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Answers to Test Your Knowledge questions for
Chapter 18 Sexual behaviour
Question 18.1
Let us briefly review what is meant by 'positive feedback', something that you have met
already (Chapter 4, 'Neurons' and in answer to questions 14.4 and 14.5). If a system is
disturbed from some reference point and the disturbance creates a force that tends to
increase the disturbance still more, this defines a positive feedback effect. In Figure 18.1,
the possibility of such positive feedback is present in that the appearance of sexual
motivation can lead to peripheral arousal at the genitals (arrow 5), which feeds back
(arrow 6) to increase sexual motivation and so on.... On a less optimistic note, a failure
of genital arousal can feed back and lower sexual motivation.
Question 18.2
Most of the features of the hypothalamic pituitary gonadal (HPG) are common in males
and females, e.g. hypothalamic trigger, secretion of FSH and LH from the pituitary,
action of pituitary hormones at the sexual organs, release of hormones from the gonads
and an effect of these hormones throughout the body, including the genitals, pituitary
gland and the brain. A difference between sexes concerns the principal hormones
secreted: testosterone in the male and oestrogen and progesterone in the female. The
secretion of hormones in the female has an intrinsic cyclicity based upon a switch of the
feedback effect from excitation to inhibition: the oestrous cycle (in humans, the
menstrual cycle).
This hormonal system is similar to the hypothalamic pituitary adrenocortical system
(HPA system) (Chapters 6,'Development' and 14, 'Stress and coping'). In each case, there
is a series of hormonal actions, starting with the release of neurohormone at the
hypothalamus, which, in turn, influences hormone release from the pituitary gland. Also
the final hormone in the sequence (in the case of the HPA system this being
corticosteroids) feeds back and influences the axis at the levels of the pituitary gland and
brain.
Question 18.3
Suppose that we set out to test for the existence of a Coolidge effect in female rats. The
female is given a period of access to a male and sexual behaviour is observed to occur.
We would might logically then wish to compare the capacity of the familiar male to
rearouse the female with the capacity of a novel male. However, how could we attempt to
standardize the behaviour of the male towards the female? Would we want to do so?
Maybe we can only define the effect in terms of the interaction between the two animals.
The familiar male has a recent history of mating and so the 'fresh' male would need to
have a similar history but with a different female. The change of female might well
increase his arousal and so cause him to behave quite differently towards the female
being tested.
A similar logic complicates any attempt to measure the effect in males.
In cattle, one can obviate the problem, since bulls will attempt to mate with an artificial
cow. Indeed, such an artificial cow is used in order to obtain their semen for artificial
insemination. One can change the characteristics (e.g. colour) of the artificial cow and
indeed note a rearousal of sexual behaviour compared with keeping the original artificial
cow.
Question 18.4
It might mean that the neurons in a region do not necessarily change their connection
with other neurons but there is an internal change within the neuron. Thus, in response to
an excitatory input, the rate at which the target neuron generates action potentials
increases. Another possibility is that androgens strengthen connections between neurons,
such that one neuron influences another more strongly. The basis of this could be that
androgens influence synaptic restructuring (Chapter 6, 'Development' and 11, 'Learning
and memory').
Question 18.5
With reference to figure 18.8, the reflex-like aspect is that triggered by tactile stimulation
at the tip of neuron 1 at the genitals. This acts via neurons 2, 3, 4 and 5. The psychogenic
aspect is that mediated via neurons 6 (descending from the brain), 3, 4 and 5.
Question 18.6
There are similarities between the control of lordosis (Figure 18.7) and erection (Figure
18.8). In each case, tactile stimulation (a) instigates a signal that is sent to the brain and
(b) evokes a response mediated via neurons within a local segment of the spinal cord. In
Figure 18.7, this sequence consists of neurons 4, 5 and 6. In Figure 18.8, it consists of
neurons 1, 2, 3, 4 and 5. In each case, the strength of the link mediated via the spinal cord
is modulated from the brain (neurons 1, 2 and 3 in the case of Figure 18.7 and 6 and 7 in
the case of Figure 18.8). In each case, this modulation reflects the influence of what
might be called motivational factors.
A difference is that in Figure 18.7, representing the somatic nervous system, only one
neuron (neuron 6) spans the distance from the CNS to the skeletal muscle, whereas in
Figure 18.8, representing the ANS, two neurons (4 and 5) span the distance to the smooth
muscle. This topic was discussed in Chapter 3, 'Coordinated action'.
Question 18.7
In each sex, central motivation depends upon a complex array of external and internal
factors. In each, it appears that the hormonal contribution to sensitivity of motivation and
desire derives from testosterone. In each sex, there is erectile tissue at the genitals and
arousal is indexed by the swelling as a result of blood vessels dilating. This is under the
control of both local and central factors. In each, there is a neuromuscular process of
orgasm linking brain and genitals, in males this being accompanied by ejaculation of
seminal fluids. In each sex, there is a process of inhibition on sexual motivation.
Question 18.8
By 'physical' presumably one means such things as damaged nerves or blocked blood
vessels at the genitals. You might make the point that it is doubtful whether doctors
would have the necessary time or equipment to investigate each case to establish whether
there really are such physical causes. The causes might prove elusive even with unlimited
resources.
You might argue that the criterion gives rise to a neat dichotomy that does not reflect
reality. That is to say, it suggests a clear distinction between physical and non-physical
disorders. Figure 18.1 would suggest a loop of information flow in the system underlying
sexual motivation and response. A slight 'physical' change at the genitals might set up a
vicious circle of effects involving anxiety and manifest as a psychological problem.
Would a low sensitization of neurons in the brain by testosterone be considered physical
or non-physical? It clearly has a physical basis and yet might be manifest as a
psychological problem involving the loop of factors. Is an abnormal ANS activity
induced by stress in the work-place physical or non-physical?
Question 18.9
As a general point, a circle of effects as represented in Figure 18.1 means that sexual
motivation arises in part from the consequences of sexual behaviour. Thus, an active
sexual life could tend to exert a self-reinforcing effect. Such things as smoking and
excessive alcohol intake might impair performance with consequent feedback effects
throughout the system. Physical activity (e.g. regular exercise) would be expected to
maintain the condition of the circulation with beneficial effects throughout. Hormone
replacement or supplementation might play a role in maintaining both central and
peripheral structures in a healthy condition.
Question 18.10
The issue of the comparison of identical and fraternal twins was introduced in Chapter 6,
'Development'. Identical twins but not fraternal twins are identical genetically. Hence, if
there were genes contributing to a homosexual orientation, one might expect to see a
higher concordance in sexual orientation between identical twins as compared to
fraternals.