CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.2. Factors that Influence the Force of Muscle Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healings
5.7.4

Male Reproductive Endocrine Axis

Before discussing the role of the various hormones involved in regulating the male reproductive system lets review the cells of the testes. Recall that within the testes are numerous, coiled seminiferous tubules (see image below). Surprisingly if all of these tubes were connected end-to-end they would stretch nearly a half a mile. Seminiferous tubules contain two types of cells.

The Sertoli cells (also known as sustentacular cells) attach to the basal lamina (basement membrane) of the tubules and surround the germ cells. Within the adult testes, the germ cells can be found at various stages of development (this will be explained later). The Sertoli cells protect and nourish the developing germ cells.

Outside of the seminiferous tubules are the testosterone producing Leydig cells or interstitial cells of Leydig. Several hormones are involved in regulating the male reproductive processes, they include: Gonadotropin Releasing Hormone (GnRH) secreted by the hypothalamus; Follicle Stimulating Hormone (FSH) for sperm production in Sertoli cells; and Luteinizing Hormone (LH) which binds to Leydig (interstitial) cells and for the production of testosterone.  Both FSH and LH are secreted by the anterior pituitary (Note that these two hormones are named after their functions in the female); Testosterone secreted by the Leydig Cells; and Inhibin secreted by the Sertoli cells.

image245.gif
Male Reproductive Hormones Chart.
Author: Unknown. [Public domain], via Wikimedia Commons                                                      Link: https://commons.wikimedia.org/wiki/File%3AMale_reproductive_hormone_chart.gif

The male testes become fully grown and functionally mature during puberty. The exact combination of biological processes that initiate puberty is still a mystery but it is believed that as the brain matures, connections (synapses) are made in the hypothalamus which eventually lead to the onset of puberty. Stimulation of the hypothalamus causes the secretion of GnRH. GnRH travels through special blood vessels that make up the hypothalamo hypophysial portal system to the anterior pituitary gland. Gonadotrophic cells in the anterior pituitary gland respond to GnRH by producing and releasing LH and FSH. FSH and LH circulate in the blood until they contact the tissues of the testes. LH binds to receptors on the Leydig cells and stimulates the secretion of Testosterone, the male sex steroid. FSH binds to receptors on the Sertoli cells.

In response to FSH, the Sertoli cells produce several proteins including: growth factors that initiate and support spermatogenesis; androgen-binding protein that helps keep the levels of testosterone high in the seminiferous tubules; an enzyme that converts testosterone to estrogen (ironically estrogen is thought to be very important in spermatogenesis); and inhibin which selectively inhibits FSH secretion. All of this is regulated by negative feedback of testosterone on the hypothalamus and pituitary to inhibit GnRH, FSH and LH secretion. Therefore, if testosterone levels increase, GnRH secretion decreases (negative feedback) resulting in reduction in FSH and LH secretion. Reduction in LH results in decreased testosterone secretion and levels come back to normal. The reverse would happen if testosterone dropped below normal.

Inhibin from the Sertoli cells selectively inhibits the release of FSH from the anterior pituitary and allows for differential secretion of FSH and LH (GnRH stimulates both FSH and LH, therefore if inhibin is present LH secretion can continue while FSH secretion is slowed). It is of interest that prior to puberty the hypothalamus is extremely sensitive to testosterone and tiny amounts are sufficient to inhibit GnRH secretion. At puberty the hypothalamus becomes much less sensitive and much higher concentrations of testosterone are required to turn off GnRH, hence the sudden increase in testosterone levels at puberty. However, as mentioned above we still don't know the exact cause of this change.

image246.jpg
Regulation of Male Testosterone Production. 
Author: OpenStax Anatomy and Physiology. License: Creative Commons Attribution License 4.0 license. Link: https://cnx.org/resources/054a534ef1de1524a8f5cdf8805d793f4aaf8610/Figure_28_01_07.JPG

Actions of Testosterone

As described above, testosterone is secreted by the Leydig cells and is the primary sex hormone in males and as such has a number of important actions. During fetal development it is testosterone that is responsible for descent of the testes into the scrotum. Interestingly, Human Chorionic Gonadotropin (hCG), a hormone secreted by the placenta, is nearly identical to LH. During fetal development hCG stimulates the Leydig cells to secrete testosterone which is then responsible for testicular descent. Once the baby is born it is no longer exposed to hCG and testosterone secretion essentially ceases until puberty.

Most of the changes associated with puberty can be ascribed to increased testosterone production. Listed below are the known effects of Testosterone.

  1. Stimulation of growth of the genitalia and the male duct system.
  2. With FSH it is required for spermatogenesis.
  3. Stimulation of growth of pubic and axillary hair.
  4. It is also responsible for the male secondary sexual characteristics, including:

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