Endocrine System Introduction

• The endocrine system works with the nervous system to oversee bodily functions.

Endocrine System Introduction

• Endocrine glands secrete hormones directly into the bloodstream.
• Endocrine signaling is slower and less targeted than cell-to-cell signaling in the nervous system.
• Hormones affect receptor cells within seconds to minutes.
• Hormone effects can persist from minutes to days.
• Hormones are distributed broadly in the blood, influencing any cells with matching receptors.

Exocrine and Endocrine Glands

• Glands produce and secrete material.
• There are two primary types: exocrine and endocrine.
• Exocrine glands secrete substances onto the body's surface through ducts, like salivary, sweat, and mammary glands.
• Endocrine glands release substances into the surrounding extracellular fluid or directly into the bloodstream.
• Materials secreted by endocrine glands diffuse quickly into the circulatory system.
• Endocrine glands produce hormones that regulate other cells.
• Endocrine glands include the hypothalamus, pituitary gland, pineal gland, thyroid, thymus, adrenal glands, pancreas, and gonads.

Endocrine Signaling

• Endocrine cells are categorized as nonneural and neurosecretory.
• Nonneural endocrine cells synthesize and secrete hormones when stimulated by other hormones or neural input.
• Nonneural endocrine cells do not generate action potentials.
• Neurosecretory endocrine cells are neuronal cells in the CNS that synthesize and secrete hormones.
• Neurosecretory cells generate action potentials but release hormones instead of neurotransmitters.
• Neurosecretory endocrine cells provide a direct link between the nervous and endocrine systems.

Hormones

• Hormones are very effective at low concentrations.
• Hormone levels are determined by production and breakdown rates.
• Hormones can be produced at varying rates.
• Hormones are enzymatically degraded in minutes or weeks.
• Hormones bind noncovalently to receptor proteins on target cells.
• Hormones interact with receptors specific to target cells.
• Target cells can modify receptor protein quantity to adjust hormone sensitivity.
• Cells may produce several types of receptors for a single hormone.
• The same hormone can have different effects on different cells based on the receptors present.
• Hormones exhibit paracrine effects, influencing nearby cells, and autocrine effects, influencing the original secreting cell.
• Most hormones are steroids, peptides, or amines.

Steroid Hormones

• Steroid hormones are synthesized from cholesterol.
• Steroid hormones are lipid-soluble, readily diffusing through cell membranes, and may require transporters.
• Examples are hormones from the adrenal glands (corticosteroids), gonads (sex steroids) in vertebrates and hormones that induce molt in insects.
• Carrier proteins bind noncovalently to lipid-soluble hormones, ensuring their solubility in plasma.
• Hormones bound to carrier proteins are less prone to degradation.
• Carrier proteins can also bind to water-soluble proteins in the plasma.

Steroid Hormone Production

• Steroid hormones are synthesized from cholesterol.
• Cholesterol can be obtained from diet or produced by the liver and some endocrine cells.
• In cells that synthesize steroid hormones, enzymes cleave cholesterol's six-carbon side chain to form pregnenolone.
• Pregnenolone serves as a precursor for various steroid hormones.
• Cells produce the necessary enzymes to convert pregnenolone to the appropriate steroid hormone.

Peptide Hormones

• Peptide hormones are chains of amino acids.
• Peptide hormones range from three to almost 200 amino acids in length.
• Peptide hormone examples include antidiuretic hormones, insulin, and growth hormone in vertebrates, and diuretic hormones for insects.

Peptide Hormone Production

• Peptide hormones are produced (like other strings of amino acids) by ribosomes and packaged into secretory vesicles by the Golgi apparatus
• Preprohormones are created from mRNA that codes for them, then are modified through translation into prohormone
• Then it is modified again and finally becomes a mature hormone
• Preprohormones can be converted into hormones by one cell or by a series of cells.

Peptide Hormone Production (Insulin Example)

• Insulin (which stimulates cells to take up glucose) is produced and secreted by beta cells in the islets of Langerhans in the pancreas.
• Preproinsulin is synthesized in the lumen of the ER.
• The "p" segment of preproinsulin is removed to produce proinsulin, which is then moved to the Golgi apparatus.
• The proinsulin is folded in the Golgi apparatus, and vesicles containing it bud off the Golgi.
• Enzymes in the vesicle cleave the proinsulin to produce insulin.
• Pancreatic beta cell action potentials trigger the exocytosis of secretory vesicles containing insulin.

Amine Hormones

• Amine hormones are derived from individual amino acids.
• Examples include melatonin (secreted by the pineal gland and promotes sleep).
• Catecholamines are derived from tyrosine and can serve as both neurotransmitters and hormones.
• Dopamine, epinephrine, and norepinephrine are all catecholamines.
• Iodothyronines are hormones secreted by the thyroid gland, such as thyroxine (T4) and triiodothyronine (T3).

Hormone Receptors

• Hormone receptors are necessary for a cell to respond to hormones, and come in three primary types:
• G protein-coupled membrane receptors bind to water-soluble hormones, use G proteins for intracellular signaling pathways.
• Enzyme-linked membrane receptors bind to water-soluble hormones, their interaction changes their catalytic ability.
• Intracellular receptors typically bind to lipid-soluble hormones (steroids and iodothyronines)
• Hormone-receptor complexes act as transcription factors to mediate gene expression.

Production of Hormones in the Brain: Hypothalamus

• The nervous system directly controls the release of hormones.
• The hypothalamus links the nervous and endocrine systems through pituitary control.
• Hormones secreted by the posterior pituitary are produced in neurosecretory cells in the hypothalamus.
• The axons of these cells extend into and terminate in the posterior pituitary.
• These cells are excited by other neurons with axon terminals in the hypothalamus.
• The hypothalamus controls the secretion of hormones from the anterior pituitary.

Production of Hormones in the Brain: PItuitary

• The pituitary is divided into two parts:
• Posterior pituitary (neurohypophysis; connected to the hypothalamus), consists of bundles and terminations of axons of neurosecretory cells that originate in the hypothalamus, as well as a network of capillaries.
• Axons run through the median eminence and the infundibular stalk.
• Oxytocin and vasopressin are released from the pars nervosa.
• Vasopressin limits urine production
• Oxytocin is important for social behaviors and induces labor/stimulates the secretion of breastmilk.
• These hormones are released because of an action potential.
• Anterior pituitary (adenohypophysis) is nonneural endocrine tissue that produces and secretes peptide hormones, consisting of pars distalis, pars intermedia, and pars tuberalis
• All hormones secreted from the anterior pituitary are produced there
• Hormone production and secretion in the anterior pituitary are controlled by hormones released by neurosecretory cells in the hypothalamus.
• Direct-acting hormones exert their principal effects on nonendocrine tissues.
• Growth hormone influences growth and metabolism in muscle and fat cells.
• Prolactin stimulates the production of milk and helps regulate reproduction and water balance.
• Tropic hormones control other endocrine tissues.
• Thyroid-stimulating hormone stimulates the thyroid to secrete thyroid hormones.