A&P Endocrine System

Intercellular communication

Communication between cells that is vital for maintaining the functions of organs and overall body homeostasis. The human body contains about 100 trillion cells organized into four basic tissues, which form organs. There are six major forms of intercellular communication.

Direct communication

Communication mediated by gap junctions. Pores in the cell membrane allow small signaling chemicals to move directly from one cell to another.

Autocrine communication

Communication mediated by local hormones such as paracrine factors or cytokines that are secreted from a cell and act on that same cell as the target.

Paracrine communication

Communication mediated by local hormones such as paracrine factors or cytokines that are secreted into extracellular fluid and affect nearby target cells.

Juxtacrine communication

Communication mediated by local hormones that remain attached to the plasma membrane. The target cell binds or links to the membrane-bound factor. This type is involved in growth and differentiation.

Synaptic communication

Communication mediated by neurotransmitters released from a presynaptic cell, usually a neuron. The neurotransmitter travels across a gap and binds to a postsynaptic target cell such as another neuron or a muscle cell.

Endocrine communication

Communication mediated by hormones. Hormones are secreted into extracellular fluid, enter blood vessels, and travel through the circulation to affect distant target cells.

Receptor

A protein on a target cell that binds a chemical messenger or signal. Binding activates the receptor and produces intracellular events that change the activities of the target cell.

Signal transduction

The intracellular events and changes in cell activity that occur after a chemical messenger binds to its receptor.

Three components of the endocrine system

1) Endocrine glands or tissues that produce hormones; 2) hormones that act as chemical messengers; 3) target cells that contain receptors for the hormone.

Hormone

A chemical messenger secreted into the bloodstream that stimulates a response in another tissue or organ.

Target cell

A cell that contains receptors for a particular hormone and can therefore respond to that hormone.

Endocrine gland

A gland or tissue without ducts that releases hormones into tissue fluid. Hormones enter highly permeable capillary networks and are distributed through the blood.

Exocrine gland

A gland that uses ducts to carry secretions to a body surface or organ cavity. Its products are released onto epithelial tissue and produce extracellular effects such as food digestion.

Endocrine vs. exocrine glands

Endocrine glands have no ducts and release hormones into blood for effects on target cells. Exocrine glands use ducts to deliver secretions to surfaces or organ cavities, producing extracellular effects.

Nervous system communication

Electrical and chemical communication. It reacts very quickly, approximately 1–10 milliseconds, and its effects usually stop quickly. Responses are targeted and specific.

Endocrine system communication

Chemical communication only. Hormone release may take seconds to days, and effects can continue for weeks. Effects are generally widespread and may involve many organs.

Nervous vs. endocrine adaptation to long-term stimuli

The nervous system tends to adapt quickly, causing the response to decline. The endocrine system tends to maintain its response to long-term stimuli.

Chemicals that can act as both hormones and neurotransmitters

Norepinephrine (NE), cholecystokinin, thyrotropin-releasing hormone, dopamine, and ADH.

Hormones secreted by neuroendocrine cells

Oxytocin and catecholamines can be secreted by neuroendocrine cells, which are neurons that function in endocrine signaling.

Interaction between nervous and endocrine systems

Neurons can trigger hormone secretion, while hormones can stimulate or inhibit neurons. Both systems can also produce overlapping effects on the same target cells.

Example of overlapping nervous and endocrine effects

Norepinephrine and glucagon both cause glycogen hydrolysis in the liver.

Endocrine functions

The endocrine system controls reproduction, growth, development, activation of body defenses, salt and water balance, nutrient balance, and cellular metabolism.

Major endocrine organs

Hypothalamus, pituitary gland, pineal gland, thyroid gland, parathyroid glands, thymus, adrenal glands, pancreas, ovaries, and testes.

Pituitary gland divisions

Adenohypophysis (anterior pituitary) and neurohypophysis (posterior pituitary).

Adrenal gland divisions

Adrenal cortex and adrenal medulla.

Hypothalamus location

The hypothalamus is shaped like a flattened funnel and forms the floor and walls of the third ventricle.

Endocrine role of the hypothalamus

1) Releases hormones that control the anterior pituitary; 2) produces and controls the release of oxytocin and ADH from the posterior pituitary; 3) controls epinephrine and norepinephrine release from the adrenal medulla; 4) controls parasympathetic versus sympathetic tone.

Pituitary gland (hypophysis)

A gland suspended from the hypothalamus by a stalk called the infundibulum. It is housed in the sella turcica of the sphenoid bone and is approximately 1.3 cm in diameter.

Infundibulum

The stalk that connects the pituitary gland to the hypothalamus.

Sella turcica

The depression in the sphenoid bone that houses the pituitary gland.

Adenohypophysis

The anterior pituitary. It arises from the hypophyseal pouch, an outgrowth of the pharynx.

Neurohypophysis

The posterior pituitary. It develops as a downward growth of the hypothalamus and consists of neural tissue.

Hypothalamo-hypophyseal portal system

A vascular system in which hormones from the hypothalamus enter a capillary bed and travel through a portal system to the anterior pituitary. Hormones secreted by the anterior pituitary then enter a second capillary bed and join the general circulation.

Trophic hormone

A hormone that targets another endocrine gland. Examples include FSH, LH, TSH, and ACTH.

Anterior pituitary hormones

FSH, LH, TSH, ACTH, PRL, and GH.

FSH (follicle-stimulating hormone)

An anterior pituitary hormone secreted by gonadotrope cells. It stimulates production of egg or sperm cells.

LH (luteinizing hormone)

An anterior pituitary hormone secreted by gonadotrope cells that mainly stimulates hormone production. In females, it stimulates ovulation and causes the corpus luteum to secrete progesterone and estrogen. In males, it stimulates interstitial cells of the testes to secrete testosterone.

TSH (thyroid-stimulating hormone)

An anterior pituitary hormone secreted by thyrotropes. It stimulates growth of the thyroid gland and secretion of thyroid hormones.

ACTH (adrenocorticotropic hormone/corticotropin)

An anterior pituitary hormone secreted by corticotropes. It regulates the response to stress and stimulates the adrenal cortex.

PRL (prolactin)

An anterior pituitary hormone secreted by lactotropes. In females, it promotes milk synthesis after delivery. In males, it increases LH sensitivity and therefore increases testosterone secretion.

GH (growth hormone/somatotropin)

An anterior pituitary hormone secreted by somatotropes. It promotes tissue growth and affects protein, lipid, carbohydrate, and electrolyte metabolism.

Growth hormone effects on tissue growth

GH promotes mitosis and cellular differentiation and stimulates the liver to produce IGF-I and IGF-II (somatomedins).

Growth hormone effects on protein metabolism

GH increases DNA transcription and mRNA production, promotes protein synthesis, enhances amino acid transport into cells, and decreases protein catabolism.

Growth hormone effects on lipid metabolism

GH stimulates release of free fatty acids and glycerol from adipocytes, helping spare protein.

Growth hormone effects on carbohydrate metabolism

GH produces a glucose-sparing effect, meaning less glucose is used for energy.

Growth hormone effects on electrolytes

GH promotes retention of Na+, K+, and Cl− and promotes Ca2+ absorption.

GH during childhood and adolescence

GH promotes growth of bone, cartilage, and muscle and stimulates growth at the epiphyseal plates.

GH during adulthood

GH increases osteoblastic activity and appositional growth, affecting bone thickening and remodeling.

GH levels with aging

Blood concentration of GH decreases with age and is approximately one-fourth of adolescent levels by age 75.

Factors that increase GH levels

GH levels are higher during deep sleep, after high-protein meals, and after vigorous exercise.

Factor that lowers GH levels

GH levels are lower after high-carbohydrate meals.

Posterior pituitary

Also called the neurohypophysis. It stores and releases oxytocin and ADH, which are produced in the hypothalamus.

Posterior pituitary hormones

Oxytocin (OT) and antidiuretic hormone (ADH).

Production and transport of ADH and oxytocin

Both hormones are produced in the hypothalamus and transported through the hypothalamo-hypophyseal tract to the posterior pituitary, where they are stored and released.

ADH (antidiuretic hormone)

A posterior pituitary hormone produced in the hypothalamus. It targets the kidneys, increases water retention, and reduces urine production. It also functions as a neurotransmitter.

Oxytocin

A posterior pituitary hormone produced in the hypothalamus. It promotes labor contractions and lactation and may also have roles in sperm transport and emotional bonding.

Control of pituitary hormone secretion

Pituitary hormones are not secreted at a constant rate. GH is highest at night, while LH, FSH, and estrogen vary during the menstrual cycle. Timing is regulated by hormones from the hypothalamus and other brain centers connected to it.

Anterior pituitary control

The anterior pituitary is controlled by releasing hormones and inhibiting hormones released from the hypothalamus.

Posterior pituitary control

The posterior pituitary is controlled by neuroendocrine reflexes, in which nervous system signals cause hormone release.

Oxytocin milk-ejection reflex

Suckling stimulates nerve endings, which activate the hypothalamus. The hypothalamus signals the posterior pituitary to release oxytocin, causing milk ejection. Milk ejection leads to more suckling and further oxytocin release.

Baby’s cry and oxytocin

Higher brain centers can stimulate the hypothalamus in response to a baby’s cry, causing oxytocin release and milk ejection.

ADH release during dehydration

Dehydration increases blood osmolarity, activating osmoreceptors in the hypothalamus. The osmoreceptors trigger the posterior pituitary to release ADH, which promotes water conservation by the kidneys.

Pineal gland hormone production

The pineal gland produces serotonin during the day and converts it to melatonin at night.

Pineal gland secretion with age

Pineal gland secretion peaks at ages 1–5 and is approximately 75% lower by puberty.

Pineal gland and puberty

The pineal gland may regulate the timing of puberty. Removal of the gland causes premature sexual maturation.

Melatonin and seasonal affective disorder (SAD)

Melatonin levels increase in seasonal affective disorder and decrease with phototherapy. SAD is associated with depression, sleepiness, irritability, and carbohydrate craving.

Thymus location

The thymus is located in the mediastinum superior to the heart.

Thymus after puberty

The thymus undergoes involution after puberty.

Thymus hormones

The thymus secretes thymopoietin, thymulin, and thymosins, which regulate development and later activation of T-lymphocytes.

Thyroid gland

The largest endocrine gland. It has a high rate of blood flow and is located on the anterior and lateral sides of the trachea. It has two large lobes connected by an isthmus.

Embryonic origin of thyroid gland

The thyroid gland arises from the root of the embryonic tongue.

Thyroid follicles

Structures filled with protein colloid called thyroglobulin and lined with simple cuboidal epithelial follicular cells.

Follicular cells of thyroid

Cells that secrete the thyroid hormones T3 and T4.

T3 and T4

The two thyroid hormones secreted by thyroid follicular cells. They increase metabolic rate and oxygen consumption and have widespread effects on metabolism, cardiovascular activity, respiration, nervous system activity, appetite, tissue growth, and heat production.

Thyroid hormone effects on metabolism

Thyroid hormone increases the body’s metabolic rate and oxygen consumption and produces a calorigenic effect, meaning increased heat production.

Thyroid hormone effects on cardiovascular system

Thyroid hormones increase heart rate and contraction strength.

Thyroid hormone effects on respiration

Thyroid hormones increase respiratory rate.

Thyroid hormone effects on nervous system

Thyroid hormones increase activity of the nervous system.

Thyroid hormone effects on nutrients

Thyroid hormones stimulate appetite and breakdown of carbohydrates, lipids, and proteins.

Thyroid hormone effects on growth

Thyroid hormones stimulate growth of bone, skin, hair, nails, teeth, and the nervous system and trigger release of growth hormone.

C cells/parafollicular cells

Thyroid cells that produce calcitonin.

Calcitonin

A hormone produced by thyroid C cells that decreases blood Ca2+ levels and promotes Ca2+ deposition and bone formation, especially in children.

Parathyroid hormone (PTH)

A hormone released by the parathyroid glands that increases blood Ca2+ levels.

PTH actions

PTH promotes calcitriol synthesis by the kidneys, increases intestinal absorption of Ca2+, decreases urinary Ca2+ excretion, and increases bone resorption.

Adrenal gland

The adrenal gland consists of two endocrine regions: the adrenal cortex, which is epithelial in origin, and the adrenal medulla, which is neural in origin.

Adrenal medulla

The inner portion of the adrenal gland. It is essentially a sympathetic ganglion innervated by sympathetic preganglionic fibers.

Chromaffin cells

Modified neurons of the adrenal medulla that release catecholamines when stimulated.

Adrenal medulla catecholamines

Epinephrine, norepinephrine, and a small amount of dopamine.

Effects of adrenal medulla catecholamines

Catecholamines increase alertness, anxiety or fear, blood pressure, heart rate, airflow, and metabolic rate. They inhibit insulin secretion and digestion and stimulate gluconeogenesis and glycogenolysis.

Adrenal medulla and stress

Stress causes adrenal medullary cells to stimulate the adrenal cortex.

Adrenal cortex

The outer portion of the adrenal gland. It is epithelial in origin and contains three layers: zona glomerulosa, zona fasciculata, and zona reticularis.

Adrenal cortex layers

From outer to inner: zona glomerulosa, zona fasciculata, and zona reticularis.

Zona glomerulosa

The outer layer of the adrenal cortex. It produces mineralocorticoids such as aldosterone.

Zona fasciculata

The middle layer of the adrenal cortex. It produces glucocorticoids, especially cortisol.

Zona reticularis

The inner layer of the adrenal cortex. It produces sex steroids such as androgens and estrogen.

Mineralocorticoids

Corticosteroids produced by the zona glomerulosa that control electrolyte balance. Aldosterone promotes Na+ retention and K+ excretion.

Aldosterone

A mineralocorticoid that promotes sodium retention and potassium excretion, helping regulate electrolyte balance.

Glucocorticoids

Corticosteroids produced by the zona fasciculata. Cortisol is the major example and helps the body deal with stress.

Cortisol

A glucocorticoid that stimulates fat and protein catabolism, gluconeogenesis, and release of fatty acids and glucose into the blood. It helps the body deal with stress and heal.