A4. Endocrine System Lecture Notes

The Endocrine System

Overview

  • The endocrine system utilizes hormones to facilitate cellular communication and control, working alongside the nervous system.

Organs of the Endocrine System

  • Adrenal gland

  • Hypothalamus

  • Ovary

  • Pancreas

  • Parathyroid gland

  • Pineal gland

  • Pituitary gland

  • Testis

  • Thymus

  • Thyroid gland

Introduction to the Endocrine System

  • The nervous system sends rapid messages using neurons.

  • The endocrine system secretes hormones directly into the bloodstream, which transports them throughout the body.

  • When hormones reach target cells, specific responses are triggered.

  • Example: Pancreas produces insulin, which increases the uptake of glucose.

  • The endocrine system consists of endocrine glands and the hormones they secrete.

  • Nervous system = rapid response.

  • Endocrine system = slower and longer response.

  • Homeostasis depends on a close relationship between the nervous and endocrine systems.

  • Overlap of two systems: some nervous system tissues secrete hormones; several chemicals act as hormones and neurotransmitter molecules (e.g., epinephrine).

Hormones and Homeostasis

  • Homeostasis: the body maintains a constant balance or steady state to adjust to a fluctuating environment.

  • Requires constant monitoring and feedback, often referred to as dynamic equilibrium.

  • A state of stability within fluctuating limits.

Hormone: Human Growth Hormone (hGH)

  • Stimulus: Time for a growth spurt!

  • The hypothalamus releases a releasing hormone (factor) to stimulate the pituitary gland.

  • The pituitary gland releases hGH.

  • hGH promotes growth of bones, tissues, muscles, etc.

Negative Feedback

  • If the desired result is achieved, hGH release needs to be stopped.

  • A new message is sent back to the hypothalamus to stop the release of the releasing hormone.

  • This is called negative feedback.

  • Represented on diagrams by a dotted arrow from the result back to the hypothalamus.

HYPERTROPISM – Gigantism

  • Robert Pershing Wadlow: the tallest person in medical history with irrefutable evidence.

  • Wadlow reached 8 feet 11.1 inches (2.72 m) in height and weighed 490 pounds (220 kg) at his death.

  • His size and continued growth in adulthood was due to hypertrophy of his pituitary gland, resulting in an abnormally high level of human growth hormone.

  • Showed no indication of an end to his growth even at the time of his death.

Pituitary Dwarfism

  • An abnormally short height may occur if the pituitary gland does not produce enough hGH.

  • It can be caused by a variety of genetic mutations, absence of the pituitary gland, or severe brain injury, but in most cases, no underlying cause of the deficiency is found.

  • Does not affect mental or sexual development.

  • Jyoti Amge – Indian actress

Feedback Loops

  • Two types of feedback loops: NEGATIVE and POSITIVE.

  • Terms to know:

    • Stimulus: a variable that will cause a response.

    • Receptor/sensor: sensory organs that detect a stimulus. This information is sent to the control center (brain).

    • Effector: muscle or gland that will respond.

    • Response: changes (decreases or increases) the effect of the stimulus.

  • Example of negative feedback loop: body temperature regulation when body temperature exceeds 37°C37°C. Nerve cells in the skin and brain act as sensors, the temperature regulatory control center is in the brain, and sweat glands throughout the body are the effectors.

The Hypothalamus and the Pituitary Gland

  • The pituitary gland exercises control over other endocrine glands.

  • Small sac-like structure connected by a stalk to the hypothalamus.

  • Hypothalamus stimulates the release of hormones by the pituitary gland using nerves and releasing hormones.

  • The master controller of the endocrine system is the hypothalamus.

Hypothalamus-Pituitary Interaction

  • The hypothalamus releases a releasing hormone that stimulates the pituitary gland to release a stimulating hormone.

  • The stimulating hormone then acts on a target gland which releases a target gland hormone.

  • Feedback mechanisms inhibit the release of hormones at various levels in the pathway.

Details of Hormone Release

  • Hypothalamus:

    • Neurosecretory cells produce ADH and oxytocin.

    • Neurosecretory cells produce releasing and release-inhibiting hormones.

  • Posterior pituitary:

    • ADH and oxytocin are secreted from axon endings into the bloodstream.

  • Anterior pituitary:

    • Hypothalamic hormones are secreted into a portal system.

    • Each type of hypothalamic hormone either stimulates or inhibits production and secretion of an anterior pituitary hormone.

    • The anterior pituitary secretes its hormones into the bloodstream.

    • Hormones produced: antidiuretic hormone (ADH), oxytocin, thyroid-stimulating hormone (TSH), follicle-stimulating hormone and luteinizing hormone (FSH and LH), human growth hormone (hGH), prolactin (PRL), adrenocorticotropic hormone (ACTH).

The Pituitary Gland

  • The pituitary gland has two parts (lobes): anterior pituitary and posterior pituitary.

Posterior Pituitary

  • Stores and releases hormones produced by the hypothalamus.

  • Hormones: ADH (antidiuretic hormone) and Oxytocin.

Anterior Pituitary

  • Makes its own hormones.

  • Hormones: hGH (human growth hormone), TSH (thyroid-stimulating hormone), ACTH (adrenocorticotropic hormone), FSH (follicle-stimulating hormone), LH (luteinizing hormone), PRL (prolactin).

  • Release of these hormones from the pituitary is regulated by the hypothalamus.

  • Releasing hormones (factors) are transported in blood to the pituitary gland.

Hormones

  • Chemical regulators are produced by cells that target/affect other cells in another part of the body.

  • Endocrine Hormones: hormones/chemicals produced by the endocrine glands and secreted directly into the bloodstream. Example: hGH (human growth hormone).

  • Trophic Hormones: hormones that target other adrenal glands to release hormones. Example: TSH (thyroid-stimulating hormone), ACTH (adrenocorticotropic hormone), FSH (follicle-stimulating hormone).

Hormonal Regulation of Blood Sugar

  • The pancreas regulates the amount of sugar in the blood.

  • The pancreas contains two types of cells:

    • Cells for secretion of digestive enzymes (Bio 20).

    • Cells that produce hormones.

  • The pancreatic cells that produce hormones are located in the structures called islets of Langerhans.

Islets of Langerhans

  • Alpha cells – produce the hormone glucagon (increases blood sugar level).

  • Beta cells – produce the hormone insulin (decreases blood sugar level).

Insulin and Glucagon

  • When blood sugar is low, glucagon is released, which promotes the conversion of glycogen to glucose, and the glucose is released into the bloodstream.

  • When blood sugar is high, insulin lowers the blood sugar level by causing the cells of the muscles, the liver, and other organs to become permeable to glucose.

  • In the liver, glucose is converted into glycogen (stored form of glucose).

Hormonal Imbalances: Diabetes Mellitus

  • The disorder when the body does not produce enough insulin or does not respond properly to insulin.

  • Inadequate production of insulin.

  • Occurs when insulin-producing cells within islets of Langerhans deteriorate.

  • Due to a lack of insulin, it can cause an increase in blood sugar called hyperglycemia.

Types of Diabetes Mellitus

  • Type 1 Diabetes (early-onset diabetes):

    • "Juvenile diabetes / insulin dependent".

    • Occurs when the immune system produces antibodies that attack and destroy the beta cells and are unable to produce insulin.

    • Caused by early degeneration of beta cells in the islets of Langerhans.

    • Can only be treated by insulin injections.

  • Type 2 Diabetes (maturity-onset diabetes):

    • "Adult onset and noninsulin dependent" develops usually over time due to the insulin receptors not responding.

    • People with maturity-onset diabetes can be helped by oral drugs, known as sulfonamides.

    • Stimulate residual function of islets of Langerhans in older people.

  • Gestational Diabetes:

    • A condition in which a woman without diabetes develops high blood sugar levels during pregnancy.

    • Temporary condition that occurs in 2 – 4 % of pregnancies.

    • Increases risk of type 2 diabetes in both mother and child.

  • Juvenile or Type 1 diabetes is 2nd leading cause of blindness in Canada.

  • Other side effects of the disease: kidney and heart failure, stroke, and peripheral nerve damage.

  • Transplanted islet cells could replace the body’s natural mechanism for monitoring and producing insulin.

  • Islet cell transplantation.

Barriers to Islet Cell Transplantation

  • Immune rejection.

  • Current anti-rejection drugs are toxic and harmful to islet function.

  • Immunosuppression leaves recipients susceptible to invading microbes.

  • U of A uses a combination of 3 drugs to:

    • Prevent rejection of transplanted islet cells.

    • Prevent diabetes from returning.

  • Success depends on methods of isolating and transplanting pancreatic cells.

Risks to Islet Cell Transplantation

  • Islet cells are extracted from the pancreas of a donor and infused into the recipient’s liver by way of a large vein.

  • The surgeon uses ultrasound to view the vein leading to the liver.

  • The skin is frozen.

  • A syringe is used to put new cells in place.

  • The patient can usually return home the next day.

  • The liver is used since if it is damaged, it can regenerate itself by building new blood vessels and cells.

  • New blood vessels and nerve cells connect to transplanted islets in the liver and produce enough insulin.

Adrenal Glands

  • Another gland that affects blood sugar is the adrenal glands.

  • Located on top of the kidneys.

  • Each gland is composed of an: inner layer (adrenal medulla) and an outer layer (adrenal cortex).

Adrenal Cortex

  • Outer layer.

  • Secretes 3 hormones:

    • Glucocorticoids (e.g., cortisol).

    • Mineralocorticoids (e.g., aldosterone).

    • Gonadocorticoids (for sexual characteristics).

  • Related to sustained physiological responses (long term stress response).

  • This is also the site for the release of female and male sex hormones (gonadocorticoids).

Aldosterone

  • Blood pressure drops.

  • Low Blood Pressure is detected by the blood pressure receptors in the nephrons.

  • Adrenal glands release Aldosterone.

  • Na+Na^+ is reabsorbed in the nephrons, water follows.

  • Blood pressure increases.

Cortisol

  • The hypothalamus secretes a releasing hormone.

  • Stimulates the anterior pituitary gland to secrete adrenocorticotropic hormone (ACTH).

  • ACTH targets the adrenal gland, which stimulates it to release CORTISOL.

  • Cortisol affects liver and muscle cells to raise blood glucose levels but has a longer effect.

  • Raising levels of cortisol turn off the response via negative feedback.

Adrenal Medulla

  • Inner layer.

  • Secretes 2 hormones – epinephrine (adrenaline) and norepinephrine (noradrenaline).

  • Responsible for the flight or fight response.

  • Raises blood sugar level.

Note: Epinephrine is also produced by the nervous system as a neurotransmitter. Actually, the nervous system and the adrenal medulla work together in times of stress. The nervous system rapidly adjusts to stress, while the adrenal medulla provides a more sustained (also somewhat slower) response to stress. Both regulate the