16 The Endocrine System: Part A
Pineal gland Hypothalamus Pituitary gland Thyroid gland Parathyroid glands (on dorsal aspect of thyroid gland) Thymus Adrenal glands Pancreas Ovary (female) Testis (male) Figure 16.1
Endocrine System: Overview • Endo = within • Crine = to secrete • Endocrine glands are ductless glands, secrete into blood stream • Exocrine glands are glands with ducts, secrete out of the body (generally) • Endocrine glands: pancreas, pineal, hypothalamus, pituitary, thyroid, parathyroid, and adrenal glands, and ovaries and testes
Endocrine System: Overview • Definition: ductless glands and tissues that secrete hormones which influence metabolic activities • Hormones are long distance chemical signals that travel in blood and lymph
Endocrine System: Overview • Hormones • Regulate growth and development • Regulate cellular metabolism and energy balance • Regulate reproduction • Mobilize the immune system • Maintain balance of electrolytes, water, and nutrients in the blood
Endocrine System: Overview • Other tissues and organs that produce hormones include: • adipose cells • thymus • cells in the walls of the small intestine • stomach • kidneys • heart
Endocrine System: Overview • Some organs produce both endocrine (hormones) and exocrine products (secretions into ducts) • e.g., pancreas and gonads • Some glands have both nervous and endocrine function • e.g., the hypothalamus and adrenal glands
Endocrine System: Overview Comparison of Endocrine and Nervous System
hormones metabolic activities of cells acts in seconds to minutes to hours to days to months long-lasting effects neurotransmitters muscle contractions and glandular secretions acts in milliseconds brief effects NERVOUS SYSTEM VS. ENDOCRINE SYSTEM Together, they coordinate functions of all body systems. - Both send chemical signals - Both affect specific target organs or tissues- Both work to maintain Homeostasis in the body NERVOUS ENDOCRINE
Nervous System vs. Endocrine System • Together the nervous and endocrine systems coordinate functions of all body systems. • The nervous system controls homeostasis through nerve impulses (action potentials) conducted along axons of neurons. • In contrast, the endocrine system releases its hormones into the bloodstream. The circulating blood then delivers hormones to virtually all cells throughout the body. • Certain parts of the nervous system stimulate or inhibit the release of hormones. Hormones in turn may promote or inhibit the generation of nerve impulses. (Nervous and Endocrine interact!)
Nervous System Modulation • The nervous system can override normal endocrine controls • For example, control of blood glucose levels • Normally the endocrine system maintains blood glucose • Under stress, the body needs more glucose • The hypothalamus and the sympathetic nervous system are activated to supply ample glucose
Endocrine System: Overview Endocrine action: the hormone is distributed in blood and binds to distant target cells.Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.Autocrine action: the hormone acts on the same cell that produced it.
Classes of Hormones • Two main classes 1. Amino acid-based hormones • Amines, thyroxine (T4), peptides, and proteins • Insulin! 2. Steroids • Synthesized from cholesterol • Gonadal and adrenocortical hormones
Hormone Classification • All steroid hormones are made initially from the precursor cholesterol.
Steroid hormone synthesis from cholesterol • Important steroid end products: • Aldosterone • Cortisol • DHEA • Testosterone • Estrone (E3) and Estraidol (E2)
Mechanisms of Hormone Action • Hormone action on target cells • Alter plasma membrane permeability of membrane potential by opening or closing ion channels • Stimulate synthesis of proteins or regulatory molecules • Activate or deactivate enzyme systems • Induce secretory activity • Stimulate mitosis
Mechanisms of Hormone Action • Two mechanisms, depending on their chemical nature • Water-soluble hormones (all amino acid–based hormones except thyroid hormone) • Act on plasma membrane receptors • Cannot enter the target cells (hydrophilic molecules trying to pass through a hydrophobic membrane) • Use second messenger systems to get message to target cell
receptor hormone adenylate cyclase G protein cAMP protein kinase phosphodiesterase altered cell function 5’-AMP (inactive) nucleus Mechanisms Of Water-Soluble Hormone Action Water-soluble hormones catecholamine, peptide, and protein hormones target cells use membrane-bound receptors first messenger vs. second messenger G protein adenylate cyclase cyclic AMP protein kinase phosphodiesterase ATP converted to
Extracellular fluid 1 Hormone (1st messenger)binds receptor. Adenylate cyclase G protein (GS) 5 cAMP acti-vates proteinkinases. Receptor Activeproteinkinase GDP Inactiveprotein kinase 2 3 4 Receptoractivates Gprotein (GS). G proteinactivatesadenylatecyclase. Adenylatecyclaseconverts ATPto cAMP (2ndmessenger). Hormones thatact via cAMPmechanisms: Triggers responses oftarget cell (activatesenzymes, stimulatescellular secretion,opens ion channel,etc.) GlucagonPTHTSHCalcitonin EpinephrineACTHFSHLH Cytoplasm Figure 16.2
Water Soluble Hormones • Glucagon • Insulin • PTH • TSH • Calcitonin • Epinephrine • ACTH • FSH • LH
Second Messenger Systems • 2 main types: • cAMP (cyclic AMP) • PIP2-calcium signaling mechanism • Involves calcium!
Mechanisms of Hormone Action • Lipid-soluble hormones (steroid and thyroid hormones) • Act on intracellular receptors that directly activate genes • Hydrophobic molecule can pass across hydrophobic membrane
hormone binds to receptor, which translocates to gene receptor nucleus mRNA diffusion diffusion protein hormone hormone DNA target cell target cell Mechanisms Of Steroid Hormone Action Lipid-soluble hormones steroid and thyroid hormones target cells use intracellular receptors hormone-receptor complexes altered gene expression
Intracellular Receptors and Direct Gene Activation • Steroid hormones and thyroid hormone • Diffuse into their target cells and bind with intracellular receptors • Receptor-hormone complex enters the nucleus • Receptor-hormone complex binds to a specific region of DNA • This prompts DNA transcription to produce mRNA • The mRNA directs protein synthesis
Target Cell Specificity • Target cells must have specific receptors for the hormones to bind to the cell • ACTH receptors are only found on certain cells of the adrenal cortex • Thyroxine (T4) receptors are found on nearly all cells of the body
Target Cell Activation • Target cell activation depends on three factors • Blood levels of the hormone • Relative number of receptors on or in the target cell • Affinity of binding between receptor and hormone
Target Cell Activation • Hormones influence the number of their receptors • Up-regulation—target cells form more receptors in response to the hormone • Down-regulation—target cells lose receptors in response to the hormone
Hormones in the Blood • Hormones circulate in the blood either free or bound • Steroids and thyroid hormone are attached to plasma proteins (hydrophobic molecules need escorts through a hydrophilic environment) • All others circulate without carriers (they are water soluble) • The concentration of a circulating hormone reflects: • Rate of release • Speed of inactivation and removal from the body
Hormones in the Blood • Hormones are removed from the blood by • Degrading enzymes • Kidneys • Liver • Half-life—the time required for a substance’s blood level to decrease by half • Water soluble hormones have the shortest half life
Interaction of Hormones at Target Cells • Multiple hormones may interact in several ways • Permissiveness: one hormone cannot exert its effects without another hormone being present • Synergism: more than one hormone produces the same effects on a target cell • Antagonism: one or more hormones opposes the action of another hormone
Control of Hormone Release • Blood levels of hormones • Are controlled by negative feedback systems • Vary only within a narrow desirable range • Hormones are synthesized and released in response to • Humoral stimuli (body) • Neural stimuli • Hormonal stimuli
Humoral Stimuli • Changing blood levels of ions and nutrients directly stimulates secretion of hormones • Example: Ca2+ in the blood • Declining blood Ca2+ concentration stimulates the parathyroid glands to secrete PTH (parathyroid hormone) • PTH causes Ca2+ concentrations to rise and the stimulus is removed
(a) Humoral Stimulus Capillary blood contains low concentration of Ca2+, which stimulates… 1 Capillary (low Ca2+ in blood) Thyroid gland (posterior view) Parathyroid glands Parathyroidglands PTH …secretion of parathyroid hormone (PTH) by parathyroid glands* 2 Figure 16.4a
Neural Stimuli • Nerve fibers stimulate hormone release • Sympathetic nervous system fibers stimulate the adrenal medulla to secrete catecholamines
Hormonal Stimuli • Hormones stimulate other endocrine organs to release their hormones • Hypothalamic hormones stimulate the release of most anterior pituitary hormones • Anterior pituitary hormones stimulate targets to secrete still more hormones • Hypothalamic-pituitary-target endocrine organ feedback loop: hormones from the final target organs inhibit the release of the anterior pituitary hormones
(c) Hormonal Stimulus The hypothalamus secretes hormones that… 1 Hypothalamus …stimulate the anterior pituitary gland to secrete hormones that… 2 Pituitary gland Thyroid gland Adrenal cortex Gonad (Testis) …stimulate other endocrine glands to secrete hormones 3 Figure 16.4c
Pineal Gland • Small gland hanging from the roof of the third ventricle • Pinealocytes secrete melatonin, derived from serotonin • Melatonin may affect • Timing of sexual maturation and puberty • Day/night cycles • Physiological processes that show rhythmic variations (body temperature, sleep, appetite)
PINEAL Hypothalamus Pituitary Adrenal Ovary/ Testes Thyroid
Hypothalamus • In the lower central part of the brain • The main link between the endocrine and the nervous systems. • Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.
Hypothalamic Hormones • GnRH (gonadotrophic releasing hormone) • SS (somatostatin) • PRF (prolactin releasing factor) • PIH (prolactin releasing inhibiting hormone) • TRH (thyrotrophin releasing hormone) • CRH (corticotrophin releasing hormone) • GHRH (growth hormone releasing hormone)
Hypothalamic Hormones Hormones from Hypothalamus
Pituitary Gland • Size of a pea • Located at the base of the brain, and the most important part of the entire endocrine system. • AKA: The master gland because it makes hormones that control other endocrine glands. • The production of hormones and secretions can be affected by emotions and seasons change.
Pituitary Gland Pituitary gland and hypothalamus are connected via a stalk, also known as the infundibulum.
Pituitary Gland • The pituitary gland (hypophysis) has two major lobes • Posterior pituitary (lobe) (neurohypophysis) • Neural tissue • Neuro = nervous • Anterior pituitary (lobe) (adenohypophysis) • Glandular tissue • Adeno = gland
Pituitary-Hypothalamic Relationships • Posterior lobe • A downgrowth of hypothalamic neural tissue • Neural connection to the hypothalamus (hypothalamic-hypophyseal tract) • Nuclei of the hypothalamus synthesize the neurohormones oxytocin and antidiuretic hormone (ADH) • Neurohormones are transported to the posterior pituitary