A patient sustains a traumatic injury that damages the right side of the spinal cord at the thoracic level (Brown-Séquard syndrome). Which sensory deficit pattern would you expect below the level of injury?
ALoss of all sensation on both sides — both pathways are disrupted regardless of laterality
BLoss of pain and temperature on the right, loss of fine touch on the left
CLoss of fine touch on the right, loss of pain and temperature on the left
DLoss of all sensation on the right side only, because both pathways run ipsilaterally
The two ascending pathways cross at different levels, which is why the deficit pattern is crossed. Fine touch and proprioception travel ipsilaterally in the dorsal columns and cross in the brainstem (medulla) — so a right-sided lesion interrupts right-side fine touch before it crosses, causing ipsilateral (right) fine touch loss. Pain and temperature fibers synapse in the dorsal horn and cross within the spinal cord before ascending — so a right-sided lesion cuts the already-crossed left-side pain fibers, causing contralateral (left) pain/temperature loss. This dissociation is clinically pathognomonic for Brown-Séquard syndrome.
Question 2 Multiple Choice
Why does the sensory homunculus depict grossly enlarged hands, lips, and tongue relative to the back and trunk?
AThese areas evolved first and therefore received more cortical territory during brain development
BThe thalamus preferentially amplifies signals from the extremities and face before relaying them to cortex
CPrimary somatosensory cortex allocates space proportional to receptor density and acuity demands — areas with more sensory receptors require more cortical processing
DThese regions have more pain receptors than the trunk, so they require more nociceptive processing
The somatotopic map in S1 is distorted by receptor density, not body size. The fingertips have an extraordinarily high density of mechanoreceptors (Meissner's corpuscles, Merkel cells) enabling fine texture discrimination — this means more sensory axons, more thalamic relay neurons, and more cortical space dedicated to processing their input. The lips and tongue similarly have very high innervation density, reflecting the survival and social importance of oral sensation. The back has far fewer receptors per square centimeter, so it occupies little cortical territory despite its large surface area. The map directly reflects the peripheral innervation pattern.
Question 3 True / False
Touch signals from the right hand cross to the left side of the nervous system within the spinal cord before ascending toward the brain.
TTrue
FFalse
Answer: False
Fine touch and proprioception travel via the dorsal column-medial lemniscal pathway, which ascends IPSILATERALLY in the dorsal columns all the way to the brainstem (specifically the medulla), where the fibers synapse and then cross to the contralateral side. They do NOT cross in the spinal cord. Contrast this with pain and temperature signals, which travel via the spinothalamic tract: these fibers DO cross within the spinal cord (in the anterior commissure) shortly after entering. This is the anatomical basis for the crossed deficit pattern in Brown-Séquard syndrome.
Question 4 True / False
The somatotopic map in primary somatosensory cortex is fixed throughout life and cannot be altered by experience or injury.
TTrue
FFalse
Answer: False
The somatotopic map is dynamically plastic. Following amputation of a limb, the cortical territory previously dedicated to that limb is gradually invaded by neighboring body representations — a phenomenon called cortical remapping. Conversely, intensive use of a body part (e.g., string musicians' left hand fingertips) can expand its cortical representation. This plasticity is clinically relevant: phantom limb pain may partly reflect reorganization of cortical maps after amputation, and sensory training regimens in rehabilitation exploit plasticity to restore function. Plasticity connects somatotopic organization to broader principles of experience-dependent cortical modification.
Question 5 Short Answer
A spinal cord injury on the left side at the cervical level results in loss of fine touch in the left arm but loss of pain sensation in the right arm below the injury. Explain why the deficits are on opposite sides.
Think about your answer, then reveal below.
Model answer: The two ascending pathways cross at different anatomical levels, producing the crossed pattern. Fine touch from the left arm travels ipsilaterally (on the left) in the dorsal columns until the brainstem, where it crosses. A left-sided cervical lesion cuts these fibers before they cross — eliminating fine touch ipsilaterally (left). Pain and temperature from the left arm enters the spinal cord, synapses in the dorsal horn, and crosses to the right side within the cervical cord before ascending. A left-sided lesion cuts the already-crossed right-side pain fibers ascending from below — eliminating pain sensation contralaterally (right). The crossing levels differ, so one pathway's deficit is ipsilateral and the other's is contralateral.
This crossed dissociation is a direct readout of neuroanatomy: dorsal columns cross in brainstem (ipsilateral deficit with ipsilateral lesion), spinothalamic crosses in spinal cord (contralateral deficit with ipsilateral lesion). Understanding the crossing levels is not just anatomical trivia — it localizes spinal cord injuries clinically without imaging, because the pattern of sensory loss tells you which tract is damaged and at what level.