The hypothalamus therefore has much control over sexual behaviour. It governs the actions of the pituitary gland and, in turn, spurring the release of other hormones from the gonads, illustrated in figure 2. However, there are slight differences between the hypothalamus of the male and the female, defined above, which result in slight differences in its function.
An explanation for this difference was put forward by Gorski et al (1978) that there was an area in the medial preoptic area of the hypothalamus that was greater in size in males than in females, this was termed the ‘sexually dimorphic nucleus’ (Pinel, J. P. J. 2003. P.344). The rats are born with the same size nucleus, but the male sexually dimorphic nucleus then increases in size at a much more rapid rate then the female. The size of male rat’s nucleus is due to the levels of testosterone, as further tests by Gorski (1980), where male rats have been castrated and female rats injected with testosterone, have indicated.
There have also been a number of other differences between the hypothalamus of males and females discovered since. With respect to humans, there have been nuclei found in the preoptic, suprachiasmatic and anterior parts that are significantly smaller in females than in males. Therefore, when looking at the role the hypothalamus plays in sexual behaviour, we will consider males and females separately.
In males, the medial preoptic area of the hypothalamus is of great importance; this was found by Malsbury (1971) when electrical stimulation of this area encouraged male sexual behaviour. As tests by Hull et al (1999) have illustrated, if this is destroyed in any way, males no longer appear to be able to copulate. Although they attempted to receive females in a study by Everitt and Stacey (1987), for an unknown reason they still could not copulate. This area carries a signal to the lateral tegmental field, if the connection between the two is destroyed, sexual activity is affected, as discovered by Brackett & Edwards (1984).
The medial amygdala is also sexually dimorphic, again much larger in male rats, discovered by Hines, Allen and Gorski (1992). Again, manipulation of this area affects sexual behaviour, De Jonge et al (1992) asserted that the time taken to mount and ejaculate increased.
In females, the medial preoptic area does not appear to have such a significant affect. Yang & Clemens (2000) found that it merely decreases the time that they spend with males. It is in fact the section of the hypothalamus called the ventromedial nucleus that affects sexual activity. A study by Pfaff & Modianos (1985) illustrated this as electrical stimulation of this area increased sexual behaviour and lesions caused it to decrease, females do not show signs of lordosis.
Artificially inserting estradiol into the ventromedial nucleus increases the receptivity to progesterone in the female, again highlighting its importance, shown in a study by Blaustein et al (1988). Further studies by Pleim & Barfield (1988) showed this to be true even in overiectomised rats. This area appears to take affect through a connection with the periaqueductal grey section of the tegmentum. Lesions of the connection or the periaqueductal grey itself stop sexual behaviour taking place, as illustrated in studies by Hennessey et al (1990) and Sakuma & Pfaff (1979).
It is easy to see then why ‘the brain is often referred to as the largest human sex organ’ (Pinel, J. P. J. 2003. P.343). Although many would believe the gonads are often regarded as the most important, the brain determines the activity that occurs here, so is a pivotal component in governing sexual behaviour.
Bibliography
Blaustein, J. D., King, J. C., Toft, D. O. & Turcotte, J. (1988) ‘Immunocytochemical Localisation of Estrogen-Induced Progestin Receptors in Guinea Pig Brain’ in ‘Brain Research’ 474, 1-15 Brackett, N. L. & Edwards, D. A. (1984) ‘Medial Preoptic Connections with the Midbrain Tegmentum are Essentia for Male Sexual Behaviour’ in ‘Physiology & Behaviour’ 32, 79-84 Brown, R. E. (1994) ‘An Introduction to Neuroendocrinology’ (Cambridge: Cambridge University Press) Carlson, N. R. (1992) ‘Foundations of Physiological Psychology (2nd Edition)’ (Massachusetts: Allyn & Bacon) Carlson, N. R. (2001) ‘Physiology of Behaviour (7th Edition)’ (Massachusetts: Allyn & Bacon)