Spinothalamic Tract
The Spinothalamic Tract is one of three main tracts (Spinomesencephalic Tract for pain inhibition; terminates in the periaqueductal grey) and Spinoreticular Tract for autonomic pain mediation; terminates in the thalamus)) of the anterolateral (anterior and bilateral spinal cord position) system. The spinothalamic system is an afferent (ascending), protopathic (coarse stimuli) alert system dedicated to nociceptive (pain) and thermal (heat) sensation.
The system is activated from sensation from a distributed population of principal receptors embedded in the skin of the body (except for the face which are activated by the trigeminal ganglion). The receptors are a mixture of myelinated and unmyelinated (free) nerve endings that can discriminate the quality and severity of somatic sensations; heat (warm, hot), cold (cool, cold) and pain (pinch, squeeze, puncture) in various degrees (discomfort to excruciating). These sensations are conducted via fibers to the dorsal root ganglion (primary neurons) which have distinct morphological and molecular cells dedicated to these modalities before entry into the spinal cord.
Lissauer's fasciculus (dorsolateral) is the impulses point of entry to the spinal cord which ascends at least one segment before entering the dorsal horn and synapsing on secondary neurons (interneurons) of the marginal zone and substantia gelatinosa (laminae I and II; ***IV, V and VII***). Following the synapse at the dorsal horn is a decussation at the same level across the white commissure to the contralateral anterolateral quadrant (sacral dermatomes ventromedially, cervical dermatomes ventromedially) where the impulse ascends lateral to the medial lemniscus in the medulla, pons and midbrain for a second synapse at third order neurons of the ventral posterior lateral nucleus of the thalamus.
The thalamic nuclei transmit the impulses through the internal capsule and corona radiata to synapse at the grey matter (third synapse) of the somatosensory cerebral cortex. The synapses can activate diffuse areas of motor and somatosensory cortex in the pre and post-central gyrus for a unified representation of nociceptive or thermal sensation. These sensations can be modified by our emotional state and conditioned; a soldier that suppresses pain to save lives and carry out their mission, a tour de france cyclist, a yogi resting upon a bed of nails or a body builder.
*The axons of the anterolateral system derive from dorsal horn neurons and decussate in the ventral (anterior) commissure. The anterolateral system ascends in the lateral column. Spinal cord hemi-section has a differential effect on the somatic sensory modalities caudal to the lesion, producing loss of touch and position senses on the side of the lesion and loss of pain and temperature senses on the opposite side. Central spinal lesions can selectively disrupt the decussating axons of the anterolateral system.*
Fibers of the anterolateral system terminate in the reticular formation and thalamus.
Axons of the spinothalamic tract synapse in the three principal thalamic nuclei. Two of these nuclei - the ventral posterior lateral nucleus which projects to the primary somatic sensory cortex and the ventromedial posterior nucleus which projects to the insular cortex - are important in stimulus perception. The third, the medial dorsal nucleus projects to the cingulate cortex for the emotional responses to pain. The insular and anterior cortical regions are also important in the behavioral and autonomic responses to pain, temperature, and itch sensations and the emotions and memories these stimuli evoke.
Inputs from thalamus arrive at layer IV of the cortex. Efferent projections from the somatic sensory cortical areas arise from specific cortical layers. Corticocortical association connections (with other cortical areas on the same side of the cerebral cortex) are made by neurons in layers II and III. Callosal connections (with the other side of the cerebral cortex are also made by neurons in layers II and III.) Descending projections to the striatum, brain stem, and spinal cord originate from neurons located in layer V, whereas the projection to the thalamus originates from neurons located in layer VI.
Free nerve endings translate dermal pain from receptors innervated by ganglion cells that give rise to group C fibers. The receptors are encapsulated, are manifested in pressure receptors in joints and tendons innervated by ganglion cells that lie in the dorsal root ganglia beside the spinal cord. There are receptors for the special senses, inner ear hair cells innervated by spiral ganglion cells of Corti. Effector receptor interactions include the muscle spindle (length-sensitive) receptor is innervated by a dorsal root ganglion cell which projects to a spinal motoneuron which in turn regulates the excitability of the muscle spindle.
The system is activated from sensation from a distributed population of principal receptors embedded in the skin of the body (except for the face which are activated by the trigeminal ganglion). The receptors are a mixture of myelinated and unmyelinated (free) nerve endings that can discriminate the quality and severity of somatic sensations; heat (warm, hot), cold (cool, cold) and pain (pinch, squeeze, puncture) in various degrees (discomfort to excruciating). These sensations are conducted via fibers to the dorsal root ganglion (primary neurons) which have distinct morphological and molecular cells dedicated to these modalities before entry into the spinal cord.
Lissauer's fasciculus (dorsolateral) is the impulses point of entry to the spinal cord which ascends at least one segment before entering the dorsal horn and synapsing on secondary neurons (interneurons) of the marginal zone and substantia gelatinosa (laminae I and II; ***IV, V and VII***). Following the synapse at the dorsal horn is a decussation at the same level across the white commissure to the contralateral anterolateral quadrant (sacral dermatomes ventromedially, cervical dermatomes ventromedially) where the impulse ascends lateral to the medial lemniscus in the medulla, pons and midbrain for a second synapse at third order neurons of the ventral posterior lateral nucleus of the thalamus.
The thalamic nuclei transmit the impulses through the internal capsule and corona radiata to synapse at the grey matter (third synapse) of the somatosensory cerebral cortex. The synapses can activate diffuse areas of motor and somatosensory cortex in the pre and post-central gyrus for a unified representation of nociceptive or thermal sensation. These sensations can be modified by our emotional state and conditioned; a soldier that suppresses pain to save lives and carry out their mission, a tour de france cyclist, a yogi resting upon a bed of nails or a body builder.
*The axons of the anterolateral system derive from dorsal horn neurons and decussate in the ventral (anterior) commissure. The anterolateral system ascends in the lateral column. Spinal cord hemi-section has a differential effect on the somatic sensory modalities caudal to the lesion, producing loss of touch and position senses on the side of the lesion and loss of pain and temperature senses on the opposite side. Central spinal lesions can selectively disrupt the decussating axons of the anterolateral system.*
Fibers of the anterolateral system terminate in the reticular formation and thalamus.
Axons of the spinothalamic tract synapse in the three principal thalamic nuclei. Two of these nuclei - the ventral posterior lateral nucleus which projects to the primary somatic sensory cortex and the ventromedial posterior nucleus which projects to the insular cortex - are important in stimulus perception. The third, the medial dorsal nucleus projects to the cingulate cortex for the emotional responses to pain. The insular and anterior cortical regions are also important in the behavioral and autonomic responses to pain, temperature, and itch sensations and the emotions and memories these stimuli evoke.
Inputs from thalamus arrive at layer IV of the cortex. Efferent projections from the somatic sensory cortical areas arise from specific cortical layers. Corticocortical association connections (with other cortical areas on the same side of the cerebral cortex) are made by neurons in layers II and III. Callosal connections (with the other side of the cerebral cortex are also made by neurons in layers II and III.) Descending projections to the striatum, brain stem, and spinal cord originate from neurons located in layer V, whereas the projection to the thalamus originates from neurons located in layer VI.
Free nerve endings translate dermal pain from receptors innervated by ganglion cells that give rise to group C fibers. The receptors are encapsulated, are manifested in pressure receptors in joints and tendons innervated by ganglion cells that lie in the dorsal root ganglia beside the spinal cord. There are receptors for the special senses, inner ear hair cells innervated by spiral ganglion cells of Corti. Effector receptor interactions include the muscle spindle (length-sensitive) receptor is innervated by a dorsal root ganglion cell which projects to a spinal motoneuron which in turn regulates the excitability of the muscle spindle.
Spinothalamic Tract Synapses
1. Dorsal Horn of the Spinal Cord (Decussation to Anterolateral System)
2. Ventral Posterior Lateral Nucleus of the Thalamus
3. Cortical processing at Somatosensory Cortex
1. Dorsal Horn of the Spinal Cord (Decussation to Anterolateral System)
2. Ventral Posterior Lateral Nucleus of the Thalamus
3. Cortical processing at Somatosensory Cortex