The nervous system detects changes (stimuli) in the environment and coordinates rapid responses.
It is divided into two main parts:
CENTRAL NERVOUS SYSTEM (CNS): the BRAIN and SPINAL CORD. Processes all incoming information and determines the appropriate response.
PERIPHERAL NERVOUS SYSTEM (PNS): all the NERVES that connect the CNS to the rest of the body — carrying signals to and from every organ, muscle and sense organ.
The nervous system communicates via ELECTRICAL IMPULSES — signals that travel along specialised cells called NEURONES at speeds of up to 120 m/s.
This is much faster than hormonal communication, making the nervous system ideal for rapid, precise responses.
Types of Neurone
There are three types of neurone, each with a specific role:
SENSORY NEURONE:
Carries electrical impulses FROM receptors (sense organs) TO the CNS.
Long dendron carrying signal towards the cell body, then axon to the CNS.
Example: carries pain signal from finger to spinal cord when you burn yourself.
RELAY NEURONE:
Found entirely WITHIN the CNS (brain and spinal cord).
Connects sensory neurones to motor neurones.
Acts as a processing junction — signals can be passed on, filtered or directed to the appropriate response pathway.
MOTOR NEURONE:
Carries impulses FROM the CNS TO effectors (muscles or glands).
Long axon — can be over 1 metre long (e.g. from spinal cord to foot).
Covered in a MYELIN SHEATH — a fatty insulating layer that speeds up signal conduction.
Example: carries signal to leg muscle to pull foot away from a sharp object.
The Synapse
A SYNAPSE is the junction between two neurones — a tiny gap called the SYNAPTIC CLEFT.
Electrical impulses CANNOT jump across this gap — transmission across a synapse is CHEMICAL.
How a synapse works:
1. An electrical impulse arrives at the end of the pre-synaptic neurone (the sending neurone).
2. Vesicles containing NEUROTRANSMITTER molecules fuse with the membrane.
3. Neurotransmitters are released into the synaptic cleft.
4. They DIFFUSE across the gap.
5. They bind to RECEPTOR PROTEINS on the post-synaptic neurone (the receiving neurone).
6. This triggers a new electrical impulse in the next neurone.
7. Neurotransmitters are then broken down by enzymes or reabsorbed — resetting the synapse.
Transmission is ONE-WAY only — neurotransmitters are released from the pre-synaptic side and receptors are only on the post-synaptic side.
How Drugs Affect Synapses
Many drugs affect the nervous system by interfering with synapse function:
Fewer impulses transmitted → slowed reactions, reduced anxiety, drowsiness, loss of coordination.
Alcohol specifically slows the CNS — increasing reaction time significantly.
SSRIs (antidepressants, e.g. fluoxetine):
Block the reabsorption of serotonin after it has been released.
Serotonin stays in the synapse longer — improves mood.
Used to treat depression and anxiety.
⚠️ Common Mistake
Transmission WITHIN a neurone is electrical (an impulse). Transmission ACROSS a synapse is CHEMICAL (neurotransmitters). Students often say 'electrical signals cross the synapse' — they cannot. The synapse is a gap and must be crossed using chemical neurotransmitters that then trigger a new electrical impulse in the next neurone.
📌 Key Note
CNS = brain + spinal cord. PNS = all nerves. Sensory: receptor → CNS. Relay: within CNS. Motor: CNS → effector. Synapse: electrical → chemical (neurotransmitter) → electrical. One-way transmission.
🎯 Matching Activity — Match the Nervous System Component
Match each component to its correct role. — drag the symbols on the right to match the component names on the left.
Sensory neurone
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Relay neurone
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Motor neurone
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Synapse
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Myelin sheath
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CNS
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Fatty insulating layer around axon — speeds up impulse conduction
Brain and spinal cord — processes information and coordinates responses
Connects sensory and motor neurones — found within the CNS
Carries impulse FROM receptor TO CNS
Junction between neurones — signal crosses via chemical neurotransmitters
Carries impulse FROM CNS TO effector (muscle or gland)
⭐ Higher Tier Only
Synapses allow one-way signal transmission between neurones via neurotransmitter diffusion. Drugs affecting synapses: stimulants (caffeine, nicotine, cocaine) increase neurotransmitter activity; depressants (alcohol, benzodiazepines) reduce synaptic activity; SSRIs block serotonin reabsorption increasing its concentration. The myelin sheath insulates axons and allows faster signal conduction by saltatory (jumping) transmission.
🎯 Test Yourself
Question 1 of 3
1. What is the correct pathway for a nerve signal from receptor to effector?
2. How does a nerve signal cross a synapse?
3. Why is synaptic transmission one-directional?
⭐ How Well Do You Understand This Topic?
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