Cellular Communication Mechanisms: Signaling Types

Overview of Endocrine System

• Communication can occur between cells through gap junctions, neurotransmitters, paracrines, and hormones
• Gap junctions contain membrane pores allowing movement of molecules, nutrients, and electrolytes between cells
• Neurotransmitters are secreted from neurons and travel across the cleft to reach subsequent cells
• Paracrines are secreted into tissue fluids to affect nearby cells
• Hormones are chemical messengers that travel in the bloodstream and stimulate responses in other tissues/organs

What is Endocrine?

• The endocrine system includes glands, tissues, and cells secreting hormones
• Endocrine glands function as hormone sources

Endocrine vs. Exocrine Glands

• Endocrine glands lack ducts and have internal secretions
• Exocrine glands possess ducts
• Endocrine glands contain dense, fenestrated capillary networks for hormone uptake into the bloodstream
• Exocrine glands carry secretions to an epithelial surface or digestive tract mucosa, involving external secretion
• Endocrine glands have intracellular effects
• Exocrine glands have extracellular effects such as food digestion

Endocrine vs. Nervous System

• The endocrine system responds slowly (seconds to days) while effects last longer
• The nervous system responds quickly (milliseconds) and stops rapidly
• Endocrine system responses persist and adapt slowly to long-term stimuli
• Nervous system responses decline rapidly to long-term stimuli
• Endocrine system effects are general and widespread
• Nervous system effects are targeted and specific

Hypothalamus

• Regulates primitive functions like water balance and reproductive functions
• Forms the floor and walls of the brain's third ventricle

Pituitary Gland

• The pituitary gland performs hypothalamic functions
• Extends from the hypothalamus via the infundibulum stalk
• The anterior pituitary is linked to the hypothalamus by blood vessels of the hypophyseal portal system
• Hypothalamic hormones regulate anterior pituitary cells
• Nine hormones are produced in the hypothalamus
• Seven hypothalamic hormones regulate the anterior pituitary
• Gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone (CRH), prolactin-releasing hormone (PRH), prolactin-inhibiting hormone (PIH), growth hormone-releasing hormone (GHRH), and somatostatin
• Oxytocin and antidiuretic hormone are stored in the posterior pituitary and released on demand
• Nerve cell bodies pass down the stalk as the hypothalamohypophyseal tract, ending in the posterior lobe
• Hypothalamic neurons secrete hormones, stored and released in the blood
• Releasing hormones from the hypothalamus trigger special pituitary cells to secrete their own hormones
• Anterior pituitary hormones usually target endocrine glands in the body

Posterior Pituitary Gland

• Antidiuretic hormone (ADH) increases water retention by kidneys to reduce urine volume and prevent dehydration
• Oxytocin (OT) is released during arousal and orgasm to promote sexual satisfaction and promotes emotional bonding between partners
• Oxytocin stimulates labor contractions during childbirth and milk flow during lactation

Control of Pituitary Secretion

• The hypothalamus releases ACTH when the amygdala is triggered by stress
• ACTH causes the release of cortisol

Growth Hormones

• Growth hormone affects many body tissues including cartilage, bone, muscle, and fat
• Growth hormone induces the liver to produce growth stimulants, including insulin-like growth factors (IGF-I and IGF-II)
• Growth hormone boosts DNA transcription/RNA translation, increasing amino acid uptake by cells
• Growth hormone stimulates adipocytes to catabolize, releasing fatty acids for fuel, and mobilizing fatty acids to reduce dependence on glucose
• Growth hormone promotes sodium, calcium, and chloride retention by the kidneys, along with calcium absorption in the small intestine
• Growth hormone secretion is high during the first 2 hours of sleep
• Growth hormone can peak in response to vigorous exercise and is activated by ghrelin
• Growth hormone levels decline with age, and lack of protein synthesis may contribute to aging

Pineal Gland

• Attached to the third ventricle beneath the posterior end of the corpus callosum
• After age 7, it undergoes involution (shrinkage)
• Functions to synthesize melatonin at night
• The pineal gland may influence the timing of puberty in humans

Thymus

• Shrinks after puberty (involution)
• Is where T cells mature that are important in the immune system
• Secretes hormones (thymopoietin, thymosin, and thymulin)
• Stimulates development of other lymphoid organs and activity of T cells

Thyroid Gland

• Adjacent to the trachea, immediately below the larynx
• Consists of two lobes connected by a narrow bridge called the isthmus
• Looks dark reddish-brown color because of rich blood supply
• Composed of sacs called thyroid follicles
• The thyroid follicles contain a protein-rich colloid surrounded by simple cuboidal epithelium of follicular cells which secrete parathyroid hormone
• Follicular cells secrete Thyroid Hormone (TH)
• TH release is influenced by TRH from the hypothalamus and in response to TSH
• TH increases metabolic rate, oxygen consumption, and heat production
• TH increases appetite, growth hormone secretion, alertness, and reflex speed
• Parafollicular cells secrete calcitonin in response to rising blood calcium and stimulate osteoblast activity and bone formation

Parathyroid Glands

• Four small, ovoid glands partially embedded in the posterior surface of the thyroid gland
• They are not regulated by the pituitary
• secrete parathyroid hormone (PTH)
• PTH regulates blood calcium level by stimulating calcium release from bones and decreasing calcium loss in urine via kidneys

Adrenal Glands

• Sit on the superior pole of the kidneys
• Adrenal Medulla is the inner core, 10% to 20% of the gland
• Adrenal Cortex is the thicker outer core, 80% to 90% of the gland
• The adrenal medulla acts as both an endocrine gland and a ganglion of the sympathetic nervous system (neuroendocrine)
• The adrenal medulla is Innervated by sympathetic preganglionic fiber
• When stimulated by fear, pain, or stress, the adrenal medulla releases catecholamines, 75% epinephrine, 25% norepinephrine
• Zona Glomerulosa secretes mineralocorticoids that regulate electrolyte balance and aldosterone
• Zona Fasciculata secretes glucocorticoids, regulating fuel metabolism and gonadocorticoids (sex hormones) like cortisol.
• Zona Reticularis secretes glucocorticoids and gonadocorticoids including androgens and sex cells
• Catecholamines increase alertness and prepare the body for physical activity
• Catecholamines mobilize high-energy fuels (lactate, fatty acids, glucose) and boost glucose levels through the liver
• Epinephrine inhibits insulin secretion
• Aldosterone is produced in response to falling blood pressure
• Decreased digestion and urine production
• Falling blood pressure causes the release of angiotensin II, which stimulates aldosterone release from the adrenal cortex

Cortisol

• Cortisol is the most potent glucocorticoid
• Secreted from zona fasciculate and zona reticularis in response to ACTH from the pituitary gland
• Stimulates fat and protein catabolism, gluconeogenesis, and release of fatty acids and glucose into blood
• Helps the body adapt to stress and repair damaged tissues
• Cortisol Has anti-inflammatory effects, however Excessive secretion or medical use suppresses the immune system
• Androgens are primary adrenal sex steroids, but also produce amounts of estrogen
• Secreted from zona fasciculate and zona reticularis in response to ACTH from pituitary
• Dehydroepiandrosterone (DHEA) is a major androgen, converted to a more potent form in target tissues
• At puberty, DHEA stimulates pubic, and axillary hair growth, gland development, and libido
• Estradiol is a major adrenal estrogen and A significant source of estrogen in women after menopause

Pancreatic Islets

• The pancreas is an endocrine gland
• Endocrine cell clusters scattered throughout are called pancreatic islets
• Hormones are critically important in the regulation of glycemia
• Alpha cells (20% of islet cells) result in raising blood glucose levels by releasing it into the circulation and stimulating fat catabolism in adipose tissue
• The pancreas Also releases in response to rising amino acid levels and promotes amino acid absorption
• Alpha (a) cells (glucagon cells) secrete glucagon between meals when blood glucose concentration falls and stimulates fat catabolism the liver
• Beta cells (70%) secrete insulin and amylin
• During/after meals, the beta cells release is stimulated when glucose and amino acid blood levels are rising
• Insulin stimulates cells to absorb nutrients and store or metabolize them, lowering blood glucose levels
• Insulin Promotes synthesis of glycogen, fat, and protein and suppress use of already-stored fuels
• Amylin reduces glucose spikes, slows stomach emptying and modulates gastric secretions, inhibits glucagon secretion, and signals satiety
• Delta cells (5%) is A small number of polypeptide cells and others

Gonads - Ovarian and Testicular Hormones

• Gonads (ovaries and testes) have both endocrine and exocrine functions
• Endocrine product: egg and sperm
• Ovarian hormones include estradiol, progesterone, and inhibin
• The egg develops in a follicle that is lined by granulosa cells and surrounded by a capsule (theca)
• After ovulation, the remains make the corpus luteum
• Secretes progesterone for 12 days following ovulation
• Shifts to weeks if a pregnancy results
• Testicular hormones include testosterone, weaker androgens, estrogen, and inhibin
• Seminiferous tubules produce sperm
• Interstitial endocrine cells secrete testosterone and other sex steroids
• Testosterone stimulates the development of the male reproductive system and sustains sperm production and Inhibin limits FSH secretion

Endocrine Function of Other Tissues/Organs

• Skin keratinocytes convert a cholesterol-like steroid into cholecalciferol using UV from the sun
• The liver converts cholecalciferol to calcidiol, secretses angiotensinogen, secrets 15% of erththopoeitin, and is a Source of insulin-like growth factor I (IGF-I)
• Kidneys convert calcidiol to calcitriol (vitamin D3) and also, secreates renin
• Heart atrial muscles secrete two natriuretic peptides in response to increased BP
• Skeletal Muscles secrete myokines that mobilize fuel from the liver and adipose tissue
• Stomach and small intestine secrete at least 10 enteric hormones
• Placenta secretes estrogen, progesterone, and pregnancy-regulating hormones

Hormone Chemistry

• Three chemical classes of hormones: steroids, monoamines, and peptides
• Steroid Hormones (lipids) synthesized from cholesterol
• Monoamines (biogenic amines) synthesized from amino acids
• Peptide Hormones are created from chains of amino acids
• Hormone secretion fluctuates according to circadian or monthly rhythms (woman's cycle)
• Neural stimuli involving nerve fibers can trigger release of hormones from endocrine glands
• Hormonal stimuli involve hormones from the hypothalamus regulating secretion by the anterior pituitary gland

Hormone Transport and Action

Travel in the blood, which will be mostly water Most monoamines and peptides are hydrophilic Steroids and thyroid hormones are hydrophobic Hormones stimulate solely their designated target cells Receptors and the targeted hormones result in direct interaction

• Specificity- receptors for one hormone will not bind to another hormone, all receptor molecules are occupied; full saturation
• With signal Amplification (Cascade Effect) Very small stimuli can produce a very large effect Increased amount of Up-Regulation causes the number of receptors to be increased
• Decreased amount of Down-regulation causes the number of receptors to be reduced

Hormone Interactions and Adaption

• Synergistic Effects- Multiple hormones act together for greater effect
• Permissive Effects- One hormone enhances the target organ's response to a second hormone
• Antagonistic Effects- One hormone opposes the action of another
• Stress any situation that upsets homeostasis
• Stress Response (generally adaptation syndrome, GAS) will generally have elevated levels of epinephrine and cortisol
• Initial response to stress Sympathetic nervous system will Release norepinephrine and epinephrine Prepare body for fight (increased HR, increased respiratory rate, pupil dialation, hyper awareness) Conserves sodium and hastens the elimination of potassium and hydrogen
• Hypothalamus releases corticotropin Long Term elevation of cortisol- Depresses immune function, increasing susceptibiligy to infections
• Eicosanoids is a family of paracrine secretion that can Mediate allergic and inflammatory reactions
• Act as a Prostacyclin and override prostacyclin and stimulate vasoconstriction

Explore cellular communication: endocrine (hormones via bloodstream), direct (nutrient/electrolyte exchange), paracrine (local signaling molecules). Contrast neurotransmitters' targeted delivery with hormones' broader reach. Understand metabolic adaptations during exercise and hormonal influences on blood pressure and metabolism.