A 32-year-old man is brought to the ER after collapsing during a marathon. Labs show serum sodium of 130 mEq/L, potassium of 2.8 mEq/L, chloride of 95 mEq/L, and bicarbonate of 22 mEq/L. The physician explains that the patient’s muscle weakness is due to a major disturbance in the most abundant intracellular cation, which plays a critical role in maintaining resting membrane potential. Which of the following ions is primarily intracellular and most directly explains his symptoms?

A 24-year-old man sustains a penetrating injury to his lower back that damages several spinal motor neurons. During evaluation of the injury in a neurophysiology lab, intracellular recordings reveal that the neuronal resting potential is altered, becoming less negative than normal. This depolarization increases the likelihood of spontaneous action potential firing, contributing to muscle fasciculations. What is the normal resting membrane potential of a spinal motor neuron under physiologic conditions?

A 19-year-old college student presents with new-onset seizures. MRI of the brain is normal. An EEG shows generalized hyperexcitability of cortical neurons. The neurologist explains that the seizures are likely due to excessive activity of the brain’s main excitatory neurotransmitter. Which neurotransmitter is primarily responsible for this excitatory activity?

A 5-month-old infant is brought to the emergency department with constipation, poor feeding, and generalized hypotonia (“floppy baby”). The mother reports that she occasionally gives the infant honey. The physician suspects infant botulism due to ingestion of spores producing botulinum toxin A, which interferes with acetylcholine release at the neuromuscular junction. This toxin acts by cleaving which component of the SNARE complex?

A 42-year-old man presents with acute onset diplopia, ptosis, and difficulty swallowing after eating home-canned vegetables. Neurologic exam shows symmetric descending paralysis. The attending physician explains that the patient’s symptoms are due to impaired neurotransmitter release at the neuromuscular junction. Normally, the entry of which ion into the presynaptic terminal is essential for triggering synaptic vesicle exocytosis?

A 28-year-old man presents with severe anxiety and is prescribed a benzodiazepine. This medication enhances the effect of GABA at the GABA-A receptor. When the drug binds, the receptor opens its associated ion channel, leading to hyperpolarization of the postsynaptic neuron and reduced neuronal excitability. Which of the following ion channel changes best explains the inhibitory effect of benzodiazepines?

A 54-year-old man presents with recurrent episodes of severe, unilateral, stabbing facial pain lasting seconds, often triggered by chewing. He is diagnosed with trigeminal neuralgia. The pain is mediated by neurotransmitters, including a class of neuropeptides that modulate nociception and inflammation. Which of the following is classified as a neuropeptide neurotransmitter?

A 12-year-old boy sustains a deep puncture wound to his foot while playing outdoors. Five days later, he develops trismus (lockjaw), risus sardonicus, and generalized muscle spasms. The mechanism of his symptoms involves blockade of inhibitory neurotransmitter release from spinal interneurons, leading to unchecked motor neuron firing.

Which part of synaptic transmission is inhibited by tetanus toxin?

A 20-year-old college student is evaluated for epilepsy after experiencing recurrent generalized seizures. EEG reveals that simultaneous activation of multiple excitatory presynaptic inputs onto cortical pyramidal neurons leads to cumulative depolarization, eventually reaching threshold to trigger an action potential. The neurologist explains that this represents a form of synaptic integration in which excitatory postsynaptic potentials (EPSPs) from several neurons combine and accumulate. Which of the following best describes this process?