Topic 2: Chpt 6-7 Flashcards
(177 cards)
1
Q
What are the four basic methods of cell-to-cell communication in the body?
A
-Local communication via gap junctions, allowing direct cytoplasmic transfer of signals between adjacent cells.
-Contact-dependent signals, occurring when surface molecules on one cell bind to surface molecules on another cell.
-Communication through chemicals that diffuse through the extracellular fluid to act on nearby cells.
-Long-distance communication, utilizing chemical and electrical signals carried by nerve cells and chemical signals transported in the blood.
2
Q
How are gap junctions formed, and what molecules can pass through them?
A
Gap junctions are formed from the union of membrane-spanning proteins called connexins on two adjacent cells. These connexins create protein channels called connexons, through which ions and small molecules such as amino acids, ATP, and cyclic AMP (cAMP) can diffuse directly from the cytoplasm of one cell to the cytoplasm of the next.
3
Q
What is the function of connexins in gap junctions?
A
Connexins are membrane-spanning proteins that form the protein channels (connexons) of gap junctions. They allow for the direct transfer of electrical and chemical signals between adjacent cells, enabling coordinated responses among cell populations.
4
Q
How can the movement of molecules and electrical signals through gap junctions be regulated?
A
the movement of molecules and electrical signals through gap junctions can be modulated or shut off completely. While ions and small molecules can freely diffuse through open gap junctions, larger molecules are unable to pass. Additionally, the opening and closing of gap junctions can be regulated, influencing the extent of communication between neighboring cells.
5
Q
Where are gap junctions found in the body, and what is their significance?
A
Gap junctions are found in almost every cell type in mammals, including heart muscle, some types of smooth muscle, lung, liver, and neurons of the brain. Their widespread presence underscores their importance in facilitating direct cell-to-cell communication, essential for coordinated physiological processes and responses.
6
Q
What is contact-dependent signaling, and where does it occur?
A
Contact-dependent signaling involves the binding of surface molecules on one cell membrane to a membrane protein of another cell. It occurs in various biological processes, including immune responses and during growth and development, such as in the extension of nerve cell projections
7
Q
What are cell adhesion molecules (CAMs), and how do they participate in cell-to-cell signaling?
A
Cell adhesion molecules (CAMs) are proteins initially recognized for their role in cell-to-cell adhesion. They have been found to act as receptors in cell-to-cell signaling, facilitating communication between neighboring cells. CAMs are linked to the cytoskeleton or intracellular enzymes, allowing for bidirectional signal transmission across cell membranes.
8
Q
What is another term for contact-dependent signaling?
A
Contact-dependent signaling is also known as juxtacrine signaling. It describes the process by which signals are transmitted directly between neighboring cells through physical contact between their membrane-bound molecules
9
Q
How do cell adhesion molecules transfer signals across cell membranes?
A
Cell adhesion molecules (CAMs) transfer signals across cell membranes through linkages to the cytoskeleton or intracellular enzymes. These linkages allow for the bidirectional transmission of signals, enabling coordinated responses between adjacent cells.
10
Q
Can you provide an example of contact-dependent signaling in biological processes?
A
An example of contact-dependent signaling is observed during growth and development when nerve cells extend long projections that must grow from the central axis of the body to the distant ends of developing limbs. In this process, interactions between membrane-bound molecules on neighboring cells guide the directional growth of nerve cell extensions.
11
Q
What is paracrine signaling?
A
Paracrine signaling involves the release of chemical signals by a cell, which act on nearby cells in the immediate vicinity of the secreting cell. These signals, called paracrine signals, diffuse through the interstitial fluid to reach their target cells
12
Q
Define autocrine signaling.
A
Autocrine signaling occurs when a cell releases chemical signals that act on the same cell that secreted them. The signals, known as autocrine signals, bind to receptors on the secreting cell, affecting its own behavior or function.
13
Q
What is the function of hormones in the endocrine system?
A
Hormones are chemical signals secreted into the bloodstream by endocrine glands. They travel throughout the body and interact with target cells that possess specific receptors for the hormone. Hormones regulate various physiological processes and coordinate the activities of different organs and tissues.
14
Q
How do cells become target cells for hormones?
A
Target cells for hormones are those cells that possess specific receptors for the hormone. These receptors are typically proteins located on the cell membrane or inside the cell. Only cells with receptors for a particular hormone can respond to its signaling effects.
15
Q
What is the difference between a neurotransmitter and a neuromodulator?
A
A neurotransmitter is a neurocrine molecule that diffuses across a narrow extracellular space from a neuron to a target cell and has a rapid-onset effect. In contrast, a neuromodulator acts more slowly as an autocrine or paracrine signal, influencing the activity of nearby neurons or modulating neurotransmitter release
16
Q
What is a neurohormone, and how does it differ from other neurocrine molecules?
A
A neurohormone is a neurocrine molecule that diffuses into the bloodstream for body-wide distribution. Unlike neurotransmitters, which act locally, neurohormones have systemic effects and can influence distant target cells throughout the body.
17
Q
What are cytokines, and how have they been broadly classified?
A
Cytokines are regulatory peptides that modulate immune responses and control cell development and differentiation. They are characterized by their structure of four or more α-helix bundles. Cytokines have been classified into families, including interferons, interleukins, colony-stimulating factors, growth factors, tumor necrosis factors, and chemokines.
18
Q
What is the primary association of cytokines in the body?
A
Cytokines are primarily associated with immune responses, such as inflammation. However, they also play crucial roles in cell development and differentiation processes.
19
Q
How do cytokines differ from classic hormones?
A
Cytokines differ from classic hormones in several ways. Firstly, cytokines are not produced exclusively by specialized epithelial cells; instead, any nucleated cell can secrete cytokines at some point in its life span. Secondly, cytokines are made on demand, unlike protein or peptide hormones that are pre-made and stored in endocrine cells until needed. Finally, the intracellular signal pathways for cytokines typically differ from those for hormones.
20
Q
What are the common features shared by all signal pathways?
A
-The signal molecule, also known as a ligand, binds to a protein receptor. The ligand serves as the first messenger, bringing information to the target cell.
-Ligand-receptor binding activates the receptor.
-The activated receptor subsequently activates one or more intracellular signal molecules.
-The final signal molecule in the pathway generates a response by modifying existing proteins or initiating the synthesis of new proteins.
21
Q
How do lipophilic and lipophobic signal molecules differ in their interaction with cells?
A
Lipophilic signal molecules enter cells by simple diffusion through the phospholipid bilayer of the cell membrane, where they bind to cytosolic or nuclear receptors. On the other hand, lipophobic signal molecules remain in the extracellular fluid and bind to receptor proteins on the cell membrane.
22
Q
What are the major categories of membrane receptors, and how do they function?
A
The major categories of membrane receptors include chemically gated ion channels (receptor-channels), G protein-coupled receptors, receptor-enzymes, and integrin receptors. These receptors transmit information from signal molecules across the membrane to initiate an intracellular response, a process known as signal transduction.
23
Q
What is signal transduction, and how does it occur?
A
Signal transduction is the process by which an extracellular signal molecule activates a membrane receptor, leading to alterations in intracellular molecules and ultimately resulting in a cellular response. Extracellular signal molecules are known as first messengers, while the intracellular molecules form a second messenger system
24
Q
What is the role of transducers in signal transduction?
A
Transducers in biological systems, typically membrane proteins, convert the message of extracellular signals into intracellular messenger molecules. These transducers essentially “translate” the extracellular signal into a form that can initiate cellular responses.
25
The basic pattern of a signal transduction pathway involves the following events:
1. An extracellular signal molecule, known as the first messenger, binds to and activates a membrane receptor.
2. The activated membrane receptor initiates an intracellular cascade of second messengers by activating associated proteins.
3. The last second messenger in the cascade acts on intracellular targets to generate a cellular response.
26
The intracellular events in basic signal transduction pathways include:
1. Membrane receptors and associated proteins activate either:
(a.) Protein kinases, enzymes that transfer a phosphate group from ATP to a protein, thereby regulating cellular processes through phosphorylation.
(b.) Amplifier enzymes that generate intracellular second messengers.
2. Second messenger molecules can:
(a.) Alter the gating of ion channels, affecting the cell's membrane potential.
(b.) Increase intracellular calcium levels, leading to changes in protein function.
(c.) Change enzyme activity, particularly of protein kinases or protein phosphatases, which can modify protein configuration through phosphorylation or dephosphorylation.
3. Proteins modified by calcium binding and phosphorylation are responsible for the cellular response to the signal, which can include alterations in enzyme activity and the opening or closing of gated ion channels.
27
How does a signaling cascade typically start, and what does it involve?
A signaling cascade begins when a stimulus, often a signal molecule, converts an inactive molecule (A, typically the receptor) into its active form. Active A then converts an inactive molecule (B) into its active form. This process continues as active B converts inactive molecule C into active C, and so forth, until the final step where a substrate is converted into a product. Signaling cascades are common in many intracellular signaling pathways, where a series of molecular events amplify and propagate the initial signal.
28
What is signal amplification in signal transduction pathways, and why is it important?
Signal amplification in signal transduction pathways refers to the process where one signal molecule, typically the first messenger ligand, activates an amplifier enzyme, which in turn activates multiple molecules downstream. This amplification results in the production of multiple second messenger molecules, creating a larger effect compared to a 1:1 ratio between each step. Signal amplification is crucial as it enables a small amount of ligand to produce a significant physiological response, maximizing the efficiency of cellular signaling.
29
What are ligand-gated ion channels, and how do they initiate intracellular responses?
Ligand-gated ion channels are receptors typically found in nerve and muscle cells. When an extracellular ligand binds to these receptor-channel proteins, a channel gate opens or closes, altering the cell's permeability to an ion. This change in ion permeability rapidly modifies the cell's membrane potential, generating an electrical signal that affects voltage-sensitive proteins. This rapid response mechanism triggers various cellular processes, such as muscle contraction in skeletal muscle cells. Examples include acetylcholine-gated ion channels, which allow Na+ and K+ ions to flow through, leading to depolarization and muscle contraction.
30
What are G protein-coupled receptors (GPCRs), and how do they function?
G protein-coupled receptors (GPCRs) are membrane-spanning proteins that cross the phospholipid bilayer seven times. The cytoplasmic tail of the receptor protein is linked to a three-part membrane transducer molecule known as a G protein. GPCRs bind various ligands, including hormones, growth factors, olfactory molecules, visual pigments, and neurotransmitters. Upon ligand binding, GPCRs activate G proteins, leading to downstream signaling events.
31
How are G proteins activated?
G proteins are activated when they exchange guanosine diphosphate (GDP) for guanosine triphosphate (GTP). This activation occurs upon ligand binding to the associated GPCR. Inactive G proteins are bound to GDP, while activation results in the exchange of GDP for GTP, leading to conformational changes that enable G protein signaling.
32
What are the two main actions of activated G proteins?
Upon activation, G proteins either (1) open an ion channel in the membrane or (2) alter enzyme activity on the cytoplasmic side of the membrane. These actions facilitate signal transduction and downstream cellular responses to extracellular stimuli.
33
What is the significance of G proteins in signal transduction?
G proteins play a crucial role in signal transduction by mediating the response of cells to extracellular stimuli. They serve as intermediaries between activated GPCRs and downstream effector proteins, such as ion channels or enzymes. The activation of G proteins initiates signaling cascades that regulate various cellular processes.
34
What are the two most common amplifier enzymes associated with G protein-coupled receptors?
Adenylyl cyclase and phospholipase C. These enzymes are activated by G proteins upon ligand binding to GPCRs and catalyze the production of second messengers, such as cyclic AMP (cAMP) and inositol trisphosphate (IP3), which mediate intracellular signaling pathways.
35
What is the role of adenylyl cyclase in the G protein-coupled adenylyl cyclase-cAMP system?
Adenylyl cyclase is the amplifier enzyme in the G protein-coupled adenylyl cyclase-cAMP system. It converts adenosine triphosphate (ATP) to cyclic AMP (cAMP), which serves as a second messenger molecule in intracellular signaling pathways
36
What is cyclic AMP (cAMP), and what role does it play in cellular signaling?
Cyclic AMP (cAMP) is a second messenger molecule generated by adenylyl cyclase in the G protein-coupled adenylyl cyclase-cAMP system. It activates protein kinase A (PKA), which phosphorylates other intracellular proteins as part of the signal cascade, leading to cellular responses to extracellular stimuli.
37
What is the significance of the G protein-coupled adenylyl cyclase-cAMP system in cellular signaling?
The G protein-coupled adenylyl cyclase-cAMP system is a fundamental signaling pathway activated by protein hormones. It regulates various cellular processes by mediating the production of cyclic AMP (cAMP) and subsequent activation of protein kinase A (PKA), leading to phosphorylation of intracellular proteins and cellular responses to extracellular stimuli.
38
What is the role of phospholipase C (PLC) in some G protein-coupled receptor signaling pathways?
Phospholipase C (PLC) is an amplifier enzyme linked to some G protein-coupled receptors. When activated by a signal molecule, PLC converts a membrane phospholipid, phosphatidylinositol bisphosphate, into two lipid-derived second messenger molecules: diacylglycerol (DAG) and inositol trisphosphate (IP3).
39
What are the lipid-derived second messenger molecules generated by phospholipase C (PLC) activity?
Phospholipase C (PLC) activity produces two lipid-derived second messenger molecules: diacylglycerol (DAG) and inositol trisphosphate (IP3). DAG remains in the lipid portion of the membrane and interacts with protein kinase C (PKC), while IP3 enters the cytoplasm and binds to a calcium channel on the endoplasmic reticulum (ER).
40
How does diacylglycerol (DAG) function as a second messenger in cellular signaling?
Diacylglycerol (DAG) is a nonpolar diglyceride produced by phospholipase C (PLC) activity. It remains in the lipid portion of the membrane and interacts with protein kinase C (PKC), which is activated by calcium ions (Ca2+). PKC phosphorylates cytosolic proteins, propagating the signal cascade.
41
What is the role of inositol trisphosphate (IP3) in cellular signaling?
Inositol trisphosphate (IP3) is a water-soluble messenger molecule generated by phospholipase C (PLC) activity. IP3 binds to a calcium channel on the endoplasmic reticulum (ER), causing it to open and allowing calcium ions (Ca2+) to diffuse out of the ER into the cytosol, where they participate in various cellular processes.
42
How do diacylglycerol (DAG) and inositol trisphosphate (IP3) contribute to intracellular signaling pathways?
Diacylglycerol (DAG) and inositol trisphosphate (IP3) are second messenger molecules generated by phospholipase C (PLC) activity. They activate downstream signaling pathways, with DAG activating protein kinase C (PKC) and IP3 mobilizing intracellular calcium ions (Ca2+), collectively regulating numerous cellular processes.
43
What are catalytic receptors, and how do they function?
Catalytic receptors are a type of receptor-enzyme that possess both a receptor region on the extracellular side of the cell membrane and an enzyme region on the cytoplasmic side. Ligand binding to the receptor activates the intracellular enzyme, initiating signaling cascades within the cell
44
What are the two main types of catalytic receptor enzymes mentioned in the text?
The two main types of catalytic receptor enzymes mentioned are protein kinases, such as tyrosine kinase, and guanylyl cyclase. Tyrosine kinase phosphorylates tyrosine residues on target proteins, while guanylyl cyclase converts GTP to cyclic GMP (cGMP), serving as an amplifier enzyme.
45
How does the insulin receptor exemplify a catalytic receptor?
The insulin receptor is an example of a catalytic receptor where both the extracellular binding region and the intracellular enzyme region are parts of the same protein molecule. The insulin receptor possesses intrinsic tyrosine kinase activity, which is activated upon ligand binding.
46
What are cytokine receptors, and how are they associated with intracellular enzymes?
Cytokine receptors are a type of catalytic receptor associated with cytosolic enzymes. Most cytokine receptors are associated with Janus family tyrosine kinases (JAK kinases). Upon ligand binding, cytokine receptors activate JAK kinases, initiating intracellular signaling pathways.
47
What are the two categories of catalytic receptors based on the organization of their extracellular binding region and intracellular enzyme region?
In one type, such as the insulin receptor, both regions are part of the same protein molecule. In the other type, exemplified by cytokine receptors, the enzyme region is a separate protein activated by ligand binding.
48
What are integrins, and what roles do they play in cell function?
Integrins are membrane-spanning proteins involved in various cellular processes, including blood clotting, wound repair, cell adhesion and recognition in the immune response, and cell movement during development. These receptors are classified as catalytic receptors and bind to extracellular matrix proteins or ligands such as antibodies and molecules involved in blood clotting. Inside the cell, integrins attach to the cytoskeleton via anchor proteins, enabling them to transmit signals and regulate cytoskeletal organization upon ligand binding.
49
How do integrin receptors contribute to blood clotting, and what happens when these receptors are absent?
Integrin receptors play a crucial role in blood clotting by mediating platelet function, which is essential for clot formation. In individuals with inherited conditions where integrin receptors are absent, platelets—cell fragments involved in blood clotting—lack these receptors, leading to defective blood clotting. This deficiency highlights the importance of integrin receptors in ensuring proper hemostasis and clot formation.
50
Describe the structure of integrin receptors and their mechanism of action upon ligand binding.
Integrin receptors span the cell membrane and bind to extracellular matrix proteins or ligands, while inside the cell, they attach to the cytoskeleton via anchor proteins. Upon ligand binding, integrins can activate intracellular enzymes or modulate the organization of the cytoskeleton, thereby initiating signaling cascades that regulate cellular responses and functions.
51
How are cell responses controlled in basic signal transduction, and what are the categories of modified proteins involved?
Cell responses in basic signal transduction are controlled by modified proteins that can be broadly categorized into four groups: metabolic enzymes, motor proteins for muscle contraction and cytoskeletal movement, proteins regulating gene activity and protein synthesis, and membrane transport and receptor proteins. These modified proteins play essential roles in mediating the cellular responses to extracellular stimuli, thereby regulating various physiological processes within the cell.
52
What are the different ways calcium ions enter a cell?
Calcium ions can enter the cell through voltage-gated, ligand-gated, or mechanically gated calcium channels. Additionally, calcium can be released from intracellular compartments by second messengers such as IP3. Most intracellular calcium is stored in the endoplasmic reticulum, where it is concentrated by active transport.
53
What happens when calcium is released into the cytosol?
Release of calcium into the cytosol creates a calcium signal or "spark," which can be recorded using special calcium imaging techniques. The calcium ions combine with cytoplasmic calcium-binding proteins to exert various effects.
54
How does calcium exert its effects in the cell?
Calcium binds to cytoplasmic calcium-binding proteins to exert various effects. It can bind to calmodulin, regulatory proteins, ion channels, and other targets to alter enzyme activity, trigger muscle contraction, initiate exocytosis, and regulate ion channel gating.
55
Provide examples of calcium-dependent events in the cell.
Calcium-dependent events in the cell include muscle contraction initiated by calcium binding to troponin, exocytosis of secretory vesicles such as insulin release from pancreatic beta cells, and alteration of ion channel gating states, among others. Calcium entry into a fertilized egg initiates development of the embryo.
56
What are some examples of soluble gases that act as paracrine/autocrine signal molecules?
nitric oxide (NO), carbon monoxide, and hydrogen sulfide.
57
What was the initially named short-lived signal molecule produced by endothelial cells lining blood vessels?
Endothelial-derived relaxing factor (EDRF), which was later identified as nitric oxide (NO).
58
How is nitric oxide (NO) synthesized in tissues?
Nitric oxide (NO) is synthesized in tissues by the action of the enzyme nitric oxide synthase (NOS) on the amino acid arginine.
59
What is the role of nitric oxide (NO) in target cells?
Nitric oxide (NO) diffuses into target cells, where it binds to intracellular proteins. In many cases, it binds to the cytosolic form of guanylyl cyclase and causes the formation of the second messenger cGMP. Nitric oxide (NO) also acts as a neurotransmitter and neuromodulator in the brain, in addition to relaxing blood vessels.
60
What are some effects of carbon monoxide (CO) as a signal molecule?
Carbon monoxide (CO) activates guanylyl cyclase and cGMP, similar to nitric oxide (NO), but it may also work independently to exert its effects. Carbon monoxide targets smooth muscle and neural tissue.
61
What is the role of hydrogen sulfide (H2S) as a gaseous signal molecule?
Hydrogen sulfide (H2S) also acts in the cardiovascular system to relax blood vessels.
62
What are orphan receptors?
Orphan receptors are receptors that have no known ligand. Scientists are attempting to identify the ligands that bind to these orphan receptors by working backward through signal pathways.
63
What are eicosanoids, and how are they derived?
Eicosanoids are lipid-derived paracrine signals that play important roles in many physiological processes. They are derived from arachidonic acid, a 20-carbon fatty acid, and act on their target cells using G protein-coupled receptors.
64
Describe the synthesis process of eicosanoids.
The synthesis process for eicosanoids involves the arachidonic acid cascade. Arachidonic acid is produced from membrane phospholipids by the enzyme phospholipase A2 (PLA2). Arachidonic acid may act directly as a second messenger or be converted into eicosanoid paracrine signals.
65
What are the two major groups of arachidonic acid-derived paracrine molecules?
The two major groups are leukotrienes and prostanoids. Leukotrienes are produced by the enzyme lipoxygenase and play a significant role in asthma and anaphylaxis. Prostanoids, produced by the enzyme cyclooxygenase (COX), include prostaglandins and thromboxanes, which act on various tissues in the body.
66
How do nonsteroidal anti-inflammatory drugs (NSAIDs) work, and what are their potential side effects?
NSAIDs, such as aspirin and ibuprofen, prevent inflammation by inhibiting COX enzymes and decreasing prostaglandin synthesis. However, they are not specific and may cause serious side effects, such as bleeding in the stomach.
67
What is the significance of the discovery of COX1 and COX2 isozymes?
The discovery of COX1 and COX2 isozymes enabled the design of drugs that target a specific COX isozyme. By inhibiting only COX2, which produces inflammatory prostaglandins, physicians hoped to treat inflammation with fewer side effects.
68
Why are COX2 inhibitors not recommended for long-term use despite their potential benefits in reducing inflammation?
Although COX2 inhibitors target the enzyme responsible for producing inflammatory prostaglandins and were designed to have fewer side effects than traditional NSAIDs, studies have shown that some patients who take these drugs have an increased risk of heart attacks and strokes, leading to the recommendation against long-term use.
69
Besides eicosanoids, what other lipid signal molecules are involved in regulating inflammation and other cellular processes?
Sphingolipids also act as extracellular signals to help regulate inflammation, cell adhesion and migration, and cell growth and death. Like eicosanoids, sphingolipids combine with G protein-coupled receptors in the membranes of their target cells.
70
Explain the concept of agonists and antagonists in receptor-ligand interactions.
When a ligand binds to a receptor, it can either activate the receptor and elicit a response (agonist) or occupy the binding site and prevent the receptor from responding (antagonist). Agonists compete with the primary ligand for binding sites and elicit a response, while antagonists block receptor activity.
71
How do pharmacologists use the principle of competing agonists in drug design?
Pharmacologists use competing agonists to design drugs that are longer-acting and more resistant to degradation than endogenous ligands produced by the body. For example, modified estrogens in birth control pills are agonists of naturally occurring estrogens but have chemical groups added to extend their active life and protect them from breakdown.
72
Describe the general protein-binding characteristics exhibited in receptor-ligand binding.
Receptor-ligand binding exhibits specificity, competition, and saturation, similar to protein-binding reactions observed in enzymes and transporters. Receptors, like enzymes and transporters, come as families of related isoforms and have binding sites for their ligands, allowing different ligand molecules with similar structures to bind to the same receptor.
73
Provide an example illustrating the specificity of receptor-ligand binding.
An example of specificity in receptor-ligand binding involves the neurotransmitter norepinephrine and its cousin, the neurohormone epinephrine (adrenaline). Both molecules bind to a class of receptors called adrenergic receptors, demonstrating the specificity of these receptors. Adrenergic receptors come in two major isoforms designated alpha (α) and beta (β), with differing affinities for norepinephrine and epinephrine.
74
What determines the response of a target cell to a signal molecule?
The response of a target cell to a signal molecule depends primarily on its receptor and associated intracellular pathways rather than the ligand itself. Different tissues or cells may respond differently to the same signal molecule based on variations in receptor isoforms and downstream signaling pathways.
75
What is saturation of proteins and how does it impact cellular responses to chemical signals?
Saturation of proteins refers to the phenomenon where protein activity reaches a maximum rate due to the limited number of protein molecules present in cells. This phenomenon is observable with enzymes, transporters, and receptors. In cells, the number of receptors can vary between 500 and 100,000 on the cell membrane, with additional receptors present in the cytosol and nucleus. Saturation impacts cellular responses by limiting the cell's ability to respond to chemical signals, as the available receptors can become fully occupied, preventing further response to increased signal concentrations.
76
How do cells respond to abnormally high concentrations of signal molecules over time?
When cells are exposed to abnormally high concentrations of signal molecules for a sustained period, they may undergo down-regulation or desensitization. Down-regulation involves a decrease in receptor number, achieved through receptor removal from the membrane via endocytosis. Desensitization occurs when a chemical modulator binds to the receptor protein, diminishing the cell's response despite continued exposure to high signal concentrations. These regulatory mechanisms help maintain cellular homeostasis and prevent overstimulation in response to excessive signaling.
77
What is up-regulation and when does it occur?
Up-regulation is a cellular response mechanism where target cells increase the number of receptors in response to decreased ligand concentrations. This process aims to maintain a normal response level despite reduced signaling. Up-regulation involves inserting more receptors into the cell membrane, enhancing the cell's responsiveness to available neurotransmitters or other signal molecules. This mechanism may occur in various contexts, such as neuronal damage or during development, to adjust the cell's sensitivity to growth factors and other signaling molecules
78
What are some examples of diseases linked to problems with signal pathways?
Diseases can arise from alterations in receptors, G proteins, or second messenger pathways. A single change in the amino acid sequence of a receptor protein can affect its binding site, modifying its activity. For instance, beta blockers are used to block beta-adrenergic receptors for conditions like high blood pressure. Calcium-channel blockers are also utilized for hypertension treatment. Additionally, SERMs (selective estrogen receptor modulators) treat estrogen-dependent cancers, while H2 receptor antagonists decrease stomach acid secretion
79
How do cellular signal mechanisms contribute to maintaining homeostasis?
Cellular signal mechanisms play a crucial role in maintaining homeostasis, often representing only one component of the body's broader signaling systems. In local control mechanisms, a localized change triggers the release of chemical paracrine or autocrine signals, which act within the same tissue or nearby cells. However, in more complex reflex control pathways, information transmission occurs throughout the body using chemical signals, electrical signaling, or a combination of both. This comprehensive approach ensures coordinated responses to maintain internal balance and function efficiently.
80
What are Walter Cannon's four postulates regarding homeostatic control systems?
1. Role of the nervous system: The nervous system plays a vital role in maintaining the "fitness" of the internal environment, ensuring conditions conducive to normal function. It coordinates and integrates various regulated variables such as blood volume, blood osmolarity, blood pressure, and body temperature.
2. Tonic control: Some systems in the body are under tonic control, where an agent maintains moderate activity that can be adjusted up or down. This concept is likened to the volume control on a radio, where the level of activity can be increased or decreased. An example is the minute-to-minute regulation of blood vessel diameter by the nervous system.
3. Antagonistic control: Systems not under tonic control are often under antagonistic control, where factors with opposing effects counterbalance each other. This can be mediated by hormones or the nervous system. For instance, sympathetic and parasympathetic divisions of the nervous system have opposing effects on heart rate regulation.
4. Differential effects of chemical signals: One chemical signal can produce different effects in different tissues. Cannon observed that homeostatic agents antagonistic in one region of the body may be cooperative in another. This phenomenon became clearer with the understanding of cell receptors, where a single chemical signal can elicit diverse responses based on the receptor and intracellular pathway of the target cell.
81
What are the three major components of reflex pathway response loops, and how are they subdivided into detailed steps?
The three major components of reflex pathway response loops are as follows: input, integration, and output. These components can be further subdivided into seven detailed steps:
1. Stimulus: The stimulus is the disturbance or change that initiates the pathway. It could be a change in temperature, oxygen content, blood pressure, or any regulated variable.
2. Sensor or receptor: A sensor or sensory receptor continuously monitors the environment for a specific variable. When activated by a change, the sensor sends an input (afferent) signal to the integrating center.
3. Input signal: The input signal is transmitted from the sensor to the integrating center, providing information about the detected change.
4. Integrating center: The integrating center compares the input signal with the setpoint or desired value of the variable. If the variable has deviated from the acceptable range, the integrating center initiates an output signal.
5. Output signal: The output signal, also known as the efferent signal, is an electrical and/or chemical signal that travels from the integrating center to the target.
6. Target: The target, or effector, is the cell or tissue that carries out the appropriate response to bring the variable back within normal limits.
7. Response: The response is the action taken by the target in response to the output signal, aiming to restore the regulated variable to its setpoint or desired value.
82
What is the first step in a physiological response loop? How do sensory receptors differ from protein receptors involved in signal transduction?
-The first step in a physiological response loop is the activation of a sensor or receptor by a stimulus.
-Sensory receptors in neural reflexes are specialized cells, parts of cells, or complex multicellular receptors that respond to changes in their environment. They detect various stimuli throughout the body and have a threshold, below which no response loop is initiated. Unlike protein receptors involved in signal transduction, sensory receptors do not bind to signal molecules but rather respond directly to environmental changes.
83
What determines the nature of the input signal in a reflex?
The nature of the input signal in a reflex varies depending on the type of reflex. In a neural pathway, such as in response to pin touch, the input signal consists of electrical and chemical information transmitted by a sensory neuron. In contrast, in an endocrine reflex, there is no distinct input pathway because the stimulus acts directly on the endocrine cell, which functions as both the sensor and the integrating center.
84
What is the integrating center in a reflex pathway?
The integrating center in a reflex pathway is the cell responsible for receiving information about the regulated variable and initiating an appropriate response. In endocrine reflexes, the integrating center is typically the endocrine cell, whereas in neural reflexes, it is usually located within the central nervous system (CNS), consisting of the brain and spinal cord.
85
How does the integrating center function when conflicting signals are received?
When conflicting signals are received from different sources, the integrating center evaluates each signal based on its strength and importance. It then synthesizes this information to generate an appropriate response that integrates inputs from all contributing receptors. This process is akin to decision-making, where one must prioritize and decide among multiple competing options based on their significance and urgency.
86
What constitutes the output signal in the nervous system?
In the nervous system, the output signal consists of electrical and chemical signals transmitted by an efferent neuron. These signals travel along specific anatomical pathways determined by the route through which the neuron delivers its signa
87
How are output pathways in the nervous system named?
Output pathways in the nervous system are named based on the anatomical route of the nerve that carries the signal. For example, the vagus nerve carries neural signals to the heart, so we refer to the output pathway as "vagal control of heart rate."
88
What characterizes the output signal pathway in the endocrine system?
In the endocrine system, the output signal pathway is consistent—the hormone travels through the bloodstream to reach its target. Output pathways in the endocrine system are distinguished by the chemical nature of the hormone that carries the message, and they are named after the specific hormone involved.
89
What are the targets of reflex control pathways, and what types of tissues can neural pathways target?
The targets of reflex control pathways are the cells or tissues that execute the response triggered by the pathway. Neural pathways can target various types of tissues, including muscles, both endocrine and exocrine glands, and adipose tissue.
90
What are the multiple levels of response for any reflex control pathway, using the example of a neurotransmitter acting on a blood vessel?
The levels of response include the cellular response, where the target cell reacts to the signal; the tissue or organ response, where the specific tissue or organ reacts accordingly; and the systemic response, which describes the overall effect on the organism as a whole.
91
What systems mediate physiological reflex control pathways?
Physiological reflex control pathways are mediated by the nervous system, the endocrine system, or a combination of both.
92
How does neural control differ from endocrine control in terms of specificity and distribution?
Neural control is very specific because each neuron has a specific target cell or cells to which it sends its message, allowing for a clear anatomical pathway from origin to termination. In contrast, endocrine control is more general because the chemical messenger is released into the blood and can reach virtually every cell in the body, allowing multiple tissues to respond to a hormone simultaneously.
93
How do the nervous system and the endocrine system differ in terms of the signals they use to transmit information?
The nervous system uses both electrical and chemical signals. Electrical signals travel long distances through neurons, while chemical signals (neurotransmitters) are released by neurons and diffuse across small gaps to target cells. In contrast, the endocrine system solely uses chemical signals in the form of hormones, which are secreted into the blood by endocrine glands or cells.
94
How do neural reflexes compare to endocrine reflexes in terms of speed?
Neural reflexes are much faster than endocrine reflexes. Electrical signals in the nervous system travel rapidly, up to 120 meters per second, while neurotransmitters create rapid responses in milliseconds. In contrast, hormones in endocrine reflexes distribute more slowly through the circulatory system and take minutes to hours to produce a measurable response in target tissues.
95
How does the duration of neural control compare to endocrine control?
Neural control is of shorter duration than endocrine control. The neurotransmitter released by a neuron initiates a response on the target cell, but the response is usually brief because the neurotransmitter is rapidly removed from the vicinity of the receptor. In contrast, endocrine reflexes are slower to start but last longer, controlling ongoing, long-term functions of the body such as metabolism and reproduction.
96
How do control systems convey information about increasing stimulus intensity to the integrating center?
As a stimulus increases in intensity, control systems convey this information to the integrating center by adjusting the frequency of signaling through the afferent neuron in neural pathways. In the endocrine system, stimulus intensity is reflected by the amount of hormone released: the stronger the stimulus, the more hormone is released.
97
What are the steps involved in a simple neural reflex pathway, using the knee jerk reflex as an example?
In a simple neural reflex pathway, such as the knee jerk reflex, the steps involved are as follows:
1. Stimulus: A blow to the knee activates a stretch receptor.
2. Input Pathway: The sensory neuron transmits a signal to the spinal cord, acting as the integrating center.
3. Integrating Center: The spinal cord evaluates the signal.
4. Output Pathway: If the stimulus exceeds the threshold, an efferent neuron carries a signal from the spinal cord to the muscles of the thigh.
5. Effector (Target): The muscles contract, causing the lower leg to kick outward.
98
Describe a simple endocrine reflex pathway, using insulin secretion in response to changes in blood glucose level as an example.
In a simple endocrine reflex pathway, such as insulin secretion in response to changes in blood glucose level, the steps involved are as follows:
1. Sensor/Integrating Center: The pancreatic beta cells monitor blood glucose concentrations.
2. Output Pathway: When blood glucose levels increase, intracellular ATP production exceeds the threshold, leading to the secretion of insulin into the blood.
3. Target: Any cell in the body with insulin receptors responds to the hormone by initiating processes that take glucose out of the blood.
99
Explain a simple neuroendocrine reflex pathway, using the release of breast milk in response to a baby’s suckling as an example.
In a simple neuroendocrine reflex pathway, such as the release of breast milk in response to a baby’s suckling, the steps involved are as follows:
1. Sensor/Integrating Center: Sensory signals from the baby’s mouth on the nipple travel through sensory neurons to the brain.
2. Output Pathway: An electrical signal in the efferent neuron triggers the release of the neurohormone oxytocin from the brain into the circulation.
3. Target: Oxytocin causes contraction of smooth muscles in the breast, resulting in the ejection of milk.
100
What characterizes complex neuroendocrine reflex pathways?
Complex neuroendocrine reflex pathways involve multiple integrating centers and output pathways. These pathways may combine neural and endocrine reflexes or involve the coordination of multiple hormones. They can include feedback loops and feedforward mechanisms to regulate physiological processes.
101
What are hormones?
Hormones are chemical messengers secreted into the blood by specialized epithelial cells.
102
What functions are primarily under hormonal control?
Processes primarily under hormonal control include metabolism, regulation of the internal environment (temperature, water balance, ions), reproduction, growth, and development.
103
How do hormones act on their target cells?
Hormones act on their target cells in one of three basic ways: (1) by controlling the rates of enzymatic reactions, (2) by controlling the transport of ions or molecules across cell membranes, or (3) by controlling gene expression and the synthesis of proteins.
104
What are the classic steps for identifying an endocrine gland?
The classic steps for identifying an endocrine gland are: 1. Remove the suspected gland to induce a state of hormone deficiency. 2. Replace the hormone by placing the gland back or administering an extract, which should eliminate the symptoms of hormone deficiency. 3. Create a state of hormone excess to observe symptoms characteristic of hormone excess.
105
What are hormones identified by purifying extracts of glands and testing for activity called?
sometimes called classic hormones.
106
Which glands are included in the classic hormones?
Classic hormones include hormones of the pancreas, thyroid, adrenal glands, pituitary, and gonads.
107
Why have some hormones been slower to discover?
Some hormones have been slower to discover because they are secreted by endocrine cells scattered throughout the wall of the stomach or intestine, making them difficult to identify and isolate
108
What is the traditional definition of a hormone?
The traditional definition of a hormone is a chemical secreted by a cell or group of cells into the blood for transport to a distant target, where it exerts its effect at very low concentrations.
109
What has traditionally been the focus of the field of endocrinology? Besides classic endocrine glands, what other sources secrete molecules that act as hormones?
- Traditionally, the field of endocrinology has focused on chemical messengers secreted by endocrine glands, which are discrete and readily identifiable tissues derived from epithelial tissue.
-Molecules that act as hormones are also secreted by isolated endocrine cells (hormones of the diffuse endocrine system), by neurons (neurohormones), and occasionally by cells of the immune system (cytokines).
110
What is the term given to signal molecules that are secreted into the external environment?
ectohormones.
111
What are pheromones, and what is their function?
Pheromones are specialized ectohormones that act on other organisms of the same species to elicit a physiological or behavioral response. They are used for various purposes, including signaling danger, attracting mates, and marking trails to food sources.
112
What are molecules suspected of being hormones but not fully accepted as such called?
called candidate hormones.
113
What are growth factors, and how are they currently being studied in relation to hormone classification?
Growth factors are substances that influence cell growth and division. Currently, they are being studied to determine if they meet all the criteria for hormones, as many of them act locally as autocrine or paracrine signals and may not be widely distributed in the circulation.
114
Can you provide an example of a molecule that acts as a hormone in one location but as a paracrine or autocrine signal in another location?
Cholecystokinin (CCK) is an example of such a molecule. Initially discovered in extracts of the intestine where it caused gallbladder contraction, it was known only as an intestinal hormone. However, it was later found in neurons of the brain, where it acts as a neurotransmitter or neuromodulator.
115
What concentration range characterizes hormones?
Hormones typically act at concentrations in the nanomolar (10^-9 M) to picomolar (10^-12 M) range.
116
Why are some chemical signals transported in the blood not considered hormones?
Some chemical signals transported in the blood are not considered hormones because they must be present in relatively high concentrations before an effect is noticed. For example, histamine released during severe allergic reactions may act on cells throughout the body, but its concentration exceeds the accepted range for a hormone.
117
Why are cytokines not considered hormones by experts in cytokine research?
Cytokines are not considered hormones by experts in cytokine research because cytokines are synthesized and released on demand, in contrast to classic peptide hormones, which are made in advance and stored in the parent endocrine cell.
118
What is the cellular mechanism of action of hormones?
Hormones bind to target cell receptors and initiate biochemical responses.
119
How do the effects of hormones vary in different tissues or stages of development?
The effects of hormones may vary in different tissues or at different stages of development. Additionally, a hormone may have no effect at all in a particular cell.
120
Can you provide an example of a hormone with varied effects?
Insulin is an example of a hormone with varied effects. In muscle and adipose tissues, insulin alters glucose transport proteins and enzymes for glucose metabolism. In the liver, it modulates enzyme activity but has no direct effect on glucose transport proteins. In the brain and certain other tissues, glucose metabolism is totally independent of insulin.
121
How does the body prevent blood glucose levels from falling too low due to prolonged insulin activity?
By limiting insulin secretion, removing or inactivating insulin circulating in the blood, and terminating insulin activity in target cells.
122
What is indicated by a hormone's half-life in the circulation?
A hormone's half-life in the circulation indicates the amount of time required to reduce the concentration of the hormone by one-half. It is one indicator of how long a hormone remains active in the body.
123
How are hormones bound to target membrane receptors terminated?
Hormones bound to target membrane receptors can be terminated by enzymes present in the plasma that degrade peptide hormones bound to cell membrane receptors. In some cases, the receptor-hormone complex is brought into the cell by endocytosis, and the hormone is then digested in lysosomes. Additionally, intracellular enzymes can metabolize hormones that enter cells.
124
How can hormones be classified according to different schemes?
Hormones can be classified according to different schemes. One scheme groups them based on their source, another scheme divides them into those whose release is controlled by the brain and those whose release is not, while another scheme categorizes them based on the type of receptors they bind to (such as G protein-coupled receptors, tyrosine kinase-linked receptors, or intracellular receptors), and so on. Additionally, hormones can be classified into three main chemical classes: peptide/protein hormones, steroid hormones, and amino acid–derived hormones.
125
How are peptide hormones synthesized and stored within endocrine cells?
The synthesis and packaging of peptide hormones into membrane-bound secretory vesicles is similar to that of other proteins. The initial peptide that comes off the ribosome is a large inactive protein known as a preprohormone. Preprohormones contain one or more copies of a peptide hormone, a signal sequence that directs the protein into the lumen of the rough endoplasmic reticulum, and other peptide sequences that may or may not have biological activity. As the inactive preprohormone moves through the endoplasmic reticulum, the signal sequence is removed, creating a smaller, still-inactive molecule called a prohormone. In the Golgi complex, the prohormone is packaged into secretory vesicles along with proteolytic enzymes that chop the prohormone into active hormone and other fragments. These secretory vesicles containing peptides are stored in the cytoplasm of the endocrine cell until the cell receives a signal for secretion. At that time, the vesicles move to the cell membrane and release their contents by calcium-dependent exocytosis. All of the peptide fragments created from the prohormone are released together into the extracellular fluid, in a process known as co-secretion
126
What are some examples of prohormones and their active peptides?
Some prohormones, such as that for thyrotropin-releasing hormone (TRH), contain multiple copies of the hormone. Another example is pro-opiomelanocortin, which splits into three active peptides plus an inactive fragment. Additionally, proinsulin is cleaved into active insulin and an inactive fragment known as C-peptide.
127
How are peptide hormones transported in the blood and what is their typical half-life?
Peptide hormones are water soluble and dissolve easily in the extracellular fluid for transport throughout the body. Their half-life is usually quite short, in the range of several minutes.
128
Describe the cellular mechanism of action of peptide hormones.
Peptide hormones usually cannot enter the target cell due to being lipophobic. Instead, they bind to surface membrane receptors. The hormone-receptor complex initiates the cellular response through a signal transduction system. Many peptide hormones work through cAMP second messenger systems, while some receptors, like that of insulin, have tyrosine kinase activity or work through other signal transduction pathways. The response of cells to peptide hormones is usually rapid because second messenger systems modify existing proteins. These changes include opening or closing membrane channels and modulating metabolic enzymes or transport proteins. Additionally, some peptide hormones have longer-lasting effects when their second messenger systems activate genes and direct the synthesis of new proteins.
129
What is the common chemical precursor for steroid hormones?
Cholesterol serves as the common chemical precursor for steroid hormones.
130
Where are steroid hormones primarily synthesized in the body?
Steroid hormones are primarily synthesized in specific organs such as the adrenal cortex (located on the outer portion of the adrenal glands), the gonads (which produce sex steroids like estrogens, progesterone, and androgens), and the skin (which can synthesize vitamin D). Additionally, during pregnancy, the placenta serves as another source of steroid hormones.
131
How are steroid hormones synthesized and released by cells?
Cells that secrete steroid hormones have abundant smooth endoplasmic reticulum, where steroids are synthesized. Steroid hormones are lipophilic and can diffuse easily across membranes, both into and out of their parent cells. Unlike peptide hormones, steroid-secreting cells cannot store hormones in secretory vesicles. Instead, they synthesize hormones as needed, rapidly converting precursors in the cytoplasm to active hormone when stimulated. The synthesized hormones then diffuse out of the cell.
132
How are steroid hormones transported in the blood, and what is their half-life?
Steroid hormones are not very soluble in plasma, so most are transported bound to protein carrier molecules. Some hormones have specific carriers, while others bind to general plasma proteins like albumin. This binding protects the hormone from degradation and extends its half-life. For example, cortisol, a hormone from the adrenal cortex, has a half-life of 60–90 minutes. Binding to carrier proteins blocks the entry of steroid hormones into target cells because carrier proteins cannot diffuse through membranes. Only unbound hormone molecules can enter cells. As unbound hormone leaves the plasma, carriers release bound steroid to maintain a constant ratio of unbound to bound hormone in the blood.
133
What is the cellular mechanism of action of steroid hormones?
Steroid hormone receptors are found within cells, either in the cytoplasm or nucleus. Upon hormone binding, the receptor-hormone complex translocates to the nucleus, where it acts as a transcription factor, binding to DNA and regulating gene expression. This process leads to the synthesis of new proteins, resulting in genomic effects on the target cell. Genomic effects have a lag time of up to 90 minutes between hormone-receptor binding and the first measurable biological effects. However, some steroid hormones, like estrogens and aldosterone, also have cell membrane receptors linked to signal transduction pathways, enabling rapid nongenomic responses in addition to their slower genomic effects.
134
How are amino acid-derived hormones synthesized, and what are their sources?
Amino acid-derived hormones are small molecules created from either tryptophan or tyrosine. Tryptophan-derived hormones include melatonin, produced by the pineal gland. Tyrosine-derived hormones include the catecholamines (epinephrine, norepinephrine, and dopamine) and thyroid hormones. Catecholamines are modifications of a single tyrosine molecule, while thyroid hormones combine two tyrosine molecules with iodine atoms.
135
How do catecholamines and thyroid hormones differ in their mechanisms of action?
Catecholamines, such as epinephrine, norepinephrine, and dopamine, are neurohormones that bind to cell membrane receptors similar to peptide hormones. They act quickly and produce rapid responses in target cells. In contrast, thyroid hormones, produced by the thyroid gland, behave more like steroid hormones. They have intracellular receptors that activate genes, leading to slower genomic effects on target cells.
136
What are the components of reflex pathways in the endocrine system, and how do simple endocrine reflexes work?
Reflex pathways in the endocrine system consist of a stimulus, a sensor (which is often the endocrine cell itself), an input signal, integration of the signal, an output signal (usually a hormone or neurohormone), one or more targets, and a response. In simple endocrine reflexes, the endocrine cell acts as both the sensor and the integrating center. When the endocrine cell directly senses a stimulus, it responds by secreting its hormone. The hormone serves as the output signal, and the response typically functions as a negative feedback signal that turns off the reflex. An example of a hormone that follows a simple endocrine reflex pattern is parathyroid hormone (PTH), which regulates calcium homeostasis. The parathyroid endocrine cells monitor plasma calcium concentration and secrete PTH when calcium levels fall below a certain threshold. PTH then acts on bone, kidney, and intestine to increase plasma calcium levels, thereby completing the negative feedback loop. Other hormones, such as insulin and glucagon, also follow a simple endocrine reflex pattern, where pancreatic endocrine cells monitor blood glucose concentration and secrete insulin in response to increased glucose levels.
137
What is the pituitary gland composed of, and how did it originate?
The pituitary gland is composed of two distinct tissue types: the anterior pituitary and the posterior pituitary. The anterior pituitary, also known as the adenohypophysis, is a true endocrine gland of epithelial origin derived from embryonic tissue that formed the roof of the mouth. The posterior pituitary, or neurohypophysis, is an extension of the neural tissue of the brain.
138
What are the functions of the anterior and posterior pituitary glands?
The anterior pituitary secretes adenohypophyseal hormones, while the posterior pituitary releases neurohormones produced in the hypothalamus.
139
How did Richard Lower contribute to our understanding of the pituitary gland?
Richard Lower, an experimental physiologist at Oxford University, provided one of the earliest accurate descriptions of the pituitary gland's function. He theorized that substances produced in the brain passed down the stalk into the gland and from there into the blood.
140
What are neurohormones, and where are they secreted from?
Neurohormones are chemical signals released into the blood by neurons. They are secreted from specialized groups of neurons, including those in the adrenal medulla and the hypothalamus.
141
What role does the hypothalamus play in relation to the pituitary gland?
The hypothalamus, a region of the brain, controls many homeostatic functions and produces neurohormones that regulate hormone release from the anterior and posterior pituitary glands.
142
What are the two neurohormones stored and released by the posterior pituitary?
Oxytocin and vasopressin (also known as antidiuretic hormone or ADH)
143
Where are the neurons producing oxytocin and vasopressin located?
The neurons producing oxytocin and vasopressin are clustered together in areas of the hypothalamus known as the paraventricular and supraoptic nuclei.
144
Describe the process of oxytocin and vasopressin release from the posterior pituitary.
Oxytocin and vasopressin are synthesized and packaged into secretory vesicles in neurons in the hypothalamus. These vesicles are transported to the posterior pituitary through axons. When a stimulus reaches the hypothalamus, it triggers depolarization of the axon terminal, leading to the opening of voltage-gated Ca2+ channels. Ca2+ entry induces exocytosis, releasing the vesicle contents into the circulation.
145
What are the physiological roles of oxytocin and vasopressin?
Vasopressin regulates water balance in the body by acting on the kidneys. Oxytocin controls milk ejection during breastfeeding and uterine contractions during labor and delivery in women. Additionally, oxytocin is involved in social, sexual, and maternal behaviors.
146
How does oxytocin function as a neurotransmitter in the brain?
In addition to its role as a neurohormone released from the posterior pituitary, oxytocin is also released as a neurotransmitter or neuromodulator by a few neurons onto other neurons in the brain. This release of oxytocin in the brain is implicated in various social, sexual, and maternal behaviors, as well as potential roles in conditions like autism.
147
How many physiologically significant hormones does the anterior pituitary secrete?
The anterior pituitary secretes six physiologically significant hormones: prolactin (PRL), thyrotropin (TSH), adrenocorticotropin (ACTH), growth hormone (GH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH).
148
What are the primary targets for the hypothalamic neurohormones that control the release of anterior pituitary hormones?
The anterior pituitary glands.
149
What are trophic hormones, and how are they named?
Trophic hormones are hormones that control the secretion of other hormones. They are named based on the target tissue they "nourish" or stimulate. Trophic hormones often have names that end with the suffix "-tropin," such as gonadotropin, where the root word indicates the target tissue.
150
What is the significance of the suffix "-tropin" in hormone names?
The suffix "-tropin" in hormone names indicates that the hormone is trophic to a specific target tissue. For example, gonadotropins are hormones that stimulate the gonads.
151
What is the purpose of the hypothalamic-hypophyseal portal system?
The purpose of the hypothalamic-hypophyseal portal system is to transport hypothalamic neurohormones to the anterior pituitary without dilution, allowing for precise control of anterior pituitary hormone secretion.
152
How does the hypothalamic-hypophyseal portal system work?
The hypothalamic-hypophyseal portal system consists of two sets of capillaries connected in series by a set of small veins. Hypothalamic neurohormones enter the blood at the first set of capillaries in the hypothalamus and travel directly to the second capillary bed in the anterior pituitary via the portal veins. This arrangement maintains a concentrated amount of hormone in the portal blood, ensuring efficient delivery to the anterior pituitary.
153
Why did researchers face challenges in isolating and analyzing hormones secreted into the hypothalamic portal system?
Researchers faced challenges in isolating and analyzing hormones secreted into the hypothalamic portal system due to the minute amounts of hormone present in this system. Significant amounts of tissue were required to obtain enough hormone for analysis. For example, Roger Guillemin and Andrew Schally had to process large quantities of hypothalamic tissue, including more than 50 tons of sheep hypothalami and more than 1 million pig hypothalami donated by a major meat packer, to isolate and identify the amino acid sequence of thyrotropin-releasing hormone (TRH).
154
What is another name for the hypothalamic-hypophyseal portal system?
The hypothalamic-anterior pituitary portal system.
155
Besides the hypothalamic-hypophyseal portal system, what are the two additional portal systems in the body mentioned in the text?
The two additional portal systems in the body mentioned in the text are the portal system in the kidneys and the portal system in the digestive tract.
156
Why is the pituitary often referred to as the master gland of the body?
The pituitary gland is often referred to as the master gland of the body because its hormones control numerous vital functions, including metabolism, growth, and reproduction.
157
Which anterior pituitary hormones have hypothalamic release-inhibiting hormones?
Prolactin (PRL) and growth hormone (GH) are the only two anterior pituitary hormones with hypothalamic release-inhibiting hormones.
158
What are the primary targets of the gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH)?
The primary targets of the gonadotropins, FSH and LH, are the ovaries and testes, regulating reproductive functions.
159
What hormone controls hormone synthesis and secretion in the thyroid gland?
Thyroid-stimulating hormone (TSH), also known as thyrotropin
160
How do feedback loops operate in the hypothalamic-pituitary pathway?
In the hypothalamic-pituitary pathway, feedback loops involve long-loop negative feedback, where the hormone secreted by the peripheral endocrine gland suppresses secretion of anterior pituitary and hypothalamic hormones. Additionally, short-loop negative feedback occurs when a pituitary hormone suppresses hormone secretion by the hypothalamus. These feedback mechanisms help maintain hormonal balance in the body.
161
Define synergism in hormone interaction.
Synergism occurs when two or more hormones, acting on the same target cell, produce a combined effect that is greater than the sum of their individual effects.
162
provide an example of synergism involving hormones.
An example of synergism involving hormones is the combined effect of epinephrine, glucagon, and cortisol in elevating blood glucose levels.
163
How are synergistic effects often linked in peptide hormones?
Synergistic effects in peptide hormones are often linked to overlapping effects on target cell second messenger systems.
164
What is permissiveness in hormone interaction?
Permissiveness in hormone interaction occurs when one hormone cannot fully exert its effects unless a second hormone is present, even though the second hormone has no apparent action on its own.
165
Provide an example of permissiveness in hormone interaction.
An example of permissiveness in hormone interaction is the role of thyroid hormone in sexual maturation. Maturation of the reproductive system is controlled by gonadotropin-releasing hormone, gonadotropins, and steroid hormones, but if thyroid hormone is not present in sufficient amounts, maturation of the reproductive system is delayed. Thyroid hormone itself cannot stimulate maturation of the reproductive system, but it is necessary for the full effects of the other hormones. Therefore, thyroid hormone is considered to have a permissive effect on sexual maturation.
166
What is antagonism in hormone interaction? How do hormones exhibit antagonism in endocrinology?
-Antagonism in hormone interaction occurs when two molecules work against each other, with one diminishing the effectiveness of the other.
- In endocrinology, two hormones are considered functional antagonists if they have opposing physiological actions. For example, both glucagon and growth hormone raise the concentration of glucose in the blood, opposing the action of insulin, which lowers blood glucose concentration.
167
What are the three basic patterns of endocrine pathology?
-Hormone excess
-Hormone deficiency
-Abnormal responsiveness of target tissues to a hormone
168
What is hormone excess, and what can cause it?
Hormone excess involves the exaggerated effects of a hormone due to its presence in excessive amounts. It can be caused by various factors such as:
-Benign or cancerous tumors of the endocrine glands
-Secretion by nonendocrine tumors
-Exogenous administration of hormones or agonists
169
What is an example of hormone excess leading to iatrogenic conditions?
An example of hormone excess leading to iatrogenic conditions is the administration of exogenous cortisol as a drug. This can act as a negative feedback signal, shutting off the production of corticotropin-releasing hormone (CRH) and adrenocorticotropin (ACTH), leading to suppression of the pituitary and adrenal gland.
170
What is hyposecretion, and where along the endocrine control pathway can it occur?
Hyposecretion involves the diminished or eliminated effects of a hormone due to its insufficient secretion. It can occur anywhere along the endocrine control pathway, including in the hypothalamus, pituitary, or other endocrine glands.
171
What is a common cause of hyposecretion pathologies, and how does it affect negative feedback pathways?
Atrophy of the gland due to some disease process is a common cause of hyposecretion pathologies. In hyposecretion, the absence of negative feedback causes trophic hormone levels to rise as they attempt to stimulate the defective gland into producing more hormone.
172
What are some examples of problems that can cause abnormal tissue responsiveness to hormones?
Abnormal tissue responsiveness to hormones can result from:
-Down-regulation, where target cells decrease the number of receptors in response to sustained high hormone levels.
-Receptor abnormalities, such as mutations altering the receptor protein sequence or absence of functional receptors.
-Signal transduction abnormalities, where genetic alterations in signal transduction pathways impair cellular responses to hormones.
173
What is an example of an inherited endocrine pathology due to problems with hormone action in the target cell?
Androgen insensitivity syndrome is an example of an inherited endocrine pathology where androgen receptors are nonfunctional due to a genetic mutation. This results in the inability of androgens to influence the development of genitalia in affected individuals.
174
What is a primary pathology in the context of endocrine disorders?
A primary pathology occurs when dysfunction arises in the last endocrine gland in a complex reflex pathway. For instance, if excessive cortisol production originates from a tumor in the adrenal cortex, it is termed primary hypersecretion.
175
Describe the process of diagnosing the cause of excess cortisol secretion.
To diagnose the cause of excess cortisol secretion, clinicians assess the levels of cortisol, corticotropin-releasing hormone (CRH), and adrenocorticotropic hormone (ACTH) in the control pathway. Elevated cortisol with low trophic hormone levels indicates a primary disorder, which can stem from endogenous cortisol hypersecretion or exogenous cortisol administration.
176
What are the possible etiologies for excess cortisol secretion in a patient with high cortisol levels and low trophic hormone levels?
The possible etiologies include endogenous cortisol hypersecretion, often due to an adrenal tumor that produces cortisol independently of trophic hormones, or exogenous cortisol administration for therapeutic purposes.
177
What are the characteristics of a secondary hypersecretion of cortisol?
In a secondary hypersecretion of cortisol, high levels of ACTH from a pituitary tumor lead to elevated cortisol production. However, high cortisol levels exert negative feedback on the hypothalamus, decreasing CRH production. This pattern isolates the problem to the pituitary.
