Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
332
conducts impulses over long distances. A motor neurone
with its cell body in your spinal cord might have its axon
running all the way to one of your toes, so axons may be
extremely long. Within the cytoplasm of an axon there
are some organelles such as mitochondria. The ends of the
branches of the axon have large numbers of mitochondria,
together with many vesicles containing chemicals called
transmitter substances. These vesicles are involved in
passing impulses to an effector cell such as a muscle cell
or a gland.
A sensory neurone has the same basic structure as a
motor neurone, but it has one long axon with a cell body
that may be near the source of stimuli or in a swelling
of a spinal nerve known as a ganglion (Figure 15.8).
(Note that the term 'dendron' is no longer used.)
Relay neurones are found entirely within the central
nervous system.
QUESTION
15.1 Make a table to compare the structure and function
of motor and sensory neurones.
Myelin
For most of their length, the axons of motor and sensory
neurones are protected within nerves. Figure 15.6 shows a
cross-section of a nerve full of neurones. You can see that
some of these are surrounded by thick dark rings. This
is myelin, which is made by specialised cells – Schwann
cells – that surround the axons of some neurones. You can
see these Schwann cells surrounding the motor neurone in
Figures 15.4 and 15.7.
Not all axons are protected by myelin. You can see
in Figure 15.6 that there are some neurones without
dark rings; these are unmyelinated neurones. About
two-thirds of our motor and sensory neurones are
unmyelinated.
Myelin is made when Schwann cells wrap themselves
around the axon all along its length. Figure 15.7 shows
one such cell, viewed as the axon is cut transversely.
The Schwann cell spirals around, enclosing the axon in
many layers of its cell surface membrane. This enclosing
sheath, called the myelin sheath, is made largely of lipid,
together with some proteins. The sheath affects the speed
of conduction of the nerve impulse (page 337). The small,
uncovered areas of axon between Schwann cells are called
nodes of Ranvier. They occur about every 1–3 mm in
human neurones. The nodes themselves are very small,
around 2–3 μm long.
cytoplasm
of axon
cell surface
membrane of axon
cytoplasm of
Schwann cell
cell surface membrane
of Schwann cell
nucleus of Schwann cell
Figure 15.7 Transverse section of the axon of a
myelinated neurone.
Figure 15.6 A photomicrograph of a transverse section (TS)
of a nerve (× 500). The circles are axons of sensory and motor
neurones in cross-section. Some of these are myelinated (the
ones with dark lines around) and some are not. Each group of
axons is surrounded by a perineurium (red lines). Several such
groups make a complete nerve.