Sensory Spinal Tract (Posterior - Medial Lemniscal and Anterior - Lateral Lemniscal)

Question 1

What is the job of the Posterior - Medial Lemniscal tract? What is the point of this?

Answer 1

It transfers general sensory information about discriminative touch, vibration, pressure and proprioception. Sensory information will travel to the frontal cortex making us conscious of the feelings of vibration, discriminative touch, proprioception, etc.

Question 2

Describe the PMLT in terms of speed, fibers used, number of synapses and resolution.

Answer 2

It is high velocity and fast conducting, the fibers travels through the Aß fibers and have limited synapses. It has very high resolution.

Question 3

How does information in the PMLT enter the spinal cord?

Answer 3

From the receptors it enters the spinal ganglion at its respective spinal level, then enters the posterior horn of the spinal cord.

Question 4

Once the information is in the dorsal horn of the spinal cord, where does it travel in the PMLT? Is there a difference in the upper and lower body information processing?

Answer 4

Some enter the anterior horn for the spinal reflex arc, but most ascend up the POSTERIOR TRACT. Information from T6 and below enter the FASCICULUS GRACILIS, located medial to the FASCICULUS CUNAETUS which carries information from T5 and above (but not the head). This is at the level of the spinal cord.

Question 5

Where does the PMLT cross the midline and what is that fiber called?

Answer 5

Once the fibers synapse at their respective nucleus gracilis or cunaetus in the MEDULLA, it will cross the midline as the INTERNAL ARCUATE FIBERS which are second order neurons.

Question 6

Trace the path of the PMLT once it is in the dorsal horn of the spinal cord.

Answer 6

Once in the dorsal horn, most of the fibers will ascend in the posterior column, fibers below T6 in F. Gracilis, and T5 and up to head in F. Cunaetus. These first order Aß fibers will then synapse in their nucleus gracilis/cunaetus located in the MEDULLA. 2nd order neurons from these nucleus will CROSS THE MIDLINE as the INTERNAL ARCUATE FIBERS and then ascend as the MEDIAL LEMNISCUS to the VPL of the THALAMUS.

Question 7

Once in the medulla, how does the positioning of information of upper/lower body change as the fibers travel up the medial lemniscus? Relate that to the positioning in the Spinal Cord.

Answer 7

In the Dorsal Column of the Spinal cord the F. Cuneutus is lateral to the F. Gracilis, which is medial. In the medulla however, in the medial lemniscus tract this information seems to rotate 90 degrees, so that the lower body information is now anterior part of the medial lemniscus tract, and the upper body information is in the posterior ML tract.

Question 8

What happens to the fiber tracts of the ML as it ascends up rostrally into the brain?

Answer 8

It rotates again, so that upper body sensory information is in the medial part and the lower body is in the lateral part.

Question 9

What is the final destination of the ML tract and what happens from there?

Answer 9

Final destination is the VPL of the thalamus, where they synapse with third order neurons which travels up to the PRIMARY SOMATOSENSORY CORTEX for conscious perception of all this sensory information.

Question 10

Despite the function of the PMLT, why is a lesion here not as severe as anticipated?

Answer 10

Part of the tactile information and discriminative touch travels along the anterior lateral system.

Question 11

Describe the speed of conduction and type of fibers in the Anteriolateral system?

Answer 11

They are slow, either from lightly myelinated A delta fibers or unmyelinated C fibers.

Question 12

What is the point of the anteriolateral system?

Answer 12

Non discriminative touch such as pain and noxious stimulus.

Question 13

How does A delta and C fibers differ in their pain conduction?

Answer 13

A delta fibers are “fast” pain pathway, well localized and associated with “sharp” pain (because it is fast). C fibers are poorly localized and they react to CHEMICALS like bradykinin, histamine, changes in pH, etc. This results in dull, poorly localized pain (because it is slow).

Question 14

The major tract of the anteriolateral system is _________ and even though this tract enters the dorsal horn like the PMLT, how does it differ? Where does it cross over?

Answer 14

Once in the dorsal horn, it can ascend or descend a few spinal levels in the LISSAUER TRACT, and after that they synapse primarily in laminae I,II, but also III and IV (called nucleus proprius). Fibers from lamina I, III and IV crosses over at the anterior white comissure in the SPINAL CORD and ascends the anteriolateral system. Ergo, crossing over occurs BEFORE we reach the brain.

Question 15

Where do the fibers of the lower and upper limb reside in the anteriolateral tract?

Answer 15

Lower limb is lateral, upper limb is medial.

Question 16

Where does the spinothalamic tract of the ALT end up and where does it go from there?

Answer 16

It ends up in the VPL of the thalamus like the MLT, and from there it projects through the posterior limb of INTERNAL CAPSULE and CORONA RADIATA to the PRIMARY SOMATOSENSORY CORTEX.

Question 17

Where would the brain register pain of a lesion in the Spinothalamic Tract?

Answer 17

Since fibers of the Spinothalamic tract will go up and down the LISSAEUR TRACT before synapsing and then cross over before reaching the brain, a lesion in the STT will result in a loss of pain and sensation in the CONTRALATERAL SIDE a few levels above or below the lesion.

Question 18

A Hemisection of the spinal cord is called the “Brown Sequard Syndrome.” Describe this syndrome and what tracts are affected and why.

Answer 18

A hemisection of the spinal cord will result in loss of propriception/tactile information from the ipsilateral side and pain/temperature sensation from the contralateral side, due to damage to the PMLT and the spinothalamic tract of the ALT. Further, the motor tract (corticospinal tract) will result in ipsilateral hemiparesis.