Somatosensory Ascending Pathways Flashcards by Chelsea Garner

Pathways for conscious sensation have three or more neurons not counting interneurons

Primary afferent neuron:
- Cell body in the dorsal root ganglion of the spinal nerve
- sensation from head/neck:
- cell body located in the trigeminal ganglion or other CN ganglia
Secondary afferent neuron
- cell body either in the spinal cord or in the brainstem.
- Ascending axon decussates CNS in the spinal cord or brainstem.
- Axons conveying conscious sensation terminate in the thalamus.
Third neuron:
- Cell body in the thalamus
- projects to the primary somatosensory cortex.
  - Side receiving somatosensory information is contralateral to the side of the sensory receptor.

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Basic Components of Somatosensory System

Composed of receptors and processing centers
Produce sensory modalities
receptors in skin, muscles, bones, joints, organs, & CV system
Reacts to stimuli w/ different receptors
- transmits info thru afferent nerves to spinal cord & brain.
- Conscious sensation sent to postcentral gyrus
- unconscious proprioception is sent to the cerebellum.

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List the somatosensory modalities.

conscious & unconscious sensory experiences that result from activation of somatic sensory receptors
Touch
Crude or light touch
Discriminative touch (two-point tactile discrimination)
Unconscious proprioception (unconscious perception of movement and spatial orientation)
Conscious proprioception (awareness of limb movement and body position in space)
Vibration
Nociception (pain)
Temperature

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Somatosensory Receptor types

Mechanoreceptors respond to tissue displacement as an optimal stimulus;
crude touch, two-point touch vibration, proprioception, and tickle sensory information.
Nociceptors respond to tissue damage; convey pain sensation.
Thermal receptors respond to temperature changes; convey perceptions of warmth and cold sensation.

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A-α Somatosensory Mechanoreceptors

Alternate name: I
Fiber diameter 13-20µm
Myelinated
Receptors: Muscle Spindle & Golgi Tendon Organ
Sensory modality: Proprioception
Fiber tract: Posterior Columns

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A-β Somatosensory Mechanoreceptors

Alternate name: II
Fiber diameter: 6-12µm
Myelinated
Receptors and Respective Sensory Modality:
- Muscle Spindle: Proprioception
- Meissner Corpuscle: Touch
- Merkel Disk: Touch
- Ruffini Ending: Touch, Vibration
- Pacinian Corpuscle: Touch, Vibration
Fiber tract: Posterior Columns

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A-δ Somatosensory Mechanoreceptors

Alternate name: III
Fiber diameter: 1-5µm
Myelinated
Receptors and respective sensory modality:
- Hair Follicle Receptor: Touch
- Free nerve endings: Pain and temperature
Fiber tract: ALSTS

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C Somatosensory Mechanoreceptors

Alternate name: IV
Fiber diameter: 0.2-1.5µm
Unmyelinated
Receptors and respective sensory modality:
- Free Nerve Ending: Pain & Temperature
Fiber Tract: ALSTS

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Characteristics of Somatosensory Primary Afferent Neurons

pseudounipolar neurons w/ peripheral and central processes.
peripheral processes located chiefly in spinal nerves
- receptors/free nerve endings in skin, muscle spindles or tendon
cell bodies located in the dorsal root ganglia.
central processes enter spinal cord thru dorsal roots of spinal n.
Dermatome: peripheral region innervated by sensory fibers from a single nerve root.
- different spinal cord levels form a map over the surface of body
- useful in localizing lesions of the nerve roots.
Most primary afferents convey ascending tract info & reflex arc formation (by axon collaterals).

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Organization of the primary afferent neurons in the dorsal roots and the spinal cord entry zone

Large (Group Ia&Ib) & intermediate (Group I&II) myelinated fibers:
- enter the spinal cord medial to the Zone of Lissauer.
- Group Ia & Ib fibers convey unconscious proprioception.
- GroupI&II fibers convey vibration, tactile & kinesthetic sense.
Small thinly myelinated Group III (A-δ) & small unmyelinated Group IV(C) fibers:
- occupy the lateral part of the dorsal root.
- enter the spinal cord through the Zone of Lissauer.
- A-δ fibers send light touch, sharp (fast) pain, & thermal sense.
- C-fibers) convey burning (slow) pain, & thermal sense.

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Posterior column/medial lemniscus system Sensory Modalities:

Vibration, tactile discrimination & conscious position sense

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Posterior column/medial lemniscus system Primary Afferent Neurons:

Location: In DRG at ALL spinal levels
Fiber Size: Large-intermediate myelinated fibers: Group I & II
- enter spinal cord medial to the Zone of Lissauer
Fiber Course: Ascends in the ipsilateral posterior funiculus
- NO SYNAPSE in the spinal cord
- DO NOT CROSS in the spinal cord
Somatotopic Arrangement in the Posterior Funiculus Below T6
- Below T6 the posterior funiculus = Fasciculus Gracilis (FG).
- afferent fibers from sacral nerves in the most medial part of FG.
- fibers from lumbar nerves course in the intermediate part of FG.
- fibers from lower thoracic nerves course in lateral part of FG.
Somatotopic Arrangement in Posterior Funiculus From C1 - T6
- T6 & above posterior funiculus: FG & Fasciculus Cuneatus (FC).
- afferent fibers from spinal nerves C1–T6 course w/in FC
- afferent fibers from upper thoracic nn in medial part of FC.
- fibers from lower cervical nn course in intermediate part of FC.
- fibers from upper cervical nn course in lateral part of FC.

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Posterior column/medial lemniscus system Secondary Afferent Neurons:

Location:
- 1˚ afferents in FC synapse on 2˚ afferents in Nucleus Cuneatus
  - ipsilateral lower medulla.
- 1˚ afferents in FG synapse on 2˚ afferents in Nucleus Gracilis
  - ipsilateral lower medulla.
Fiber Course:
- Axons arc anteromedially to midline (internal arcuate fibers)
- decussate & ascend to the thalamus as the medial lemniscus.
Termination: Thalamus: ventral posterolateral nucleus (VPL)
- Axons of the secondary afferents cross in the medulla

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Posterior column/medial lemniscus system Tertiary Afferent Neurons:

Location: Thalamus: VPL
Fiber Course: traverse posterior limb of the internal capsule
Termination: synapse on neurons in the somatosensory cortex

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Posterior column/medial lemniscus system CNS Final Target

1˚ Somatosensory Cortex in the postcentral gyrus & posterior parcentral gyrus.
cortex has somatotopic body surface in a foot to tongue pattern along the medial to lateral axis
Blood supply to the primary somatosensory cortex from the anterior and middle cerebral arteries
Lesions of the MCA will cause tactile loss over the contralateral face, upper limb and trunk
Lesions of the anterior cerebral artery will affect the contralateral lower extremity and genitalia

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Anterolateral spinothalamic system Sensory Modalities:

Pain, thermal sense, light touch

Anterolateral spinothalamic system Primary Afferent Neurons

Location: In DRG at ALL spinal levels
- enter the spinal cord through the Zone of Lissauer
- synapse on neurons in the dorsal horn of the spinal cord.
Fiber Size: Small, thinly myelinated Group III (A-δ fibers) & unmyelinated Group IV C fibers
Fiber Course: Fibers synapse on dorsal horn neurons
- Synapse in the spinal cord

Anterolateral spinothalamic system Secondary Afferent Neurons

Location: Axons of neurons in diverse laminae of the dorsal horn
Course: cross in the anterior white commissure
- form the contralateral anterolateral spinothalamic tract
- ascends to the thalamus
- the axons CROSS in the spinal cord
- ascend thru the spinal cord without synapsing in the cord
Termination: Thalamus: ventral posterolateral nucleus (VPL)

Anterolateral spinothalamic system Tertiary Afferent Neurons

Location: Thalamus:VPL
Fiber Course: Axons traverse posterior limb of internal capsule
- synapse on neurons in the somatosensory cortex

Anterolateral spinothalamic system final CNS target

-Primary Somatosensory Cortex

Anterolateral spinothalamic system Somatotopic Arrangement

Somatotopic arrangement is retained along the entire pathway
cruder than the dorsal column/medial lemniscal system
input from segments sacral & lumbar segments occupy lateral position in the tract
- rostral segments occupy the more medial positions

Spinoreticular Tract Sensory Modalities:

Pain- older pain pathway that conveys emotional and arousal aspects of pain.

Spinoreticular Tract Primary Afferent Neurons:

Location: In DRG at ALL spinal levels
Fiber Size: Group III (A-δ fibers); Group IV (C fibers).
- Fibers enter spinal cord through the Zone of Lissauer
Fiber Course: Fibers synapse on dorsal horn neurons

Spinoreticular Tract Secondary Afferent Neurons

Location:
Course: Axons in intermediate & ventral areas of spinal cord cross in the anterior white commissure
*course with the ALSTT axons.
*ascend through the spinal cord without synapsing in the cord
-Termination: Medullary and Pontine Reticular Formation

Spinoreticular Tract Reticulothalamic Fibers:

- pain info carried to the thalamus by reticulothalamic fibers - Synapse on intralaminar nuclei of the thalamus. - intralaminar nuclei project diffusely to the cerebral cortex * involved in behavioral arousal. - Connection b/t spinoreticular pathway & reticulothalamic * conveys emotional & arousal aspects of pain.

Spinomesencephalic Tract Sensory Modalities:

Pain older pain pathway that conveys emotional and arousal aspects of pain.

Spinomesencephalic Tract Primary Afferent Neurons:

- Location: In DRG at ALL spinal levels - Fiber Size: Group III (A-δ fibers); Group IV (C fibers) * Fibers enter spinal cord through the Zone of Lissauer - Fiber Course Fibers synapse on dorsal horn neurons

Spinomesencephalic Tract Secondary Afferent Neurons:

- Location: Begin in DRG - Course: Axons in laminae of dorsal horn cross in the anterior white commissure * course with the ALSTT axons. * Axons ascend thru the spinal cord w/o synapsing in the cord - Termination: Midbrain periaqueductal gray matter

Spinomesencephalic Tract Periaqueductal Gray Matter:

- Participates in central modulation of pain. | - Inhibits pain transmission in the dorsal horn.

Anterior spinocerebellar tract Sensory Modalities:

- Unconscious muscle feedback from Golgi Tendon Organs & muscle spindles - gross coordination of posture related to lower extremity as a whole

Anterior spinocerebellar tract Primary Afferent Neurons:

- Location: DRG of spinal nerves pf lower extremity (L1-S5) - Fiber Size: Large myelinated fibers: Group Ia & Ib * Fibers enter spinal cord medial to Zone of Lissauer * in Fasciculus Gracilis - Fiber Course: Fibers enter the spinal cord in the FG * course ipsilaterally * synapse in the dorsal horn & intermediate zone of spinal cord

Anterior spinocerebellar tract secondary Afferent Neurons:

- From neurons in dorsal horn & intermediate zone of spinal cord - Course: Axons cross in the anterior white commissure - form the contralateral ASCT in the lateral funiculus ventral to the PSCT - ASCT seen at all levels of the spinal cord but tract only has axons from neurons in L1 to S5 segments

Anterior spinocerebellar tract CNS Final Target

-Cerebellum (for unconscious proprioception)

Posterior spinocerebellar tract: Sensory Modalities

- Unconscious proprioception from Golgi Tendon Organs & muscle spindles - fine coordination of posture related to individual muscles of lower extremity

Posterior spinocerebellar tract: Primary Afferent Neurons

-Location: DRG of spinal nerves of the lower extremity (L1-S5) -Fiber Size: Group Ia & Ib *enter spinal cord medial to the Zone of Lissauer -Fiber Course: ascend in ipsilateral posterior funiculus *synapse in the Dorsal Nucleus of Clark (DCN) –located in Lamina VII in spinal cord segments C8 – L2 -Fibers entering below L2 ascend in FG until DCN L1 –L2 *synapse in DCN & give rise to the PCST at upper lumbar levels *PCST not present in the lower lumbar and sacral spinal cord. -Fiber Termination: Dorsal Nucleus of Clark (DCN)

Posterior spinocerebellar tract: Secondary Afferent Neurons

- Location: DCN neurons | - Course: Axons of DCN neurons course in the ipsilateral lateral funiculus and form the PSCT.

Posterior spinocerebellar tract: CNS Final Target

- Cerebellum (unconscious proprioception) | - Upper Limb Homologue of PSCT