Module 2.1-2.3 : Endocrine System

Autocrine Signaling

Local Signaling : ligands diffuse across a short distance to target the very same cell that released them.

Endocrine Signaling

Long Distance Signaling : ligands released into the blood and travel anywhere in the body; used in growth, reproduction, and blood pressure regulation.

Paracrine Signaling

Local Signaling : ligands diffuse to nearby target cells within the same tissue or organ, but the secreting cell and target cell are different types.

Synaptic Signaling

Local Signaling : neurons release neurotransmitters that diffuse across a synapse to reach a target cell such as muscle, tissue, or gland

Neuroendocrine Signaling

Long Distance Signaling : ligands are made and stored by neurons but travel through the bloodstream to reach distant target cells

Pheromone Signaling

ligands are released into the external environment to affect other organisms, such as marking territory or attracting mates

Peptides, amino acid derivatives, or amines

Neurotransmitters are complex ligands that can take on what types of structures?

Excitatory decrease the electrical threshold (increase excitability); Inhibitory increase the electrical threshold (decrease excitability)

What is the functional differnce between excitatory and inhibitory neurotransmitters?

Local, synaptic signaling

Through what type of signaling do all neurotransmitters exert their effects?

In the pre-synaptic neuron, stored in small vesicles.

Where are neurotransmitters synthesized and stored in a neuron?

Stimulation of the presynaptic neuron and an increase in action potential frequency.

What triggers the release of neurotransmitters into the synaptic cleft?

A small gap that serves as the communication area between the presynaptic cell and the post synaptic cell.

What is the synaptic cleft?

They bind to cell surface receptors on the post synaptic cell

How do neurotransmitters affect the post synaptic cell after being released?

Neurotransmitters act locally at synapses, while neurohormones travel through the bloodstream to act on many tissues/organs.

What is the key difference between neurotransmitters and neurohormones?

Amino acid derivatives: epinephrine / Peptides: oxytocin / Lipid soluble: testosterone

What are three structural types of neurohormones, with an example of each?

The hypothalamus and the pituitary gland.

Which two brain regions are primarily responsible for synthesizing?

Neurohormones from the hypothalamus that travel to the pituitary gland to regulate hormone release.

What are “releasing hormones” and where are they produced?

Dopamine and Norepinephrine

Give two examples of molecules that can act as both neurotransmitters and neurohormones.

Negative Feedback

regulatory mechanism that results in a decrease in function

Negative Feedback

“stop” mechanism important for maintaining homeostasis

Positive Feedback

the final response produced by the target cell reinforces and increases the functional output of the initial stimulus (breast feeding)

Tropic Effects

signals from hypothalamus are redirected to specific endocrine glands, and then the glands release others hormones that target specific cells. (boss giving orders to workers)

Non-Tropic Effects

occur when a hormone released front he anterior pituitary causes a direct response of the target tissue. (workers doing the job directly)

Prolactin (non-tropic)

stimulates milk production and prolonged progesterone secretion after ovulation and during early pregnancy.

Growth Hormone : GH (can be tropic and non tropic)

increases proteins production, breakdown of lipids, increases blood glucose levels.

Thyroid-stimulating Hormone (TSH) (anterior pituitary)

Target Tissue: thyroid gland

Response: release of T3 and T4

Adrenocorticotropic Hormone (ACTH) (anterior pituitary)

Target Tissue: adrenal gland

Response: secretion of cortisol

Luteinizing Hormone (LH) (anterior pituitary)

Target Tissue: gonads

Response: females - ovulation / males - sperm production

follicle-stimulating Hormone (FSH) (anterior pituitary)

Target Tissue: gonads

Response: females - estrogen secretion and egg cell maturation / males - testosterone synthesis and support of sperm production

Hypothalamic-pituitary portal system

responsible for transportation and communication of anterior pituitary hormones.

Thyrotropin-releasing Hormone (TRH)

released by the hypothalamus neurons into the blood capillaries of the hypothalamic pituitary portal when T3 and T4 levels are low

Regulation of Thyroid Hormone Secretion from the Anterior Pituitary (#1)

A stimulus, specifically a drop in thyroid hormone levels, is detected.

Regulation of Thyroid Hormone Secretion from the Anterior Pituitary (#2)

TRH is released from the hypothalamus to the anterior pituitary

Regulation of Thyroid Hormone Secretion from the Anterior Pituitary (#3)

TRH causes TSH to be secreted from the anterior pituitary

Regulation of Thyroid Hormone Secretion from the Anterior Pituitary (#4)

TSH binds to target cells of the thyroid gland, stimulating the release of thyroid hormones (T3/T4)

Regulation of Thyroid Hormone Secretion from the Anterior Pituitary (#5)

As the hormones circulate in the blood, they bind to target cells of the blood vessels, heart, digestive system, and reproductive system

Regulation of Thyroid Hormone Secretion from the Anterior Pituitary (#6)

As thyroid hormone levels return to normal, they bind to cells of the hypothalamus to block the release of TRH and to cells of the anterior pituitary to block the release of TSH. (Negative Feedback Loop)

Hypothyroidism

thyroid hormone levels are low; characterized by a decrease in the size of the thyroid gland.

Hyperthyroidism

thyroid hormone levels are high; characterized by an enlarged thyroid.

Grave’s Disease (hyperthyroidism)

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

antibody

A _________ produced by the patient’s own body has a structure like that of TSH. The antibody binds to cells of the thyroid gland, causing the release of large amounts of thyroid hormone.

Symptoms of Graves Disease

Weight loss, excessive sweating, rapid heartbeat, flushing, muscle atrophy, goiter (enlarged thyroid gland), exophthalmia (bulging eyes)

(1) Stimulus —> Endocrine Cell —> Target Cell

(2) Stimulus —> Hypothalamus —> Pituitary —> Target Cell

What are the two main pathway types in the endocrine system?

By secreting a hormone that travels through the body to bind target cell receptors

How do endocrine cells respond to a stimulus?

The target cells responds by secreting a ligand, which contributes to feedback regulation.

What happens once a hormone binds to its target cell receptor?

Negative Feedback

What type of feedback decreases the initial stimulus and prevents overstimulation?

Positive feedback amplifies the initial stimulus and increases the functional response.

How does a positive feedback loop differ from a negative feedback loop?

It maintains homeostasis by preventing overstimulation and over-secretion of ligands

Why is negative feedback important in endocrine pathways?

Tropic hormones signal other endocrine glands to release hormones, while non-tropic hormones directly act on target tissues.

What is the difference between tropic and non-tropic effects of anterior pituitary hormones?

Growth Hormone (GH) and prolactin

Which anterior pituitary hormones have non-tropic effects?

Increases protein production, lipid breakdown, and blood glucose; affects most body tissues

What are the major effects of growth hormone (GH)?

Stimulates milk production and prolongs progesterone secretion after ovulation and during early pregnancy.

What role does prolactin play in humans?

Thyroid-stimulating hormone (TSH) / Adrenocorticotropic hormone (ACTH) / Luteinizing hormone (LH) / Follicle-stimulating hormone (FSH)

Which anterior pituitary hormones are considered tropic?

Hypothyroidism (underactive thyroid)

Cause: thyroid gland fails to produce enough T3/T4 despite TSH stim.

Symptoms: fatigue, weight gain, cold intolerance, slowed heart rate, difficulty concentrating, dry skin, brittle hair

Lab Results: low t3/t4, high TSH (thyroid issue); low TRH, low TSH, low T3/T4 (hypothalamus issue)

Tx: hormone replacement therapy

TRH

Which hormone is secreted by the hypothalamus to stimulate the anterior pituitary?

thyroid

Elevated TSH but low T3/T4 suggests a problem in which gland?

Does not regulate production of insulin

does regulate energy metabolism, muscle mass maintenance

Thyroid hormones are responsible for regulating all of the following except?

Negative feedback

What type of feedback loop regulates thyroid hormone secretion?

feeling cold and fatigued

In hypothyroidism, which of the following symptoms is most likely?

decrease

If the anterior pituitary fails to release TSH, what will happen to thyroid levels?

Hyperthyroidism

If thyroid hormones fail to inhibit TRH and TSH release, which of the following occurs?

TSH

Which hormone directly stimulates the thyroid gland?

hypothalamus

A patient with low TRH, low TSH, and low T3/T4 likely has dysfunction in the ?

It prevents overstimulation and maintains homeostasis

Why is negative feedback critical in endocrine pathways?

Hyperthyroidism (overactive thyroid)

Cause: thyroid gland produces excessive T3/T4 (often autoimmune stim)

Symptoms: unexplained weight loss, heat intolerance, rapid heartbeat, nervousness, irritability, sweating

Lab Results: high T3/T4, low TSH (due to negative feedback)

Tx: anti-thyroid meds, radioactive iodine therapy, surgery.

homeostasis

When thyroid hormone levels are too high or low, symptoms emerge because the regulatory system cannot maintain ______________.

thyroid gland

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

T3/T4

primary managers of regulatory metabolism

calcitonin

lowers concentration of calcium in the blood.

parathyroid glands

specialized structures located on the thyroid gland; responsible for secreting parathyroid hormone (PTH) —> raises blood calcium levels

adrenal glands

affixed directly on top of the kidneys; have two main types of endocrine tissues: medulla and cortex

adrenal medulla

releases epinephrine and norepinephrine

adrenal cortex

releases glucocorticoids, which raise the blood glucose level

releases mineralocorticoids, which target the kidneys to increase sodium resorption and potassium excretion.

gonads

reproductive structures, namely, the testes in males and ovaries in females.

testes

secrete androgens (testosterone); promotes sperm production / responsible for secondary sex characteristics (body hair, deep voice, increased muscle mass)

ovaries

secrete estrogens; responsible for secondary sex characteristics (breasts, ovulation)

pineal gland

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

Responsible for secreting melatonin

melatonin

a peptide that promotes circadian rhythms (sleep-wake cycle)

hypothalamus

located on the undersurface of the brain; complex region with a high level of specialization and multiple autonomous nervous system functions.

hypothalamus

gate keeper for the release or inhibition of pituitary hormones.

releasing factors / inhibiting factors

The hypothalamus primarily circulates ___________________ that promote activity in the pituitary or _________________ that prevent the release of specific pituitary hormones.

pituitary gland

pea-sized structure located below the hypothalamus that protrudes from a long stalk called the infundibulum.

posterior pituitary

neurohypophysis =

extension of the hypothalamus

anterior pituitary

adenohypophysis =

slightly larger and is not an extension of the hypothalamus

antidiuretic hormone (ADH)

posterior pituitary releases ________________, which targets the tubules of the kidneys to increase water absorption (less water excreted)

oxytocin

targets the uterus to increase contractions during childbirth and mammary tissues to stimulate milk “let down”; released from posterior region

ADH and oxytocin

synthesized within the hypothalamus and are then transported to the posterior pituitary via axons

stored in the posterior region until their release is stimulated by messages from hypo.

anterior pituitary

responsible for releasing hormones that are part of intricate feedback loops.

hormones released from here are not synthesized within the hypothalamus as other neurohormones are.

endocrine cells of the anterior pituitary gland itself.

hormones of the anterior pituitary are synthesized from?

pancreas

releases insulin and glucagon

insulin

targets most tissues, especially liver cells, adipose cells, and skeletal muscles —> 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.