Page 223 - Biology_F5

Biology for Advanced Level Secondary Schools

collected information from different parts. throughout the body of an organism and
The collected information is processed form a communication network. Neurones
and integrated in the CNS, and finally are the basic structural and functional
the information is transmitted to effectors units of the nervous system. They are
(muscles and glands) which produce responsible for transmission of impulses
appropriate responses. from one part of the body to another.
FOR ONLINE READING ONLY
Functions of the nervous system The main portion of the neurone is the
The nervous system has the following cell body, which contains a nucleus. The
functions:
(a) It receives stimuli from the environment cytoplasm of the cell bodies contains
using receptor cells or sensory input. granules called Nissl’s bodies. Extending
from the cell body are one or more short
(b) It converts the stimuli into electrical extensions called dendrites. These receive
impulses by the process called signals from the sensory receptors and
transduction. transmit electrical signals or impulses to
(c) It transmits nerve impulses. The the cell body. The neurone also contains
impulse is transmitted from the sensory long extension called axon. In some cells,
receptor to the CNS and then to the axons are covered by a fatty layer of
effector, which is capable of producing material known as myelin sheath. Within
an appropriate response. the cell body there are fine neurofibrils that
(d) It stores information so that behaviour extend from the dendrites to the axon. An
can be modified according to the past axon is a long, stem-like part of the nerve
stimuli. cell that sends action potential signals to
the next cell. Outside the myelin sheath
Neurones is a cellular layer called neurilemma,
Task 4.2 containing sheath of Schwan cells which
Search for information from internet are essential for rapid propagation of
sources and the library on neurones. Note nerve impulse as well as nourishment and
their categories and specific functions insulation of the axon. The myelin sheath
in the nervous system. together with neurilemma constitute the

Nerve cells or neurones are conducting medullary sheath (Figure 4.2). Inside the
cells of the nervous system found between axon there is a space containing charged
the receptors and effectors. These spread ions called axoplasm.
Dendrites
Schwan
cell nucleus
Nucleus Cell body
Myelin sheath Axon terminal
Node of
Node of
Ranvier
Ranvier
Nissil’s
bodies Axon

Figure 4.2: Structure of a typical neurone

Form Five Student’s Book
216