Control of Sexual Behavior - Biospych

Sexual Behavior

• Appetitive Behavior: behavior aimed at finding and obtaining a sexual partner

• Consummatory Behavior: behavior of the actual sexual act (intercourse)

Pheromones

Species-typical molecules that trigger a specific response in another animal of the same species

• detected by the vomeronasal organ (might not be present anymore due to evolution) and olfactory epithelium

Role of Pheromones in Human Sexual Interest

• Women’s t-shirts smell sexier during fertile phase of menstrual cycle

• Androstadienone (in men’s sweat) triggers alertness, good mood and sexual arousal in women (but poor mood in men)

• Estratetraene (only present in women’s urine and third trimester) activates male, but not female hypothalamic areas

Appetitive Behavior: Sexual Motivation

External Stimuli

• usually related to opposite-sex conspecifics

• can be chemical stimuli like pheromones

• can also be learned of different sensory modalities (liking someone’s smell because you like them)

Internal Motivation

• hormones

• brain mechanisms

Role of Sex Hormones in Sexual Interest

Males:

• suppression of T reduces sexual interest but not performance

• anticipation of sexual encounter increases T

• No correlation with individual differences in T

Females:

• sexual interest fluctuates with cycling ovarian hormones (oestradiol)

• androgens (T from ovaries, androstenedione from adrenal gland) increase effect of oestradiol up to 3 times

• incerase in T in females also with anticipation of sexual activity

*the role of sex hormones in increasing sexual motivation is activational

Consummatory Sexual Behavior: 4 Stages of Arousal

1. Excitement

2. Plateau

3. Orgasm

4. Resolution

Excitement and Plateau

Parasympathetic Nervous System - relaxation and sexual arousal

Preganglionic neurons carry acetylcholine (nicotinic receptors) (cell bodies in CNS that go into the body) make synapses onto post-ganglionic neurons that carry acetylcholine (muscarinic receptors)(make synapses onto every organ in the body)

Excitement and Plateau: Mechanics

• Parasympathetic stimulation relaxes smooth muscle in blood vessels

• More blood enters the genitals

• Penile/clitoral erection

• Engorgement of labia

• Vaginal labrication

• Smooth muscle relaxation involves signalling by Nitric Oxide

• This increases the levels of cyclic GMP (cGMP)

• Viagra inhibits the breakdown of cGMP by phosphodiesterase 5 (PDE5), keeping the smooth muscle relaxed

Sensory Neurons in the Penis/Clitoris

• Krause corpuscules are specialized genital vibrotacticle sensors

• Most sensitive to vibrations between 40-80 Hz

• Density in clitoris is much higher

Excitement and Plateau: Central and Peripheral Influences

1) Sensory stimulation of the genitals is detected by the sensory neuron which starts firing action potentials

2) It releases glutamate exciting the dorsal horn neuron. This neuron sends APs to brain but also to synapses with a spinal interneuron

3) This interneuron integrates signals from the genitalia (excitatory), but also from the brain (excitatory and inhibitory)

4) Depending on the overall input to the interneuron, it will either fire APs or won’t

5) If it does, it stimulates the parasympathetic preganglionic neuron which in turn stimulates the post-ganglionic parasympathetic neuron (acetylcholine in use).

6) The post-ganglionic neuron, by releasing acetylcholine onto the smooth muscles, causes these smooth muscles to relax whihc allow blood into the cavities which leads to the formation of an erection (penile or clitoris)

Anti-androgen drugs

They make a man uninterested in sex

What are the smooth muscle receptors that trigger relaxation?

Musarinic acetycholine receptors

Erectile Dysfunction

• Always “physical”

• Stress leads to increased sympathetic activity, which causes vasoconstriction - smooth muscles won’t relax and blood won’t enter into the cavities in penis or clitoris

• Learned aversions can have central effects

• Causes can be different so solutions are different

Orgasm

It is triggered by the sympathetic nervous system - SNS excitatory pelvic muscles resulting in orgasm

• Triggered by combination of local stimulation and central input

• Sympathetic stimulation (NA) and pulsatile release of oxycotin results in series of smooth muscle contractions in the pelvic floor

• Orgasm requires disinhibition by the CNS

Central Inhibition of Orgasm

• Nucleus paragigantocellularis (nPGi) in medulla inhibits orgasm circuitry in lumbar spina cord (neurotransmitter: serotonin)

• During intercourse, hypothalamus inhibits the peri-aqueductal gray (PAG), which normally stimulates nPGi

Resolution: Satiety

• Active inhibition of sexual motivation

• During orgasm: blood oxycotin levels peak

• During orgasm: prolactin is released - high levels suppress sexual motivation