What is the human body’s most important sex organ?
In this, the first in a series of brainposts investigating how our brains orchestrate sexual behaviour, I provide a short introduction into this fascinating (and often overlooked) branch of neuroscience.
Libido, lust, desire is a drive that promotes relatively indiscriminate sexual activity and is fundamentally dependent on adequate levels of testosterone in men and women to ensure neural pathways that orchestrate sexual arousal are well maintained. Lust ensured that every single one of your direct line of ancestors managed to pass their genes on to the next generation, by hook or by crook. It is not an emotion, nor a feeling, so much as a fundamental, goal-directed, motivation to act – hence the term sex drive.
Love evolved much later to promote more selective sexual behaviours. Romantic love is thought to involve elevated levels of dopamine and noradrenaline in combination with low serotonin to produce sensations of intense euphoria, energy and excitement whilst also compelling us to obsess over our single specially-chosen beloved deemed superior to all the others.
Long term bonding is a third system, seemingly dependent on the action of the neurotransmitter vasopressin, which mediates behaviours promoting long-term partnership with one individual; known as monogamous pair bonding. This evolutionary benefit of such behaviour relates to the importance of nuture, i.e. ensuring that parenting duties are fulfilled. Committed parenting helps offspring to fulfil their genetic potential, improving the quality of mates that they attract, facilitating passage of genes into a third generation.
These three systems, although inter-related, are largely independent of each other. At any stage in post-pubertal life the three systems can be perfectly aligned or pull in different directions. No wonder the tabloids are full of stories of public figures sleeping around, and /or falling in love with people other than those with whom they are engaged in long term relationship. Understanding how we are wired up for sex, love and commitment can help us navigate the traps and pitfalls of the love game. If we can anticipate the antagonisms that develop between these systems, we can develop decision-making strategies that favour contentment in the long term, as opposed to immediate, but fleeting, satisfaction. For further reading on the evidence behind this conception of coupling as a three stage process I would highly recommend Helen Fisher’s books such as “Why We Love.”
Sex drive, or libido, evolved in mammals to promote the act of sex, or coitus. Coitus is captured in the below image, which is, believe it or not, an MRI image of a couple having sex inside the scanner! I’ve taken the liberty of adding some pink coloured dots to delineate the border of the woman’s body and blue dots to delineate the outer surface of the man. The green dots show the path that the sperm takes at the point of ejaculation: passing from the testis up and over the pelvic bone (white oval) and bladder (black/grey semi-circle), through the seminal vesicle / past the prostate gland (which together inject the constituents fluids of which semen consists), along the length of the penis where it is deposited right next to the cervix (neck) of the woman’s uterus (womb). Under optimal conditions the uterus rhythmically contracts to “suck” this seminal fluid up into the uterus and even on into whichever of the two fallopian tubes is most likely to have an ovum (egg) ready for fertilisation.
I added the red dots to help readers orient themselves with regard to the key erogenous zones in the female anatomy. The clitoris is marked with a large red dot and the position of the G-spot has been illustrated with several smaller red dots to convey that fact that it’s exact location on the anterior (front) wall of the vagina appears to be highly variable from person to person.
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