Anatomy & Physiology Lecture Outline: Week #5

• The Endocrine System

LSc 107 Anatomy and Physiology Spring 1999

Thibodeau Ch 16

What is the endocrine system?

Glands and tissues which secrete hormones either into the bloodstream or locally

Fig. 16-2, Table 16-2 - major endocrine glands

What is a hormone? chemical which affects the physiology of a cell (target) other than the one in which it was produced

systemic hormone produced by major glands, ex: thyroxine

local hormone produced by tissues, and acting locally, ex: prostaglandins

What does the endocrine system control, and in what way is its control different from nervous system control? See Table 16-1

Connections between the nervous system and the endocrine system

• Hypothalamus controls the secretion of the pituitary gland
• Adrenal cortex is directly stimulated by sympathetic nerves

 

Chemical structure of hormones Fig.16-3

• Polypeptides– majority of hormones (proteins, glycoproteins, and shorter peptides)
• Steroids –sterone in the name, sex homones (estrogen, testosterone, progesterone, testosterone), hydocortisone (cortisol), aldosterone
• Amines – thyroid (T3, T4) and adrenal medulla hormones (epinephrine and norepinephrine), melatonin

Target Cells for hormones

Target cells for a particular hormone have a specific receptor for that hormone either on the cell membrane or inside the cell

Ex: Prolactin affects only certain cells of the body

Ex: Insulin and thyroid hormone affect almost all cells of the body

How steroid hormones work

• Derived from cholesterol
• Fat soluble, pass into cells easily
• Receptor molecules inside target cell. Receptor and steroid go to nucleus, where they control the uncoiling of certain sections (genes) of the genetic material for transcription and translation into proteins
• Steroid hormone controls what proteins (functional, structural) are made
• Often a slow process - hours to days

How polypeptide hormones work

• Hormone molecules is the first messenger
• Arrives at the cell membrane where the receptor protein is
• Hormone does not enter the cell, receptor binding triggers a second messenger inside the cell (cyclic AMP or Ca⁺⁺), activates a cascade of enzymes reactions resulting in function
• Fast method
• Amplification

 

What stimulates a gland to produce its hormone?

• Substance in the blood which is affected by the hormone
  Ex: blood sugar and insulin
  Ex: calcium levels and parathyroid hormone
• Hormone from another gland
  Ex: ovaries are stimulated to produce estrogens by pituitary hormones, pituitary is influenced by hormones from hypothalamus

What controls how well a target cell responds to a stimulus? Box 16-1

Number of receptor molecules on surface for that particular hormone

What controls how much hormone is produced by a gland? Fig. 16-10

Negative feedback loops Fig. 16-11

• Short feedback loop - high blood sugar stimulates production of insulin, reduces blood sugar, stimulus to insulin reduced
• Long feedback loop - hypothalamus causes pituitary gland to produce a hormone ® stimulates adrenal gland to produce its hormone ® Adrenal hormone feeds back onto hypothalamus and pituitary damping the loop

Endocrine Glands and their Hormones

Pituitary Gland (hypophysis)

Structure

• 1.5 cm., 0.5 gram
• Connected to hypothalamus by infundibulum
• Anterior part - derived from roof of mouth - glandular tissue
• Posterior part derived from brain - neural tissue

Control of secretion of the anterior pituitary

• Connections with hypothalamus - Fig. 16-18
• Portal system of veins brings a releasing hormone for each of the ant pit hormones
• Table 16-3 - hormones of the hypothalamus
• Negative feedback loops and Psychological or emotional factors (hypothalamus is part of brain) controls the secretion of the hypothalamus

Pituitary Hormones Fig. 16-15, Table 16-4

Anterior pituitary

Growth hormone (GH - somatotrophin)

• variety of target cells: bone, muscle and other tissues
• promotes protein anabolism
• promotes lipid catabolism
• indirectly inhibits glucose metabolism - blood level rises
• produced high in infancy - peaks in childhood - still present in adult
• stimulated by sleep, certain amino acids, low blood sugar

Prolactin

• target cells: mammary glands
• manufacture milk
• stimulated by pregnancy
• inhibits other sex hormones, including testosterone

Thyroid stimulating hormone TSH

• target cells: thyroid; tropic - acts on another gland
• maintains growth development of thyroid gland, and causes it to produce its hormone

Adrenocorticotropic hormone ACTH (tropic)

• target cells: adrenal cortex
• maintains growth and development of the adrenal cortex
• stimulates gland to produce cortical hormones

Follicle stimulating hormone FSH (tropic) gonadotrophin

• target cells: ovary and testis
• stimulates follicles in the ovary to continue development of egg, and to produce estrogen
• in male - stimulates development of seminiferous tubules and sperm

Luteinizing hormone LH (tropic) gonadotrophin

• target cells: ovary and testis
• maintenance of corpus luteum - produces estrogen and progesterone after egg is shed
• in male - production of testosterone

Melanocyte stimulating hormone MSH

• target cells: skin
• influences pigmentation of skin
• absence causes pallor, oversecretion causes deep "tan"

Note: For each of these hormones produced by the anterior pituitary there is a corresponding releasing or inhibiting hormone produced by the hypothalamus and sent to the pituitary in the portal system. Table 16-3

Posterior pituitary

• Post pit is nervous tissue
• Hormones made in neurons in hypothalamus, sent down axons into the post pit. (no releasing hormones - direct neuronal stimulation)

Antidiuretic hormone ADH

• target cells: kidney tubules
• water retention - mechanism should be familiar!
• osmoreceptors of hypothalamus

Oxytocin OT

• target cells: uterus and mammary glands
• letdown of milk from breast tissue into ducts (prolactin makes milk)
• contraction of uterus during and after labor
• Positive feedback mechanism in both actions

Pineal gland

• has both nervous and endocrine function
• nervous: responds to light - information from optic nerve
• concerned with biological clock - timing of ovulation, sleeping patterns
• produces melatonin which inhibits LH (thought to induce sleep)
• possible connection with SAD

Thyroid Gland Fig. 16-20, 16-21

• stimulated by ant pit TSH
• inhibited by feedback of its hormones on ant pit and hypothalamus

Thyroid Hormones (TH) Table 16-5

• T4 (thyroxine)
• T3 (triiodothyronine)
  • target cells: all cells
  • structure of thyroid hormones - amines, with 3 or 4 iodine atoms attached
  • regulate (increase) speed of metabolism
• calcitonin – target cells: bone tissue
  • reduces blood Ca⁺⁺ level
  • increases calcium storage in bones

Note: goiter (Fig. 16-23) does NOT indicate whether thyroid underacting, overacting or normal, can result from a variety of thyroid disorders. Iodine deficiency is only one (loss of negative feedback of T3 and T4 means TSH increases and stimulates growth of gland)

Parathyroid Gland Fig. 16-20, 16-22

• separate from thyroid - own blood supply
• stimulated by low blood Ca⁺⁺ level
• inhibited by high blood Ca⁺⁺ level
• parathormone PTH (parathyroid hormone) calcitonin antagonist
  • target cells: intestine, kidney tubules, bone
  • increases blood Ca⁺⁺

Adrenal Gland Fig. 16-25

• adrenal cortex – outer layer
  • stimulated by ACTH from ant pit
  • inhibited by negative feedback of its hormones on ant pit and hypothalamus
• medulla – inner layer
  • nervous tissue - postganglionic sympathetic fibers
  • stimulated by sympathetic activation from any cause

Hormones of the Adrenal Glands Table 16-6

Adrenal Cortex -- corticosteriods

Aldosterone

• secreted from zona glomerulosa
• target cells: kidney tubules
• mineralocorticoid
• increases reabsorption of sodium by kidney (then triggers ADH and water reabsorption)
• controlled by renin-angiotensin mechanism (Fig. 16-26)

Glucocorticoids – cortisol

• secreted from zona fasciculata
• target cells: general
• Functions:
  • increases blood glucose
  • increases protein breakdown - muscle loss, gluconeogenesis
  • increases fat breakdown for use as fuel - gluconeogenesis
  • part of stress response - mobilizes fuel
  • antiinflammatory effect
  • excess cortisol (Cushing Syndrome), deposition of fat on trunk and face

Adrenal Androgens (gonadocorticoid)

• secreted from zona reticularis
• target cells: sex organs
• possible role in sexual function

Adrenal Estrogens (gonadocorticoid)

• secreted from zona reticularis
• target cells: sex organs
• probably physiologically insignificant

 

"Steroid abuse"

Some steroid hormones - not all- have protein anabolic effects, and are taken by athletes for muscle building. They are effective, but have severe side effects, including atrophy of the testes, and liver tumors

Adrenal Medulla

Epinephrine (adrenaline) and Norepinephrine (noradrenaline)

• target cells: sympathetic effectors
• enhance and prolong effects of sympathetic autonomic nervous system

 

Pancreas Fig. 16-29 Islets of Langerhans are the endocrine part - 1% of total mass

Pancreatic Hormones: Table 16-7

glucagon

• secreted by alpha cells
• target cells: general
• increases blood glucose by breakdown of glycogen in liver
• increases gluconeogenesis in liver
• stimulated by low blood sugar, inhibited by high blood sugar

 

insulin

• secreted by beta cells
• target cells: general
• removes glucose, fat, and protein out of blood into cells
• lowers blood glucose
• stimulated by high blood sugar, inhibited by low blood sugar

somatostatin

• secreted by delta cells
• target cells: pancreatic cells and other effectors
• regulates other pancreatic hormones and growth hormone
• may effect tumors

 

Diabetes mellitus

• 3-6% of the population
• characterized by lack of effect of insulin
• leading to increased blood sugar, and fat breakdown for use as fuel
• presence of glucose in urine causes osmotic diuresis, which causes dehydration, and depletion of Na⁺

IDDM - insulin dependent, ~10 % of diabetics

• usually young people
• pancreas produces little or no insulin
• large scale fat breakdown causes ketoacidosis
• dehydration from osmotic diuresis
• loss of Na⁺
• diuresis leads to thirst, inability to take food into cells leads to weight loss
• can be life threatening

 

NIDDM - non-insulin dependent

• majority ~90% of all diabetics
• older people
• produce insulin, but receptors damaged or absent, and insulin ineffective
• ketoacidosis rare, obesity common

Complications of disordered metabolism of diabetes:

• small vessel disease - blood vessels to kidneys, feet, nerves, eyes damaged
• large vessel disease - early artherosclerosis reduces blood flow

 

Gonads (testes, ovary)

Testes

• seminiferous tubules - sperm production
• interstitial cells - production of testosterone
• stimulated by LH from ant pit
• effect of testosterone - male secondary sexual characteristics

Ovary

• development of follicle
• stimulated by LH and FSH from ant pit
• regulates female reproductive cycle

Hormones:

• estrogens - female secondary sexual characteristics
• progesterones - maintains uterus ready for pregnancy

 

Other Endocrine functioning Glands and Tissues:

Placenta

• layer of the developing embryo which embeds into uterine lining
• interface circulations of mother and child
• secretes human chorionic gonadotrophin HCG - maintains lining of uterus
• high in early part of pregnancy - basis for early testing

 

Thymus Gland

• large gland in children until puberty, atrophies throughout adulthood
• hormone - thymosin important in T cell development

GI Hormones

• gastrin, secretin, cholecystokinin CCK, GIP
• regulate and coordinate secretory and motor activities of digestion

Heart

• secretes atrial natrurietic hormone ANH
• stimulated by- stretch of atrium
• effect - Na⁺ and water loss
• opposes ADH and aldosterone

The Big Picture - p. 514

Endocrine diseases

• hypersecretion and hyposecretion
• Examples of Endocrine dysfunction Table 16-8

 

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