Corticospinal Tract
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Overview
Corticospinal tract (or pyramidal tract), as the name suggests, arises in the cerebral cortex and ends at all levels of the spinal cord. It is the major motor (descending) pathway.
Upper motor neuron: mainly from the pyramidal (Betz) cells of the precentral gyrus (motor cortex).
Lower motor neuron: from the anterior horn cells of the spinal cord.
Pathway
Upper motor neurons descend mainly in the posterior limb of the internal capsule, then in the anterior aspect of the brainstem ie cerebral peduncle of the midbrain, basis pons of the pons and pyramid of the medulla.
In the medulla, most (85%) of the fibres decussate (cross over) to the opposite side forming the lateral corticospinal tract.
Remaining 15% continue as uncrossed anterior corticospinal tract.
Both tracts eventually end at the anterior horn cells (AHC) of the spinal cord grey matter. The anterior corticospinal tract fibres, however, end on either side of the spinal cord thereby innervating both right and left sides.
From the AHC, lower motor neurons arise and exit the spinal cord to innervate the muscles. These fibres are the ones forming the peripheral nerves (ex. ulnar nerve).
Lateral corticospinal tract mainly innervates the limb muscles; the anterior corticospinal tract innervates the trunk/ midline muscles.
Clinical anatomy
Lesion to the corticospinal tract above the level of pyramidal decussation of the medulla (ex. stroke or head injury) paralyses the contralateral (opposite) side of the body. Lesion below this level (ex. Brown-Sequard syndrome) affects the ipsilateral (same) side of the body.
Lesion to the upper motor neurons result in a spastic paralysis where the tone of the affected muscle group and the deep tendon reflexes increase. Lesion to the lower motor neurons result in a flaccid paralysis where the tone of the affected muscle group and the deep tendon reflexes decrease/ absent.
Spinothalamic Tract
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This is a major ascending (sensory) pathway, carrying pain sensation. It arises from all levels of the spinal cord and travels to the thalamus (hence the name) and finally to the cerebral cortex.
1st order neuron: In the dorsal root ganglia, ending in the posterior horn of the spinal cord.
2nd order neuron: From the posterior horn of the spinal cord to the thalamus.
3rd order neuron: From the thalamus to the sensory cortex.
Pathway
Sensory fibres arising from the skin receptors have their cell bodies in the dorsal root ganglion; they end in the posterior horn of the spinal cord grey matter.
The second order neurons cross to opposite side, ascend as the lateral spinothalamic tract placed on the lateral side of the white matter (hence the name lateral spinothalamic tract).
These fibres end in the ventroposterolateral (VPL) nucleus of the thalamus.
Fibres from the thalamus run through the posterior limb of the internal capsule to end in the somatosensory cortex (post central gyrus) of the cerebral hemisphere.
The spinothalamic tract carries the pain and temperature sensation to the sensory cortex.
Clinical anatomy
Injury to the spinothalamic tract in the spinal cord affects the pain sensation in the opposite side of the body as the fibres have decussated (crossed over) as soon as they entered the spinal cord.
Crossing of fibres follow a specific route, for example, fibres in the cervical level cross closer to the central canal of the spinal cord and the fibres of the thoracic/lumbar levels cross more anteriorly. In conditions like syringomyelia, where the central canal of the spinal cord is affected, the cervical fibres are involved first. This means, there will be loss of pain sensation in the upper limb, not in the lower limb.
Dorsalcolumn Tract
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Overview
Dorsal column (= gracile and cuneate tracts) is a pair of tracts that lie in the posterior aspect of the spinal cord (hence the name).
This is a major ascending (sensory) pathway, carrying proprioception and vibration.
The gracile tract (aka fasciculus gracilis/ tract of Goll) carries these sensations from the lower limb; the cuneate tract (aka fasciculus cuneatus/ tract of Burdach) from the upper limb.
1st order neuron: In the dorsal root ganglia. They enter the posterior horn of the spinal cord, ascending to terminate in the nucleus gracilis (for lower limb) or cuneatus (for upper limb) of the medulla.
2nd order neuron: From the nucleus cuneatus and gracilis, they cross to the opposite side and terminate in the thalamus.
3rd order neuron: From the thalamus to the sensory cortex.
Pathway
Dendrites arising from the joint and muscle receptors have their cell bodies in the dorsal root ganglion; the axons enter the dorsal horn of the spinal cord, ascend in the ipsilateral dorsal column to the medulla oblongata.
Fibres from the lower limb contribute to the gracile tract (lying medial in the dorsal column) and the upper limb fibres form the cuneate tract (lying lateral in the dorsal column).
The gracile tract terminates in the gracile nucleus, and the cuneate tract terminates in the cuneate nucleus.
In the medulla, fibres from both the nucleus gracilis and cuneatus cross to the opposite side as the internal arcuate fibres. They ascend as the medial lemniscus to the VPL of the thalamus.
From the thalamus, they travel through the posterior limb of the internal capsule and terminate in the somatosensory cortex (postcentral gyrus).
The dorsal column tract carries the proprioception and vibration sensation to the sensory cortex.
Clinical anatomy
In the spinal cord lesions, the symptoms will be ipsilateral (same side).
Injury to the dorsal column tracts in the spinal cord affects the patient’s ability to predict the position of the joint in space (with eyes closed).
In conditions like tabes dorsalis where the dorsal colum tract is affected, the patient will not be able to sense his foot placement on the ground without seeing them.
Sometimes, these patients, while walking, take a higher step and forcefully place the foot on the ground (high stepping gait) to stimulate deeper receptors in an attempt to ‘feel’ their feet.