In one breath
The CNS is built from just two kinds of cell: neurons, which carry impulses as action potentials, and neuroglia, which support, feed, insulate and protect them and far outnumber them. The four CNS glia are astrocytes, oligodendrocytes (which make CNS myelin), microglia (the scavengers) and ependymal cells; Schwann cells are the glia of the PNS. A neuron has a soma, dendrites that receive, and one axon whose initial segment fires the action potential.
Builds on: Divisions of the nervous system · Leads to: Synapses and synaptic transmission · Sensory receptors and their classification
Two kinds of cell
| Neuroglia | Neurons | |
|---|---|---|
| Job | Support and protect; keep the fluid around neurons stable | Carry impulses as action potentials |
| Number | 10–30 times more | Fewer |
| Conduct action potentials? | No | Yes |
| Can divide? | Yes | No |
- The glial cells of the nervous system are called neuroglia.
- Worth knowing, though not in your pages: modern cell counts put glia and neurons in the human brain nearer one to one. Quote your book’s 10–30 in the exam.
Glial cells
- They do not conduct action potentials and do not form functional synapses with other cells.
- They can still be passively polarised by neural activity close by.
- Their roles go beyond support and are not yet fully understood.
- Unlike neurons, they can multiply (see gliomas below).
Functions of glia
- Form the mechanical matrix in which neurons sit.
- Give neurons metabolic and nutritive support.
- May help regulate blood flow through the brain.
- Buffer the ions around neurons, taking them up or giving them out as needed.
- Insulate axons and synapses, isolating them electrically from each other.
- Phagocytose neural debris.
Types of glia
| Glial cell | Where | Main job |
|---|---|---|
| Astrocyte | Throughout brain and cord | Matrix, nutrition, insulating synapses |
| Oligodendrocyte | Beside myelinated CNS axons | Makes CNS myelin |
| Microglia | Brain; smallest CNS cells | Scavenging (phagocytosis) |
| Ependymal cell | Lines ventricles and central canal | Unclear, says your book |
| Schwann cell | PNS | Myelin of peripheral nerves |
Astrocytes
- Named for their star shape. Plentiful all through the brain and spinal cord.
- Their processes wrap around neurons and axons, and many end on blood-vessel walls.
- Functions:
- provide the mechanical matrix;
- give neurons metabolic and nutritive support;
- wrap most CNS synapses, so they insulate synapses electrically and keep them apart.
- Worth knowing, though not in your pages:
- fibrous astrocytes lie in white matter and protoplasmic astrocytes in grey matter;
- their end-feet on capillaries help maintain the blood–brain barrier (the barrier itself is the tight junctions of the capillary endothelium);
- they take up excess K⁺ and the transmitter glutamate from around neurons, and after injury they form a glial scar.
Oligodendrocytes
- Lie close to myelinated axons in the brain and spinal cord.
- Their processes wind many times round an axon to make the myelin sheath.
- Myelin insulates axons from one another and limits current flow across the axon membrane (axolemma).
- So impulses are conducted in a saltatory (jumping) way, much faster than in unmyelinated fibres.
- They do for CNS axons what Schwann cells do for peripheral nerves.
- Worth knowing, though not in your pages: one oligodendrocyte myelinates segments of many axons, but one Schwann cell myelinates a single segment of one axon. The bare gaps between segments are the nodes of Ranvier.
Microglia
- The smallest cells in the CNS.
- The scavenger cells of the brain.
- When nervous tissue is damaged or infected, they enlarge into mononuclear phagocytes and clear away debris and organisms.
- Worth knowing, though not in your pages: microglia are the brain’s own macrophages, of mesodermal origin, whereas the other glia come from neuroectoderm.
Ependymal cells
- Line the ventricles of the brain and the central canal of the spinal cord.
- Your book says their function is unclear.
- Worth knowing, though not in your pages: the choroid plexus epithelium, which secretes CSF, is modified ependyma, and ependymal cilia help move CSF along.
Applied: gliomas
Neurons cannot divide, but glia can. After a brain injury, glia multiply to fill the space that lost neurons leave behind. When they multiply too much the result is a glioma, a common malignant tumour of the brain.
Three glia traps in MCQs
- CNS myelin comes from oligodendrocytes; PNS myelin from Schwann cells.
- The brain’s scavengers are microglia, not astrocytes.
- Stellate cells are neurons (your book lists them under dendritic pattern), so in a “which is a glial cell?” question, pick the ependymal cell. Worth knowing, though not in your pages: Purkinje and basket cells are neurons of the cerebellar cortex.
Neuron: structure
The neuron (nerve cell) is the functional unit of the nervous system. It has three parts: the soma (cell body), the dendrites and the axon.
| Part | Nissl bodies | What it does |
|---|---|---|
| Soma | Present, except in the axon hillock | Makes proteins and transmitters |
| Dendrites | Proximal part only | Receive; conduct toward the soma |
| Axon | Absent | Starts the action potential; carries it to the synapse |
Soma
- Holds the nucleus and the cytoplasm.
- Organelles in the cytoplasm: endoplasmic reticulum, a prominent Golgi apparatus, many mitochondria, and a cytoskeleton of microfilaments (neurofilaments) and microtubules.
- Nissl bodies are special granules of modified rough endoplasmic reticulum. They are the neuron’s protein-making machinery, and they fill the soma except the axon hillock.
- The nucleus sits centrally or off to one side, and has a nucleolus.
Dendrites
- Tapering processes, simple or elaborate, that branch from the soma.
- They make up over 90% of the neuron’s surface area.
- Proximal dendrites (near the soma) contain Nissl granules and Golgi apparatus; distal dendrites have no Nissl granules.
- Their skeleton is mainly microfilaments and microtubules.
- With the soma they form the neuron’s receiving (receptor) zone, and they conduct impulses toward the soma.
Axon
- The longest process. It leaves the soma at the axon hillock, the tapered part of the cell body.
- The hillock leads into the initial segment, the unmyelinated first stretch of the axon.
- The initial segment is the spike initiation zone: the action potential normally starts here, once the electrical activity of the soma and dendrites has summated.
- Inside is the axon cylinder, filled with axoplasm. It holds mitochondria, neurofibrils and vesicles, but no Nissl bodies.
- Neurotransmitters are made in the soma and carried down the axoplasm to the synapse by axonal flow.
- An axon covered by myelin is a myelinated fibre; one without it is an unmyelinated fibre.
| Contains | Axon hillock | Proximal dendrites |
|---|---|---|
| Nissl granules | No | Yes |
| Endoplasmic reticulum | No | Yes |
| Golgi apparatus | No | Yes |
- Worth knowing, though not in your pages:
- the initial segment has the densest voltage-gated Na⁺ channels, which is why its threshold is the lowest;
- axonal transport runs both ways: anterograde (soma to terminal, by kinesin) and retrograde (terminal to soma, by dynein). Rabies virus and tetanus toxin reach the CNS by retrograde transport.
Draw it: a typical neuron
Draw a soma with a nucleus, a nucleolus and dots for Nissl bodies, leaving the axon hillock clear of dots. Put branching dendrites on one side with an arrow toward the soma, labelled “receiving zone”. From the hillock draw the initial segment (label it “spike initiation zone”), then a long axon wrapped in myelin segments with gaps between them, ending in terminal branches. Add an arrow along the axon, pointing away from the soma.
Types of neuron
Neurons are classified four ways: by the number of processes, the length of the axon, their function and their pattern of dendrites.
By number of processes
| Type | Processes | Example |
|---|---|---|
| Unipolar | One | Invertebrates; in vertebrates, the ANS |
| Pseudounipolar | One, which splits into central and peripheral branches | Dorsal root ganglion cell |
| Bipolar | Two | Bipolar cell of the retina |
| Multipolar | Many | Spinal motor neuron |
- Worth knowing, though not in your pages: many books loosely call the dorsal root ganglion cell “unipolar”, so read MCQ options carefully.
Draw it: four types by processes
Draw four cell bodies in a row. Unipolar: one process. Pseudounipolar: one short stem that forks like a T, a central branch to the cord and a peripheral branch to the skin. Bipolar: one process at each pole. Multipolar: many dendrites and one axon. Write an example under each.
By length of axon
| Axon | Your book’s label | Your book’s example |
|---|---|---|
| Short; dendrites end near the soma | Golgi type 1 | Cortical inhibitory neurons |
| Long | Golgi type 2 | Cortical motor neurons of the corticospinal tract |
A slip in your book: the Golgi types are reversed
Standard histology has it the other way round. Golgi type I neurons have long axons that leave their region (projection neurons such as the pyramidal cells of the Corticospinal (pyramidal) tract, Purkinje cells and spinal motor neurons). Golgi type II neurons have short axons that branch near the soma (local interneurons, many of them inhibitory). In an answer, give the standard version: type I long, type II short.
By function
- Sensory (afferent) neurons: carry impulses from receptors to the CNS.
- Motor (efferent) neurons: carry impulses from the CNS to the target organs.
By dendritic pattern
- Pyramidal cells: the dendrites spread out in a pyramid, as in hippocampal pyramidal neurons.
- Stellate cells: the dendrites radiate like a star, as in the stellate cells of the cortex.
Exam-answer skeleton: "Neuroglia: types and functions" (short note)
- Define neuroglia: the supporting cells of the nervous system, 10–30 times as many as neurons. They conduct no action potentials and form no synapses, but they can divide.
- List the six general functions of glia.
- Types: astrocytes, oligodendrocytes, microglia and ependymal cells in the CNS; Schwann cells in the PNS.
- Astrocytes: star-shaped; matrix, nutrition and insulation of synapses.
- Oligodendrocytes: CNS myelin, and so saltatory conduction.
- Microglia: the smallest cells; scavengers that turn into phagocytes.
- Ependymal cells: line the ventricles and the central canal.
- Applied: gliomas.
Asked in exams
- Neuroglia: types and functions: Dec 2015, 2 marks · Jan/Feb 2022, MCQ · May 2022, MCQ · May 2022, 2 marks · Oct 2024, MCQ
- The parts of a neuron and what each does: Mar 2024, 5 marks