Physiology Flashcards

Question

What are the first cleavage products of proopiomelanocortin? A. β-lipotrophin, α-MSH, and γ-MSH B. ACTH, α-MSH, and γ-MSH C. α-MSH, β-MSH, β-endorphin D. ACTH, β-lipotrophin, and γ-MSH E. α-MSH, β-MSH, and γ-MSH

Answer 1

D. ACTH, β-lipotrophin, and γ-MSH ## Footnote ACTH, β-lipotrophin, and γ-MSH Following the initial cleavage of pro-opiomela- nocortin into ACTH, β-lipotrophin, and γ-MSH, ACTH can be processed further to α-MSH and CLIP. β-lipotrophin can be processed into β-endorphin and γ-lipotrophin with the latter eventually becoming β-MSH.

Answer 2

A. The H-wave disappears ## Footnote The H-wave disappears The H-wave is the electrophysiological equiva- lent of the stretch reflex and represents the mus- cle’s electrical response to a square wave stimulus to the skin that first propagates in the antidromic direction (away from the muscle) to the cell bodies. The reflex arc continues with an electrical signal sent in the orthodromic direction (toward the mus- cle) to elicit a response. This response is the H-wave. As stimulation amplitude increases, the H-wave diminishes and disappears with supramaximal stimulation. It is most useful for evaluating the Ia sensory afferents. (B, D) The M-wave is the ortho- dromic response recorded in the muscle to electri- cal stimulation of the skin overlying the muscle. It bypasses the reflex arc and increases with increas- ing stimulation amplitude. (E) The F-wave increases with increases in amplitude but not to the extent seen in the M-wave. The F-wave is the result of alpha-fiber stimulation and is useful to evaluate proximal (near the spinal cord) nerve conduction velocities.

Answer 3

D. Arginine ## Footnote Arginine along with cofactors NADPH and oxy- gen react in the presence of nitric oxide synthe- tase to produce nitric oxide, NADP, and citrulline. (A, E) Tyrosine and asparagine are not involved in nitric oxide synthesis. (B, C) NADP and citrulline are products in the reaction creating nitric oxide.

Answer 4

B. The curve is wider with smooth muscle ## Footnote The curve is wider with smooth muscle, as shown in this image. (A) Tension equals the force of the muscle. (C) Tension increases to its maximum as the mus- cle is stretched beyond its ideal working length (at the point of maximal myosin and actin overlap). Tension will increase indefinitely at the far right of the tension–length curve until muscle tissues begin to tear. (D) When the single peak of active muscle tension versus length is added to the exponential relationship of tension to length in resting muscle, there is a tension peak, followed by a trough, fol- lowed by a continued rise in tension with both smooth and skeletal muscle

Answer 5

B. Glutamate ## Footnote Glutamate triggers the activation of NMDA receptors. When the magnesium ion is displaced from its channel-blocking site, there is calcium influx into the cytosol. Elevated calcium levels trigger apoptotic changes in the affected cells.

Answer 6

C. Pericytes ## Footnote Pericytes are the cells surrounding the endothelial cells of capillaries and venules throughout the body. In the brain, pericytes create and maintain the tight junctions between endothelial cells and regulate vesicle trafficking between endothelial cells. It is this role that creates the selectivity of the blood–brain barrier. Pericytes also inhibit the expression of endothelial markers that increase vascular permeability. (A) Astrocytic foot processes historically were thought to be the key component creating the blood–brain barrier; however, their role is to provide biochemical support to the endothelial cells creating the barrier. (B) Arachnoid “cap” cells are cells of the arachnoid villi from which meningiomas originate. (D) The endothelial cells themselves are not the key component to the blood–brain barrier. Instead, the highly selective tight junctions between endothelial cells allow for the specialized function of the blood–brain barrier.

Answer 7

A. Irreversible binding to and inhibition of acetylcholinesterase ## Footnote Organophosphates irreversibly bind to acetylcholinesterase and inactivate this acetylcholine-degrading enzyme. Pralidoxime can be used as an antidote in organophosphate poisoning, as it is able to reactivate the enzyme prior to degradation.

Answer 8

B. The affinity of hemoglobin for oxygen decreases, and the oxygen-hemoglobin dissociation curve shifts to the right. ## Footnote The affinity of hemoglobin for oxygen decreases, and the oxygen-hemoglobin dissociation curve shifts to the right, as shown in this image. Increasing temperature, carbon dioxide, and 2,3-DPG along with decreasing pH all decrease the binding affinity of hemoglobin for oxygen and thus shift the hemoglobin-oxygen dissociation curve to the right. Conditions opposite to these will increase binding affinity and shift the curve to the left. As blood passes through a capillary, metabolically active tissues release 2,3-DPG, which induces an allosteric change in hemoglobin and leads to oxygen unbinding. Oxygen is taken up by tissues, and carbon dioxide enters the blood. The partial pressure of oxygen thus is reduced as is the pH causing subsequent oxygen unbinding from hemoglobin. A decrease in the partial pressure of oxygen causes a shift along the curve to the left.

Answer 9

A. –65 mV; potassium ## Footnote All ions considered, the actual resting membrane potential for a neuron is between –60 and –70 mV. This is due in large part to the flux of potassium ions from inside to outside the neuron through open channels along the concentration gradient. The equilibrium potential for potassium is around –90 mV. (B) Sodium flux is from the extracellular to the intracellular space when a membrane is at rest, but there are far fewer sodium than potassium channels open when a neuron is at rest. (C) The equilibrium potential for sodium is around +60 mV, whereas it is around –60 mV for chloride. (D) Although the resting membrane potential is based largely on potassium conductance, it is more positive than the equilibrium potential for potassium alone. (E) The equilibrium potential for calcium is around +20 mV, but the ion plays little role in the membrane resting potential.

Answer 10

C. Sonic hedgehog proteins ## Footnote Sonic hedgehog proteins are secreted from the notochord and signal the development of the floor plate and motor neurons in a concentration-dependent fashion. (A, B) Bone morphogenic proteins from the ectoderm overlying the notochord and Wnt family proteins establish the dorsal axis. (D) The patched gene (Ptc) expresses the ligand-binding domain for the sonic hedgehog proteins. (E) The Ras family proteins are GTPases involved cell signal transduction and can signal aberrantly in cancers.

Answer 11

B. Increased postsynaptic activity ## Footnote Myasthenia gravis is a condition characterized by antibodies that bind to postsynaptic acetylcholine receptors and inhibit acetylcholine binding. This reduces the postsynaptic activity. Medications for myasthenia gravis act by competitively inhibiting acetylcholinesterase, thus decreasing acetylcholine breakdown and increasing acetylcholine in the synaptic cleft. Organophosphates irreversibly bind to and inhibit acetylcholinesterase and have a similar end result on the postsynaptic activity.

Answer 12

A. Its resting state would become more positive. ## Footnote Due to its high potassium relative to sodium concentration compared with perilymph and normal neuronal extracellular fluid, endolymph has a potential of 80 to 120 mV. The normal resting concentration of potassium in a neuron is about 150 mmol, which is the same concentration of potassium in endolymph. This means that there essentially would be almost no passive flow of potassium ions in this situation, and according to the Nernst equation, the resting potential due to potassium flux would be about 0 mV.

Answer 13

D. Inhibition of glycine receptors ## Footnote Strychnine is a rodenticide that works by binding to, and thus blocking, glycine and acetylcholine receptors on motor neurons in the spinal cord. This leads to a hyperactivation of skeletal musculature and death by asphyxiation due to tonic contraction of the respiratory muscles.

Answer 14

C. 18 mL/h ## Footnote Cerebrospinal fluid (CSF) is produced at an average rate of 18 mL/h, yielding a total daily production of 400 to 500 mL. The total CSF volume in an adult is approximately 140 mL at any given time. Seventy percent of CSF is produced by the choroid plexus.

Answer 15

A. Shorten the muscle spindles with skeletal muscle fiber contraction ## Footnote The cells bodies of gamma motor neurons are located in the anterior horns of the spinal cord. They are activated along with their corresponding alpha motor neurons and contract the intrafusal muscle fibers to keep the muscle spindles taut regardless of extrafusal muscle fiber length. (B) Beta motor neurons activate both extrafusal and intrafusal muscle fibers, thus shortening muscle length and keeping muscle spindles taut. They greatly are outnumbered by gamma motor neurons. (C) Gamma and alpha motor neurons are activated simultaneously in a given muscle. (D) Type 2 afferents are highly myelinated stretch receptors in the muscle that, unlike type 1a afferents, are non-adapting and thus fire continuously with a stable muscle length. (E) The Golgi tendon organs are proprioceptive sensory receptors oriented in series with skeletal muscle fibers. They are located where skeletal muscles join their tendons and sense the force on the muscles. With signaling through 1b afferents, the Golgi tendon reflex can be initiated, which inhibits further contraction of the muscle involved through inhibitory interneurons in the spinal cord.

Answer 16

B. Aromatic amino acid decarboxylase ## Footnote Aromatic amino acid decarboxylase uses activated B6 as a cofactor. (A) Tyramine β-hydroxylase converts tyramine to octopamine (an amine that acts in place of norepinephrine in sympathetic neurons with chronic monoamine oxidase inhibitors). (C) Tryptophan hydroxylase converts tryptophan to 5-hydroxy-tryptophan during serotonin synthesis. (D) Tyrosine hydroxylase converts tyrosine to levodopa during dopamine synthesis. (E) Catechol-O-methyl transferase inactivates catecholamine neurotransmitters including dopamine, levodopa, norepinephrine, and epinephrine

Answer 17

D. It allows for more precise spatial resolution. ## Footnote Presynaptic inhibition has an advantage over postsynaptic inhibition in that presynaptic inhibition downregulates only one synapse acting on the postsynaptic cell. The entire neuronal chain thus does not simply become inhibited based on one postsynaptic action. In addition, the multiple miniature end-plate potentials (MEPPs) from several presynaptic terminals (whether inhibitory or excitatory) can be grouped together at the postsynaptic cell to induce or inhibit an action potential (spatial resolution). (B) Temporal summation involves the grouping of MEPPs together in a temporal fashion to induce an action potential postsynaptically. (C) With presynaptic inhibition, there are fewer quanta of neurotransmitters entering the synaptic cleft.

Answer 18

D. Respond to depolarization and activate calcium channels on the sarcoplasmic reticulum ## Footnote As the action potential at the neuromuscular junction occurs, depolarization propagates through the T-tubules in the muscle fiber. L-type voltage-dependent calcium channels are activated, and they open ryanodine receptors in the adjacent sarcoplasmic reticulum. The calcium that then is released binds to troponin C on the myofibrils to start sarcomere contraction. (A) Once activated, calcium-release channels (ryanodine receptors) on the sarcoplasmic reticulum allow for stored calcium to be released within the muscle cells’ cytoplasm. (E) This is the role of calcium and not a channel.

Answer 19

B. Inhibits osteoblasts ## Footnote By inhibiting osteoblasts, parathyroid hormone (PTH) is not able to inhibit osteoclasts and reduce bone reabsorption. As bone is reabsorbed, calcium is released into the circulation. (A) Calcitonin is secreted by the thyroid and inhibits intestinal calcium absorption of calcium, inhibits osteoclast activity, stimulates osteoblast activity, and promotes renal secretion of calcium. All of these effects work to reduce the serum calcium concentration. Calcitonin can be thought of as having the opposite activity of PTH. (C) PTH promotes the reabsorption of calcium from the distal tubules and thick ascending limbs in the kidneys and reduces phosphate reabsorption. This leads to an increase in the calcium/ phosphate ratio, meaning that more calcium is free in the serum. Increasing renal phosphate reabsorption would have the opposite effect. (D) PTH upregulates the enzyme responsible for activating vitamin D, which in turn increases the intestinal reabsorption of calcium. (E) Osteoclast inhibition would reduce bone turnover and calcium release from the bony matrix.

Answer 20

C. Uncompetitive inhibitor ## Footnote An uncompetitive inhibitor binds to an enzyme/ substrate complex to reduce or inhibit its activity. The enzyme/substrate must first form for this type of inhibition to take place. (A) In a simple form, a competitive inhibitor binds to an enzyme and, when bound, prevents the enzyme from binding to its substrate. In other words, the enzyme may either be bound to its inhibitor or substrate but not both at the same time. (B) A noncompetitive inhibitor binds to and acts on an enzyme to reduce its activity and interaction with its substrate. A noncompetitive inhibitor binds to an enzyme equally well whether or not the enzyme is bound to its substrate. The substrate does not have to be present for a noncompetitive inhibitor to act. (D) A mixed inhibitor is a type of noncompetitive inhibitor that has a higher affinity for the enzyme in a particular state (e.g., bound to its substrate).

Answer 21

D. Vesicles and corresponding receptors each on only one side of the synaptic cleft ## Footnote Excitatory synapses have vesicles and corresponding receptors each on only one side of the synaptic cleft; thus, there always is a “signaling” end and a “receiving” end for vesicular traffic. (A) The absolute refractory period forces axons to propagate action potentials in only one direction. (C) Presynaptic neurotransmitter reuptake receptors exist but do not play a role in signal direction across a synapse.

Answer 22

E. Increase CBF and increase ICP. ## Footnote When the brain detects a rising carbon dioxide level in the blood, the cerebrovasculature dilates to increase cerebral blood flow (CBF) and thus increase the clearance of carbon dioxide. In turn, this raises intracranial pressure. With the increase in CBF, the PO2 the brain experiences also increases. This mechanism is impaired in brain death and severe stroke.

Answer 23

A. Absolute refractory period ## Footnote The absolute refractory period is determined by the inability of voltage-gated sodium channels to open for a period of time once they have opened and closed during an action potential. Without a large sodium flux from outside to inside the axon, there can be no depolarization and thus no subsequent action potential. The action potential thus must propagate in an anterograde fashion to a portion of the axon where the sodium channels have not been activated and closed recently. (B) Protein transport goes in both an anterograde and retrograde fashion in axons but is not the mechanism behind action potential direction. (C) Saltatory conduction describes the process by which the action potential “leaps” between nodes of Ranvier in a design that speeds signal conduction along axons. Action potentials are triggered only in the unmyelinated nodes, and the voltage differential produced at one node must be transmitted at a sufficient level to the next node to trigger another action potential. (D) The relative refractory period promotes an action potential to propagate in one direction but is not the absolute mechanism. The relative refractory period refers to the point at which a neuron is hyperpolarized by potassium flux with voltage-gated sodium channels that are not inactivated. The neuron is resistant to depolarization, but a sufficient depolarization would trigger an action potential. If the relative refractory period was the only mechanism present, a large depolarization would send an action potential propagating in two directions along an axon. (E) Temporal summation refers to the summation of multiple small depolarizations in the postsynaptic terminal over a given period of time with each small depolarization alone not being sufficient to trigger an action potential. It requires the summation of signal to initiate the action potential.

Answer 24

D. No postsynaptic blockade ## Footnote Hexamethonium is an antagonist of nicotinic acetylcholine receptors and thus is a ganglionic blocker in the autonomic nervous system. It binds to the nicotinic acetylcholine receptors but not in the binding site for acetylcholine. Hexamethonium has no effect on the postsynaptic muscarinic acetylcholine receptors. (C) Hexamethonium administration will decrease signaling in both the sympathetic and parasympathetic nervous systems.

Answer 25

B. By simple diffusion ## Footnote Steroids are lipophilic and thus can pass through lipophilic cell membranes without the use of channels or receptors. (A) Membrane channels for steroids do not exist. (C) Facilitated diffusion relies on membrane channels to allow molecules to pass through cell membranes along their electrochemical and/or osmotic gradients. (D) G proteins allow extracellular molecules to induce cell signaling within cells, without signaling molecules passing through the membranes. (E) Active transport requires the use of energy (usually in the form of ATP) for molecules to pass through membrane channels against a gradient.

Answer 26

D. Hyperpolarization of the retinal cells results in phototransduction ## Footnote As in the auditory and vestibular systems, the stimulus (light) induces hyperpolarization of the retinal cells, which is due to the closure of sodium channels and reduction of the inward sodium flux of sodium in the case of phototransduction. (A) Phosphodiesterases are essential for continuing the phototransduction cascade. (B) Rhodopsin contains retinal in the 11-cis conformation until light evokes a configuration change to the all-trans form. This change makes rhodopsin no longer fit into the opsin-binding site, and upon disengagement, opsin changes forms, continuing the phototransduction cascade. (C) Molecules of cGMP are hydrolyzed, which causes the closure of cGMP dependent sodium channels.

Answer 27

A. G1 phase ## Footnote The G1 (growth 1) phase is characterized by cell growth and synthesis of mRNA and proteins in preparation for DNA replication in S phase. Various checkpoint systems like the p53 tumor-suppressor protein prevent progression to S phase and DNA replication if errors are present. (B) During the S (synthetic) phase, cells replicate their DNA and are resistant to radiation due to the numerous DNA proofreading and repair mechanisms at play. (C) The G2 (growth 2) phase is characterized by cell growth and protein synthesis after cell DNA has replicated. Some rapidly dividing cells (e.g., cancer cells) may skip the G2 phase. There exist checkpoints in the G2 phase that induce cell arrest if errors are detected. (D) The M (mitotic) phase is characterized by division and separation of the cellular nuclear material and division of cells into two daughter cells. Cells are very sensitive to radiation during the M phase. (E) The G0 (resting) phase occurs when cells are in a quiescent state. Cells can enter the G0 phase due to a variety of conditions including nutrient deficiency and lack of growth factors. Most neurons in the central nervous system are in the G0 phase of the cell cycle.

Answer 28

A. A motor neuron and all of the muscle fibers innervated by that neuron ## Footnote Each motor neuron innervates several muscle fibers. There are numerous motor neurons in each peripheral nerve.

Answer 29

D. Synaptic vesicle ## Footnote During the process of conversion of dopamine to norepinephrine, the latter is transported into synaptic vesicles by the vesicular monoamine transporter. The actual synthesis of norepinephrine mostly takes place in the synaptic vesicle. (A) Dopamine synthesis from tyrosine takes place in the neuronal cytoplasm. (E) The synaptic cleft is where acetylcholine is degraded by acetylcholinesterase.

Answer 30

B. The blood–brain barrier is closed in cytotoxic edema and disrupted in vasogenic edema ## Footnote The blood–brain barrier (BBB) becomes disrupted in vasogenic edema, as breakdown of the endothelial tight junctions occurs. In addition, gliomas and certain brain tumors can secrete vascular endothelial growth factors that increase permeability. Proteins and molecules leave the intravascular space and create an oncotic force driving fluid into the extracellular space. Fluid spreads along white matter tracts, creating a large area of edema. In cytotoxic edema, alteration of cellular metabolism leads to a failure of ion pumps (notably the sodium-potassium pumps), which in turn creates an osmotic gradient and increased fluid in the intracellular space. (A) Vasogenic edema responds better to steroids, in that steroids decrease endothelial permeability by various means. (C) Cytotoxic edema is characterized by cell swelling, whereas cells remain of similar size in pure vasogenic edema. (D) Although ischemic injury initially is characterized by cytotoxic edema, when cells die and the BBB breaks down, elements of vasogenic edema arise. (E) Proteins expand the extracellular space in vasogenic edema due to disruption of the BBB.

Answer 31

B. Myosin heads are bound to actin following the power stroke ## Footnote The power stroke occurs as phosphate is released from the cocked myosin heads, and they change conformation to move relative to the actin thin filaments. Without the addition of ATP, the myosin heads cannot unbind actin. This accounts for the rigid muscles and tonic sarcomere length seen in rigor mortis. (A) The myosin heads are able to unbind from actin thin filaments with the addition of ATP and return to their resting (uncocked) positions. (C) Once cocked, myosin heads bind actin. (D) Myosin heads cock in a high-energy, ready position as ATP is hydrolyzed into ADP and phosphate.

Answer 32

E. Phenylethanolamine N-methyltransferase ## Footnote Phenylethanolamine N-methyltransferase converts norepinephrine to epinephrine creating homocysteine in the process. (A) Monoamine oxidase degrades monoamines like dopamine. (B) Tyrosine hydroxylase converts tyrosine to L-dopa in dopamine synthesis. It is the rate-limiting factor in dopamine synthesis. (C) Aromatic L-amino acid decarboxylase converts L-dopa to dopamine in dopamine synthesis. (D) Dopamine β-hydroxylase converts dopamine to norepinephrine.

Answer 33

C. Inhibition of synthesis of proteins C, S, and Z ## Footnote Vitamin K is required for the complete synthesis of the coagulation factors prothrombin (factor II), factor VII, factor IX, factor X, protein C, protein S, and protein Z. These factors are present (in the classic coagulation model) in the intrinsic, extrinsic, and common pathways. (A) Antithrombin inhibits several factors in the clotting cascade, and its effects on factors II and X are inhibited by heparin. (B) Factor XII acts on factor XI to begin the clotting cascade in the intrinsic pathway. (D) Bleeding time and platelet function are unaffected by a vitamin K deficiency. (E) With a vitamin K deficiency, prothrombin time (a measure of the extrinsic pathway) and partial thromboplastin time (a measure of the intrinsic pathway) both are prolonged, with the former being affected to a greater extent.

Answer 34

B. Astrocytic scarring ## Footnote Astrocytic scarring disrupts the axonal tracts following wallerian degeneration, and astrocytes secrete axonal growth inhibitory proteins in the central nervous system. (A) There is a relative paucity of axonal growth–promoting factors in the central nervous system, but this is not due to, nor does it change relative to, the presence of axonal injury. (C) There is a lack of progenitor cells in the central nervous system, but axons in the peripheral nervous system regenerate without such progenitor cells. (D) Directional markers and signaling molecules do degrade following neurogenesis, but axons still can regenerate in the peripheral nervous system along axon tracts following the supporting matrix (e.g., endoneurium and other such components) after wallerian degeneration.

Answer 35

D. Membrane repolarization ## Footnote As the sodium channels close, the open potassium channels are able to keep the net flux of positive ions out of the cell, resulting in membrane repolarization toward the resting membrane potential of potassium. (B) Potassium channels are open at this point, which allows for cellular repolarization and eventual hyperpolarization as the sodium channels close. (C) The relative refractory period is the time at which the cell is hyperpolarized so that the membrane depolarization required to trigger a subsequent action potential is greater than it is when the cell is at its resting membrane potential. (E) Voltage-gated calcium channels open with depolarization and not repolarization.

Answer 36

E. Inhibition of the sodium-chloride cotransporters in the distal tubules ## Footnote By competing for the chloride-binding site and blocking the sodium-chloride transporters, hydrochlorothiazide prevents the reabsorption of sodium in the distal convoluted tubules. This maintains the osmotic gradient in the tubules, thus promoting natriuresis. In addition, hydrochlorothiazide increases calcium reabsorption in the tubules through an unrelated mechanism. (A) One mechanism by which vasopressin acts is by promoting the insertion of aquaporin channels in the apical membrane of the distal convoluted tubules and collecting ducts, allowing water to be reabsorbed into the blood along its osmotic gradient. This creates an antidiuretic effect. (B) Spironolactone acts on the distal nephron on the distal convoluted tubules where it inhibits the sodium-potassium exchangers. This prevents the exchangers from reabsorbing sodium and excreting potassium, resulting in a diuretic effect. It is considered a potassium-sparing diuretic. (C) Furosemide is a loop diuretic that works by inhibiting the sodium-potassium-chloride symporters in the thick ascending limb of the loops of Henle. Blockade of the transporter prevents reabsorption of sodium, chloride, and potassium. (D) Carbonic anhydrase inhibitors reduce sodium chloride and bicarbonate reabsorption in the proximal tubules by blocking the luminal conversion of bicarbonate to carbon dioxide. Without the inhibitors, carbon dioxide diffuses passively back into the cells of the proximal tubules. Without the conversion, sodium remains with the bicarbonate ions in the lumen due to their electrochemical attraction.

Answer 37

A. Large size and negative charge ## Footnote The basement membrane and podocytic epithelium in the glomerulus have a positive charge and thus resist passage of negatively charged molecules. In addition, the effective pore size in the glomerular wall is about 8 nm, preventing passage of most proteins and other large molecules. The oncotic pressure in the glomerular capillaries increases as blood passes through as proteins are retained while ions and small molecules are filtered through the wall. This causes a gradient of flow back into the capillaries at their distal ends as oncotic pressure builds. Finally, the hydrostatic pressure in the glomerular capillaries is low relative to capillaries elsewhere in the body. This resists filtration through the capillary walls.

Answer 38

A. Concentric ## Footnote Lateral geniculate nucleus cells have concentric (center-surround) receptive fields that are either on-center or off-center, with the surrounding region being antagonistic. Similar receptive fields are seen in retinal ganglion cells. (B) Rectangular receptive fields are seen in simple cells of the visual cortex. (C, D) Motion, direction, and orientation receptive fields are seen in complex cells of the visual cortex. Cells are dedicated and specialized to respond to certain aspects of visual stimuli.

Answer 39

D. Calcium ## Footnote During the secondary (delayed) phase of traumatic brain injury, neurons spill the neurotransmitter glutamate that in turn causes a large calcium flux into neurons. Excess intracellular calcium can initiate apoptosis and open mitochondrial pores, leading to metabolic dysfunction. (A) Glutamate is the neurotransmitter involved in excitotoxic brain injury, but the molecule only activates the receptor that opens calcium channels. (B, C) During the primary phase of traumatic brain injury, there is cell damage and cellular membrane damage due to the direct forces involved. Cellular membrane damage can allow numerous ions and molecules to pass freely along their concentration and electrochemical gradients.

Answer 40

E. Retrograde transport ## Footnote Retrograde transport of vesicles using dynein is an ATP-dependent process. (A) There are two types of slow anterograde transport. The slower mode carries microtubules and neurofilaments, whereas the faster slow transport mechanism carries actin and a variety of other proteins. (B) There is no single carrier protein that serves bidirectional axoplasmic transport. (C) Both slow and fast anterograde transport of molecules in vesicles are mediated by kinesins along microtubules

Answer 41

A. C-band ## Footnote The C-band enlarges with sarcomere shortening, as shown in this image. It is the region of the A-band not including the H-band. (B) The A-band is the region of the sarcomere containing myosin thick filaments with actin thin filament overlaps at the ends of the band. The A-band remains the same size during sarcomere contraction. (C) The I-band is the portion of the sarcomere with actin thin filaments that are not overlapping with myosin thick filaments. It decreases in size with sarcomere contraction. (D) The H-band is the portion of the sarcomere containing myosin thick filaments that are not overlapping actin thin filaments. It shortens with contraction. (E) The M-band is the center anchoring point of the myosin thick filaments and does not change in size as sarcomere length changes.

Answer 42

C. Reverse transcriptases ## Footnote Reverse transcriptases create cDNA from RNA templates in the very error-prone process of reverse transcription. (A) RNA polymerases are involved in RNA translation from DNA. (B) DNA polymerases are involved in transcription and DNA replication. (D) Integrases allow the DNA created from retroviruses to be incorporated into host cell genomes. (E) Helicases separate two annealed strands of DNA, RNA, or DNA-RNA hybrids, and are involved in the processes of transcription and translation among other roles that they play.

Answer 43

B. Tyrosine hydroxylase ## Footnote Tyrosine hydroxylase converts tyrosine to L-dopa. (A) Tyrosine is abundant and does not limit the rate of dopamine synthesis. (C) Dopa decarboxylase converts L-dopa to dopamine. (D) Dopamine β-hydroxylase converts dopamine to norepinephrine. (E) Tetrahydrobiopterin is a cofactor for the conversion of tyrosine to L-dopa, but being widely available, it does not limit the reaction.

Answer 44

B. Exogenous administration that mimics the endogenous activity ## Footnote To prove that the action seen in a neuronal chain is due to signaling by a neurotransmitter, the effect from exogenous administration must be the same as endogenous activity. (A) Neurotransmitters are present in the presynaptic terminal and only released in a controlled amount and fashion to exert a particular effect. (C) Some neurotransmitters can be seen throughout the nervous system, whereas others are found in only a few select locations. (D) Although a neurotransmitter may be excitatory or inhibitory at a particular synapse, the downstream effects can vary depending on the arrangement of the neural network. (E) An effect distant from the site of production/release is part of the definition of a hormone.

Answer 45

A. Promoting arousal ## Footnote Orexin/hypocretin is a neurotransmitter that promotes arousal and wakefulness along with appetite. Despite decreased levels of production in the brains of narcoleptics, narcoleptics tend to have higher body mass indices, highlighting that the chemical plays more of a role in arousal and thermogenesis than in promoting that appetite. The weight gain effect is due to the lowered metabolic rate in orexin-deficient individuals.

Answer 46

C. Resting membrane potential would increase, and the threshold to trigger an action potential would be reached with fewer MEPPs ## Footnote The equilibrium potential for an ion is determined by the relative concentration of the ion outside compared with inside the neuron. If the extracellular concentration of potassium increases and the intracellular concentration remains constant, the equilibrium potential would become more positive. Because the resting membrane potential for a neuron is determined mainly by potassium ions, it also would increase. The threshold for trigging an action potential would remain the same but now would be closer to the resting potential.