The cellular machinery behind the brain — how the body wires, sends, and coordinates its signals
Our guide to the human brain covers the major structures inside the skull and what each one does. This guide zooms in further, to the cellular level: the individual nerve cells that make up the entire nervous system, the electrical and chemical signals they use to communicate, and how those cells are organised into the wider network running from the brain down to every muscle and sense organ in the body.
The nervous system is conventionally split into two parts, based purely on physical location, which then divide further based on function.
| Division | Comprises | Role |
|---|---|---|
| Central Nervous System (CNS) | Brain and spinal cord | Processes information and issues commands |
| Peripheral Nervous System (PNS) | All nerves outside the brain and spinal cord | Carries signals between the CNS and the rest of the body |
The PNS itself splits further into the somatic nervous system, which carries voluntary motor commands to skeletal muscle and sensory information back, and the autonomic nervous system, which governs involuntary functions — heart rate, digestion, gland secretion — without any conscious input at all. The autonomic system is covered in more detail toward the end of this guide.
The nervous system's basic working unit is the neuron, a cell specialised almost entirely around receiving, conducting, and passing on electrical signals. An adult human brain contains somewhere around 86 billion neurons, each typically built from the same basic parts.
| Part | Function |
|---|---|
| Cell body (soma) | Contains the nucleus and the cell's main metabolic machinery |
| Dendrites | Branching extensions that receive signals from other neurons |
| Axon | A single long fibre that carries the outgoing signal away from the cell body, sometimes over a metre in length |
| Myelin sheath | A fatty insulating layer wrapped around the axon that dramatically speeds signal transmission |
| Axon terminals | Branched endings that pass the signal on to the next cell, typically via a synapse |
Neurons don't work alone. They're matched in number by roughly as many glial cells (the long-cited "10 glia per neuron" figure is now known to be a myth — modern whole-brain cell counts put the ratio at close to 1:1, though it varies by region), which don't transmit signals themselves but support, insulate, and maintain the neurons around them.
| Glial cell | Location | Role |
|---|---|---|
| Oligodendrocytes | CNS | Produce the myelin sheath around CNS axons; each cell can myelinate several neurons |
| Schwann cells | PNS | Produce myelin in the peripheral nervous system; each wraps a single segment of one axon |
| Astrocytes | CNS | Regulate the chemical environment around neurons and help form the blood-brain barrier |
| Microglia | CNS | The CNS's resident immune cells, clearing debris and pathogens |
| Ependymal cells | CNS | Line fluid-filled cavities in the brain and produce cerebrospinal fluid |
A neuron at rest maintains a voltage difference of about -70mV across its cell membrane — the inside slightly negative relative to the outside — called the resting membrane potential. This is actively maintained by the sodium-potassium pump, which continuously exports three sodium ions for every two potassium ions it imports, and by the membrane's much greater permeability to potassium than sodium at rest. A nerve signal, called an action potential, is a brief, self-propagating reversal of this voltage that travels along the axon.
Myelin dramatically speeds this process up. Because myelin insulates most of the axon, ions can only cross the membrane at small unmyelinated gaps called nodes of Ranvier, spaced at regular intervals along the axon. Rather than regenerating continuously along every point of the membrane, the signal effectively jumps from node to node — a mechanism called saltatory conduction (from the Latin saltare, "to leap"). This lets myelinated axons conduct signals at up to roughly 120 metres per second, compared to as little as 1 metre per second in unmyelinated fibres of similar diameter.
An action potential eventually reaches the end of the axon, but neurons don't physically touch the next cell in the chain — they're separated by a microscopic gap called the synaptic cleft, crossed chemically rather than electrically at the great majority of synapses in the human nervous system.
A single neuron typically receives thousands of these synaptic connections simultaneously, from many other neurons at once, and it fires only when the combined excitatory and inhibitory input tips its membrane voltage past threshold — a constant, ongoing tally rather than a simple relay.
| Neurotransmitter | Typical effect | Notable role |
|---|---|---|
| Glutamate | Excitatory | The brain's main excitatory signal; central to learning and memory |
| GABA | Inhibitory | The brain's main inhibitory signal; target of anti-anxiety medications |
| Acetylcholine | Excitatory (varies) | Triggers skeletal muscle contraction at the neuromuscular junction |
| Dopamine | Varies | Reward, motivation, and voluntary motor control; depleted in Parkinson's disease |
| Serotonin | Varies | Mood, appetite, and sleep regulation; a common target of antidepressants |
| Norepinephrine | Excitatory (varies) | Alertness and the body's stress and arousal response |
The spinal cord isn't just a passive cable relaying signals to and from the brain — it can also process and act on certain signals entirely on its own. A reflex arc is a fixed circuit that lets the body react to a stimulus without waiting for the brain to get involved at all: touching a hot surface triggers sensory neurons that synapse directly onto motor neurons within the spinal cord itself, pulling the hand away before the pain signal has even finished travelling up to the brain to be consciously felt. The simplest reflex arcs, such as the knee-jerk reflex, involve just one synapse between a sensory and a motor neuron; withdrawal reflexes like the hot-surface example typically route through at least one additional connecting neuron in between.
The autonomic nervous system controls the body's internal organs without conscious direction, and is itself split into two branches that generally act in opposition, both regulated ultimately by the hypothalamus and brainstem covered in the brain guide.
| Organ / system | Sympathetic ("fight or flight") | Parasympathetic ("rest and digest") |
|---|---|---|
| Heart rate | Increases | Decreases |
| Pupils | Dilate | Constrict |
| Digestion | Slows | Stimulates |
| Airways | Dilate | Constrict |
| Blood flow | Redirected to muscles | Redirected to digestive organs |
The sympathetic branch dominates during acute stress or exertion, rapidly mobilising the body's resources for immediate physical action. The parasympathetic branch dominates during rest, promoting digestion, recovery, and energy storage. Both branches are constantly active to some degree, and it's the balance between them — rather than a simple on/off switch — that determines the body's overall state at any given moment.
Touch and pain signals travel along different types of nerve fibre at different speeds — fast, thickly myelinated fibres carry touch and pressure, while thinner, more slowly conducting fibres carry sharp pain. This difference is large enough to be noticeable: the sensation of touching something is often perceptibly faster than the sharp pain that follows a fraction of a second later.
Drugs like lidocaine work by directly blocking the voltage-gated sodium channels responsible for depolarisation, physically preventing an action potential from starting in the treated area at all — numbing the region without needing to affect the brain or consciousness in any way.
Peripheral nerves, supported by Schwann cells, can regrow slowly after injury — typically about 1mm per day — allowing feeling and function to sometimes return even after a nerve is severed. Neurons within the central nervous system have far more limited capacity to regenerate after injury, which is why spinal cord and brain injuries are typically so much more permanent than peripheral nerve damage.
This document provides a general scientific overview of the nervous system for educational purposes, and is not medical advice.