Somatosensory Ascending Pathways Flashcards
0
Q
Pathways for conscious sensation have three or more neurons not counting interneurons
A
- 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.
1
Q
Basic Components of Somatosensory System
A
- 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.
2
Q
List the somatosensory modalities.
A
- 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
3
Q
Somatosensory Receptor types
A
- 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.
4
Q
A-α Somatosensory Mechanoreceptors
A
- Alternate name: I
- Fiber diameter 13-20µm
- Myelinated
- Receptors: Muscle Spindle & Golgi Tendon Organ
- Sensory modality: Proprioception
- Fiber tract: Posterior Columns
5
Q
A-β Somatosensory Mechanoreceptors
A
- 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
6
Q
A-δ Somatosensory Mechanoreceptors
A
- 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
7
Q
C Somatosensory Mechanoreceptors
A
- Alternate name: IV
- Fiber diameter: 0.2-1.5µm
- Unmyelinated
- Receptors and respective sensory modality:
- Free Nerve Ending: Pain & Temperature
- Fiber Tract: ALSTS
8
Q
Characteristics of Somatosensory Primary Afferent Neurons
A
- 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).
9
Q
Organization of the primary afferent neurons in the dorsal roots and the spinal cord entry zone
A
- 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.
10
Q
Posterior column/medial lemniscus system Sensory Modalities:
A
Vibration, tactile discrimination & conscious position sense
11
Q
Posterior column/medial lemniscus system Primary Afferent Neurons:
A
- 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.
12
Q
Posterior column/medial lemniscus system Secondary Afferent Neurons:
A
- 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
13
Q
Posterior column/medial lemniscus system Tertiary Afferent Neurons:
A
- Location: Thalamus: VPL
- Fiber Course: traverse posterior limb of the internal capsule
- Termination: synapse on neurons in the somatosensory cortex
14
Q
Posterior column/medial lemniscus system CNS Final Target
A
- 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
