Endocrine System, Hormones & Feedback
Endocrine System : It includes
Feedback Loops
Regulating Estrogen
Fear Response
Feedback Mechanisms
Basic Endocrine System (ES) and Neuro secretion (NS) = 2 main control systems of body Endocrine organs located throughout body actions mediate all tissues Control of ES through feedback mechanisms
II. Endocrine gland – Secreting Hormones
Hypothalamus,
Pituitary,
Pineal Gland,
Thyroid,
Parathyroid,
Thymus,
Adrenal,
Pancreas,
Testes,
Ovaries •
Chemical messengers • Blood stream
Hormones
• Chemical messenger
• Secreted by endocrine gland
• Specific to target
• Activate cellular change
Classes of Hormones
• Peptide/ Protein
• Steroid
• Amine
• Eicosanoid
Protein/Peptide Hormones
• Hydrophilic
• Large
• Can't fit through membrane
• Most hormones
• Example: Insulin
Peptide Hormones
Short 1/2-life
Examples
• Pancreas • Insulin/glucagon
• Hypothalamus • RH (releasing hormones)
• RIH (release inhibiting hormones)
Steroid Hormones
Small
Hydrophobic/Lipophilic
Travel in blood with carrier
Cytoplasmic receptors
change protein synthesis
Example: estradiol
Steroid hormones
Genomic effect
Activates genes
Directs synthesis of new proteins
Lag time between hormone binding and effect long time.
Gonads & placenta
Adrenal cortex
Amine
Synthesized from a single amino acid
Melatonin from tryptophan
Thyroid hormone from tyrosine
Catecholamines
Released from adrenal medulla / adrenal gland
Epinephrine, Norepinephrine, Dopamine
Eicosanoid
• Produced from 20-carbon fatty acid
• Produced in all cells except RBCs
• Inflammation
• (Omega 3, Omega 6)
• Essential Fatty Acids
The Hypothalamic-Pituitary Axis (H-P-A)
• Most feedback loops run through this axis
• HPA mediates growth, metabolism, stress response, reproduction.
• is secondarily in charge of almost everything else.
D. Neurosecretory Cells:
Specialized neurosecretory cells Synthesize and secrete hormones Extend from hypothalamus to posterior pituitary
2. Neurosecretory cells in Hypothalamus
• Nuclei synthesize and secrete hormones
• AntidiureticHormone (ADH) • Responsible for increasing water absorption in the collecting ducts of the kidney nephron
• Oxytocin • Social Behavior • Sexual behavior – Pair bonding?
Importance of Hypothalamus
• Secretes regulatory hormones
• RH = Pituitary releasing hormones
• RIH = Pituitary release inhibiting hormones
•"Directs" pituitary •
Ex. A releasing hormone controls the release of other hormones.
Mechanism of stimulus
Hypothalamus
↓
Releasing Hormone (Release-Inhibiting Hormone)
↓
Pituitary Stimulating Hormone
↓
Gland Hormone
↓
Target
Hypothalamic Hormomes-
• Release Inhibiting Hormones
• Somatostatin
• growth hormone-inhibiting hormone
• Prolactin release inhibiting hormone-PIH
• Peptide hormone involved in lactation • Releasing Hormones • Thyrotropin releasing hormone-TRH • Growth hormone releasing hormone-GHRH
E. Pituitary gland • MASTER GLAND
• Anterior and posterior portions
• Posterior connected to hypothalamus by infundibulum
• Anterior connected via blood stream
Control of Endocrine Function; Feedback Mechanism
Positive Feedback mechanisms
Negative Feedback mechanisms
Self-regulating system
STIMULUS
↓
Hypothalamus Releasing Hormone (Release-Inhibiting Hormone)
↓
Pituitary Stimulating Hormone
↓
Gland
↓
Hormone
↓
Target
Positive Feedback
• Not common
• Classic example: Action of OXYTOCIN on uterine muscle during birth.
Positive Feedback (Mechanism)
Baby pushes on cervix
↓
Nervous signal to Hypothalamus
↓
Hypothalmus manufactures OXY
↓
OXY transported to POSTERIOR PITUITARY & released
↓
OXY stimulates uterine contraction
↓
Loop stops when baby leaves birth canal
Negative Feedback
• Most common control mechanism
• Level of hormone in blood or body’s return to homeostasis shuts off loop at hypothalamus and pituitary
Basic Structure of Feedback Loop
Environmental Stimulus
↓
Stimulates Control Center (Brain-hypothal.)
↓
Hypothalamic hormones stem.
↓
Pituitary
↓
Pituitary hormone stem
↓
Target area
↓
Target area produces change
↓
Change acts negatively or positively on the cycle.
IV. Specific Endocrine Events • Thyroid Hormone • Growth Hormone • Adrenal Cortex Hormones • Sex Steroids
Feedback Loops
The endocrine system controls many bodily functions, including:
reproduction
sexual development
growth and maintenance
metabolism
responding to external stimuli
Some endocrine actions, such as reproductive cycles or growth, occur over long periods of time and can take months or years to complete. Other endocrine actions, such as your body's reaction to fear, are fast, sometimes occurring within seconds.
For every function it controls, the endocrine system follows this general pattern:
a signal is received
a hormonal response takes place
a reaction occurs
Most endocrine activities are regulated by a series of complex feedback loops. These feedback loops work like a thermostat that responds to temperature changes by telling a furnace to turn on and off. When it's cold, the thermostat signals the furnace to turn on and make heat. As the temperature rises above the thermostat's set point, the signal turns off and the furnace shuts down. When the temperature falls below the set point, the thermostat again signals the furnace to turn on and start another feedback cycle.
Endocrine glands react to hormonal changes in the blood in much the same way that a thermostat reacts to temperature changes. The glands, which do not constantly secrete hormones, rely on the presence or absence of hormones in the blood to turn their secretions on and off. If there is not enough hormone circulating in the blood, the endocrine glands make more, increasing blood hormone levels. If there is too much hormone, the glands stop producing it, leading to lower blood hormone levels. The liver also plays a role by removing hormones from the blood and breaking them apart.
Regulating Estrogen
Estradiol, the female estrogenic hormone, is regulated through a complex series of chemical indicators that involve at least three glands:
The hypothalamus
The pituitary
The ovary
The process starts when the hypothalamus notices a low level of estradiol in the blood and begins releasing a hormone known as gonadotropin releasing hormone (GnRH), which notifies the pituitary gland to make and release two more hormones: LH or luteinizing hormone and FSH or folicle-stimulating hormone.
In females, LH and FSH tell the ovaries to secrete estradiol and progesterone, which stimulates the growth of the egg-producing ovarian follicle and prepares the uterus for pregnancy. In males, LH and FSH notify the testes to secrete testosterone, which stimulates sperm production.
The ovaries (and testes) make and release estradiol (and testosterone) until a certain level is reached in the bloodstream.
The hypothalamus and pituitary notice the increase and stop secreting GnRH, LH and FSH hormones. This causes the ovaries to stop releasing estradiol and progesterone (or the testes to stop releasing testosterone).
This feedback loop is also influenced by the liver. As part of its normal cleaning function, the liver degrades or takes apart some of the hormone molecules, removing them from the blood and lowering the amounts found in the blood.
Thyroid-Stimulating Hormone
Fig- Control of thyroxine secretion: a negative feedback loop. Steps involved in the stimulation of hormone secretion by the thyroid gland, showing negative feedback at each step, both by the thyroid hormones (left) and other related hormones (right).
TSH controls the thyroid and TSH secretion is reg-ulated by the hypothalamus via thyroid-releasinghormone. Receptors in the hypothalamus controllevels of circulating thyroxine. When these levels are low, receptors signal the hypothalamus to release TSH releasing hormone. As thyroxine levels increase, TSH releasing hormone secretion declines (FIGURE 16-10) in a process called negative feedback control of TSH secretion. The secretion of TSH releasing hormone is also stimulated by cold and stress. TSH is also known as thyrotropin.
Adrenocorticotropic Hormone
ACTH controls hormone secretion from the cortex of the adrenal gland, partly via corticotropin-releasinghormone (CRH) from the hypothalamus. Stress mayalso increase ACTH secretion. Negative feedback con-trols the secretion of ACTH (FIGURE 16 -11). ACTH is also known as corticotropin, because it is secreted by the corticotropic cells. It stimulates the release of corticosteroid hormones from the adrenal cortex, of which glucocorticoids are most important because they play a role in resisting stressors. Every day, the release of ACTH occurs in a rhythm, wherein levels are highest in the morning just before we wake up. As levels of glucocorticoids rise, CRH secretion is blocked, as is ACTH release. However, normal ACTH rhythm can be altered by factors such as fever, all types of stressors, and hypoglycemia.
Posterior Pituitary and Hypothalamic Hormones
The posterior pituitary differs from the anterior pituitary, in that it is made up of mostly nerve fibers and neuroglial cells called pituicytes .The hypothalamic neurons are located in supra-optic or paraventricular nuclei, synthesizing oxy-tocin and antidiuretic hormone (ADH). These neurohormones are received from the hypothalamus.The posterior pituitary actually functions as a storage area for hormones instead of a manufacturing area. Together, the infundibulum and the posterior lobe of the pituitary gland make up the neurohypophysis. This term is often used to describe just the posterior lobe itself, but this is incorrect. The posterior lobe is actually part of the brain and is formed by a down growth of hypothalamic tissue. Its neural connection with the hypothalamus is via a nerve bundle called the hypothalamic-hypophyseal tract, which runs through the infundibulum. Therefore, if there is a transection of the infundibulum, oxytocin and ADH would be lost. The hypothalamic-hypophyseal tract is formed by neurons in the supraoptic and paraventric-ular nuclei of the hypothalamus.
Fear Response
How and why your heart pounds when you are scared or nervous or exercising is a good example of the endocrine system producing a quick response. Your heart rate increases because the endocrine system, in response to stimuli outside of your body, releases hormones that cause noticeable bodily changes. How does this happen?
Your adrenal gland notices chemical signals put out by the nervous system in response to any bodily stress: hot, cold, life threatening danger or pleasure
Once signaled, the adrenal gland releases the hormones adrenaline and noradrenaline into your bloodstream
These hormones travel to a target cell, bind with the cell's receptor site and cause complex chemical chain reactions that mobilize glucose and fatty acids
Cellular energy reserves are increased and your muscles contract
These unconscious and almost instantaneous chemical responses bring about all the familiar feelings: the ball in the pit of your stomach, a tensing of muscles, goose bumps and a faster heart beat.
All this happens in a matter of seconds as the body responds to an outside stimuli and decides.
