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Endocrine System: Hormones, Pathways, and Regulation

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Endocrine System

Overview of the Endocrine System

The endocrine system is a communication network that uses chemical signals, known as hormones, to target specific cells or tissues that have the appropriate receptors. This system plays a crucial role in maintaining homeostasis through feedback mechanisms.

  • Hormone Types:

    • Lipophilic hormones: Bind to cell surface or intracellular receptors.

    • Lipophobic hormones: Bind to cell surface receptors only.

  • Control Mechanisms:

    • Hormones regulated by tonic control.

    • Homeostasis maintained through negative feedback loops.

  • Hormone Activity:

    • Synthesis and secretion rates.

    • Hormone storage.

    • Receptor availability on target cells.

Hormone Classification

Peptide Hormones

Peptide hormones are synthesized in advance and stored in secretory vesicles. They are released by exocytosis and transported dissolved in plasma. Their receptors are located on the cell membrane, and they act via second messenger systems. Peptide hormones generally have a short half-life.

  • Synthesis and Storage: Synthesized in advance, stored in vesicles.

  • Release/Secretion: Via exocytosis.

  • Transport in Blood: Dissolved in plasma.

  • Receptor Location: Cell membrane.

  • Cellular Mechanism of Action: Activate second messenger systems.

  • Half-life: Short.

Steroid Hormones

Steroid hormones are synthesized on demand from precursors and are not stored. They are released by simple diffusion and transported in the blood bound to carrier proteins. Their receptors are located in the cytoplasm or nucleus, and they activate genes for transcription and translation. Steroid hormones have a long half-life.

  • Synthesis and Storage: Synthesized on demand, not stored.

  • Release/Secretion: Released by simple diffusion.

  • Transport in Blood: Bound to carrier proteins.

  • Receptor Location: Cytoplasm or nucleus; few have membrane receptors.

  • Cellular Mechanism of Action: Activate genes for transcription and translation.

  • Half-life: Long.

Amino Acid-Derived Hormones

Amino acid-derived hormones include catecholamines and thyroid hormones. Catecholamines are made in advance and stored, while thyroid hormones have precursors stored. Catecholamines are released by exocytosis and dissolve in plasma, whereas thyroid hormones are transported bound to carrier proteins. Catecholamine receptors are on the cell membrane, while thyroid hormone receptors are in the nucleus. Catecholamines activate second messenger systems, and thyroid hormones activate genes. Catecholamines have a short half-life; thyroid hormones have a long half-life.

  • Synthesis and Storage:

    • Catecholamines: Made in advance and stored.

    • Thyroid hormones: Precursors stored.

  • Release/Secretion:

    • Catecholamines: Exocytosis.

    • Thyroid hormones: Transport proteins.

  • Transport in Blood:

    • Catecholamines: Dissolve in plasma.

    • Thyroid hormones: Bind to carrier proteins.

  • Receptor Location:

    • Catecholamines: Cell membrane.

    • Thyroid hormones: In the nucleus.

  • Cellular Mechanism of Action:

    • Catecholamines: Activate second messenger systems.

    • Thyroid hormones: Activate genes.

  • Half-life:

    • Catecholamines: Short.

    • Thyroid hormones: Long.

Hormone Pathways and Effects

Stimulus and Pathways

Hormone pathways begin with a stimulus, which is detected by a sensor or sensory receptor. The signal is integrated and sent to an effector, which produces a response to restore homeostasis.

  • Stimulus: Change or disturbance in the environment (e.g., temperature, blood pressure).

  • Afferent Pathway (Input Signal): Signal detected by sensor, sent to integrating center.

  • Integrator (Integrating Center): Compares input signal to setpoint, initiates output signal.

  • Effector Pathway: Output signal travels to effector.

  • Effector (Target): Cell or tissue responds to output signal, restoring variable to normal levels.

Hormone Effects on Effectors

  • Altering Enzymatic Reactions: Hormones can increase or decrease the rate of specific reactions.

  • Transport: Modify transport of substances across membranes.

  • Gene Expression and Protein Synthesis: Influence gene expression, leading to changes in protein synthesis.

  • Receptor Binding: Varied tissue responses; a single hormone can have different effects on different tissues.

Hormone Interactions

Types of Hormone Interactions

  • Synergism: Combined effect of two or more hormones is greater than individual effects.

  • Permissiveness: One hormone enables another to exert its full effect.

  • Antagonism: Hormones have opposing effects; one hormone inhibits or counteracts the effect of another.

Predicting Target Cell Response

  • Synergistic Hormones: Amplify effects.

  • Permissive Hormones: Enable full effects.

  • Antagonistic Hormones: Counteract each other.

Hormone Metabolism and Regulation

Importance of Hormone Metabolism

Hormones must be metabolized or broken down to ensure their signal activity is limited. This is crucial for the body to adapt to changes in its internal environment.

  • If insulin remains active for too long, it can cause blood glucose to drop excessively.

  • Hormones are usually degraded into inactive metabolites by enzymes in the liver and kidneys.

  • If hormones are maintained chronically, target cells may downregulate their receptors, leading to reduced sensitivity (e.g., high insulin levels).

Hormonal Axes

Major Hormonal Axes

  • HPA Axis (Hypothalamic-Pituitary-Adrenal): CRH → ACTH → Cortisol

  • HPG Axis (Hypothalamic-Pituitary-Gonadal): GnRH → FSH/LH → Androgens, Estrogens, Progesterone

  • HPT Axis (Hypothalamic-Pituitary-Thyroid): TRH → TSH → T4

Hormones of the Anterior and Posterior Pituitary

Anterior Pituitary Hormones

  1. Prolactin (PRL):

    • Signal for release: Dopamine inhibits its release.

    • Primary target: Mammary glands.

    • Receptor location: Breast tissue.

  2. Thyroid Stimulating Hormone (TSH):

    • Signal for release: Thyrotropin-releasing hormone (TRH).

    • Primary target: Thyroid gland.

    • Receptor location: Thyroid cells.

  3. Adrenocorticotropic Hormone (ACTH):

    • Signal for release: Corticotropin-releasing hormone (CRH).

    • Primary target: Adrenal cortex.

    • Receptor location: Adrenal cortex cells.

  4. Growth Hormone (GH):

    • Signal for release: Growth-hormone-releasing hormone (GHRH).

    • Primary target: Liver and skeletal muscle.

    • Receptor location: Liver and various tissues.

  5. Gonadotropin-Releasing Hormones (GnRH):

    • Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH).

    • Primary target: Ovaries and testes.

Posterior Pituitary Hormones

  • Oxytocin:

    • Primary target: Mammary glands.

    • Receptor location: Mammary and uterine tissues.

  • Vasopressin (ADH):

    • Primary target: Kidneys.

    • Receptor location: Kidney cells.

Reflex Pathways: Neural, Endocrine, and Neuroendocrine

Simple Neural Reflex

  • Sensor: Sensory neuron.

  • Communication: Direct neurotransmitter release onto target cells.

  • Integrating Center: CNS.

  • Speed/Duration: Fast and short-lived.

  • Example: Knee-jerk reflex.

Simple Endocrine Reflex

  • Sensor: Endocrine cell.

  • Communication: Hormones released into the blood.

  • Integrating Center: Endocrine gland.

  • Speed/Duration: Slower, long-lasting.

  • Example: Insulin release due to high blood glucose.

Complex Neuroendocrine Reflex

  • Sensor: Sensory neuron.

  • Communication: Neurohormones and hormones in the blood.

  • Integrating Center: Endocrine gland, CNS.

  • Speed/Duration: Variable, depending on pathway.

  • Example: Insulin secretion influenced by brain signals.

Negative Feedback in Hormone Regulation

  • Role: Maintains homeostasis by reducing hormone secretion when levels are high.

  • Short-loop feedback: Hormone acts on the pituitary or hypothalamus.

  • Long-loop feedback: Hormone acts on both the pituitary and the hypothalamus.

Primary, Secondary, and Tertiary Pathologies

Pathology Classification

Endocrine pathologies are classified based on the location of dysfunction in the hormonal axis:

Pathology Type

Location of Dysfunction

Example

Hormone Levels

Primary

Endocrine gland (e.g., adrenal cortex)

Adrenal insufficiency

Low cortisol, high ACTH, high CRH

Secondary

Pituitary gland

Pituitary adenoma

Low ACTH, low cortisol, high CRH

Tertiary

Hypothalamus

Hypothalamic dysfunction

Low CRH, low ACTH, low cortisol

Key Equations

  • Hormone Concentration:

Summary Table: Hormone Types

Hormone Type

Synthesis & Storage

Release/Secretion

Transport in Blood

Receptor Location

Mechanism of Action

Half-life

Peptide

Advance, stored

Exocytosis

Dissolved in plasma

Cell membrane

Second messenger

Short

Steroid

On demand, not stored

Diffusion

Carrier proteins

Cytoplasm/nucleus

Gene activation

Long

Catecholamine

Advance, stored

Exocytosis

Dissolved in plasma

Cell membrane

Second messenger

Short

Thyroid

Precursors stored

Transport proteins

Carrier proteins

Nucleus

Gene activation

Long

Additional info: Academic context and definitions have been expanded for clarity and completeness.

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