The Body's Communication Network
The nervous system is the body's fastest communication system. It detects changes in the environment (stimuli), processes the information, and coordinates responses — all in fractions of a second. Without it, we could not sense the world, think, move, or control our internal organs.
The nervous system has two main divisions:
- Central Nervous System (CNS) — the brain and spinal cord
- Peripheral Nervous System (PNS) — all the nerves that branch out from the CNS to the rest of the body
Neurons: The Cells of the Nervous System
The nervous system is built from specialised cells called neurons (nerve cells). There are three main types:
Sensory neurons — carry electrical signals from sense organs (eyes, ears, skin, etc.) toward the CNS.
Motor neurons — carry signals from the CNS to muscles and glands, causing them to respond (contract or secrete).
Relay neurons (interneurons) — found entirely within the CNS; connect sensory and motor neurons, processing and relaying signals.
A typical neuron has a cell body containing the nucleus, branching dendrites that receive signals, and a long axon that carries the signal away. Many axons are wrapped in a fatty myelin sheath that speeds up signal transmission.
Nerve impulses travel as electrical signals along the axon at speeds of up to 120 metres per second (about 430 km/h) in myelinated fibres. Non-myelinated fibres conduct signals much more slowly.
The Brain and Spinal Cord
The brain is the control centre of the nervous system, containing about 86 billion neurons. Key regions include:
- Cerebrum — largest region; responsible for conscious thought, language, memory, and voluntary movement.
- Cerebellum — coordinates balance and smooth, coordinated movement.
- Brain stem — controls automatic (involuntary) functions: breathing, heart rate, swallowing.
The spinal cord runs through the vertebral column, relaying signals between the brain and the rest of the body. It also coordinates reflex actions.
Reflex Actions
A reflex is a rapid, automatic response to a stimulus that does not require conscious thought — no signal goes to the brain first. This makes reflexes much faster than conscious responses.
Reflex arc path: Stimulus → sensory receptor → sensory neuron → relay neuron (in spinal cord) → motor neuron → effector (muscle or gland) → response
Example: Touching a hot surface → pain receptors fire → signal travels to spinal cord → motor signal returns → hand withdraws (all in about 0.1–0.2 seconds, before your brain registers the pain).
Reflexes protect us from injury. Other examples: blinking when an object approaches the eye, the knee-jerk reflex when tapped below the kneecap, and pupils narrowing in bright light.
The Peripheral Nervous System
The PNS includes:
- Somatic nervous system — controls voluntary skeletal muscles.
- Autonomic nervous system — automatically regulates internal organs (heart rate, digestion, glands). It has two branches: the sympathetic (activates "fight or flight" responses) and parasympathetic (promotes "rest and digest") systems.
Real world: The knee-jerk reflex at the doctor
When a doctor taps just below your kneecap with a rubber hammer, your leg kicks forward automatically — this is the patellar (knee-jerk) reflex. It tests whether your nervous system is functioning properly. The signal travels from the knee to the spinal cord and back as a reflex arc — it never goes to the brain first, which is why the leg moves even if you try not to let it. Doctors use this reflex routinely to assess neurological function.
Real world: Reaction time in sport
A sprinter at the starting blocks must react to the starter's gun in under 0.1 seconds — any faster is considered a false start (the nervous system cannot consciously respond faster). Reaction time measures the speed of the complete neural path: stimulus (sound) → sensory neurons → brain processing → motor neurons → muscle response. Sports scientists use reaction time tests to measure reflexes and assess fatigue or neurological health.
Real world: Concussion in contact sports
Concussion — a brain injury from a blow to the head — is taken extremely seriously in rugby, football, and boxing. The brain (the CNS's control centre) is briefly disrupted, causing symptoms like confusion, blurred vision, memory problems, and slow reaction times. Players are immediately removed from sport and tested under concussion protocols — a real-world application of understanding what the brain controls and why protecting it matters.
Where do you see this in real life? Have a friend hold a ruler vertically between your fingers without warning you when they'll drop it. Your reaction time to catch it (measured in centimetres fallen) tests how fast your sensory-motor nerve circuit is. Typical reaction time is about 0.15–0.25 seconds. Test before and after coffee or exercise — the nervous system's speed changes with alertness.
Practice Problems
answer="Stimulus → sensory receptor → sensory neuron → relay neuron (in spinal cord) → motor neuron → effector (muscle or gland) → response." />
What is the role of the myelin sheath around an axon?