Neural tissue: neurons and neuroglia

The tissue that carries messages

Touch a hot cup, and you pull your hand away before you can think. Something has carried a message from the skin to the spinal cord and back to the muscle. That something is neural tissue.

Neural tissue has the greatest control over how the body responds to change. It has two kinds of cell.

• Neurons are excitable cells. The neuron is the unit of the nervous system.
• Neuroglia are the supporting cells. They protect and support the neurons. They make up more than half the volume of neural tissue.

A neuron

When a neuron is suitably stimulated, an electrical disturbance, the nerve impulse, travels quickly along its plasma membrane. At the end of the neuron the impulse can stimulate another neuron, make a muscle fibre contract or make an endocrine gland secrete.

A neuron has three main regions.

• The cell body, also called the cyton, holds the nucleus. It also has small dots called Nissl granules.
• The dendrites are short, branched processes. They bring impulses towards the cell body.
• The axon is a long process. It carries impulses away from the cell body. At its end the axon divides into axon terminals, which have small swellings. These pass the impulse to the next cell.

By the number of processes, neurons are multipolar (one axon and two or more dendrites), bipolar (one axon and one dendrite) and unipolar (the cell body gives off a single process).

The myelin sheath

Many axons are covered by a fatty layer called the myelin sheath. It is formed by Schwann cells in the nerves of the body. The sheath is not continuous. There are small gaps called nodes of Ranvier.

In a myelinated fibre the impulse jumps from one node to the next. This is faster than the way an impulse crawls along a fibre with no myelin. So a myelinated fibre conducts much faster. Large myelinated fibres conduct at about $100\ \text{m/s}$, and fine unmyelinated fibres at about $1\ \text{m/s}$.

The helper cells

Neuroglia do not carry impulses, but a neuron cannot live without them. Many books name the kinds: astrocytes support the neurons, oligodendrocytes make myelin in the brain and spinal cord, microglia engulf germs and debris, and ependymal cells line the fluid spaces. The Schwann cells above are the neuroglia of the nerves of the body. NCERT does not name these kinds, but A-Level and IB do.

Notice how fast the curve falls. A fibre of $1.2\ \text{m}$ takes $1.2$ seconds at $1\ \text{m/s}$, but only $12\ \text{ms}$ at $100\ \text{m/s}$. Speed matters, because the body must react quickly to danger.

Try it in Python

The program finds the time taken by an impulse in two kinds of fibre.

Practise with new numbers

Common mistakes

• Mixing up dendrites and axons. Dendrites bring impulses to the cell body. The axon takes them away.
• Saying that neuroglia carry impulses. They support and protect.
• Saying that the myelin sheath is a continuous layer. It has gaps, the nodes of Ranvier.
• Saying that the impulse is a flow of electrons along the fibre. It is a disturbance of the membrane.
• Forgetting that the neuroglia are more than half of the neural tissue.

Remember these

• Neuron: cyton (nucleus, Nissl granules), dendrites (towards the cell body), axon (away), axon terminals.
• Myelin sheath: made by Schwann cells, with nodes of Ranvier. It makes conduction faster.
• Neuroglia: protect and support neurons, and make up more than half of the neural tissue.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the two kinds of cell in neural tissue.

2. Which part of a neuron holds the nucleus and the Nissl granules?

3. In which direction do dendrites and axons carry the impulse?

4. Why does a myelinated fibre conduct faster than an unmyelinated one?

5. An impulse travels 0.9 m at 90 m/s. How long does it take?

References

• OpenStax Biology 2e, chapter 33, The Animal Body: Basic Form and Function, section 33.2 Animal Primary Tissues.
• NCERT (2023). Biology, Textbook for Class XI, chapter 7, Structural Organisation in Animals, and chapter 18, Neural Control and Coordination.