Page 30 - Biology_Form_3

Biology for Secondary Schools

Procedure with at least two cytoplasmic processes. The
1. Put several books of the same size cell body may be angular, oval, polygonal or
in an upright position on a table in a star-shaped with dense granulated cytoplasm.
straight line. The cell body processes the nerve impulses it

2. Push the first book so that it hits the receives. Extending from the cell body are
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second book. one or more extensions called dendrites.
Questions These receive signals from the sensory
receptors and direct them to the cell body.
1. What happens to the third book and A neurone also contains long extension
the rest? Comment on the results. called an axon as shown in Figure 2.3. An

2. What would happen if the chain of axon is a long, stem-like part of the cell
coordination components is broken? that sends signals or nerve impulses to the
other cells. Inside the axon there is a space
Structure of neurones called axoplasm which contains charged

Task 2.3 ions. In some cells, axons are covered by a
fatty layer known as myelin sheath. Inside
Search from the library and internet the myelin sheath there is a cellular layer
sources for the information about the called neurilemma which form the outer
structure and functions of neurones. Write layer of Schwann cells. The myelin sheath
short notes on the searched information. has periodic gaps called nodes of Ranvier
in the insulating myelin sheath. These serve
The nervous system is composed of to facilitate the rapid transmission of nerve
specialised cells called neurones or nerve impulses.
cells. Neurones are the conducting cells
of the nervous system found between the The node of Ranvier enhance leaping of
receptors and effectors. They are basic nerve impulses from node to node. This is
structural and functional units of the nervous
system. They are found throughout the much faster and energy efficient impulse
body of an organism forming a network of transmission than continuous transmission
communication. Neurones have properties of impulses along the axon. Myelin sheaths
which distinguish them from other cells in are therefore essential for rapid transmission
the animal’s body. Compared to other cells, of nerve impulses. They also nourish and
they are highly responsive to stimuli. They insulate the axon. The terminal end of the
are also highly capable of conducting nerve axon is extended to form axon terminal, also
impulses (messages). They are responsible called synaptic ending or synaptic knob.
for transmission of impulses from one part
of the body to another.

Generally, the structure of a neurone consists
of the cell body which contains a nucleus

23 Student’s Book - Form Three

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