Page 222 - Biology_F5

Coordination and Irritability

(b) Astrocytes Examples of effectors include a muscle
These are star-shaped macroglial cells with and a gland.
many processes (Figure 4.1b). They are (b) The cells of the nervous tissues are
the most abundant glial cells in the CNS. tightly packed for effective working of
Their roles are to provide metabolic and the tissue.
structural support to the neurones. (c) The cells of the nervous tissues have
FOR ONLINE READING ONLY
large number (high concentration) of
(c) Oligodendrocytes mitochondria which help to generate
These are cells with very few processes energy. This is important because
(Figure 4.1c). They are found in the CNS. nervous tissues require large amount
They form myelin sheath on the axons of a of energy for efficient functioning.
neurone, which have lipid-based insulation (d) They have neuroglia (glial cells) that
for increasing the speed at which the action provide protection and support to the
potential can travel down the axons. tissues.
(e) The cells in the PNS are capable of
(d) Ependymal cells regenerating themselves. This is due
These are ciliated cells which line up the to the presence of neurolemma (also
central cavities of the brain and spinal cord known as neurilemma).
where they form a fairly permeable barrier (f) The cells in the nervous tissues
between cerebrospinal fluid that fills these produce neurotransmitter chemicals
cavities (Figure 4.1d). which act as conveyors that carry

impulses from one neurone to another
(e) Schwann cells across the synaptic gap.
These are equivalent to oligodendrocytes (g) Nerve cells or neurones have nodes
(Figure 4.1e). They surround nerve fibres of Ranvier and fatty myelin sheath
in the PNS. They help to maintain axons which facilitate rapid transmission of
and form myelin sheaths in the PNS.
impulses.

(f) Satellite cells The Central Nervous System (CNS)
They line the surface of neuronal cell The CNS, which consists of the brain and
bodies in ganglia within the PNS (Figure the spinal cord has the grey matter and
4.1f). They are analogous to astrocytes. white matter. The grey matter is comprised
of cell bodies, dendrites, unmyelinated
Adaptive features of nervous tissues axons, and very few myelinated axons.
The nervous tissues have the following In contrast, the white matter is comprised
adaptive features: of myelinated axons. The main function
(a) They have nerve cells (neurones) that of the CNS is to integrate information
receive information from sensory from various sources. The collection, both
parts and transmit it to the CNS for from internal environment and external
interpretation and then to the effector environment, is done by receptors.
for a response. An effector is any part They usually form the sensory system
of the body that produces response. along with neurones, which transmit the

Form Five Student’s Book
215