Sensory loss may be due to disorders of the central or peripheral nervous system. As in most of neurology, the initial goal when confronted with a patient with sensory loss is localization of the lesion using information from the history and physical examination. This narrows the differential diagnosis and directs further investigations. Localization requires an understanding of the anatomy of the sensory system.
ANATOMY OF THE SENSORY SYSTEM
Perception of a somatic sensation depends on detection of a stimulus by specialized receptors in the skin, muscle, or joints. Information from these receptors is then transmitted via peripheral nerves to the central nervous system. With the exception of the mesencephalic nucleus, whose projections carry proprioceptive information from the muscles of mastication , the cell bodies of the primary sensory neurons that constitute the peripheral nerves reside in ganglia outside of the central nervous system. Projections from these ganglia then enter the central nervous system to synapse with second order neurons.
The peripheral nerves are made up of large myelinated fibers that transmit proprioceptive, vibratory, pressure, and touch stimuli, and small, poorly myelinated fibers that transmit pain, temperature, and touch stimuli. (Note that touch is transmitted by both large and small fibers.) Projections from the cell bodies receiving pain, temperature, and touch stimuli enter the spinal cord via the dorsal nerve root. These fibers terminate in the dorsal horns, fanning out over several segments . They synapse with the second order neurons in the dorsal horns. These neurons then cross the midline of the cord in the anterior commissure in front of the central canal, and these second order neurons ascend (now on the opposite side of the spinal cord to the peripheral nerve) in either the anterior spinothalamic tract (touch) or the lateral spinothalamic tract (pain and temperature) to the ventral posterolateral nucleus of the thalamus. Here, they synapse with neurons which ascend to the primary sensory cortex in the parietal lobe (figure 1).
Projections from the dorsal root ganglia that carry proprioceptive, vibratory, pressure, and touch stimuli directly enter the dorsal columns from the dorsal roots . The gracile column is medial and carries stimuli from the lumbar and thoracic region; the cuneate column begins laterally in the cervical region, which it subserves. Thus, the dorsal columns consist of first order neurons traveling ipsilateral to the peripheral nerve from which they originate. These neurons synapse with second order neurons in the cuneate and gracile nuclei of the medulla. These second order neurons cross in the dorsal midline of the medulla and ascend through the brainstem as the medial lemniscus to the ventral posterolateral nucleus of the thalamus where they synapse with third order neurons which project through the internal capsule and the centrum semiovale to the primary sensory cortex in the parietal lobe (figure 2).
As with the motor cortex, the primary sensory cortex is arranged somatotopically, with the face represented laterally, close to the Sylvian fissure. The hand and arm are represented just above the region for the face and the leg is represented medially, similar to the layout of the motor homunculus.