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Biology of Reproduction, lecture on Lactation
text: Human Reproductive Biology 2nd Edition
- R.E. Jones: Read pages 365-450 for this lecture
XXIII. Sexual Response and Behavior back to XXII. Lactation
A. requires integration of multi-sensory input
1. balanced sensory and physiological coordination
a. \ integration of neural control and endocrine modulation
i. seasonal cues (e.g. light and/or To) via sensory
receptors must activate HPG axis ®
follicular growth and spermatogenesis
(1) or puberty
(2) pulsatile GnRH & LH secretion required
ii. +feedback is necessary for cyclicity
iii. appropriate cycle phase means
appropriate in hormone
iv. hormone (may be) required to prime neural
centers for sensory information for arousal
and/or behavior
v. signal stimuli are sensory information which
can now activate complete circuits for arousal
vi. sense mediated arousal must be coordinated
with sensory behavioral information
vii. efferent signals stimulate endocrine
and motor outputs for behavior
viii. behavioral elements (there may be many) and
arousal must be coordinated for successful
sexual response between male and female
b. many components of sexual behavior and responsivity
are still unknown
i. e.g. the role of hormones in human female sexual
desire is controversial
B. Lordosis (an example)
1. Reflex - stimulated by tactile sensory input of male
body against females rump (rats)
a. this reflex requires input from the brain
i. input from the brain is dependent on hormone
priming and sensory stimulation of arousal
2. afferent information in fibers of anterolateral spinal columns
a. to 3 sites:
i. lateral vestibular nucleus (LVN): postural control
ii. medullary and midbrain reticular formations
+ midbrain central gray
(1) midbrain central gray: integrates autonomic,
limbic, sensory & motor information
(2) midbrain reticular formation: brain arousal
(3) medullary (+ pontine) reticular formations:
modulation of muscular reflexes
3. E2 primes neurons of the ventromedial hypothalamus,
raising the level of tonic activity
a. axon terminal levels of GnRH, and up-regulates P-R
b. GnRH potentiates neurons of the midbrain central gray
i. threshold facilitation necessary
4. E2 is followed by a short burst of P
prior to lordosis (luteinization precedes ovulation)
a. P: fast response - intracellular
and membrane receptors
b. P inhibits 5-HT
i. via GABAA
ii. 5-HT tonically inhibits behavior:
\ P disinhibits system
5. accessory olfactory (vomeronasal) information
potentiates E2/P effects
a. olfactory/accessory olfactory neurons secrete GnRH
i. hypothalamic GnRH develops
ontogenetically from olfactory tissue
b. visual input, e.g. presence of the male,
also facilitates response
6. midbrain reticular formation (reflexive arousal)
+ midbrain central gray (hormonal + neuromodulator
potentiation) neurons activate medullary reticular neurons
7. Reflex can now be completed via medullary reticular formation
a. project through reticulospinal tract to ventral
horn motor neurons
8. Efferent spinal neurons stimulate contraction of back muscles
a. facilitated by LVN neurons
C. Endocrine roles in sexual responsiveness and behavior
1. steroid hormones primarily influence sexuality of males
and females by affecting sexual desire
a. in some animals, e.g. rodents, steroids may influence
both the ability to engage in sex and sexual interest
i. in nonprimate mammals orchidectomy completely
abolishes sexual behavior
(1) restored by exogenous hormone
ii. encephalization of sexual behavior hypothesis
(Frank Beach)
(1) as neocortex becomes elaborated hormones
have less influence on sexual behavior
(2) cortical mechanisms substitute for hormonally
regulated reflexive stereotypical behavior
(a) increased flexiblity and lability of sexual
motivation and behavior
(b) cortical mechanisms are modulated by hormones
and neurogenic steroids and peptides
(i) GnRH, AVP/AVT, DHEA, E2
are made centrally
2. hormonal effects on sexual behavior are influenced
strongly by social context
a. under some circumstances hormone levels will accurately
predict the occurrence of sexual behavior
i. in different social contexts the same hormonal
conditions will appear unrelated to the occurrence
of sexual activity
ii. e.g. ovarian cycle and hormones are more important
for sexual activity in multifemale groups than
male-female pairs
(1) female primates in pairs mate at any time;
even without ovaries
(2) grouped females show greatest sexual
behavior midcycle; no behavior at all
during early follicular or luteal phases
(a) female initiation behavior = proceptivity
posively correlates with [E2],
negatively with [P]
(b) human females exhibit libido
midcycle & pre- and/or post-menstrually;
peak coital rate on the day of greatest
[E2] and LH surge
(c) adrenal androgens may be the most
potent stimulators of human female
sexual motivation
(i) further uncoupling sexual desire
and activity from cyclicity
(ii) adrenalectomy eliminates sexual interest
(iii) adx + aromatizable androgen
replacement restores libido;
menopausal women experience
an in sexual desire
(iv) necessity of aromatization suggests E-R
iii. testicular steroids are more important sexual
responsivity in mulitmale-multifemale groups
than male-female pairs
iv. erotic arousal stimulates LH + T secretion in men
(1) T decreases with age,
but not in sexually active men
3. hormones are not strict regulators of sexual behavior
a. castrated male humans acheive penile erection
in response to sexual stimulation
i. excess androgen does not produce excessive sex drive
b. regulate some physical aspects of sexual functioning
i. vaginal lubrication
ii. rigidity of erection
D. Erection - hemotumescence (another example)
1. vasocongestion leading to hemotumescence
® arterial flow into tissue > veinous drainage
2. male and female
3. spinal cord reflex
a. erection reflex center in the sacral spinal cord
i. erotic neural input may be direct
or indirect via the brain
ii. sympathetic/parasympathetic
(1) innervate arterioles to corpus cavernosum,
corpus spongiosum, labia minora
b. erotic stimuli ® parasympathetic action
i. release ACh
(1) ACh colocalized with VIP
(2) VIP also released
(a) VIP ACh secretion and effectiveness
(b) histamine, 5-HT, substance P, adenosine,
and ATP may also mediate erection
ii. sympathetic (= fight or flight) can block arousal
c. ACh binds to muscarinic receptors
i. M1,3,5-R activate PLC (phospholipase C) ®
IP3 (inositol triphosphate) ® opens endoplasmic
and membrane Ca++ channels
ii. Ca++ activates NOS (nitric oxide synthase)
iii. NOS converts Arg to Cit; giving off NO
iv. NO diffuses to all nearby cells (smooth muscle cells)
v. NO binds to the heme in GC (guanylate cyclase)
(1) GC ® cGMP ® relax arteriole smooth muscle
(a) by ¯ Ca++, dephosphorylating myosin,
or K+ ® hyperpolarizing cell
(b) sildenafil citrate (viagra) potentiates
erection by inhibiting PDE5 (phosphodiesterase5);
PDE5 catabolizes cGMP
d. \ NO ® arterioles dilate ® vasocongestion
e. T, DHT enhance penile sensory feedback
i. DHT (not aromatizable to E2) can restore
penile function in castrates without reinstating
male sexual behavior
(1) castration eliminates erection and behavior
in rats, only reduces them in
primates (including humans)
(a) transection of the dorsal nerve of the
penis eliminates erection and intromission
in rats, but not in primates
(2) complete penile sensory feedback is necessary
for/enhances erection, intromission, ejaculation
(a) rats without senory feedback (e.g. anesthesia)
mount without completing intromission
and ejaculation
ii. aromatizable androgens enhance sexual behavior
(rats, some primates)
(1) T ® both penile function and sexual behavior
(2) in rats aromatization blockers can eliminate
sexual behavior
(3) context (read neuromodulation) strongly
affects the role of androgens
(a) testicular suppression results in ¯ sexual
behavior fast (1 wk in rhesus monkeys)
in multimale groups, slower (7 wks)
male-female pairs
(b) A-R blockers ¯ sexual desire and interest
in human males, but do not effect
erections in response to sexually
explicit material
E. Phases of sexual response in humans
1. excitement
a. female
i. vaginal lubrication (10-30 s)
ii. vaginal barrel in length & width (inner 2/3)
iii. uterus/cervix ascends (tenting)
also length of vagina
iv. vasocongestion of clitoris, vagina, uterus,
labia minora, nipples
v. tumescence of clitoris (corpus cavernosum and glans),
labia minora, nipples, and breasts (by 25%)
vi. retraction of labia majora
vii. uterine contractile fibrilliation
viii. darkening of vagina, areola; sex flush or
reddening abdomen, throat, chest, face, shoulders,
arms, thighs (74%)
ix. myotonia
b. male
i. vasocongestion ® tumescence of the penis
(1) corpus cavernosum and corpus spongiosum
ii. urethral meatus widens
iii. cremaster muscle contraction elevates testes
iv. scrotal skin becomes congested/thick;
nipples erect (60%)
v. sex flush (50-60%)
vi. HR, BP, breathing
vii. myotonia
2. plateau
a. female
i. orgasmic platform = imminent orgasm: engorgment
of outer 1/3 of vaginal wall,
labia minora ® vaginal
cavity reduced, labia become larger
ii. clitoris retracts (¯ length 50%), covered by
clitoral foreskin or hood
iii. HR, BP, breathing
iv. uterine fibrillation and tenting, nipple erection,
breast size (maximal), darkening areola,
sex flush & myotonia
b. male
i. slight in glans of the penis, darkens,
coronal ridge swells
ii. urethral bulb 3X
iii. 1st stage of ejaculation
(1) few drops from bulbourethral gland
(a) may contain some sperm
iv. HR, BP, breathing, myotonia, sex flush,
testes elevation - which rotate slightly
and in volume (50%)
3. orgasmic
a. female
i. strong muscular contractions of
outer 1/3 vaginal barrel
(1) 1st contraction 2-4 s
(2) followed by rhythmic contractions
at 0.8 s intervals
(a) same frequency as male ejaculatory contractions
(i) 3 -15 contractions typically
(ii) status orgasmus - sustained
orgasm for up to 1 min
ii. expansion of inner 2/3 of the vaginal barrel
iii. Oxy stimulates uterine contractions
(1) ejaculation - a small amount of fluid mostly
from lesser vestibular (Skene's) gland
into the vestibule
(2) cervical os dips
iv. release of neuromuscular tension
b. male
i. loss of neuromuscular tension and voluntary control
ii. ejaculation
(1) ejaculation reflex
(a) spinal cord ejaculatory center
(above erection center)
(i) sympathetic neural stimulation to
muscles at the base of the penis
(2) 2 phases
(a) emmission stage: OXY + PGF2a ®
sequential contraction of smooth muscle
of walls of testes, epididymis, vas deferens,
seminal vesicles, prostate, bulbourethral
gland, ejaculatory duct; and simultaneously
bladder sphincter
(b) expulsion stage: rhythmic contractions
of bulbocavernous muscle at the base of the
penis ® expulsion of semen
4. resolution
a. female
i. no refractory period
(1) may return to plateau
ii. rapid return to normal: ceasation of vaginal
contractions, clitoris leaves retracted position,
¯ HR, BP, respiration, lightening of labia minora
ii. cervical os dilates immediately
iii. slower: decreased muscle tension,
breast & uterine size, vasocongestion of
clitoris, vagina and labia minora,
labia majora return to normal position
b. male
i. erection center sympathetically dominated
(1) constricted arterioles
(a) loss of tumescence
ii. rapid: ¯ penis size (50%), HR, BP, respiration,
muscle tension, sex flush
iii. slower: final ¯ penis size, loss of nipple
erection, testes/scrotal descent