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Clinically, there are 2 major somatosensory pathways that are examined.
The first is the spinothalamic (ST) part of the anterolateral system
and the second is the dorsal column-medial lemniscus (DCML) system.
The principle sensory modalities for the ST system are pain and temperature.
The principle sensory modalities for DCML system are vibratory, position
sense and discriminatory or integrative sensation.
The anatomical pathways for the 2 major sensory systems is as follows: ST- the axons from the 1st order neuron located in the dorsal root
ganglion enter the dorsal root entry zone and within several segments synapse
with 2nd order neurons in the dorsal horn. Axons from the 2nd order neuron
cross immediately via the ventral white commissure to the anterolateral
quadrant of the spinal cord then ascend as the spinothalamic tract to the
ventral posterior lateral nucleus (VPL) of the thalamus. The axons of the
3rd order neurons project to the postcentral gyrus or somatosensory cortex
(there are also projections to the insular and anterior cingulate cortex
but we are mainly focusing on the primary somatosensory cortex).
The axons from the 1st order neurons located in the dorsal root ganglion
enter the dorsal root entry zone and then ascend in the dorsal columns on
the same side of the cord until they reach the 2nd order neurons in the
medulla. Axons from the 2nd order neurons cross at the level of the medulla
and then travel near the midline in the medial lemniscus. By the time the
medial lemniscus reaches the rostral pons it has moved laterally and at
this point it is in close proximity to the spinothalamic tract as it ascends
to the VPL of the thalamus. The 3rd order neuron projects to the primary
somatosensory cortex in the postcentral gyrus.
System The trigeminal system is the somatosensory system for the face, which
is clinically tested in the cranial nerve exam. For the trigeminal system
it is important to remember that the descending tract of the trigeminal
nerve, which serves pain and temperature, descends to the level of the upper
cervical spinal cord and then axons from the 2nd order neurons cross over
to the opposite side and ascend to the ventral posterior medial (VPM) nucleus
of the thalamus.
Level of Crossing
The following are important anatomical points to remember that have significant
power in localizing lesions:
The level of crossing
of the axons of the 2nd order neurons is immediate for the ST system
and not until the medulla for the DCML system.
Location of Tracts
The ST tract is
lateral in the cord and lower brainstem while the DCML system is dorsal
and medial in the cord and medial in the lower brainstem. It is not
until the rostral pons that the 2 tracts are anatomically close to each
trigeminal tract is ipsilateral to its origin and axons from the 2nd
order neurons cross at the lower medulla-upper cervical spinal cord
The above anatomical points translate into the following clinical findings:
Spinal cord and
lower brainstem lesions can result in sensory dissociation, which
means one sensory system is affected without the other one.
alternating findings. For example one side of the face is affected
and the opposite side of the body for brainstem lesions. In the spinal
cord, lesions can cause DCML system findings on one side of the body
and ST findings on the opposite side.
Exam Tests The ST is examined by testing:
The DCML is
examined by testing:
sensation (must have intact DCML plus intact parietal cortex):
Light touch is represented
in both the ST and DCML system so it is OK for sensory screening but not
specific for either system.
The sensory exam is
perhaps the most subjective of the entire neurological exam so patient
response can be difficult to interpret or at times be misleading.
A sensory level is
valuable in determining if there is spinal cord disease. Pain (sharp)
is used to determine a sensory level. The sensory level on examination
is usually 1-2 spinal cord segments below the actual spinal cord lesion.
A sensory deficit
from a spinal nerve lesion will be in a dermatome distribution.
A sensory deficit
from a peripheral nerve lesion will be in the distribution of that peripheral
A sensory deficit
from a polyneuropathy will have a stocking and glove distribution because
the longest axons are the most affected.