USMLE Step 1 Quiz
Practice Neurophysiology in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 34-year-old man undergoes a dental procedure. The dentist injects lidocaine, a local anesthetic, into the gingival tissue. The patient reports a loss of sensation in the area, allowing the procedure to be completed without pain.
Lidocaine exerts its anesthetic effect by primarily altering which of the following properties of neuronal action potentials?
This quiz focuses on Neurophysiology, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.
A 34-year-old man undergoes a dental procedure. The dentist injects lidocaine, a local anesthetic, into the gingival tissue. The patient reports a loss of sensation in the area, allowing the procedure to be completed without pain.
Lidocaine exerts its anesthetic effect by primarily altering which of the following properties of neuronal action potentials?
Explanation: Lidocaine and other local anesthetics are voltage-gated sodium channel blockers. By blocking these channels, they prevent the rapid influx of sodium ions that is necessary for the rising phase (depolarization) of an action potential. This slows the rate of depolarization and prevents the membrane potential from reaching the threshold required to fire an action potential, thus blocking nerve conduction and sensation.
A 2-year-old boy presents with recurrent, difficult-to-control seizures. Genetic testing reveals a gain-of-function mutation in a gene encoding a neuronal voltage-gated sodium channel. The mutation results in incomplete and delayed channel inactivation following depolarization.
This genetic defect leads to neuronal hyperexcitability primarily by causing which of the following changes to the action potential?
Explanation: Incomplete inactivation of voltage-gated sodium channels allows for a persistent influx of positive sodium ions into the neuron after the initial upstroke of the action potential. This sustained inward current opposes the repolarizing outward potassium current, thereby prolonging the period of depolarization. This keeps the neuron in an excited state for longer, facilitating repetitive firing and leading to the hyperexcitability that manifests as seizures.
A 28-year-old woman presents with ptosis and diplopia that worsen as the day progresses. She also reports difficulty chewing her food. Administration of edrophonium, a short-acting acetylcholinesterase inhibitor, transiently improves her muscle strength.
The pathophysiology of this patient's condition involves an autoimmune-mediated reduction in the number of which of the following structures, leading to a decreased amplitude of the end-plate potential?
Explanation: This clinical presentation is classic for myasthenia gravis, an autoimmune disorder where antibodies target and destroy postsynaptic nicotinic acetylcholine receptors at the neuromuscular junction. The reduction in available receptors means that even with normal acetylcholine release, the resulting end-plate potential is smaller and may fail to reach the threshold for muscle fiber contraction, causing fatigable weakness.
A neurophysiology experiment is conducted on a postsynaptic neuron with a resting membrane potential of -70 mV and a threshold potential of -55 mV. Two separate excitatory presynaptic neurons (Neuron X and Neuron Y) synapse onto it. Firing Neuron X alone produces a 10 mV excitatory postsynaptic potential (EPSP). Firing Neuron Y alone also produces a 10 mV EPSP.
Which of the following events would be most likely to trigger an action potential in the postsynaptic neuron?
Explanation: An action potential is triggered when the neuron's membrane potential reaches the threshold of -55 mV. A depolarization of at least 15 mV (from -70 mV) is required. A single EPSP of 10 mV is subthreshold. When Neuron X and Neuron Y fire simultaneously, their individual 10 mV EPSPs summate at the axon hillock (spatial summation). The combined depolarization of 20 mV brings the membrane potential to -50 mV, which is above the threshold, triggering an action potential.
A 32-year-old woman presents with a 3-day history of blurry vision in her right eye and pain with eye movement. She recalls an episode of tingling in her legs 6 months ago that resolved spontaneously. An MRI of the brain reveals multiple periventricular white matter plaques consistent with multiple sclerosis.
The demyelination process seen in this patient impairs neuronal signal propagation primarily by which of the following mechanisms?
Explanation: Multiple sclerosis is an autoimmune disease characterized by the destruction of myelin sheaths in the central nervous system. Myelin acts as an electrical insulator, allowing the action potential to propagate rapidly via saltatory conduction, where it 'jumps' between the unmyelinated nodes of Ranvier. When myelin is destroyed, this saltatory conduction is disrupted. The electrical signal dissipates as it travels along the now-uninsulated axon, leading to slowed or completely blocked nerve impulse transmission.
A 68-year-old man presents with a resting tremor in his right hand, slowness of movement, and a shuffling gait. Physical examination reveals cogwheel rigidity. His symptoms are attributed to Parkinson's disease, which involves the degeneration of dopaminergic neurons.
The loss of neurons in which of the following structures is most directly responsible for this patient's motor symptoms?
Explanation: The cardinal motor features of Parkinson's disease (bradykinesia, resting tremor, rigidity) are caused by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNc). These neurons project to the striatum (caudate and putamen) as part of the nigrostriatal pathway, which is a critical component of the basal ganglia motor loop responsible for initiating and modulating movement.
A 42-year-old woman is started on fluoxetine for major depressive disorder. After several weeks, she reports an improvement in her mood and energy levels. Fluoxetine is a selective serotonin reuptake inhibitor (SSRI).
The therapeutic action of this medication is primarily due to an increase in the concentration of serotonin in which of the following locations?
Explanation: Serotonin's action in the synapse is terminated primarily by its reuptake into the presynaptic neuron via the serotonin transporter (SERT). SSRIs, like fluoxetine, selectively block SERT. This inhibition of reuptake prevents serotonin from being cleared from the synaptic cleft, leading to an increased concentration of the neurotransmitter in the cleft. This enhances and prolongs its effect on postsynaptic receptors, which is believed to mediate the drug's antidepressant effects over time.
A 35-year-old farmer is brought to the emergency department with confusion, sweating, lacrimation, and muscle fasciculations. His heart rate is 45/min and he has pinpoint pupils. He was recently exposed to an organophosphate insecticide.
The patient's symptoms are caused by excessive stimulation of both muscarinic and nicotinic receptors. Which of the following clinical findings is primarily mediated by the overstimulation of nicotinic receptors?
Explanation: Organophosphates irreversibly inhibit acetylcholinesterase, causing an excess of acetylcholine at all cholinergic synapses. Muscle fasciculations, followed by paralysis (depolarizing blockade), are characteristic signs of nicotinic receptor overstimulation at the neuromuscular junction. The other symptoms listed—bradycardia, miosis, and excessive salivation—are classic muscarinic effects resulting from overstimulation of the parasympathetic nervous system.
A 40-year-old woman with multiple sclerosis is prescribed baclofen to manage severe spasticity in her lower limbs. The drug is effective in reducing her muscle tone and spasms.
Baclofen exerts its muscle relaxant effect by acting as an agonist at which of the following receptor types, leading to hyperpolarization via increased potassium conductance?
Explanation: Baclofen is a selective agonist for GABA-B receptors. Unlike GABA-A receptors (which are ligand-gated ion channels), GABA-B receptors are G-protein coupled receptors. Their activation in the spinal cord leads to hyperpolarization of motor neurons by opening potassium channels (increasing K+ efflux) and also inhibits presynaptic calcium influx, which reduces neurotransmitter release. This overall inhibitory effect reduces motor neuron excitability and relieves spasticity.
A 72-year-old man suffers an ischemic stroke. In the hours following the event, neurons in the under-perfused area surrounding the infarct (the ischemic penumbra) begin to die via a process known as excitotoxicity.
Excitotoxicity in this setting is primarily driven by the excessive influx of which ion through overstimulated NMDA receptors?
Explanation: During ischemia, lack of ATP impairs ion pumps, leading to neuronal depolarization and massive release of the excitatory neurotransmitter glutamate. Glutamate overstimulates its receptors, particularly the NMDA receptor. This leads to a large and sustained influx of calcium (Ca2+). High intracellular Ca2+ is toxic because it activates a cascade of degradative enzymes, including proteases, phospholipases, and endonucleases, ultimately leading to apoptosis and neuronal death.
A researcher is studying signal transduction pathways in the central nervous system. A novel drug is found to be a selective agonist for the D1 dopamine receptor. Application of this drug to cultured striatal neurons leads to an increase in the intracellular concentration of cyclic AMP (cAMP).
The D1 receptor mediates this intracellular effect by coupling to which of the following G-proteins?
Explanation: Dopamine receptors are G-protein coupled receptors (GPCRs). D1-like receptors (D1 and D5) are coupled to the Gs (stimulatory) protein. When activated, Gs stimulates the enzyme adenylyl cyclase, which converts ATP to cyclic AMP (cAMP). cAMP then acts as a second messenger. In contrast, D2-like receptors (D2, D3, D4) are coupled to Gi (inhibitory) proteins, which inhibit adenylyl cyclase and decrease cAMP levels.
A 60-year-old man with poorly controlled type 2 diabetes presents with a 1-year history of progressive numbness in his feet. He reports difficulty sensing where his feet are without looking at them and frequently stumbles in the dark. On examination, he has loss of vibration and position sense in both feet, but pain and temperature sensation are relatively intact. A Romberg test is positive.
This patient's signs and symptoms are best explained by pathologic changes in which of the following neural pathways?
Explanation: The dorsal column-medial lemniscus (DCML) pathway transmits sensory information for fine touch, vibration, and proprioception (position sense). Diabetic neuropathy often affects the large, myelinated nerve fibers that constitute this pathway. The patient's loss of vibration and proprioception, with preserved pain and temperature (carried by the anterolateral pathway), is a classic dissociative sensory loss pointing to DCML pathology. The positive Romberg test confirms the loss of proprioception.
A 45-year-old man sustains a spinal cord injury. An MRI reveals a small, focal lesion that has damaged the anterior white commissure at the C5-C6 level, while sparing the dorsal columns and lateral tracts.
Which of the following sensory deficits would be most expected in this patient?
Explanation: The spinothalamic tract fibers, which convey pain and temperature sensation, decussate (cross over) in the anterior white commissure near the spinal cord level where they enter. A lesion at C5-C6 that specifically damages this crossing point will interrupt the fibers from both sides corresponding to those dermatomes. This results in a bilateral loss of pain and temperature sensation in the affected dermatomes (C5-C6 correspond to the shoulders and lateral arms), creating a 'cape-like' sensory deficit.
A 65-year-old woman with a history of hypertension suddenly develops right-sided weakness. A physical examination reveals increased muscle tone, hyperreflexia, and a positive Babinski sign on the right side. An MRI of the brain confirms an ischemic stroke in the left internal capsule.
The patient's physical examination findings are most consistent with a lesion affecting which of the following?
Explanation: The clinical findings—spasticity (increased tone), hyperreflexia, and a positive Babinski sign (extensor plantar response)—are the classic signs of an upper motor neuron (UMN) lesion. UMNs originate in the cerebral cortex and their axons travel down through structures like the internal capsule. Because the main motor pathway (the corticospinal tract) decussates in the medulla, a lesion in the left cerebral hemisphere results in contralateral (right-sided) UMN signs.
A 44-year-old man is diagnosed with early-stage Huntington's disease, exhibiting characteristic chorea (involuntary, jerky movements). Pathologic studies in this disease show selective atrophy of the striatum (caudate and putamen).
The chorea in early Huntington's disease results from the preferential loss of neurons of the indirect basal ganglia pathway. This loss leads to which of the following net effects on the motor circuit?
Explanation: In the indirect pathway, the striatum normally inhibits the globus pallidus externus (GPe). In early Huntington's, loss of these striatal neurons disinhibits the GPe. The now overactive GPe overly inhibits the subthalamic nucleus (STN). The inhibited STN provides less excitatory drive to the globus pallidus internus (GPi). This reduced activity of the GPi results in decreased tonic inhibition of the thalamus. This thalamic disinhibition leads to excessive excitation of the motor cortex, causing the hyperkinetic movements (chorea).
A 55-year-old man with a history of chronic alcoholism presents with a wide-based, unsteady gait that has worsened over several months. On examination, he has marked difficulty with the heel-to-shin test bilaterally. Coordination in his upper extremities, speech, and eye movements are normal.
This patient's selective gait and lower limb ataxia are most consistent with alcohol-induced degenerative changes in which of the following structures?
Explanation: The clinical picture describes truncal ataxia (wide-based gait) and lower limb dysmetria (impaired heel-to-shin test) with relative sparing of the upper limbs and speech. This specific pattern is highly characteristic of alcoholic cerebellar degeneration, which preferentially affects the Purkinje cells of the anterior lobe and superior vermis of the cerebellum. These regions are somatotopically organized to coordinate the movements of the trunk and legs.
A 50-year-old woman experiences a seizure characterized by a tingling sensation that begins in her left hand and spreads up her left arm to her face over the course of about a minute. She remains fully conscious during the episode. An EEG confirms focal seizure activity.
The seizure focus is most likely located in the primary somatosensory cortex of which cerebral hemisphere and region?
Explanation: The primary somatosensory cortex is located in the postcentral gyrus. Sensory pathways are contralateral, so left-sided symptoms point to a focus in the right cerebral hemisphere. The seizure's progression from hand to arm to face (a 'sensory march' or Jacksonian march) reflects the somatotopic organization (homunculus) of the cortex. The areas representing the hand and face are located adjacent to each other on the lateral aspect of the gyrus, while the leg is represented more medially.
A 25-year-old man with generalized anxiety disorder is prescribed alprazolam, a benzodiazepine. He reports a significant reduction in his anxiety symptoms after starting the medication.
The therapeutic effect of this drug is achieved by potentiating the action of receptors that, when activated, primarily mediate an influx of which ion into the postsynaptic neuron?
Explanation: Benzodiazepines are positive allosteric modulators of the GABA-A receptor, which is an ionotropic receptor coupled to a chloride (Cl-) channel. GABA is the main inhibitory neurotransmitter in the CNS. Benzodiazepines increase the frequency of Cl- channel opening in the presence of GABA. The resulting influx of negatively charged Cl- ions hyperpolarizes the postsynaptic neuron, creating an inhibitory postsynaptic potential (IPSP) and making it less likely to fire an action potential, which produces a calming, anxiolytic effect.
A 12-year-old boy with attention-deficit/hyperactivity disorder (ADHD) is treated with mixed amphetamine salts. His teacher reports significant improvement in his ability to focus and a decrease in impulsive behaviors.
The therapeutic effect of amphetamine in this condition is primarily mediated by increasing the synaptic availability of norepinephrine and dopamine through which mechanism?
Explanation: Amphetamines increase the synaptic concentrations of dopamine and norepinephrine primarily by promoting their release from the presynaptic terminal. They enter the presynaptic neuron, disrupt the storage of these neurotransmitters in vesicles by inhibiting VMAT, and reverse the direction of the dopamine (DAT) and norepinephrine (NET) transporters, causing a massive, non-vesicular efflux of neurotransmitter into the synaptic cleft.
A 45-year-old man is bitten by a black widow spider while gardening. He develops severe muscle cramps, abdominal pain, and hypertension. The venom, α-latrotoxin, is known to form pores in the presynaptic terminal membrane of motor neurons.
The formation of these pores leads to a massive, uncontrolled release of neurotransmitter primarily by allowing the unregulated influx of which of the following ions?
Explanation: Neurotransmitter release from the presynaptic terminal is a tightly regulated process triggered by the influx of calcium (Ca2+) through voltage-gated channels. α-latrotoxin bypasses this regulation by inserting itself into the presynaptic membrane and forming pores that are highly permeable to Ca2+. The resulting massive and uncontrolled influx of Ca2+ into the terminal causes widespread fusion of synaptic vesicles and massive release of acetylcholine, leading to spastic paralysis and autonomic symptoms.