Unit 7 Asma Nervous And Endocrine Function

Front 1

Describe the basic physics of membrane potentials.

Back 1

Let us assume that a membrane is permeable to potassium ions but not to any other ions, and the potassium concentration is high inside the membrane and low outside. There is a strong tendency for the potassium ions to diffuse outward, down their concentration gradient. As they do so, they carry positive charges to the outside, thus creating a state of electropositivity outside the membrane and electronegativity inside because of negative anions that remain behind. The new potential difference, positive outside and negative inside, repels the positively charged potassium ions that are diffusing outward back in the opposite direction. Within a millisecond or so, the potential change becomes great enough to block further net diffusion to the exterior despite the high potassium ion concentration gradient.

Front 2

What do positively charged sodium ions do if the membrane is permeable to sodium ions but not any other ions and the sodium concentration is high outside the membrane and low inside?

Back 2

The positively charged sodium ions readily diffuse down their concentration gradient to the inside of the membrane creating a membrane potential of opposite polarity than potassium, with negative outside and positive inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside.

Front 3

What can cause the creation of a membrane potential?

Back 3

Thus, in both parts we see that a concentration difference of ions across a selectively permeable membrane can, under appropriate conditions, cause the creation of a membrane potential. The rapid changes in membrane potentials observed during the course of nerve and muscle impulse transmission result from the occurrence of rapidly changing diffusion

Front 4

Describe the use for/purpose of the Nernst and Goldman equations.

Back 4

The potential level across the membrane that prevents net diffusion of an ion in either direction through the membrane is called the Nernst potential for that ion. The magnitude of this potential is determined by the ratio of the ion concentrations on the two sides of the membrane – the greater this ratio, the greater the tendency for the ions to diffuse in one direction, and therefore the greater is the Nernst potential.

Front 5

State the Nernst and Goldman equations.

Back 5

EMF= Electromotive Force
EMF (millivolts) = (+/-) 61 log * Conc. Inside/Conc. Outside

Front 6

When a membrane is permeable to several different ions, the diffusion potential that develops depends on what three factors?

Back 6

(1) the polarity of the electrical charge of each ion, (2) the permeability of the membrane to each ion, and (3) the concentrations of the respective ions on the inside and outside of the membrane.

Front 7

What does the Goldman equation give?

Back 7

The Goldman equation gives the calculated membrane potential when taking into account several different ions interacting with a membrane at the same time.

Front 8

When can membrane potential be calculated?

Back 8

The membrane potential and be calculated when two univalent positive ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl-), are involved. Sodium, potassium, and chloride ions are the ions most importantly involved in the development of membrane potentials in nerve and muscle fibers as well as in neuronal cells in the CNS.

Front 9

Describe the determinants of a resting membrane potential.

Back 9

The membrane potential of large nerve fibers when they are not transmitting nerve signals is about -90 millivolts. That is, the potential inside the fiber is 90 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber. The membrane potential is caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na+ K+ pump.

Front 10

What are the concentration gradients of sodium and potassium seen inside and outside of a resting nerve fiber are as follows

Back 10

Outside Inside
Na+ 142 mEq/L 14 mEq/L
K+ 4 mEq/L 140 mEq/L

Front 11

Define potassium-sodium leak channels

Back 11

A channel protein spans the cell membrane through which potassium and sodium ions can leak, called potassium-sodium “leak” channels. The emphasis is on potassium leakage out of the nerve fiber because, on the average, the channels are far more permeable to potassium than to sodium, normally about 100 times as permeable.

Front 12

Define the sodium-potassium pump

Back 12

All cell membranes of the body have a powerful sodium- potassium pump that continually pumps sodium to the outside of the fiber and potassium to the inside. This is an electrogenic pump because more positive charges are pumped to the outside than to the inside (three Na+ ions to the outside for each two K+ ions to the inside), leaving a net deficit of positive ions on the inside; this causes a negative charge inside the cell membrane.

Front 13

If the membrane is highly permeable to potassium but only slightly permeable to sodium which will contribute more to the membrane potential?

Back 13

Tthe diffusion of potassium will contribute far more to the membrane potential than will the diffusion of sodium.

Front 14

In the normal nerve fiber the permeability of the membrane to potassium is about ____ times as great as to sodium.

Back 14

100 times

Front 15

Using the value of 100 times more potassium than sodium in the normal nerve fiber in the Goldman equation gives an internal membrane potential of what?

Back 15

-86 millivolts

Front 16

There is continuous pumping of ____ sodium ions to the outside for each ____ potassium ions pumped to the inside of the membrane. What does this cause?

Back 16

Three to two. The fact that more sodium ions are bring pumped to the outside than potassium to the inside causes a continual loss of positive charges from inside the membrane; this creates an additional degree of negativity (about -4 millivolts additional) on the inside beyond that which can be accounted for by diffusion alone.

Front 17

What is the net resting membrane potential and why?

Back 17

The diffusion potentials alone caused by potassium and sodium diffusion would give a membrane potential of about -86 millivolts, almost all of this being determined by potassium diffusion. Then, an additional -4 millivolts is contributed to the membrane potential by the electrogenic Na+ K+ pump, giving a net resting membrane potential of -90 millivolts.

Front 18

Nerve signals are transmited by what?

Back 18

Nerve signals are transmitted by action potential.

Front 19

What are action potentials?

Back 19

Rapid changes in the membrane potential.

Front 20

Each action potential begins with what?

Back 20

Each action potential begins with a sudden change from the normal resting negative potential to a positive membrane potential and then ends with an almost equally rapid change back to the negative potential.

Front 21

What are the 3 stages for action potentials?

Back 21

Resting stage, depolarization stage, and repolarization stage

Front 22

Describe the resting stage of an action potential

Back 22

RESTING STAGE: This is the resting membrane potential before the action potential occurs. The membrane is said to be “polarized” during this stage because of the large negative membrane potential that is present.

Front 23

Describe the depolarization stage of an action potential

Back 23

DEPOLARIZATION STAGE: At this time, the membrane suddenly becomes permeable to sodium ions, allowing tremendous numbers of positively charged sodium ions to flow to the interior of the axon. The normal “polarized” state of -90 millivolts is lost, with the potential rising rapidly in the positive direction. This is called depolarization.

Front 24

Describe the repolarization stage of an action potential

Back 24

REPOLARIZATION STAGE: Within a few 10,000ths of a second after the membrane becomes highly permeable to sodium ions. The sodium channels begin to close and the potassium channels open more than they normally do. Then, rapid diffusion of potassium ions to the exterior re-establishes the normal negative resting membrane potential. This is called repolarization.

Front 25

What is the necessary actor in causing both depolarization and repolarization of the nerve membrane during the action potential

Back 25

The voltage-gated sodium channel.

Front 26

The voltage-gated potassium channel also plays and important role in what?

Back 26

Increasing the rapidity of repolarization of the membrane.

Front 27

Describe the functioning of the voltage-gated sodium and potassium channels

Back 27

Voltage-Gated Sodium Channel: This channel has two gates, one near the outside of the channel called the activation gate and another near the inside called the inactivation gate. These two voltage-gated channels are in addition to the Na+ K+ pump and the Na+ K+ leak channels.

Front 28

Describe the activation gate of the voltage-gated sodium channel

Back 28

The activation gate is closed in the normal resting membrane, which prevents any entry of sodium ions to the interior of the fiber through these sodium channels. When the membrane potential becomes less negative than during the resting state, rising from -90 millivolts toward somewhere between -70 and -50 millivolts, this causes a sudden conformational change in the activation gate, flipping it to the open position. This is called the activated state; during this sate, sodium ions can literally pour inward through the channel, increasing the sodium permeability of the membrane as much as 500 to 5000 fold.

Front 29

Describe the inactivation gate of the voltage-gated sodium channel

Back 29

The same increase in voltage that opens the activation gate also closes the inactivation gate. The inactivation gate, however, closes a few 10,000ths of a second after the activation gate opens. Therefore, after the sodium channel has remained open for a few 10,000ths of a second, it closes and sodium ions can no longer pour to the inside of the membrane. At this point, the membrane potential begins to recover back toward the resting membrane state, which is the repolarization process.

Front 30

Describe the voltage-gated potassium channel

Back 30

VOLTAGE-GATED POTASSIUM CHANNEL: During the resting state, the gate of the potassium channel is closed, and potassium ions are prevented from passing through this channel to the exterior. When the membrane potential rises from -90 millivolts toward zero, this voltage change causes a slow conformational opening of the gate and allows increased potassium diffusion outward through the channel. Because of the slowness of opening of these potassium channels, they mainly open just at that same time that the sodium channels are beginning to close because of inactivation. Thus, the decrease in sodium entry to the cell and simultaneous increase in potassium exit from the cell greatly speed the repolarization, leading to full recovery of the resting membrane potential within a few 10,000ths of a second.

Front 31

Describe the positive-feedback aspects of the action potential.

Back 31

If any event causes enough initial rise in the membrane potential from -90 millivolts up toward the zero level, the rising voltage itself will cause many voltage-gated sodium channels to begin opening. This allows rapid inflow of sodium ions, which causes still further rise of the membrane potential, this opening still more voltage-gated sodium channels and allowing more streaming of sodium ions to the interior of the fiber. This process is a positive-feedback vicious circle that, once the feedback is strong enough, will continue until all the voltage-gated sodium channels have become activated (opened).

Front 32

Describe the threshold for the propagation of an action potential.

Back 32

An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback vicious circle. This occurs when the number of Na+ ions entering the fiber becomes greater than the number of K+ ions leaving the fiber. A sudden rise in membrane potential of 15 to 30 millivolts usually is required and therefore, a sudden increase in the membrane potential in a large nerve fiber from -90 millivolts up to about -65 millivolts usually causes the explosive development of the action potential. This level of -65 millivolts is said to be the threshold for stimulation.

Front 33

Describe the all-or-nothing principle of action potentials.

Back 33

Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process will travel over the entire membrane if conditions are right, or it might not travel at all if conditions are not right. This is called the all-or-nothing principle, and it applies to all normal excitable tissues. Occasionally, the action potential will reach a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane. When this occurs, the spread of depolarization stops.

Front 34

Describe the mechanism for restoring the sodium/potassium concentrations following action potentials.

Back 34

The transmission of each impulse along the nerve fiber reduces infinitesimally the concentration differences of sodium and potassium between the inside and outside of the membrane because of diffusion of sodium ions to the inside during depolarization and diffusion of potassium ions to the outside during repolarization. For a single action potential, this effect is so minute that is cannot be measured. Indeed, 100,000 to 50 million impulses can be transmitted by nerve fibers before the concentration differences have run down to the point that action potential conduction ceases. Even so, with time it becomes necessary to re-establish the sodium and potassium membrane concentration differences. This is achieved by the action of the Na+ K+ pump in the same way as that described earlier for original establishment of the resting potential. That is, the sodium ions that have diffused to the interior of the cell during the action potentials and the potassium ions that have diffused to the exterior are retuned to their original state by the Na+ K+ pump. Because this pump requires energy for operation, the process of “recharging” the nerve fiber is an active metabolic one, using energy derived from the adenosine triphosphate (ATP) energy system of the cell.

Front 35

What is a special feature of the sodium-potassium ATPase pump

Back 35

A special feature of the sodium-potassium ATPase pump is that its degree of activity is strongly stimulated when excess sodium ions accumulate inside the call membrane. In fact, the pumping activity increases approximately in proportion to the third power of the sodium concentration.

Front 36

What is a myelin sheath?

Back 36

The axon of some nerve cells are surrounded by a myelin sheath that is often thicker than the axon itself, and about once every 1 to 2 millimeters along the length of the axon the myelin sheath is interrupted by a node of Ranvier. The myelin sheath is deposited around the axon by Schwann cells that lay down multiple layers of cellular membrane containing the lipid substance sphingomyelin. This substance is an excellent electrical insulator.

Front 37

What is the node of Ranvier?

Back 37

At the juncture between each two successive Schwann cells along the axon, a small uninsulated are only 2 to 3 micrometers in length remains where ions can still flow with ease between the extra cellular fluid and the axon.

Front 38

What is saltatory conduction and why is it important?

Back 38

Even though ions cannot flow significantly through the thick myelin sheaths of myelinated nerves, they can flow with considerable ease through the nodes of Ranvier. Therefore, action potential can occur only at the nodes. Yet the action potentials are conducted from node to node, this is called saltatory conduction. That is, electrical current flows through the surrounding extracellular fluids outside the myelin sheath as well as through the axoplasm from node to node, exciting successive nodes one after another. Thus, the nerve impulse jumps down the fiber.

Front 39

What are the two reasons saltatory conduction is of value?

Back 39

Saltatory conduction is of value for two reasons. First, by causing the depolarization process to jump long intervals along the axis of the nerve fiber, this mechanism increases the velocity of nerve transmission in myelinated fibers as much as 5-fold to 50-fold. Second saltatory conduction conserves energy for the axon because only the nodes depolarize, allowing perhaps a hundred times smaller loss of ions than would otherwise be necessary and therefore requiring little metabolism for reestablishing the sodium and potassium concentration differences across the membrane after a series of nerve impulses. The excellent insulation afforded by the myelin membrane and the 50-fold decrease in membrane capacitance allows repolarization to occur with little transfer of ions.

Front 40

Describe the functional anatomy of the cochlea

Back 40

The cochlea is a system consisting of three tubes coiled side by side: (1) the scala vestibuli, (2) the scala media, and (3) the scala tympani. The scala vestibuli and scala media are separated from each other by Reissner’s membrane (also called the vestibular membrane). The scala tympani and scala media are separated from each other by the basilar membrane. On the surface of the basilar membrane lies the organ of Corti, which contains a series of electromechanically sensitive cells, the hair cells. They are the receptive end organs that generate nerve impulses.

Front 41

Why are the scala vestibuli and scala media considered to be a single chamber?

Back 41

Reissner’s membrane is so thin and so easily moved that it does not obstruct the passage of sound vibrations from the scala vestibuli into the scala media. Therefore, so far as the conduction of sound is concerned, the scala vestibuli and scala media are considered to be a single chamber.

Front 42

Sound vibrations enter the scala vestibula from where?

Back 42

Sound vibrations enter the scala vestibuli from the faceplate of the stapes at the oval window. The faceplate covers the window and is connected with the window’s edges by a relatively loose annular ligament so that it can move inward and outward with the sound vibrations. Inward movement causes the fluid to move into the scala vestibuli and scala medial, and outward movement causes the fluid to move backward.

Front 43

What is the basilar membrane?

Back 43

The basilar membrane is a fibrous membrane that separated the scala medial from the scala tympani. It contains 20,000 to 30,000 basilar fibers that project from the bony center of the cochlea, the modiolus, toward the outer wall. These fibers are stiff, elastic, reed like structures that are fixed at their basal ends in the central bony structure of the cochlea ( the modiolus) but not fixed at their distal ends, except that the distal ends are embedded in the loose basilar membrane. Because the fibers are stiff and free at one end, they can vibrate like reeds of a harmonica. The lengths of the basilar fibers increase progressively as one goes from the base of the cochlea to its apex. The diameters of the fibers, on the other hand, decrease from the base to the helicotream, so that their overall stiffness decreases. As a result, the stiff, short fibers near the oval window of the cochlea vibrate best at a high frequency, whereas the long limber fibers near the tip of the cochlea vibrate best at low frequency.

Front 44

Describe the function of the organ of Corti in the acquisition of sound waves.

Back 44

The organ of Corti is the receptor organ that generates nerve impulses in response to vibration of the basilar membrane. Note that the organ of Corti lies on the surface of the basilar fibers and basilar membrane.

Front 45

What are the two types of hair cells in the organ of Corti?

Back 45

The actual sensory receptors in the organ of Corti are two types of hair cells: a single row of internal (or “inner”) hair cells, numbering about 3500 and measuring about 12 micrometers in diameter, and three to four rows of external (or “outer”) hair cells, numbering about 12,000 and having diameters of only about 8 micrometers. The bases and sides of the hair cells synapse with a network of cochlear nerve endings. The nerve fibers from these endings lead to the spiral ganglion of Corti, which lies in the modiolus (the center) of the cochlea. The spiral ganglion in turn sends axons, a total of about 30,000 into the cochlear nerve and then into the central nervous system at the level of the upper medulla.

Front 46

What part does stereocilia play in hearing?

Back 46

Minute hairs, or stereocilia, project upward from the hair cells and either touch or are embedded in the surface gel coating of the tectorial membrane which lies above the stereocilia in the scala medial. Bending the hairs in one direction depolarizes the hair cells, and being them in the opposite direction hyperpolarizes them. This in turn excites the nerve fiber synapsing with their bases. The upper ends of the hair cells are fixed tightly in a rigid structure composed of a flat plate, called the reticular lamina, supported by triangular rods of Corti, which in turn are attached tightly to the bases of the basilar fibers. Therefore, the basilar fiber, the rod of Corti, and the reticular lamina all move as a rigid unit. Upward movement of the basilar fiber rocks the reticular lamina upward an inward toward the modiolus. Then, when the basilar membrane moved downward, the reticular lamina rocks downward and outward. The inward and outward motion causes the hairs to shear back and forth against the tectorial membrane. Thus, the hair cells are excited whenever the basilar membrane vibrates.

Front 47

Describe the central nervous components of the sense of hearing.

Back 47

Nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nuclei located in the upper part of the medulla. At this point all the fibers synapse, and second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus. Some second-order fibers also pass ipsilaterally to the superior olivary nucleus on the same side. From the superior olivary nucleus, the auditory pathway then passes upward through the lateral lemniscus: some of the fibers terminate in the nucleus of the lateral lemniscus. Many bypass the nucleus and pass on to the inferior colliculus, where all or almost all of them terminate. From here, the pathway passes to the medial geniculate nucleus, where all the fibers again synapse. And finally the pathway proceeds by way of the auditory radiation of the auditory cortex, located mainly in the superior gyrus of the temporal lobe.
Several points of importance should be noted. First, signals from both ears are transmitted through the pathways of both sided of the brain with slight preponderance of transmission in the contra lateral pathway. In at least three places in the brain stem, crossing-over occurs between the two pathways. Second, many collateral fibers from the auditory tracts pass directly into the reticular activating system of the brain stem. This system projects diffusely upward in the brain stem and downward into the spinal cord and activates the entire nervous system in response to a loud sound. Other collaterals go to the vermis of the cerebellum, which is also activated instantaneously in the event of a sudden noise. Third, a high degree of spatial orientation is maintained in the fiber tracts from the cochlea all the way to the cortex.

Front 69

State the gland of origin and basic function for Growth Hormone

Back 69

(Anterior Pituitary) causes growth of almost all cells and tissues of the body.

Front 70

Describe the newer pathway of the olfactory system

Back 70

THE NEWER PATHWAY
Still a newer olfactory pathway has now been found that passes through the thalamus, passing to the dorsomedial thalamic nucleus and then to the lateroposterior quadrant of the orbitofrontal cortex. Based on studies in monkeys, this newer system probably helps especially in the conscious analysis of odor.

Front 71

State the gland of origin and basic function for Adrenocorticotropin

Back 71

(Anterior Pituitary) causes the adrenal cortex to secrete adrenocortical hormones.

Front 72

State the gland of origin and basic function for Thyroid-stimulating Hormone

Back 72

(Anterior Pituitary) causes the thyroid gland to secrete thyroxine and triidothyronine.

Front 73

List the nine primary endocrine glands.

Back 73

1.      Pituitary (size of a grape and hangs by a stalk from the inferior surface of the brain)
2.      Thyroid Gland (base of the throat, just inferior of the Adam’s apple)
3.      Parathyroid Gland (behind thyroid gland)
4.      Adrenal (bean shaped glands that curve over the top of each kidney)
5.      Pineal (roof of third ventricle of brain)
6.      Thymus (upper thorax)
7.      Islets of Langerhans in the Pancreas (pancreas which is located close to the stomach)
8.      Ovaries
9.      Testes

Front 74

State the gland of origin and basic function for Antidiuretic Hormone

Back 74

(Posterior Pituitary) also called vasopressin, causes the kidneys to retain water, thus increasing the water content of the body; also, in high concentrations, causes constriction of the blood vessels throughout the body and elevates the blood pressure.

Front 75

State the gland of origin and basic function for Cortisol

Back 75

(Adrenal Cortex) has multiple metabolic functions for control of the metabolism of proteins, carbohydrates, and fats.

Front 76

State the gland of origin and basic function for Aldosterone

Back 76

(Adrenal Cortex) reduces sodium excretion by the kidneys and increases potassium excretion, thus increasing sodium in the body while decreasing the amount of potassium.

Front 77

State the gland of origin and basic function for Thyroxine and Triidothyronine

Back 77

(Thyroid) increases the rates of chemical reactions in almost all cells of the body, thus increasing the general level of body metabolism.

Front 78

State the gland of origin and basic function for Calcitonin

Back 78

(Thyroid) promotes the deposition of calcium in the bones and there by decreases calcium concentration in the extra cellular fluid.

Front 79

State the gland of origin and basic function for Insulin

Back 79

(Islets of Langerhans in the Pancreas) promotes glucose entry into most cells of the body, in this way controlling the rate of metabolism of most carbohydrates.

Front 80

State the gland of origin and basic function for Glucagon

Back 80

(Islets of Langerhans in the Pancreas) increases the synthesis and release of glucose from the liver into the circulating body fluids.

Front 81

State the gland of origin and basic function for Parathormone

Back 81

(Parathyroid) controls the calcium ion concentration in the extracellular fluid by controlling (a) absorption of calcium from the gut, (b) excretion of calcium by the kidneys, and (c) release of calcium from the bones.

Front 82

State the gland of origin and basic function for Epinephrine and norepinephrine

Back 82

(Adrenal medulla) secreted in response to stimulation by the sympathetic nervous system when you feel threatened physically or emotionally bringing about the “fight-or-flight” response. These hormones increase heart rate, blood pressure, and blood flow levels and dilate the small passageways of the lungs. These events result in more oxygen and glucose in the blood and a faster circulation of blood to the body organs.

Front 83

Describe the basic mechanism by which hormones affect target tissues.

Back 83

Almost without exception, a hormone affects its target tissues by first activating target receptors in the tissue cells. This alters the function of the receptor itself, and this receptor is then the direct cause of the hormonal effects.

Front 84

Describe the hormonal effect on the change in membrane permeability

Back 84

Virtually all the neurotransmitter substances, which are themselves local hormones, combine with receptors in the postsynaptic membrane. Almost always this causes a conformational change in the protein structure of the receptor, usually opening or closing a channel for one or more ions. A few of the general hormones also have similar effects in opening or closing membrane ion channels.

Front 85

Describe the hormonal effect on the activation of an intracellular enzyme when a hormone combines with a membrane receptor

Back 85

Another common effect of membrane receptor binding is activation (or occasionally inactivation) of an enzyme immediately inside the cell membrane. One example of a widely used hormonal control of cell function is for the hormone to bind with a special transmembrane receptor that then becomes the activated enzyme adenyl cyclase at its end that protruded to the interior of the cell. This cyclase in turn causes the formation of the substance cyclic adenosine monophosphate (cAMP). And cAMP has a multitude of effects inside the cell to control cell activity. The cAMP is called a “second messenger” because it is not the hormone itself that directly institutes the intracellular changes; instead, it is the cAMP that serves as a “second messenger” to cause these effects.

Front 86

Describe the hormonal effect on the activation of genes by binding with intracellular receptors

Back 86

Several hormones, especially the steroid hormones and thyroid hormones, bind with protein receptors inside the cell, not in the cell membrane. The activated hormone-receptor complex then binds with or activates specific portions of the DNA strands of the cell nucleus which in turn initiates transcription of specific genes to from messenger RNA. Therefore, minutes, hours, or even days after the hormone has entered the cell, newly formed proteins appears in the cell and become the controllers of new of increased cellular functions.

Front 87

Describe the second messenger mechanisms for mediating hormonal functions.

Back 87

One of the means by which hormones exert intracellular actions is to cause the “second messenger” cAMP to be formed inside the cell membrane. Then the cAMP in turn causes all or most of the intracellular effects of the hormones. Thus, the only direct effect that the hormone has on the cell is to activate a single type of membrane receptor. The second messenger does the rest.

Front 88

What are 2 othersecond messangers used by the different hormones?

Back 88

(a) calcium ions and associated calmodulin and (b) products of membrane phospholipid breakdown.

Front 89

What is cyclic amp?

Back 89

The stimulating hormone first binds with a specific “receptor” for that hormone on the membrane surface of the target cell. The specificity of the receptor determines which hormone will affect the target cell. After binding with the membrane receptor, the portion of the receptor that protrudes to the interior of the cell membrane is activated to become the protein enzyme adenyl cyclase. This enzyme in turn causes immediate conversion of a small amount of the cytoplasmic adenosine triphosphate into cAMP, which is the compound 3’, 5’-adenosine monophosphate. Once cAMP is formed inside the cell, it activates still other enzymes. In fact, it usually activates a cascade of enzymes. In a few instances, cyclic guanosine monophosphate (cGMP), which is only slightly different for cAMP, serves in a similar manner as a “second messenger”. In this way, even the slightest amount of hormone acting on the cell surface can initiate a powerful cascading activating force of the entire cell.

Front 90

What is calcium ion and calmodulin?

Back 90

Another second messenger system operates in response to the entry of calcium ions into cells. The calcium entry may be initiated by a change in the membrane electrical potential that opens membrane calcium channels or by hormones interacting with membrane receptors that similarly open calcium channels. On entering the cell, the calcium ions bind with a protein called calmodulin. This protein has four calcium sites; when as many as three or four of these sites have bound with calcium, a conformational change occurs that activates the calmodulin, causing multiple effects inside the cell in the same way that cAMP functions. However, it activates a different set of enzymes from those activated by cAMP, thus causing a different set of intracellular reactions.

Front 91

What is phospholipid breakdown

Back 91

Some hormones activate transmembrane receptors that then activate the enzyme phospholipase C attached to the inside projections of the receptors. This enzyme in turn causes some phospholipids in the cell membrane itself to split into smaller substances that have widespread “second messenger” intracellular effects. The types of hormones that cause this effect are mainly local hormones, most notably hormonal factors released by tissue immune and allergic reactions.

Front 92

Descrine the pituitary gland

Back 92

The pituitary gland, also called the hypophysis, is a small gland – about 1 cm in diameter and 0.5 to 1 gm in weight – that lies in the sella turcica, a bony cavity at the base of the brain, and is connected to the hypothalamus by the pituitary (or hypophysial) stalk. Physiologically, the pituitary gland is divisible into two distinct portions: the anterior pituitary, also known as the adenohypophysis, and the posterior pituitary, also known at the neurohypophysis. Six important hormones plus several less important ones are secreted by the anterior pituitary and two important hormones are secreted by the posterior pituitary.

Front 93

What contolls the secretion by the pituitary?

Back 93

Almost all secretion by the pituitary is controlled by either hormonal or nervous signals from the hypothalamus. Secretion from the posterior pituitary is controlled by nerve signals that originate in the hypothalamus and terminate in the posterior pituitary. In contrast, secretion by the anterior pituitary is controlled by hormones called hypothalamic releasing and inhibitory hormones (or factors) secreted within the hypothalamus itself and then conducted to the anterior pituitary thought minute blood vessels called hypothalamic-hypophysial portal vessels. In the anterior pituitary, these releasing and inhibitory hormones act on the glandular cells to control their secretion.

Front 94

What does the hypothalamus do?

Back 94

The hypothalamus in turn receives signals from almost all possible sources in the nervous system. Thus, when a person is exposed to pain, a portion of the pain signal is transmitted into the hypothalamus. Likewise, when a person experiences some powerful depressing or exciting thought, a portion of the signal is transmitted into the hypothalamus. Olfactory stimuli denoting pleasant or unpleasant smells transmit strong signal components directly and through the amygdaloid nuclei into the hypothalamus. Even the concentrations of nutrients, electrolytes, water, and various hormones in the blood excite or inhibit various portions of the hypothalamus. Thus, the hypothalamus is a collecting center for information concerned with the internal well-being of the body, and in turn, much of this information is used to control secretions of the many globally important pituitary hormones.

Front 95

Where is the thyroid gland located?

Back 95

The thyroid gland, which is located immediately below the larynx on either side of and anterior to the trachea

Front 96

What hormones does the thyroid gland secrete?

Back 96

Two significant hormones, thyroxine and triiodothyronine, commonly called T4 and T3 that have the profound effect of increasing the metabolic rate of the body.

Front 97

Thyroid secretion is controlled primarily by what?

Back 97

Thyroid-stimulating hormone (TSH) secreted by the anterior pituitary gland

Front 98

What are the percentages of the hormones secreted by the thyroid gland?

Back 98

About 93% of the metabolically active hormones secreted by the thyroid gland is thyroxine and 7% is triiodothyronine. However, almost all the thyroxine is eventually converted to triiodothyronine in the tissues, so that both are important functionally.

Front 99

What is the difference in functions between thyroxine and triiodothyronine?

Back 99

The functions of these two hormones are qualitatively the same, but they differ in rapidity and intensity of action. Triiodothyronine is about four times a potent as thyroxine, but it is present in the blood in much small quantities and persists for a much shorter time than does thyroxine.

Front 100

How much iodide is required to produce normal quantities of thyroxine?

Back 100

To form normal quantities of thyroxine, about 50 milligrams of ingested iodine in the form of iodides are required each year, or about 1 mg/week. To prevent iodine deficiency, common table salt is iodized with about 1 part sodium iodide to every 100,000 parts sodium chloride.

Front 101

Describe the thyroid cells

Back 101

The thyroid cells are typical protein-secreting glandular cells. The endoplasmic reticulum and Golgi apparatus synthesize and secrete into the follicles a large glyco-protein molecule called thyroglobulin. Each molecule of thyroglobulin contains 70 tyrosine amino acids, and they are the major substrates that combine with iodine to form the thyroid hormones, which form within the thyroglobulin molecule. That is, the thyroxine and triiodothyronine hormones formed from the tyrosine amino acids remain part of the thyroglobulin molecule during synthesis of the thyroid hormones and even afterward as stored hormones in the follicular colloid.

Front 102

Is thyroglobulin relased into the circulating blood? List the process

Back 102

Thyroglobulin itself is not released into the circulating blood in measurable amounts; instead, thyroxine and triiodothyronine are first cleaved from the thyroglobulin molecule, and then these free hormones are released. This process occurs as follows: Pinocytic (fluid) vesicles are formed that contain protein digesting enzymes and the thyroglobulin molecules. The thyroglobulin molecules are digested and release the thyroxine and triiodothyronine, which then diffuse through the base of the thyroid cell into the surrounding capillaries. Thus the thyroid hormones are released into the blood.

Front 103

What is the general effect of the thyroid hormones?

Back 103

The general effect of the thyroid hormones is to cause nuclear transcription of large numbers of genes. Therefore, in virtually all cells of the body, great numbers of protein enzymes, structural proteins, transport proteins, and other substance increase. The net result of all this is a generalized increase in functional activity throughout the body. The thyroid hormones increase the metabolic activities of all or almost all the tissues of the body. The basal metabolic rate can increase to 60 to 100 % above normal when large quantities of the hormones are secreted. The rate of utilization of foods for energy is greatly accelerated. Although the rate of protein synthesis is increased, at the same time the rate of protein catabolism is also increased. The growth rate of young people is greatly accelerated. The mental processes are excited, and the activity of most of the endocrine glands is increased.