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Wednesday, 9 August 2017

Sensory system - Transmission of somatic sensations

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Transmission of somatic sensations

       All somatic sensation transmitted in the form of nerve impulses:
       1/ from their specific receptors along afferent nerve fibers to the CNS (brain & SC)
       2/ then ascend to their final destination in the cerebral cortex along somatosensory tracts.
1/ somatic sensory afferents
       Each receptor is usually a peripheral beginning of an afferent nerve which carry information from receptors to CNS (brain & SC) in the form of a nerve impulses
       Somatic sensory afferent fibers are either peripheral branches of the posterior root ganglion or the sensory fibers of the cranial nerves

       11/ Somatosensory pathway= tracts :

       All sensory signal are transmitted to brain by one of two major pathways. Each tract carry specific sensation :
       Dorsal column = lamniscal tracts: show unimodality as it carries mechanoceptive sensation
       Fine toch & fine pressure
       Spinothalamic = anterolateral tracts: shows polymodality as it carries different types of sensations along its two divisions
       The lateral spinothalamic tracts carries:
       Temperature include cold & warm
       The venteral spinothalamic tracts carries
       Crude touch & crude pressure
       Tickle & itch
       Each pathway is composed of 3 order neurons starting from the afferent fiber to the last destination in the brain

Dorsal column lamniscal pathway

       The fiber are large myelinated Aβ fibers. They are actually for the first order neuron.
       First order neurons: afferent fibers enter the Sc via the medial portion of the dorsal root. They enter the ipsilateral (same side) dorsal column & ascend upwards as gracile & cuneate tracts to end respectively in the gracile & cuneate nuclei in the medulla
       Second order neurons: arise from gracile & cuneate nuclei, cross to the opposite side forming the sensory decussation & then pass upwards as the medial lemniscus which transverse the midbrain to end in the ventrobasal nuclei of the thalamus
       In its pathway through brain stem the medial lamniscus is joined by additional fibers from the sensory nuclei of the trigeminal nerve, these fibers carry the same sensation from the head that the dorsal column carry from the body
       3- third order neurons: these arise from ventrobasal nuclear complex of the thalamus & pass in the posterior limb of the internal capsule as part of the sensory radiation to the somatosensory cortex

The spinothalamic (ventrolateral) pathway

       The fibers are small myelinated type Aδ & unmyelinated type Cfibers.
       First order neuron: afferent fibers enter SC via the lateral division of the dorsal root. Fibers then ascend or descend a few segments in lissauer’s tract before synapsing in the posterior(dorsal) horn cells of the SC (dorsal horn lamina 1, 11, 111, 1V, V, V1)
       Second order neurons: arise from posterior horn cells cross to the opposite side infront of the central canal (ie anterior commissure) then ascend as the spinothalamic tract through the anterolateral columns of SC. Fibers then ascend through the brain stem to end in the ventrobasal nuclear complex of the thalamus
       In the anterior commissure of SC the temperature fibers are nearest to central canal, next to these are pain fibers & the furthest from central canal are touch fibers. Therefore in syringomyelia a developmental defect of the central canal the temp- & pain fibers are damaged & touch fibers escape à loss of temp- & pain sensation with preservation of other sensation a phenomenon known as dissociated aneathesia or dissociated sensory loss
       Third order neuron: arise from ventrobasal nuclie of thalamus & pass upwards in the sensory radiation (in internal capsule) to reach somatosensory cortex
       NB: on their pathway through brain stem fibers within the dorsal column & ventrolateral column are joined by additional fibers from nuclei of trigeminal nerve carrying sensation from the head

Comparison between the pathways

Dorsal column
       Concerned with fine type of transmission (ie there is a high degree of spatial orientation of nerve fibers in respect of their origin on the surface of the body)
       Graduation of the intensity are acute
       Responsiveness is not greatly altered by stimuli from other areas of the nervous system
       Fibers from the lower parts of the body lie towards the center, while fibers from higher levels form successive layers laterally
       Receives afferent sensory fibers from the dorsal roots which belong to A beta which ascend directly in the dorsal column
       Have v fast velocity of transmission, 30-110m/s
       Have great ability to transmit rapidly repetitive sensation (eg vibration sense)
       Is limited to transmission of mechanoceptive sensation only (eg touch, pressure etc)


       Is concerned with crude type of transmission (ie poor degree of spatial orientation)
       Graduation of intensities are far less acute
       Responsiveness can be greatly altered by stimuli from other areas of the nervous system (brain & SC analgesic system)
       Fibers from the lower parts lies laterally while those from the higher levels form successive layer towards the center
       Receives afferent sensory fibers from the dorsal roots which are thin myelinated Adelta or unmyelinated C fibers, these travel in lissauer’s tract & relay in the dorsal horn cells to give rise to anterolateral pathway
       Have relatively slow velocity of transmission, 8-40m/s
       Has poor ability to transmit rapidly repetitive sensations.
       Has ability to transmit broad spectrum of modalities ( eg pain, thermal, crude touch etc)

       Role of the thalamus & sensory cortex in appreciation of sensation:

       All sensory tracts except the olfactory pathway synapse in the thalamus on their way to cerebral cortex
       When impulse mediating a given sensation reaches the thalamus the subject become crudely aware of the sensation but can’t perceive all of its fine details eg a person will be aware of a change in temperature if he contact a hot object but perception of how hot the object is, will be less acute
       Gradation and spatial & temporal characteristics are appreciated at the level of sensory cortex & not at the level of thalamus
       Pain & to lesser extent temperature seem to be the only sensations that are fully appreciated at the thalamus & probably even at the reticular formation level or even lower. Still interpretation of the quality and localization of these sensations at level of the cerebral cortex .

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