The Nervous System
The Nervous System
The nervous system is the body's rapid communication and control network. It detects stimuli from the environment, processes information, and coordinates responses — all within fractions of a second.
It works alongside the endocrine (hormonal) system, but the nervous system is faster (milliseconds vs. seconds/minutes) and more targeted.
Divisions of the Nervous System
Central Nervous System (CNS):
- Brain — the main processing centre
- Spinal cord — connects the brain to the rest of the body; also processes reflexes
Peripheral Nervous System (PNS):
- All nerves outside the CNS
- Connects receptors (sense organs) to the CNS, and the CNS to effectors (muscles and glands)
- Divided into somatic (voluntary, conscious control) and autonomic (involuntary, automatic) branches
Neurons (Nerve Cells)
Neurons are specialised cells that carry electrical impulses. There are three types:
| Neuron type | Direction of signal | Location |
|---|---|---|
| Sensory neuron | Receptor → CNS | PNS |
| Relay (interneuron) | Within CNS | Brain and spinal cord |
| Motor neuron | CNS → Effector | PNS |
Structure of a neuron:
- Cell body — contains nucleus
- Dendrites — short branches that receive signals from other neurons
- Axon — long fibre that carries impulses away from the cell body
- Myelin sheath — fatty insulating layer that speeds up impulse transmission
- Axon terminals — ends that release neurotransmitters at synapses
The Synapse
Neurons do not physically touch each other. The tiny gap between them is called a synapse. Signals cross it chemically:
- An electrical impulse reaches the end of the pre-synaptic neuron
- Neurotransmitters (chemical messengers, e.g., acetylcholine, dopamine) are released into the synaptic cleft
- They diffuse across the gap and bind to receptors on the post-synaptic neuron
- This triggers a new electrical impulse in the next neuron
Synapses only allow signals to travel in one direction (from pre- to post-synaptic neuron) because neurotransmitters are only released from one side.
Drugs and toxins often work by affecting synapses — e.g., caffeine blocks certain inhibitory neurotransmitters; some poisons block acetylcholine receptors.
Reflex Arcs
Reflexes are rapid, automatic, involuntary responses to stimuli. They protect the body from harm without waiting for the brain to process information.
Reflex arc pathway:
Stimulus → Receptor → Sensory neuron → Relay neuron (spinal cord) → Motor neuron → Effector → Response
Example: Touching a hot surface
- Receptor (skin pain receptors) detects heat/pain
- Sensory neuron carries signal to spinal cord
- Relay neuron in spinal cord processes signal
- Motor neuron carries signal to arm muscles (effector)
- Muscles contract, withdrawing hand
- Signal also travels to brain — you feel pain a moment later
The response occurs before conscious awareness because the signal is processed in the spinal cord, not the brain. This makes reflexes much faster than voluntary responses.
Other reflexes: pupil dilation in bright light, knee-jerk reflex, blinking when something approaches the eye, salivating at the smell of food.
The Brain
The brain is divided into regions with specific functions:
| Region | Key functions |
|---|---|
| Cerebrum | Thinking, memory, personality, voluntary movement, language, sensory perception |
| Cerebellum | Balance, coordination, fine motor control |
| Medulla oblongata | Automatic functions: heart rate, breathing rate, blood pressure |
| Hypothalamus | Temperature regulation, hunger, thirst, sleep; controls the pituitary gland |
| Hippocampus | Formation of new memories |
The cerebrum is divided into two hemispheres and further into lobes:
- Frontal lobe: decision-making, personality, voluntary movement
- Parietal lobe: touch and spatial awareness
- Temporal lobe: hearing and language
- Occipital lobe: vision
Key Terms
- Stimulus — a change in the environment detected by receptors
- Receptor — a cell or organ that detects a stimulus (e.g., eye, skin)
- Effector — a muscle or gland that produces a response
- Neuron — a nerve cell that carries electrical impulses
- Synapse — the gap between two neurons; signals cross it chemically
- Neurotransmitter — chemical that carries a signal across a synapse
- Reflex — a rapid, automatic, involuntary response to a stimulus
- Myelin sheath — insulating layer around axons that speeds up impulse transmission
Common Mistakes
- Saying the signal in a reflex goes to the brain first — it goes through the spinal cord and the brain is informed afterwards
- Confusing receptors (detect stimuli) with effectors (produce responses)
- Thinking synapses transmit signals electrically — the signal is chemical across the gap
- Confusing the cerebrum (thinking, voluntary movement) with the cerebellum (balance and coordination)
Tips and Tricks
- Remember the reflex arc order with: R-S-R-M-E (Receptor, Sensory, Relay, Motor, Effector)
- Synapse signals travel one way only — neurotransmitters are released from one side, receptors are on the other
- The medulla controls the vital automatic functions — if it is damaged, breathing and heart rate can stop
- Myelin sheath = faster impulses; conditions that damage myelin (like MS) slow neural communication