Types of Neurone

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(a)–(b) 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.

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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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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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.

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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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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.

Reference: OCR A-Level Biology A (H420) specification 5.1.3(a)–(b).