The Wave That Resets Itself As It Travels

Every organ and organ system in the body needs to work in step with every other one, and coordination is simply the process through which two or more organs interact to complement each other's functions. Physical exercise makes the underlying logic vivid: increased muscular activity raises the body's energy demand, which raises its oxygen demand, which in turn requires a faster breathing rate, a faster heartbeat and increased blood flow, and the moment exercise stops, every one of these systems, nerves, lungs, heart, kidneys, gradually settles back to its resting baseline together. Two separate body systems jointly manage this kind of whole-body coordination: the neural system, providing fast, precise, point-to-point connections, and the endocrine system, providing slower, broader chemical coordination through hormones released into the blood. Neural organisation itself varies enormously in complexity across the animal kingdom. Hydra's entire neural system is just a diffuse network of neurons with no central control point at all. Insects show a genuinely more organised system, with an actual brain working alongside a number of separate ganglia and neural tissue clusters. Vertebrates carry the most developed neural systems of all, and the human neural system specifically divides into two major parts. The central neural system, or CNS, made up of the brain and spinal cord, is where information actually gets processed and controlling decisions actually get made. The peripheral neural system, or PNS, covers every nerve connecting the CNS to the rest of the body, split further by direction of travel into afferent fibres, carrying signals from tissues and organs toward the CNS, and efferent fibres, carrying regulatory signals back out from the CNS to the body. The PNS divides again by target: the somatic neural system relays signals specifically to skeletal muscle, under voluntary control, while the autonomic neural system relays signals to involuntary organs and smooth muscle, itself further split into a sympathetic branch and a parasympathetic branch, and the visceral nervous system specifically covers the nerves, ganglia and plexuses carrying signals to and from the body's internal organs.

A neuron, the basic structural and functional unit of the whole neural system, is built from three main parts, each with a genuinely specific job. The cell body contains the usual cell organelles plus distinctive granular structures called Nissl's granules. Short, repeatedly branching fibres projecting from the cell body, called dendrites, also contain Nissl's granules and specifically carry impulses toward the cell body. The axon, by contrast, is a single long fibre whose far end branches out, each branch ending in a bulb-shaped synaptic knob packed with synaptic vesicles holding chemicals called neurotransmitters, and axons specifically carry impulses away from the cell body, toward a synapse or a neuromuscular junction. Neurons sort into three types based simply on how many axons and dendrites they carry. Multipolar neurons, with one axon and two or more dendrites, are the type found in the cerebral cortex. Bipolar neurons, with exactly one axon and one dendrite, are found in the eye's retina. Unipolar neurons, with a cell body carrying just one axon and no separate dendrites, appear mainly during embryonic development. Axons themselves come in two further varieties based on insulation. Myelinated axons are wrapped by Schwann cells that form a fatty myelin sheath around the axon, interrupted at regular intervals by small gaps called nodes of Ranvier, and this type is found in spinal and cranial nerves specifically. Unmyelinated axons are still enclosed by a Schwann cell, but that cell does not wrap the axon in an actual myelin sheath, and this type is found mainly in the autonomic and somatic neural systems.

Neurons are excitable cells specifically because their membranes maintain a deliberately unequal, polarised electrical state, and understanding why requires looking at exactly which ions can cross the membrane and which cannot. A resting, non-conducting neuron's axonal membrane is comparatively permeable to potassium ions but nearly impermeable to sodium ions, and it is also impermeable to the negatively charged proteins floating in the axoplasm. The consequence is a genuinely stark imbalance: the axoplasm inside the axon ends up with a high concentration of potassium ions and negatively charged proteins but a low concentration of sodium ions, while the fluid outside carries the reverse, a low potassium concentration and a high sodium concentration, together forming a real concentration gradient across the membrane. This gradient does not simply happen and stay put; it is actively maintained by the sodium-potassium pump, which continuously transports three sodium ions out of the cell for every two potassium ions it brings in, using energy to work directly against both ions' natural diffusion gradients. The net effect of all this ion movement is that the axonal membrane's outer surface ends up carrying a net positive charge while its inner surface carries a net negative charge, a state called polarisation, and the specific electrical potential difference across this resting, polarised membrane is called the resting potential.

A nerve impulse is really nothing more than a travelling, self-resetting disturbance in this same resting polarisation, and following it step by step reveals a genuinely elegant mechanism. When a stimulus hits one specific site on a polarised membrane, that site suddenly becomes freely permeable to sodium ions, and sodium rushes rapidly inward, reversing the local polarity entirely: the outer surface at that exact site turns negative and the inner surface turns positive, a state called depolarisation, and the specific electrical potential difference generated at this depolarised site is called the action potential, which is really just another name for the nerve impulse itself. This local reversal does not stay contained to a single point, either. At the very next site along the axon, still in its original resting, polarised state, the outer surface stays positive while the just-depolarised site's outer surface has turned negative, and this local difference in charge drives a current to flow along the membrane's inner surface from the depolarised site toward this next site, and along the outer surface in the opposite direction, completing an electrical circuit. That current flow is itself enough to trigger the exact same permeability change, and therefore the exact same depolarisation, at this next site, effectively passing the impulse one step further down the axon. Repeat this same handoff at every successive site along the axon's length and the impulse travels the axon's entire length as a genuine wave. Sodium's spike in permeability is extremely short-lived, though; it is quickly followed by a rise in potassium permeability instead, and potassium ions diffuse back out through the membrane within a fraction of a second, restoring the original resting potential at that site, called repolarisation, and leaving that stretch of membrane ready to respond to a fresh stimulus all over again. The impulse, in other words, does not simply move forward, it moves forward while actively repairing the polarisation behind it, which is exactly what lets a single axon carry impulse after impulse in rapid succession.

An impulse travelling down one neuron's axon eventually has to cross over into a completely separate neuron, and that crossing happens at a specialised junction called a synapse, formed where a pre-synaptic neuron's membrane meets a post-synaptic neuron's membrane, sometimes separated by an actual physical gap called the synaptic cleft and sometimes not. Synapses come in two genuinely different types. At an electrical synapse, the two neurons' membranes sit in extremely close proximity, letting electrical current flow directly from one neuron straight into the next, a transmission mechanism very similar to impulse conduction along a single axon and, as a result, always faster than transmission at a chemical synapse, though electrical synapses are actually fairly rare in the human system. At a chemical synapse, by contrast, an actual fluid-filled synaptic cleft genuinely separates the two neurons, and crossing that physical gap requires a real chemical messenger. When an action potential reaches the axon terminal of a pre-synaptic neuron, it triggers synaptic vesicles stored there to move toward the membrane and fuse with it, releasing their stored neurotransmitters directly into the synaptic cleft. These released neurotransmitters then diffuse across the cleft and bind to specific receptor proteins on the post-synaptic neuron's membrane, and this binding event opens ion channels there, letting ions flow in and generate an entirely new electrical potential in the post-synaptic neuron, a potential that can turn out to be either excitatory, making that neuron more likely to fire its own impulse, or inhibitory, making it less likely.

The brain functions as the body's central command and control system, responsible for an extraordinary range of jobs at once: voluntary movement, balance, the functioning of vital involuntary organs like the lungs, heart and kidneys, thermoregulation, hunger and thirst, the body's roughly 24-hour circadian rhythm, the activity of several endocrine glands, and the entirety of human behaviour, alongside processing vision, hearing, speech, memory, intelligence, emotion and thought. Physically, the brain sits protected inside the skull and, immediately beneath that bony protection, inside three further protective membrane layers collectively called the cranial meninges: an outer dura mater, a very thin middle arachnoid layer, and an inner pia mater sitting in direct contact with brain tissue itself. Structurally, the whole brain divides into three major regions, forebrain, midbrain and hindbrain, each covered in more detail across the next two sections of this chapter, together forming a genuinely layered command structure running from the most complex, evolutionarily newest processing at the front down to the most basic, life-sustaining functions at the back, closest to the spinal cord.

The forebrain, the brain's largest and most complex region, is itself built from three genuinely distinct structures. The cerebrum forms the major bulk of the human brain, split longitudinally by a deep cleft into left and right cerebral hemispheres, connected to each other by a thick tract of nerve fibres called the corpus callosum. Each hemisphere's outer layer, the cerebral cortex, is thrown into extensive folds and appears distinctly grey, called grey matter, because neuron cell bodies are densely concentrated there, and this cortex itself contains motor areas, sensory areas, and large association areas that are neither purely sensory nor purely motor, instead handling complex work like combining information across different senses, memory and communication. Beneath the cortex, myelinated fibre tracts give the cerebrum's inner layer an opaque, pale appearance, called white matter. Wrapped by the cerebrum is the thalamus, functioning as a major relay and coordinating centre for sensory and motor signals passing through the brain, and sitting at the thalamus's base is the hypothalamus, containing dedicated centres that regulate body temperature and the urge to eat and drink, alongside clusters of neurosecretory cells that release their own hypothalamic hormones directly. A further complex of inner cerebral structures, including the amygdala and hippocampus, together forms the limbic system, which works alongside the hypothalamus to regulate sexual behaviour, express emotional reactions like excitement, pleasure, rage and fear, and drive motivation generally.

Below the forebrain sit two further regions running the brain's more automatic, life-sustaining business. The midbrain sits between the forebrain's thalamus and hypothalamus above and the hindbrain's pons below, with a canal called the cerebral aqueduct running through it and, on its dorsal surface, four rounded swellings called the corpora quadrigemina. The hindbrain comprises three further structures. Pons is built from fibre tracts that interconnect different regions of the brain, functioning essentially as a communication relay hub. The cerebellum carries a genuinely highly convoluted, folded surface specifically to fit far more neurons into a limited space than a smooth surface ever could, and it integrates sensory information, including signals from the ear's balance-sensing semicircular canals and the auditory system, to help coordinate movement and maintain balance. The medulla, also called the medulla oblongata, connects directly to the spinal cord and houses genuinely vital centres controlling respiration, cardiovascular reflexes and gastric secretions, the same respiratory rhythm centre and cardiac regulation centre already introduced earlier in this thread's chapters on breathing and circulation. Midbrain, pons and medulla together are collectively called the brain stem, forming the direct physical and functional connection between the brain proper and the spinal cord below it, and, fittingly, running some of the most basic functions a body needs simply to stay alive, entirely without needing any conscious input at all.