Lect 7 Descending Motor Systems Flashcards
corticospinal tracts
motor tract
pathways originating in the cerebral cortex
corticospinal tract (anterior & lateral) corticobulbar tract
Pathways originating in the brainstem
aka bulbospinal rubrospinal tract reticulospoinal tract vestibulospinal tracts tectospinal tract
lateral corticospinal tract
largest tract
90% of the fibers traveling from motor cortices to the spinal cord course through here
fibers cross in the pyramidal decussation
travels to spinal cord levels (anterior horn)
C-S fibers originate from the primary motor cortex (area 4), premotor cortex and supplementary motor cortex
also passes through internal capsule
Upper motor neuron
UMN
these neurons originate in the motor cortices and terminate on neurons (interneurons or anterior horn cells) in the anterior horn of the spinal cord
terminate a brainstem motor nuclei
lower motor neurons
LMN
are the axons of anterior horn cells; these axons course to skeletal muscle
Primary motor cortex
Experimentally, movement is elicited with relatively low stimulus
-produce small and localized movements
Premotor cortex
Lateral
Requires stimulation to induce movement
Elicits movements involving multiple joints
Non-human primate studies shown that neurons fire BEFORE the onset of movement, particularly in response to external cues; thus this area is thought to contribute to the selection of movements based on external cues
area of brain thought to contribute to movements based on external cues
premotor cortex
Supplementary motor cortex
require higher stimulation to induce movement
elicits complex movements involving multiple joints, including movements that require bilateral coordination
Activity recorded during mental rehearsal of movement sequences (without motion) and is important in the internal memory/retrieval of motor sequences
Betz cells
large projection of neurons located only in the primary motor cortex (precentral gyrus, area 4)
give rise to axons that contribute to the lateral and anterior corticospinal tracts
pass through the internal capsule before going to brainstem
pyramidal neurons
area 6 give rise to corticospinal tract axons
pass through the internal capsule before going to brainstem
cerebral peduncle
betz and pyramidal axons contribute to the peduncle in the midbrain
anterior horn cells
lower motor neurons
axons of anterior horn cells project their axons into the periphery and together with the afferent (sensory) fibers, form a peripheral nerve
anterior corticospinal tract
- 10% of fibers from motor cortex to spinal cord
- fibers do NOT cross in pyramids
- Many fibers cross in the anterior white commissure (some stay ipsilateral)
- Fibers terminate in the cervical and upper thoracic spine only
primary control of neck and shoulder muscle?
anterior corticospinal tract
does unilateral damage of the anterior corticospinal tract display weakness?
no bc bilateral tract and overlap with lateral C-S tract
spasticity
increased muscle tone
what accompanies and upper motor nuclei lesion?
increased muscle tone (spasticity) and increased reflexes (hyperreflexia
Believed reasons of spasticity and hyperreflexia in response to an UMN lesion
- up regulation of receptors on anterior horn cells
- axonal sprout to pre-existing synaptic sites (sites that have been left vacant by injury) and to new receptor sites. This sprouting can come from preserved descending motor collaterals and sensory afferents (competing)
- Because the sensory afferent collaterals will predominate following an UMN lesion, the anterior horn cells receive excessive sensory input. Additionally, the descending inhibitory fibers are lost
Rubrospinal tract
influences voluntary limb movements
primarily influences distal flexor muscles
small in human
starts in brain stem
Pontine reticulospinal tract
modulates activity of anterior horn cells via facilitation of axial extensor musculature
medullary reticulospinal tract
modulates activity of anterior horn cells mainly via suppression of reflex activity
some fibers cross laterally
Reticular formation
reticular nuclei receive widespread cerebral cortical input as well as subcortical input (basal ganglia)
reticuloospinal tract
believed to be important route by which output from anterior horn cells is regulated
vestibular nuclei
2 of 4 are involved in descending motor path
Lateral Vestibular nucleus
Medial vestibular nucleus
Lateral vestibular nucleus
facilitate postural/axial extensor muscles
Projects ipsilaterally to all spinal cord levels
medial vestibular nucleus
projects axons bilaterally to cervical and upper thoracic spinal cord levels, therefore this nucleus influences motor output to muscles of neck
projects axons bilaterally to cranial nerve motor nuclei (3,4,6). therefore, this nucleus contributes to the control of head/neck and eye position in response to changes in body position (this pathway will be reviewed with cranial nerves
-together provide stable platform for eyes
decoorticate posturing
occurs with a lesion above the red nucleus (pontine)
with this lesion, the brainstem centers are intact but there is a lack of cortical modulation of these brainstem regions
decerebrate posturing
occurs with a lesion below the red nucleus but above the vestibulo & reticulo spinal tract nuclei
there is a complete loss of activity of te rubrospinal tract and loss of cortical modulation of the vestibulo and reticulo spinal nuclei
bulbospinal tracts that are still intact with a patient with decerebate posturing - Pontine reticulospinal tract
receives inhibitory input from the cortex - loss of this inhibition control results in facilitation of axial/postural extensor musculature
Medullary reticulospinal tract and decerebrate posturing
recieves excitatory input from the cortex. Loss of excitatory input (which functions to suppress reflex activity) lacks suppression of reflex activity and increases output from anterior horn cells
lateral vestibulospinal tract & decerebrate posturing
receives inhibitory input from the cortex. Loss of this inhibitory control results in excessive facilitation of axial/postural extensor musculature
tectospinal tract
initiates in superior colliculus (midbrain)
functions in orienting the head to visual stimuli
exits at cervical spinal cord levels
