honors endocrine signaling & regulation unit 1

Primary function of the endocrine system

Coordinates and integrates body activities by releasing hormones into the bloodstream.

Comparison between endocrine and nervous system responses

Endocrine responses are generally slower to initiate but longer-lasting in duration.

Endocrinology

The scientific study of hormones and the endocrine organs.

Major physiological processes governed by the endocrine system

Reproduction, growth and development, electrolyte/nutrient balance in blood, cellular metabolism, energy balance, and mobilization of body defenses.

Exocrine glands

Glands that produce nonhormonal substances (such as sweat and saliva) and route them through ducts to a membrane surface.

Endocrine glands

Ductless glands that release hormones directly into surrounding tissue fluid and blood.

The five major primary endocrine glands

Pituitary, thyroid, parathyroid, adrenal, and pineal glands.

Endocrine classification of the hypothalamus

A neuroendocrine organ that bridges neural signaling and hormonal control.

Primary organ exhibiting both prominent exocrine and endocrine functions

The pancreas.

Two major chemical classifications of hormones

Amino acid-based hormones and steroids.

Lipid precursor from which steroid hormones are synthesized

Cholesterol.

Primary anatomical sources of steroid hormones

The gonads and the adrenal cortex.

Target cell

A cell or tissue expressing specific receptors capable of binding and responding to a given hormone.

Factor determining whether a body cell responds to a circulating hormone

The presence of specific complementary receptors for that hormone on or within the cell.

Major cellular changes triggered by hormone receptor activation

Altering membrane permeability or electrical state, activating or deactivating enzymes, inducing protein synthesis, stimulating secretory activity, and initiating mitosis.

Water-soluble hormones

All amino acid-based hormones except thyroid hormone; unable to cross the lipid bilayer, they bind plasma membrane receptors and act via second-messenger systems.

Lipid-soluble hormones

Steroid hormones and thyroid hormone; capable of diffusing directly through plasma membranes to bind intracellular receptors.

Direct mechanism of lipid-soluble hormones inside a target cell

The hormone-receptor complex binds specific DNA regions to initiate gene transcription and direct protein synthesis.

First messenger in second-messenger systems

The hormone itself, which binds to a cell-surface receptor.

Second messenger produced by adenylate cyclase

Cyclic AMP (cAMP), synthesized from ATP.

Membrane enzyme activated by a G protein in the cAMP signaling pathway

Adenylate cyclase.

Intracellular targets activated directly by cAMP

Protein kinases, which phosphorylate other proteins to alter their functional activity.

Signaling cascade amplification

A phenomenon where a single hormone-receptor binding event activates enzymes that produce hundreds of secondary messengers, creating an exponentially magnified response.

Primary homeostatic feedback mechanism regulating circulating hormone levels

Negative feedback systems, where increased target organ effects inhibit further secretion of the initial hormone.

Humoral stimulus

Hormone release triggered directly by changing blood levels of specific ions or nutrients (e.g., declining blood $Ca^{2+}$ triggering parathyroid hormone release).

Neural stimulus

Hormone release prompted directly by nerve fiber stimulation (e.g., sympathetic nervous stimulation triggering catecholamine release from the adrenal medulla).

Hormonal stimulus

Hormone release provoked by the arrival of other tropic hormones (e.g., hypothalamic releasing hormones prompting anterior pituitary hormone release).

Hypothalamic-pituitary-target organ pathway sequence

Hypothalamus $\rightarrow$ anterior pituitary $\rightarrow$ target endocrine organ $\rightarrow$ peripheral hormone secretion.

Nervous system modulation

The capacity of neural signaling to override normal endocrine set points and controls during exceptional circumstances (such as severe physical stress).

Three primary factors determining the degree of target-cell activation

Hormone blood concentration, relative receptor density on/in the cell, and the receptor affinity for the hormone.

Receptor affinity

The binding strength and attraction between a hormone and its target receptor.

Up-regulation

The process by which target cells increase their receptor number in response to persistently low hormone levels.

Down-regulation

The process by which target cells decrease their receptor number in response to persistently elevated hormone levels, desensitizing the cell.

Circulating state of lipid-soluble hormones in the bloodstream

Bound to plasma transport proteins.

Circulating state of most water-soluble hormones in the bloodstream

Free and unbound in blood plasma.

Organs primarily responsible for clearing and degrading hormones from blood

The liver and the kidneys.

Hormone half-life

The time required for a hormone's concentration in the bloodstream to decrease by 50%.

Permissiveness (hormone interaction)

A relationship where one hormone cannot exert its complete effects unless a second hormone is present (e.g., reproductive hormones requiring thyroid hormone).

Synergism (hormone interaction)

A situation where multiple hormones produce similar effects and their combined action produces an amplified outcome (e.g., glucagon and epinephrine acting together on liver glycogenolysis).

Antagonism (hormone interaction)

A relationship where one hormone counteracts or opposes the physiological effects of another hormone (e.g., insulin lowering blood glucose while glucagon raises it).