NSCI 201 Chapter Notes - Chapter 17: Upper Motor Neuron, Alpha Motor Neuron, Premotor Cortex

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NSCI 201 (15.17)
Upper Motor Neuron Circuits
10th March 2016
Upper Motor Neuron Circuits!
!This lecture: motor cortex/ brainstem, top-down decision making. Refer to 14.16 for
organization of neural structures. !
!
Recall: Local circuit neurons in spinal cord gray matter!
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Medial circuits are bilateral, extend along larger segments of spinal cord to
control posture and balance.!
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Lateral circuits are ipsilateral, cover less segments, shorter rostral-caudal
distances to control limbs and distal structures. !
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Projections from brain are contralateral; crossover happens in medulla
(Brainstem)!
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Overview of descending motor control!
Information from brain sent through lateral and medial white matter tracts. Lateral: from motor
cortex to distal motor neurons, fine motor control. Medial: from brainstem to proximal motor
neurons, much more unconscious control. !
!
Unlike gray matter, white matter tracts innervating motor neurons are not continuous— can have
damage to one segment and only lose function there. !
!
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NSCI 201 (15.17)
Upper Motor Neuron Circuits
10th March 2016
Primary motor cortex and premotor areas in cerebral cortex!
Primary motor cortex: just anterior of primary somatosensory, dorsal to central sulcus, produces
movement and motor generation. More direct connections to spinal cord (70%), lower stimulation
threshold.!
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Premotor: just anterior of M1, also produces movement and motor generation. Less direct
connections to spinal cord, higher stimulation threshold.!
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Differ in cytoarchitecture— different cells exist in different organisations!
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Cytoarchitectonic appearance of M1 in the human brain!
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Most distinctive in L5: Appearance of betz
cells. !
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Betz cells exist only in CNS, largest and most
recognisable in M1, used as identifying
marker. !
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NSCI 201 (15.17)
Upper Motor Neuron Circuits
10th March 2016
!
Corticospinal and Corticobulbar Tracts!
!
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Two separate tracts leading down spinal cord from
brainstem: Corticospinal and corticobulbar!
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Corticobulbar: Cortex to brainstem. !
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Conveys information through cranial nerves; represents
face, head, and neck. !
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Important connections: Descends into pons and
medulla, contacts trigeminal and hypoglossal nuclei. !
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Corticospinal: Cortex to spinal cord. !
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Majority are projections through cortex > spinal cord >
local circuit neurons. Small portion are direct
connections from cortex to alpha motor neurons,
controlling fine motions (eg. Fingertips). !
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Important connections: Reticular formation in medulla,
Bifurcation in caudal medulla: 90% crossover to join
lateral white matter tracts to extremities; 10% stay ipsi-
and innervate medial wm, terminating i/c.!
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Topographic map of movement in M1!
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Corticobulbar tract arises in lower, more lateral region of M1.!
Corticospinal is represented in more dorsal/ superior part. !
Rough resemblance to homunculus in S1; not as topographically precise. !
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