Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Where biology meets psychology
We have five senses: touch, sight, hearing, taste and
smell. It’s a controversial view, but some people
believe in extrasensory perception (ESP), telepathy
and having premonitions as a ‘sixth sense’. Recent
research suggests that we detect subtle changes,
which we cannot put into words, so imagine it is an
extra sense. Some people also have synaesthesia – a
condition where stimulation of, say, hearing also
produces a visual response (Figure 15.1).
But we do have a genuine sixth sense, one which
we take for granted. In his essay ‘The Disembodied
Lady’, the neurologist Oliver Sacks relates the story
of a woman who woke up one day to find she had lost
any sense of having a body. All the sensory neurones
from the receptors in her muscles and joints had
stopped sending impulses. She had no feedback from
her muscles and could not coordinate her movements.
The only way she could live without this sixth sense
was to train herself to rely entirely on her eyesight
for coordinating her muscles. A man with the same
condition describes the efforts needed to do this as
equivalent to running a marathon every day. Curiously,
the night before Oliver Sacks’s patient found she had
total loss of body awareness, she dreamt about it.
Figure 15.1 Crossed wires? By studying electrical activity in
the brain, researchers have found that some people do indeed
hear colour and see sound.
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Most animals and plants are complex organisms, made up
of many millions of cells. Different parts of the organism
perform different functions. It is essential that information
can pass between these different parts, so that their
activities are coordinated. Sometimes, the purpose of
this information transfer is to coordinate the regulation
of substances within the organism, such as the control of
blood glucose concentrations in mammals. Sometimes,
the purpose may be to change the activity of some part of
the organism in response to an external stimulus, such as
moving away from something that may do harm.
There are communication systems within animals that
coordinate the activities of receptors and effectors. The
information they receive comes from the internal and the
external environment. So there are receptors that detect
stimuli inside the body and receptors that detect stimuli in
the surrounding environment. There are examples of these
in Table 15.1 on page 338.
In animals, including mammals, there are two types of
information transfer that are used to coordinate the body’s
activities:
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nerves that transmit information in the form of
electrical impulses
chemical messengers called hormones that travel in the
blood.
In Chapter 14, you saw that aspects of homeostasis are
controlled by hormones that are secreted into the blood
by glands, such as the pituitary glands and the pancreas.
The glands that secrete hormones make up the body’s
endocrine system. In this chapter, we look first at the
nervous system and then, on page 349, at further aspects
of the endocrine system.
Coordination in plants also involves the use of
electrical impulses for fast responses and hormones (also
known as plant growth regulators) for coordinating
slower responses to stimuli. We look at these methods of
coordination at the end of this chapter.
Nervous communication
The mammalian nervous system is made up of the brain
and spinal cord, which form the central nervous system
(CNS), and the cranial and spinal nerves, which form the
peripheral nervous system (PNS) (Figure 15.2). Cranial
nerves are attached to the brain and spinal nerves to the
spinal cord. Information is transferred in the form of
nerve impulses, which travel along nerve cells at very high
speeds. Nerve cells are also known as neurones, and they
carry information directly to their target cells. Neurones
coordinate the activities of sensory receptors such as
those in the eye, decision-making centres in the CNS, and
effectors such as muscles and glands.