The HumanNervous System

Now we have most of the neuroanatomy we need to understand our behavior and to look for the seat of consciousness. It’s time to think about the neural bases of sex, violence, and our appetitive behaviors. These behaviors can be called survival behaviors, controlled mostly by the autonomic nervous system and sometimes summarized with the mnemonic of the four F’s: feeding, fighting, fleeing, and fucking. While we tend to think of the autonomic nervous system as being responsible for these behaviors, there is much more going on than just the sympathetic and parasympathetic nerves firing. Let’s back up and take a deep dive into these behaviors and their neural underpinnings.

We’ve seen in previous blogs that we do not have direct access to the true nature of things. We approach the world from information obtained through the narrow windows of our senses, and we can confirm that other animals chop up the electromagnetic spectrum of energy differently. The things that stimulate our nervous systems for sex or violent behaviors are filtered though this lens, shaped by evolution and development.

Our eyes and other senses, just like our brains, don’t just process information, they create information. Our sexual, violent, and appetitive behaviors are shared, in some form or another, with other animals, and the neural mechanisms are similar. These behaviors of ours are as old as humanity, and the mechanisms they share are as old as mammalian life.

In embryological development, the endoderm (inside layer) and mesoderm (middle layer) split into structures that will become either somatic, meaning the body on the outside, or visceral, meaning the guts on the inside. The nervous system, developing from the overlying ectoderm (outside layer), differentiates into nerves going to and from somatic structures, like voluntary muscles (also called striated from their stiped appearance), and nerves going to and from visceral structures, like smooth muscles and organs. We have separate somatic and visceral nervous systems. The visceral nervous system is what we want to talk about, along with its interactions with deep brain structures like the hypothalamus, the pituitary glands, and the amygdala.

The visceral nervous system is also called the autonomic nervous system and is divided into two, often competing divisions, the sympathetic and parasympathetic nervous systems. The sympathetic division revs up the heart and blood pressure, making fight and flight behavior more capable. It dilates the pupils and widens the bronchial tubes. The parasympathetic division slows down the heart and reduces blood pressure and breathing rates and increases digestion, making us juicy. It is often referred to as the rest and digest division of the autonomic nervous system. Both divisions are involved in sex, with the parasympathetic division responsible for the engorging of blood in the male and female genitals (erection), and the sympathetic division responsible for orgasm, or at least the ejaculatory mechanisms involved. Medical students remember this with the mnemonic “Point (parasympathetic) and Shoot (sympathetic)”. None of this, of course, happens in isolation. There’s a lot going on, to say the least. A symphony of somatic and visceral nervous systems, with cortex, deep brain structures, brainstem, and spinal cord are all involved to produce the behavior we call sex.

The parasympathetic innervation of the genitals comes out of the sacral spinal cord. The sacral cord is the end of the spinal cord. In adults, the spinal cord ends at about waist level. The parasympathetic nerves come out of both sides of spinal segments S2, S3, and S4 and join a plexus of other nerves to innervate the pelvis and the external genitals.

The pudendal nerve, a somatic nerve, provides motor function to the anal and urethral sphincter muscles, bringing them under (usually) voluntary control, and sensory function from the genitals, anus, and the skin in between (the perineum).

The sympathetic innervation is body wide, increasing pupil size, increasing heart rate and breathing rate, stimulating small smooth muscles in the skin that raise the hair on your arms and neck, gearing us up to trigger orgasm.

During sex (and at other times) receptors dangling in your nose send impulses directly into the amygdala and the underside (ventral) of the frontal lobe of the brain. The amygdala is a small oval pool of neuron cell bodies in the deep temporal lobe. Recall from other posts that the cortex is only the top 6 cell layers on the outside of the brain, and that deeper inside are these paired pools of neurons, for example, the basal nuclei, the hippocampus, and the amygdala. On both sides of your brain, in the temporal lobes, the amygdala is just in front of (rostral to) the hippocampus, which you know is an important memory structure. The olfactory input to the amygdala is most like the mechanism that synchronizes the menstrual cycles of women living in close proximity. Since human females have hidden estrus (no visible cues that they are sexually viable), males must rely on secondary sexual characteristics, including olfactory and visual stimulation.

The amygdala is part of the limbic system, which connects memory and emotion in our brains. The amygdala is involved in the production of fear and anxiety, in concert with the sympathetic nervous system. As you now know, it also processes some aspects of olfaction, our sense of smell. The connections of memory, emotion, and smell, runs deep. All of this is connected to the brainstem, survival and attention, and to other parts of the frontal lobe via the dopamine pathway which rewards and reinforces the behavior. Damage to the amygdala can cause odd behaviors, such as fascination with the mouth and sucking on objects, as well as hypersexuality (Kluver-Bucy Syndrome). The amygdala has also been linked to violence.

The topic of violence brings us back to the fight-or-flight function of the sympathetic division of the autonomic nervous system. As with other behaviors, the autonomic nervous system does not produce the behavior, it speeds it up or calms it down. Violence is an emergent behavior produced by the brain. The frontal lobe, which inhibits behavior is certainly involved, as are the dopamine pathway, and the hypothalamus.

The hypothalamus, as the name indicates, is below the thalamus. It is a multinucleated structure (in neuroanatomy terms) involved in temperature control, hunger, rage, non-REM sleep, and sex (specifically male sexual receptiveness). The medial preoptic nucleus of the hypothalamus is large in human males, and small in human females, and this can very according to sexual orientation. The hypothalamus is one of the targets of general anesthetics. The hypothalamus, along with the pituitary and adrenal glands, form an axis of control with the autonomic motor system to modulate our response to stress, including violent behavior.

What stresses us? Other people. Our limbic system, connecting our emotion, memory, and survival behaviors, including sex and violence, has been shaped by our species’ social experiences, over millions of years. The environmental challenges that stressed us then, for example getting eaten by an animal, starving, and freezing to death, are not the same as they are now. The sociality that protected us from these threats has become the dominant source of our stress. As Neil Young says, “The same thing that make you live/ Can kill you in the end.”

Sex and violence are social behaviors, even when others are not involved. Sex and violence are also cognitive behaviors, behaviors that we think about, observe, read and write about. Our brains are involved, heavily involved, even when some of the neural and mechanical components are damaged or missing. Sex and violence have taken on an importance beyond their contributions to our survival. They were perhaps historically connected through aggression in mate competition, and have more modernly been linked in horror movies, sexual frustration theories of violent crime, in sexual sadism, masochism, and in consensual forms of sex play. The amygdala has been accused of being responsible for this. However, as we have discussed, something else, something more, perhaps something causally emergent is responsible for our behavior. Not a part of our brain or nervous system. You can’t blame your amygdala.

What about our other appetitive behaviors, like eating and drinking? These behaviors, like sex and violence, have become uncoupled from our survival. We sometimes eat when we are hungry, but also when we are sad, happy, stressed, and bored. Our eating and drinking behaviors also have strong social connections. Eating cooked food around a fire for protection and comfort has done more to shape our modern behavior and anatomy than any other behavior. The next blog post will go into detail about these behaviors and continue to look for the elusive ghost in the machine.