Sensory Pathways

pathway

The “route” is designated and suports the code of labeled lines concept

tracts

the “roads” are the different highways the information takes

synapses

The “exchanges”, where you have to change roads, are the _ between neurons

tract

a bundle of axons in the CNS with a common origin, course, destination or termination and usually function; often named according to origin and ending, and these terms usually identify direction as well

neuron #1

1. Neuron #1

• a Primary Sensory Afferent with Cell Body in Dorsal Root Ganglion

2. Neuron #2

• a Tract {T} Cell with Cell Body in a Central Nucleus

3. Neuron #3

• a Cell with Origin in the Thalamus traveling to Cortex over Posterior Limb of Internal Capsule

4. Primary Sensory Cortex Area 3, 1, 2

neuron #2

a Tract {T} Cell with Cell Body in a Central Nucleus

pathway

A series of synaptically linked neurons that share a common function. Tracts are often included as parts of pathways. Pathways may mediate reflexes, or may serve sensory, motor or other functions.

spinoreticular terminations

_ in the reticular formation act to initiate an alerting response to pain

brainstem motor neurons

in charge of quick reflexes to facilitate withdrawal from noxious stimuli

spinomesencephalic terminations

in the periaqueductal gray are onto brainstem neurons in charge of descending modulation of pain relief: alleviates pain

terminations in the intralaminar thalamic nuclei

( e.g. centremedian); also contribute to an alerting response: they project to widespread areas of cortex

anterolateral system

• Some sensory fibers travel up/down briefly in Lissauer’s tract before synapsing in the dorsal horn

• 2nd-order neurons cross midline (decussate) and move upward (rostrally) as they ascend

• Because fibers enter → travel → then cross, the pathway is slightly “shifted” upward

Clinical Insight (IMPORTANT)

• Cutting the ALS at a certain level does NOT eliminate pain from that exact level

• Why? → fibers haven’t crossed yet at that point

➡ Cordotomy rule:

• Must cut a few segments ABOVE (rostral to) the pain level to be effective

first order neuron (sesnory afferent)

neuron #1 is a small diameter afferent from type A Delta or C nerve endings

second order neuron

neuron #2 crosses AT or NEAR the segment of entry in the spinal cord

third order neuron

Neuron #3 is in the Ventral Posterior Lateral (VPL of the Thalamus)

ALS

• A three neuron pathway running from the periphery of the body to primary S1 cortex.

• Lightly myelinated or unmyelinated fbers enter segmentally into the the lateral division of the dorsal horn either directly or after ascending or descending one or two segments via Lissauer’s tract

• They synapse onto neurons in laminae I-VII (predominantly I,II,V) of the dorsal horn

• Also carries itch, tickle, and sexual sensations

exclusively free nerve ending receptors

give rise to nociceptve, innocuous and harmful thermal sensations and some mechanoreceptive light, poorly localizing non-discriminatve touch sensations

merkel cells

Non-discriminatve touch includes very lightly felt sensatons such as a breeze or an insect on the skin; _ and endings are prime encoders of light touch

2nd order neurons

• Synapse in dorsal horn

• Axons cross midline (anterior white commissure)

• Then ascend in anterior part of lateral funiculus

thalamus (VPL)

• Fibers go to VPL of thalamus

• ALS and DC-ML terminate in different areas

• Some multimodal neurons receive both inputs

collateral pathways

Along the way, fibers send branches to:

• Reticular formation → alertness/arousal

• Periaqueductal gray → pain modulation

• Intralaminar nuclei (thalamus) → diffuse pain awareness

• pain does not just mean sensation

3rd order neurons

• Leave VPL via PLIC (posterior limb of internal capsule)

• Travel to primary sensory cortex

• Organized somatotopically (body map)

VPL

The ascending ALS fibers are headed to (and synapse in) the ventral posterolateral nucleus (VPL) of the thalamus.

• The main “relay station” for body sensation

• ALS fibers end there (2nd → 3rd order neuron synapse)

• Then signals go from VPL → cortex

ALS pathway

• 1st-order neuron

• Starts at receptors in skin (pain/temp)

• Travels through peripheral nerve

• Cell body in dorsal root ganglion (DRG)

• Enters spinal cord

• May travel briefly in Lissauer's tract

• Synapses in dorsal horn

—

• 2nd-order neurons start in dorsal horn

  • Cross midline → anterior white commissure

  • Ascend to ventral posterolateral nucleus (VPL)

—

• 3rd-order neurons

  • Start in VPL

  • Travel via posterior limb of the internal capsule (PLIC)

  • Go to primary somatosensory cortex

  • ➡ ALS terminates in VPL (thalamus)

anterior part of ALS

contains more crude touch info

lateral part of ALS

contains more pain and temperature

thalamus; reticular formation

The ALS contains fibers that go to the cortex via the _ (spinothalamic) but ALSO fibers that end in the brainstem _ (spinoreticular)

pain

• Transmited by a non-discrete system, meaning it has a lot of “rest stops” along its pathway, particularly with the reticular formation

• The result of that is that it serves as an arousing stimulus as do itch and tickle

fasciculus of lissauer

when the axons of the primary afferents enter the cord, before they synapse on the 2nd order neuron, they sometimes jump a segment or two in a “local road” known as the posterolateral tract

• This makes it difficult to diminish pain by cuttng just one dorsal root

A delta and C fibers

Pain and temperature from the face is detected by _ and _ in the trigeminal nerve CN V

pain/temp detection of face

1. Pain & temperature from the face are detected by A-delta and C fibers in trigeminal nerve

2. Signals enter the brainstem at the pons

3. Then descend to the spinal trigeminal nucleus & tract → synapse (2nd-order neuron)

4. Cross midline

5. Ascend as ventral trigeminothalamic tract

6. Go to ventral posteromedial nucleus (VPM)

7. 3rd-order neurons → primary somatosensory cortex (face area, 3,1,2)

dorsal column medial lemniscal (DC-ML) system

conveys conscious perception of vibraton discriminative touch and proprioception to the cortex

proprioception

• Position sense

• It is how we know where and how our bodies are positoned in space

• So important that it is also carried to the cerebellum and it underlies local spinal cord reflexes

It is brought to us by:

• Muscle spindles

• Golgi tendon organs

• Joint receptors

visual and vestibulary systems

Information about the positon of our bodies in space (propriocpetion) is also carried via the_ (the three-legged stool)

discriminative touch

Identfying objects or letters on the basis of how they feel

three destinations for proprioceptive data

• spinal cord (initiates reflexes)

• cerebellum (unconscious proprioception via spinocerebellar pathways)

• cortex (conscious proprioception via DC-ML)

fasciculus gracilis

Fibers entering BELOW the T6 segment form a tract known as the _

fasciculus cuneatus

Fibers entering ABOVE T6 layer onto the outside of the ascending fasciculus gracilis laterally, forming a bundle of fibers known as the _

anterior spinal artery

• supplies anterior 2/3 of the spinal cord

• most of the ALS

• midline artery; a lesion here would affect bilaterally

posterior spinal artery

Supplies:

• Posterior 1/3 of spinal cord

Includes:

• DC-ML pathway → vibration, proprioception, fine touch

If damaged:

➡ Loss of vibration & proprioception
➡ Pain & temperature intact
➡Ipsilateral deficit

discriminative touch from face

1. Stimulus detected by A-beta fibers in trigeminal nerve

2. Signals enter the brainstem at the pons

3. Synapse in the Chief (Principal) Sensory Nucleus of V

4. 2nd-order neurons ascend IPSILATERALLY (do NOT cross)

5. Travel as dorsal trigeminothalamic tract

6. Synapse in ventral posteromedial nucleus (VPM)

7. 3rd-order neurons go to primary somatosensory cortex (face area)

DC-ML

What it carries

• Fine/discriminative touch (stereognosis, graphesthesia)

• Vibration

• Proprioception (position sense)

DC-ML pathway

• Step 1:
  1st-order neurons enter spinal cord

  • Go ipsilaterally in:

    • Gracile fasciculus (below T6)

    • Cuneate fasciculus (above T6)

• Step 2:
  Synapse in:

  • Gracile nucleus or Cuneate nucleus (in medulla)

• Step 3:
  2nd-order neurons cross (decussate) as → medial arcuate fibers

• Step 4:
  Ascend in brainstem to ventral posterolateral nucleus (VPL)

• Step 5:
  3rd-order neurons leave VPL via posterior limb of the internal capsule

• Step 6:
  Terminate in primary somatosensory cortex (somatotopic)

➡ DC-ML crosses in the MEDULLA

sensation

is fundamental; it is usually at the detection, discriminaton, and localization level [some would argue it is only detection]

YOU CANNOT TEST PERCEPTION IF _ IS NOT INTACT

reporting results of sesnory testing

• Modality: Light touch, pin prick most common; discriminatve touch, temperature and proprioception less often

• Location/Somatotopy: Ipsilateral, contralateral, bilateral/Body Part

• Quality: Burning, painful, etc.

• Intensity: Dull vs. Clear

peripheral nerves

lose all modalities in a nerve field distribution; usually reports analgesia

dorsal roots

lose all modalities, but with dermatomal distribrution; reports hypoesthesia or analgesia

CNS

side and extent of deficit depends on level of leison

spinal cord

Lose all modalites; dermatome distribution; analgesia

cortex

Usually less discrete; often report abnormalites of sensation, such as a simple touch feels burning

anesthesia

Absence of Sensation

• damage to PN, nerve root, or a cord

• stroke rarely is the cause

hypoalgesia

Diminished Sensation

paresthesia

Abnormal sensaton; prickles; pins and needles think your arm fell asleep

• central lesions almost always associated

hyperesthesia

Hypersensitvity

• around lesions

lesions in the ALS pathway

• Often “jump” a few segments

  • They can go up or down a few levels in
    → Lissauer's tract

• Cross at or near segment of entry; therefore, lesions are contralateral to the side damaged through the cord

  • e.g. if the damage is on the right side of the SC, the loss of pain and temperature would be on the left, below the level of the lesion

• And stays contralateral throughout the neuraxis

common lesions to ALS pathway

1. Anterior spinal artery

• Would cause BILATERAL LOSS of pain and temperature

• BUT some touch would be preserved through the DCML; as would propriocepton

• Motor Paralysis

2. Syringomyelia

• This is a progressive cavitaton in the center of the cord and it catches the crossing ALS fibers, so BILATERAL LOSS of pain and temperature

• Specifc Motor Deficits

lesions in the DCML pathway

• IPSILATERAL through the spinal cord

• Don’t become contralateral until after the pathway crosses in the medulla

• Loss of propriocepton [causes ataxia]; 2 point discrimination

• Decreased touch

• Preserved pain and temperature

common lesions to DCML pathway

1. Posterior spinal arteries (uncommonly lesioned in isolation)

• If one artery infarcts, would be ipsilateral to side of infarct

• If both arteries infarct, would be bilateral

2. Tables dorsalis/tabetic ataxia

• Ataxic gait

• Poor balance- post synpalatic deteroriation

below

will NOT SEE fisculus cuneatus _ t6

tavis dorsalis

disruption of movement motor disorder of ataxia and loss of conscious sensation