Endocrine Glands, Hormone Actions, and Receptor Mechanisms

What are the characteristics of endocrine glands?

Ductless, secrete hormones into the blood, help regulate body metabolism, growth, and reproduction.

What effects do hormones have on physiological function?

Cause ions to move down concentration gradients to change membrane potentials; sometimes act as neurotransmitters.

How do hormones reach their target cells/organs?

They must have a specific target receptor protein to interact with the chemical, causing a specific change in the target cells. Must have a turn off mechanism

What are steroid hormones?

Lipids derived from cholesterol, secreted by the adrenal cortex and gonads.

What are prohormones?

Inactive hormones that must be cut and spliced together to be active.

What are prehormones?

Inactive hormones that must be modified within their target cells.

What is a synergistic hormone interaction?

2+ hormones work together to produce a particular effect, can be additive or complementary.

What is a permissive hormone interaction?

One hormone makes target cell more responsive to a second hormone.

What is an antagonistic hormone interaction?

Hormones working in opposite directions.

What does half-life describe about hormone concentrations?

It is the time required for the plasma concentration of a given amount of hormone to be reduced by half.

What can result from unnaturally high concentrations of a hormone?

Binding to receptors of related hormones that have similar structures, leading to widespread side effects.

What is priming/upregulation?

Increasing the number of receptors for a particular hormone to enhance sensitivity.

What is desensitizing/downregulation?

Decreased number of receptors for a particular hormone.

What is pulsatile secretion?

Hormones are sometimes released in spurts.

What benefit does pulsatile secretion provide?

Avoids desensitization of hormone receptors.

What is a common example of desensitization of a hormone?

Getting 'used to' caffeine.

What are the three characteristics of hormonal action?

Binding is highly specific, high affinity, and low capacity.

Where are the receptors of steroid hormones and thyroid hormones found?

Within the nucleus of the target cell.

How are steroid hormones and thyroid hormones transported to their receptors?

Carrier proteins carry these hormones to the nuclear hormone receptors. Eventually they dissociate + diffuse across the membrane to nuclear hormone receptors

What does 'transcription factor' indicate about a hormone's effect?

The hormone produces new proteins, usually enzymes that change metabolism within the cell.

What is the structure of nuclear hormone receptors?

Has a ligand binding domain and a DNA binding domain.

What is the process of thyroxine (T4) being transported to its target cells?

Travels via thyroxine-binding globulin, dissociates into T4 and T3, enters heterodimer target cells, and alters protein production.

What is a homodimer in relation to nuclear hormone receptors?

Two identical receptors, such as steroid hormones.

What is a heterodimer in relation to nuclear hormone receptors?

Two different receptors, such as T3 and 9-cis-retinoic acid.

What molecule forms a heterodimer with T3?

Trilodothyronine.

What are the three possible second-messenger mechanisms for hormones?

Adenylate cyclase, phospholipase C, and tyrosine kinase.

What systems does epinephrine utilize?

Alpha and beta adrenergic systems.

What is the result of epinephrine activation in hepatocytes?

Phosphorylates glycogen to create free glucose for the bloodstream. Can be done via protein kinases from cAMP (beta) or calmodulin from Ca2+ (alpha)

What hormone utilizes tyrosine kinase as its receptor?

Insulin.

What are the two lobes of the pituitary gland?

Anterior lobe and posterior lobe.

What hormones are associated with the anterior lobe of the pituitary gland?

Growth hormone, thyroid stimulating hormone, adrenocorticotropic hormone, follicle stimulating hormone, luteinizing hormone, and prolactin.

What hormones are associated with the posterior lobe of the pituitary gland?

Antidiuretic hormone and oxytocin.

How does hormone storage differ between the two lobes of the pituitary gland?

Anterior releases hormones through the hypothalamo-hypophyseal portal system; posterior releases via neuroendocrine reflexes.

What effects does thyroid-stimulating hormone produce?

Stimulates secretion of thyroid hormones.

What effects does adrenocorticotropic hormone produce?

Stimulates secretion of glucocorticoids.

What effects does antidiuretic hormone produce?

Promotes retention of water in the kidneys.

What effects does oxytocin produce?

Stimulates contractions during childbirth and milk let-down in lactation.

What is the hypothalamo-hypophyseal portal system?

A venous capillary system from the hypothalamus to the anterior pituitary.

What is the venous capillary system from the hypothalamus responsible for?

Transporting hormones from the anterior pituitary gland.

What are the releasing and inhibiting hormones associated with the hypothalamus?

CRH, GRH, PIH (dopamine), TRH, somatostatin, and GHRH.

What hormones are secreted by the adrenal cortex?

Cortisol, Aldosterone, Epinephrine, and Norepinephrine.

What effects does cortisol produce?

Stimulates protein degradation, gluconeogenesis, inhibits glucose utilization, stimulates lipolysis, suppresses immune response, inhibits inflammation.

What effects does aldosterone have?

Retention of Na+, excretion of K+ to regulate blood pressure, blood volume, and electrolyte balance.

What effects do epinephrine and norepinephrine produce?

Increased cardiac output, respiratory rate, glucose release, mental alertness, dilation of coronary blood vessels, elevation of metabolic rate.

What are the three hormones of the thyroid?

Thyroxine (T4), Calcitonin, Triiodothyronine (T3).

What is thyroglobulin?

A protein made by follicular cells where iodine attaches to tyrosine and serves as a binding site for T3 and T4.

What are T3 and T4?

Transcription factors that lead to the synthesis of thyroid hormones.

What are the main steps for T3 and T4 production and release?

T3 = diiodotyrosine + monoiodotyrosine; T4 = 2 diiodotyrosine. They dissociate from thyroglobulin when TSH stimulates the thyroid gland. DIT and MIT is produced via oxidation of iodine.

What effects do T3 and T4 produce?

Stimulates protein synthesis, promotes maturation of the nervous system, increases rate of cellular respiration, elevates basal metabolic rate.

What effect does calcitonin produce?

Inhibits osteoclast activity to preserve bone density and stimulates calcium excretion in kidneys.

What is hypothyroidism characterized by?

Low metabolic rates, weight gain, lethargy, poor adaptation to cold, and myxedema.

How can insufficient iodine lead to hypothyroidism?

Iodine deficiency causes overstimulation of the thyroid gland, leading to goiter and hypothyroidism due to lack of negative feedback.

What effect does parathyroid hormone produce?

Increases blood calcium levels via bones, kidneys, and intestine.

What hormones are produced by the alpha and beta cells of the pancreas?

Alpha: glucagon; Beta: insulin.

What is the effect of insulin release?

Lowers blood glucose levels, promotes glycogenesis and lipogenesis.

What is the effect of glucagon release?

Increases blood glucose levels, promotes glycogenolysis and lipolysis.

What occurs when insulin binds to its receptor?

Tyrosine kinase receptor produces ADP from ATP, triggering GLUT4 vesicles to translocate to the plasma membrane.

What is the difference between Type 1 and Type 2 diabetes mellitus?

Type 1 is due to damage to beta cells; Type 2 is insulin resistance.

What hormone is secreted by the pineal gland?

Melatonin.

What effect does melatonin produce?

Makes you drowsy and facilitates falling asleep.

What structure of the hypothalamus regulates the pineal gland?

Suprachiasmatic nucleus. Light stimulus

What is the structural organization of skeletal muscle from largest to smallest?

Tendon → skeletal muscle → fascicle → fiber → myofibrils → myofilaments.

What are the unique characteristics of skeletal muscle?

Sarcolemma plasma membrane, multinucleated, striated.

What produces the striations seen in skeletal muscle?

I/light bands, A/dark bands, Z-lines.

What is the biggest factor in contraction strength?

Motor unit recruitment.

What is the relationship between muscular control and muscular strength?

Finer control requires smaller and fewer motor units; greater strength requires larger and greater motor units. Control and strength are trade offs

What are the components of the sarcomere?

Striations, myofibrils with thick and thin filaments, M line.

What components of the sarcomere change when a muscle contracts?

A bands move closer, I bands shorten, actin and myosin do not change in length, H bands shorten or disappear

Briefly describe the sliding filament theory.

Shortening of myofibrils is caused by sliding of myofilaments, produced by myosin cross bridges pulling actin.

What are the steps to the activation/action of the myosin heads?

ATP breaks down to ADP + Pi, allowing myosin to bind to actin and produce a power stroke. Repeats

What is the structure of actin (thin filament)?

300-400 subunits arranged in a double row twisted to form a helix.

What are the components of troponin?

Troponin C interacts with calcium during contraction. Troponin I inhibits binding of myosin, and Troponin T binds to tropomyosin

Which component interacts with calcium (Ca2+) during contraction?

Troponin C binds to calcium (Ca2+).

What happens when calcium (Ca2+) binds to troponin C?

It causes a conformational change in troponin and tropomyosin, allowing myosin to cross-bridge with actin.

What occurs at the neuromuscular junction when a somatic motor neuron stimulates contraction?

ACh is released from the motor neuron, leading to action potentials at the end plate.

What happens in T-tubules during muscle contraction?

The action potential travels down T-tubules, opening voltage-gated calcium channels and releasing calcium.

What is the role of the sarcoplasmic reticulum in muscle contraction?

Ca2+ diffuses out of calcium release channels; at the end of contraction, Ca2+ is pumped back into the SR.

What neurotransmitter is used at the neuromuscular junction?

Acetylcholine.

What occurs when muscular contraction ceases?

Calcium channels close, Ca2+ is pumped back into the SR, and tropomyosin blocks cross bridges from myosin to actin.

What does the term tetanus refer to?

Consistent muscle contractions in response to a constant stimulus, leading to fatigue.

What does asynchronous activation communicate about muscular contraction?

Some motor units contract while others relax, producing continuous contraction of the whole muscle.

What does the force-velocity curve indicate about muscle contraction?

As force increases, the velocity of muscle shortening decreases. In order for a muscle to contract, it must generate a force greater than the opposing forces

What is an isotonic contraction?

Muscle fibers shorten when the tension produced is greater than the load.

What is an isometric contraction?

Muscles do not shorten when exerting tension because the load is too great.

What is the series elastic component in muscle?

Noncontractile parts of the muscle and tendons that must be pulled tightly for movement and can snap back to resting length.

What factors influence the strength of a muscular contraction?

Number of fibers recruited, frequency of stimulation, muscle fiber thickness, and initial fiber length.

How does muscle length influence tension/force development?

Tension is maximal at normal resting length (80-120% of rest length). Too short leads to increased fluid pressure and increased distance between actin and myosin. Too long leads to decreases in muscle tension

What is energy (ATP) used for in skeletal muscle?

70% for myosin ATPase contraction, 30% for Ca2+ pump and Na+/K+ pump.

What is the primary energy substrate for skeletal muscle at rest?

Fatty acids via aerobic respiration.

What adaptations occur in skeletal muscle during high exercise intensity?

GLUT4 channels are inserted into the sarcolemma for increased glucose intake.

What factors influence maximal oxygen uptake in muscle tissue?

Exercise intensity, age, sex, size, and athletic training.

What are characteristics of Slow Twitch (Type I) muscle fibers?

Slower contraction speed, fatigue-resistant, rich capillary supply, high oxidative capacity, more mitochondria, more enzymes and more myoglobin

What are characteristics of Fast-Oxidative (Type IIA) muscle fibers?

High oxidative capacity, more resistant to fatigue, red fibers.

What are characteristics of Fast-Glycolytic (Type IIX) muscle fibers?

Faster contraction speed, quick fatigue, less capillaries and mitochondria, white fibers. Have more glycogen stores, metabolizes anaerobically, greatest rate of ATP and phosphocreatine consumption

What contributes to muscular fatigue?

Accumulation of extracellular K+ reduces excitability from a decrease in the membrane potential, reduced SR Ca2+ release from buildup of ATP, lactic acid accumulation, and decreased muscle glycogen.

How does aging affect muscular function?

Reduced muscle mass, especially type II fibers, and reduced capillary blood supply.

What stimulus do muscle spindles respond to?

Changes in muscle length by stretching.

What stimulus do Golgi Tendon Organs respond to?

Tension a muscle puts on a tendon.

Describe the structure of a muscle spindle.

Intrafusal fibers with no contractile units, parallel to extrafusal fibers. Intrafusal has primary and secondary.

How do muscle spindles produce their response?

Alpha motor neurons innervate extrafusal fibers; gamma motor neurons innervate intrafusal fibers.