Descending tracts Flashcards
(11 cards)
The corticospinal tract is what?
The primary pathway that carries the motor commands that underlie voluntary movement.
Originating in the neocortex, the corticospinal tract is the longest and one of the largest central nervous system (CNS) tracts (X axons in humans).
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Two thirds of the axons in the corticospinal tract originate where?
Where are most of the rest from?
Motor cortex (areas 4 and 6).
Somatosensory areas of the parietal lobe.
Indirect corticorubrospinal pathway has largely been …
Replaced by the direct corticospinal path over the course of primate evolution.
What did Lawrence & Kupers show the effects of both corticospinal an corticorubral lesions were in monkeys in the 1960s?
Rendered them unable to make fractionated movements of the arms and hands; that is, they could not move their shoulders, elbows, wrists, and fingers independently. Voluntary movements were also slower and less accurate.
Despite this, the animals could sit upright and stand with normal posture.
What did L&K show was only permanent deficit after CST lesion?
Only permanent deficit was some weakness of the distal flexors and an inability to move the fingers independently.
Lateral corticospinal tract is responsible for the control of the X musculature and the anterior corticospinal tract is responsible for the control of the Y musculature.
Distal, proximal.
CST is the only descending pathway in which …
Axons make synaptic contacts directly onto alpha motor neurons.
The fact that the percentage of axons in the corticospinal tract that innervate alpha motor neurons directly is greater in humans and nonhuman primates than in other mammals, presumably reflecting what?
What did Kuypers et al (2011) show to illustrate this?
The increased manual dexterity of primates.
Possums have limited dexterity and only use their forepaws to deliver food to their mouth. Have far fewer terminations of corticospinal fibres in the cervical spinal cord (upper limb control). Kuypers, 2011.
Describe the CST.
Axons originate in motor cortex then pass through the internal capsule, cerebral peduncle, pons, and medullary pyramid.
At the medulla-spinal cord junction, they decussate, enabling contralateral control.
Axons terminate in the dorsolateral ventral horn, directly synapsing onto alpha motor neurons and intermediate grey matter, influencing motor neurons controlling distal muscles, particularly the flexors.
Describe the RST.
Originates in the red nucleus of the midbrain, decussate in pons, parallel those in the corticospinal tract in the lateral column of SC.
