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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.
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:
OCR recognises three types of neurone based on where they carry information within the reflex arc.
Sensory neurones carry impulses from receptors to the central nervous system (CNS).
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.
Motor neurones carry impulses from the CNS to effectors (muscles or glands).
| 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 |
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]
Regardless of functional type, every neurone has a cell body (soma) that contains:
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.
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.
The three types of neurone work together in a reflex arc, a neural pathway that produces a rapid, involuntary response to a stimulus:
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.
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.
Reference: OCR A-Level Biology A (H420) specification 5.1.3(a)–(b).