bio142_-_unit_1_notes

Overview and Comparison of Extracellular Communication Systems

Foundations of Homeostatic Control:
- The nervous system and endocrine system operate together to regulate and maintain homeostasis across nearly every regulated physiological variable in the human body.
- Both systems utilize chemical messengers to communicate with target cells.
- Both systems can function cooperatively, amplifying or modulating each other's physiological effects.
Key Differences Between Nervous and Endocrine Communication:
- Nervous System Characteristics:
  - Operates through precise anatomical networks of neurons.
  - Directly affects target cells via the release of neurotransmitters across microscopic synaptic clefts.
  - Exerts immediate effects, typically occurring within milliseconds.
  - Demonstrates short-lived responses unless stimulation is continuously repeated.
- Endocrine System Characteristics:
  - Endocrine cells do not make direct physical or close synaptic contact with target cells.
  - Alters target cell activity through the secretion of hormones into extracellular fluid and the bloodstream.
  - Displays a delayed onset of action, requiring seconds, hours, or even days to elicit physiological effects.
  - Produces prolonged, long-lasting cellular effects relative to neural signals.
Four Steps of Endocrine Hormone Transport:
- Step 1: Endocrine cells synthesize and secrete hormones into the surrounding interstitial fluid, from which they diffuse across endothelial walls into blood capillaries.
- Step 2: Deoxygenated systemic capillary blood carries the hormones through systemic venules and veins back to the heart.
- Step 3: The heart pumps blood into the pulmonary circuit for oxygenation, returns it to systemic circulation, and distributes the hormones throughout the entire body via systemic arteries.
- Step 4: Within peripheral capillary beds, hormones diffuse out of the blood plasma, cross into the interstitial fluid, and bind specifically to matching receptors located on or inside target cells.

Mechanism of hormone secretion into interstitial fluid, circulation through the heart and vasculature, and target cell binding

General Organization and Chemical Signaling of the Endocrine System

General Functions of the Endocrine System:
- Regulates fluid, electrolyte, and acid-base homeostasis.
- Controls growth, development, and tissue remodeling.
- Regulates metabolic reactions and energetic balance.
- Coordinates biological responses to physical, psychological, and physiological stressors.
Local Chemical Signaling Mechanisms:
- Autocrine Secretion: A form of signaling in which a chemical messenger influences the function of the very same cell or identical cell type that synthesized and secreted it.
- Paracrine Secretion: A mode of signaling wherein a chemical messenger is released into the extracellular fluid to act locally on nearby, distinct cell types without entering the systemic circulation.
Classification of Endocrine Tissues and Organs:
- Primary Endocrine Organs: Glands and organs whose primary physiological role is the synthesis and secretion of hormones:
  - Anterior pituitary gland
  - Thyroid gland
  - Three to five small parathyroid glands
  - Paired adrenal cortices
  - Endocrine pancreas (pancreatic islets)
  - Thymus
  - Paired ovaries (in females) or testes (in males)
- Neuroendocrine Organs: Neural tissues that contain specialized neurosecretory cells producing hormones:
  - Hypothalamus
  - Pineal gland
  - Adrenal medulla
- Secondary Endocrine Tissues: Organs and structures that have primary non-endocrine functions but contain cells that secrete hormones:
  - Heart
  - Kidneys
  - Small intestine
  - Adipose tissue
  - Cancer cells (notably originating from lung and gastrointestinal malignancies)
- Dual-Classification Organs: Both the testes and ovaries serve as primary endocrine organs (secreting sex steroids) and secondary reproductive organs (producing gametes).

Hormone Chemistry, Transport, and Cellular Receptor Mechanisms

Chemical Classification of Hormones:
- Amino Acid-Based Hormones:
  - Composed of one or more amino acids.
  - Span a wide size spectrum: single modified amino acids (amine hormones), short peptide chains (peptide hormones), or large complex proteins (protein hormones).
  - Synthesized by the anterior pituitary gland, pancreas, thymus, thyroid gland, and parathyroid glands.
  - The majority possess polar chemical structures making them hydrophilic (water-soluble, interacting freely with water molecules).
  - Exception: Thyroid hormone is derived from the nonpolar amino acid tyrosine, making it hydrophobic (lipid-soluble).
- Steroid Hormones:
  - Derived from cholesterol and characterized by a core four-hydrocarbon-ring backbone.
  - Synthesized by the adrenal cortices, testes, and ovaries.
  - Inherently hydrophobic (lipid-soluble) and unable to interact directly with water molecules.
  - Readily mix with nonpolar lipids and can be stored within adipose tissue deposits.
Hormone Dynamics in Blood Circulation:
- Free Hormones:
  - Generally small, hydrophilic amino acid-based hormones.
  - Dissolve and travel freely through the watery plasma of the bloodstream.
- Bound Hormones (Protein-Hormone Complexes):
  - Hydrophobic hormones that reversible bind to hydrophilic plasma transport proteins.
  - Select hydrophilic hormones, such as growth hormone, are also carried bound to plasma proteins.
- Physiological Characteristics of Bound Hormones:
  - Facilitate safe and uniform transit of hydrophobic molecules through the aqueous environment of blood plasma.
  - Establish a circulating reservoir of hormone that buffers plasma levels, preventing rapid or extreme fluctuations in free hormone concentration.
  - Substantially extend the biological half-life of hormones by retarding metabolic degradation and renal clearance.
- Hormone Half-Life: The exact amount of time required for the plasma concentration of a circulating hormone to decrease by $50\%$ .
Mechanism of Action: Hydrophobic Hormones:
- Receptor Location: Hydrophobic hormones cross the phospholipid bilayer because they are soluble in the membrane's hydrophobic fatty acid tails; their receptors are located intracellularly in the cytosol or nucleus.
- Cellular Cascade:
  - The free hormone diffuses across the plasma membrane into the target cell.
  - The hormone binds to an intracellular receptor, creating a hormone-receptor complex.
  - The complex translocates into the nucleus (if not already localized there).
  - The complex binds to specific regulatory DNA sequences termed hormone-response elements.
  - This binding alters gene transcription, modulating the rate of synthesis of messenger RNA ( $mRNA$ ) and resulting cellular proteins.
- Representative Example: Thyroid hormones ( $T_3$ and $T_4$ ).

Mechanism of hydrophobic hormone diffusion across the plasma membrane, intracellular receptor binding, and nuclear gene activation

Mechanism of Action: Hydrophilic Hormones:
- Receptor Location: Hydrophilic hormones are repelled by the hydrophobic fatty acid interior of the plasma membrane; they bind to the extracellular domain of transmembrane receptor proteins on the cell surface.
- Second-Messenger Transduction:
  - The hormone serves as the first messenger by binding its cell-surface receptor.
  - Receptor activation causes a membrane-associated G-protein to split into active subunits.
  - A G-protein subunit activates a membrane enzyme, such as adenylate cyclase.
  - Adenylate cyclase catalyzes the conversion of adenosine triphosphate ( $ATP$ ) into cyclic adenosine monophosphate ( $cAMP$ ), which acts as the second messenger.
  - $cAMP$ activates downstream effector enzymes, specifically protein kinase A ( $PKA$ ).
  - Protein kinase A phosphorylates specific intracellular proteins, modifying their enzymatic or structural activity and triggering a coordinated cellular response.
- Ion Channel Modulation: Select hydrophilic hormone receptors function directly or indirectly as ligand-gated ion channels, opening or closing upon binding to alter intracellular ion concentrations and membrane potential.
- Representative Example: The adenylate cyclase- $cAMP$ signaling system.

Hydrophilic hormone signaling cascade via G-protein activation, adenylate cyclase, cAMP second messenger generation, and protein kinase A

Five Major Cellular Effects of Hormone Action:
- Stimulation of secretory activity from endocrine or exocrine cells.
- Enzymatic activation or inhibition within metabolic pathways.
- Stimulation or inhibition of cell division via mitosis or meiosis.
- Opening or closing of plasma membrane ion channels, thereby altering electrical membrane potential.
- Initiation or suppression of gene expression through altered transcription of $RNA$ and protein synthesis.
Patterns of Hormone Interaction:
- Synergistic Interactions: Occur when two or more distinct hormones act upon the same target cell to elicit the same effect, producing a cumulative physiological outcome that is greater than the sum of their individual effects (e.g., prolactin and oxytocin acting collaboratively during lactation).
- Antagonistic Interactions: Occur when two distinct hormones act on the same target cells to produce opposite biological actions (e.g., insulin lowering blood glucose levels while glucagon raises them).

Regulation of Hormone Secretion and Feedback Loops

Three Primary Stimuli for Hormone Secretion:
- Hormonal Stimuli: Endocrine cells modulate hormone release in direct response to other upstream hormones (e.g., hypothalamic growth hormone-releasing hormone stimulating pituitary growth hormone release, or somatostatin inhibiting it).
- Humoral Stimuli: Endocrine cells monitor and respond directly to concentrations of specific ions or chemical compounds circulating in the blood or extracellular fluid (e.g., pancreatic beta cells detecting elevated glucose and secreting insulin).
- Neural Stimuli: Endocrine cells release hormones in direct response to nerve impulses (e.g., sympathetic preganglionic neurons stimulating adrenal medullary chromaffin cells to release epinephrine and norepinephrine).

Hormonal, humoral, and neural stimuli triggering endocrine secretion

Five Steps of a Homeostatic Negative Feedback Loop:
- 1. Stimulus: A regulated physiological variable drifts outside its set point or normal homeostatic range (either increasing or decreasing).
- 2. Receptor: Cellular sensors on specialized endocrine cells detect the deviation of the regulated variable.
- 3. Control Center: The endocrine cell (often acting simultaneously as both receptor and control center) adjusts its rate of hormone synthesis and secretion (increasing or decreasing release).
- 4. Effector/Response: The secreted hormone travels through the circulation and triggers cellular changes in distant target tissues that counteract the original deviation.
- 5. Return to Homeostatic Range: As the physiological variable returns to its baseline range, the reduction in stimulus intensity feeds back to the control center, downregulating effector hormone output.

The Hypothalamus and Pituitary Gland

Anatomical Relationship:
- The hypothalamus resides in the anteroinferior region of the diencephalon of the brain.
- It is physically connected to the inferiorly located pituitary gland by an anatomical stalk termed the infundibulum.
- The pituitary gland sits nestled within the sella turcica of the sphenoid bone and is divided into the anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis).
Hypothalamus and the Posterior Pituitary Gland:
- The posterior pituitary is nervous tissue that stores and releases neurohormones synthesized in the hypothalamic supraoptic and paraventricular nuclei.
- Antidiuretic Hormone (ADH / Vasopressin):
  - Stimulus for Release: Increased solute concentration (osmolarity) of the blood plasma detected by hypothalamic osmoreceptors.
  - Inhibitor of Release: Decreased solute concentration of the blood.
  - Target Tissues: Kidney collecting tubules and brain cells.
  - Physiological Effects: Stimulates water reabsorption from renal tubular filtrate, thereby reducing urine volume and elevating blood volume and pressure.
- Oxytocin:
  - Stimulus for Release: Mechanical stretching of the cervix and uterus during labor, and infant suckling at the breast.
  - Inhibitor of Release: Absence of sensory stretching or suckling stimuli.
  - Target Tissues: Uterine smooth muscle (myometrium) and specialized myoepithelial cells of the mammary glands.
  - Physiological Effects: Induces rhythmic uterine smooth muscle contractions during child delivery; triggers the milk let-down reflex (milk ejection) in lactating females.
Hypothalamus and the Anterior Pituitary Gland:
- The hypothalamus communicates with the anterior pituitary via the hypophyseal portal system, delivering releasing and inhibiting hormones.
- Hypothalamic Regulatory Hormones:
  - Thyrotropin-Releasing Hormone (TRH): Stimulates secretion of thyroid-stimulating hormone ( $TSH$ ).
  - Somatostatin (Growth Hormone-Inhibiting Hormone / GHIH): Inhibits secretion of growth hormone ( $GH$ ) and $TSH$ .
  - Corticotropin-Releasing Hormone (CRH): Stimulates secretion of adrenocorticotropic hormone ( $ACTH$ ).
  - Prolactin-Releasing Hormone (PRH): Stimulates secretion of prolactin ( $PRL$ ).
  - Prolactin-Inhibiting Hormone (PIH / Prolactin-Inhibiting Factor / Dopamine): Continuously inhibits secretion of prolactin ( $PRL$ ).
  - Gonadotropin-Releasing Hormone (GnRH): Stimulates secretion of luteinizing hormone ( $LH$ ) and follicle-stimulating hormone ( $FSH$ ).
  - Growth Hormone-Releasing Hormone (GHRH): Stimulates secretion of growth hormone ( $GH$ ).
Anterior Pituitary Hormones and Endocrine Axes:
- Thyroid Axis:
  - Hypothalamic $TRH$ triggers anterior pituitary $TSH$ release.
  - $TSH$ acts on thyroid follicular cells to stimulate thyroid gland development and the synthesis and secretion of thyroid hormones ( $T_3$ and $T_4$ ).
  - Elevated blood $T_3$ and $T_4$ levels exert negative feedback, inhibiting both hypothalamic $TRH$ and pituitary $TSH$ release.
- Adrenal Cortical Axis:
  - Hypothalamic $CRH$ triggers anterior pituitary $ACTH$ release.
  - $ACTH$ targets the adrenal cortex, stimulating development of the cortex and the secretion of corticosteroids, primarily glucocorticoids (cortisol).
  - Elevated cortisol inhibits further release of $CRH$ and $ACTH$ .
- Prolactin Axis:
  - Under dominant hypothalamic inhibition by $PIH$ (dopamine); stimulation occurs via $PRH$ and infant suckling.
  - $PRL$ targets mammary gland tissue, stimulating mammary gland growth, the initiation of milk synthesis following parturition, and the continuous production of milk during lactation.
- Gonadotropic Axis:
  - Hypothalamic $GnRH$ triggers release of anterior pituitary gonadotropins: $FSH$ and $LH$ .
  - In Males: $LH$ stimulates interstitial Leydig cells to produce testosterone; $FSH$ stimulates Sertoli cells to produce factors that bind and concentrate testosterone.
  - In Females: $LH$ and $FSH$ jointly promote estrogen secretion; $FSH$ triggers ovarian follicle maturation; an $LH$ surge triggers ovulation and stimulates the corpus luteum to produce progesterone and estrogens.
- Somatotropic Axis:
  - Hypothalamic $GHRH$ stimulates $GH$ release (elevated by fasting, stress, exercise, and protein-rich meals); somatostatin inhibits it.
  - $GH$ regulates total body growth directly and stimulates the liver and peripheral tissues to secrete insulin-like growth factor ( $IGF$ ).
Growth Hormone Dynamics and Pathology:
- Short-Term Metabolic Effects of GH:
  - Skeletal Muscle: Inhibits glucose uptake, sparing glucose for the brain.
  - Liver: Stimulates gluconeogenesis to elevate systemic blood glucose concentration.
  - Adipose Tissue: Stimulates lipolysis, increasing circulating free fatty acids.
- Long-Term Growth-Promoting Effects of GH and IGF:
  - All Body Cells: Promotes glucose uptake, enhances amino acid transport into cells, stimulates cellular division, and upregulates protein synthesis.
  - Bones and Skeletal Muscle: Promotes longitudinal bone growth at epiphyseal plates and increases muscle mass in children and adolescents.

Short-term metabolic versus long-term growth-promoting effects of growth hormone and insulin-like growth factor

Disorders of Growth Hormone Hypersecretion:
- Pituitary Gigantism: Results from a $GH$ -secreting pituitary adenoma before closure of the epiphyseal growth plates in children. Characterized by excessive longitudinal bone growth, tall stature (often exceeding $7\text{ feet}$ ), and general organomegaly (including cardiomegaly). Untreated individuals typically die early from congestive heart failure. Surgical resection of the adenoma can halt progression if diagnosed early.
- Acromegaly: Results from a $GH$ -secreting pituitary tumor after closure of epiphyseal growth plates in adults. Longitudinal bone elongation cannot occur; instead, bones and soft tissues broaden and increase in girth. Clinical features include enlargement of the hands, feet, cranium, facial bones, thickening of the tongue, coarsening of skin, hypertension, headaches, and eventual heart failure.

The Thyroid and Parathyroid Glands

Thyroid Gland Anatomy and Hormones:
- Location and Anatomy: Located in the anterior neck, superficial to the larynx. It is a butterfly-shaped organ composed of right and left lateral lobes connected across the trachea by a central tissue bridge termed the isthmus.
- Chemical Structure: Composed of an amino acid tyrosine core covalently coupled to iodine atoms:
  - Triiodothyronine ( $T_3$ ): Possesses three bound iodine atoms; represents the physiologically more active form.
  - Thyroxine ( $T_4$ ): Possesses four bound iodine atoms; secreted in greater quantities and converted to $T_3$ in target tissues.
- Major Physiological Functions:
  - Calorigenic Effect and Thermoregulation: Establishes basal metabolic rate ( $BMR$ ), the