USMLE® Step 1 style questions USMLE
A 39-year-old woman comes to the emergency department because of altitudinal visual field defect in both eyes and bilateral quadriplegia and sensory loss. Her cervical spine magnetic resonance imaging reveals a longitudinally extensive transverse myelitis. In her serum, antibodies targeting the water channel aquaporin-4 are found. Which of the following is the most likely diagnosis?
The name of the disorder transverse myelitis can be broken down. Transverse means extending completely across something - in this case, it refers to going across the spinal cord, and myelitis means inflammation of myelin which is the fatty substance surrounding nerves.
So, in transverse myelitis there’s inflammation that damages the myelin as well as the rest of the neuron across a section of the spinal cord.
Now, neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and nerve fibers, which are projections that extend out from the neuron cell body.
Nerve fibers are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons.
Where two neurons come together is called a synapse, and that’s where one end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.
The axons are intermittently wrapped in a fatty substance called myelin.
Myelin is extremely important to neurons, because it helps to allow an action potential to propagate much faster.
An action potential is an electrical signal that races down the axon, triggering the release of neurotransmitters or a chemical signal, on the other end.
Without myelin this signal propagation is very slow and inefficient.
Since some of these neurons can be very long, especially ones that go from the spinal cord to the toes, the fact that myelin helps speed up action potentials is super important!
Now, the spinal cord is composed of both grey and white matter.
Grey matter consists of cell bodies. It’s in the middle of the spinal cord and is shaped like a butterfly.
Surrounding the grey matter is white matter, which consists of the myelinated axons of various neurons.
The neurons in the spinal cord form different neural tracts that carry information to and from the brain.
There are three main tracts to remember. The corticospinal tract is a descending pathway that carries motor information from the brain to different muscles in the body and it controls voluntary muscle movement.
Finally, the spinothalamic tract is another ascending pathway and it’s divided into two parts.
The lateral tract carries sensory information for pain, pressure, and temperature, while the anterior tract carries information for crude touch--or the sense one has been touched, but without being able to localize where they were touched.
These neurons hitch a ride with the various tracts, but their cell bodies are found in the spinal cord.
For example, the sympathetic division, or the fight response, has its cell bodies in the thoracic and lumbar regions and make up the lateral horns of the grey matter.