Endocrine System Overview and Comparison

Endocrine System Overview

• The endocrine system works with the nervous system to coordinate and integrate body cell activities.
• It influences metabolic activities through hormones carried in the blood.
• Responses are slower but longer-lasting than nervous system responses.
• Endocrinology is the study of hormones and endocrine organs.

Comparison of Nervous and Endocrine Systems

• The nervous system initiates responses rapidly, whereas the endocrine system initiates responses slowly.
• Nervous system responses are short-duration, while endocrine system responses are long-duration.
• Nervous system communication is via action potentials and neurotransmitters, while endocrine system communication is via hormones released into the blood.
• Nervous system actions target specific locations, determined by axon pathways, while endocrine system actions can target cells anywhere blood reaches.
• Neurotransmitters act over very short distances, whereas hormones act over longer distances.

Endocrine System Control and Integration

• The endocrine system controls and integrates:
  • Reproduction
  • Growth and development
  • Maintenance of electrolyte, water, and nutrient balance of blood
  • Regulation of cellular metabolism and energy balance
  • Mobilization of body defenses

Exocrine and Endocrine Glands

• Exocrine glands:
  • Produce nonhormonal substances (e.g., sweat, saliva)
  • Have ducts to carry secretions to the surface.
• Endocrine glands:
  • Produce hormones
  • Lack ducts

Endocrine Glands

• Endocrine glands include:
  • Pituitary, thyroid, parathyroid, adrenal, and pineal glands
  • Hypothalamus (a neuroendocrine organ)
  • Pancreas, gonads, placenta
  • Other tissues and organs (adipose cells, thymus, cells in walls of small intestine, stomach, kidneys, and heart).

Chemical Messengers

• Hormones: long-distance chemical signals, traveling in blood or lymph
• Autocrines: chemicals that affect the same cells that secrete them
• Paracrines: chemicals that affect cells other than those that secrete them
  • Autocrines and paracrines are local chemical messengers, not considered part of the endocrine system

Classes of Hormones

• Amino acid-based hormones:
  • Amino acid derivatives, peptides, proteins
• Steroids: synthesized from cholesterol
  • Gonadal and adrenocortical hormones
• A possible third class (eicosanoids) is considered a hormone by some, otherwise classified as paracrine

Target Cells

• Though hormones circulate systemically, only cells with specific receptors are affected.
• These are target cells (tissues with receptors for a specific hormone)
• Hormones alter target cell activity.

Hormone Action on Target Cells

• Hormones can:
  • Alter plasma membrane permeability and/or membrane potential by opening or closing ion channels
  • Stimulate synthesis of enzymes or other proteins
  • Activate or deactivate enzymes
  • Induce secretory activity
  • Stimulate mitosis

Water-Soluble and Lipid-Soluble Hormones

• Water-soluble hormones (all amino acid-based hormones except thyroid hormone):
  • Act on plasma membrane receptors
  • Use G protein second messengers
  • Cannot enter cells
• Lipid-soluble hormones (steroid and thyroid hormones):
  • Act on intracellular receptors directly activating genes
  • May act on plasma membrane receptors
  • Can enter cells

Plasma Membrane Receptors and Second-Messenger Systems

• Amino acid-based hormones (except thyroid hormone) exert their effects using second-messenger systems.
• Two main second-messenger systems include:
  • Cyclic AMP (cAMP)
  • PIP2-calcium

Cyclic AMP (cAMP) Signaling

• Hormone binds to a receptor.
• Receptor activates a G protein.
• G protein activates adenylate cyclase.
• Adenylate cyclase converts ATP to cAMP (a second messenger).
• cAMP activates protein kinases, which phosphorylate other proteins.
• Phosphorylated proteins are either activated or inactivated.
• cAMP is degraded by phosphodiesterase, stopping the cascade.

PIP2-Calcium Signaling

• Hormone activates a G protein that activates phospholipase C.
• Phospholipase C splits PIP₂ into DAG and IP₃.
• DAG activates protein kinases.
• IP₃ causes Ca²⁺ release from intracellular storage sites.
• Calcium ions act as another second messenger, altering enzyme activity or binding to calmodulin.
• Calcium-bound calmodulin activates enzymes for a cellular response.

Other Signaling Mechanisms

• cGMP (cyclic guanosine monophosphate) is a secondary messenger for some hormones.
• Some hormones, notably insulin, work without a secondary messenger, using tyrosine kinase enzymes.

Intracellular Receptors and Direct Gene Activation

• Lipid-soluble hormones (steroid and thyroid hormones) diffuse into target cells.
• Receptor-hormone complex enters the nucleus and binds to specific DNA regions.
• Binding initiates DNA transcription to produce mRNA.
• mRNA directs the synthesis of new proteins with various functions.

Hormone Release: Control by Negative Feedback

• Blood hormone levels are usually regulated by negative feedback systems.
• Increasing hormone effects on target organs can inhibit further hormone release.
• Hormone levels are within a narrow, desirable range

Endocrine Gland Stimuli

• Three types of stimuli trigger endocrine gland secretion:
  • Humoral stimuli (blood levels of ions or nutrients).
  • Neural stimuli (nerve fibers).
  • Hormonal stimuli (hormones).

Nervous System Modulation of Hormone Release

• The nervous system can adjust hormone levels.
• It can modify stimulation or inhibition of endocrine glands.
• It can override normal endocrine controls (e.g. stress).

Target Cell Specificity

• Target cells must have specific receptors for a hormone to bind.
• Three influencing factors to target cell activation:
  • Blood levels of hormone,
  • Relative number of receptors,
  • Affinity (strength) of binding between hormone and receptor.
• Hormone amount influences receptor numbers (up-regulation or down-regulation).

Ligand-receptor Binding

• The number of receptors on a cell can change (up-regulation or down-regulation).
• Up-regulated cells are more sensitive to hormones.
• Down-regulation can reduce sensitivity to hormones.

Hypothalamus

• Connects to the pituitary gland via the infundibulum.
• Has two major lobes (posterior and anterior).
• Posterior pituitary:composed of neural tissue, secreting neurohormones (oxytocin and ADH)
• Anterior pituitary (adenohypophysis): consists of glandular tissue, regulated by hypothalamus.

Pituitary-Hypothalamic Relationships

• Posterior: neural pathway from hypothalamus --> posterior pituitary.
  • Hormones oxytocin and ADH are stored in axon terminals
  • Released into blood when neurons fire.
• Anterior: hypophyseal portal system (primary + secondary capillary plexuses and hypophyseal portal veins) connects hypothalamus –> anterior pituitary.
  • Hypothalamus secretes releasing and inhibiting hormones –> anterior pituitary to control hormone secretion.

Posterior Pituitary and Hypothalamic Hormones

• Posterior pituitary hormones: oxytocin and antidiuretic hormone (ADH).
  • Oxytocin: stimulates uterine contractions, milk ejection, neurotransmitter.
  • ADH: increases water reabsorption in kidneys, also called vasopressin, regulated by osmotic pressure.

Anterior Pituitary Hormones:

• All six anterior pituitary hormones are peptide hormones except growth hormone
• Six of them regulate other endocrine glands.
• Growth hormone: has direct + indirect actions on metabolism; and stimulating growth by producing insulin growth factors.
• TSH: stimulates thyroid gland to release T3 & T4
• ACTH: stimulates adrenal cortex to secrete glucocorticoids (e.g., cortisol)
• FSH: stimulates gamete production and gonadal hormone production.
• LH: stimulates hormone production.
• PRL: stimulates milk production.

Clinical Issues

• Hypersecretion or hyposecretion of growth hormone can lead to gigantism or acromegaly in adults, and pituitary dwarfism in children.
• Disorders of pituitary growth hormone have been documented.

Explore the intricacies of the endocrine system and how it operates alongside the nervous system. This quiz delves into the differences in response time, communication methods, and the role of hormones in regulating body functions. Test your knowledge on these key concepts and gain a deeper understanding of human physiology.