Ascending Sensory Pathways

How is the posterior limb positioned?

Positioned between the globus pallidus and the thalamus. Contains ascending fibers running from the thalamus to the cortex

What is an injury to the posterior limb common with?

Common to see some impact with the thalamus

In most cases, what do somatosensory pathways look like?

• Are modality-specific

  • Diverge when the neurons enter the spinal cord

• Majority of then consist of 3 neurons

  • Primary, secondary, tertiary

  • First order, second order, third order

When do most somatosensory pathways cross midline? How are they organized?

• Cross midline at level of 2nd order neuron (spinal cord or brainstem)

• Are organized somatotopically to bring in sensation from the body and head

Name the primary ascending pathways

• Dorsal column system

  • Medial lemniscus pathway (DCML)

• Anterolateral column system

  • Neospinothalamic pathway (NST)

• Trigeminal system

Name the secondary ascending pathways

• Spinothalamic Reticular tract

• Paleospinothalamic pathway

What is the function of the dorsal column (medial lemniscus) DCML and what kind of receptors and myelination?

• Function: proprioception, fine touch, vibration

• Cutaneous mechanoreceptors and proprioceptors

• Thick myelinated receptors (Aα, Aβ)

Describe the 1st and 2nd order of the DCML

• 1st Order: Ascend the spinal cord medially in the dorsal column

• 2nd Order: Cross midline at the medulla

What is the function of the anterolateral column (neospinothalamic) NST and what kind of receptors and myelination?

• Function: Deep pressure, crude touch, sharp pain, tickle, temperature (cold)

• Nociceptors and thermoreceptors

• Thin myelinated receptors (Aδ)

Describe the 1st and 2nd order of the NST

• 1st Order: Terminate laterally in the spinal cord

• 2nd Order: Cross midline in spinal cord

  • Ascend in the anterolateral column

What is the function of the anterolateral column (paleospinothalamic and spinothalamic reticular) and what kind of receptors and myelination?

• Function: Dull pain, temperature (warm), pain awareness

• Nociceptors and thermoreceptors

• Thick unmyelinated receptors (C)

Describe the 1st and 2nd order of the Anterolateral column (Paleospinothalamic and Spinothalamic Reticular)

• 1st Order: Synapse laterally in the spinal cord

• 2nd Order: Cross midline in spinal cord

• Ascend in the anterolateral column

What is the principal nucleus in the trigeminal system?

• Receives and transmits fine touch, proprioception

• Second order neuron pons to thalamus

What is the spinal nucleus in the trigeminal system?

• Receives and transmits pain, temperature

• Second order neuron medulla to thalamus

What makes up the gracile tract?

T7-S5

What makes up the cuneate tract?

C1-T6

Of the cervical, thoracic, lumbar, and sacral which one is a part of the cuneate tract (would synapse at the cuneate nucleus)?

Cervical & Thoracic C1-T6

Of the cervical, thoracic, lumbar, and sacral which one is a part of the gracile tract (would synapse at the gracile nucleus)?

Thoracic, lumbar, and sacral T7-S5

DCML first order neurons follow somatotopically from most medial to lateral sacral, lumbar, thoracic, and cervical. What happens when it crosses midline at level of 2nd order neuron in the medulla?

It flips so now cervical is most medial followed by, thoracic, lumbar, and sacral going lateral.

Where is the medial lemniscus found?

On the contralateral side of the medulla on the 2nd order

What is VPL (ventral posterior lateral) nucleus?

Where in the thalamus the 3rd order neurons are synapsing before projecting to the somatosensory cortex (S1)

In the homunculus the order from medial to lateral is lower extremity, upper extremity, and face. At the VPL nucleus the order of the 2nd neurons are as followed medial to lateral cervical, thoracic, lumbar, and sacral. What must happen?

They need to fan out to return back to the proper order of medial to lateral to match the homunculus postcentral gyrus (S1)

Sacral - lower extremity

Lumbar - lower extremity

Thoracic - upper extremity

Cervical - face

After leaving the thalamus, where do the 3rd order neurons travel through?

Posterior limb of the internal capsule (PLIC)

Describe in the spinal cord, what is the orientation of the lower and upper extremity order neurons?

Lower extremity is medial and upper extremity is lateral

Describe at the medulla, what is the orientation of the lower and upper extremity order neurons?

Lower extremity is lateral and upper extremity is medial

Describe at the cortex, what is the orientation of the lower and upper extremity order neurons?

Lower extremity is medial and upper extremity is lateral

In the anterolateral column system (NST) going from 1st to 2nd neuron how does it cross midline? At the same level, above or below

Same level of the spinal cord ex. comes in the dorsal side of the SC and crosses midline (gray matter) and ends up on the opposite side (from left to right) in the white matter to ascend up

In the NST where is the Lissauer tract found?

In the any of the columns ascending with all the fibers in the NST

In the NST what happens at the 3rd order neurons?

They need to fan out. Entering into the thalamus it is in the order of sacral, lumbar, thoracic, and cervical. To get to S1 they go to cervical, thoracic, lumbar, and sacral

Describe the orientation of the fibers in the spinal cord and cortex in the NST.

Spinal cord - lower extremity is lateral and upper extremity is medial. In the cortex is flips

As the paleospinothalamic (PST) (only medulla) and spinothalamic reticular (STR) ascend into the medulla and above what does it do?

Gives off collaterals in the lower processing parts of the brain to tell it that something is wrong (reticular system)

Where does the PST and STR synapse within the medulla?

medial dorsal nucleus (MD)

In 3rd order neuron for the PST and STR what does it give off?

Instead of going to S1 (primary somatosensory) and fans out information