Somatosensory Systems Flashcards
Describe the anterior cerebral artery - what does it supply?
Medial surface + narrow strip of dorsal surface = LOWER LIMB
Describe the middle cerebral artery - what does it supply?
Lateral surface = rest of body
Somatosensory Pathways:
- What is the posterior column medial lemniscal path?
Proprioception, fine touch, vibration
Somatosensory Pathways:
- What is he anterolateral system
(major componenet = spinothalamic tract)
PAin (sharp and dull), temperature, crude touch
Somatosensory Pathways:
- what is the spinocerebellar pathway?
Non conscious proprioception and touch
Descrube the pathway of the Posterior column-medial lemniscus
conscious proprioception, 2 point discrimination, touch, vibration.
- Primary sensory neuron ->
- examples (all myelinated): Abeta fibre - pacinian corpuscle (vibration), meissners corpuscle (light touch), merkel ending (touch) ; Ia, II: muscle spindles; Ib- golgi tendon organs
- ascend ipsilaterally up the spinal cord in the fasciculus gracilis and fasciculus cuneatus (above T6) - 2nd order neurons in caudal medulla : nucleus gracilis and cuneatus (swellings on dorsal surface)
- somatotopic arrangement - cross over to the medial lemniscus and ascend brainstem - Third order neurons in ventroposterolateral (VPL) nucleus of thalamus
- -> travel in posterior limb of internal capsule - terminate in SSC
Describe what would happen in damage to posterior column-medial lemniscal pathway
- Impairment of proprioception & discriminative tactile function
- Distinctive type of ataxia – incoordination of movement due to inadequate sensory feedback e.g. difficulty in moving cursor to target on computer screen
Describe Romberg’s sign
- Test of proprioception from lower extremities
- Swaying when standing erect with feet together, eyes shut.
Sense of position & movement is mediated by: - Vestibular system
- Visual system
- Somatosensory system
These 3 systems work together. If you take one away (i.e. posterior column/medial lemniscus) then the other 2 are sufficient to give you a sense of position. However if you take away two (M/l & vision – i.e. eyes closed) → you fall over
Describe the processing of proprioceptive and touch from the FACE
- primary cell bodies in trigeminal ganglion
- synapse in the main sensory nucleus of CNV
- > efferent fibres cross over at mid pons & join medial lemniscal pathway -> - tertiary sensory cell fibres are in the VPM of the thalamus
- Fibres from VPM project to primary somatosensory cortex
Describe the proproception from muscles of mastication (this is different)
- 1° cell bodies in mesencephalic nucleus of V (located in the midbrain)
- Fibers descend to Pons & synapse with 2° neurons in main sensory nucleus of V
o Result of this = crossing over at different points
o Hence an injury in the medulla/SC may lead → loss of touch & proprioception from the body BUT NOT FROM THE FACE (as the medulla is below the crossing point of the pons)
Describe the pathway of the anterolateral tract
PAIN, TEMPERATURE AND COARSE TOUCH
1. Primary afferents in cell bodies in DRG
- > a delta fibres: fast pain
-> C ibres are slow pain
Central processes enter the spinal cord and travel in LISSAURS TRACT -> synapse on neurons in the Substantia gelatinosa (DORSAL HORN of spinal cord)
- Secondary neurons in substantia gelatinosa - cross over within one spinal segment through the ANTERIOR WHITE COMMISURE to form the STT
- Axons form the spinothalamic tract - located in anterior funiculus. Somatotopically organised: lower limb: medial and posterior, neck: medioanterior
- > extends rstrally to medully & Pons
- > in PONS - STT close to medial lemniscus - Terminates in VPL of thalamus (3 neurons)
- projects to somatosensory cortex (post central gyrus of the parietal lobe)
Describe processing of Pain and Temperature from the FACE
- 1˚ cell bodies in the trigeminal ganglion
- 2˚ cell bodies in the descending nucleus of V (medulla, upper cervical spinal cord)
o Majority of fibers cross over in the medulla
o After crossing, these axons of second order neurons run in the TRIGEMINOTHALAMIC TRACT which is near the STT - Terminate at VPM (3˚ neurons)
- Fibers from VPM project to the primary somatosensory cortex
Subset of the anterolateral system is DULL PAIN through C fibres-?
Polysynaptic, absence of somatotopic organization
- Once they synapse in the dorsal horn, dull pain is processed differently.
- It is polysnaptic (many synapses are involved as information is transferred from one synapse to another)
- There is no somatotopic organization
- 2 tracts pointed out are a component of this dull pain
Describe the spinoreticular tract
- Reticular formation (makes up much of the core of the brainstem)
- Changes in level of attention in response to pain
- In the spinal cord (medulla, pons, tegmentum & midbrain), there is an area called the reticular formation (pink)
- Tracts that terminate in the RF change our levels of attention/arousal (pain/dull pain)
Spinomesencephalic tract is responsible for the modulation of pain -> how?
- Bilateral projection (in the midbrain, terminates around cerebral aqueduct)
- Modulation of pain
o Spinomesencephalic fibers → Periaqueductal grey matter (PAG) in midbrain
• PAG also receives input from hypothalamus & cortical areas
o Surrounding interneurons contain endogenous opioid peptides: encephalin & dynorphin → when these interneurons are stimulated, they release the opioid peptides
o The PAG & nucleus raphe magnus have opiate receptors
o Opioid peptides released by interneurons → receptors on PAG, nucleus raphe magnus → suppression of transmission of pain information between first & second order neurons - This system of suppression of pain information is in place to protect the body so you don’t have to endure extremes of pain → opioids dull transmission
