Sex Differences in the Brain

Sex differences exist in every brain lobe. For instance in the “cognitive regions” such as the hippocampus, amygdala and neocortex. The hippocampus has an important function in memory, the amygdala in mood. Both structures lie deep within the brain. The cortex is the “outside” of the brain. The part you can look at from the outside. Important for instance for stimulation with rTMS.

Brain and gender

Red structures that are larger in the healthy female brain, relative to cerebrum size
Blue structures that are larger in the healthy male brain, relative to cerebrum size

Sex differences in the brain can also be relatively global in nature. For example, widespread areas of the neocortex are significantly thicker in women than in men. Ratios of grey to white matter also differ significantly between the sexes in diverse regions of the human cortex or the outermost layer of the brain.

Grey matter (or gray matter) is a major component of the central nervous system, consisting mostly of nerve cell bodies (neurons). White matter is composed of bundles of myelinated nerve cell processes (or axons), which connect various gray matter areas.

These structural differences are not as much fun as differences in functionality. So we will focus in structural and functional sex differences:

  • The hippocampus in men and women differ significantly in their anatomical structure, their neurochemical make-up and their reactivity to stressful situations. In women the hippocampus is larger than in men when adjusted for total brain size. Sex influences the role of the hippocampus in learning and memory in animals. Whether and how sex influences hippocampal function in humans has not yet been systematically examined. Another hippocampal sex difference is the reaction to chronic stress. In both rats and monkeys, chronic stress causes damage to the hippocampus in males, but does so far less, if at all, in females. This is of interest since susceptibility of hippocampal cells to chronic stress has been suggested to have a role in two debilitating disorders — post-traumatic stress disorder (PTSD) and clinical depression. Both disorders disproportionately affect women, but animal models for these disorders continue to use male subjects almost exclusively.
  • The amygdala is significantly larger in men than in women adjusted for total brain size. The amygdala can modulate the storage of memory for emotional events, and does so through interactions with endogenous stress hormones released during stressful events. Studies have indicated a preferential involvement of the left amygdala in memory for emotional material (generally visual images) in women, but a preferential involvement of the right amygdala in memory for the same material in men. So it seems that the amygdala has laterality, ‘women left, men right’.
  • The Prefrontal cortex is rich in sex hormone receptors, and has among the highest concentration of oestrogen receptors in the human brain. These sex differences might influence working memory, a function thought to depend on the prefrontal cortex. The prefrontal cortex is also associated with sex differences in its response to stress, and might develop at different rates in males and females.

Now why is this important?
These are only some structural sex differences in the brain probably resulting in functional differences. Not that women or men are superior in one way or the other when it concerns cognitive tasks. They probably have the same results but the way they get their can be different.

Since many psychiatric and neurological disorders show sex differences in their incidence and/or nature requires us to examine sex influences in both our basic and clinical research to fully understand, and treat, the disorder (e.g. depression, schizophrenia, Alzheimer’s disease).

It is also important to realize that for instance studying depression in male rats is probably not a very good idea since this disorder affects mostly women.

The next post about: Sex, Chocolate and the Brain or Why Women Prefer Chocolate, will be on Friday October 31.