Endocrine System
Overview
• Two major controlling systems in the body: nervous and endocrine.
• Nervous system regulates the activity of muscle and glands by means of
electrochemical impulses delivered by neurons.
• The endocrine system influences the metabolic activities of cells by means
of hormones (chemical messengers).
• The endocrine glands are ductless glands and include pituitary, thyroid,
parathyroid, adrenal, pineal, and thymus.
• Organs that contain endocrine tissue include the pancreas, gonads (testes
and ovaries) and digestive organs (stomach and intestines).
• Pituitary secretions regulated by a neuroendocrine gland, the
hypothalamus.
Hormones
• Hormones - chemical substances secreted by cells into the
extracellular fluids and regulate the metabolic function of other cells
in the body
• Two chemical classes:
• Amino acid based - protein, peptide, and catecholamines (epinephrine and
norepinephrine)
• Steroids - most hormones synthesized from cholesterol, lipid soluble passing
through cell membranes (estrogen, testosterone)
Hormone Target Cell Specificity
• Hormone is received by a target cell and evokes a response
• Cell response is dependent upon protein receptors on the plasma membrane or
receptors in the nucleus and if receptors present, will bind hormones
complimentarily
• Extent of target-cell activation (response) by hormone-receptor interaction
depends on....
• blood levels of the hormone
• relative numbers of receptors for the specific hormone
• affinity for the hormone by receptor
• Regulation of cell receptors:
• Up regulation - increase in the number of receptors with increase in hormone concentration
• Down regulation - prolonged exposure, cells become desensitized and there is a loss of
receptors
Mechanism of Hormone Action
• A hormonal stimulus typically produces one or more of the following
changes:
• changes in plasma membrane permeability and/or electrical state
(membrane potential)
• synthesis of proteins or certain regulatory molecules (such as enzymes)
within the cell
• enzyme regulation (activation or deactivation)
• induce secretion
• stimulation of mitosis
Mechanism of Hormone Action
• Two major mechanisms leading to hormone action:
1. Second messenger systems (mediate the target cell's response to hormone)
• Hormone binds to a receptor on the cell membrane
• Receptor causes adenylate cyclase in the membrane to be active and convert ATP into
cAMP
• cAMP activates protein kinases
A.C.
• Protein kinases activates enzymes,
stimulate cellular secretion, opens ion channels, etc.....
Mechanism of Hormone Action
2. Direct gene activation
• Steroid hormones diffuse easily into their target cells
• They bind to receptors within the nucleus
• The activated complex then interacts with
the nuclear chromatin binding to a receptor protein
• Binding to chromatin "turns on" DNA Transcription
of mRNA which is used to direct the synthesis of other proteins
(e.g. enzymes).
Control of Hormones
• Synthesis and release of most hormones are regulated by some
type of negative feedback
• The output of the system feeds back and decreases the input into the
system
• Three types of stimuli that initiates/stimulates the manufacture and
release of hormones:
• Hormonal - hypothalamus produces releasing and inhibiting hormones
• Humoral - changing in blood levels of ions and nutrients
• Neural - neuronal control (e.g. sympathetic nervous system release of
catecholamines during stress)
Pituitary Gland (Hypophysis) –
The Master Gland
• Housed in the sella turcica of the sphenoid bone
• Pituitary is connected the hypothalamus by the infundibulum and is
divided into two lobes:
• Posterior Pituitary (neurohypophysis) - neuroglia and nerve fibers release
neurohormones premade from hypothalamus
• Anterior Pituitary (adenohypophysis) - glandular tissue
Pituitary Gland (Hypophysis) –
The Master Gland
• Pituitary-Hypothalamic Relationships
• Posterior Pituitary (neurohypophysis) -Connected with hypothalamus via a
nerve bundle (hypothalamic-hypophyseal tract) which runs through the
infundibulum
• ADH-anti-diuretic hormone, promotes reabsorption of water in kidney resulting in less
urine volume
• Oxytocin- stimulates birth contractions in uterus and allows for milk ejection from breast
• Anterior Pituitary (adenohypophysis)
• No neural connection, rather vascular connection
• Adenohypophyseal hormones (all second messenger hormones) - six distinct
adenohypophyseal hormones:
• (GH) - growth hormone, increases growth and maintenance of organs
• (PRL) – prolactin, stimulates mammary development and milk
• Tropic (regulates the hormone function of other endocrine glands)
• (TSH) - thyroid-stimulating hormone, stimulates secretion of thyroid hormone from
thyroid
• (ACTH) – adrenocorticotropic hormone, promotes growth of adrenal cortex and
secretion of cortical hormones
• gonadotropic (regulate the function of gonads)
• (FSH) - follicle-stimulating hormone, stimulates ova to develop and secretion of
estrogen; stimulates sperm production and section of testosterone
• (LH) – luteinizing hormone, stimulates corpus luteum after ovulation and secretion
of progesterone; stimulates secretion of testosterone
Thyroid
• Butterfly-shaped and largest endocrine gland located
in the anterior neck overlying the trachea just inferior
to the larynx.
• The thyroid gland has two lateral lobes
• Synthesizes two thyroid hormones (thyroxin and
triodothyronine, require iodine) which are collectively
called thyroid hormone.
• Thyroxine (T4) and triodothyronine (T3) - thyroid
hormone stimulates enzymes concerned with glucose
oxidation and therefore increases basal metabolic
rates and heat production (calorigenic effect)
• Calcitonin (thyrocalcitonin) - polypeptide hormone
that lowers blood calcium and phosphate levels and is
a direct antagonist of parathyroid hormone
Parathyroid
• Embedded on the posterior surface of the lobes of the thyroid gland
(usually four).
• Secretes parathyroid hormone (PTH)
• PTH controls the calcium balance of the blood and is triggered by
decreased calcium
• PTH stimulates three target organs: skeleton, kidneys, and intestines.
• PTH release results in...
• Activation of osteoclasts to digest bony matrix and release calcium ions and
phosphate
• Enhanced resorption of calcium ions
• Increased absorption of calcium by intestinal mucosal cells
Adrenal Gland
• Pyramid-shaped organs located at superior border of each kidney and
is surrounded by connective tissue and embedded in fat
• There two separate portions: inner medulla and an outer cortex.
• Adrenal Cortex
• Synthesizes steroid hormones (corticosteroids) from cholesterol
• Glucocorticoids- produce cortisol, released under stress and low blood sugar; and acts to
increase blood sugar, suppress immune system and aid in metabolism.
• Mineralocorticoids- produce aldosterone causing sodium retention, water retention, and
increase in blood pressure
• Gonadocorticoids- produces sex hormones, androgens (testosterone), some estrogens
and progesterone.
Adrenal Gland
• Adrenal Medulla
• Secretes epinephrine (adrenaline) and norepinephrine (noradrenaline)
• The catechlolamines secreted increase heart rate and blood pressure (via
vasoconstriction).
Pancreas
• Contains acinar cells that produce enzymes that are ducted into the
small intestine, therefore is a exocrine gland
• Among the acinar cells are islets of Langerhans that produce
pancreatic hormones, therefore endocrine gland
• Cells of islets of Langerhans:
• Alpha cells – make glucagon to promote raising blood sugar
• Beta cells – make insulin to promote lowering blood sugar
Gonads
• Produce gonadal sex hormones:
• Ovaries - estrogens and progesterone
• Testes - testosterone
Pineal Gland
• Roof of third ventricle in cranium and contains pinealocytes
• Secrete melatonin
• Involved in sleep-wake cycle
Thymus Gland
• Thymic epithelial cells secrete the peptide hormones thymopoeitin
and thymosin used in the development of immunity