Neural and Hormonal Control of Sexual Behaviour

Appetitive Behaviour: Sexual Motivation

Behaviour that aims to find a sexual mate/ aids reproduction

Opposite-sex Conspecifics

Members of the same species that are of the opposite sex

External Stimuli: Pheromones

Chemical messengers to opposite-sex conspecifics that trigger a specific response like sexual interest. Mammals have been observed to have a vomeronasal organ that can detect pheromones.

Pheromones in Humans

Largely debated as to whether humans use pheromones. there is no clear evidence for a functional vomeronasal organ but it is suggested that an olfactory epithelium may allow for pigs and humans to detect them.

Older studies show that Women’s shirts smell better when they are ovulating, more recent research finds that being exposed to mens sweat releases androstadienone in women which increases alertness, good mood and sexual arousal.

Furthermore, estratraene release in men seems to have some impact on the hypothalamus

Androstadienone

Steroid compound from testosterone, potentially a human steroid. Studies found being exposed to a male’s sweat increases androstadienone in women, improving mood, cortisol levels and brain activity (and sexual arousal)

External Stimuli: Learning

Over time, people can become more sexually interested in the kinks, fetishes and smells of their partners. They learn to associate these external stimuli with sex and they become more sexually motivating over time.

Internal Motivation: Male hormones

Individual testosterone levels have been seen to increase sexual interest, but not necessarily performance. Testosterone increases in anticipation of sex. There does not seem to be a correlation between individual differences in testosterone and sexual interest.

Internal Motivation: Female Hormones

Sexual interest in women appears to vary across the menstrual cycle. During the ovulatory period, sex was more frequently initiated by women, and this fell significantly during the midluteal stage.

Like men, testosterone in women increases in anticipation of sex.

Testosterone and androstenedione appear to increase the sexually motivating effects of estradiol by 3x.

The Paraympathetic Nervous System

Brain to base of spinal cord, neural connections in the PsNS consist of long pre-ganglionic neurons and short post ganglionic neurons, so the ganglia is close to the target organs.

Rest and digest, this system is responsible for relaxing smooth and cardiac muscles, increasing saliva, vasodilation, increasing fat stores etc.

Works with acetylcholide neurotransmitters.

The Sympathetic Nervous System

Fight or flight, vasoconstriction, increased blood pressure, reduced saliva, muscle constriction, fast metabolism, pupil dilation.

Short pre-ganglionic neurons and long post-ganglionic neurons, ganglia is close to the spinal cord.

The Sympathetic nervous system works from the thoracic and lumbar parts of the spine.

In the sympathetic nervous system, epinephrine is secreted at the pre-ganglionic terminal buttons and acetylcholine is secreted at the post ganglionic.

Preganglionic Neurons

Nicotinic receptors, epinephrine in the SNS, acetylcholine in the PsNS. Long in the PsNS, short in the SNS.

Postganglionic Neurons

Always secretes acetylcholine. Short in PsNS, long in SNS.

The Mechanics of Excitement and Plateau

3 sources of information on an interneuron that excites the pre-ganglionic neuron. One synapse from a sensory neuron in the peripheral nervous system, that reacts to genital stimulation the. Two synapses from the brain (CNS), one excitatory, one inhibitory, also signal to the interneuron. Depending on the overall input, the interneuron may excite the pre-ganglionic neuron, and the parasympathetic system kicks in. Going to the bathroom- no PsNS as it’s not sexy context.

When PsNS is active, Nitric Oxide triggers vasodilation and smooth muscle relaxation, allowing for blood to collect in the genitals, causing clitoral or penile erection, along with vaginal lubrication.

Signaling by Nitric Oxide increases cyclic GMP concentration, which maintains the erection. Viagra stops the breakdown of this cyclic GMP, keeping the smooth muscle relaxed.

Erectile Dysfunction

Physiological cause, stress triggers SNS, causing vasoconstriction and preventing erection.

The Mechanics: Orgasm

Orgasm is constantly being inhibited by the nPGi -in the medulla- via serotonin neurotransmitters (why SSRIs may delay orgasm- so would drug use) that synapse in the lumbar spinal chord.

As a result of stimulation and intercourse, the hypothalamus inhibits the PAG (peri-aqueductyal gray) which usually inhibits the nPGi. These leads to a release of oxytocin that results in contractions of the smooth muscle in the pelvic floor as the CNS becomes disinhibited.

Resolution

Oxytocin is released at a very high concentration into the blood during orgasm- aiding pair bonding. Then, prolactin is released suppressing sexual motivation.

The Central and Peripheral Influences on Excitement and Plateau

Peripheral- sensory neurons in the genitals signal both to the brain and local interneurons- so the brain ‘feels’ the action and the interneuron that stimulates the parasympathetic nervous system receives the feedback

Central- The brain has two synapses with the interneuron that determines whether the PsNS is excited, one excitatory and one inhibitory. Depending on context, this determines whether the interneuron activates the PsNS.

Vomeronasal Organ

Identified in most mammals as the organ that detects pheromones. Debate to whether a functional Vomeronasal organ exists in humans.

Olfactory Epithelium

Might be responsible for pheromone detection in pigs and humans.

Estratetraene

Potential pheromone, causes changes in the male hypothalamus when they are exposed, not the same in females.

Nicotinic Receptors

Preganglionic neurons use these.

Muscarinic receptors

Postganglionic neurons use these.

Nitric Oxide

Stimulates smooth muscle relaxation and vasodilation in the parasympathetic nervous system

Clyclic GMP Phosphodieseterase

Regulates shirt time small muscle relaxation, when broken down, vasodilation stops.

Nucleus paragigantocellularis

nPGi- Responsible for inhibiting the orgasm circuit in the lumbar spinal cord. Found in the Medulla.