MODULE 3: DORSAL COLUMN –
MEDIAL LEMNISCUS (DC-ML)
General Organization of the Somatosensory System
Two major pathways bring sensation into the brain:
o Dorsal coloumn medial leminiscus pathway:
Contains:
Dorsal coloumns (make up the
posterior funciulus)
Medial leminiscus: lies in the
brainstem
Is newer than other pathways
Is in charge of the EPICRATIC sensations:
Vibration
Two point tactile discrimination
Proprioception
Fine touch
Stereognosis (the ability to identify
something based on touch alone
without seeing it)
graphesthesia
o Anterolateral system
Contains a number of tracts that pass
through the anterolateral spinal cord
Conveys PROTOPATHIC sensations:
Temperature
Pain
Crude touch (nondiscriminative)
Tickle
Components of the DC-ML Pathway
There are three orders of neurons that originate at the surface of the body (i.e skin)
and terminate in the cerebral cortex.
Internuncials are in between the long neurons but are not considered, but have
been found to increase synaptic delay.
First order neurons:
o Have cell bodies in the dorsal root ganglia (outside the CNS)
Dorsal root ganglia is a collection of pseudounipolar neurons
that have a peripheral (ends in the sensory receptors) and central
process (enters the spinal cord).
o Are peripheral nerves that are invested with neurolemma (the thin sheath
around a nerve axon (including myelin where this is present) and NOT
neuroglia.
o Have central processes that enter the brain or spinal cord and terminate
ipsilaterally (same side) to the origin.
o Their centeral process enters via the MEDIAL portion of the dorsal root
, entry zone (as opposed to the ALS where it enters through the lateral part
of the dorsal root entry zone)
o Do not decussate
o Have neurons that enter below T6 and:
Travel rostrally within the (medial) fasiculus gracilis of the dorsal
columns
Terminate on the cells of the nucleus
gracilis in the caudal medulla
o Have neurons that enter above T6 that:
Enter the (lateral) fasiculus cuneatus
Synapse on the cells of the nucleus
cuneatus (rostral and alongside the
nucleus gracilis)
o Enter through the medial dorsal horn ascend
and pass through the dorsal column
o Therefore, any lesion in the fasiculus gracilis
or cuneatus will give sensory loss to the same
side of the body
o A lesion in the MEDIAL part of the dorsal
root will result in loss of epicratic sensations
where a lesion in the LATERAL part of the
dorsal root will result in a loss of protopathic
sensations)
o To distinguish between the sections at different
spinal cord levels you look at the ratio of white
matter to grey matter. Cervical segments of the
spine have more white matter.
o The fibers carrying sensory information are added first – therefore occupy
the medial part of the dorsal coloumn. Other parts of the body are added
subsequently.
MEDIAL LEMNISCUS (DC-ML)
General Organization of the Somatosensory System
Two major pathways bring sensation into the brain:
o Dorsal coloumn medial leminiscus pathway:
Contains:
Dorsal coloumns (make up the
posterior funciulus)
Medial leminiscus: lies in the
brainstem
Is newer than other pathways
Is in charge of the EPICRATIC sensations:
Vibration
Two point tactile discrimination
Proprioception
Fine touch
Stereognosis (the ability to identify
something based on touch alone
without seeing it)
graphesthesia
o Anterolateral system
Contains a number of tracts that pass
through the anterolateral spinal cord
Conveys PROTOPATHIC sensations:
Temperature
Pain
Crude touch (nondiscriminative)
Tickle
Components of the DC-ML Pathway
There are three orders of neurons that originate at the surface of the body (i.e skin)
and terminate in the cerebral cortex.
Internuncials are in between the long neurons but are not considered, but have
been found to increase synaptic delay.
First order neurons:
o Have cell bodies in the dorsal root ganglia (outside the CNS)
Dorsal root ganglia is a collection of pseudounipolar neurons
that have a peripheral (ends in the sensory receptors) and central
process (enters the spinal cord).
o Are peripheral nerves that are invested with neurolemma (the thin sheath
around a nerve axon (including myelin where this is present) and NOT
neuroglia.
o Have central processes that enter the brain or spinal cord and terminate
ipsilaterally (same side) to the origin.
o Their centeral process enters via the MEDIAL portion of the dorsal root
, entry zone (as opposed to the ALS where it enters through the lateral part
of the dorsal root entry zone)
o Do not decussate
o Have neurons that enter below T6 and:
Travel rostrally within the (medial) fasiculus gracilis of the dorsal
columns
Terminate on the cells of the nucleus
gracilis in the caudal medulla
o Have neurons that enter above T6 that:
Enter the (lateral) fasiculus cuneatus
Synapse on the cells of the nucleus
cuneatus (rostral and alongside the
nucleus gracilis)
o Enter through the medial dorsal horn ascend
and pass through the dorsal column
o Therefore, any lesion in the fasiculus gracilis
or cuneatus will give sensory loss to the same
side of the body
o A lesion in the MEDIAL part of the dorsal
root will result in loss of epicratic sensations
where a lesion in the LATERAL part of the
dorsal root will result in a loss of protopathic
sensations)
o To distinguish between the sections at different
spinal cord levels you look at the ratio of white
matter to grey matter. Cervical segments of the
spine have more white matter.
o The fibers carrying sensory information are added first – therefore occupy
the medial part of the dorsal coloumn. Other parts of the body are added
subsequently.
