The Body's Communication Network
Imagine needing to react instantly when a hot pan burns your finger — you pull your hand away before you even consciously think about it. This lightning-fast response is made possible by the nervous system, the body's electrical communication network.
The nervous system has two main divisions. The central nervous system (CNS) consists of the brain and spinal cord — these are the command centres that process information and decide on responses. The peripheral nervous system (PNS) consists of all the nerves that branch out from the CNS to every part of the body, carrying signals to and from the organs, muscles, and sensors.
Neurons — The Nerve Cells
The basic working unit of the nervous system is the neuron (nerve cell). Neurons are highly specialised cells designed to carry electrical signals called nerve impulses at very high speeds — up to 120 metres per second in some neurons.
A typical neuron has a central cell body (containing the nucleus), several short branching dendrites (which receive signals from other neurons), and a long fibre called an axon (which carries the signal away from the cell body). Many axons are wrapped in a fatty myelin sheath that speeds up the transmission of signals.
There are three types of neuron: sensory neurons carry signals from receptors (sense organs) to the CNS; motor neurons carry signals from the CNS to effectors (muscles or glands); and relay neurons (interneurons) connect sensory and motor neurons inside the CNS.
Your brain contains approximately 86 billion neurons, each connected to thousands of others. The total number of connections (synapses) in the brain is estimated to be around 100 trillion — more than the number of stars in the Milky Way galaxy.
Synapses — How Neurons Communicate
Neurons do not actually touch each other. There is a tiny gap between neurons called a synapse. When an electrical impulse reaches the end of one neuron, it triggers the release of chemical messengers called neurotransmitters into the synapse. These chemicals diffuse across the gap and bind to receptors on the next neuron, triggering a new electrical impulse. This is how signals are relayed from one neuron to the next.
Reflex Actions
A reflex is a rapid, automatic response to a stimulus that does not require conscious thought from the brain. Reflexes are faster than conscious responses because the signal travels a short path — the reflex arc — through the spinal cord rather than all the way to the brain.
The pathway for a reflex arc is: stimulus → receptor → sensory neuron → relay neuron (in spinal cord) → motor neuron → effector (muscle or gland) → response. Examples include pulling your hand away from something hot, blinking when something approaches your eye, and the knee-jerk reflex.
To remember the order of neurons in a reflex: S → R → M — Sensory neuron takes information in, Relay neuron links them in the CNS, Motor neuron sends the command out to the muscle.
Students often say the brain controls all reflexes. In fact, most reflexes are controlled at the level of the spinal cord — the brain becomes aware of the reflex after it has already happened. This is why reflex actions are so fast.
Real world: corrective glasses and lenses
The eye's retina contains photoreceptor cells (rods and cones) that detect light and convert it into nerve impulses transmitted by the optic nerve to the brain's visual cortex. In short-sightedness, the signal from a distant object reaches the retina out of focus — an optician prescribes lenses to correct this. Understanding the full sensory-to-brain pathway is what makes ophthalmology possible.
Real world: hearing aids and cochlear implants
Sound waves vibrate hair cells in the cochlea, generating nerve impulses in the auditory nerve. In profound deafness, a cochlear implant electrically stimulates auditory nerve fibres, bypassing damaged hair cells. The processor decodes sounds and sends the correct electrical pattern. This technology directly replaces the sensory neuron stage of the hearing pathway.
Real world: reflex testing in medical diagnosis
A doctor tests the patellar (knee-jerk) reflex by tapping below the kneecap. If the leg kicks involuntarily, it confirms the spinal reflex arc is intact. An absent or exaggerated reflex suggests damage to specific spinal cord segments or nerve pathways, helping neurologists localise the site of injury. Clinical reflex tests are direct diagnostic windows into nervous system health.
Practice Problems
What are the three types of neuron and what does each do?
Show answer
Why are reflex actions faster than voluntary actions?