Leo was in an accident while riding his motorcycle. He now needs a ventilator to breathe because his spinal cord damage is above C3. Which nerve, important to respiration, is not functioning properly?

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Here is the step-by-step analysis of each option in the context of respiration and spinal cord injury above C3.
Option 1: Musculocutaneous nerve. This nerve mainly innervates the anterior compartment of the arm (biceps brachii, brachialis, coracobrachialis) and provides elbow flexion and some sensation on the lateral forearm. It has no direct role in controlling the muscles required for breathing, so its dysfunction would not by itself explain the need for a ventilator.
Option 2: Femoral nerve. The femoral nerve primarily supplies the anterior thigh muscles (quadriceps group) and provides hip flexion and knee extension, along with some sensory input from the anterior thigh and leg. Its territory is far removed from the respiratory system, so damage here would not explain ventilator dependence.
Option 3: Tibial nerve. The tibial nerve serves the posterior compartment of the leg and foot, contributing to plantarflexion and sensory input to the sole of the foot. It has no central role in inspiratory mechanics, so impairment would not directly cause the need for a ventilator unless there were widespread neurological injury, which would present with broader deficits than isolated diaphragmatic failure.
Option 4: Sciatic nerve. The sciatic nerve is the major nerve of the posterior thigh, giving motor supply to many muscles of the leg and foot via its branches. While a large lesion can cause extensive motor deficits, it does not control respiration directly. Respiratory failure from high cervical injury is not explained by isolated sciatic nerve dysfunction.
Option 5: Phrenic nerve. The phrenic nerve arises mainly from cervical spinal levels C3–C5 and provides innervation to the diaphragm, the primary muscle of respiration. If spinal cord injury is above C3, diaphragmatic function can be severely compromised or lost, leading to ventilator dependence. This aligns with the clinical scenario where airway and breathing require mechanical support due to diaphragm paralysis or weakness.
In summary, the options dealing with lower limb nerves (musculocutaneous, femoral, tibial, sciatic) do not account for diaphragmatic control and would not explain ventilator dependency in a high cervical injury. The nerve that directly governs diaphragmatic movement and would be affected by a lesion above C3 is the one responsible for diaphragmatic innervation, which is the phrenic nerve. The reasoning emphasizes the distinct anatomical and functional roles of these nerves, with respiration-specific control localized to the phrenic nerve.

BIOL 251 Module 7 Problem Set

Leo was in an accident while riding his motorcycle. He now needs a ventilator to breathe because his spinal cord damage is above C3. Which nerve, important to respiration, is not functioning properly?

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