A 10-year-old boy is brought to the emergency department due to a fall while riding his bicycle. The patient hit his forehead on a concrete road barrier and sustained a laceration but did not lose consciousness. He reports no headache, nausea, vomiting, vision disturbances, or focal weakness or numbness. The patient received a tetanus vaccine at age 6. Vital signs are within normal limits. Physical examination reveals a 2-cm laceration above the left eyebrow. There are no other injuries. Neurologic examination shows no abnormalities. Prior to laceration repair with suture, lidocaine is injected in the subcutaneous tissue around the laceration. The patient reports immediate relief of pain, and there is loss of sensation to touch in the area infiltrated with the medication. These effects are most likely due to which of the following changes within the affected neurons?

Question

A 10-year-old boy is brought to the emergency department due to a fall while riding his bicycle.  The patient hit his forehead on a concrete road barrier and sustained a laceration but did not lose consciousness.  He reports no headache, nausea, vomiting, vision disturbances, or focal weakness or numbness.  The patient received a tetanus vaccine at age 6.  Vital signs are within normal limits.  Physical examination reveals a 2-cm laceration above the left eyebrow.  There are no other injuries.  Neurologic examination shows no abnormalities.  Prior to laceration repair with suture, lidocaine is injected in the subcutaneous tissue around the laceration.  The patient reports immediate relief of pain, and there is loss of sensation to touch in the area infiltrated with the medication.  These effects are most likely due to which of the following changes within the affected neurons?

Quizplus · Accepted Answer

11ec160b_3c07_ad15_9249_5bc4d1789e80_MD0001_00 Lidocaine is a local anesthetic that interrupts nerve fiber transmission by blocking sodium channels in the neuronal cell membrane, preventing depolarization of the nerve. Local anesthetics have a greater effect on small myelinated nerves, so they preferentially inhibit neurons that carry pain and temperature sensation; pressure sensation and motor function are often preserved at typical doses. Local anesthetics are weak bases that exist in charged and uncharged forms. When in the uncharged form, they diffuse through the cell membrane to block sodium channels from the inside of the cell. Increased acidity in the surrounding tissue, which occurs in infected wounds, results in more anesthetic molecules remaining in the charged form, rendering them unable to penetrate the cell membrane to exert an effect. Therefore, the dose required to achieve adequate anesthesia is higher in infected tissue. (Choice A) Blockade of potassium channels would inhibit repolarization, thereby prolonging action potentials and increasing neurotransmitter release at the neuromuscular junction rather than halting action potential propagation. (Choice C) Decreased calcium current slows the action potential propagation in cells that rely primarily on voltage-gated calcium channels for depolarization, such as cardiac slow response tissues (eg, sinoatrial and atrioventricular nodes); peripheral nerve depolarization is driven primarily by sodium, rather than calcium, influx. (Choice D) GABA is the primary inhibitory neurotransmitter in the CNS. Activation of the GABA-A receptor, which is the target of some sedating medications (eg, benzodiazepines), activates voltage-gated chloride channels. The influx of chloride ions causes hyperpolarization, which inhibits action potentials. (Choice E) Interruption of axonal transport delays the delivery of organelles (eg, mitochondria) and synaptic vesicles to the distal axon, causing impaired nerve impulse propagation. This occurs in some neurodegenerative diseases (eg, sporadic Alzheimer) or as an adverse effect of some chemotherapeutic agents (eg, vincristine). Educational objective: Local anesthetics diffuse through the neuronal cell membrane to block the influx of sodium into the cell. This prevents depolarization and inhibits action potential propagation. __________ References: Molecular mechanisms of local anesthesia: a review. (http://www.ncbi.nlm.nih.gov/pubmed/2157353) Interactions of local anesthetics with voltage-gated Na+ channels. (http://www.ncbi.nlm.nih.gov/pubmed/15635807)

A 10-Year-Old Boy Is Brought to the Emergency Department Due