The Nervous System: Quick Revision

A brief note on The Nervous System. Read it before a test. Try each question at the end before you open its answer.

Messages along nerves

• A stimulus is a change that the body responds to.
• Receptors pick it up: in the eye, ear, nose, tongue and skin.
• Effectors respond: muscles move and glands release substances.
• A neuron has dendrites, a cell body, an axon and nerve endings.
• Inside a neuron the message is an electrical impulse.
• At a synapse a chemical carries it to the next neuron, in one direction only.

Reflexes

• A reflex is a fast, automatic response, done without thinking.
• Reflex arc: receptor, sensory neuron, relay neuron (in the spinal cord), motor neuron, effector.
• The spinal cord answers at once. A message still reaches the brain, so you feel it later.

The brain

• The brain and the spinal cord form the central nervous system.
• Forebrain (cerebrum): thinking, the senses, voluntary actions, the hunger centre.
• Hindbrain: the medulla controls blood pressure, salivation and vomiting.
• The cerebellum keeps posture and balance.
• Protection: the skull, a fluid around the brain, and the vertebral column.
• Muscles: impulses make proteins change shape, so the cell shortens.

A worked example

Lata pricks her finger on a thorn. The message travels $0.9$ m of nerve at $60$ m/s and crosses $3$ synapses of $1$ ms each. How long does her reflex take, and when does she feel the pain?

1. Along the nerves. $\dfrac{0.9}{60} = 0.015$ s $= 15$ ms.
2. At the synapses. $3 \times 1 = 3$ ms.
3. In all. $15 + 3 = 18$ ms, and her hand moves.
4. The pain. The message to her brain travels farther, so she feels the pain a moment after her hand has moved.

Practice with new numbers

A program to try

The program compares a reflex with a thought-out response.

Mistakes to avoid

• The axon does not pick up messages. Dendrites do; the axon carries them away.
• The impulse does not jump the synapse. A chemical crosses it.
• Not every reflex goes to the brain first. The spinal cord handles many reflexes.
• Balance is not the cerebrum's job. It belongs to the cerebellum.
• Muscles do not move by themselves. They contract when impulses reach them.

Check yourself

1. Name the gap between two neurons, and what carries a message across it.

2. Why is the spinal cord, and not the brain, used in most reflexes?

3. Which part of the brain controls salivation?

4. Give one voluntary action and one involuntary action.

5. What protects the brain inside the skull?