Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
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Synapses are involved in memory and learning.
Despite much research, little is yet known about how
memory operates. However, there is much evidence
that it involves synapses. For example, if your brain
frequently receives information about two things at
the same time, say the sound of a particular voice
and the sight of a particular face, then it is thought
that new synapses form in your brain that link the
neurones involved in the passing of information along
the particular pathways from your ears and eyes. In
future, when you hear the voice, information flowing
from your ears along this pathway automatically flows
into the other pathway too, so that your brain ‘pictures’
the face which goes with the voice. Compare how you
respond when talking on the phone to someone you
know well and someone you have never met.
QUESTION
15.6 Suggest why:
a impulses travel in only one direction at synapses
b if action potentials arrive repeatedly at a synapse,
the synapse eventually becomes unable to
transmit the impulse to the next neurone.
Muscle contraction
This section concerns the contraction of striated muscle.
This type of muscle tissue makes up the many muscles in
the body that are attached to the skeleton. Striated muscle
only contracts when it is stimulated to do so by impulses
that arrive via motor neurones. Muscle tissue like this is
described as being neurogenic. You have already seen how
the cardiac muscle in the heart is myogenic – it contracts
and relaxes automatically, with no need for impulses
arriving from neurones (Chapter 8 page 177). We have also
mentioned a third type of muscle tissue, smooth muscle,
which is found throughout the body in organs, such as
in the gas exchange system (page 188), alimentary canal
and in the walls of the arteries, arterioles and veins. Most
smooth muscle only contracts when it receives impulses
in motor neurones. However, smooth muscle in arteries
also contracts when it is stretched by the pressure of blood
surging through them. This happens without any input
from the nervous system. This type of muscle is called
smooth because, unlike the other two types of muscle
tissue, it has no striations. Smooth muscle does not form
smooth linings of tubular structures, such as the trachea
and arteries; the lining of these structures is always formed
by an epithelium. The structures and functions of the three
types of muscle tissue are compared in Table 15.2.
Appearance in the light
microscope
Cell structure
Shape of cells
Organisation of
contractile proteins
inside the cell
Distribution in the body
Type of muscle
striated cardiac smooth
stripes (striations) at regular
intervals
multinucleate (syncytium)
long, unbranched cylinder
organised into parallel
bundles of myofibrils
muscles attached to the
skeleton
stripes (striations) at regular
intervals
uninucleate cells joined by
intercalated discs (Figure 8.23,
page 173)
cells are shorter with branches
that connect to adjacent cells
organised into parallel bundles
of myofibrils
heart
Table 15.2 Mammals have three types of muscle tissue: striated, cardiac and smooth.
no striations
uninucleate cells
Control neurogenic myogenic neurogenic
long, unbranched cells that
taper at either end
contractile proteins not
organised into myofibrils
tubular structures e.g. blood
vessels (arteries, arterioles and
veins), airways, gut, Fallopian
tubes (oviducts), uterus