Module 2 - Endocrine System: Hormones, Targets, and Regulation

Types of signaling used by the endocrine system

-Endocrine signaling -Paracrine signaling -Autocrine signaling -Synaptic signaling -Neuroendocrine signaling -Pheromone signaling

Symptoms of Grave's Disease

-Weight loss, accompanied by an increase in appetite. -Cardiovascular system is taxed as the excess thyroid hormones cause increases in metabolic activity, including rapid heartbeat and palpitations -Muscle atrophy can occur as muscles are broken down and used to provide energy for the unusually high metabolic processes of other body systems -Thyroid gland can become enlarged, making it difficult to swallow -Exophthalmia as a result of connective tissue components being deposited behind the eyes as the thyroid gland enlarges

Regulation of thyroid hormone (T3 and T4) secretion from the anterior pituitary

1) Thyroid hormone levels drop 2) Neurosecretory cells secrete TRH into the blood, which carries it to the anterior pituitary 3) TRH causes the anterior pituitary to secrete TSH (also known as thyrotropin) 4) TSH stimulates endocrine cells in the thyroid gland to secrete thyroid hormone 5) Thyroid hormone levels increase in the blood and body tissues 6) Thyroid hormone blocks TRH release and TSH release preventing overproduction of thyroid hormone

What are the two ways the endocrine system exerts its control?

1) Various types of signaling 2) Chemical ligands

Amine

A class of hormones that may be either water-soluble or lipid-soluble and share characteristics of both steroid and peptide hormones. All have a similar structure and are derivatives of the amino acid tyrosine.

Endocrine signaling

A form of long-distance signaling in which ligands are released and travel through the blood to reach a target cell, which could be anywhere within the body. Uses: Regulatory processes, long-term regulatory mechanisms, and responding to environmental threats and the onset of labor and delivery

Neurotransmitters

A group of complex ligands that can embody a variety of structures. Can be peptides, amino acid derivatives, or amines.

Pituitary gland

A pea-sized structure located below the hypothalamus that protrudes from a long stalk called the infundibulum. Consists of the posterior pituitary and the anterior pituitary.

Melatonin

A peptide that promotes circadian rhythms such as the sleep-wake cycle

Negative feedback loop

A regulatory mechanism that results in a decrease in function. 1. Stimulus sets off a signaling cascade in an endocrine cell 2. Endocrine cell releases hormones into the bloodstream 3. Hormone circulates throughout the body until it reaches its target cell 4. Hormone acting on the target cell produces a response that causes a decrease in the initial stimuli which started the entire process

Synaptic cleft

A small gap that serves as an area of communication between the pre-synaptic cell and the post-synaptic cell

Paracrine signaling

A type of local signaling in which secreted ligands diffuse to target cells that are close to the secreting cells. The ligand is not transported through the blood but rather relies solely on diffusion into local tissue spaces.

Autocrine signaling

A type of local signaling in which the cell releasing the ligand is also the target cell of the very same ligand. Ligand is not transported through the blood but instead diffuses across a short distance.

Synaptic signaling

A type of local signaling in which the ligand diffuses across a synapse. Neurotransmitters diffuse a very short distance across the synapse to reach the target cell.

Neuroendocrine signaling

A type of long-distance signaling in which ligands are transported through the blood to any part of the body. The ligands, however, are specifically manufactured by and stored within the neurons of the nervous system.

Adrenal glands

Affixed directly on top of the kidneys. Have two main types of endocrine tissues, namely, the adrenal medulla and the adrenal cortex.

Grave's Disease

An autoimmune disorder that results from an over-secretion of thyroid hormones.

Hyperthyroidism

Associated with an enlarged thyroid gland. Results in high levels of thyroid hormone.

Exophthalmia

Bulging of the eyes

Thyroid gland

Butterfly-shaped organ located in the neck that produces thyroxine (T4), triiodothyronine (T3), and calcitonin.

Hypothyroidism

Characterized by a decrease in the size of the thyroid gland. Results in low levels of thyroid hormone

Polypeptide hormones

Composed of short chains of amino acids. Soluble in water, will bind to external GPCRs of the target cell. Examples are insulin, adrenocorticotropic, and luteinizing hormone.

Endocrine system

Consists of glands, organs, and tissues that are responsible for the regulation and coordination of all other body systems. Characterized by slow responses and long-lasting effects spanning across multiple organs and tissues.

Classes of local regulators

Cytokines, prostaglandins, and nitric oxide

excitatory neurotransmitters

Decrease the electrical threshold of post-synaptic cells, allowing action potentials to be propagated more quickly.

Endocrine glands

Ductless structures that secrete hormone products directly into the surrounding extracellular fluid or bloodstream.

Epinephrine signaling in the liver

Epinephrine is water-soluble and is transported through the bloodstream to hepatocytes. Epinephrine binds to GPCRs anchored in the plasma membrane, which initiates a signal cascade in which cAMP is a second messenger. cAMP activates protein kinase A, which activates other enzymes to cause the inhibition of glycogen synthesis and promotion of glycogen breakdown. Glycogen is released as glucose into the bloodstream.

Exocrine glands

Glands that secrete their products into a duct system rather than directly into the bloodstream

Examples of hormones with non-tropic effects

Growth hormone (GH) and prolactin

Estradiol and intracellular signaling in liver cells of female birds and frogs

Hydrophobic estradiol migrates through the plasma membrane of a hepatocyte. Once inside the target cell, estradiol binds to a cytosolic receptor and is translocated into the nucleus. The estradiol-receptor complex binds directly to DNA to increase gene expression for the vitellogenin protein. Once transcription is complete, the translation of the transcript occurs, and a functional protein is released from the cell.

Classes of neurotransmitters

Includes excitatory and inhibitory

inhibitory neurotransmitters

Increase the electrical threshold of the post-synaptic neuron, making it more difficult for action potentials to be propagated

Growth hormone (GH)

Increases protein production, breakdown of lipids, and increases blood glucose levels. Most of the body's cells have receptors that bind GH.

Posterior pituitary

Known as the neurohypophysis. Releases antidiuretic hormone (ADH) and oxytocin, which are both synthesized within the hypothalamus and are then transported to the posterior pituitary via axons.

Pheromones

Ligands that many animal species release into the external environment to mark territory or attract mates. The presence of pheromones in humans is controversial within the research community

Steroid hormones

Lipid-soluble derivatives of cholesterol. Since they are non-polar, they require carrier molecules to assist with transport through the bloodstream. Pass through the cell membrane and bind intracellularly to directly change gene expression in the target cell. Examples are cortisol, aldosterone, estrogen, progesterone, and testosterone.

Hydrophobic hormones

Lipid-soluble. Include steroid hormones and other kinds of amines. Will diffuse through the plasma membrane and bind to an intracellular receptor of the target cell. Require the assistance of protein carriers so that they can travel through the aqueous environment of the blood.

Pineal gland

Located deep within the brain, situated between the two cerebral hemispheres. Secretes melatonin.

Hypothalamus

Located on the undersurface of the brain. It is a complex region with a high level of specialization and multiple autonomous nervous system functions. Primarily circulates releasing factors or inhibiting factors.

Classes of neurohormones

May be amino acid derivatives such as epinephrine, peptides such as oxytocin, or lipid-soluble such as the sex hormones testosterone and estradiol.

Prostaglandins

Modified fatty acids and are lipid-soluble. These ligands are released when body tissue is injured or infectious invaders are identified.

Hormones

Molecules that are synthesized within the body and secreted into the blood where they act as signaling molecules. Hormones may be hydrophilic or hydrophobic.

Releasing hormones

Neurohormones from the hypothalamus that are transported along axons, where they are released to the cells of the pituitary gland.

Non-tropic effects

Occur when a hormone released from the anterior pituitary causes a direct response of the target tissue

Example of positive feedback

Oxytocin is released in response to suckling, causing milk release in mammary glands

Prolactin

Plays a role in the reproductive cycles of humans by stimulating milk production and prolonged progesterone secretion after ovulation and during early pregnancy

Inhibiting factors

Prevent the release of specific pituitary hormones

Releasing factors

Promote activity in the pituitary gland

Parathyroid hormone (PTH)

Raises blood calcium levels

Cytokines

Released during inflammatory immune responses and can stimulate a greater response from other immune system cells.

Adrenal medulla

Releases epinephrine and norepinephrine

Adrenal cortex

Releases glucocorticoids, which raise the blood glucose level, and mineralocorticoids, which target the kidneys to increase sodium reabsorption and potassium excretion

Pancreas

Releases insulin and glucagon

Gonads

Reproductive structures, namely, the testes in males and ovaries in females

Calcitonin

Responsible for lowering the concentration of calcium in the blood

Anterior pituitary

Responsible for releasing hormones that are part of intricate feedback loops. Releases hormones that it produces itself.

Hypothalamic-pituitary portal system

Responsible for transport and communication of anterior pituitary hormones

Testes

Secrete androgens, such as testosterone, which promote sperm production and are responsible for producing secondary male sex characteristics, such as body hair, deepened voice, and increased muscle mass

Ovaries

Secrete estrogens, which are responsible for secondary female sex characteristics, such as the development of breasts and ovulation

Tropic effects

Similar to a cascade pathway; signals from the hypothalamus are redirected to specific endocrine glands, and then the glands release other hormones that target specific cells

Parathyroid glands

Specialized structures located on the thyroid gland that are responsible for secreting parathyroid hormone (PTH)

Nitric oxide (NO)

Synthesized within cells when oxygen levels fall. Diffuses to nearby smooth muscles to cause dilation and increased blood flow, thus increasing oxygen.

Insulin

Targets most tissues, especially liver cells, adipose cells, and skeletal muscles, and stimulates the uptake and use of glucose from the blood

Glucagon

Targets the cells of the liver to increase the breakdown of stored glycogen into glucose, so that the glucose may be released into the bloodstream to be used for energy

antidiuretic hormone (ADH)

Targets the tubules of the kidneys to increase water reabsorption (less water is excreted)

Oxytocin

Targets the uterus to increase contractions during childbirth and mammary tissues to stimulate milk "let down"

Positive feedback loop

The final response produced by the target cell actually reinforces and increases the functional output of the initial stimulus.

Example of negative feedback

The hormone secretin is released by S cells of duodenum when they detect low pH in duodenum

Where are neurohormones synthesized?

The hypothalamus or pituitary gland of the brain

T3 and T4

The primary managers of regulatory metabolism

Which glands are exclusive members of the endocrine system?

Thyroid and parathyroid glands

Examples of hormones with tropic effects

Thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH)

Examples of lipid-soluble amines

Thyroxine and triiodothyronine (thyroid gland)

Neurohormones

Very similar to neurotransmitters. Often transported throughout the entire body and can affect many different tissues and organs.

Specific things that the endocrine system is responsible for

Water balance, ion regulation, growth and metabolism, heart rate and blood pressure, blood glucose regulation, reproductive functions, uterine contractions during labor, and milk release post-labor

Hydrophilic hormones

Water-soluble. Includes polypeptides and some amine hormones. Will bind to an external receptor anchored within the plasma membrane of target cells.

Goiter

enlargement of the thyroid gland

Example of a water-soluble amine

epinephrine (adrenal gland)