BASIC SOMATOSENSORY PATHWAY
A. The big picture:
The somatosensory system includes multiple
types of sensation from the body - light touch, pain, pressure,
temperature, and joint and muscle position sense (also called
proprioception). However, these modalities are lumped into three
different pathways in the spinal cord and have different targets
in the brain. The first modality is called discriminative
touch, which includes touch, pressure, and vibration perception,
and enables us to "read" raised letters with our fingertips,
or describe the shape and texture of an object without seeing
it. The second grouping is pain and temperature, which
is just what it sounds like, and also includes the sensations
of itch and tickle. The third modality is called proprioception,
and includes receptors for what happens below the body surface:
muscle stretch, joint position, tendon tension, etc. This modality
primarily targets the cerebellum, which needs minute-by-minute
feedback on what the muscles are doing.
These modalities differ in their receptors, pathways,
and targets, and also in the level of crossing. Any sensory system
going to the cerebral cortex will have to cross over at some point,
because the cerebral cortex operates on a contralateral (opposite
side) basis. The discriminative touch system crosses high
- in the medulla. The pain system crosses low - in the
spinal cord. The proprioceptive system is going to the cerebellum,
which (surprise!) works ipsilaterally (same side). Therefore
this system doesn't cross.
B. Discriminative touch:
As an introduction to the somatosensory system, we
will start by looking in some detail at the discriminative touch
system. The system that is carried in the spinal cord includes
the entire body from the neck down; face information is carried
by cranial nerves, and we will come back to it later. Overall,
the pathway looks like this:
|Sensation enters the periphery via sensory axons. All sensory neurons have their cell bodies sitting outside the spinal cord in a clump called a dorsal root ganglion. There is one such ganglion for every spinal nerve. The sensory neurons are unique because unlike most neurons, the signal does not pass through the cell body. Instead the cell body sits off to one side, without dendrites, and the signal passes directly from the distal axon process to the proximal process.|
The proximal end of the axon enters the dorsal half of the spinal cord, and immediately turns up the cord towards the brain. These axons are called the primary afferents (pink), because they are the same axons that brought the signal into the cord. (In general, afferent means towards the brain, and efferent means away from it.) The axons ascend in the dorsal white matter of the spinal cord.
At the medulla, the primary afferents finally synapse. The neurons receiving the synapse are now called the secondary afferents (purple). The secondary afferents cross immediately, and form a new tract on the other side of the brainstem.
|This tract of secondary afferents will ascend all the way to the thalamus, which is the clearinghouse for eveything that wants to get into cortex. Once in thalamus, they will synapse, and a third and final neuron (lavender) will go to cerebral cortex, the final target.|
C. Names and faces:
Now let's give names and images to the pathways and
The location of the pathway in the spinal cord has
several names. Since the tracts are on the dorsal side of the
cord, they are sometimes called the dorsal columns. In
upright humans, we call dorsal "posterior", so they
are also called the posterior columns. Finally, they have
Latin names. If we look at a cervical cord section (below) the
posterior columns can actually be divided into two separate tracts.
The midline tracts are tall and thin, and were given the name
gracile fasciculus (gracile means slender, and fasciculus
means a collection of axons). The outer tracts are more wedge
shaped, and were given the name cuneate fasciculus (cuneate
The gracile fasciculus is carrying all of the information
from the lower half of the body (legs and trunk), while the cuneate
fasciculus is carrying information from the upper half (arms and
trunk). This explains why you will only see the cuneate fasciculus
in thoracic and cervical sections, even though the gracile fasciculus
starts down in the sacral cord.
In the medulla, each tract synapses in a nucleus of the same name. The gracile fasciculus axons synapse in the gracile nucleus, and the cuneate axons synapse in the cuneate nucleus.
The secondary afferents leave these nuclei and immediately
cross, lining up in the ventral medulla. The new tract that they
form is called the medial lemniscus ("midline ribbon"),
and it will ascend all the way through the brainstem. Here is
how it looks in the upper medulla:
In the pons, the medial lemniscus begins to flatten out as the pontine nuclei enlarge beneath it.
By the time we get to the midbrain, the medial lemniscus
is getting pushed way up laterally and dorsally, which will position
it to enter the thalamus.
Once in the thalamus, the secondary afferents synapse
in a thalamic nucleus called the ventrolateral posterior nucleus
(VPL). The thalamocortical afferents (from thalamus to
cortex) travel up through the internal capsule to get to primary
somatosensory cortex, the end of the pathway.
Primary somatosensory cortex is located in the post-central
gyrus, which is the fold of cortex just posterior to the central
D. A diagrammatic review:
Here is the entire pathway in schematic form:
Now, think briefly about some possible lesions.
If you cut this pathway, most of the discriminative touch sensation
will be lost (not all, because some sneaks into other pathways
for redundancy). Where would the sensory loss be if you cut:
1) The left gracile fasciculus?
2) The left dorsal columns (gracile & cuneate)?
3) The right medial lemniscus, in the medulla?
4) The left internal capsule?
Answer these for yourself, with the diagram if necessary,
then scroll down.
1) The left leg and lower left trunk.
2) The left side of the body below the level of the cut.
3) The entire left body, from the neck down.
4) The entire right body (including the face, because
the face joins the pathway in the pons, but we will get to that