Lecture 9: Upper motor systems

Question 1

Corticospinal tract

Answer 1

terminates in spinal cord. 90% of axons cross to the contralateral side at the caudal medulla form the lateral corticospinal tract 10% remain ipsolateral and form the anterior corticospinal tract. Corticospinal tracts synapse onto neurons within the lateral region of the ventral horn. Mostly on local spinal circuit interneurons Some (hand and arm) synapse directly onto alpha motor neurons Some of these projections aren’t involved in motor control of lower motor neurons (proprioception)

Question 2

Corticobulbar tract

Answer 2

Terminates in brainstem

Question 3

Organization of descending motor control

Answer 3

Upper motor neurons in the brainstem mostly stay ipsolateral in the anterior-medial white matter of the spinal cord Upper motor neurons from the cerebral cortex cross over (90%) to the lateral white matter of the spinal cord. (skilled movement)

Question 4

Medullary pyramids

Answer 4

….

Question 5

Lateral corticospinal tract

Answer 5

90% of the corticospinal tract neurons cross to contralateral side to form this tract. Innervates lower motor neurons in the lateral ventral horn. involved in distal limb muscles, skilled movement.

Question 6

ventral corticospinal tract

Answer 6

……

Question 7

Functional organization of primary motor cortex

Answer 7

Motor maps within cortex • Largely understood via stimulation studies • Also astute observations in seizing patients take recordings of brain while someone is doing a motor action, or stimulate areas of the motor cortex and see what action results. Directional tuning of the upper neurons in primary motor cortex is also prominent (respond to specific direction of planned movement)

Question 8

What is the premotor cortex

Answer 8

Influences motor behavior • Indirectly • Extensive reciprocal connections with primary motor cortex • Directly • Via axons that project through the corticobulbar and corticospinal tract • Thought to use information from various cortical regions to select movements appropriate to the context of action • Have directional specificity, but occurs BEFORE initiation of movement • Damage to premotor cortex impairs cued movement execution • Also the location of mirror neuron networks ..• Ventrolateral portion of premotor cortex

Question 9

Describe motor control in the brainstem

Answer 9

Balance posture and gaze Vestibulospinal tract Reticular formation (superior colliculus and rubrospinal tract)

Question 10

What is upper motor neuron syndrome?

Answer 10

compared to lower motor neuron damage

Injury to the upper motor neurons is common because

• Large amount of cortex
• Pathways extend from cerebral cortex to lower spinal cord

• Associated with a collection of clinical features markedly different from
lower motor neuron syndrome

Question 11

Axons from primary and premotor cortex descend via..

Answer 11

corticobulbar and corticospinal tracts Upper motor neurons in cerebral cortex reside in adjacent interconnected areas in the posterior frontal lobe • Planning and control of voluntary movements

Question 12

Describe mirror neurons in the premotor cortex

Answer 12

neurons that fire while we observe someone else doing a natural action. They’re the same neurons that we need to do the action as well.

Question 13

Rostral-lateral premotor cortex

Answer 13

specialized for the production of speech (Broca’s area)

Question 14

Medial premotor cortex

Answer 14

Mediates selection of movements (similar to lateral areas) specific to internal cues as opposed to external cues (voluntary movements) Ablating cortex in monkey reduces spontaneous movement Human functional neuroimaging reveals this region is activated when performing sequences of movements from memory

Question 15

Anterior cingulate cortex

Answer 15

evaluation of emotions, causes our own facial expressions with emotions

Question 16

frontal eye fields

Answer 16

responsible for planning eye movements in response to stimuli

Question 17

Medial vestibular nuclei

Answer 17

Descends the brain stem/spinal cord via the medial vestibulospinal tract • Terminates in bilateral medial ventral horn of cervical spinal cord • Involved in reflexive action of neck muscles in response to vestibular stimulation

Question 18

Lateral vestibular nuclei

Answer 18

• Exit via the lateral vestibulospinal tract
• Terminate in the medial lower motor neuronal pools that control proximal muscles of the limbs • Include “antigravity” muscles

Question 19

Reticulospinal tract

Answer 19

Involved in the control of both
axial and proximal limb muscles

• Reticular formation neurons are
widely distributed

• Also widely varied function

• Postural control and balance

• Modulation of sensory-motor
reflexes

• Coordination of eye movements
• Cardiovascular / Respiration

• Regulating sleeping and waking
states

Question 20

Reticular formation maintains….

Answer 20

posture during movements
• Entails feedforward (anticipatory) mechanisms

Question 21

Collicuspinal tract

Answer 21

From (deep layers) of superior colliculus to the spinal cord

• Most projections from the superior colliculus are mediated by the
reticular formation

• Involved in orienting of the head

Question 22

Rubrospinal tract

Answer 22

Observed in non-human primates and other mammals

• From red nucleus to cervical level of the spinal cord
• Not really seen in humans, so if it exists, it is marginally related to motor control

Question 23

What happens in upper motor neuron syndrome?

Answer 23

• Immediately following damage we see “spinal shock”
• Period of hypotonia

• Reflective of decreased activity of spinal circuits that have suddenly lost
input from cortex and brain stem

• Following initial period, we see a resurgence of spinal cord
activity

• Result of strengthening of remaining connections and the formation of
new connections?

• Result in a specific pattern of motor signs and symptoms

Question 24

The babinski sign

Answer 24

Stroke the sole, toes normally go down (flexion)

• Following damage to the descending upper motor neuron pathways (or
pre-development of them) the toes point up!

Question 25

Spasticity: associated with upper motor neuron syndrome

Answer 25

Increases in muscle tone, hyperactive stretch reflexes, clonus

• Caused by the removal of suppressive influences of the cortex on the
postural control centers of vestibular nuclei and reticular formation

• Decerebrate rigidity

Question 26

Why is there a loss of ability to perform fine movements with upper motor neuron syndrome?

Answer 26

• If damage includes descending pathways that control the lower motor to
the upper limbs, that ability to execute fine motor commands is lost