Nervous System
ICSE Class 8 · Biology · Chapter 7
What this chapter covers
- Control and coordination in the body
- The neuron and the structure of a motor neuron
- Types of nerves
- Cranial and spinal nerves
- The central nervous system: the brain
- The central nervous system: the spinal cord
- Reflex action
- Taking care of the nervous system
Key definitions
Dendrites are short, thin, branched threads that stick out from the cell body.
Sensory nerves are nerves that carry impulses from the sense organs and other receptors towards the brain or spinal cord.
Motor nerves are nerves that carry impulses from the brain or spinal cord to muscles and glands.
Mixed nerves are nerves that contain both sensory fibres and motor fibres in the same bundle.
Key points
- Coordination is brought about by the nervous system (fast, through impulses) and the endocrine system (slow, through hormones).
- The central nervous system is the brain and the spinal cord; the peripheral nervous system is the nerves.
- A neuron is a nerve cell; a motor neuron has dendrites, a cell body with a nucleus, one long axon with a myelin sheath, and axon endings.
- Sensory nerves carry impulses to the CNS, motor nerves carry them from the CNS, and mixed nerves carry both.
- There are 12 pairs of cranial nerves, from the brain, and 31 pairs of spinal nerves, from the spinal cord.
- The brain is protected by the cranium, the meninges and the cerebrospinal fluid.
- The cerebrum controls thinking, memory, senses and voluntary actions; the cerebellum controls balance; the medulla oblongata controls heartbeat, breathing and other involuntary actions.
- The spinal cord carries messages to and from the brain and controls many reflex actions.
- A reflex action is quick, automatic and involuntary; the path of a spinal reflex is stimulus, receptor, sensory neuron, spinal cord, motor neuron, effector, response, while reflexes such as blinking and sneezing pass through the brain.
- A conditioned reflex is learned by repeated experience, as in Pavlov's experiment with a dog and a bell.
Questions answered
Name the two systems that bring about coordination in the human body. How do they differ in speed?
The nervous system and the endocrine system. The nervous system acts within a fraction of a second through nerve impulses; the endocrine system acts more slowly through hormones carried by the blood.
Name the parts of a motor neuron in the order in which an impulse passes through them.
Dendrites, cell body (cyton), axon (covered by the myelin sheath), axon endings.
What is the function of the myelin sheath?
It protects the axon, like the coating on a wire, and helps the impulse travel faster.
Distinguish between a sensory nerve and a motor nerve.
A sensory nerve carries impulses from receptors to the brain or spinal cord; a motor nerve carries impulses from the brain or spinal cord to muscles or glands.
Why are most nerves of the legs called mixed nerves?
They contain both sensory and motor fibres, so they carry feeling from the legs to the CNS and orders from the CNS to the leg muscles.
Define cranial nerves and spinal nerves, and give the number of each.
Cranial nerves arise directly from the brain; there are 12 pairs. Spinal nerves arise from the spinal cord; there are 31 pairs.
Which part of the brain is affected if a person (a) cannot walk steadily, (b) cannot remember things, (c) has trouble breathing regularly?
(a) Cerebellum. (b) Cerebrum. (c) Medulla oblongata.
Name the three ways in which the brain is protected.
The bony cranium, the three meninges and the cerebrospinal fluid.