.

Examples of Reflexes

• Pulling your hand away from a hot object
• Blinking when something approaches your eye
• Pupil constricting in bright light
• Knee-jerk reflex
• Dropping a very hot plate

The Reflex Arc

The pathway of a reflex action is called the reflex arc:

$\text{Stimulus} \rightarrow \text{Receptor} \rightarrow \text{Sensory neuron} \rightarrow \text{Relay neuron (in CNS)} \rightarrow \text{Motor neuron} \rightarrow \text{Effector} \rightarrow \text{Response}$

Example: Touching a hot object

1. Stimulus: Hot object (heat)
2. Receptor: Temperature receptors in the skin of the hand detect the heat
3. Sensory neuron: Carries the electrical impulse from the receptor to the spinal cord (CNS)
4. Relay neuron: In the spinal cord, the sensory neuron synapses with a relay neuron, which passes the impulse to a motor neuron
5. Motor neuron: Carries the impulse from the spinal cord to the effector (muscle in the arm)
6. Effector: The muscle contracts
7. Response: The hand is pulled away from the hot object

Why Are Reflexes Important?

• They are faster than voluntary responses because they don't involve the brain making a conscious decision
• The impulse travels through the spinal cord, not up to the brain (the brain is informed afterwards, but the response doesn't wait for this)
• They protect the body from damage (e.g., burning, eye injury)
• They are automatic — you don't have to think about them

You may be tested on an investigation to measure reaction time using the ruler drop test.

Method

1. Person A holds a ruler vertically at the 0 cm mark at the top
2. Person B places their thumb and finger at the bottom of the ruler (at the 30 cm mark), without touching it
3. Person A drops the ruler without warning
4. Person B catches the ruler as quickly as possible
5. Record the distance the ruler fell (in cm)
6. Repeat at least 5 times and calculate the mean
7. A shorter distance = faster reaction time

Variables

• Independent variable: The factor you're testing (e.g., practice, caffeine, distractions)
• Dependent variable: The distance the ruler falls (cm)
• Control variables: Same ruler, same person catching, same starting position, same gap between fingers

Improving Reliability

• Repeat the test multiple times and calculate the mean
• Discard anomalous results (outliers)
• Use a consistent method each time

Question: Describe the reflex arc involved when you step on a sharp object. (5 marks)

Solution:

When you step on a sharp object, pain receptors in the skin of the foot detect the stimulus. An electrical impulse travels along a sensory neuron from the receptor to the spinal cord (CNS). In the spinal cord, the sensory neuron synapses with a relay neuron, which transmits the impulse to a motor neuron. The motor neuron carries the impulse to the effector — a muscle in the leg. The muscle contracts, causing you to lift your foot away from the sharp object (the response). This happens automatically and rapidly, before the brain is consciously aware of the pain.

• Learn the reflex arc sequence: Stimulus → Receptor → Sensory neuron → Relay neuron → Motor neuron → Effector → Response.
• At a synapse, the signal changes from electrical (in the neuron) to chemical (neurotransmitters across the gap) to electrical again.
• Reflexes involve the spinal cord, not the brain — this is what makes them fast.
• In the ruler drop test, shorter distance = faster reaction time. Convert distance to actual reaction time using a conversion table if provided.

• The nervous system detects stimuli and coordinates rapid responses via the CNS (brain + spinal cord) and PNS (nerves).
• Three neuron types: sensory (receptor → CNS), relay (within CNS), motor (CNS → effector).
• Synapses transmit signals chemically using neurotransmitters across the gap between neurons.
• Reflex actions are rapid, automatic responses processed by the spinal cord (reflex arc), protecting the body from harm.
• Reaction time can be measured using the ruler drop test.