Neuron: structure and types
A neuron is the structural and functional unit of the nervous system. It has a cell body, dendrites and an axon, and carries messages as electrical impulses.
Parts of a neuron
| Part | What it is | Its job |
|---|---|---|
| Cyton (cell body) | Rounded part with a nucleus and cytoplasm. It has Nissl's granules (groups of ribosomes). | Controls the cell and makes proteins |
| Dendrites | Short, branched threads from the cyton | Receive messages and carry them towards the cyton |
| Axon | One long fibre from the cyton | Carries messages away from the cyton |
| Myelin sheath | Fatty white covering round the axon, made by Schwann cells | Insulates the axon and speeds up the impulse |
| Nodes of Ranvier | Small gaps in the myelin sheath | Let the impulse jump along the axon quickly |
| Axon terminals | Branched end of the axon with swollen synaptic knobs | Pass the message on to the next cell |
Types of neurons by number of processes
| Type | Processes (threads) | Where found |
|---|---|---|
| Unipolar | Only one process | Early embryo |
| Bipolar | Two processes: one dendrite and one axon | Retina of the eye; lining of the nose (smell) |
| Multipolar | Many dendrites and one axon | Brain (cerebral cortex) and spinal cord |
Myelinated vs non-myelinated neurons
In the brain, grey matter is outside and white matter inside. The spinal cord has the opposite.
How to draw a neuron
- 1Draw a rounded or star-shaped cell body on the left. Put a dark nucleus in it. Label cyton and nucleus.
- 2Add small dots in the cytoplasm. Label Nissl's granules.
- 3Draw 4 to 6 short branched threads from the cyton. Label dendrites.
- 4From the right side, draw one long tube. Label axon.
- 5Cover the axon with sausage-shaped blocks with small gaps. Label myelin sheath, Schwann cell and node of Ranvier.
- 6Branch the far end into fine threads with round knobs. Label axon terminals and synaptic knobs.
- 7Draw an arrow along the axon. Write direction of impulse.
When a mosquito bites your arm, sensory neurons carry the message to your brain in a fraction of a second. Vitamin B12 (from milk, curd, eggs and fish) helps keep the myelin sheath healthy.
2 marks: “What are Nissl's granules? Where are they found?” Groups of ribosomes in the cytoplasm of the cyton of a neuron. They help make proteins.
2 marks: “Distinguish between myelinated and non-myelinated neurons.” Myelinated: myelin sheath present, form white matter, fast impulse. Non-myelinated: no myelin sheath, found in grey matter, slower impulse.
7 marks: “Draw the structure of a neuron and explain its parts.” Labelled diagram (cyton, nucleus, Nissl's granules, dendrites, axon, myelin sheath, Schwann cell, node of Ranvier, synaptic knob); job of each part; unipolar, bipolar and multipolar types.
Dendrites bring messages in (towards the cell body); the Axon carries them Away.