The Nervous System Explained
Every thought you think, every movement you make, and every sensation you feel depends on your nervous system. It is the body's rapid communication network — a system of specialised cells that transmit electrical and chemical signals at speeds of up to 120 metres per second, coordinating everything from a heartbeat to a philosophical argument.
Organisation: Central and Peripheral
The nervous system is divided into two main parts. The central nervous system (CNS) consists of the brain and spinal cord. The CNS is the processing centre: it receives information from the body, interprets it, and generates responses. The peripheral nervous system (PNS) consists of all the nerves that extend from the CNS to the rest of the body, carrying signals to and from muscles, organs, and sense organs.
The peripheral nervous system is itself divided into two branches. The somatic nervous system controls voluntary movements — the skeletal muscles you consciously direct. The autonomic nervous system controls involuntary functions such as heart rate, digestion, and breathing. The autonomic system has two subdivisions: the sympathetic division, which prepares the body for action in stressful situations (the "fight or flight" response), and the parasympathetic division, which promotes rest and digestion ("rest and digest").
Neurons: The Signalling Cells
The functional unit of the nervous system is the neuron (nerve cell). The human brain contains roughly 86 billion neurons, and the body holds tens of billions more. A neuron has three main structural regions.
The cell body (soma) contains the nucleus and carries out routine cellular functions. Branching from the cell body are dendrites — short, tree-like extensions that receive incoming signals from other neurons. Extending from the cell body is the axon, a single long fibre that carries outgoing signals away from the cell body toward the next neuron or target tissue. Axons can range from less than a millimetre to over a metre long (the sciatic nerve axons running from the spinal cord to the foot are among the longest cells in the human body).
Many axons are wrapped in a fatty sheath called myelin, produced by Schwann cells in the PNS and oligodendrocytes in the CNS. Myelin acts as electrical insulation, dramatically increasing the speed at which signals travel. Gaps in the myelin sheath called nodes of Ranvier force the electrical signal to jump from node to node — a process called saltatory conduction — which is far faster than continuous conduction along an uninsulated fibre. Multiple sclerosis is a disease in which the immune system attacks the myelin sheath, disrupting signal transmission and causing a range of neurological symptoms.
Neurons are functionally classified into three types: sensory (afferent) neurons carry signals from receptors (skin, eyes, ears) toward the CNS; motor (efferent) neurons carry signals from the CNS to effectors (muscles, glands); and interneurons connect neurons within the CNS and make up the vast majority of brain neurons.
The Action Potential
Neurons communicate using electrical signals called action potentials. In its resting state, a neuron maintains a voltage difference across its membrane called the resting membrane potential, typically about −70 millivolts (mV) inside relative to outside. This is maintained by the sodium-potassium pump, which actively moves three sodium ions (Na+) out of the cell and two potassium ions (K+) in for each ATP molecule used.
When the neuron receives a sufficiently large stimulus, voltage-gated sodium channels in the axon membrane open. Na+ rushes into the cell (it is attracted both by its concentration gradient and by the negative charge inside), and the membrane potential rapidly reverses to about +30 mV. This is depolarisation.
The depolarisation triggers adjacent sodium channels to open, propagating the signal along the axon as a wave. Almost immediately after depolarisation, sodium channels close and potassium channels open, allowing K+ to flow out, restoring the negative interior. This is repolarisation. For a brief period (the refractory period), the neuron cannot fire again, which ensures the action potential travels in only one direction and limits the maximum firing rate.
A key principle is the all-or-nothing law: a neuron either fires completely or does not fire at all. The strength of a stimulus is coded not by the size of each action potential (they are always the same size) but by the frequency of firing — a strong stimulus produces a rapid burst of action potentials; a weak one produces fewer per second.
Neurons do not touch. The gap between the axon terminal of one neuron and the dendrite of the next is called the synapse (specifically the synaptic cleft, typically 20–40 nanometres wide). When an action potential reaches the axon terminal, it triggers the release of chemical messengers called neurotransmitters from vesicles into the synaptic cleft. Neurotransmitters diffuse across the cleft and bind to receptor proteins on the receiving neuron. Depending on the neurotransmitter and receptor type, this can either excite the next neuron (making it more likely to fire) or inhibit it (making it less likely to fire). After binding, neurotransmitters are either broken down by enzymes or reabsorbed by the sending neuron (reuptake). Common neurotransmitters include glutamate (excitatory), GABA (inhibitory), dopamine, serotonin, and acetylcholine. Many drugs and toxins act by mimicking, blocking, or altering the reuptake of neurotransmitters — from caffeine (blocks adenosine receptors) to selective serotonin reuptake inhibitors (SSRIs) used in treating depression.
Reflex Arcs
A reflex is a rapid, automatic response to a stimulus that does not require conscious processing by the brain. Reflexes are mediated by the spinal cord, which allows them to occur much faster than if the signal had to travel all the way to the brain and back.
The simplest reflex circuit is the spinal reflex arc. When you touch something very hot, sensory receptors in your fingertip send a signal via a sensory neuron to the spinal cord. In the spinal cord, an interneuron connects the sensory neuron directly to a motor neuron, which sends a signal to the flexor muscles of your arm to contract and pull your hand away — all before the pain signal has even reached the brain. The brain receives the information a split second later, but the protective withdrawal has already happened.
The knee-jerk reflex (patellar reflex) is even simpler: it involves only a sensory neuron and a motor neuron, with no interneuron. A tap on the patellar tendon stretches the quadriceps muscle, triggering a reflex contraction. Doctors use it to test the integrity of the spinal cord and peripheral nerves.
The Brain
The brain is the most complex structure known. Its major regions each have distinct functions. The cerebral cortex — the deeply folded outer layer of the cerebrum — handles conscious thought, language, voluntary movement, and the interpretation of sensory input. It is divided into two hemispheres, each with four lobes: frontal (reasoning, planning, personality), parietal (sensory processing, spatial awareness), temporal (hearing, memory, language), and occipital (vision).
The cerebellum at the back of the brain coordinates balance and fine motor control — learning to ride a bicycle depends heavily on cerebellar plasticity. The brainstem (medulla oblongata, pons, midbrain) controls vital automatic functions: heart rate, blood pressure, breathing, and swallowing. The hypothalamus links the nervous system to the endocrine (hormonal) system and regulates hunger, thirst, body temperature, and the sleep-wake cycle.
Summary
The nervous system divides into the central nervous system (brain and spinal cord) and the peripheral nervous system (all other nerves). Neurons transmit electrical signals called action potentials along their axons; the all-or-nothing law means signal strength is encoded in firing frequency, not amplitude. Signals cross from neuron to neuron at synapses via chemical neurotransmitters. Reflex arcs provide rapid automatic responses by bypassing the brain. The brain's cortex handles conscious functions; the cerebellum coordinates movement; the brainstem controls vital autonomic functions. Understanding this architecture is the foundation for topics ranging from pharmacology to neurological disease.