Point
1
Intro
What happens when there
is a lesion anywhere in the system involving the peripheral
processes of the dorsal root neurons, the fasciculus
gracilis, the fasciculus cuneatus, the nucleus gracilis and
the nucleus cuneatus? Let's take the peripheral processes
first and include all alpha-beta fibers which can carry
TWO POINT DISCRIMINATION, VIBRATION, AND CONSCIOUS
PROPRIOCEPTION. Such a lesion would result in
interruption of the information from the region of the body
innervated by that dorsal root. This is called a
dermatome. While there
is overlap of adjacent dermatomes, don't worry about that
now. Think about the distribution of the peripheral process
of each dorsal root.
Following a lesion of the dorsal root the resulting deficits are manifest on the same side as the lesion = IPSILATERAL
Deficits that result from a lesion in the dorsal column system (i.e. in the spinal cord) would differ depending on the precise location of the lesion. For instance, following a lesion at spinal cord level C2 which damages both fasciculi, information from the entire ipsilateral side of the body (and the back of the head, which is innervated by C2) would not reach the nucleus gracilis and nucleus cuneatus, and thus we would never feel the sensations (they don't reach consciousness via pathways from the medulla to the cerebral cortex). If the spinal cord lesion involves only fasciculus gracilis at spinal segment C2, only the information from spinal segments T7 and below (all the way down) is lost. Information from the arm is OK because fasciculus cuneatus is fine. If the lesion lies at S1, then only the ascending information from spinal segments S1 and below are affected. Information coming in above S1 (toward the head) gets in OK and ascends to the caudal medulla.
In addition to the loss
of 2 pt. discrimination, vibration and conscious
proprioception, you should know that dorsal column
lesions result in astereognosia (Gr. steros = solid,
gnosis = recognition), which is the inability to recognize
objects or forms by touch. Put a key in your hand with your
eyes closed and you can identify it as a key. Another
problem is called agraphesthesia (inability to
recognize letters, numbers, etc., drawn on the skin). In the
upper extremity, such ataxia results in clumsiness. Finally,
damage to the dorsal columns sometimes presents as
paresthesia (Gr.- para = abnormal, aisthesis =
sensation) which is tingling and numbness. Think of
this as resulting from irritation of the fibers as they die.
Use the diagram to the right to draw your own lesions (hopefully you will be more creative than me!) and think about the resulting deficits.