Somatosensory Ascending Pathways Flashcards
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.
- projects to the primary somatosensory cortex.
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.
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
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.
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
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
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
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
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).
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.
Posterior column/medial lemniscus system Sensory Modalities:
Vibration, tactile discrimination & conscious position sense
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.
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.
- 1˚ afferents in FC synapse on 2˚ afferents in Nucleus Cuneatus
- 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
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
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
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
