Phys BRS Questions
Differentiate between the terms osmole, osmolarity, osmolality and tonicity. List the typical value and normal range for plasma osmolality.
-Osmole: a unit of osmotic pressure equivalent to the amount of solute that dissociates in solution to form one mole (Avogadro's number) of particles (molecules and ions). -Osmolarity: the concentration of a solution expressed as the total number of solute particles per liter. -Osmolality: refers to the number of particles of solute per kilogram of solvent. -Tonicity: a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a semipermeable cell membrane. In other words, tonicity is the relative concentration of solutes dissolved in solution which determine the direction and extent of diffusion. -275-299 milli-osmoles per kilogram.
Which of the following responses is mediated by parasympathetic muscarinic receptors? (A) Dilation of bronchiolar smooth muscle (B) Erection (C) Ejaculation (D) Constriction of gastrointestinal (GI) sphincters (E) Increased cardiac contractility
The answer is B - Erection is a parasympathetic muscarinic response. Dilation of the bronchioles, ejaculation, constriction of the gastrointestinal (GI) sphincters, and increased cardiac contractility are all sympathetic α or β responses.
25. Which of the following would produce maximum excitation of the hair cells in the right horizontal semicircular canal? (A) Hyperpolarization of the hair cells (B) Bending the stereocilia away from the kinocilia (C) Rapid ascent in an elevator (D) Rotating the head to the right
The answer is D The semicircular canals are involved in angular acceleration or rotation. Hair cells of the right semicircular canal are excited (depolarized) when there is rotation to the right. This rotation causes bending of the stereocilia toward the kinocilia, and this bending produces depolarization of the hair cell. Ascent in an elevator would activate the saccules, which detect linear acceleration.
6. Which of the following statements best describes the basilar membrane of the organ of Corti? (A) The apex responds better to low frequencies than the base does (B) The base is wider than the apex (C) The base is more compliant than the apex (D) High frequencies produce maximal displacement of the basilar membrane near the helicotrema (E) The apex is relatively stiff compared to the base
The answer is A Sound frequencies can be encoded by the organ of Corti because of differences in properties along the basilar membrane. The base of the basilar membrane is narrow and stiff, and hair cells on it are activated by high frequencies. The apex of the basilar membrane is wide and compliant, and hair cells on it are activated by low frequencies.
1. Which of the following characteristics is shared by simple and facilitated diffusion of glucose? (A) Occurs down an electrochemical gradient (B) Is saturable (C) Requires metabolic energy (D) Is inhibited by the presence of galactose (E) Requires a Na+ gradient
The answer is A [II A 1, C]. Both types of transport occur down an electrochemical gradient ("downhill") and do not require metabolic energy. Saturability and inhibition by other sugars are characteristic only of carrier-mediated glucose transport; thus, facilitated diffusion is saturable and inhibited by galactose, whereas simple diffusion is not.
What process is responsible for the change in membrane potential that occurs in the first uphill region of an action potential graph? (A) Movement of Na+ into the cell (B) Movement of Na+ out of the cell (C) Movement of K+ into the cell (D) Movement of K+ out of the cell (E) Activation of the Na+ -K+ pump (F) Inhibition of the Na+ -K+ pump
The answer is A. The upstroke of the nerve action potential is caused by opening of the Na+ channels (once the membrane is depolarized to threshold). When the Na+ channels open, Na+ moves into the cell down its electrochemical gradient, driving the membrane potential toward the Na+ equilibrium potential.
Degeneration of dopaminergic neurons has been implicated in (A) schizophrenia (B) Parkinson disease (C) myasthenia gravis (D) curare poisoning
The answer is B Dopaminergic neurons and D2 receptors are deficient in people with Parkinson disease. Schizophrenia involves increased levels of D2 receptors. Myasthenia gravis and curare poisoning involve the neuromuscular junction, which uses acetylcholine (ACh) as a neurotransmitter.
8. Which autonomic receptor mediates an increase in heart rate? (A) Adrenergic α receptors (B) Adrenergic β1 receptors (C) Adrenergic β2 receptors (D) Cholinergic muscarinic receptors (E) Cholinergic nicotinic receptors
The answer is B Heart rate is increased by the stimulatory effect of norepinephrine on β1 receptors in the sinoatrial (SA) node. There are also sympathetic β1 receptors in the heart that regulate contractility.
Which of the following will double the permeability of a solute in a lipid bilayer? (A) Doubling the molecular radius of the solute (B) Doubling the oil/water partition coefficient of the solute (C) Doubling the thickness of the bilayer (D) Doubling the concentration difference of the solute across the bilayer
The answer is B Increasing oil/water partition coefficient increases solubility in a lipid bilayer and therefore increases permeability. Increasing molecular radius and increased membrane thickness decrease permeability. The concentration difference of the solute has no effect on permeability.
4. Which of the following is a property of C fibers? (A) Have the slowest conduction velocity of any nerve fiber type (B) Have the largest diameter of any nerve fiber type (C) Are afferent nerves from muscle spindles (D) Are afferent nerves from Golgi tendon organs (E) Are preganglionic autonomic fibers
The answer is A C fibers (slow pain) are the smallest nerve fibers and therefore have the slowest conduction velocity.
Cutting which structure on the left side causes total blindness in the left eye? (A) Optic nerve (B) Optic chiasm (C) Optic tract (D) Geniculocalcarine tract
The answer is A Cutting the optic nerve from the left eye causes blindness in the left eye because the fibers have not yet crossed at the optic chiasm.
Which of the following statements about the olfactory system is true? (A) The receptor cells are neurons (B) The receptor cells are sloughed off and are not replaced (C) Axons of cranial nerve (CN) I are A-delta fibers (D) Axons from receptor cells synapse in the prepiriform cortex (E) Fractures of the cribriform plate can cause inability to detect ammonia
The answer is A Cranial nerve (CN) I innervates the olfactory epithelium. Its axons are C fibers. Fracture of the cribriform plate can tear the delicate olfactory nerves and thereby eliminate the sense of smell (anosmia); however, the ability to detect ammonia is left intact. Olfactory receptor cells are unique in that they are true neurons that are continuously replaced from undifferentiated stem cells.
5. When compared with the cones of the retina, the rods (A) are more sensitive to low-intensity light (B) adapt to darkness before the cones (C) are most highly concentrated on the fovea (D) are primarily involved in color vision
The answer is A Of the two types of photoreceptors, the rods are more sensitive to low-intensity light and therefore are more important than the cones for night vision. They adapt to darkness after the cones. Rods are not present in the fovea. The cones are primarily involved in color vision.
A newly developed local anesthetic blocks Na+ channels in nerves. Which of the following effects on the action potential would it be expected to produce? (A) Decrease the rate of rise of the upstroke of the action potential (B) Shorten the absolute refractory period (C) Abolish the hyperpolarizing afterpotential (D) Increase the Na+ equilibrium potential (E) Decrease the Na+ equilibrium potential
The answer is A Blockade of the Na+ channels would prevent action potentials. The upstroke of the action potential depends on the entry of Na+ into the cell through these channels and therefore would also be reduced or abolished. The absolute refractory period would be lengthened because it is based on the availability of the Na+ channels. The hyperpolarizing afterpotential is related to increased K+ permeability. The Na+ equilibrium potential is calculated from the Nernst equation and is the theoretical potential at electrochemical equilibrium (and does not depend on whether the Na+ channels are open or closed).
4. The correct temporal sequence for events at the neuromuscular junction is (A) action potential in the motor nerve; depolarization of the muscle end plate; uptake of Ca2+ into the presynaptic nerve terminal (B) uptake of Ca2+ into the presynaptic terminal; release of acetylcholine (ACh); depolarization of the muscle end plate (C) release of ACh; action potential in the motor nerve; action potential in the muscle (D) uptake of Ca2+ into the motor end plate; action potential in the motor end plate; action potential in the muscle (E) release of ACh; action potential in the muscle end plate; action potential in the muscle
The answer is B [V B 1-6]. Acetylcholine (ACh) is stored in vesicles and is released when an action potential in the motor nerve opens Ca2+ channels in the presynaptic terminal. ACh diffuses across the synaptic cleft and opens Na+ and K+ channels in the muscle end plate, depolarizing it (but not producing an action potential). Depolarization of the muscle end plate causes local currents in adjacent muscle membrane, depolarizing the membrane to threshold and producing action potentials.
A 42-year-old man with myasthenia gravis notes increased muscle strength when he is treated with an acetylcholinesterase (AChE) inhibitor. The basis for his improvement is increased (A) amount of acetylcholine (ACh) released from motor nerves (B) levels of ACh at the muscle end plates (C) number of ACh receptors on the muscle end plates (D) amount of norepinephrine released from motor nerves (E) synthesis of norepinephrine in motor nerves
The answer is B [V B 8]. Myasthenia gravis is characterized by a decreased density of acetylcholine (ACh) receptors at the muscle end plate. An acetylcholinesterase (AChE) inhibitor blocks degradation of ACh in the neuromuscular junction, so levels at the muscle end plate remain high, partially compensating for the deficiency of receptors.
Which of the following would occur as a result of the inhibition of Na+ K+ -ATPase? (A) Decreased intracellular Na+ concentration (B) Increased intracellular K+ concentration (C) Increased intracellular Ca2+ concentration (D) Increased Na+ -glucose cotransport (E) Increased Na+ -Ca2+ exchange
The answer is C Inhibition of Na+, K+ -adenosine triphosphatase (ATPase) leads to an increase in intracellular Na+ concentration. Increased intracellular Na+ concentration decreases the Na+ gradient across the cell membrane, thereby inhibiting Na+ Ca2+ exchange and causing an increase in intracellular Ca2+ concentration. Increased intracellular Na+ concentration also inhibits Na+ -glucose cotransport.
27. Which autonomic receptor is activated by low concentrations of epinephrine released from the adrenal medulla and causes vasodilation? (A) Adrenergic α receptors (B) Adrenergic β1 receptors (C) Adrenergic β2 receptors (D) Cholinergic muscarinic receptors (E) Cholinergic nicotinic receptors
The answer is C β2 Receptors on vascular smooth muscle produce vasodilation. α Receptors on vascular smooth muscle produce vasoconstriction. Because β2 receptors are more sensitive to epinephrine than are α receptors, low doses of epinephrine produce vasodilation, and high doses produce vasoconstriction.
Which of the following is an inhibitory neurotransmitter in the central nervous system (CNS)? (A) Norepinephrine (B) Glutamate (C) γ-Aminobutyric acid (GABA) (D) Serotonin (E) Histamine
The answer is C γ-Aminobutyric acid (GABA) is an inhibitory neurotransmitter. Norepinephrine, glutamate, serotonin, and histamine are excitatory neurotransmitters.
26. The inability to perform rapidly alternating movements (dysdiadochokinesia) is associated with lesions of the (A) premotor cortex (B) motor cortex (C) cerebellum (D) substantia nigra (E) medulla
The answer is C Coordination of movement (synergy) is the function of the cerebellum. Lesions of the cerebellum cause ataxia, lack of coordination, poor execution of movement, delay in initiation of movement, and inability to perform rapidly alternating movements. The premotor and motor cortices plan and execute movements. Lesions of the substantia nigra, a component of the basal ganglia, result in tremors, lead-pipe rigidity, and poor muscle tone (Parkinson disease).
Which of the following parts of the body has cortical motoneurons with the largest representation on the primary motor cortex (area 4)? (A) Shoulder (B) Ankle (C) Fingers (D) Elbow (E) Knee
The answer is C. Representation on the motor homunculus is greatest for those structures that are involved in the most complicated movements—the fingers, hands, and face.
32. A new drug is developed that blocks the transporter for H+ secretion in gastric parietal cells. Which of the following transport processes is being inhibited? (A) Simple diffusion (B) Facilitated diffusion (C) Primary active transport (D) Cotransport (E) Countertransport
The answer is C. H+ secretion by gastric parietal cells occurs by H+ -K+ adenosine triphosphatase (ATPase), a primary active transporter.
29. Sensory receptor potentials (A) are action potentials (B) always bring the membrane potential of a receptor cell toward threshold (C) always bring the membrane potential of a receptor cell away from threshold (D) are graded in size, depending on stimulus intensity (E) are all-or-none
The answer is D Receptor potentials are graded potentials that may bring the membrane potential of the receptor cell either toward (depolarizing) or away from (hyperpolarizing) threshold. Receptor potentials are not action potentials, although action potentials (which are all-or-none) may result if the membrane potential reaches threshold.
What process is responsible for the change in membrane potential that occurs during repolarization? (A) Movement of Na+ into the cell (B) Movement of Na+ out of the cell (C) Movement of K+ into the cell (D) Movement of K+ out of the cell (E) Activation of the Na+ -K+ pump (F) Inhibition of the Na+ -K+ pump
The answer is D The process responsible for repolarization is the opening of K+ channels. The K+ permeability becomes very high and drives the membrane potential toward the K+ equilibrium potential by flow of K+ out of the cell.
Which of the following transport processes is involved if transport of glucose from the intestinal lumen into a small intestinal cell is inhibited by abolishing the usual Na+ gradient across the cell membrane? (A) Simple diffusion (B) Facilitated diffusion (C) Primary active transport (D) Cotransport (E) Countertransport
The answer is D In the "usual" Na+ gradient, the [Na+] is higher in extracellular than in intracellular fluid (maintained by the Na+ -K+ pump). Two forms of transport are energized by this Na+ gradient—cotransport and countertransport. Because glucose is moving in the same direction as Na+ one can conclude that it is cotransport.
At the muscle end plate, acetylcholine (ACh) causes the opening of (A) Na+ channels and depolarization toward the Na+ equilibrium potential (B) K+ channels and depolarization toward the K+ equilibrium potential (C) Ca2+ channels and depolarization toward the Ca2+ equilibrium potential (D) Na+ and K+ channels and depolarization to a value halfway between the Na+ and K+ equilibrium potentials (E) Na+ and K+ channels and hyperpolarization to a value halfway between the Na+ and K+ equilibrium potentials
The answer is D Binding of acetylcholine (ACh) to receptors in the muscle end plate opens channels that allow passage of both Na+ and K+ ions. Na+ ions will flow into the cell down its electrochemical gradient, and K+ ions will flow out of the cell down its electrochemical gradient. The resulting membrane potential will be depolarized to a value that is approximately halfway between their respective equilibrium potentials. Equilibrium (or reversal) potentials. For each ion, the equilibrium (or reversal) potential is the membrane potential where the net flow through any open channels is 0. In other words, at Erev, the chemical and electrical forces are in balance.
Solutions A and B are separated by a semipermeable membrane. Solution A contains 1 mM sucrose and 1 mM urea. Solution B contains 1 mM sucrose. The reflection coefficient for sucrose is one, and the reflection coefficient for urea is zero. Which of the following statements about these solutions is correct? (A) Solution A has a higher effective osmotic pressure than solution B (B) Solution A has a lower effective osmotic pressure than solution B (C) Solutions A and B are isosmotic (D) Solution A is hyperosmotic with respect to solution B, and the solutions are isotonic (E) Solution A is hyposmotic with respect to solution B, and the solutions are isotonic
The answer is D Solution A contains both sucrose and urea at concentrations of 1 mM, whereas solution B contains only sucrose at a concentration of 1 mM. The calculated osmolarity of solution A is 2 mOsm/L, and the calculated osmolarity of solution B is 1 mOsm/L. Therefore, solution A, which has a higher osmolarity, is hyperosmotic with respect to solution B. Actually, solutions A and B have the same effective osmotic pressure (i.e., they are isotonic) because the only "effective" solute is sucrose, which has the same concentration in both solutions. Urea is not an effective solute because its reflection coefficient is zero. Reflection coefficient: index of the effectiveness of a solute in generating an osmotic driving force. of a solution to pull water across a biologic membrane. hyperosmotic: refers to a solution that has more solutes, or components of a solution, than a similar solution.
An inhibitory postsynaptic potential (A) depolarizes the postsynaptic membrane by opening Na+ channels (B) depolarizes the postsynaptic membrane by opening K+ channels (C) hyperpolarizes the postsynaptic membrane by opening Ca2+ channels (D) hyperpolarizes the postsynaptic membrane by opening Cl− channels
The answer is D. An inhibitory postsynaptic potential hyperpolarizes the postsynaptic membrane, taking it farther from threshold. Opening Cl− channels would hyperpolarize the postsynaptic membrane by driving the membrane potential toward the Cl− equilibrium potential (about −90 mV). Opening Ca2+ channels would depolarize the postsynaptic membrane by driving it toward the Ca2+ equilibrium potential.
Which of the following has a much lower concentration in the cerebrospinal fluid (CSF) than in cerebral capillary blood? (A) Na+ (B) K+ (C) Osmolarity (D) Protein (E) Mg2+
The answer is D. Cerebrospinal fluid (CSF) is similar in composition to the interstitial fluid of the brain. Therefore, it is similar to an ultrafiltrate of plasma and has a very low protein concentration because large protein molecules cannot cross the blood-brain barrier. There are other differences in composition between CSF and blood that are created by transporters in the choroid plexus, but the low protein concentration of CSF is the most dramatic difference.
31. Which of the following structures has a primary function to coordinate rate, range, force, and direction of movement? (A) Primary motor cortex (B) Premotor cortex and supplementary motor cortex (C) Prefrontal cortex (D) Basal ganglia (E) Cerebellum
The answer is E Output of Purkinje cells from the cerebellar cortex to deep cerebellar nuclei is inhibitory. This output modulates movement and is responsible for the coordination that allows one to "catch a fly."
Which autonomic receptor mediates secretion of epinephrine by the adrenal medulla? (A) Adrenergic α receptors (B) Adrenergic β1 receptors (C) Adrenergic β2 receptors (D) Cholinergic muscarinic receptors (E) Cholinergic nicotinic receptors
The answer is E Preganglionic sympathetic fibers synapse on the chromaffin cells of the adrenal medulla at a nicotinic receptor. Epinephrine and, to a lesser extent, norepinephrine are released into the circulation.
During a nerve action potential, a stimulus is delivered as indicated by the arrow shown in the following figure. In response to the stimulus, a second action potential (A) of smaller magnitude will occur (B) of normal magnitude will occur (C) of normal magnitude will occur but will be delayed (D) will occur but will not have an overshoot (E) will not occur
The answer is E Because the stimulus was delivered during the absolute refractory period, no action potential occurs. The inactivation gates of the Na+ channel were closed by depolarization and remain closed until the membrane is repolarized. As long as the inactivation gates are closed, the Na+ channels cannot be opened to allow for another action potential.
7. Which of the following is a feature of the sympathetic, but not the parasympathetic nervous system? (A) Ganglia located in the effector organs (B) Long preganglionic neurons (C) Preganglionic neurons release norepinephrine (D) Preganglionic neurons release acetylcholine (ACh) (E) Preganglionic neurons originate in the thoracolumbar spinal cord (F) Postganglionic neurons synapse on effector organs (G) Postganglionic neurons release epinephrine (H) Postganglionic neurons release ACh
The answer is E Sympathetic preganglionic neurons originate in spinal cord segments T1-L3. Thus, the designation is thoracolumbar. The sympathetic nervous system is further characterized by short preganglionic neurons that synapse in ganglia located in the paravertebral chain (not in the effector organs) and postganglionic neurons that release norepinephrine (not epinephrine). Common features of the sympathetic and parasympathetic nervous systems are preganglionic neurons that release acetylcholine (ACh) and postganglionic neurons that synapse in effector organs.
Administration of which of the following drugs is contraindicated in a 10-year-old child with a history of asthma? (A) Albuterol (B) Epinephrine (C) Isoproterenol (D) Norepinephrine (E) Propranolol
The answer is E. Asthma, a disease involving increased resistance of the upper airways, is treated by administering drugs that produce bronchiolar dilation (i.e., β2 agonists). β2 Agonists include isoproterenol, albuterol, epinephrine, and, to a lesser extent, norepinephrine. β2 Antagonists, such as propranolol, are strictly contraindicated because they cause constriction of the bronchioles.
Adenosine triphosphate (ATP) is used indirectly for which of the following processes? (A) Accumulation of Ca2+ by the sarcoplasmic reticulum (SR) (B) Transport of Na+ from intracellular to extracellular fluid (C) Transport of K+ from extracellular to intracellular fluid (D) Transport of H+ from parietal cells into the lumen of the stomach (E) Absorption of glucose by intestinal epithelial cells
The answer is E. All of the processes listed are examples of primary active transport (and therefore use adenosine triphosphate [ATP] directly), except for absorption of glucose by intestinal epithelial cells, which occurs by secondary active transport (i.e., cotransport). Secondary active transport uses the Na+ gradient as an energy source and, therefore, uses ATP indirectly (to maintain the Na+ gradient).
A 56-year-old woman with severe muscle weakness is hospitalized. The only abnormality in her laboratory values is an elevated serum K+ concentration. The elevated serum K+ causes muscle weakness because (A) the resting membrane potential is hyperpolarized (B) the K+ equilibrium potential is hyperpolarized (C) the Na+ equilibrium potential is hyperpolarized (D) K+ channels are closed by depolarization (E) K+ channels are opened by depolarization (F) Na+ channels are closed by depolarization (G) Na+ channels are opened by depolarization
The answer is F. Elevated serum K+ concentration causes depolarization of the K+ equilibrium potential and therefore depolarization of the resting membrane potential in skeletal muscle. Sustained depolarization closes the inactivation gates on Na+ channels and prevents the occurrence of action potentials in the muscle.
A 42-year-old woman consults a dermatologist to evaluate and treat the frown lines on her forehead just above the nose. After the treatment options are explained to her, the patient asks the dermatologist to administer botulinum type A (Botox). Botox smooths out glabellar lines by which of the following mechanisms? a. Blocking the release of synaptic transmitter from α-motoneurons b. Decreasing the amount of calcium released from the sarcoplasmic reticulum c. Enhancing the enzymatic hydrolysis of acetylcholine at the neuromuscular junction d. Increasing the flow of blood into facial muscles e. Preventing the opening of sodium channels on muscle membranes
The answer is a. Botulinum toxin inhibits the release of acetylcholine from α-motoneurons by blocking one of the proteins responsible for the fusion of the synaptic channel with the presynaptic membrane. Botulinum toxin also inhibits the release of acetylcholine from the neurons of the autonomic nervous system. Botulinum toxins are used for a variety of cosmetic and therapeutic purposes. Generalized botulism-like weakness (iatrogenic botulism) is a rare complication of these procedures. Botulinum and tetanus toxin are released from the same class of bacteria (Clostridium). Illness begins with cranial nerve involvement and proceeds caudally to involve the extremities. Cases may be classified as food borne, wound botulism, and intestinal botulism. Because of its extraordinary potency, botulinum toxin has also been used as an agent of bioterrorism or biological warfare that could be acquired by inhalation or ingestion. Features of outbreaks suggesting deliberate release of botulinum toxin include infection with an unusual toxin type, outbreak of a large number of cases of acute flaccid paralysis with prominent bulbar palsies with a common geographic factor but without a common dietary exposure, or multiple simultaneous outbreaks without a common source. Tetanus toxin produces an increase in skeletal muscle contraction by blocking the release of inhibitory neurotransmitter from spinal interneurons.
A 17-year-old boy is admitted to the hospital with a traumatic brain injury, sustained when he fell off his motorcycle. He develops a fever of 102.2°F (39°C), which is unrelated to an infection or inflammation. The fever is most likely due to a lesion of which of the following? a. Anterior hypothalamus b. Arcuate nucleus c. Lateral hypothalamus d. Paraventricular nucleus e. Posterior nucleus
The answer is a. The hypothalamus regulates body temperature. Core body temperature, the temperature of the deep tissues of the body, is detected by thermoreceptors located within the anterior hypothalamus. The anterior hypothalamus also contains neurons responsible for initiating reflexes, such as vasodilation and sweating, which are designed to reduce body temperature. Heat-producing reflexes, such as shivering, and heat-maintenance reflexes, such as vasoconstriction, are initiated by neurons located within the posterior hypothalamus. The posterior hypothalamus regulates catecholamine secretion. The paraventricular nucleus regulates TSH secretion. The lateral hypothalamus regulates thirst, and the arcuate nucleus exerts neuroendocrine control of prolactin secretion.
A 20-year-old boxer presents at a neurologist's office complaining of dizziness and a problem with his balance. He indicates that in a recent match, he suffered several blows to the ears. Which of the following normally happens when a person slowly rotates toward the right? a. Both the left and right eyes deviate toward the left b. The endolymph in the left and right horizontal semicircular canals moves in opposite directions c. The hair cells in the left horizontal semicircular canal become depolarized d. The stereocilia on the hair cells in the right horizontal semicircular canal bend away from the kinocilium e. The visual image on the retina becomes unfocused
The answer is a. When the head rotates in one direction, the hair cells mounted on the cristae rotate along with the head. However, the flow of endolymph is delayed and as a result, the cupula is moved in a direction opposite to the movement of the head. When the head moves to the right, the cupula moves toward the left; this bends the stereo-cilia on the hair cells in the right horizontal canal toward the kinocilium and bends the stereocilia on the hair cells in the left horizontal canal toward the kinocilium. As a result, the hair cells in the right horizontal canal depolarize and those in the left horizontal canal hyperpolarize. The depolarization of the hair cells in the right horizontal canal stimulates the right vestibular nerve, which in turn causes the eyes to deviate toward the left. The movement of the eyes toward the left as the head deviates toward the right keeps the image on the retina in focus.
A 10-year-old boy sprains his ankle while running. History reveals that he has difficulty running, jumping, and keeping up with other children in races. His mother reports that she is also clumsy. Physical examination demonstrates foot drop, weakness, sensory loss, and reduced reflexes. The boy is found to have a decrease in nerve conduction velocity and an X-linked mutation of connexin 32, consistent with Charcot-Marie-Tooth (CMT) disease. The neuropathy and gait disorder result because connexin is an important component of which of the following? a. Gap junction b. Microtubule c. Sarcoplasmic reticulum d. Sodium channel e. Synaptic vesicle
The answer is a. - Connexin is a membrane-spanning protein that is used to create gap junction channels. The gap junction channel creates a cytoplasmic passage between two cells. Each cell membrane contains half of the channel. The channel, called a connexon, is constructed from six connexin molecules that form a cylinder with a pore at its center. CMT disease comprises a heterogeneous group of inherited peripheral neuropathies. Approximately 1 in 2500 persons has some form of CMT, making it one of the most frequently occurring inherited neuromuscular disorders. Demyelinating forms of CMT are classified as CMT1 and axonal forms as CMT2. Transmission is most frequently autosomal dominant, but it may also be autosomal recessive or X-linked, like the mutation affecting the connexin 32 (Cx32), located in the folds of the Schwann cell cytoplasm around the nodes of Ranvier. This localization suggests a role for gap junctions composed of Cx32 in ion and nutrient transfer around and across the myelin sheath of peripheral nerves.
A 16-year-old, highly allergic girl who is stung by a bee gives herself a shot of epinephrine prescribed by her physician. Because epinephrine activates β- adrenergic receptors, it will relieve the effects of the bee sting by decreasing which of the following? a. Contraction of airway smooth muscle b. Rate of depolarization in the SA node c. Rate of glycogenolysis in the liver d. Strength of ventricular muscle contraction e. Transport of calcium into skeletal muscle fibers
The answer is a. Epinephrine (adrenalin) acts on both α-and β-adrenergic receptors, but has a greater affinity for β-adrenergic receptors. Activation of β2 -adrenergic receptors leads to relaxation of smooth muscle in the bronchi, vasculature, intestine, uterus, and bladder; to increased pancreatic insulin and glucagon secretion; and an increase in liver glycogenolysis. The bronchodilator effects of epinephrine are key in the treatment of the life-threatening effects of anaphylactic shock. Activation of β1- and β-2-adrenergic receptors in the heart leads to an increase in the rate of SA nodal phase 4 depolarization and thus heart rate (positive chronotropic response), an increase in contractility (positive inotropic response), an increase in conduction velocity (positive dromotropic response), and an increase in cardiac excitability/irritability. The transport of Ca 2+ into skeletal muscle fibers is not affected by β-receptors.
A 42-year-old man sees his ophthalmologist because it is becoming increasingly difficult for him to read the newspaper. His vision problem most likely results from an inability to contract which of the following? a. Ciliary body b. Extraocular muscles c. Iris d. Pupils e. Suspensory ligaments
The answer is a. The ciliary body contains the ciliary muscle, which changes the shape of the lens when your eyes focus on something, a process called accommodation. Contracting the ciliary body increases the refractive power of the eye for near vision. When the ciliary muscle contracts, it pulls the suspensory ligaments toward the cornea, which causes the lens surface to bulge, increasing its refractive power. Contraction of the ciliary muscle, which causes short-range focus, is mediated by M3 cholinergic muscarinic receptors, and relaxation of the ciliary muscle, which causes long-range focus, is mediated by β2 -adrenergic receptors. The muscles of the iris control the size of the pupils, and the extraocular muscles control the position of the eye in the socket. Sympathetic activation causes dilation of the pupil (mydriasis) by stimulating α1 -adrenergic receptors, which lead to contraction of the radial muscle in the iris of the eye. Parasympathetic stimulation causes constriction of the pupil (miosis) due to contraction of the circular muscle in the eye mediated by M3 cholinergic muscarinic receptors.
At a first-grade parent-teacher conference, the teacher of a 6-year-old boy indicates that the boy seems to have difficulty hearing. His parents take him to the pediatrician, who refers the boy to an otolaryngologist. The boy is found to have a significant hearing deficit accompanying a middle ear infection that also involves the middle ear bones. Which of the following is the primary function of the middle ear bones? a. Amplifying sounds b. Enhancing frequency discrimination c. Filtering high-frequency sounds d. Localizing sound e. Protecting the ear from load sounds
The answer is a. When sound waves pass from air to water, most of the energy contained in the sound stimulus is lost. Because the auditory receptors within the inner ear are bathed in liquid, most of the energy in the sound stimulus could be lost as the sound travels from air to water. The bones of the middle ear significantly reduce the amount of loss by amplifying the sound stimulus. Audiologists refer to this amplification phenomenon as impedance matching. Sound localization is carried out by the CNS, which integrates information from both ears. Frequency discrimination is a function of the basilar membrane. The stapedius and tensor tympani muscles protect the ear from loud sounds.
A 22-year-old woman presents at the student medical center with tachycardia and palpitations. She reports that she has been taking a diet supplement containing ephedrine that she purchased from an Internet site. Activation of the sympathetic nervous system by ephedrine causes smooth muscle contraction in which of the following? a. Arterioles b. Bronchioles c. Ciliary bodies d. Intestines e. Pupils
The answer is a. - The catecholamine, norepinephrine (and epinephrine), activates both α- and β-adrenergic receptors. α1 -Adrenergic receptors predominate on arteriolar smooth muscle, so these muscles contract when stimulated with norepinephrine. Ephedrine is both a direct and indirectly acting sympathomimetic amine. Its direct action to activate postsynaptic α-receptors and β-receptors is weak. Ephedrine's actions are primarily due to its effects as an indirect sympathomimetic, which involves its uptake into the presynaptic nerve terminal, where it is packaged and released with norepinephrine from the sympathetic nerve terminals. The effect of adding ephedrine is to increase the number of vesicles released during each action potential and possibly to extend the duration of action of norepinephrine by prolonging its inactivation via the neuronal reuptake process. Therefore, the actions of norepinephrine are enhanced in the presence of ephedrine. There are no α-receptors in the bronchioles, pupils, or ciliary smooth muscles in the ciliary body of the eye, so norepinephrine does not cause contraction in these areas, but rather smooth muscle relaxation caused by activation of β-receptors in these tissues. Intestinal smooth muscles have α2 -adrenergic receptor, which mediates relaxation when bound to norepinephrine.
A 16-year-old adolescent boy is brought to the emergency room by ambulance after suffering a concussion during a football game. When he awoke, he was able to understand and follow commands, including repeating language spoken to him, but he had difficulty with spelling, mild word-finding difficulty, and difficulty understanding written language and pictures. His condition is most likely caused by damage to which of the following? a. Angular gyrus in the categorical hemisphere b. Arcuate fasciculus connectingBroca and Wernicke areas c. Broca area in the frontal lobe d. Hippocampus e. Wernicke area at the posterior end of the superior temporal gyrus
The answer is a. Aphasias are language disorders in which a person is unable to properly express or understand certain aspects of written or spoken language. Aphasias are caused by lesions to the language centers, which are located in the categorical hemisphere of the neocortex. There are a number of different classifications of aphasias, but one divides them into fluent, nonfluent, and anomic aphasias. In this case, the boy developed an anomic aphasia, in which he was able to understand and follow commands, but he had difficulty understanding written language and pictures. There may be mild word-finding difficulty with this aphasia as well. Anomic aphasia is the single most common language disturbance seen in head trauma, metabolic encephalopathy, and Alzheimer disease. Anomic aphasia can be caused by lesions anywhere within the language network, but often is caused by damage to the angular gyrus without damage to Broca or Wernicke areas. A lesion in Broca area leads to nonfluent aphasia, and would have resulted in a severe impairment in expressive language. Fluent aphasias are due to lesions in Wernicke area or to lesions in and around the auditory cortex. Language disorders caused by memory loss, which could be the result of a hippocampal lesion, are not classified as aphasias, nor are language disorders caused by vision or hearing abnormalities or motor paralysis. Damage to the arcuate fasciculus would be incorrect because the patient was able to verbally repeat language spoken to him.
A 16-year-old adolescent girl with epilepsy has an EEG recording done during a routine visit to her neurologist. The α-rhythm appearing on an EEG has which of the following characteristics? a. It disappears when a patient's eyes open b. It is associated with deep sleep c. It is replaced by slower, larger waves duringREM sleep d. It produces 20 to 30 waves per second e. It represents activity that is most pronounced in the frontal region of the brain
The answer is a. As shown in the EEG below, in a totally relaxed adult with eyes closed, the major component of the EEG will be a regular pattern of 8 to 12 waves per second observed over the posterior (occipital) brain regions, called the α-rhythm or posterior dominant rhythm. The α-rhythm disappears when the eyes are opened. It is most prominent in the occipital region. In deep sleep, the α-rhythm is replaced by larger, slower waves called delta waves. In REM sleep, the EEG will show fast, irregular activity.
A 24-year-old male medical student develops apprehension, restlessness, tachycardia, and tachypnea as he enters the testing center for his initial licensure examination. Activation of which of the following receptors would be expected to decrease his anxiety? a. GABAA b. Glutamate c. Histamine d. Neurokinin 1 (NK-1, substance P) e. Neuropeptide Y
The answer is a. Gamma-aminobutyric acid (GABA) is the major inhibitory mediator in the brain. GABAA receptors are pentameric Cl− ion channels that are widely distributed in the CNS. The increase in Cl− conductance produced by GABAA receptors is potentiated by the anxiolytic drug, diazepam, and other benzodiazepines. Gluta-mate is the major excitatory transmitter in the brain. Neuropeptide Y is an excitatory neurotransmitter that has a stimulatory effect on food intake. CNS actions of histamine have been implicated in arousal, sexual behavior, drinking, pain thresholds, and the sensation of itch. Antagonism of central NK-1 receptors has antidepressant activity in humans.
A 62-year-old man with COPD presents to the emergency room in respiratory distress. The attending physician uses succinylcholine to produce skeletal muscle relaxation prior to tracheal intubation. Soon after infusion of the succinylcholine, the patient develops a severe bradycardia. Which of the following drugs would counteract the bradycardia without affecting muscle relaxation? a. Acetylcholine b. Atropine c. Curare d. Dopamine e. Epinephrine
The answer is b. Succinylcholine is a rapidly acting neuromuscular-blocking agent with a very short duration of action. Respiratory paralysis can be produced in less than 60 seconds and normal respiration typically returns within 15 minutes. Because succinylcholine can also stimulate autonomic postganglionic fibers, vagal fibers innervating the heart are stimulated. The vagal fibers release acetylcholine, which binds to muscarinic receptors on the SA node, slowing down the heart. The bradycardia can be prevented by administering atropine, which blocks the muscarinic receptors on the SA node.
A 19-year-old woman with a history of diplopia and paresthesia is diagnosed with MS. Immersion of an affected limb in a cold bath restores nerve conduction in many MS patients. The explanation often cited for this effect is that cold increases the duration of the action potential. Which of the following best explains why increasing the duration of the action potential can restore nerve conduction in patients with MS? a. The amount of sodium entering the nerve with each action potential increases b. The capacitance of the nerve fiber membrane is increased c. The duration of the refractory period is increased d. The potassium conductance of the membrane is increased e. The membrane potential becomes more positive
The answer is b. Vitamin B12 is necessary for normal neurological function because it is involved in myelin synthesis and repair. Vitamin B12 deficiency causes damage to the white matter of the spinal cord and peripheral neuropathy. Treatment consists of vitamin B12 administration. Although folate may be an adequate substitute for treatment of the megaloblastic anemia characteristic of vitamin B12 deficiency, folate should not be used instead of vitamin B12 because the neurological deficits will persist and progress. Vitamin B12 does not modulate the sodium/potassium pump. Tau hyper-phosphorylation causes neuritic tangles, as seen in the brains of dementia patients. Vitamin B12 or its deficiency does not stimulate formation of ANA.
A 62-year-old man with a history of hypertension and hyperlipidemia is admitted to the hospital for evaluation after demonstrating signs and symptoms of a stroke. Subsequent CT scans, perceptual tests, and a neurological examination provide evidence for impairment of the otolith pathways. The otolith organs (utricle and saccule) are responsible for which of the following? a. Detecting angular acceleration b. Detecting the position of the head in space c. Producing rotary nystagmus d. Producing the stretch reflex e. Producing the vestibulo-ocular reflex
The answer is b. The otolith organs provide information about the position of the head with respect to gravity. When the head is bent away from its normal upright position, otoliths (small calcium carbonate crystals within the utricle and saccule) are pulled downward by gravity. The crystals bend the stereocilia on the hair cells, causing the hair cells to depolarize. Depolarization of the hair cells stimulates the vestibular nerve fibers. Bending the head in different directions causes different otoliths to move. Therefore, the particular group of vestibular nerve fibers that is stimulated signals the direction in which the head bends.
A 59-year-old woman with bilateral glaucoma is treated with drops of the parasympathetic agent pilocarpine. Cholinergic stimulation of the pupil causes which of the following? a. Absence of the pupillary response to light b. Inequality of pupil size c. Pupillary constriction (miosis) d. Pupillary dilation (mydriasis) e. Tonic pupil (slow redilation after exposure to light)
The answer is c. Parasympathetic stimulation or cholinergic muscarinic agonists used to treat glaucoma produce miosis, that is, pupillary constriction caused by contraction of the sphincter muscle of the iris. Anticholinergic agents (eg, atropine) or sympathetic stimulation produce mydriasis, that is, pupillary dilation, in which the increase in pupil size results from contraction of the radial muscle of the iris mediated by α1 - adrenergic receptors.
A 27-year-old musician visits an otolaryngologist complaining of ringing in his ear. An audiometry test reveals a high-frequency hearing loss in which the threshold for hearing high-frequency sounds is raised by 1000 times. If a patient is unable to hear high-frequency sounds, the damage to the basilar membrane is closest to which of the following structures? a. Helicotrema b. Modiolus c. Oval window d. Spiral ganglion e. Stria vascularis
The answer is c. The portion of the basilar membrane vibrated by a sound depends on the frequency of the sound. High-frequency sounds produce a vibration of the basilar membrane at the base of the cochlea (near the oval and round windows); low-frequency sounds produce a vibration of the basilar membrane at the apex of the cochlea (near the helicotrema). The modiolus is the bony center of the cochlea from which the basilar membrane emerges, the spiral ganglion contains the cell bodies of the auditory nerve fibers, and the stria vascularis is the vascular bed located on the outer wall of the scala media of the cochlea responsible for endolymph secretion.
10. A 43-year-old pregnant woman develops preeclampsia at 32 weeks' gestation. Intravenous labetalol is given to reduce blood pressure, and magnesium sulfate, which blocks N-methyl-D-aspartate (NMDA) receptors in the central nervous system, is ordered for the prevention of eclamptic seizures until the fetus can be delivered. Which of the following activates the NMDA receptor? a. Acetylcholine b. Gamma-aminobutyric acid (GABA) c. Glutamate d. Glycine e. Kainate
The answer is c. Glutamate is the major neurotransmitter that mediates synaptic excitation in the central nervous system, and glutamate receptors are also known as excitatory amino acid receptors. The NMDA (N-methyl-D-aspartate) receptor channel is one of the five different classes of excitatory amino acid receptors. The NMDA receptor is a large channel permeable to Ca 2+, K+, and Na+ . It is activated by glutamate, but unlike other glutamate receptor channels, the NMDA channel is blocked by Mg
A 26-year-old woman presents with unilateral facial weakness. She states that whenever she tries to close her eyes, the upper eyelid on the affected side rolls upward. Electromyography on the affected side shows evidence of axonal degeneration. Which of the following characteristics of an axon is most dependent on its diameter? a. The magnitude of its resting potential b. The duration of its refractory period c. The conduction velocity of its action potential d. The overshoot of its action potential e. The activity of its sodium-potassium pump
The answer is c. The conduction velocity of an action potential along an axon is proportional to the axon's diameter for both nonmyelinated and myelinated axons. For any given axon diameter, conduction velocity is greater in myelinated than in nonmyelinated fibers. Propagation via saltatory conduction in myelinated fibers is faster than propagation in nonmyelinated fibers of the same axon diameter because less charge leaks out through the myelin-covered sections of the membrane.
A 20-year-old woman complains of altered taste following extraction of her wisdom teeth. Which of the following is the most likely cause of the dysgeusia? a. A disturbed salivary milieu b. Damage to the central gustatory pathways c. Damage to the gustatory afferent nerves d. Impaired transport of the tastant to the receptor cells in the taste bud e. Injury to the taste receptor cells
The answer is c. Trauma to the chorda tympani branch of the facial nerve during third molar extractions or middle ear surgery is relatively common and can cause dysgeusia. Other mechanisms of disorders of the sense of taste, besides damage to the gustatory afferent nerves, include damage to central gustatory pathways (trauma, diabetes mellitus, hypothyroidism, stroke, CNS disorders), sensory losses (aging, Candidiasis, viral infections, many drugs especially those that interfere with cell turnover such as antineoplastic and antithyroid agents), and transport gustatory losses (interference with access of tastant to receptor cells, such as with xerostomia, Sjögren syndrome, heavy metal intoxication, oral radiation therapy). No effective therapies exist for the sensorineural disorders of taste. Altered taste due to surgical stretch of the chorda tympani nerve usually improves within 3 to 4 months.
The morning after a rock concert, a 20-year-old college student notices difficulty hearing his professor during lecture. The physician at the student health center suspects possible damage to his hair cells by the loud music. Depolarization of the hair cells in the cochlea is caused primarily by the flow of which of the following? a. Ca 2+ into the hair cell b. Cl− out of the hair cell c. K+ into the hair cell d. K+ out of the hair cell e. Na+ into the hair cell
The answer is c. When the hair cells are bent, K+ -selective channels open, K+ flows into the cell, and the cell depolarizes. This unusual situation occurs because the apical surface of the hair cells, on which the stereocilia are located, is bathed in endolymph, which contains a high concentration of K+ Moreover, the endolymph is positively charged with respect to the perilymph, which surrounds the basal lateral portion of the hair cell. Because the intracellular concentration of K+ is similar to the extracellular concentration of K+ the electrical gradient determines the direction of K+ flow. Because the endolymph is positively charged and the intracellular fluid is negatively charged, K+ flows into the cell.
A 2-day-old infant starts having brief tonic-clonic seizures throughout the day. His neurological function in between seizures is normal, and he has no other medical or neurological problems. The history reveals no readily apparent causes for the seizures, though the mother recalled that her first baby also developed seizures shortly after birth that only lasted for 2 weeks, with no subsequent episodes or developmental problems. Genetic analysis revealed a mutation of voltage-gated K + channels consistent with a diagnosis of benign familial neonatal seizures. Which of the following would cause an immediate reduction in the amount of potassium leaking out of a cell? a. Decreasing the extracellular potassium concentration b. Decreasing the extracellular sodium concentration c. Hyperpolarizing the membrane potential d. Increasing the permeability of the membrane to potassium e. Reducing the activity of the sodium-potassium pump
The answer is c. (Barrett, pp 9-10, 276-278. McPhee and Hammer, pp 169-171.) The amount of potassium moving out of the cell depends on its membrane potential, its concentration gradient, and its membrane conductance. According to the Nernst equation, the electrical gradient for K + is inward and the concentration gradient is outward. Hyperpolarizing the membrane makes the inside of the cell more negative and therefore makes it more difficult for potassium to flow out of the cell. Answer (a) is incorrect because decreasing the extracellular potassium concentration would increase the flow of potassium out of the cell, as would increasing the permeability of the membrane to potassium (answer d). Decreasing the activity of the sodium-potassium pump (answer e) has no immediate effect on potassium efflux, but ultimately results in depolarization of the membrane, resulting in an increased flow of potassium out of the cell. Altering the extracellular sodium concentration (answer b) has no immediate effect on the flow of potassium across the membrane. Seizures are paroxysmal disturbances in cerebral function caused by an abnormal synchronous discharge of cortical neurons. The epilepsies are a group of disorders characterized by recurrent seizures. Benign familial neonatal seizures constitute a rare type of idiopathic epilepsy linked to autosomal dominant mutations of voltage-gated K + channels. The seizures are paroxysmal, generally appearing within the first week to month of life, and generally resolve spontaneously within days to weeks after onset. The seizures are brief tonic-clonic seizures with little or no postictal state. There are generally no resultant developmental problems, though there may be a predisposition to developing epilepsy later in life.
A 41-year-old man complains to his physician about jet lag whenever he flies long distances to meetings. Melatonin is prescribed as a way to reset his circadian rhythm. The circadian rhythm is controlled by which of the following nuclei? a. Arcuate b. Lateral c. Paraventricular d. Suprachiasmatic e. Ventromedial
The answer is d. A variety of physiological functions, such as alertness (the sleep-wake cycle), body temperature, and secretion of hormones, exhibits cyclic activity that varies over a 24-hour period of time. These variations in activity are called circadian rhythms and are controlled by the suprachiasmatic nucleus of the hypothalamus. The paraventricular nucleus secretes oxytocin and vasopressin, the ventromedial and lateral nuclei control food intake, and the arcuate nucleus secretes gonadotropin-releasing hormone.
A 49-year-old man in end-stage renal failure is able to perform peritoneal dialysis at home. The osmolality of the solution chosen for peritoneal dialysis will determine the rate of ultrafiltration. Which of the following statements best characterizes a molecule whose osmolality is zero? a. It will not permeate the membrane. b. It can only cross the membrane through the lipid bilayer. c. It causes water to flow across the membrane. d. It is as diffusible through the membrane as water. e. It is transported across the membrane by a carrier.
The answer is d. The osmolality of a substance is the number of osmoles per kg of solvent. One osmole (Osm) equals the gram molecular weight of a substance divided by the number of free-moving particles that each molecule liberates in solution. Osmotically active substances in the body are dissolved in water, and the density of water is 1. Thus, osmolar concentrations can be expressed as osmoles (or milliosmoles) per liter of water. If the osmolality is zero, there are no free-moving particles and, thus, the molecule is as diffusible as water through the membrane.
A 10-year-old girl with type I diabetes develops a neuropathy of sensory neurons with free nerve endings. As a result, quantitative sensory testing will reveal higher-than-normal thresholds for detection of which of the following stimuli? a. Fine touch b. Muscle length c. Pressure d. Temperature e. Vibration
The answer is d. Free nerve endings are small diameter nerves that contain receptors for temperature, pain, and crude touch. Fine touch, pressure, and vibration are detected by nerve endings contained within specialized capsules that transmit the stimulus to the sensory receptors. Muscle length is encoded by the primary nerve endings of Ia fibers, which are located on intrafusal fibers within the muscle spindle.
A 41-year-old man is seen by his physician complaining of "always feeling tired" and having "vivid dreams when he is sleeping." He is referred to the hospital's sleep center for evaluation. He is diagnosed with narcolepsy based on his clinical history and the presence of rapid eye movements (REM) as soon as he falls asleep. Which of the following signs will be observed when the patient is exhibiting REM sleep? a. High-amplitude electroencephalogram (EEG) waves b. Hyperventilation c. Low frequency EEG waves d. Periods of loss of skeletal muscle tone e. Slow but steady heart rate
The answer is d. In a normal sleep cycle, a person passes through the three stages of non-REM sleep before entering REM sleep. In narcolepsy, a person may pass directly from the waking state to REM sleep. REM sleep is characterized by irregular heartbeats and respiration and atonia (loss of muscle tone). Hypoventilation is characteristic of both REM and non-REM sleep because sleep depresses the central chemoreceptors. It is also the state of sleep in which dreaming occurs. High-amplitude EEG waves occur in the late stages of slow-wave sleep. EEG during REM sleep is low amplitude, high frequency.
A 13-year-old boy on the junior high wrestling team experienced attacks of proximal muscle weakness that lasted from 30 minutes to as long as 4 hours following exercise and fasting. The trainer attributed it to the symptoms of fatigue, but his mother recalled having similar symptoms when she was on a diet and exercise regime. Genetic testing revealed an inherited channelopathy. Electrically excitable gates are normally involved in which of the following? a. Depolarization of the end-plate membrane by acetylcholine b. Hyperpolarization of rods by light c. Increase in nerve cell potassium conductance caused by membrane depolarization d. Release of calcium from ventricular muscle sarcoplasmic reticulum e. Transport of glucose into cells by a sodium-dependent, secondary active transport system
The answer is d. Electrically excitable gates are those that respond to a change in membrane potential. The most notable electrically excitable gates are those on the sodium and potassium channels that produce the nerve action potential. The potassium channel gate is opened by depolarization. Ventricular muscle sarcoplasmic reticulum releases its calcium in response to an increase in intracellular calcium. The gates opened by ACh are chemically excitable gates. In rods, sodium channels are closed when cGMP is hydrolyzed. Electrically excitable gates do not regulate the active transport of glucose.
81. An 82-year-old woman with end-stage renal disease is brought to the emergency department complaining of nausea, vomiting, muscle cramps, and generalized weakness. Laboratory analysis reveals significant hyper-kalemia. Elevations of extracellular potassium ion concentration will have which of the following effects on nerve membranes? a. The activity of the Na+-K+pump will decrease b. The membrane potential will become more negative c. The membrane will become more excitable d. Potassium conductance will increase e. Sodium conductance will increase
The answer is d. (Barrett, pp 88-89. Longo, pp 355-359.) Because the resting membrane potential is related to the ratio of ICF to ECF K+ concentration, an increase in extracellular K+ partially depolarizes the cell membrane, that is, makes the membrane potential more positive. Depolarizing the membrane opens K+ channels, causing an increase in membrane conductance to potassium. Prolonged depolarization, whether caused by an increase in extracellular K+ or by an action potential, inactivates Na+ channels and decreases the excitability of the nerve membrane, which manifests as weakness, and which may progress to flaccid paralysis. The activity of the Na+-K+ pump is reduced in hypokalemia, not in hyperkalemia.
An 86-year-old woman develops unilateral vesicular eruption on the trunk in a T8 dermatomal pattern. Staining of the skin scrapings confirms a diagnosis of herpes zoster. The woman complains of significant pain, as well as increased sensitivity to touch. Which of the following receptors is responsible for measuring the intensity of a steady pressure on the skin surface? a. Krause ending b. Meissner corpuscle c. Merkel disk d. Pacinian corpuscle e. Ruffini ending
The answer is e. The Ruffini ending is a tonic receptor that produces a train of action potentials proportional to the intensity of pressure applied to the skin. The Pacinian corpuscle is a very rapidly adapting receptor that fires once or twice in response to skin deformation, but can produce a continuous train of action potentials if the stimulus is repetitively applied and withdrawn. Therefore, the Pacinian corpuscle is used to encode vibration.
A 52-year-old man has a motor vehicle accident and closed head injury, after which he notes decreased sense of smell. Anosmia after head injury is most likely associated with which of the following? a. Direct injury to the olfactory bulbs b. Fractures of the cribriform plate c. Low Glasgow coma scale d. Recovery of olfaction over time to baseline e. Shearing of the olfactory fila as they pass through the cribriform plate
The answer is e. "The physiologic basis for most head trauma-related smelling losses is the shearing and subsequent scarring of the olfactory fila as they pass from the nasal cavity into the brain cavity. The cribriform plate does not have to be fractured or show pathology for smell loss to be present. Severity of trauma, as indexed by a poor Glasgow Coma Rating on presentation, and the length of posttraumatic amnesia, is associated with a higher risk of olfactory impairment. Fewer than 10% of posttraumatic anosmic patients recover age-related normal function over time."
A 72-year-old man develops selective loss of the large pyramidal cells in the precentral gyrus and degeneration of the corticospinal and corticobulbar projections. Other neuronal systems are spared. He is told that the progression of the disease is variable, and that the worst prognosis is about a 3-year survival. The precentral gyrus and corticospinal and corticobulbar tracts are essential for which of the following? a. Auditory identification b. Kinesthesia c. Olfaction d. Vision e. Voluntary movement
The answer is e. The precentral gyrus is the motor area of the cortex that contains the cell bodies of the neurons that form the corticospinal tract (also referred to as the pyramidal tract). The corticospinal tract contains axons that cross to the contralateral side of the brain within the pyramids and end within the motor areas of the spinal cord. These structures are essential for the generation of fine voluntary movements. Kinesthesia, the sense of movement and position of the limbs, is handled primarily by the Ia and Ib afferents that innervate the muscle spindles and Golgi tendon organs, respectively, and by the parietal lobe. Olfaction is processed in the frontal lobe. Cortical localization of visual function is within the occipital lobes, and the optic tracts. Primary lateral sclerosis (PLS) is a rare disorder arising sporadically in midto-late life. PLS is characterized clinically by progressive spastic weakness of the limbs, preceded or followed by spastic dysarthria and dysphagia, indicating combined involvement of corticospinal and corticobulbar tracts. Sensory changes are absent and neither EMG nor muscle biopsy shows denervation.
A 48-year-old woman with multiple sclerosis (MS) and increasing spasticity is treated with an intrathecal infusion of baclofen, a GABAB agonist that mediates presynaptic inhibition. Presynaptic inhibition in the central nervous system (CNS) affects the firing rate of α-motoneurons by which of the following mechanisms? a. Decreasing the frequency of action potentials by the presynaptic nerve ending b. Decreasing the potassium permeability of the α-motoneuron c. Hyperpolarizing the membrane potential of the α-motoneuron d. Increasing the amount of the neurotransmitter released by the presynaptic nerve ending e. Increasing the chloride permeability of the presynaptic nerve ending
The answer is e. Presynaptic inhibition is caused by interneurons that secrete a transmitter that increases the Cl − conductance of the presynaptic nerve ending. The increase in Cl− conductance causes a partial depolarization of the presynaptic nerve ending and a decrease in the magnitude of the action potential in the pre-synaptic nerve ending. Because the number of synaptic vesicles released from the presynaptic neuron is proportional to the magnitude of the action potential, fewer vesicles are released and magnitude of the postsynaptic potential is reduced. Reducing the magnitude of the postsynaptic potential decreases the probability that an action potential will be generated by the postsynaptic cell. Presynaptic inhibition does not change the membrane potential of the α-motoneuron.
A 27-year-old man with severe epilepsy, characterized by major convulsions and lapses of consciousness every few minutes, underwent experimental neurosurgery to help relieve his seizures. The operation had a significant, beneficial effect on the epilepsy, but led to a devastating memory deficit. He had normal procedural memory, maintained long-term memory for events that occurred prior to surgery, and his short-term memory was intact, but he could not commit new events to long-term memory (loss of declarative memory). Which of the following areas of the brain was bilaterally resected in this patient? a. Cerebral cortex b. Cingulate gyrus c. Hypothalamus d. Parietal lobe e. Temporal lobe
The answer is e. The patient underwent bilateral surgical removal of the amygdala, large portions of the hippocampal formation, and portions of the association area of the temporal cortex. His case was the first to bring attention to the critical role of the temporal lobes in formation of long-term declarative memories and to implicate this region in the conversion of short- to long-term memories. Later work showed that the hippocampus is the primary structure within the temporal lobe involved in this conversion. Because the patient retained memories from before surgery, his case also shows that the hippocampus is not involved in the storage of declarative memory.
A 72-year-old man visits his physician because he finds it difficult to hold his hand steady when painting. Examination reveals a resting tremor and rigidity. The symptoms are relieved by a single dose of levodopa. This patient's neurological signs are most likely related to a lesion within which of the following? a. Caudate nucleus and putamen b. Cerebellum c. Hippocampus d. Premotor area e. Substantia nigra
The answer is e. Longo, pp 3317-3335.) These findings are consistent with the presence of Parkinson disease, which is characterized by resting tremor rigidity and bradykinesia. It is caused by destruction of the dopa-mine-secreting neurons within the substantia nigra pars compacta. Levo (L)-dopa is a precursor for dopamine. L-dopa, rather than dopamine, is administered because it can cross the blood-brain barrier, but dopamine cannot. In contrast to the resting tremor of Parkinson disease, cerebellar disease is characterized by an intention tremor. In contrast to damage to the nigrostriatal dopaminergic system in Parkinson disease, Huntington disease results in a loss of the intrastriatal GABAergic and cholinergic neurons in the caudate nucleus and putamen of the basal ganglion, and is not responsive to treatment with L-dopa. Hippocampal lesions do not cause Parkinson disease.
A 58-year-old woman goes to her physician because she is having difficulty threading needles. An eye examination leads to the diagnosis of presbyopia (old eyes). Her condition is most likely caused by which of the following? a. Ciliary muscle paralysis b. Clouding of the vitreous c. Degeneration of the macula d. Retinal detachment e. Stiffening of the lens
The answer is e. The increase in lens power that normally occurs when objects are placed close to the eye (the accommodation reflex) does not take place in presbyopia. The failure of the accommodation reflex occurs because the lens and lens capsule stiffen with age. There are some reports of ciliary muscle weakness accompanying presbyopia, but there are none indicating that presbyopia is caused by ciliary muscle paralysis.
A fireman suffers extensive burns, resulting in a fluid and electrolyte imbalance. Which of the following electrolyte imbalances will produce a decrease in the magnitude of a nerve membrane action potential? a. Hyperkalemia b. Hypernatremia c. Hypocalcemia d. Hypokalemia e. Hyponatremia
The answer is e. (Barrett, pp 87-89. Kaufman, pp 246-247.) The upstroke of the action potential is caused by an inward flow of sodium ions, and therefore its magnitude depends on the extracellular sodium concentration. Decreasing the external Na+ concentration decreases the size of the action potential, but has little effect on the resting membrane potential because the permeability of the membrane to Na+ at rest is low. Conversely, increasing the external K+ concentration decreases the resting membrane potential. Changes in external Ca 2+ concentration affect the excitability of nerve and muscle cells, but not the magnitude of the resting potential or the action potential.
