Flashcards in Pyramidal Tract: Corticospinal and Corticobulbar Tracts Deck (24)
What does the corticobulbar / corticospinal refer to?
Whether the upper motor neurons of the pyramidal tract terminate in the spinal cord or brainstem
What are the fates of upper motor neurons?
They terminate on sensory neurons, interneurons, or lower motor neurons
What is the overall effect of the pyramidal tract and when does it develop?
Effects on reflexes, muscle tone, and cutaneous reflex. Develops late, with its myelination not being completed until year 2 postnatally
Where do pyramidal cells originate in the neocortex?
Cell layer V of the neocortex, in different area of the brain, a few percentage from several Brodmann's areas
What is area 4? Where is it? What does it do?
Primary motor cortex, gives rise to about 20% of PT fibers. First motor cortex, anterior to central sulcus
Leads to contraction of a small number of specific muscles on the motor homunculus, same organization as sensory homunculus. It does not plan or initiate movements, which is done in area 6.
How much do areas 3,1,2 contribute?
About 30% of PT
What does area 6 include? Where is it?
Premotor area (PMA) which is more lateral, and supplementary motor area (SMA) which is more medial
It is adjacent to area 4, anterior to central sulcus
PMA is involved in patterned movements involving many muscles
SMA is involved in planning of movement and recall of programs to move
How much do areas 5,7 contribute?
About 15%, superior parietal lobe
What is the path of pyramidal fibers to the corticospinal / corticobulbar tracts?
Pass thru white matter of ipsilateral cerebral hemisphere as corona radiata fibers, then converge on internal capsule.
Corticospinal - primarily in posterior limb of internal capsule
Corticobulbar - primarily in genu of internal capsule (between anterior and posterior limbs)
Where do the corticospinal / corticobulbar tracts go after the internal capsule?
The cerebral peduncle (crus cerebri), at the rostral midbrain level.
Below the rostral midbrain, the corticospinal tract can be found in the basilar pons region, while the corticobulbar tract follows the tegmentum
What is the organization of fibers in the cerebral peduncle?
Face muscles medially, leg muscles laterally (kinda opposite the motor homunculus)
Where do collaterals of PT fibers also terminate?
Basal ganglia, thalamus, red nucleus, and reticular formation
What makes up the "pyramid" on the anterior surface of the medulla?
primarily corticospinal fibers, as the the corticobulbar fibers terminate higher
Where is the decussation of the pyramidal tract and what happens here?
Happens at junction of medulla and spinal cord, where 75-90% of the PT fibers cross midline, descend as lateral corticospinal tract (LCST) in lateral funiculus.
ACST is formed by 10-25%, will not cross midline until their level of spinal termination.
Where does the Anterior Corticospinal Tract (ACST) end?
Cervical and upper thoracic spinal levels
Where do most corticospinal fibers terminate?
Rexed's laminae 4-8, but a few on 9 where there are spinal motor neurons
What is the facilitation bias and what does it mean for PT lesions?
There is a bias towards flexors in the upper limb and extensors in the lower limb. When there is a PT lesion, they will be affected more severely than their antagonists.
What does a PT lesion lead to clinically?
Spastic hemiparesis of the contralateral side, augmented DTR's, loss of abdominal + cremastic reflexes, Babinski reflex, clonus. Clasp knife reflex will still work in paralyzed limb
What causes spasticity a few days after PT lesion?
Due to hyperinnervation of spinal motor neurons by surviving fibers (sensory and other motor). This is due to collateral sprouting of non-lesioned fibers.
Which spinal motor nuclei do not receive PT fibers?
Eye muscles: CN 3, 4, 6
Where do the corticobulbar fibers actually go and what is their function?
Terminate in sensory relay nuclei, reticular formation, and CN5, 7, ambiguus, 11, and 12
How do descending corticobulbar fibers distribute with respect to bilateral / crossed?
Equally bilaterally, except for 5 and 12 they are more crossed than uncrossed (and 7, special case)
What is the corticobulbar innervation of motor nucleus of 7?
Dorsal 1/2 receives bilateral innervation, which goes to upper face -> check with forehead wrinkle
Ventral 1/2 receives only contralateral innervation, which goes to lower face -> check with smile