WEEK 7 ENDOCRINE SYSTEM

Regulatory Systems

• Main Regulatory Centers:

  • Nervous System: Responsible for immediate responses to stimuli and maintaining rapid communication within the body.

  • Somatic Nervous System: Provides voluntary control over skeletal muscle contractions, enabling conscious movements and actions.

  • Autonomic Nervous System: Functions involuntarily to maintain homeostasis by regulating internal organ functions.

    • Sympathetic System: Activates "fight-or-flight" responses during stress or physical activity, increasing heart rate, respiration, and alertness.

    • Parasympathetic System: Activates "rest and digest" functions, promoting digestion, energy storage, and relaxation.

  • Endocrine System:

  • Function: Secretes hormones into the bloodstream to target tissues and regulate various physiological processes over longer durations.

  • Definition: Derived from Greek words endo (within) and krino (to secrete), highlighting its mechanism of internal secretion.

CNS vs Endocrine System

• Similarities:

  • Both use the brain and hypothalamus for regulation of bodily functions and coordination of responses.

  • Some molecules act as both neurotransmitters and hormones, indicating functional overlap and integrated signaling.

  • Both systems aid in regulating bodily functions, ensuring homeostasis and coordinated responses to internal and external stimuli.

• Differences:

  • Transport Mode:

  • Endocrine: Hormones are released into the bloodstream, allowing them to reach distant target cells throughout the body.

  • Nervous System: Neurotransmitters are released directly onto target cells at synapses, enabling rapid and localized communication.

  • Response Speed:

  • Endocrine: Slower responses due to the time required for hormones to travel through the bloodstream and bind to target receptors.

  • Nervous: Faster responses due to the direct and rapid transmission of electrical signals along neurons.

  • Response Duration:

  • Endocrine: Hormone effects last minutes to weeks, providing sustained regulation of physiological processes.

  • Nervous: Active as long as action potentials are sent, allowing for immediate but transient responses.

Functions of the Endocrine System

• Regulations include:

  • Metabolism: Regulates metabolic rate and energy balance.

  • Control of food intake and digestion: Influences appetite, digestion, and nutrient absorption.

  • Tissue development: Regulates growth, differentiation, and maintenance of tissues.

  • Ion levels and water balance: Maintains optimal ion concentrations and fluid balance in the body.

  • Heart rate and blood pressure adjustments: Modulates cardiovascular function to meet physiological demands.

  • Control of blood glucose and other nutrients: Regulates blood sugar levels and nutrient storage.

  • Regulation of reproductive functions (e.g., uterine contractions, milk release): Coordinates reproductive processes such as ovulation, pregnancy, and lactation.

  • Immune system response: Modulates immune cell activity and inflammatory responses.

Classes of Chemical Messengers

• Autocrine: Effects the same cell from which it was secreted, allowing for self-regulation.

• Paracrine: Affects neighboring cells within local tissues, enabling localized communication and coordination.

• Neurotransmitter: Synaptic transmission affecting adjacent cells in the nervous system, facilitating rapid and precise signaling.

• Endocrine: Hormones that travel through the bloodstream to distant target tissues, enabling systemic regulation of various physiological processes.

Hormone Secretion Patterns

• Chronic Secretion: Stable hormone levels maintained over time, typically seen with lipid-soluble hormones; essential for maintaining baseline physiological functions.

• Acute Secretion: Irregular and dramatic hormone concentration changes, characteristic of water-soluble hormones; allows for rapid responses to specific stimuli.

• Episodic Secretion: Regular interval hormone release, observed with some lipid-soluble hormones; contributes to cyclical physiological processes.

Hormonal Regulation Mechanisms

• Neural Activation: Neurons release neurotransmitters to initiate hormone release; secretion stops when the stimulus ends, ensuring precise control.

• Hormonal Activation: Hormones stimulate other endocrine organs to release hormones (tropic hormones), forming complex regulatory cascades.

• Humoral Control: Blood-borne molecules stimulate hormone release, sensitive to blood substance levels; allows for direct monitoring and adjustment of internal environment.

Control of Hormone Release

• Increased Demand: Hormone secretion stimulated by relevant stimuli to meet physiological needs.

• Inhibition Mechanisms: Endocrine glands receive messages to stop secretion, maintaining hormone homeostasis and preventing overproduction.

• Feedback Mechanisms:

  • Positive Feedback: Hormones enhance their own production, amplifying the initial response until a specific endpoint is reached.

  • Negative Feedback: Inhibition of hormone secretion when levels are adequate, maintaining stable hormone concentrations within a narrow range.

Receptors for Hormones

• Hormonal Binding: Hormones bind to specific protein receptors on target cells; only cells with matching receptors can be stimulated, ensuring specificity.

• Receptor Regulation:

  • Down-Regulation: Decreased number of receptors lowers hormone sensitivity, preventing overstimulation.

  • Up-Regulation: Increased receptor synthesis enhances sensitivity, boosting the response to low hormone levels.

Endocrine Glands and Functions

• Hypothalamus:

  • Major control site; integrates information from hormones and CNS to regulate various physiological functions.

  • Connected to the pituitary gland via the infundibulum, facilitating communication and hormonal control.

• Pituitary Gland:

  • Posterior Pituitary Gland (Neurohypophysis):

  • Secretes neurohormones synthesized in the hypothalamus; functions as an extension of the hypothalamus.

  • Anterior Pituitary Gland:

  • Secretes various hormones like GH, TSH, ACTH under hypothalamic control; regulates growth, metabolism, and stress responses.

Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH):

  • Responds to blood osmolality and volume changes, regulating water balance.

  • Target Tissue: Kidneys; increases water reabsorption, reducing urine output and conserving water.

  • Helps regulate blood pressure by promoting vasoconstriction; contributes to maintaining circulatory stability.

Anterior Pituitary Hormones

• Growth Hormone (GH):

  • Influences growth and metabolism; secreted in response to stress or low blood glucose; promotes tissue repair and energy mobilization.

• Thyroid-Stimulating Hormone (TSH):

  • Stimulates thyroid gland; controlled by TRH and inhibited by thyroid hormones; essential for maintaining thyroid hormone levels and metabolic rate.

• Adrenocorticotropic Hormone (ACTH):

  • Stimulates secretion from adrenal cortex; regulates stress responses and cortisol production.

Thyroid Gland

• Produces thyroid hormones (T3 and T4) and calcitonin; regulates metabolism, growth, and calcium homeostasis.

• Functions: Regulates metabolism, growth, and development of tissues; influences energy expenditure and protein synthesis.

• Regulation: Controlled by TRH and TSH through negative feedback; ensures stable thyroid hormone levels.

• Secretion Issues:

  • Hypersecretion: Increased metabolism, rapid heart rate, weight loss; leads to hyperthyroidism.

  • Hyposecretion: Decreased metabolism, weight gain, low body temperature; results in hypothyroidism.

Parathyroid Gland

• Hormone: Parathyroid Hormone (PTH)

  • Regulates calcium levels; stimulates bone resorption and renal absorption of calcium; essential for maintaining calcium homeostasis.

Adrenal Gland

• Medulla: Produces catecholamines (epinephrine, norepinephrine) for stress response; activates the sympathetic nervous system.

• Cortex: Produces mineralocorticoids (e.g., aldosterone), glucocorticoids (e.g., cortisol), and androgens; regulates electrolyte balance, stress response, and sex hormone production.

Pancreas

• Functions as both endocrine and exocrine gland; regulates blood glucose levels via insulin and glucagon; crucial for maintaining metabolic homeostasis.