Sexual behaviour has been studied in a number of different subject areas and derives from the instinct of survival through reproduction. It is often divided into two main categories: the ‘developmental’ and ‘activational’ effects (Pinel, J. P. J. 2003. P.325). The former refers to the progress from conception to full sexual development, distinguishing one as male or female. The latter is concerned with the act of copulation itself. In this essay we will consider the part that the brain has to play in this.
There are three main areas of the brain that affect sexual behaviour; these are the pituitary gland, the hypothalamus and the pineal. The brains main purpose with respect to sexual behaviour is to release steroid hormones into the body. As Brown (1994) asserted, these are those hormones, unlike most others, that directly effect genes and have ‘especially diverse and long-lasting effects on cellular function’ (Pinel, J. P. J. 2003. P.326).
There are two sections to the pituitary gland: the anterior pituitary and the posterior pituitary. The posterior pituitary grows from the bottom of the hypothalamus and the anterior pituitary attaches itself to this. The primary function of the pituitary gland is to release tropic hormones, these are those that bring about the release of other hormones elsewhere in the body. However, it the anterior pituitary that directly affects sexual behaviour as this is where most of these hormones are actually released from.
There are overtly apparent differences in the male and female with respect to sexual behaviour. Firstly, the female pituitary gland releases hormones on a cyclic basis, typically on a 28-day system, controlling the menstrual cycle. Males, on the other hand, have a somewhat steadier system of hormone release, not changing very much at all. However, this does not mean it does not vary at all as hormones are released through a system of pulsatile hormone release which means it can vary at different times in the day, identified by Koolhaas, Schuurman & Wierpkema (1980).
Due to the fact that the anterior pituitary has control over hormone release, it was once believed that there was a difference between this area in males and females. Raisman (1997) identified through the works of Harris, however, that this was not the case by swapping the anterior pituitaries of male and female rats to no observable difference in hormone release. This then implied that there was a difference somewhere else which was controlling the pituitary.
The nervous system was considered as research on birds, reptiles, amphibians and fish by Cassone (1990) was performed. The pineal gland, which regulates circadian rhythms through its release of melatonin, was then implicated. Although in most mammals, including humans, it does not have an effect on reproductive activity, it does in these due to the effect of light and dark in seasonal changes. This, in turn, lead to the hypothalamus being an obvious option, as the pituitary was attached to this it made sense. Various experiments soon confirmed this, though how it was able to do was still not apparent, as there was no neural connection between the hypothalamus and the anterior pituitary, only the posterior pituitary, shown in figure 1.
The hormones vasopressin and oxytocin are released from the posterior pituitary after being synthesised in the paraventricular and supraoptic nuclei of the hypothalamus. Oxytocin would control contractions during labour and suckling of young, whereas vasopressin would aid water reabsorbtion in the kidneys. This could be explained easily, as there was a neural connection between the hypothalamus and the posterior pituitary.
The anterior pituitary, on the other hand, had no neural connection with the hypothalamus. Harris (1955) still hypothesised that this was regulated by the hypothalamus. The hypothalamopituitary portal system was soon discovered whereby capillaries in the hypothalamus transported information through portal veins to capillaries in the anterior pituitary. This was proven when cutting the portal vein meant hormone release was affected.
Once this connection was identified, it was suggested that there were specific chemicals in the hypothalamus that triggered the release or stopped the release of hormones from the anterior pituitary, called releasing hormones or inhibiting hormones respectively. Schally, Kastin and Arimura (1971) identified that gonadotropin-releasing hormone in the hypothalamus caused the release of gonadotropins from the anterior pituitary, namely follicle-stimulating hormone and luteinising hormone.