Sexual Behaviour and the Brain

The behaviour known as lordosis is performed by a receptive female rat when mounted by a male. It is species-typical and triggered by stimulation of the female's lower back. The stimulation of the sensory nerves in the spine, produces an appropriate response in the motor nerves controlling the back muscles.

The sensory neurons also connect to the rat's hypothalamus and two other regions in its brain, which normally inhibit this spinal reflex. In a sexually receptive female, estrogens sensitise the hypothalamus, which in turn sensitises the nerve junctions controlling the reflex, allowing it to occur. During early life, this system will have developed preferentially in the female, the organising function of estrogen. During estrus, estrogen is also the activating medium.

This behaviour is often used to measure the effect of studies which involve injecting laboratory rats with hormones. Thus injecting a male rat with estrogens makes it more likely to display lordosis. It may be argued that the fact that administered hormones produce a given behaviour in a rat does not mean it has been 'feminised'. It simply means that the inhibition on a sexual reflex has been lifted, especially given the sizes of the doses often used. The question arises, can parallels be truly drawn between such behaviours and those of humans? How much human behaviour is predetermined in this way?

What constitutes the basis of human sexual behaviour? Is it gentleness or boldness? Is it nurturing or uncaring? Is it insertion or receiving? Is it a greater or lesser tendency to sexual arousal?

Perhaps there is special sensitivity to different sensory cues, like, among other things, smell. It seems likely that humans are much more sensitive to odour than used to be thought. One of the effects of testosterone is to modify sebaceous glands, giving rise to the teenage bane of acne, and body odour. Some male-to-female transsexuals have reported that their armpits become less smelly when they are taking estrogen medication.

It seems that the hypothalamus acts as a monitor, alerted by certain signals on the neurons by which it monitors sensory inputs. These may be genetically mediated, but equally they may be ones that the cortex recognises as potential mate material. That is, stimuli that learning has found attractive. It is likely to be alerted by the presence of certain 'thoughts' in the cortex, and to encourage certain behaviours like a more erect stance, a flush of the cheeks. It may even trigger a learned behaviour like straightening the tie.

Male reflexes.

Both human and rat males have the appropriate mechanisms for penile erection and ejaculation. These are not part of the brain, but are in the spine. That is, they are reflex mechanisms.

Bancroft(1)differentiates between sexual appetite and sexual interest. In his view, while appetite is controlled by testosterone, he suggests that, in humans, sexual interest is psychological. This is relevant to the treatment of sex offenders. The voluntary use of medication will help the person to control appetite, but the diversion of interest depends on a great deal of cognitive work. Moreover, as a reflex mechanism, erection and orgasm can still take place, in spite of massive doses of hormones, if interest is strong enough. The removal of the testicles, in the gender confirmation operation for a male-to-female transsexual, amounts to castration, although a small amount of testosterone continues to be produced by the adrenal glands. A good surgeon can use the nerves of the removed penis to innervate a constructed clitoris. Many patients report a satisfactory level of sensitivity, and some report the ability to climax.

1. Bancroft J. (1989) Human Sexuality and its Problems, (p69) Edinburgh: Churchill Livingstone.(bookshelf)

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Bland, J., (1998) About Gender: Sexual Behaviour and the Brain
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Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
Last amended 08.04.98