Chapter 17

Introduction

  • The endocrine and nervous systems in humans specialize in communication and coordination.

    • The endocrine system uses hormones for communication.

    • The nervous system relies on neurotransmitters.

  • Chapter coverage includes:

    • Basics of glands, hormones, and effects.

    • Details of hormone chemistry, production, transportation, and mechanism of action.

    • Endocrine system's role in stress.

    • Paracrine secretions.

    • Endocrine dysfunctions.

Overview of the Endocrine System

  • Expected Learning Outcomes:

    • Define hormone and endocrine system.

    • Name several organs of the endocrine system.

    • Contrast endocrine with exocrine glands.

    • Recognize standard abbreviations for many hormones.

    • Compare and contrast the nervous and endocrine systems.

  • Principal Mechanisms of Cell Communication:

    • Gap Junctions:

    • Pores in the cell membrane allow signaling molecules, nutrients, and electrolytes to move between cells.

    • Neurotransmitters:

    • Released from neurons to traverse the synaptic cleft to a second cell.

    • Paracrines:

    • Secreted into tissue fluids to affect nearby cells.

    • Hormones:

    • Chemical messengers that travel in the bloodstream to other tissues and organs.

Endocrine System Characteristics

  • Endocrine System:

    • Comprises glands, tissues, and cells that secrete hormones.

  • Endocrinology:

    • The study of the endocrine system and disorders.

  • Endocrine Glands:

    • Organs serving as traditional sources of hormones.

  • Hormones:

    • Chemical messengers transported by the bloodstream that stimulate physiological responses in distant cells.

Main Organs of the Endocrine System

  • Pineal gland

  • Pituitary gland

  • Hypothalamus

  • Thyroid gland

  • Thymus

  • Adrenal gland

  • Pancreas

  • Gonads (ovary and testis)

  • Parathyroid glands

Comparison of Endocrine and Exocrine Glands

  • Exocrine Glands:

    • Have ducts to carry secretions to an epithelial surface or digestive tract (external secretions).

    • Effects are extracellular (e.g., food digestion).

  • Endocrine Glands:

    • No ducts; have dense capillary networks for easy hormone uptake.

    • Internal secretions with intracellular effects (e.g., altering target cell metabolism).

    • Liver functions atypically, releasing hormones while also secreting bile into ducts and other factors into blood.

Comparison of Nervous and Endocrine Systems

  • Similarities:

    • Both systems facilitate internal communication.

  • Differences:

    • Response Speed:

    • Nervous: Quick (milliseconds), short-lived effects.

    • Endocrine: Slower (seconds to days), effects can last longer.

    • Adaptation to Stimuli:

    • Nervous: Adapts quickly to stimuli.

    • Endocrine: Adapts slowly.

    • Area of Effect:

    • Nervous: Targeted to specific organs.

    • Endocrine: Widespread effects on multiple organs.

  • Hormones as Neurotransmitters:

    • Some chemicals function as both hormones and neurotransmitters (e.g., norepinephrine, dopamine, and antidiuretic hormone).

    • Similar effects on target cells (e.g., norepinephrine and glucagon promote glycogen hydrolysis in the liver).

    • The two systems can have regulatory influences over each other.

    • Neuroendocrine cells share characteristics with both systems.

  • Target Organs/Cells:

    • Organs or cells with receptors for a hormone that can respond to it; these cells may possess enzymes to convert hormones to their active forms.

The Hypothalamus and Pituitary Gland

  • Expected Learning Outcomes:

    • Describe anatomical relationships between the hypothalamus and pituitary gland.

    • Distinguish between anterior and posterior pituitary lobes.

    • List hormones produced by the hypothalamus and pituitary lobes; identify their functions.

    • Explain how the pituitary gland is controlled by the hypothalamus and its target organs.

    • Describe growth hormone effects.

  • Hypothalamus Structure:

    • Shaped like a flattened funnel; forms the floor and walls of the third brain ventricle.

    • Regulates primitive functions, such as water balance, thermoregulation, sex drive, and childbirth.

  • Pituitary Gland Structure:

    • Suspended from the hypothalamus by a stalk (infundibulum).

    • Located in the sella turcica of the sphenoid bone; kidney bean-sized.

    • Consists of two structures:

    • Adenohypophysis (anterior pituitary): constitutes the anterior three-quarters of the gland. Regulated by the hypophyseal portal system.

      • Primary capillaries in hypothalamus connected to secondary capillaries in adenohypophysis via portal venules.

    • Neurohypophysis (posterior pituitary): comprises the posterior one-quarter; not a true gland but nerve tissue.

      • Has nerve cell bodies from the hypothalamus traveling down the stalk and ending in posterior lobe; stores hormones until release.

Hormones of the Hypothalamus

  • Produced Hormones: Eight hormones total, six regulate the anterior pituitary, two released into capillaries in the posterior pituitary.

    • Release and Inhibit: Hormones include:

    • Thyrotropin-releasing hormone (TRH)

    • Corticotropin-releasing hormone (CRH)

    • Gonadotropin-releasing hormone (GnRH)

    • Growth hormone-releasing hormone (GHRH)

    • Prolactin-inhibiting hormone (PIH)

    • Somatostatin (inhibits TSH and GH).

  • Posterior Pituitary Hormones:

    • Antidiuretic hormone (ADH):

    • Increases water retention, reduces urine volume, prevents dehydration; also known as vasopressin.

    • Produced in the supraoptic nuclei.

    • Oxytocin (OT):

    • Released during sexual arousal, childbirth, stimulates contractions duringbirth, and milk ejection during lactation.

    • Produced in paraventricular nuclei.

Control of Pituitary Secretion

  • Secretion Rates: Not constant; regulated by the hypothalamus and feedback from target organs.

  • Hypothalamic and Cerebral Control:

    • Brain monitors conditions affecting anterior pituitary function (e.g., stress triggers ACTH release).

    • Responses in pregnancy also invoke prolactin secretion.

  • Negative Feedback:

    • Increased levels of target organ hormones can inhibit hypothalamic or pituitary hormone release.

    • Example: Thyroid hormones inhibit TRH and TSH release.

  • Positive Feedback:

    • E.g., Uterine stretching enhances OT release, driving further contractions until delivery.

Growth Hormone Overview

  • Growth Hormone Effects:

    • Widespread effects on cartilage, bone, muscle, fat.

    • Induces liver to produce insulin-like growth factors (IGFs).

    • Encourages protein synthesis and fat catabolism, stimulates glucose-sparing for carbohydrate metabolism.

    • Bone growth and muscle mass increase during childhood and adolescence.

Other Endocrine Glands

  • Pineal Gland:

    • Located in the brain, influences circadian rhythms and synthesizes melatonin.

  • Thymus:

    • Matures T-cells for immune response; produces thymopoietin, thymosin, and thymulin.

  • Thyroid Gland:

    • Primary endocrine gland; secretes T3 and T4. Affects metabolic rate, growth, and development.

  • Parathyroid Glands:

    • Secrete parathyroid hormone (PTH) which regulates calcium levels in the blood.

  • Adrenal Glands:

    • Secrete epinephrine and norepinephrine; consist of cortex (corticosteroids) and medulla.

Hormonal Functions and Disorders

  • Diabetes Mellitus:

    • Type 1 (IDDM): Autoimmune destruction of pancreatic beta cells; treated with insulin.

    • Type 2 (NIDDM): Insulin resistance with lifestyle risk factors; managed through diet, exercise, and medication.

  • Common Endocrine Disorders:

    • Hyposecretion (e.g., diabetes insipidus).

    • Hypersecretion (e.g., Cushing syndrome, acromegaly).