Types of Neurone

Spec Mapping — OCR H420 Module 5.1.3 — Neuronal communication, content statements covering the structure and function of sensory, relay (intermediate) and motor neurones, including the role of myelinated and non-myelinated neurones in the mammalian nervous system (refer to the official OCR H420 specification document for exact wording). This lesson establishes the anatomical vocabulary that every later neurone lesson — resting potential, action potential, saltatory conduction, synapse, reflex arc — assumes you already command.

The mammalian nervous system owes its speed and precision to neurones — highly specialised cells that generate and conduct electrical impulses called action potentials. OCR A-Level Biology A specification module 5.1.3 requires you to describe the structures of sensory, relay (intermediate) and motor neurones, and to relate these structures to their functions. This lesson provides the foundation for every subsequent topic in neuronal communication: without a clear mental map of what a neurone looks like, nothing that follows will make sense.

The conceptual lineage of the modern neurone doctrine is short but distinguished. The Spanish histologist Santiago Ramón y Cajal (Nobel 1906, shared with Camillo Golgi) established that the nervous system is composed of discrete cells separated by tiny gaps — the neurone doctrine — paraphrased here from his Golgi-stained drawings of cerebellar Purkinje cells and pyramidal cortical neurones. Otto Loewi (1921) demonstrated, through his elegant vagus-stimulation experiment on isolated frog hearts, that signalling between neurones and effectors is chemical rather than electrical; he named the unknown released substance "Vagusstoff", later identified as acetylcholine (paraphrase). Henry Dale (1936 Nobel, shared with Loewi) extended this work by classifying neurones as cholinergic (releasing acetylcholine) or adrenergic (releasing noradrenaline), a classification still used today (paraphrase). And Alan Hodgkin and Andrew Huxley (1952, Nobel 1963) characterised the ionic basis of the action potential using the voltage-clamp technique on the squid giant axon — a paradigm-defining piece of work to be revisited in detail in the resting/action potential lesson (paraphrase). For now, simply hold these scientists in mind as you build the anatomical foundation in this lesson; their experiments only become interpretable once you know what a neurone looks like.

Key Definitions:

• Neurone — an excitable cell that transmits electrical impulses (action potentials) along its plasma membrane.
• Nerve — a bundle of many neurones wrapped in connective tissue; not a single cell.
• Dendron / Dendrite — a cytoplasmic extension that carries impulses towards the cell body.
• Axon — a long cytoplasmic extension that carries impulses away from the cell body.
• Cell body (soma) — the region containing the nucleus and the bulk of the cytoplasm, rough endoplasmic reticulum and mitochondria.

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Why Neurones Are Specialised

Every cell in the body has a membrane potential, but only excitable cells — neurones and muscle cells — can generate and propagate action potentials. Several structural features make neurones ideal for rapid, long-distance communication:

• Elongated shape — a single motor neurone axon can stretch from the spinal cord to the toe, approaching one metre long. This allows signalling over long distances without relay.
• Cell membrane with voltage-gated ion channels — Na⁺ and K⁺ channels open in response to changes in membrane voltage, producing the characteristic action potential.
• Abundant mitochondria — providing ATP for the Na⁺/K⁺ pump, which maintains ion gradients that make action potentials possible.
• Abundant rough ER and Nissl bodies — for synthesis of membrane proteins and neurotransmitters.
• Myelin sheath (many but not all neurones) — insulating wraps made by Schwann cells that speed up conduction by forcing action potentials to jump between nodes of Ranvier.

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The Three Functional Types

OCR recognises three types of neurone based on where they carry information within the reflex arc.

1. Sensory Neurones

Sensory neurones carry impulses from receptors to the central nervous system (CNS).

• Have one long dendron leading from a sensory receptor (e.g. a temperature receptor in the skin) to the cell body.
• Have one short axon leading from the cell body into the CNS.
• The cell body lies in a dorsal root ganglion just outside the spinal cord — a characteristic feature that is often asked about in exams.
• Shape: pseudo-unipolar or bipolar (OCR does not require you to use these terms but recognising the characteristic shape is useful).

2. Relay (Intermediate) Neurones

Relay neurones lie within the CNS and connect sensory to motor neurones. OCR uses the term "relay neurone"; "intermediate neurone" and "interneurone" mean the same thing.

• Have many short dendrites and one short axon.
• Entire neurone lies within the brain or spinal cord.
• Form complex networks — each relay neurone may make thousands of synapses with other neurones.
• Usually not myelinated, because the distances travelled are small.

3. Motor Neurones

Motor neurones carry impulses from the CNS to effectors (muscles or glands).

• Have many short dendrites receiving input from relay neurones at the cell body in the CNS (in the ventral horn of the spinal cord for spinal motor neurones).
• Have one long axon leaving the CNS and extending to the effector.
• Axon terminates at a neuromuscular junction (if the effector is a skeletal muscle), which is structurally similar to a cholinergic synapse.
• Heavily myelinated to maximise conduction speed.

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Comparison Table

Feature	Sensory neurone	Relay neurone	Motor neurone
Direction of impulse	Receptor → CNS	Within CNS	CNS → effector
Cell body location	Dorsal root ganglion (outside CNS)	Inside CNS	Inside CNS (e.g. ventral horn)
Number of dendrons / dendrites	One long dendron	Many short dendrites	Many short dendrites
Axon length	Short	Short	Long
Myelination	Yes (peripherally)	Usually not	Yes
Typical speed	~60 m s⁻¹	Low (short paths)	Up to 120 m s⁻¹
Example function	Sensing a pinprick	Coordinating the reflex in the spinal cord	Contracting the biceps

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Diagrammatic Overview

flowchart LR
    R[Sensory receptor in skin] -->|Generator potential| S["Sensory neurone<br/>Cell body in DRG"]
    S -->|Action potential| RL[Relay neurone in spinal cord]
    RL -->|Action potential| M[Motor neurone]
    M -->|Synaptic transmission| E[Effector: skeletal muscle]
    E --> RESP[Response: withdraw limb]

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Detailed Motor Neurone Anatomy — Inline Diagram

The motor neurone is the canonical "named example" you will be expected to draw and label in exams. Study the labelled diagram below and commit it to memory; almost every diagram-based question on this topic asks for a subset of these labels.

Legend: orange = cell body / dendrites; blue = Schwann cell + myelin; gaps = nodes of Ranvier; pink = synaptic knobs.

The labelled features above map onto OCR's standard mark scheme: an unlabelled diagram is a zero-mark answer no matter how artistically rendered. Practise sketching this from scratch on plain paper until you can label all eight features (dendrites, cell body, nucleus, axon hillock, axon, Schwann cell, node of Ranvier, axon terminal) in under two minutes.

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The Cell Body in Detail

Regardless of functional type, every neurone has a cell body (soma) that contains:

• Nucleus — with euchromatin indicating active transcription.
• Rough endoplasmic reticulum (Nissl substance) — highly developed; neurones are protein-secreting cells that must continuously replace membrane proteins, enzymes and neurotransmitters.
• Golgi apparatus — packages neurotransmitters into vesicles that travel down the axon.
• Mitochondria — exceptionally numerous; neurones demand large amounts of ATP to run the Na⁺/K⁺ pump.
• Neurofilaments and microtubules — cytoskeletal elements that provide structural support and act as railways for axonal transport.

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Axonal Transport

The cell body sits at one end of the neurone, but mitochondria, membrane proteins and vesicles must reach the distant axon terminal. This happens via axonal transport along microtubules, powered by motor proteins (kinesin for anterograde, dynein for retrograde). OCR does not require the names of motor proteins but understanding that material moves along the axon explains why damage to the cell body ultimately kills the whole neurone.

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Myelin Sheath and Schwann Cells

In myelinated neurones, a specialised glial cell called a Schwann cell wraps itself around the axon many times, forming a fatty insulating layer. Between adjacent Schwann cells lie small gaps called nodes of Ranvier, which is where voltage-gated ion channels are concentrated. The next lesson explores how this arrangement enables saltatory conduction; for now, simply appreciate that most sensory and motor neurones are myelinated, whereas most relay neurones are not.

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The Reflex Arc in Context

The three types of neurone work together in a reflex arc, a neural pathway that produces a rapid, involuntary response to a stimulus:

1. Stimulus detected by a receptor (e.g. pain from a drawing pin).
2. Sensory neurone carries the impulse to the spinal cord.
3. Relay neurone in the spinal cord passes the impulse to a motor neurone.
4. Motor neurone carries the impulse to the effector.
5. Effector (a muscle) contracts, removing the body part from harm.

The reflex bypasses the brain, making it much faster than a conscious response. The brain still receives information about what happened via ascending tracts, which is why you then feel pain after the reflex.

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The Axon Hillock — Spike-Initiation Zone

The axon hillock is the narrow tapering region where the axon emerges from the cell body. It is the most important computational location in the entire neurone, yet it is rarely emphasised in introductory accounts. Three properties make it the spike-initiation zone:

1. Highest density of voltage-gated Na⁺ channels. While dendrites and the soma carry mostly ligand-gated channels (responding to neurotransmitters), the axon hillock carries voltage-gated channels at high density. This means it has the lowest threshold of any part of the neurone — a relatively small depolarisation here is enough to fire an action potential.
2. Convergence point. All the EPSPs and IPSPs that arrive at the dendrites and soma must travel passively to the axon hillock. Passive spread weakens these signals (a phenomenon called electrotonic decay), so by the time signals reach the axon hillock they have already been integrated by the geometry of the cell. This is the biological basis of summation discussed in the synaptic transmission lesson.
3. Fire/no-fire decision. If the integrated depolarisation at the axon hillock reaches the threshold (typically around −55 mV), an action potential fires and propagates down the axon. If not, no action potential is generated — the all-or-nothing principle. This is the moment at which thousands of graded synaptic inputs are reduced to a single binary output.

In exam answers, you can earn a discriminator mark by mentioning the axon hillock as the trigger zone — most candidates omit it, even though it is structurally distinct and labelled in standard textbook diagrams.

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Glial Cells — The Forgotten Half of the Nervous System

For every neurone in the human nervous system there are roughly equally many glial cells (an old "10:1 glia:neurone" figure is now known to be inaccurate; the true ratio is close to 1:1 brain-wide, with regional variation). Glial cells are not directly excitable, but they perform essential supporting roles:

• Schwann cells — myelinate peripheral axons (one cell per internodal segment, ~1 mm). Featured in the OCR spec.
• Oligodendrocytes — the CNS equivalent of Schwann cells; each oligodendrocyte myelinates several CNS axons.
• Astrocytes — star-shaped CNS cells that buffer extracellular K⁺, recycle neurotransmitters, and form part of the blood–brain barrier.
• Microglia — the resident immune cells of the brain, derived from macrophage lineage.
• Satellite cells — surround sensory neurone cell bodies in dorsal root ganglia.

OCR focuses on Schwann cells, but understanding that glia exist and that not all neural cells are neurones is useful for synoptic answers and for the brain lesson later in this module.

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Exam Tip

In diagrams, always label the direction of the impulse with an arrow. For the sensory neurone, show the cell body sitting to the side on a short branch — a common mistake is to draw it in line, like a motor neurone. Label the dorsal root ganglion as the location of the sensory neurone's cell body, and remember to put the axon hillock on the motor neurone diagram.

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Common Exam Mistakes

• Confusing a neurone with a nerve. A nerve is a bundle of neurones held together by connective tissue.
• Placing the sensory neurone cell body within the CNS. It lies in the dorsal root ganglion, outside the spinal cord.
• Drawing the motor neurone with its cell body at the effector end. The cell body is in the CNS; the long axon projects to the muscle.
• Saying relay neurones are myelinated. They are usually unmyelinated — short distances do not benefit from saltatory conduction.
• Forgetting to name the Schwann cell as the source of myelin in the peripheral nervous system. (In the CNS, oligodendrocytes do this, but OCR focuses on peripheral neurones.)

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Synoptic Links

Synoptic Links — Connects to:

• ocr-alevel-biology-cell-structure / ultrastructure-nucleus-er — the neurone cell body is a textbook example of a secretory cell, packed with rough ER (Nissl substance) for neurotransmitter and membrane-protein synthesis. The Golgi packages neurotransmitter precursors into vesicles destined for axonal transport.
• ocr-alevel-biology-membranes-cell-division / fluid-mosaic-model — the entire neurone story depends on a fluid-mosaic membrane studded with voltage-gated and ligand-gated channels; you will not understand action potentials without understanding the membrane.
• ocr-alevel-biology-biological-molecules / lipids — myelin is ~80% lipid (sphingomyelin, cholesterol); the Schwann cell wraps its phospholipid bilayer many times to form an excellent electrical insulator. The exam-style link is "why is the myelin sheath rich in lipid?" — answer: lipids are non-polar and resist ion flow, so the membrane acts as a capacitive/resistive insulator.

Practical Activity Group anchor

Practical Activity Group anchor: PAG 1 — Microscopy techniques. Prepared slides of teased-out motor neurones (often spinal cord smears) can be examined at low magnification with a light microscope to identify cell body, dendrites and axon. The cell body is the most prominent feature; with phase-contrast or a basophilic stain (cresyl violet) the Nissl substance becomes visible inside the soma. This is the principal practical anchor for this lesson and is suitable for CPAC (Common Practical Assessment Criteria) work on biological drawing.

Specimen question modelled on the OCR H420 paper format

Question (6 marks): Figure 1 (above) shows a myelinated motor neurone. Identify three structural features of the motor neurone and explain how each contributes to its function as a fast-conducting peripheral neurone. Give one structural feature that distinguishes a sensory neurone from a motor neurone.

AO breakdown

Mark	AO	Awarded for
1	AO1	Correctly identifies the first structural feature (e.g. long myelinated axon, or Schwann cell, or axon hillock)
2	AO2	Links feature 1 to function (e.g. myelin insulates the axon, forcing saltatory conduction)
3	AO1	Correctly identifies the second structural feature
4	AO2	Links feature 2 to function
5	AO1	Identifies third feature + links to function (combined mark)
6	AO2	Distinguishing feature of sensory neurone — cell body in dorsal root ganglion, outside the CNS (not in the ventral horn)

AO split: AO1 = 3, AO2 = 3.

Mid-band response

The motor neurone has a long axon that runs from the spinal cord all the way to a muscle. This is useful because it means the impulse only has to travel through one cell, which is fast. The axon is covered in a myelin sheath made by Schwann cells, which insulates the axon so the action potential jumps from node to node. This is called saltatory conduction and it makes the neurone much faster than an unmyelinated one. The cell body is in the CNS (the spinal cord). The axon hillock is where the action potential is first generated when enough input has arrived at the dendrites. One way that a sensory neurone is different is that its cell body is in the dorsal root ganglion outside the spinal cord, on a small side branch, not in the main path of the axon.

Examiner commentary: M1 (long axon identified) + M2 (link to single-cell speed); M1 (myelin sheath identified) + M2 (saltatory conduction link, although phrased loosely — "jumps from node to node" is sufficient at C-grade); M1+M2 combined for axon hillock and impulse generation. M3 secured by the DRG location of the sensory neurone cell body. Around 6/6 mechanically, but the answer never uses the words "excitable", "graded EPSP" or "trigger zone" — terminology weak. The "this is fast" reasoning is correct but vague. This is exactly the kind of structured-but-imprecise answer that ends up at the C/B borderline depending on rounding.

Top-band response

The myelinated motor neurone exhibits three structural specialisations that together optimise it as a peripheral effector-bound fast-conducting neurone. First, the long single axon projects from the ventral horn of the spinal cord to the neuromuscular junction without intervening synapses; this minimises synaptic delay (each chemical synapse adds ~0.5 ms) and exploits axonal rather than network conduction for raw speed. Second, the myelin sheath — formed by many Schwann cells, each contributing ~100 layers of phospholipid bilayer — generates an internodal segment of very low capacitance and high transverse resistance. Action potentials cannot regenerate under the myelin because voltage-gated Na⁺ channels are absent there; instead, local-circuit current flows passively from one node of Ranvier to the next, where dense clusters of voltage-gated Na⁺ channels (~2000 per μm²) regenerate the action potential. This is saltatory conduction in the strict sense — only the active regeneration "jumps"; charge flow is continuous. The mechanism is dramatically faster (up to 120 m s⁻¹) and far more energetically economical than continuous conduction. Third, the axon hillock acts as the spike-initiation zone: it has the lowest threshold of any membrane patch in the neurone because of its high Na⁺ channel density, so it integrates dendritic and somatic EPSPs and IPSPs into a single fire/no-fire decision.

The distinguishing feature of a sensory neurone is the location of its cell body in the dorsal root ganglion, on a small side-branch outside the central nervous system. Sensory neurones are pseudo-unipolar: one long dendron carries information from the periphery to the soma, and one short axon delivers it into the spinal cord.

Examiner commentary: Full 6/6. The candidate names three distinct features (long axon, myelin sheath, axon hillock) and supplies a mechanistic AO2 link for each. The phrase "spike-initiation zone" and the quantitative channel-density figure both signal top-band command. The DRG distinguishing feature is supplied with the precise sensory-neurone shape ("pseudo-unipolar") which is not required by OCR but reads as undergraduate-level fluency. The answer also makes the important conceptual correction that "only the active regeneration jumps" — explicitly distinguishing the active regenerative step from the continuous local-circuit current flow that engineers find easy to muddle.

A-Level-depth misconceptions

The errors that distinguish A from A*:

1. Confusing neurone with nerve. A nerve is a macroscopic bundle of many axons wrapped in connective tissue (endoneurium, perineurium, epineurium). A neurone is one cell. A-grade candidates use these terms precisely; A*-grade candidates can also place the connective tissue layers.
2. Drawing the sensory neurone cell body inline with the axon. Sensory neurones are pseudo-unipolar: the cell body sits on a short branch off the main dendron/axon trajectory. The cell body is in the dorsal root ganglion, not in line with the impulse path. A candidate who draws it inline has fundamentally misunderstood the geometry.
3. Saying myelin "speeds up the action potential" without specifying mechanism. The correct mechanistic story has two parts: (i) myelin reduces transverse capacitance and increases resistance so passive current spreads efficiently to the next node; (ii) voltage-gated Na⁺ channels are concentrated at nodes of Ranvier, where the AP actively regenerates. Saying "myelin speeds it up" alone is one mark of two.
4. Treating Schwann cells as if they are neurones. Schwann cells are glial cells — supporting cells of the peripheral nervous system. They do not generate action potentials; they form the myelin. In the CNS, oligodendrocytes do the equivalent job, but OCR's named example is Schwann cells.
5. Forgetting axonal transport. The cell body manufactures all the proteins; motor proteins (kinesin anterograde, dynein retrograde) carry them along microtubules to the distant axon terminal. This is why damage to the cell body kills the whole neurone over weeks — protein supply is cut.

Common errors and mark-loss patterns

Pedagogical observations — not fabricated statistics:

• A typical pitfall is describing motor neurones as "running from the brain to the muscle". They run from the spinal cord (specifically the ventral horn for somatic motor neurones) to the muscle. The corticospinal tract from the brain to the spinal cord is a separate set of neurones synapsing onto the motor neurone in the ventral horn.
• Candidates often write "Schwann cells produce myelin". This is loose — Schwann cells are the myelin in the sense that the myelin sheath is the spirally wrapped Schwann-cell plasma membrane. There is no separate secretion step.
• A common pitfall is forgetting that not all axons are myelinated. Many sympathetic post-ganglionic neurones, many CNS interneurones, and C-fibre pain neurones are unmyelinated. The OCR shorthand "motor neurones are myelinated" is true for somatic motor neurones, but exam questions sometimes use unmyelinated examples deliberately.
• Candidates lose easy marks by failing to label the axon hillock in diagrams. This is the trigger zone where the action potential is first initiated — a single-mark label that very few candidates put on the diagram.

Going further

• Kandel, Schwartz & Jessell, Principles of Neural Science — gold-standard undergraduate neuroscience text. Chapter 4 on neurone anatomy is exactly the material above in much greater depth, with discussion of Cajal's original Golgi-stained drawings.
• Cajal's Histology of the Nervous System — facsimile edition available; the drawings are works of art and built the modern neurone doctrine.
• Oxbridge interview-style prompt: "Why is myelin not just thicker membrane?" (Answer: layered wrapping creates a series of capacitors that, electrically, behave very differently from a single thick membrane — the layered structure is the key, not the bulk lipid.)
• Undergraduate analogue: First-year Cambridge or Imperial neuroscience papers re-derive the cable equation from passive membrane properties and explicitly model the capacitance-reducing effect of myelin. The equation $\lambda = \sqrt{r_m / r_i}$λ=rm​/ri​​ gives the length constant of passive spread, and myelin increases $r_m$rm​ (transverse resistance), increasing $\lambda$λ.

Quick Recap

• Neurones are excitable cells specialised for rapid transmission of action potentials.
• Sensory neurones carry impulses from receptors to the CNS; cell body in the dorsal root ganglion.
• Relay neurones lie within the CNS and connect sensory to motor pathways.
• Motor neurones carry impulses from the CNS to effectors; long myelinated axons.
• All neurones have a cell body rich in rough ER, Golgi, mitochondria and neurofilaments.
• Schwann cells myelinate peripheral axons, leaving nodes of Ranvier between them.
• The axon hillock is the spike-initiation zone where EPSPs and IPSPs are integrated into a fire/no-fire decision.

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Reference: OCR A-Level Biology A (H420) specification 5.1.3 (refer to the official OCR H420 specification document for exact wording).