Spinothalamic, Posterior Column, Thalamo-Cortical Pathways Flashcards
2 Systems for Somesthesis
1) Protopathic: anterolateral pathways
2) Epicritic: lemniscal pathway
Somatosensory Modalities
-touch, pain, temp., vibration proprioception
Protopathic
- pain, crude touch, temperature
- low spacial & temporal resolution
- small, slowly conducting, lightly myelinated & unmyelinated
- lateral Spinothalamic tract (anterolateral funiculi)
Epicritic
-form, texture, touch, pressure, slippage, vibration, position
-high spacial & temporal resolution
-large, rapidly conducting, myelinated
-Posterior & Posterolateral columns
-Medial Lemniscus
-proprioception
In spinal cord: epicritic info travels in posterior columns that becomes a tract known as the medial lemniscus in brainstem
Motorneuron Cell Bodies
- located in the spinal cord ventral horn
- axons exit the ventral horn in the ventral roots & travel within spinal nerves to their target muscles
- “final common path” for motor activation & the efferent side of spinal reflexes
Pseudo Unipolar Sensory Neurons
- how sensory receptors communicate with the CNS
- cell bodies are located outside the spinal cord in the dorsal root ganglia (DRG)
- action potentials continue (within dorsal roots) along the central process into the dorsal horn of the spinal cord
- peripheral process may synapse in the dorsal horn or project up the spinal cord to the brainstem
Types of Sensory Receptors for Somesthesis
- exteroreceptors: external events
- proprioceptors: position of joints & muscles in space
- Enteroreceptors: state of internal organs
- Chemoreceptors
- Photoreceptors
- Thermoreceptors
- Mechanoreceptors
- Nociceptors (pain/noxious stimuli)
Pacinian Corpuscle
- large, lamellar, rapidly adapting mechanoreceptor that efficiently detects gross pressure & vibratory skin stimula (250 Hz)
- in subcutaneous skin, they are also found in joints, muscle, & mesentery
- free nerve endings encapsulated by a series of modified Schwann cells (lamellae)
- pressure to lamellae the corpuscle is distorted, causes AP at onset and adaptation until pressure is released, then 2nd AP happens
Meissner’s Corpuscle
- rapidly adapting skin mechanoreceptor that is sensitive to light touch & vibration of <50 Hz
- located in glabrous skin, right below epidermis
Merkel’s Disks
- slowly adapting mechanoreceptors located in skin & mucosa
- glabrous skin: these disks are found clustered beneath the ridges on the fingertips that make up fibgerprints
- in hairy skin: they cluster into specialized epithelial structures called “touch domes” or “hair disks”
Ruffini Organs
- slowly adapting mechanoreceptors found only in deep layers of glabrous skin
- respond to sustained pressure & skin stretch
- responsible for detecting objects slipping along the skin & contribute to position sense
Free Nerve Endings
- no accessory structures associated with them to form a sense organ
- free nerve endings are typically responsive to temperature and nociceptive stimuli
Adaptation
-a reduced response in the face of a continued, constant stimulus
Funiculi = Fasciculi
-surface feature indicating an underlying axon tract
Epicritic Somesthesis: Lemniscal Pathway
- Pacinian Corpuscle connected to large afferent axon which projects into the dorsal horn of the spinal cord
- axon continues into the ipsilateral posterior column of the spinal cord & ascends through the entire spinal cord to the ipsilateral brainstem (medulla)
Epicritic Somesthesis: Lemniscal Pathway
Synapses
-First Synapse: one of the posterior column nuclei in the medulla (for afferents that enter in the lumbar & sacral spinal cord, this synapse is in the Gracile Nucleus
Nucleus Cuneatus
- serves similar function for afferents from Throacic & Cervical levels
- for spinal cord, axons from the lumbar cord destined for the nucleus gracilis travels in the fasciculus gracilis
Fasciculus Cuneatus
-corresponding path for the throacic/cervical cord
What is the cut-off for gracilis?
-T6
gracilis (below T6)
cuneatus (above T6)
Gracile
-thin
Cuneate
-“wedge shaped”
Decussation
-crossing the midline
Protopathic Somesthesis: Spinothalamic Pathway
- free nerve endings (axons are small and slowly conducting
- afferents from skin of leg enter spinal cord in the lumbar or sacral spinal cord
- axons synapse in the ipsilateral dorsal horn (1st synapse)
- a second order neuron sends its axon across the mid-line at the same level of the spinal cord to the contralateral anterolateral funiculus
- axon turns & ascends in the anterolateral spinal cord through the brainstem to the VPL thalamus (synapse #2)
- third order neuron projects ipsilaterally to somatosensory cortex (post central gyrus, synapse #3)
-entry: pathway decussates in anterior commissure of spinal cord & rest of pathway is in contralateral to peripheral receptor
Posterior Column - Medial Lemniscal Pathway
- First Synapse: Nucleus Cuneatus or Nucleus Gracilis, the nuclei are in the caudal medulla
- decussation of axons from these nuclei occurs in caudal medulla (internal arcuate fibers)
- in rostral medulla, the medial lemniscus (axons of controlateral posterior column nuclei) runs near the midline, towards the anterior medulla
- as medial lemniscus travels rostrally its path takes it gradually away from the midline, through the pons & midbrain on its way to VPL
- Projections from VPL to cortex travel through the posterior limb of the internal capsule