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General Sensory Systems II

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Physiologists refer to this entire system as the Anterolateral System.


    The Spinothalamic system conveys the Protopathic Sensations:


(1) Crude (light) touch a form of touch mediated by low threshold (sensitive) receptors, and characterized by very large receptive fields that allow only poor         ability to localize touch


(2) Pain - mediated by nociceptors


(3) Temperature -mediated by separate warm and cold receptors

      These have a punctate distribution and preferentially detect sudden changes in temperature.


(4) Sexual sensations and Pressure sensations from the pelvis (bladder & bowel) –  conducted bilaterally in the spinal cord so that loss of the pathway 

unilaterally will not cause a total loss of sensation.


Pathway through Spinal Cord, Brainstem, Thalamus and Cerebral Cortex:


- Primary sensory neurons (touch, pain & temperature) enter the spinal cord through the dorsal root.  These synapse in the dorsal horn (Posteromarginal Nucleus, Substantia Gelatinosa or Nucleus Proprius) with interneurons or secondary sensory projection neurons. 


-Axons of projection neurons cross in the Ventral White Commissure and form the contralateral Spinothalamic Tract which traverses the spinal cord and lateral part of the medulla and lies above the basilar pons immediately lateral to the medial lemniscus (where it is sometimes called the spinal lemniscus) in the ventral pontine tegmentum.  


-Fibers terminate in the Ventral Posterolateral (VPL) thalamic nuclei which project to sensory cortex, and to Intralaminar thalamic nuclei which project in a diffuse, widespread manner to the cerebral cortex.  There are direct projections to the Hypothalamus as well.  


-Unlike the DCML pathway, a few secondary sensory neurons in this pathway do not cross in the ventral white commissure of the spinal cord but project to the thalamus on the ipsilateral side.



Neospinothalamic part of Spinothalmic Tract


-projects to VPL whose axons pass through the posterior limb of the internal capsule to synapse in SI & SII.  (SII is located within the lateral fissure on the opercular part of the postcentral gyrus.)

Functions of this part of the pathway are crude touch and fast, well localized pain mediated by projections from VPL to cortical areas SI & SII.


Paleospinothalamic part 


projects to

(a)Nonspecific (Intralaminar) thalamic nuclei

(b)Brainstem Reticular Formation (spinoreticular fibers)

(c)Tectum and Periaqueductal Gray (PAG) (spinotectal or spinomesencephalic fibers)



 The Reticular Formation (RF)


-lies in the center of the brainstem tegmentum and consists mostly of Golgi type I neurons, each with ascending and descending axon collaterals that branch repeatedly and innervate many different nuclei. 


-All of the sensory cranial nerves as well as the Spinothalamic Tract provide sensory input to the RF, and the resulting neural activity can spread widely throughout the brainstem. 


-Such activity is projected from the rostral part of the RF (Pedunculopontine and Laterodorsal tegmental nuclei) largely through the Central Tegmental Tract to the Intralaminar thalamic nuclei and to other nuclei in the basal forebrain (Meynert’s nuclei) and hypothalamus (Tuberomammillary nuclei & Lateral Hypothalamic Area) which, in turn, have widespread projections to the cerebral cortex


Ascending Reticular Activating System (ARAS)


helps to modulate the general level of activity of the cerebral cortex, producing different states of consciousness.  Extrathalamic projections from the brainstem RF (noradrenergic, serotonergic) also play a role in modulating awareness, alertness and attention.  


This pathway does not mediate any sensory function requiring high acuity (hearing, touch localization) but it does mediate slow pain sensations as well as the affective nature (unpleasantness) and emotional responses to pain.


 Projections to the Tectum


are involved in reflex head and eye movements in response to tactile stimulation.


 Projections to the PAG


provide information about pain to an Antinociceptive (pain-inhibiting) System that is centered in the PAG and projects in sequence to:

(1) Serotonergic medullary Raphe nuclei and Noradrenergic nuclei in the brainstem and

(2) the Dorsal Horn in the spinal cord where excitation of Enkephalinergic interneurons inhibits pain transmission.


 Somatosensory information to the Hypothalamus


reaches the Hypothalamus both directly and via relays in the RF so that pain elicits Autonomic Responses and influences Emotional states and perceptions of somatic stimuli as being pleasant or unpleasant.

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