Download Review Article The Neuro-Endocrine Regulation of Male Libido

Review Article
The Neuro-Endocrine Regulation of Male Libido
Manash P Baruah, Consultant Endocrinologis,t Excel center, Guwahati, India
Sanjay Kalra, Consultant Endocrinologist, BRIDE, Karnal,India
[email protected]
International Journal of Clinical Cases and Investigations 2011. Volume 3 (Issue
2), 25:27, 6th September, 2011
Introduction
Libido [ latin ] has been defined as “psychic drive or energy that is associated with sexual
desire ( Oxford dictionary ). Although libido is loosely used interchangeably with sexual
desire, it is a more complex function that includes generation of the following- (1)
spontaneous sexual thoughts and fantasie, (2) attentiveness and responsiveness to erotic
stimuli, (3) pleasure seeking behavior.1
Male sexual response consists of- (1) arousal, (2) erection, (3) ejaculation, (4) orgasm, (5)
resolution. Male sexual function is athe outcome of conjugation of a no of factors such as,
(1)psychogenic, (2)neurological, (3) endocrinological, (4)vascular, (5)cavernosal.
The Neurophysiology of sexual arousal
The word
arousal encompasses a behavioral state produced by arousal pathways,
electrophysiological mechanisms, and genetic influences. Two basic qualities of the arousal
response are protection and performance. Sexual arousal comprises a particular subset of
central nervous system arousal functions which depend on primitive, fundamental arousal
mechanisms that cause generalized brain activity, but are manifest in a sociosexual
context.As a subcategory sexual arousal is dependent on all of the above. In its specificity,
has additional dependent factors like (1) Neural (sensation, cognition) factors, (2) Hormonal
factors, (3) Genetic factors, (4) Influences of culture and context (human). Without sexual
arousal, there is no activation of the cognitive and physiologic processes that lead to sexual
behavior. The neural mechanisms for sexual arousal probably operates at different
levels(spinal cord, midbrain, hindbrain) individually and in conjugation as described below.
Cutaneous pressure on the genital dermatome leads to pressure on Ruffini endings in the
skin, and action potentials enter the spinal cord and excite second-order neurons.
Somatosensory inputs from the spinal cord ascend through the anterolateral columns
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which are eventually received by the medullary reticular formation (MRF) and the lateral
vestibular nucleus (LVN The efferent signals are carried by the axons from the MRF and the
LVN descending through the anterolateral columns to facilitate lordosis central nervous
system. The Lumbarspinothalamic (LSt) neurons which are unique to the spinal cord in
male are situated dorsolaterally to the central canal at the level of the third and fourth
lumbar spinal cord and project to the thalamus. These neurons are sexually dimorphicin the
sense that males possess a greater number. These neurons are specifically associated with
ejaculation, but not with mounting or intromission. The Lumbarspinothalamic (LSt) neurons
which are unique to the spinal cord in male are situated dorsolaterally to the central canal
at the level of the third and fourth lumbar spinal cord and project to the thalamus. These
neurons are sexually dimorphicin the sense that males possess a greater number. These
neurons are specifically associated with ejaculation, but not with mounting or intromission.2
The Endocrinology of sexual arousal: the role of testosterone
Attentiveness to erotic stimuli, spontaneous sexual thoughts and fantasies, and ability to
respond to visual erotic stimuli, and optimal arousibility threshold –all these are of
paramount importance to generation of libido. The abstract form of libido is translated to a
more physical form i.e. erection of phallus(penis in male and clitoris in female) primarily by
the influence of the efferent neural signals as described above. Penile tumescence is of basic
necessity to have a successful erection. Penile tumescence can be spontaneous (nocturnal)
or to erotic stimuli. A more advanced step in the erectile process is rigidity, which is
dependent on the patent vascular supply and sustained dilatation of the venous sinuses
within the penis. The erectile process culminates in ejaculation which coincide with orgasm
in male and is the outcome of coordinated action of neural, vascular and hormonal
processes. That the penile erections can occur in hypogonadal men, particularly in response
to appropriate erotic stimuli, is testimony to the fact that erection is a primitive reflex and
primarily dependent on neural signals and not endocrine. Orgasm and ejaculation are also
androgen-independent and can occur in the absence of a full erection. This could explain
perverted sexual behavior and fetish . However, testosterone might induce nitric oxide
synthase within the cavernosal smooth muscle and might be necessary for achievement of
optimal penile rigidity.1,2,3 Even estrogen(E2) has some interesting role to play in male. .
Estrogen(not testosterone)
along with inhibin has negative feedback on H-P-G axis
4
bringing down LH FSH
Late Onset Hypogonadism(LOH)
It is also referred to as age-associated Testosterone Deficiency Syndrome (TDS).[5] Along
with the ageing process, significant changes in the Hypothalamo Pituitary Gonadal axis take
place. To start with the pulsatile GnRH secretion is attenuated in older men. Overall the
orderly and synchronus manner
in which
the hormones at different level, i.e.
hypothalamus, pituitary and gonad is disrupted.
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Fig. 1: Gradual decline of plasma testosterone level in male with advancing age.
While on one hand the so called feedforward action of LH (of stimulating testicular androgen
secretion is ) attenuated, on he other hand the androgen mediated feedback inhibition of
pitiutry LH secretion becomes more efficient. Greater variability in LH pulse
frequency,amplitude, and secretary mass in older male in comparision to their younger
counterpart also have been observed. The gradual decline testosterone level in male is
depicted in Fig.2.1-3,5
References
1. Bhasin S, Lakshman KM, Herbst K. Age-Related Changes in the Male Reproductive
Axis. In McLachlan R (ed)
Endocrinology of Male Reproduction. Chapter 11
WWW.Endotext.com 2008.WWW.Endotext org.
2. Matsuda K, Sakamoto H, Kawata M. Androgen action in the brain and spinal cord for
the regulation of male sexual behaviors. J Current Opinion in Pharmacology 2008,
8:747–751
3. Schober JM, Pfaff D. The neurophysiology of sexual arousal. Best Practice &
Research Clinical Endocrinology & Metabolism Vol. 21, No. 3, pp. 445–461, 2007
4. Grumbach MM, Auchus RJ. Estrogen: Consequences and Implications of Human
Mutations in Synthesis and ActionJ Clin Endocrinol Metab 84(12): 4677–4694, 1999 .
5. Wang C, Nieschlag E, Swerdloff R, et al. Investigation, Treatment, and Monitoring of
Late-Onset Hypogonadism in Males: ISA, ISSAM, EAU, EAA, and ASA
Recommendations. European Urology 55 ( 2 0 0 9 ) 121–130
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