Orientation
Rapid review — the whole lesson in one panel
What you should already have
About 45 minutes
Plus the time it takes to redraw this lesson’s figures from memory, which is the fastest way to find out what you have not understood.
Where this shows up
Every later lesson assumes this vocabulary. It is also the frame the ageing nervous system is described in — brain, spinal cord, autonomic nervous system and functional impairment — and those four headings only mean something once the divisions behind them are secure.
Learning outcomes
By the end of this lesson you should be able to:
- Divide the nervous system structurally into central and peripheral, and functionally into somatic and autonomic, and state what each division carries.
- Name the parts of a neuron and relate each to its role in receiving, integrating, conducting and transmitting a signal.
- Classify neurons by structure (unipolar, bipolar, multipolar, pseudounipolar) and by function (afferent, efferent, interneuron).
- State the functions of astrocytes, oligodendrocytes, Schwann cells, microglia and ependymal cells, and why glia outnumber neurons.
- Explain axonal transport, its two directions and their speeds, and name one clinical consequence of each.
- Locate the grey and white matter of the brain and cord, and state which contains cell bodies and which contains tracts.
Together these settle one syllabus objective: Organisation of the nervous system, and the neurone and glia. Tick it on the Physiology objective list once you can do all of the above without notes.
The minimum high-yield framework
- Two structural divisions — central (brain and spinal cord) and peripheral (cranial and spinal nerves, ganglia). Two functional divisions — somatic and autonomic.
- The neuron is polarised. Dendrites and soma receive, the axon hillock decides, the axon conducts, the terminal transmits. That order is the answer to most “describe a neuron” questions.
- Structural classification — unipolar, bipolar, pseudounipolar, multipolar. Functional classification — afferent (sensory), efferent (motor), interneuron. Interneurons are the great majority.
- Five supporting cells. Astrocytes (homeostasis and the blood-brain barrier), oligodendrocytes (central myelin), Schwann cells (peripheral myelin), microglia (immune), ependymal cells (line the ventricles, secrete CSF).
- Glia outnumber neurons and occupy roughly half the brain’s volume, but consume much less energy per cell.
- The axon cannot make protein. Everything it needs arrives by anterograde transport on kinesin; everything returning to the soma travels retrogradely on dynein.
- Grey matter is cell bodies (cortex, deep nuclei, the H-shaped centre of the cord); white matter is myelinated tracts. In the brain, grey is outside and white inside. In the cord it is the other way round.
The map
How the nervous system divides, structurally and functionally
The structural division is anatomical and asks only: is this structure inside the skull and vertebral canal, or outside it? The functional division is physiological and asks what the tissue does and whether it is under voluntary control. The two overlap: the sympathetic nervous system has central components (the intermediolateral cell column of the cord) and peripheral ones (the ganglia and postganglionic fibres).
| Division | Classification | What it contains | Defining feature |
|---|---|---|---|
| Central nervous system (CNS) | Structural | Brain and spinal cord | Integration and processing. Everything that is not a peripheral nerve or ganglion. |
| Peripheral nervous system (PNS) | Structural | Cranial nerves (12 pairs), spinal nerves (31 pairs), their ganglia and plexuses | Carries information to and from the CNS. The dorsal root ganglion cell body sits here even though its central process enters the cord. |
| Somatic nervous system | Functional | Somatic afferents from skin, muscle and joint; somatic efferents to skeletal muscle | Voluntary and consciously perceived. One neuron from the CNS to the effector, always excitatory, transmitter always acetylcholine at a nicotinic receptor. |
| Autonomic nervous system (ANS) | Functional | Sympathetic, parasympathetic and enteric divisions | Involuntary. Two neurons in series with a ganglion between them, and the effector may be excited or inhibited. |
| Enteric nervous system | Functional | Myenteric (Auerbach) and submucosal (Meissner) plexuses | Can generate and sustain reflexes without any CNS input at all, which is why it is often treated as a third autonomic division. |
Within the peripheral nervous system, fibres are further named by the direction they carry information. Afferent fibres carry toward the CNS; efferent fibres carry away from it. Both words are relative to the CNS, and both apply within the autonomic system as well as the somatic one — a baroreceptor afferent and a cutaneous afferent are both afferents.
The signalling cell
The parts of a neuron, and the job each one does
The neuron and its glial partners
A multipolar neuron, the commonest type in the central nervous system, with the four cell types that support it. Information flows one way: dendrites and soma receive, the axon hillock decides, the axon conducts, the terminal transmits. Schematic — the axon of a peripheral motor neuron may be a metre long, so no scale is implied. Myelin is drawn by an oligodendrocyte here because this is a central axon; in a peripheral nerve the same job is done by Schwann cells, one cell per internode.
| Part | Structure | Function | Why it matters |
|---|---|---|---|
| Dendrites | Branching processes carrying dendritic spines | Receive. Each spine is a postsynaptic site; a single cortical neuron may carry thousands. | Receptive field of the cell. Graded potentials only — no action potentials are generated here. |
| Soma (cell body, perikaryon) | Nucleus, nucleolus, Nissl substance (rough endoplasmic reticulum), Golgi, mitochondria | Synthesise. Nissl substance is the protein factory; its loss (chromatolysis) is the histological marker of axonal injury. | Also receives synapses directly (axosomatic). |
| Axon hillock and initial segment | The cone where the axon leaves the soma, and the first unmyelinated stretch beyond it | Decide. The highest density of voltage-gated sodium channels in the neuron, so the lowest threshold. | This is where summed synaptic input is compared with threshold — the trigger zone. |
| Axon | Single process; may branch into collaterals; contains microtubules, neurofilaments, mitochondria | Conduct. Carries the action potential, unchanged in size, to the terminal. | No rough endoplasmic reticulum, so it cannot make its own protein — everything arrives by axonal transport. |
| Myelin sheath and nodes of Ranvier | Concentric wraps of glial membrane, interrupted at regular intervals | Insulate. Raises membrane resistance and lowers capacitance so current runs to the next node. | Nodes carry the sodium channels. Myelin is made by oligodendrocytes centrally and Schwann cells peripherally. |
| Axon terminal (bouton) | Expanded ending packed with vesicles and mitochondria | Transmit. Converts the electrical signal into a chemical one. | Contains the voltage-gated calcium channels whose opening triggers exocytosis. |
Two features of that list carry more weight than the rest. The first is that the axon hillock and initial segment carry the highest density of voltage-gated sodium channels in the cell, and therefore the lowest threshold. That single fact is why a neuron integrates: thousands of graded inputs arriving all over the dendritic tree and soma are summed, and the sum is tested against threshold at one place. The second is that the axon has no rough endoplasmic reticulum. An axon a metre long is entirely dependent on protein made in a soma at the far end of it, which is why axonal transport is not a piece of trivia but the axon’s lifeline.
Two axes
Classifying neurons by structure and by function
| Type | Processes from the soma | Where you find it |
|---|---|---|
| Multipolar | One axon and many dendrites | The commonest type. Motor neurons, cortical pyramidal cells, most interneurons. |
| Bipolar | One axon and one dendrite at opposite poles | Retina, olfactory epithelium, the cochlear and vestibular ganglia. |
| Pseudounipolar | A single process that divides into a peripheral and a central branch | Dorsal root ganglion cells and their trigeminal equivalent — every somatic sensory first-order neuron. |
| Unipolar | A single process | Common in invertebrates; rare in the adult mammalian nervous system. |
The pseudounipolar arrangement matters clinically and in the examination. The peripheral branch behaves like a dendrite functionally — it carries information toward the cell — but is structurally an axon, myelinated and capable of conducting action potentials. The action potential travels from the periphery straight past the cell body into the cord without being processed by the soma at all. That is why a dorsal root ganglion block interrupts sensation while leaving the cell body alive, and why the cell body sits in a ganglion outside the direct signal path.
| Type | Direction | Comment |
|---|---|---|
| Afferent (sensory) | Periphery → CNS | Cell body in a dorsal root or cranial nerve ganglion. Pseudounipolar for somatic sensation. |
| Efferent (motor) | CNS → effector | Cell body in the anterior horn or a cranial motor nucleus, or in an autonomic ganglion for postganglionic fibres. |
| Interneuron | Entirely within the CNS | By far the most numerous class. Both excitatory and inhibitory. The Renshaw cell and the substantia gelatinosa interneuron are the two you will be asked about by name. |
The supporting cells
Glia: five cell types, and why the brain needs all of them
The central nervous system contains on the order of 1011 neurons and many times that number of glial cells; glia occupy roughly half the volume of the brain. Unlike neurons, glia continue to divide in adult life, which is why the proliferative response to injury — gliosis — is glial, and why almost all primary intracranial tumours in adults are glial rather than neuronal in origin.
| Cell | Where | Structure | Functions | Clinical relevance |
|---|---|---|---|---|
| Astrocyte | CNS | Star-shaped, with end-feet on capillaries and on neurons. Fibrous type in white matter, protoplasmic in grey. | Induce and maintain the blood-brain barrier; buffer extracellular potassium; take up and recycle glutamate and GABA; supply lactate to neurons; form the glial scar; contribute to neurovascular coupling. | Glutamate uptake failure is a step in excitotoxic injury. Astrocyte swelling is the cytotoxic component of cerebral oedema. |
| Oligodendrocyte | CNS | Small cell with several processes; one cell myelinates segments of up to about 50 different axons. | Forms CNS myelin. | The target in multiple sclerosis. Poor regenerative capacity, which is why central axons do not recover the way peripheral ones can. |
| Schwann cell | PNS | One cell forms one internode of one axon; also ensheathes (without myelinating) groups of C fibres. | Forms peripheral myelin; guides peripheral axonal regeneration along the endoneurial tube. | The target in Guillain-Barré syndrome. Peripheral nerves can regenerate at roughly 1 mm per day because Schwann cells survive and guide the regrowing axon. |
| Microglia | CNS | Small, highly ramified. Derived from the monocyte-macrophage lineage, not from neuroectoderm. | The resident immune cell of the brain: phagocytosis of debris, antigen presentation, cytokine release, synaptic pruning. | Activated in ischaemia, infection and neurodegeneration; a source of the inflammatory mediators implicated in postoperative delirium and cognitive change. |
| Ependymal cell | CNS | Ciliated cuboidal-to-columnar epithelium lining the ventricles and central canal; specialised as choroid plexus epithelium. | Line the ventricular system, move CSF by ciliary beating, and — as choroid plexus epithelium — secrete CSF and form the blood-CSF barrier. | The blood-CSF barrier is ependymal, not endothelial. That is the distinction the blood-brain barrier question turns on. |
Supplying the axon
Axonal transport, in both directions
| System | Direction | Speed | Motor protein | Cargo |
|---|---|---|---|---|
| Fast anterograde | Soma → terminal | ≈ 400 mm per day | Kinesin, walking along microtubules toward the plus end | Membrane-bound organelles: synaptic vesicle precursors, mitochondria, transmitter-synthesising enzymes |
| Slow anterograde | Soma → terminal | ≈ 0.5-10 mm per day | Also kinesin-based, but with long pauses | Cytoskeletal elements and cytosolic proteins — the axon's own structure |
| Retrograde | Terminal → soma | ≈ 200 mm per day | Dynein, walking toward the microtubule minus end | Recycled membrane, neurotrophins (nerve growth factor), and signals reporting on the terminal's state |
Both systems run along microtubules, which are intrinsically polarised, and this polarity is what gives each motor its direction: kinesin walks toward the plus end (away from the soma) and dynein toward the minus end (toward it). Both are ATP-dependent, so both stop when the axon becomes ischaemic — one reason a chronically compressed or hypoperfused nerve degenerates distally.
Organisation of the tissue
Grey matter, white matter, and why the arrangement inverts
Grey matter is neuronal cell bodies, dendrites, unmyelinated axons and glia. White matter is myelinated axons running in tracts; it is white because myelin is lipid-rich. The distinction matters physiologically because the two have very different metabolic rates: cerebral blood flow and cerebral metabolic rate are both about four times greater in grey matter than in white.
| Structure | Grey matter | White matter |
|---|---|---|
| Cerebral hemisphere | Outside — the cortex, a 2-4 mm mantle. Plus deep nuclei (basal ganglia, thalamus). | Inside — the corona radiata, internal capsule, corpus callosum and association fibres. |
| Cerebellum | Outside — the cerebellar cortex. Plus the deep cerebellar nuclei. | Inside — the arbor vitae. |
| Spinal cord | Inside — the H-shaped or butterfly-shaped central region: dorsal, lateral and ventral horns. | Outside — the dorsal, lateral and ventral columns, carrying the ascending and descending tracts. |
The reason the cord inverts is developmental: the neural tube’s neurons stay where they are formed, around the central canal, and their axons grow outward to form the surrounding tracts. In the forebrain, neurons migrate outward along radial glia to form a surface cortex, leaving their axons behind in the interior. Stating that reason turns a memorised fact into an explained one, which is what the difference between a pass and a good pass usually is.
Anaesthetic and clinical application
Where this appears in the operating theatre
The patient with multiple sclerosis presenting for surgery
The lesion is oligodendrocyte and central myelin. Conduction in demyelinated central tracts is slowed, blocked, or made temperature-sensitive: a rise of a fraction of a degree can convert a marginally conducting fibre into a blocked one, which is why relapses follow fever and why a rise in core temperature perioperatively is worth avoiding actively. Because the peripheral nervous system is spared, the neuromuscular junction is structurally normal, but disuse atrophy and denervation-like upregulation of extrajunctional receptors make suxamethonium a hyperkalaemia risk in a patient with established motor deficit. Regional anaesthesia is not contraindicated, but the physiology should be understood rather than assumed.
The patient with Guillain-Barré syndrome
The same physiology, one cell type across. Here the target is the Schwann cell and peripheral myelin, so the deficit is peripheral, ascending and includes autonomic instability — the autonomic fibres are peripheral too. The same suxamethonium risk applies, for the same reason, and it persists for months after apparent recovery.
Positioning and the peripheral nerve
Perioperative nerve injury is a transport and a blood-supply problem before it is anything else. Sustained compression or stretch raises intraneural pressure, reduces endoneurial blood flow, and stops the ATP-dependent transport the distal axon depends on. Mild insults produce a conduction block that recovers in days; axonal disruption regenerates at roughly 1 mm per day, because that is the speed the Schwann cell tube and slow anterograde transport can sustain. That number is the reason a brachial plexus injury is quoted in months.
Previously examinedOctober 2020 — the physiological changes in the central nervous system of the elderly, under the headings brain, spinal cord, autonomic nervous system and functional impairment. Worked answers in the library
Consolidation
The lesson in one paragraph
The nervous system divides structurally into central and peripheral, and functionally into somatic and autonomic. Its signalling cell is the neuron, a polarised cell in which dendrites and soma receive graded inputs, the axon hillock integrates them and tests the sum against threshold, the axon conducts an all-or-none action potential, and the terminal converts it back into a chemical signal. Neurons are classified structurally as multipolar, bipolar, pseudounipolar or unipolar, and functionally as afferent, efferent or interneuron. Because the axon contains no rough endoplasmic reticulum, it depends on anterograde transport on kinesin at up to 400 mm per day and retrograde transport on dynein at about 200 mm per day. Glia outnumber neurons and occupy about half the brain’s volume: astrocytes maintain the extracellular environment and the blood-brain barrier, oligodendrocytes and Schwann cells make central and peripheral myelin respectively, microglia provide immune surveillance, and ependymal cells line the ventricles and, as choroid plexus epithelium, secrete cerebrospinal fluid. Grey matter is cell bodies and white matter is myelinated tracts; grey is peripheral in the brain and central in the cord.