Page 321 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Sexual Response, Intercourse and Sexual response in the female (! A2). Due
Fertilization to impulses similar to those in the male, the
erectile tissues of the clitoris and vestibule of
Sexual response in the male (! A1). Impulses the vagina engorge with blood during the erec-
from tactile receptors on the skin in the genital tion phase. Sexual arousal triggers the release
region (especially the glans penis) and other of secretions from glands in the labia minora
parts of the body (erogenous areas) are trans- and transudates from the vaginal wall, both of
mitted to the erection center in the sacral spinal which lubricate the vagina, and the nipples be-
cord (S2–S4), which conducts them to para- come erect. On continued stimulation, afferent
sympathetic neurons of the pelvic splanchnic impulses are transmitted to the lumbar spinal
Hormones and Reproduction brain triggered by sensual perceptions, imagi- lengthens and widens, and the uterus becomes
cord, where sympathetic impulses trigger or-
nerves, thereby triggering sexual arousal.
gasm (climax). The vaginal walls contract
Sexual arousal is decisively influenced by
rhythmically
(orgasmic
cuff),
the
vagina
stimulatory or inhibitory impulses from the
erect, thereby creating a space for the semen.
nation and other factors. Via nitric oxide
(! p. 278), efferent impulses lead to dilatation
The cervical os also widens and remains open
of deep penile artery branches (helicine arter-
for about a half an hour after orgasm. Uterine
ies) in the erectile body (corpus cavernosum),
contractions begin shortly after orgasm (and
are probably induced locally by oxytocin). Al-
while the veins are compressed to restrict the
(! 1000 mmHg) in the erectile body causes the
are similar to those in the male (see above),
penis to stiffen and rise (erection). The ejacula-
there is a wide range of variation in the or-
11 drainage of blood. The resulting high pressure though the accompanying physical reactions
tory center in the spinal cord (L2 –L3) is acti-
gasmic phase of the female. Erection and or-
vated when arousal reaches a certain thresh- gasm are not essential for conception.
old (! A2). Immediately prior to ejaculation, Fertilization. The fusion of sperm and egg usually
efferent sympathetic impulses trigger the par- occurs in the ampulla of the fallopian tube. Only a
tial evacuation of the prostate gland and the small percentage of the sperm expelled during ejacu-
emission of semen from the vas deferens to the lation (1000–10 000 out of 10 to 10 sperm) reach
8
7
posterior part of the urethra. This triggers the the fallopian tubes (sperm ascension). To do so, the
ejaculation reflex and is accompanied by or- sperm must penetrate the mucous plug sealing the
gasm, the apex of sexual excitement. The ef- cervix, which also acts as a sperm reservoir for a few
fects of orgasm can be felt throughout the en- days. In the time required for them to reach the am-
tire body, which is reflected by perspiration pullary portion of the fallopian tube (about 5 hours),
the sperm must undergo certain changes to be able
and an increase in respiratory rate, heart rate, to fertilize an ovum; this is referred to as capacita-
blood pressure, and skeletal muscle tone. tion (! p. 302).
During ejaculation, the internal sphincter After ovulation (! p. 298ff.) the ovum enters the
muscle closes off the urinary bladder while the tube to the uterus (oviduct) via the abdominal cavity.
vas deferens, seminal vesicles and bulbo- When a sperm makes contact with the egg (via
cavernous and ischiocavernous muscles con- chemotaxis), species-specific sperm-binding recep-
tract rhythmically to propel the semen out of tors on the ovum are exposed and the proteolytic
enzyme acrosin is thereby activated (acrosomal re-
the urethra. action). Acrosin allows the sperm to penetrate the
Semen. The fluid expelled during ejacula- cells surrounding the egg (corona radiata). The sperm
tion (2–6 mL) contains 35–200 million sperm bind to receptors on the envelope surrounding the
in a nutrient fluid (seminal plasma) composed ovum (zona pellucida) and enters the egg. The mem-
of various substances, such as prostaglandins branes of both cells then fuse. The ovum now under-
(from the prostate) that stimulate uterine con- goes a second meiotic division, which concludes the
traction. Once semen enters the vagina during act of fertilization. Rapid proteolytic changes in the
intercourse, the alkaline seminal plasma in- receptors on the ovum (zona pellucida reaction)
prevent other sperm from entering the egg. Fertiliza-
crease the vaginal pH to increase sperm motil- tion usually takes place on the first day after inter-
ity. At least one sperm cell must reach the course and is only possible within 24 hours after ovu-
308 ovum for fertilization to occur. lation.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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