Endocrine System

Overview

The endocrine system is the body's second major controlling system, influencing metabolic activities of cells through the release of hormones. Unlike the nervous system, endocrine responses are typically slower but longer-lasting, and the hormone-producing cells are distributed throughout the body in various glands and tissues.

• Endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands.

• The pancreas and gonads (testes and ovaries) also produce hormones.

• The hypothalamus has both neural and endocrine functions, releasing hormones that regulate other glands.

• Other hormone-producing tissues include the stomach, kidneys, heart, and adipose tissue.

Hormones

Hormones are powerful biological chemicals produced in minute quantities and released into the bloodstream, where they travel to target cells throughout the body. They regulate cellular activity and coordinate complex bodily functions.

• Hormones can have different effects depending on the target tissue and the level or quantity present.

• At times, a hormone may have opposite or entirely different functions in different tissues.

• Hormones are measured in nanograms (ng, parts per billion) and picograms (pg, parts per trillion).

• Hormones regulate the metabolic function of cells and can act within seconds or over days/weeks.

• Hormones tend to have prolonged effects on muscles and organ system function.

Types of Hormones

Hormones are classified based on their chemical structure:

• Amino acid-based hormones: Most hormones, including thyroxine, peptide, and protein hormones.

• Steroid hormones: Synthesized from cholesterol; include gonadal and adrenocortical hormones.

• Eicosanoids: Biologically active lipids (e.g., leukotrienes, prostaglandins, thromboxanes) that act like hormones but are not true hormones. Additional info: Eicosanoids are involved in inflammation and immune responses.

Hormone Action

Hormones alter cell activity by one of two main mechanisms:

• Second messengers (amino acid-based hormones):

  • Hormone binds to receptor and activates a G protein.

  • G protein activates adenylate cyclase, which generates cAMP from ATP.

  • cAMP activates protein kinases, leading to cellular responses.

  • Phospholipase C pathway: PIP2 is split into DAG and IP3; DAG activates protein kinases, IP3 triggers Ca2+ release.

• Direct gene activation (steroid hormones):

  • Steroid hormones diffuse into target cells and bind to intracellular receptors.

  • The hormone-receptor complex binds to DNA, prompting transcription and mRNA production.

  • mRNA is translated into proteins, which mediate the hormone's effects.

Hormones can:

• Alter plasma membrane permeability

• Stimulate protein synthesis

• Activate or deactivate enzyme systems

• Induce secretory activity

• Stimulate mitosis

Hormone-Target Cell Specificity

Hormones circulate throughout the body but only affect specific target cells that have appropriate receptors. These receptors may be located on the plasma membrane or inside the cell.

• Examples of hormone activity include regulation of blood glucose and calcium levels.

Target Cell Activation

Activation of target cells depends on three main factors:

• Blood levels of the hormone

• Relative number of receptors on the target cell

• Affinity of the receptors for the hormone

Cells can increase (up-regulation) or decrease (down-regulation) their sensitivity to hormones by altering receptor numbers.

• Permissiveness: One hormone requires another to exert its full effect (e.g., thyroid hormone for reproductive development).

• Synergism: Multiple hormones produce the same effect, amplifying the response.

• Antagonism: One hormone opposes the action of another (e.g., insulin vs. glucagon).

Hormone Concentrations in the Blood

The concentration of circulating hormones is determined by:

• Rate of release

• Speed of inactivation and removal from the body

• Removal by degrading enzymes, kidneys, or liver

Control of Hormone Synthesis and Release

Hormone synthesis and release are regulated by:

• Humoral stimuli: Changes in blood levels of ions or nutrients (e.g., calcium levels regulate parathyroid hormone release).

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic stimulation of adrenal medulla).

• Hormonal stimuli: Hormones stimulate other endocrine glands to release hormones (e.g., hypothalamic hormones stimulate pituitary).

Nervous System Modulation

The nervous system can override or modulate endocrine controls, especially in response to stress or changing needs (e.g., blood glucose regulation).

• The hypothalamus and sympathetic nervous system can activate the endocrine system to supply additional glucose.

Major Endocrine Organs

Pituitary (Hypophysis)

The pituitary gland is a two-lobed organ that secretes many major hormones. It consists of:

• Posterior lobe (neurohypophysis): Stores and releases neurohormones (ADH, oxytocin, vasopressin) produced by the hypothalamus.

• Anterior lobe (adenohypophysis): Synthesizes and secretes a variety of hormones.

Pituitary-Hypothalamic Relationships

• Posterior lobe: Downgrowth of hypothalamic neural tissue; has a neural connection with the hypothalamus.

• Anterior lobe: Upgrowth of oral mucosa; no direct neural contact with the hypothalamus, but connected via the hypophyseal portal system (primary capillary plexus and hypophyseal portal veins).

Adenohypophyseal Hormones

The six main hormones of the anterior pituitary are:

• Growth Hormone (GH)

• Thyroid-Stimulating Hormone (TSH)

• Adrenocorticotropic Hormone (ACTH)

• Follicle-Stimulating Hormone (FSH)

• Luteinizing Hormone (LH)

• Prolactin (PRL)

These hormones regulate the activity of other endocrine glands and tissues.

Key Terms and Examples

• cAMP (cyclic adenosine monophosphate): A second messenger generated from ATP by adenylate cyclase, mediating hormone effects.

• Permissiveness: Thyroid hormone is necessary for reproductive hormones to exert their effects.

• Synergism: Glucagon and epinephrine both stimulate the liver to release glucose; together, their effect is amplified.

• Antagonism: Insulin lowers blood glucose, while glucagon raises it.

Important Equations

• cAMP generation:

• Hormone concentration:

Table: Types of Hormones

Additional info:

• Hormone regulation is essential for homeostasis, growth, metabolism, and reproduction.

• Disorders of the endocrine system can lead to conditions such as diabetes, hypothyroidism, and Cushing's syndrome.