Answers to Test Your Knowledge questions for
Chapter 3
Question 3.1
There is a similarity between classical neurotransmitters and hormones in that, in each
case, a specific chemical is released from one site, moves to another site (a 'target'),
where it occupies receptors and thereby changes the target. At different sites, the same
chemical type can serve as either hormone or neurotransmitter.
The difference between classical neurotransmitters and hormones lies not intrinsically in
the chemical itself but in its mode of release, transport and distribution. The difference
concerns the distance from the site of release to the site of action. In the case of classical
neurotransmitters, release is from a presynaptic neuron and the substance migrates the
minute distance to an adjacent postsynaptic cell (other neuron or muscle cell) where it
exerts a local action. In the case of hormones, the substance is released into a blood
vessel, where it is transported (a relatively large distance) to a site of action. Often the
sites of action are multiple and spread throughout the body. For example, the distance can
be as large as that between the genitals (where so-called sex hormones are released) and
the brain (one of the sites of action of sex hormones). Reciprocally, hormones released in
the brain trigger hormonal activity at the genitals.
The question is designed to highlight the important similarity between these modes of
control, just described, and also the difference between two extremes of distance and
breadth of effect. Lying between these extremes are systems having features of each
mode of control, e.g. substances released from a neuron and influencing a few
surrounding neurons.
Question 3.2
Clearly, skeletal muscle needs to track very closely commands to action. For example, try
raising your arm in the air suddenly and then immediately lowering it again. The muscles
and arm should follow your commands closely. Contraction of the muscle follows the
command, which is effected through motor neurons. Contraction involves sudden release
of transmitter and relaxation involves the equally sudden halting of this release. If the
action were not a so-called 'detonating' one, muscles and thereby limbs would not
respond to commands so faithfully. Actions would be delayed and inappropriate to needs,
clearly a disastrous scenario
Question 3.3
The nervous, endocrine and immune systems interact, and events within any one of them
influence the other two. There is coordination between these systems. Although in a
study of behaviour, the nervous system is usually our principal focus of interest, we can
derive valuable insights from looking at the interaction of this system with the others.
The immune system is sensitive to psychological states and to strategies of action.
Conversely, we can sometimes better understand behaviour and mental states if we know
what is happening in the immune system. For example, infection can trigger a reaction of
lethargy and depression. This is discussed in greater detail in Chapter 14, 'Stress and
coping'.
Question 3.4
A neuron is a class of cell. It is one of the principal types of cell that make up the
nervous system. Figure 3.5b shows several neurons. A nerve is a collection of axons of
neurons that are located together and are found in the peripheral nervous system. It is
somewhat like a bundle of wires (analogous to axons) that make up a cable. For an
example of a nerve, you can see Figures 3.5a and c. Neurons such as 1 and 4 of Figure
3.5b form part of the nerve shown in part c. A tract is similar to a nerve in that it consists
of a collection of axons located together to form something like a cable. However, a tract
is located within the central nervous system. In Figure 3.5b, you can imagine the
projection of neuron 2 that goes to the brain to form part of a tract within the spinal cord.
Similarly, neuron 5 would form part of a tract. Tracts within the brain will be considered
later.
Question 3.5
The two parts of the adjective 'axo-dendritic' tell you where the synapse is formed. The
axon of a presynaptic neuron influences the dendrite of a postsynaptic neuron. Examples
of such synapses are those between (a) neurons 2, 3 and 4 and (b) neuron 1 in Figure
3.6d. (The text described an axo-somatic synapse).
Question 3.6
A warm-sensitive neuron is one whose frequency of generating action potentials
increases with increasing temperature. The implication of the term is that this is a sensory
neuron, which detects temperature at the skin. This can be compared with the coldsensitive neuron, whose activity is represented in Figure 3.7b.
Question 3.7
You might find Figures 3.5 and 3.8 useful in answering this question. In Figure 3.8, the
dorsal root is a collection of axons of sensory neurons (with cell bodies in the dorsal root
ganglion), whereas the ventral root consists of a collection of axons of motor neurons.
For example, neuron 1 of Figure 3.5b would form part of a dorsal root whereas neuron 4
would form part of a ventral root.
(a) Cutting a dorsal root would cut the axons of sensory neurons and result in a loss of
sensation in a region of the body innervated by the neurons that form part of this
particular dorsal root. Sensations would be lost from a region defined by the dermatome
corresponding to that particular dorsal root. See Figure 3.9 and 3.10.
(b) Since the ventral root carries the axons of motor neurons, there would be a loss of
motor control in a part of that body that would normally be innervated by neurons within
the cut ventral root.
(c) As shown in Figure 3.8, the spinal nerve consists of the axons of both sensory and
motor neurons. Cutting it would involve both effects described under (a) and (b), i.e. loss
of sensation and motor control in a region of the body.
Question 3.8
There is a specificity of action in both cases. A given key only fits one type of lock and a
given neurotransmitter only fits one type of receptor. Trying the wrong key is ineffective
and, by analogy, a neurotransmitter that does not fit the receptors will not attach to them
and will not influence the neuron at whose surface the receptors are located. At a given
synapse, the particular neurotransmitter released at a presynaptic neuron fits the receptors
on the immediately adjacent postsynaptic neuron. As you will see later, this specificity
prevents 'foreign' substances, which might just happen to be around, from influencing a
neuron.
Question 3.9
Inhibition at a synapse gives flexibility in that commands from the brain can override
what is otherwise being organized and stop the locally-organized response being
performed. For example, we described a reflex organizing the withdrawal of a limb from
a noxious object. This organization occurs at the level of the spinal cord. This reflex
would normally be triggered by a noxious object. However, it can sometimes be
overriden (or at least partly attenuated) by descending inhibitory influences from the
brain. Such inhibition from the brain was illustrated in Chapter 2, Figure 2.9.
Inhibition provides for much than just attenuating the strength of responses. For example,
you will later see where inhibition forms an integral part of sensory information
processing.
Question 3.10
The classical mode of action of this hormone is (1) release from the adrenal gland, (2)
transport in the blood stream, (3) occupation of adrenalin receptors at targets and (4)
influence on the activity of the target. For example, receptors at the cardiac muscle are
occupied, which increases the vigour of the heart's pumping. This sequence might be
called classical in that it is the best-known mode of action of a hormone. In more recent
years, a diversity of different hormonal actions have come to light.
Question 3.11
Insulin is a hormone which communicates information on the availability of glucose.
When there is an abundance of glucose in the blood or the anticipated arrival of glucose,
insulin is secreted. This promotes the uptake of glucose by cells of non-neural tissue.
Hence, times of availability are times that such cells can rely upon glucose. Compared to
non-neural tissue, neurons are limited in the range of substrates from which they can
derive energy. They rely largely on glucose. Therefore, from a functional perspective, it
makes sense that when there is little glucose available, this should be reserved for neural
tissue. By insulin secretion being low at such times, non-neural cells are unable to grab
glucose and must exploit other substrates. The functions served is one of distribution
according to need.
Arginine vasopressin is secreted at a high rate at times of dehydration and it inhibits
production of urine. Thus, seen in terms of function, by reducing urine production rate, it
conserves water when there is a deficiency ('need state'). Conversely, overhydration
inhibits secretion rate which promotes loss of water from the kidney. Thus, the action is
one of contributing to homeostasis and function is served by protecting the integrity of
the body.
Testosterone has various roles (discussed in more detail in Chapter 6, 'Development' and
18, 'Sexual behaviour'). These are related to reproduction and include sensitizing neurons
that are involved in sexual motivation and aggression. The full functional significance of
such effects of testosterone will appear later. Defeat lowers testosterone production,
which biases behaviour away from confrontation. It could prove adaptive to alter strategy
in this way when aggression has not been successful.
Question 3.12
As described in Chapter 2, a negative feedback system is one in which deviations from a
condition instigate action that tends to return the system to the same condition. In the
present context, deviations from normal in body-fluid level alter the secretion rate of
arginine vasopressin. When fluid volume expands, arginine vasopressin secretion is
inhibited, which causes a high rate of urine production. This tends to return body fluids to
normal. When fluid volume falls, secretion of arginine vasopressin increases which slows
urine production. This in itself cannot restore body fluid to normal. However, it slows the
rate of dehydration, since it slows urine production relative to the rate at which it occurs
normally. Only drinking, another negative feedback control, enables the deficit to be
corrected.
Question 3.13
Both neurotransmitters and neuromodulators are substances that are released from
neurons and affect other neurons by their occupation of receptors on their surface.
Neurotransmitters mediate their effects by their occupation of receptors at a postsynaptic
cell (of a neuron or muscle cell) that is a very short distance from the site of release.
Classically, neurotransmitters are described in terms of a one-to-one neurotransmittermediated link between one neuron and another, immediately adjacent, postsynaptic
neuron. The effect of a neurotransmitter is described as 'detonating', meaning a sharp
signal of excitation or inhibition at the second neuron. This signal closely tracks the
activity in the neuron releasing the neurotransmitter. Neuromodulator does not trigger
activity as such in a neuron. Rather, it sensitizes neurons making them more or less likely
to show activity in response to a detonating signal. Neuromodulator is thought to extend
considerable distances from its site of release and to influence a relatively large number
of neurons.
Question 3.14
Reflexes are fast and are an effective solution to relatively simple local problems. For
example, they organize the rapid removal of a limb from a source of tissue damage.
However, they act in a rather predetermined and relatively inflexible way. Voluntary
behaviour is somewhat slower but is flexible and can achieve a wide range of different
solutions. For example, if a thorn is stuck in the foot, voluntary behaviour permits us to
favour this part of the body in rest, to search for tweezers to pull the thorn out or to
recruit the help of others. The complementarity is seen in that having a set of local fast
solutions is appropriate where a predictable solution is possible. Reflexes solve such
problems where speed is of the essence, leaving voluntary behaviour to solve the more
complex but less pressing problems in a flexible and even creative way.
Question 3.15
There are several examples that you might have given. On the one hand, there is deep
sleep or quiet meditation. In such states, the somatic system is inactive and the activity of
the organs innervated by the ANS (e.g. force of contraction of the heart) is relatively low.
The functional end-point of rest is served in that there is a conservation of resources (e.g.
energy). There is no need to pump large amounts of blood around at such a time.
By contrast, when in a fight or flight situation, there is an active strategy of the somatic
nervous system, accompanied by excitation of the sympathetic branch of the ANS. The
functional end-point is either to win the fight to to escape from a threat. At such times,
the end-point requires relatively large amounts of energy and oxygen to be made
available to the skeletal muscles, something served by the adjustments within the ANS..
What we have discussed so far in the answer is coordination of whole-body reactions
involving global actions by the ANS.. There are also more local actions in each system.
As part of the somatic nervous system, local reflexes can solve local problems, e.g.
removing a limb from a sharp object. Similarly, a local problem within the ANS can be
solved by means of a local solution, for example, food in the mouth triggers salivation as
an output. Alterations in the circulatory system can be compensated for by local actions.
For example, unexpected loads on the heart can, in negative feedback fashion, be
compensated for by changes of activity within the neurons of the ANS that supply the
heart. Chapter 8, 'Vision' describes the local reaction of changes in the curvature of the
lens controlled from within the ANS in response to changes in visual input.
Question 3.16
In terms of neural events, there is likely to be an overall increase in activity within the
sympathetic branch of the ANS. Hormonally, there will be increased secretion of
adrenalin and noradrenalin from the adrenal gland. In terms of muscles, there will be
increased activation of cardiac muscle, causing the heart to beat at a higher frequency and
with greater vigour. The effect on the cardiac muscle is the result of the neural and
hormonal events just described. There will be alterations in the diameter of blood vessels,
mediated via smooth muscles in their walls. These will be such as to increase the flow of
blood to skeletal muscle involved in the action, e.g. arms and legs.
Question 3.17
The sympathetic nervous system excites the heart to beat with vigour, whereas the
parasympathetic system restrains the activity. A very slow heart beat would be due either
to a low activity in the sympathetic division of the ANS, a high activity in the
parasympathetic division, or a combination of both.
Question 3.18
As introduced in Chapter 2, homeostasis describes a system whereby (a) constancy is
maintained and (b) departures from normal tend to trigger action such as to return the
system to its normal state. The term 'homeostasis' is normally used to refer to such
physiological variables as body temperature, where a departure from normal triggers such
actions as shivering or sweating.
Can a similar logic be applied to the immune system? One might see invasion by bacteria
or viruses or the appearance of cancerous tissue as a disturbance to homeostasis (and
thereby to the well-being of the body). The immune system opposes these disturbances
by attacking their source. If successful the system is returned to its prepathological state.
This describes important features of homeostasis.