By Y. Saturas. American Graduate School of International Management. 2018.
These glands contribute Brain several constituents to the seminal fluid that are necessary centers for maintaining functional sperm buy levitra plus 400 mg on line. Hypothalamus REGULATION OF TESTICULAR FUNCTION GnRH Testicular function is regulated by LH and FSH purchase levitra plus 400 mg fast delivery. LH regu- lates the secretion of testosterone by the Leydig cells and Anterior pituitary FSH buy 400 mg levitra plus with amex, in synergy with testosterone purchase levitra plus 400mg free shipping, regulates the produc- tion of spermatozoa quality levitra plus 400mg. FSH LH Inhibin Follistatin Hypothalamic Neurons Produce Testes Gonadotropin-Releasing Hormone Activin Hypothalamic neurons produce gonadotropin-releasing Testosterone hormone (GnRH), a decapeptide, which regulates the se- cretion of luteinizing hormone (LH) and follicle-stimulat- Accessory ing hormone (FSH). Although neurons that produce Behavior Secondary sex GnRH can be located in various areas of the brain, their reproductive characteristics tissues highest concentration is in the medial basal hypothalamus, in the region of the infundibulum and arcuate nucleus. GnRH enters the hypothalamic-pituitary portal system and The main reproductive hormones are shown in binds to receptors on the plasma membranes of pituitary boxes. Positive and negative regulations are depicted by plus and minus signs, respectively. A variety of external cues and internal signals influence the secretion of GnRH, LH, and FSH. For example, the structures, which produce sperm and hormones; a ductal amount of GnRH, FSH, and LH secreted changes with age, system, which stores and transports sperm; and accessory stress levels, and hormonal state. Little, if any, The endocrine glands of the male reproductive system secretion of hypothalamic GnRH occurs in patients with include the hypothalamus, anterior pituitary, and testes. GnRH moves down the hypothalamic- nadal from a deficiency in LH and FSH secretion because pituitary portal system and stimulates the secretion of LH of a failure of GnRH neurons to migrate from the olfactory and FSH by the gonadotrophs of the anterior pituitary. These patients do binds to receptors on the Leydig cells and FSH binds to re- not have a sufficient hypothalamic source of GnRH to ceptors on the Sertoli cells. Leydig cells reside in the inter- maintain secretion of LH and FSH, and the testes fail to un- stitium of the testes, between seminiferous tubules, and dergo significant development. Sertoli cells are located within the GnRH originates from a large precursor molecule called seminiferous tubules, support spermatogenesis, contain preproGnRH (Fig. PreproGnRH consists of a signal FSH and testosterone receptors, and produce estradiol, al- peptide, native GnRH, and a GnRH-associated peptide beit at low levels. The signal peptide (or leader sequence) allows the Testosterone belongs to a class of steroid hormones, the protein to cross the membrane of the rough ER. Sertoli cells also produce glycoprotein hormones— nhibin, activin, and Three distinct pituitary LH- and FSH-secreting cells have follistatin—that regulate the secretion of FSH. Gonadotrophs contain either LH or FSH, The duct system that transports sperm from the testis to and some cells contain both LH and FSH. GnRH can in- the outside through the penis includes the epididymis, vas duce the secretion of both hormones simultaneously be- deferens, and urethra. The sperm acquire motility and the cause GnRH receptors are present on all of these cell types. They are trans- ferred to as alpha and beta chains, that are about 15 kDa in CHAPTER 37 The Male Reproductive System 651 Processing sites 23 AA 10 AA 56 AA Signal peptide GnRH GnRH-associated peptide (GAP) N GnRH C terminus terminus FIGURE 37. Both hormones contain the same subunit but differ- rectly indicate that GnRH pulses have occurred. Each hormone is glycosylated prior to re- human studies measuring pulsatile secretion of LH and FSH lease into the general circulation. Glycosylation regulates in peripheral blood at various times have provided much of the half-life, protein folding for receptor recognition, and the information regarding the role of LH and FSH in regu- biological activity of the hormone. However, the LH and FSH bind membrane receptors on Leydig and exact relationship between endogenous GnRH pulses and Sertoli cells, respectively. The two gonadotropin receptors are in serum and do not exhibit pulsatile secretion of LH. Pul- linked to G proteins and adenylyl cyclase for the produc- satile injections of GnRH restore LH and FSH secretion tion of cAMP from ATP. FSH pulses tend to be smaller in count for all of the actions of LH and FSH on testicular amplitude than LH pulses, mostly because FSH has a longer cells. These generating GnRH pulsatility is unknown, the presence of a factors activate the promoter region of the genes of pulse generator in the hypothalamus has been postulated. Similar signal-transducing events oc- pothalamus and is responsible for the synchronized and cur in Sertoli cells that regulate the production of estradiol. The activity of The testis converts testosterone and some other androgens the pulse generator is modified by several factors.
The dominant follicle is protected from a stigma 400 mg levitra plus with visa, the point on the follicle that actually ruptures purchase 400mg levitra plus with visa. As fall in circulating FSH levels because it has a healthy blood ovulation approaches purchase 400 mg levitra plus free shipping, the follicle enlarges and protrudes supply order levitra plus 400 mg with visa, FSH accumulated in the follicular fluid discount levitra plus 400mg visa, and an in- from the surface of the ovary at the stigma. In response to the creased density of FSH receptors on its granulosa cells. An- LH surge, plasminogen activator is produced by theca and other factor in selection is the accumulation of atretogenic granulosa cells of the dominant follicle and converts plas- androgens, such as DHT, in the nonselected follicles. Plasmin is a proteolytic enzyme that increase in DHT changes the intrafollicular ratio of estrogen acts directly on the follicular wall and stimulates the produc- to androgen and antagonizes the actions of FSH. On day 9 or 10 of the cycle, the vascu- wall facilitates the rupture of the follicle. The extrusion of the larity of the dominant follicle is twice that of the other antral oocyte-cumulus complex is aided by smooth muscle con- follicles, permitting a more efficient delivery of cholesterol traction. At the time of rupture, the oocyte-cumulus complex to theca cells and better exposure to circulating go- and follicular fluid are ejected from the follicle. At this time, the main source of circulating The LH surge triggers the resumption of the first meiosis. Since estradiol is the pri- Up to this point, the primary oocyte has been protected by mary regulator of LH and FSH secretion by positive and neg- unknown factors within the follicle from premature cell divi- ative feedback, the dominant follicle ultimately determines sion. Within a couple of hours after the initiation of circulating estradiol, and it causes multiple changes in the of the LH surge, the production of progesterone, androgens, dominant follicle, which occur within a relatively short time. Progesterone, acting These include the resumption of meiosis in the oocyte (as al- through the progesterone receptor on granulosa cells, pro- ready discussed); granulosa cell differentiation and transfor- motes ovulation by releasing mediators that increase the dis- mation into luteal cells; the activation of proteolytic en- tensibility of the follicular wall and enhance the activity of zymes that degrade the follicle wall and surrounding tissues; proteolytic enzymes. As LH levels reach their peak, plasma increased production of prostaglandins, histamine, and other estradiol levels plunge because of down-regulation by LH of local factors that cause localized hyperemia; and an increase FSH receptors on granulosa cells and the inhibition of gran- in progesterone secretion. Eventually, LH receptors on luteinizing onset of the LH surge, this coordinated series of biochemical granulosa cells escape the down-regulation, and proges- and morphological events culminates in follicular rupture terone production increases. The midcycle FSH surge is not essential for ovulation because an injection of either LH or human chori- onic gonadotropin (hCG) before the endogenous go- FORMATION OF THE CORPUS LUTEUM FROM nadotropin surge can induce normal ovulation. However, THE POSTOVULATORY FOLLICLE only follicles that have been adequately primed with FSH will ovulate because they contain sufficient numbers of LH In response to the LH and FSH surges and after ovulation, receptors for ovulation and subsequent luteinization. The granulosa cells begin to cease their proliferation cycle regulator), and a transcription factor called C/EBP and begin to undergo hypertrophy and produce proges- (CCAAT/enhancer binding protein). The ruptured follicle which these proteins interact to regulate follicular rupture are develops a rich blood supply and forms a solid structure largely unknown. However, mice with specific disruption of called the corpus luteum (yellow body). The mature corpus genes for any of these proteins fail to ovulate, and these pro- luteum develops as the result of numerous biochemical and teins are likely to have a functional role in human ovulation. The granulosa cells and theca cells in the corpus lu- surge are the release of vasodilatory substances, such as his- teum are called granulosa-lutein cells and theca-lutein tamine, bradykinin, and prostaglandins, which mediate in- cells, respectively. The highly vascu- Continued stimulation by LH is needed to ensure mor- larized dominant follicle becomes hyperemic and edematous phological integrity (healthy luteal cells) and functionality and swells to a size of at least 20 to 25 mm in diameter. If pregnancy does not occur, the 676 PART X REPRODUCTIVE PHYSIOLOGY corpus luteum regresses, a process called luteolysis or luteal LH; therefore, LH is referred to as a luteotropic hormone. Luteolysis occurs as a result of apoptosis and Lack of LH can lead to luteal insufficiency (see Clinical Fo- necrosis of the luteal cells. Luteal regression is thought to be induced corpus luteum is a transient endocrine structure formed from by locally produced luteolytic agents that inhibit LH action. It serves as the main source of cir- Several ovarian hormones, such as estrogen, oxytocin, culating steroids during the luteal (postovulatory) phase of prostaglandins, and GnRH, have been proposed, but their the cycle and is essential for maintaining pregnancy during role as luteolysins is controversial. The corpus luteum is res- the first trimester (see Case Study) as well as maintaining cued from degeneration in the late luteal phase by the action menstrual cycles of normal length. Af- mone that is produced by the embryonic trophoblast during ter acquiring a high concentration of LH receptors, granu- the implantation phase (see Chapter 39). This hormone losa cells respond to the LH surge by undergoing morpho- binds the LH receptor and increases cAMP and proges- logical and biochemical transformation. Unlike the nonvascular granulosa cells in the THE MENSTRUAL CYCLE follicle, luteal cells have a rich blood supply. Invasion by capillaries starts immediately after the LH surge and is facil- Under normal conditions, ovulation occurs at timed inter- itated by the dissolution of the basement membrane be- vals.
In either case safe 400 mg levitra plus, ventricular diastole com- during atrial diastole produces the v wave and reflects its prises the remainder of the cycle discount 400 mg levitra plus visa. The small pressure oscillation early in atrial dias- The first (S1) and second (S2) heart sounds signal the be- tole purchase levitra plus 400 mg free shipping, called the c wave buy levitra plus 400 mg on line, is caused by bulging of the mitral ginning and end of mechanical systole levitra plus 400mg online. The first heart sound valve and movements of the heart associated with ven- (usually described as a “lub”) occurs as the ventricle contracts tricular contraction. The relatively low- pitched sound associated with their closure is caused by vi- TABLE 14. Force of contraction tic and pulmonic valves close at the end of ventricular sys- 1. End-diastolic fiber length (Starling’s law, preload) tole, when the ventricles relax and pressures in the ventricles a. Contractility aortic and pulmonic valves produce the second heart sound, a. Sympathetic stimulation via norepinephrine acting on 1 which is relatively high-pitched (typically described as a receptors “dup”). Circulating epinephrine acting on 1 receptors (minor) and nearby structures contribute to these two sounds, espe- c. Intrinsic changes in contractility in response to changes cially S1; these factors include movement of the great vessels in heart rate and afterload and turbulence of the rapidly moving blood. Ventricular radius the rapid phase of ventricular filling and is associated with 2. Heart rate (and pattern of electrical excitation) heard in normal children and adolescents, its appearance in a patient older than age 35 usually signals the presence of a cardiac abnormality. It is caused by blood movement resulting from atrial contraction and, like S , is more com- Stroke Volume Is a Determinant 3 mon in patients with abnormal hearts. The force of contraction is affected by Cardiac output (CO) is defined as the volume of blood end-diastolic fiber length, contractility, and hypertrophy. The usual resting val- Afterload, the force against which the ventricle must con- ues for adults are 5 to 6 L/min, or approximately 8% of tract to eject blood, is affected by the ventricular radius and body weight per minute. When it is neces- across the aortic valve is normally small, aortic pressure is sary to normalize the value to compare the cardiac output often used as a substitute for ventricular pressure in such among individuals of different sizes, either cardiac index or considerations. Car- diac output is the product of heart rate (HR) and stroke Effect of End-Diastolic Fiber Length. The relationship volume (SV), the volume of blood ejected with each beat: between ventricular end-diastolic fiber length and stroke volume is known as Starling’s law of the heart. Within lim- CO SV HR (1) its, increases in the left ventricular end-diastolic fiber Stroke volume is the difference in the volume of blood in length augment the ventricular force of contraction, which the ventricle at the end of diastole—end-diastolic volume— increases the stroke volume. This reflects the relationship and the volume of blood in the ventricle at the end of sys- between the length of a muscle and the force of contraction tole—end-systolic volume. After reaching an optimal diastolic fiber If heart rate remains constant, cardiac output increases in length, stroke volume no longer increases with further proportion to stroke volume, and stroke volume increases stretching of the ventricle. Ejection fraction (EF) is a commonly used measure of End-diastolic pressure is the force that expands the ventri- cardiac performance. In Chapter 10, preload was de- diastolic volume (EDV), expressed as a percentage: fined as the passive force that establishes the muscle fiber length before contraction. For the intact heart, preload can EF (SV/EDV) 100 (2) be defined as end-diastolic pressure. It is de- compliance (change in volume caused by a given change in pendent on heart rate, preload, afterload, and contractility pressure), a higher end-diastolic pressure (preload) in- (all to be discussed below) and provides a nonspecific index creases both diastolic volume and fiber length. Still, it has proved to be valuable in astolic pressure depends on the degree of ventricular filling predicting the severity of heart disease in individual pa- during ventricular diastole, which is influenced largely by tients. If the stroke volume rises ther case, the relationship between ventricular filling, end- too much, the left heart begins to pump more blood than diastolic pressure, and end-diastolic volume is altered. The the right heart and left atrial pressure drops; this decreases effect is a decrease in end-diastolic fiber length and a re- left ventricular filling and reduces stroke volume. Further enlargement of the ventricle would require also be a variable other than stroke volume.