USMLE® Step 1 style questions USMLE
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A 42-year-old woman comes to the clinic because of concerns with her vision. She reports experiencing a simultaneous perception of two images of a single object that is displaced both vertically and horizontally. Physical examination shows a temperature of 36.6°C (98.0°F) and apparent misalignment of the eyes. Fundoscopic examination shows white sclera and equally symmetrical optic discs with sharp margins bilaterally. Which of the following is most likely impaired?
Content Reviewers:Rishi Desai, MD, MPH
The cranial nerves consist of 12 pair of nerves originating directly from the brain and brainstem. They supply both motor and sensory information from the brain to other parts of the body, primarily supplying the region of the head and neck.
The nervous system is divided into the central nervous system, so the brain and the spinal cord, and the peripheral nervous system, which is further divided into the somatic and the autonomic nervous systems.
Broadly speaking, the nervous system is split into an afferent and an efferent division.
The afferent division brings sensory information - like vision, smell, touch, and proprioception - from the outside world into the brain.
On the other hand, the efferent division brings motor information from the brain to the periphery, ultimately resulting in contraction of skeletal muscles to trigger movement through the somatic nervous system, as well as contraction of the smooth muscles to trigger activity of the glands and organs through the autonomic nervous system.
Neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and when there’s a group of neuron cell bodies located in the central nervous system, the whole thing is called a nucleus, while a group of neuron cell bodies that are located outside of the central nervous system is called a ganglion.
Neurons have nerve fibers that extend out from the neuron cell body - these 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 releases neurotransmitters, further relaying the signal to the dendrites or directly to the cell body of the next neuron in the series.
Now, the peripheral nervous system consists of nerves, which are enclosed bundles of axons that connect the central nervous system to every other part of the body.
Some nerves are purely sensory or purely motor, while most nerves are mixed.
Nerves are either classified as cranial nerves - those are ones that exit from the skull and innervate the head and neck - or spinal nerves, those are ones that exit from the spinal cord and innervate the rest of the body. Let’s focus on cranial nerves.
There are twelve cranial nerves pairs that are numbered based on the order in which they arise from different nuclei in the brain, except for cranial nerves eleven and twelve, which are inverted.
In order from one to twelve, the cranial nerves are called olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal. And a quick way to remember them is with the acronym “On Old Olympus’ Towering Top A Finn Van German Viewed A Hop.”
The cranial nerves exit the skull through different openings or foramina.
The primary functions of the cranial nerves - so sensory, motor, or both for mixed - can be remembered with the acronym “some say marry money, but my brother says big brains matter more.”
The first cranial nerve, the olfactory nerve, arises from the primary olfactory cortex, which is located at the temporal lobe of the brain.
Fibers from the olfactory cortex neurons form the olfactory tracts of the olfactory nerves, which run to the olfactory bulb.
The olfactory bulb lies right above the cribriform plate of the ethmoid bone of the skull. It receives information from tiny sensory nerve fibers that run through the cribriform plate to synapse with neurons in the olfactory bulb.
These sensory fibers are axons coming from the olfactory neurons in the nasal cavity, which have specific receptors that can bind to a variety of odor molecules.
The second cranial nerve, called the optic nerve, actually emerges from the retina of the eye.
The optic nerve then passes through the optic canal and meets the optic nerve coming from the other eye at the optic chiasm. Here, partial decussation occurs, meaning that the fibers coming from the temporal visual fields of eye cross to the opposite side.
After decussating, the optic nerve fibers form the optic tracts. So each optic tract carries information from the opposite visual field of each eye.
Then, the optic fibers travel and synapse at different nuclei – some synapse at the suprachiasmatic nucleus located in the hypothalamus and help regulate the sleep wake cycle, some synapse at the pretectal nucleusi in the midbrain and help regulate eye reflexes, and most fibers synapse at the lateral geniculate nucleus in the thalamus.
From here, thalamic fibers form the optic radiations, which run to the occipital visual cortex where the brain interprets what you see.
There are three purely motor cranial nerves in charge of eye movement: the third cranial nerve, called the oculomotor nerve, the fourth cranial nerve, called the trochlear nerve, and the sixth cranial nerve, called the abducens nerve.
The oculomotor nerve arises from the ventral midbrain, and then it runs through the superior orbital fissure - a foramen in the sphenoid bone of the skull, to enter the orbit.
Here, the oculomotor nerve splits into a superior and inferior branch.
The superior branch innervates the levator palpebrae superioris muscle - which raises the upper eyelid – and the superior rectus muscle that elevates the eye.
The inferior branch innervates the inferior oblique, and the inferior and medial rectus muscles, which help elevate and abduct, depress and adduct, and just adduct the eyeball, respectively.
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