Chapter 17

17.1 Neural and Endocrine Signaling

• The nervous system uses two types of intercellular communication:

  • Electrical signaling

  • Direct action of an electrical potential.

  • Chemical signaling

  • Involves neurotransmitters like serotonin and norepinephrine that act rapidly and locally.

    • Neurotransmitter action:

    • Action potentials reach synaptic terminals.

    • Neurotransmitters diffuse across the synaptic cleft to bind receptors on the post-synaptic cell.

    • Stimulation transduced into cellular responses (e.g., electrical signaling, cellular modification).

    • Response Timeliness:

    • Target cell responses occur within milliseconds and cease quickly after signaling ends.

    • Enables quick actions related to movement, sensation, and cognition.

• The endocrine system uses only one type of communication:

  • Chemical signaling, with hormones secreted into extracellular fluid.

  • Hormones are primarily transported via the bloodstream, binding to receptors on target cells, leading to slower responses compared to neural signals.

  • Response Timeliness:

    • Fast reactions (seconds) can occur, such as adrenal hormones in fight-or-flight, while some hormones like reproductive hormones may take up to 48 hours for response.

  • Generally less specific: the same hormone can impact various physiological processes.

    • Example: Oxytocin affects uterine contractions, breastfeeding, and emotional bonding.

• Connection between systems:

  • Rapid actions of the nervous system stimulate adrenal glands to release hormones quickly, allowing adaptation to changes in environment.

17.2 Structures of the Endocrine System

• The endocrine system consists of cells, tissues, and organs that produce and secrete hormones.

  • Major components include:

  • Pituitary Gland

  • Thyroid Gland

  • Parathyroid Glands

  • Adrenal Glands

  • Pineal Gland

  • Some glands have both endocrine and non-endocrine functions:

  • Pancreas: secretes insulin and glucagon for blood glucose regulation as well as digestive enzymes.

  • Other endocrine-functioning organs include hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, ovaries, and testes.

  • Recent research indicates that adipose and bone tissues also have endocrine functions.

• Ductless endocrine glands are distinct from exocrine glands that secrete through ducts (e.g., sebaceous and sweat glands).

• Additional Types of Chemical Signaling:

  • Autocrine Signaling: Chemical that elicits a response in the same cell that secreted it.

  • Example: Interleukin-1 in inflammatory response.

  • Paracrine Signaling: Induces a response in neighboring cells, typically not traveling far.

  • Example: Histamine in asthma.

  • Neurotransmitters in the synapse exemplify this.

17.3 Hormones

• Hormones affect only target cells that have specific receptors.

• They regulate physiological processes crucial for:

  • Reproduction

  • Growth and development

  • Metabolism and fluid balance

  • Sleep and many more functions.

  • Examples of Major Hormones:

• Types of Hormones

  • Chemical Structure Classification:

  • Hormones from amino acids: amines, peptides, proteins.

  • Lipid-derived hormones: steroids.

• Amine Hormones:

  • Modified amino acids, e.g., melatonin from tryptophan.

  • Include catecholamines like epinephrine, norepinephrine, and dopamine.

• Peptide and Protein Hormones:

  • Chains of amino acids, synthesized like body proteins (from DNA to mRNA to polypeptides).

  • Examples: ADH, growth hormone, FSH.

• Steroid Hormones:

  • Derived from cholesterol (lipid), e.g., testosterone, aldosterone, cortisol.

  • Hydrophobic; require transport proteins to circulate in blood.

17.4 Pathways of Hormone Action

• Hormones send messages via hormone receptors, which may be located inside cells or on cell membranes.

• Receptor Function:

  • Process hydrant ratio messages, triggering cellular responses.

  • Receptor specificity leads to different target cell outcomes.

• Intracellular Hormone Receptors:

  • Steroid hormones and thyroid hormones cross membranes to bind with receptors inside cells, initiating gene transcription.

• Cell Membrane Hormone Receptors:

  • Hydrophilic hormones bind to receptors on the cell surface, often activating secondary messengers within the cell.

  • Common Pathways:

  • cAMP signaling involves engagement of G-proteins, activating adenylyl cyclase, converting ATP to cAMP, ultimately stimulating protein kinases.

  • Examples: Calcitonin, glucagon, thyroid-stimulating hormone utilize cAMP pathway.

  • Calcium ions as second messengers involve phospholipase C, generating diacylglycerol (DAG) and inositol triphosphate (IP3).

  • Calcium ions influence cellular processes directly or via calmodulin.

17.5 Factors Affecting Target Cell Response

• Receptor Specificity: Cells must have the appropriate receptors for hormones to elicit responses.

• Receptor Regulation Mechanisms:

  • Downregulation: High circulating hormone levels can decrease receptor number, reducing tissue responsiveness.

  • Upregulation: Low hormone levels can increase receptor number for heightened response.

• Hormonal Interactions:

  • Permissive Effect: One hormone enables another to act (e.g., thyroid hormones on reproductive hormones).

  • Antagonistic Effect: Opposing effects of two hormones (e.g., insulin lowering blood glucose vs. glucagon raising it).

  • Synergistic Effect: Combined effects of hormones amplify outcomes (e.g., FSH and estrogens promoting ovum maturation).

17.6 Regulation of Hormone Secretion

• Hormone levels must be finely regulated to prevent disease.

• Mechanisms include:

  • Feedback Loops:

  • Positive Feedback: Amplifies original hormone release (e.g., oxytocin during childbirth).

  • Negative Feedback: Inhibits further secretion when adequate hormone levels are reached (e.g., glucocorticoids).

  • Hormonal Stimuli: Hormone secretion in response to levels of other hormones, especially via the hypothalamus.

  • Neural Stimuli: Nerves, such as those from the sympathetic nervous system, stimulate processes like fight-or-flight, activating adrenal gland secretion.

17.7 The Pituitary Gland and Hypothalamus

• Hypothalamus-Pituitary Complex:

  • Function: Secretes hormones affecting other glands and coordinates endocrine responses.

  • Contains neural and endocrine functions; located in the diencephalon of the brain.

  • Related anatomically and functionally to the pituitary gland, which has anterior and posterior lobes (anterior is glandular; posterior is neural tissue).

• Pituitary Hormones:

17.8 Posterior Pituitary Hormones

• Oxytocin:

  • Key hormone for childbirth inducing uterine contractions through positive feedback.

  • Release continues for milk ejection during breastfeeding and emotional bonding.

• Antidiuretic Hormone (ADH):

  • Regulates blood osmolarity, reabsorbing water in kidneys.

  • Secretion is controlled via negative feedback; high concentration inhibits its release, affecting water balance.

17.9 Anterior Pituitary Hormones

• Growth Hormone (GH):

  • Promotes growth, protein synthesis, and impacts blood glucose through diabetogenic effects.

  • Regulated by GHRH and GHIH from the hypothalamus.

• Thyroid-Stimulating Hormone (TSH):

  • Stimulates the release of thyroid hormones, regulated by TRH.

• Adrenocorticotropic Hormone (ACTH):

  • Indicates the secretion of corticosteroids; its release is controlled by corticotropin-releasing hormone (CRH).

• Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH):

  • Regulatory hormones in reproductive processes (e.g. gamete production).

• Prolactin (PRL):

  • Stimulates milk production, regulated by PIH and PRH from the hypothalamus.

17.10 Thyroid Gland

• Location and Structure:

  • Butterfly-shaped beneath the larynx; produces thyroid hormones T3 and T4 derived from iodine and thyroglobulin.

• Synthesis of Thyroid Hormones:

  1. TSH binding induces iodide uptake into follicular cells.

  2. Iodide oxidation produces iodine, which links with tyrosine in thyroglobulin.

  3. Formation of T3 (triiodothyronine) and T4 (thyroxine) through iodination and coupling.

  4. Secretion regulated by TSH; negative feedback system maintains levels.

• Functions of Thyroid Hormones:

  • Regulation of basal metabolic rate, thermogenesis, and significant roles in development and metabolism.

17.11 Calcitonin and Parathyroid Glands

• Calcitonin:

  • Produced by parafollicular cells; released in response to high blood calcium, acting to lower calcium levels.

• Parathyroid Hormone (PTH):

  • Produced by chief cells in response to low calcium levels; works to increase blood calcium through osteoclast stimulation and kidney reabsorption.

17.12 Adrenal Glands

• Structure:

  • Composed of cortex and medulla, performing distinct functions:

• Adrenal Cortex:

  • Zona Glomerulosa: Produces aldosterone for fluid/electrolyte balance.

  • Zona Fasciculata: Produces glucocorticoids like cortisol, affecting glucose metabolism and stress response.

  • Zona Reticularis: Produces androgens, influencing sex characteristics and functions in both genders.

• Adrenal Medulla:

  • Secretes epinephrine and norepinephrine in response to stress triggers.

17.13 Disorders of the Endocrine System

• Examples of endocrine dysregulation:

  • Cushing’s Disease: Hypersecretion of cortisol, leading to high glucose and associated symptoms.

  • Addison’s Disease: Hyposecretion results in low glucose and sodium, presenting with nonspecific symptoms.

17.14 The Pineal Gland

• Produces melatonin, involved in circadian rhythms, sleep regulation, and possibly functioning as an antioxidant, influenced by environmental light levels.

17.15 Gonadal Hormones

• Testes: Produce testosterone (steroid), affecting reproductive system and secondary sex characteristics.

• Ovaries: Produce estrogens and progesterone (steroids), impacting reproductive health and preparing for childbirth.

• Placental Hormones:

  • Human chorionic gonadotropin (hCG) sustains early pregnancy; relaxin aids in childbirth prep.

17.16 The Endocrine Pancreas

• Pancreatic islets secrete glucagon (alpha cells) to increase blood glucose and insulin (beta cells) to decrease blood glucose.

• Somatostatin (delta cells) inhibits glucagon and insulin secretion.

17.17 Organs with Secondary Endocrine Functions

• Key Tissues:

  • Heart: Atrial natriuretic peptide (ANP) reduces blood pressure.

  • Gastrointestinal Tract: Various hormones enhancing digestion.

  • Kidneys: Release of renin, producing erythropoietin (EPO) for red blood cell production.

  • Skeleton: Produces hormones affecting insulin sensitivity.

  • Adipose Tissue: Secretes leptin and adiponectin, regulating energy levels.

  • Skin: Produces cholecalciferol, the precursor to active vitamin D3.

• Thymus: Produces thymosins for T cell development.

• Liver: Secretes IGF, angiotensinogen, thrombopoietin, and hepcidin.

17.18 Development and Aging of the Endocrine System

• Endocrine glands arise from different germ layers; aging affects their structure and hormone production, e.g., decline in growth hormone and reproductive hormones.