Endocrine Glands: Hormone-secreting organs are called endocrine glands

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1 University of Jordan Department of Physiology and Biochemistry Nursing students, Academic year 2017/2018. ******************************************************************* Ref: Principles of Anatomy and Physiology, Tortora and Derickson, 14 th Ed: INTRODUCTION Endocrine System Part I The endocrine system and the nervous system act individually and together in regulating physiological functions The control provided by nervous system is fast and short lasting over targeted structures, while the control achieved by endocrine system is slower and longer lasting over cellular activities The endocrine system is a chemical control system. It functions in conjunction with the nervous system to control the internal environment (homeostasis). Endocrine Glands: Hormone-secreting organs are called endocrine glands (Glands without ducts). They secrete hormones directly into the extracellular fluid, where they diffuse into the blood. Then distributed to all body and acting target cells that are responding to hormones Chemical types of hormones: - Proteins and polypeptides - Amines - Steroid Hormones are acting to maintain homeostasis by altering cellular activity, they can affect: - Organ or groups of organs directly ("target organ(s)") - Other endocrine glands called "target glands" [these hormones are acting on other glands are called tropic hormones: hormones that regulate production of other hormones] Targeted function controlled by hormones is very complex: as example: Single gland may produce multiple hormones, and a hormone may be secreted by more than one gland. A single hormone can have more than one target cells and accordingly can have more than one function. 1

2 Generally the following functions are controlled by hormones : - Metabolism - Growth and development - Red cell production - Reproduction - Adaptive changes like stress responses - Fluid-electrolyte balance - Acid-base balance - Energy balance Effectiveness of a hormone depends on plasma concentration of the hormone and Responsiveness of target tissues to the hormone. -Plasma concentration depends on: -Hormonal Secretion: Usually, the rate of secretion of a hormone is highly regulated and controlled by: -Negative feed back mechanisms: The out-put changing the in-put. As example, TSH (Thyroid stimulating hormone) which released by Pituitary gland to cause release of thyroid hormone. The output (Thyroid hormone) can act to decrease release of TSH. -Neuro-Endocrine reflexes: Some hormones are needed to be released fast when there is external stimulation. These hormones are released by neural control. Example (Epinephrine release by suprarenal gland by sympathetic stimulation). - Diurnal (Circadian) rhythm of secretion: Secretion of some hormones can also be secreted by repetitive oscillation (decreases and increases) according to day-night cycles. Example: Cortisol increases during night and and the peak of secretion in the morning before getting up. Then falls during the day to its lower level before bedtime. (hypo and hypersecretion) - Peripheral conversion: some hormones are subject to conversion after their release from glands like T4 to T3 conversion - Transport (lipophylic hormones) - Inactivation - Excretion - Responsiveness to hormone depends on many factors: -Number of hormonal receptors over target cells: Responsiveness of the target cells to the hormone can vary according to the number of hormone specific receptors over target cells. As example, when hormonal concentration in plasma is elevated, the number of receptors is decreased. This mechanism is known as downregulation of the receptors over target cells. This will be like fine tuning of hormonal action over cells. 2

3 The process appears after binding of hormone to its receptors which results in receptor mediated endocytosis. In this way, the hormone-receptor complex is internalized. This mechanism also serves to intracellular degradation of hormones after internalization of complex' - Permissiveness: Some hormones are needing presence of a second hormones in adequate amounts to have responsiveness by the first hormone (it is like permissive effect by the second hormone to have the first acting over target cells ) which is called PERMISSIVENESS.(example: Thyroid hormones increases number of receptors for epinephrine over target cells) - Synergism: also is another phenomenon in hormonal responsiveness. This appears as greater responsiveness of target cells to two hormones when acting together that are having same effect over target cells than the sum of effects from each when these hormones are acting separately. Example FSH and Testosterone which are both needed to maintain sperm production by testis. -Antagonism: Appears when one hormone is reducing the effectiveness of a second hormone at target structures. Example: Progesterone which is secreted during pregnancy, reduces responsiveness of uterine smooth muscle cells to estrogen (this hormone is also increased during pregnancy and can cause contraction of uterine muscle). This effect of progesterone appears by reducing estrogen receptors over uterine smooth cells. Disorders of Endocrine System: -Disorders of secretion (Hypo- or Hypersecretion) -Disorders of responsiveness (Target cells are not responding adequately to hormone. Example: lack of receptors to hormones) Central Glands - Pituitary Gland - Pinneal Gland Pituitary Gland - (Hypophysis) [Master Gland] Location: in Sella Turcica of sphenoid bone and attached to Hypothalamus by a stalk called Infundibulum Structure: this gland is having 2 lobes: - Posterior lobe [neural part] Neurohypophysis - Anterior lobe [glandular part] - Adenohypophysis 3

4 Posterior Pituitary Lobe (Neurohypophysis): Hypothalamus and Posterior pituitary glands act as a unit to release Antidiuretic Hormone ADH (called also VASOPRESSIN) and Oxytocin, These hormones are synthesized by hypothalamic neurons (Supraoptic and paraventricular nuclei). The axons of these neurons are passing down through hypothalamic-posterior pituitary STALK to terminate in posterior pituitary gland where these hormones are stored and released. Antidiuretic Hormone (ADH) or VASOPRESSIN from its names, it has 2 major effects: -Conserves water during urine formation: This effect results by increasing Permeability of the kidney tubules to water and promoting reabsorption of the water from the urinary filtrate resulting in a smaller volume of urine. Decreased production of this hormone results in Diabetes Insipidus (condition resulting in larger volumes of urine produced). This condition can be treated by giving vasopressin. -Second effect of this hormone is over vessels: Acts over vascular smooth muscle cells to cause vasoconstriction. Stimulation of hormonal release: The release of this hormone is by activation of osmoreceptors (increased osmolarity of extracellular fluids results in releasing ADH). Also to a lesser extent, decrease in blood volume which can be sensed by left atrium as a results of decreased extracellular volume and arterial pressure may result in its release. Oxytocin - Stimulates contractions of uterine smooth muscle cells of pregnant women during childbirth at the time of delivery (labor) - Causes milk ejection from the lactating breast. In addition to these two major effects, this hormone has also effects on maternal behaviors by increase bonding between mother and child. (For that it is sometimes called Love hormone) Stimulation of release: The release of this hormone is triggered by reflexes from: Birth canal during childbirth. When child suckles breast. 4

5 Anterior Pituitary - (Adenohypophysis) Anterior pituitary gland is synthesizing hormones by itself and releasing them into the blood. There are five different populations of cells which are secreting six major peptide hormones. The secretion by each anterior pituitary glands is stimulated or inhibited by secretions from neurons of the hypothalamus (Hypothalamic releasing or inhibitory hormones (factors)). There are seven Hypothalamic Hypophysiotropic Hormones (TRH, CRH, GnRH, PRH, PIH (Dopamine), GHRH, GHIH), TRH: Thyrotropin Releasing hormone: Stimulates release of TSH and Prolactin CRH: Corticotropin Releasing Hormone: Stimulates release of ACTH GnRH: Gonadotropin Releasing Hormone. Stimulates release of Gonadotropin (FSH and LH) PRH: Prolactin Releasing Hormone: Stimulates release of Prolactin PIH (Dopamine): Prolactin Inhibiting Hormone: Inhibits release of Prolactin GHRH: Growth Hormone Releasing Hormone. Stimulates release of GH GHIH: Growth Hormone Inhibiting Hormone (Somatostatin): Inhibits release of GH and TSH. Table: showing hormones released by hypothalamus and effects over anterior pituitary gland These hormones are released in hypothalamic capillaries which rejoin to form Hypothalamic-Hypophyseal Portal System which branches again into capillaries of the anterior pituitary. The portal system is important to get these hormones transported directly toward pituitary gland. The secretion of these hormones is under neural influences (Stress as example increases release of CRH) as well as negative feedback control provided by target glands hormones which inhibit hypothalamic and anterior pituitary hormones. Hormones of ANTERIOR PITUITARY GLAND Growth Hormone (GH): - Increases Growth and Maintenance of Organs by: a. stimulating protein anabolism b. promotes fat catabolism (use of fat rather than sugars for energy) Prolactin - (Lactogenic Hormone) - Promotes breast development during pregnancy - Stimulates mammary glands to produce milk after delivery Thyriod Stimulating Hormone (TSH) - (Thyrotropin) - Promotes growth of the Thyroid Gland 5

6 - Stimulates the secretion of the Thyroid Hormone Adrenocorticotropin -- (ACTH) - Promotes growth of the Adrenal Cortex - Stimulates the secretion of Cortical Hormones - Stimulates Fat Catabolism & Glycogenesis Gonadotropins - FSH and LH * Follicle Stimulating Hormone (FSH) Female - Stimulates the Ovarian Follicles to Develop and produce ova - Stimulates the Ovarian Follicles to secrete Estrogens Male - Stimulates the production of sperm - Stimulates the secretion of Testosterone * Luteinizing Hormone (LH) Female - Associated with FSH in development of the Ovarian Follicles - Stimulates development of the Corpus Luteum following ovulation - Stimulates Corpus Luteum to secrete Progesterone Male - Stimulates the Interstitial Cells to secrete Testosterone (also called Interstitial Cell Stimulating Hormone [ICSH]) Pineal Gland: releasing MELATONIN. Located in the center of the brain and secretes MELATONIN. This hormone is investigated widely to clarify its functions. One of the accepted functions is helping body inherent with circadian rhythm by synchronization of the body with day-dark cycle. The Suprachiasmal Nucleus (SCN) (neural nucleus structure is involved in synthesis of clock proteins after stimuli that are coming from the retina during light. SCN is working together with pineal gland which releases of Melatonin that synchronizes circadian rhythm with the 24hours of day-night cycle. 6