Sexual Motivation and the Neurobiology of Arousal
Appetitive vs. Consummatory Behaviours
Two stages of sexual behaviour, applicable to any motivational system:
Appetitive behaviours: aimed at finding and obtaining a sexual partner
Consummatory behaviours: behaviour of the actual sexual act
Sexual Motivation
External motivators
Usually triggered by external stimuli.
Usually related to the characteristics of individuals by whom one is sexually stimulated (conspecifics).
These stimuli can be chemical = pheromones
Pheromones: Species-typical molecules that trigger behaviors in other organisms of the same species.
Act between organisms.
Insects: Pheromones are usually single molecules released by one sex, triggering a behaviour that leads to copulation in the other sex.
Mammals: Pheromones are usually a mix of molecules.
In most mammals, pheromones are detected by the vomeronasal organ, located within the nose and mouth cavity, separate from the olfactory epithelium used for normal smelling.
Pheromones in Humans
The existence of a vomeronasal organ in humans is debated.
Some tissue in the nasal septum might be a vestigial vomeronasal organ.
It is possible that humans can detect pheromones through the regular smell system (olfactory epithelium).
Human pheromones would involve chemical signals that are often unconsciously detected, influencing sexual attraction.
Evidence for Pheromones in Humans
T-shirt smelling studies:
Individuals smell t-shirts worn by others and rate them for attractiveness.
Studies with women who have regular menstrual cycles (not on the pill) show that men rate t-shirts worn during the fertile phase (around ovulation) as sexier.
Suggests a chemical signal influences attraction.
Two molecules touted as potential human pheromones:
Androstadienone: Found in male sweat, improves mood and sexual arousal in women, worsens mood in men.
The hairs in the underarms and groin may serve to disperse these chemicals, increasing the surface area for chemical information transfer.
Estratetraenol: Exists in female urine during the third trimester of pregnancy.
When exposed to males (but not females), it activates the hypothalamus, which controls sexual behavior and motivation.
Women tend to rate men's t-shirts as more sexy if they share fewer MHC (major histocompatibility complex) genes.
MHC Genes
MHC genes are immune-related genes.
The evolutionary explanation suggests that women may be attracted to men with dissimilar immune molecules to provide their offspring with a wider range of immune molecules.
Learned Aspects of Sexual Motivation
Most of what turns people on involves learning.
Examples:
Liking a partner's smell after being in a long-term relationship.
Attraction to different types of people.
These learned associations do not extend to sexual orientation.
Fetishes are examples of unexpected learned things that can cause sexual motivation.
Internal Stimuli: Hormones
Hormones:
Male sex hormones are relevant to sexual motivation.
Sexual orientation interacts with attraction to stimuli.
Hormonal Influence on Attractiveness
Changes in the menstrual cycle and hormonal balance (e.g., when women stop taking the pill) can alter what smells are attractive or repulsive.
Some argue that the pill mimics pregnancy, influencing attraction to kin (similar MHC complex) during pregnancy and non-kin (dissimilar MHC complex) when not pregnant.
Testosterone
In males, testosterone levels correlate with sexual motivation within an individual.
Decreasing testosterone levels (e.g., through testicular cancer treatment or anti-androgen drugs used for sex offenders) reduces sexual motivation.
Testosterone reduction does not change sexual orientation or the ability to have sex, but it reduces the interest in sex.
The opportunity to have sex can increase testosterone levels, creating a positive feedback loop.
Women also experience increased testosterone levels when anticipating sexual activity.
Sexual Interest Fluctuations in Women
Sexual interest in women fluctuates with the menstrual cycle.
Women are more likely to initiate sex when ovulating.
Hormones like estradiol and testosterone contribute to sexual motivation in women.
Combined increase in testosterone and estradiol enhances the effect of estradiol on sexual motivation.
Women make testosterone (or androstenedione) in their ovaries and adrenal glands.
External Testosterone
External testosterone has the same effect as naturally forming testosterone.
Anabolic steroids increase interest in sex and aggression.
The Consumer Stage: Physiological Aspects of Sexual Arousal
Masters and Johnson described four stages of arousal:
Excitement
Plateau
Orgasm
Resolution
Excitement and Plateau
Excitement and plateau involve physiological sexual arousal.
In men: Getting an erection
In women: Vaginal lubrication, swelling of the clitoris and labia
These stages are controlled by the parasympathetic nervous system.
The Autonomic Nervous System
The autonomic nervous system controls bodily functions and consists of the:
Parasympathetic nervous system
Sympathetic nervous system
It is organized in two stages:
Pre-ganglion neurons: Neurons that sit before the ganglion
Post-ganglion neurons: Neurons that sit after the ganglion
Ganglion: A group of neuronal cell bodies.
In the parasympathetic nervous system, neuronal cell bodies sit close to the target organs.
Both sets of neurons use acetylcholine as their neurotransmitter.
Pre-ganglion neurons release acetylcholine onto post-ganglion neurons.
The receptor is nicotinic receptors.
Post-ganglion neurons release acetylcholine onto the organs.
The receptors are muscarinic receptors.
Parasympathetic nervous system increases somatic function (relaxation, digestion).
Sympathetic nervous system dries out the mouth and speeds up heart rate. (Opposite function)
Physiological Mechanisms
Both the genitalia and internal sex organs have innervation from the parasympathetic nervous system.
Parasympathetic nervous system releases acetylcholine onto muscarinic receptors, causing the relaxation of smooth muscle in the blood vessels leading into the penis, clitoris, and labia.
Relaxation of smooth muscles expands blood vessels, allowing more blood to enter special cavities in these organs, causing them to swell.
The increase of blood flow causes the creation of an erection.
More blood also means more lubrication.
How Viagra Works
Viagra maintains smooth muscle relaxation, keeping blood in the penis or clitoris.
Viagra blocks the enzyme PDE5, which breaks down cyclic GMP.
Cyclic GMP, triggered by nitric oxide, is responsible for muscle relaxation.
Viagra does not cause an erection; it maintains one that is already there.
Neural Circuit for Arousal
The post-ganglion neuron of the parasympathetic nervous system releases acetylcholine onto the smooth muscle in the genitals.
An excitatory interneuron in the spinal cord influences the pre-ganglion neuron to fire action potentials.
The interneuron integrates information from:
Sensory neurons, which are stimulated by touching the genitals.
The brain (excitatory and inhibitory inputs).
Visual stimulation, mood/music, the right place, or the right person can excite this neuron.
Reflex Arc
Stimulation of the sensory neurons activates a neuron that sends information to the brain and the interneuron.
The brain can enhance or suppress that reflex are.
A local reflex arc exists in the lower part of the spinal cord, so people with spinal cord damage above this level will not feel anything that happens in their genitals but can still have an erection.
The brain can feed into the circuit and suppress it under the wrong circumstances or enhance it under the right circumstances.
Erectile Dysfunction
Erectile dysfunction is always physical, as all behavior is controlled by the brain.
Psychological reasons for erectile dysfunction mean the inhibitory input from the brain is too strong.
Physical reasons often refer to problems with the circulatory system.
Psychological and physical aspects are interrelated.
Orgasm
Plateau involves complete erection.
Orgasm is triggered by the sympathetic nervous system (the opposite of the parasympathetic nervous system).
Smooth Muscle
Muscles come in two major categories:
striated (skeletal/voluntary)
smooth (organs/involuntary).
Blood vessels have smooth muscle around them, and the gut has smooth muscle.
If smooth muscles contract, blood vessels get smaller, and blood pressure increases.
If smooth muscles relax, blood vessels get bigger, and blood pressure decreases.
Erectile Dysfunction in Women
Since the same mechanisms are at work in women, it can also go wrong.
It is not physically impossible for women to have sex if they do not have an engorged clitoris or labia, whereas it is not physically possible for a man to do that.
Puberty
During puberty, the threshold for inhibition for the circuit is lessened.
High motivation in puberty is because you are coming from a really low testosterone level, and testosterone is going up.
People in puberty are very interested in sex.
Asexuality
People who are asexual are not interested in sex.
If you aren't interested in sex at all with whoever, there is something different about the motivational aspects.
People who are asexual can still have orgasms and erections.
Orgasm and the Sympathetic Nervous System
Orgasm utilizes the sympathetic nervous system,
The sympathetic nervous system is structured similarly to the parasympathetic nervous system, with pre-ganglion neurons in the central nervous system and post-ganglion neurons in ganglia.
The neurotransmitter used by post-ganglion neurons in the sympathetic nervous system is noradrenaline, which increases heart rate and blood pressure.
Sympathetic ganglia are located close to the spinal cord, featuring post-ganglion neurons with long axons and pre-ganglion neurons with short axons.
Pre-ganglion neurons in the sympathetic nervous system originate mainly from the thoracic and lumbar segments of the spinal cord, unlike the parasympathetic system, which originates from the brainstem and sacral segments.
Pre-ganglion neurons in the sympathetic nervous system use acetylcholine as their neurotransmitter, interacting with nicotinic receptors, similar to the parasympathetic nervous system.
Adrenaline vs. Noradrenaline
Noradrenaline is a precursor to adrenaline; adrenaline is made from noradrenaline through enzymatic reactions.
Noradrenaline acts as a neurotransmitter in the sympathetic nervous system and the brain, while adrenaline is primarily a hormone released by the adrenal medulla into the bloodstream.
Physiology of Orgasm
Orgasm involves a series of smooth muscle contractions in the pelvic floor, triggered by the sympathetic nervous system, oxytocin release from the hypothalamus, and inhibition from the central nervous system.
The nucleus paragigantocellularis, located in the medulla, inhibits orgasm by sending axons down the spinal cord and using serotonin as its neurotransmitter.
Selective serotonin reuptake inhibitors (SSRIs) increase serotonin levels in the inhibitory synapse, making it more difficult to achieve orgasm.
During intercourse or masturbation, the hypothalamus inhibits the periaqueductal gray (PAG), which normally stimulates the nucleus paragigantocellularis, thus releasing the inhibition on orgasm.
In males, orgasm leads to ejaculation.
After orgasm, a refractory period occurs, during which obtaining a second orgasm is difficult due to the release of prolactin.
High prolactin levels are associated with difficulty in achieving orgasm.
Hormones and Pair Bonding
Oxytocin, released during orgasm, is also involved in pair bonding.
In some rodent species, pair bonds are formed at first copulation, with orgasm imprinting the male and female to stick together.
Testosterone levels are highest around ovulation, correlating with increased sexual motivation.
Accessories and Premature Ejaculation
Accessories can make orgasm more difficult, which is why they are sometimes used to help with premature ejaculation by reinforcing inhibition.
