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B. Ouabain (digoxin) is a cardiac glycoside that irreversibly blocksNa+/K+ ATPase, allowing equilibration of sodium and potassiumacross the cell membrane. This stops sodium-dependent transportprocesses and depolarizes the membrane potential.6.7.8.9.10.11.12.Section 2: The Nervous System and Muscle1. A. Reduced permeability of sodium ion will result in hyperpolarization of the cell. Ordinarily, sodium leakage into the cell contributesto the actual resting membrane potential, making it less negativethan would be predicted on the basis of potassium concentrationand permeability.
Reduction in potassium ion permeability, on theother hand, would cause depolarization of the cell, as would influxof calcium ion or increased extracellular concentration of sodiumor potassium ion.2. C. Neuronal action potentials result when the threshold potentialis reached and voltage-gated sodium channels open. Althoughaction potentials are evoked in some excitable cells by opening ofCa2+ channels, opening of voltage-gated Na+ channels is responsiblefor the phase 0 upstroke of the neuronal action potential.3. D.
Myelination of an axon results in decreased capacitance andincreased membrane resistance. As a result, current travels throughthe interior of the axon but not through the membrane. Myelinationproduces a large increase in conduction velocity.4. A. When axonal depolarization reaches the synaptic bouton,voltage-gated Ca2+ channels are opened, resulting in influx of Ca2+and vesicular release of neurotransmitter into the synaptic cleft.5. D. A single, large Na+ and K+ channel conducts the depolarizingcharge during an endplate potential while the neuronal actionpotential involves multiple ion channels and depolarization is13.14.15.mainly produced by Na+ influx.
Depolarization is rapid in bothendplate and action potentials. Depolarization to +40 mV, causation by voltage-gated channels, and repolarization associated withincreased K+ conductance are all characteristics of neuronal actionpotentials but not endplate potentials.E. The neural tissue of the spinal cord is covered by three membranes (meninges). These are the inner pia mater, the middle arachnoid membrane, and the outer dura mater.B. CSF is produced by the choroid plexus and is reabsorbed at thearachnoid granulations into the venous system, as well as into thecapillaries of the CNS and pia mater.E.
Association pathways functionally and anatomically linking thetwo cerebral hemispheres include the corpus callosum and anterior,posterior, and hippocampal commissures.C. While the pons is also involved in regulation of breathing, themedulla is the major site of autonomic regulation, including regulation and integration of cardiovascular and respiratory functions.The thalamus and hypothalamus are not part of the brainstem.A.
Meissner’s corpuscles, located in the dermal papillae, especiallyin fingertips, palms, soles, lips, face, tongue, and genital skin (nonhairy skin), are rapidly adapting receptors with small receptive fieldsthat allow point discrimination and detection of low-frequencystimuli such as flutter. Pacinian corpuscles respond to rapid changesin pressure and vibration, Merkel’s disks respond to pressure andtouch, especially indentation of the skin; hair follicle receptorsrespond to movement across the skin surface; and Ruffini’s corpuscles respond to stretch of skin and joints.B.
Photoreceptors respond to light reception through a sequenceof events that ultimately results in reduced Na+ permeability andhyperpolarization of their cell membranes, which inhibits neurotransmitter release by the photoreceptors.E. The three ducts of the membranous labyrinth of the cochlea ofthe inner ear are the scala vestibuli, scala tympani, and scala media.The scala media contains endolymph, which resembles intracellularfluid in its composition. The other two ducts contain perilymph,which resembles extracellular fluid in its composition.E. The pattern of pressure changes between the three perpendicularly oriented semicircular canals during angular acceleration of thehead is dependent on the direction of head movement.
The bendingof cilia as a result of these pressure changes produces hyperpolarization or depolarization and changes in neurotransmitter release byhair cells. As a result, the firing rate of afferent nerve fibers endingon the hair cells is altered. These signals are carried through cranialnerve VIII to the vestibular nuclei of the pons.A. When the patellar tendon of the knee is tapped, muscle spindlesin the quadriceps muscle are stretched, producing a reflex in whichtype Ia afferent nerves are depolarized, and conduct the signal tothe spinal cord.
The nerves synapse with α-motor neurons, whichconduct the signal back to the quadriceps, causing contraction. Thisstretch reflex is a monosynaptic reflex. Golgi tendon reflex is abisynaptic reflex involved in preventing muscle damage whenmuscle tension is excessive. The flexor withdrawal reflex is a polysynaptic reflex that occurs in response to painful stimuli.E. The corticospinal tract carries fibers that originate in the primarymotor cortex, premotor and supplemental motor areas, and somatosensory areas posterior to the motor cortex.
It is the most importantpathway for fine motor activity controlled by the cortex. The other37137216.17.18.19.20.Answerstracts mentioned are involved in other aspects of motor function,including brainstem control of balance and posture.C. The cerebellum coordinates and fine-tunes motion. All of theefferent signals from the cerebellar cortex are inhibitory, originatingfrom the Purkinje cells. This inhibitory output is regulated by excitatory input from climbing fibers and mossy fibers by way of thegranule cells, and inhibitory input from interneurons in the cerebellar cortex.B.
Like the cerebellum, the basal ganglia have an accessory role tothe motor cortex. With input from the motor cortex and output tothe motor cortex through the thalamus, the basal ganglia functionto produce smooth movement and regulate posture.B. Preganglionic nerves of the sympathetic nervous system originate in the intermediolateral and intermediomedial cell columns ofthe thoracolumbar spinal cord (T1 to L3). Their axons extend to thechain ganglia, where they synapse with postganglionic nerves.
Preganglionic nerves of the parasympathetic nervous system have cellbodies in the brainstem.D. Postganglionic neurons of the parasympathetic nervous system(PNS) release acetylcholine, which acts at muscarinic receptors atthe neuroeffector junction. Postganglionic neurons of the sympathetic nervous system (SNS) primarily release norepinephrine andepinephrine, which act at a variety of adrenergic receptors on effector organs and tissues. Both sympathetic and parasympathetic preganglionic nerves achieve ganglionic neurotransmission by releaseof acetylcholine, which acts at nicotinic receptors.C.
β2-adrenergic receptors mediate sympathetic bronchial dilation.Constriction of vascular smooth muscle and mydriasis are producedby norepinephrine and epinephrine release by postganglionic sympathetic nerves and subsequent binding to α1 receptors; increasedcontractility is mediated by β2 receptors.
Sweat gland secretion isstimulated by atypical sympathetic postganglionic nerve fibers thatsecrete acetylcholine.7.8.9.10.11.Section 3: Cardiovascular Physiology1. B. The systemic veins normally contain more than 60% of theblood volume and can act as a reservoir for blood. Blood can bemobilized from veins during hemorrhage or dehydration as a compensatory measure to help maintain blood pressure until bloodvolume can be restored.2. C. Contraction of the left ventricle consists of powerful constrictionof the chamber with shortening of the heart from base to apex,resulting in the high pressures required to pump blood through thehigh resistance systemic circulation.3. A. The coronary circulation receives approximately 4% of thecardiac output at rest, while the other listed parts of the circulationeach receive approximately 15% (brain) to 24% (liver and GI tract)of the circulation.4.
E. Following the plateau of the action potential, gradual inactivation of L-type Ca2+ channels leads to activation of K+ channels andrapid repolarization. Opening of Na+ channels is responsible for thephase 0 upstroke; this is accompanied by reduced conductance ofthe inwardly rectified K+ current. Opening of L-type Ca2+ channelsis mainly responsible for the plateau (phase 2); inactivation of Na+channels and opening of voltage sensitive K+ channels cause therapid repolarization to the plateau (phase 1).5. E. A multiple-lead electrocardiogram can yield information oncardiac rhythm and conduction; presence, location, and extent ofischemia; orientation of the heart and chamber size; and abnormalities resulting from altered electrolyte levels or drugs.
It does notyield direct information regarding cardiac output, stroke volume,ejection fraction, or other flow-related parameters.6. B. Conduction velocity is a function of the slope of the phase 0upstroke of the action potential. Velocity is slowed in the AV node,resulting in a pause between atrial and ventricular depolarization.12.13.14.15.16.As a result, there is a pause between atrial and ventricular contraction, allowing atrial contraction to produce the final filling of theventricles before they contract.B. Mean arterial pressure is not simply the arithmetic mean ofdiastolic and systolic arterial pressure, because the arterial pressurecurve is complex in morphology and because diastole is longer induration than systole. As a rule of thumb, at normal heart rates,mean arterial pressure is approximately equal to the diastolic pressure plus one third the pulse pressure.
In this problem, the diastolicpressure is 80 and one third of pulse pressure is 16.7; therefore, themean arterial pressure is predicted to be approximately97 mm Hg.A. The highest pulse pressure is observed in the left ventricle, wherenormal resting pressure is approximately 120/0 mm Hg. High systolic pressure is required to pump blood through the high resistancesystemic circulation, while low diastolic pressure allows fillingduring diastole. Within the pulmonary circuit, the highest pulsepressure occurs in the right ventricle (approximately 25/0 mm Hgat rest).E. According to Poiseuille’s law, flow is proportional to the fourthpower of the radius of a tube. Thus, doubling the radius of the tubewill result in an increase in flow by a factor of 16, if other variablesare held constant.
Doubling the pressure gradient, halving the viscosity of the fluid, or halving the length of the tube will double theflow; doubling the viscosity will reduce flow by 50%.C. High viscosity of fluid favors laminar flow. When the hematocritis reduced, resulting in low blood viscosity, flow may be turbulent,resulting in audible murmurs. Wide tube diameter, rapid velocityof fluid flow, high density of fluid, and pulsatility of flow all increaseReynolds number and favor development of turbulent flow.C. Wall tension (T) is defined by Laplace’s law, T = Ptr, where Pt isthe transmural pressure and r is vessel radius.
Reduction of interstitial pressure will result in increased transmural pressure, and thus,increased wall tension. Reduced radius, vascular hydrostatic pressure, or transmural pressure will be associated with lower walltension.D. The isovolumetric contraction period begins when ventricularcontraction results in ventricular pressure rising to exceed atrialpressure, causing the atrioventricular valve to close. Thus, the beginning of the isovolumetric contraction period for the right ventricleis marked by closure of the tricuspid valve.A. During the cardiac cycle, valves on the left side of the heart closebefore the corresponding valves on the right side (mitral valve closesbefore the tricuspid, and aortic valve closes before the pulmonic),whereas valves on the right side open before the correspondingvalves on the left side (pulmonic valve opens before the aortic, andtricuspid valve opens before the mitral valve).E.
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