Lect 7 Descending Motor Systems

Question 1

corticospinal tracts

Answer 1

motor tract

Question 2

pathways originating in the cerebral cortex

Answer 2

corticospinal tract (anterior & lateral)
corticobulbar tract

Question 3

Pathways originating in the brainstem

Answer 3

aka bulbospinal
rubrospinal tract 
reticulospoinal tract
vestibulospinal tracts 
tectospinal tract

Question 4

lateral corticospinal tract

Answer 4

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

Question 5

Upper motor neuron

Answer 5

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

Question 6

lower motor neurons

Answer 6

LMN

are the axons of anterior horn cells; these axons course to skeletal muscle

Question 7

Primary motor cortex

Answer 7

Experimentally, movement is elicited with relatively low stimulus
-produce small and localized movements

Question 8

Premotor cortex

Answer 8

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

Question 9

area of brain thought to contribute to movements based on external cues

Answer 9

premotor cortex

Question 10

Supplementary motor cortex

Answer 10

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

Question 11

Betz cells

Answer 11

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

Question 12

pyramidal neurons

Answer 12

area 6 give rise to corticospinal tract axons

pass through the internal capsule before going to brainstem

Question 13

cerebral peduncle

Answer 13

betz and pyramidal axons contribute to the peduncle in the midbrain

Question 14

anterior horn cells

Answer 14

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

Question 15

anterior corticospinal tract

Answer 15

• 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

Question 16

primary control of neck and shoulder muscle?

Answer 16

anterior corticospinal tract

Question 17

does unilateral damage of the anterior corticospinal tract display weakness?

Answer 17

no bc bilateral tract and overlap with lateral C-S tract

Question 18

spasticity

Answer 18

increased muscle tone

Question 19

what accompanies and upper motor nuclei lesion?

Answer 19

increased muscle tone (spasticity) and increased reflexes (hyperreflexia

Question 20

Believed reasons of spasticity and hyperreflexia in response to an UMN lesion

Answer 20

1. up regulation of receptors on anterior horn cells
2. 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)
3. 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

Question 21

Rubrospinal tract

Answer 21

influences voluntary limb movements
primarily influences distal flexor muscles
small in human
starts in brain stem

Question 22

Pontine reticulospinal tract

Answer 22

modulates activity of anterior horn cells via facilitation of axial extensor musculature

Question 23

medullary reticulospinal tract

Answer 23

modulates activity of anterior horn cells mainly via suppression of reflex activity
some fibers cross laterally

Question 24

Reticular formation

Answer 24

reticular nuclei receive widespread cerebral cortical input as well as subcortical input (basal ganglia)

Question 25

reticuloospinal tract

Answer 25

believed to be important route by which output from anterior horn cells is regulated

Question 26

vestibular nuclei

Answer 26

2 of 4 are involved in descending motor path
Lateral Vestibular nucleus
Medial vestibular nucleus

Question 27

Lateral vestibular nucleus

Answer 27

facilitate postural/axial extensor muscles

Projects ipsilaterally to all spinal cord levels

Question 28

medial vestibular nucleus

Answer 28

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

Question 29

decoorticate posturing

Answer 29

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

Question 30

decerebrate posturing

Answer 30

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

Question 31

bulbospinal tracts that are still intact with a patient with decerebate posturing - Pontine reticulospinal tract

Answer 31

receives inhibitory input from the cortex - loss of this inhibition control results in facilitation of axial/postural extensor musculature

Question 32

Medullary reticulospinal tract and decerebrate posturing

Answer 32

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

Question 33

lateral vestibulospinal tract & decerebrate posturing

Answer 33

receives inhibitory input from the cortex. Loss of this inhibitory control results in excessive facilitation of axial/postural extensor musculature

Question 34

tectospinal tract

Answer 34

initiates in superior colliculus (midbrain)
functions in orienting the head to visual stimuli
exits at cervical spinal cord levels