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Descending Motor Systems
Neuroanatomy Motor Systems Lecture
61
Anatomy
Professional
04/07/2012

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Term
Describe the decussation of the corticospinal tract:
Definition
90% crosses at the decussation of the pyramids- this is the lateral tract
10% remains uncrossed to the spinal level- this is the anterior tract
Term
What muscles are handled by the lateral corticospinal tract?
Definition
Distal muscles, fingers, toes, fine articulations
FLEXORS more than extensors
Term
What muscles are handled by the anterior corticospinal tract?
Definition
Trunk muscles, bilaterally innervated so that a lesion of one side of the tract has minimal/imperceptible manifestations
Term
What characterizes a lower motor neuron lesion?
Definition
Muscle weakness, fasciculations, atrophy, fibrillations, decreased reflexes, decreased tone
Term
What characterizes an upper motor neuron lesion?
Definition
Muscle weakness, increased tone, hyperreflexia (spasticity), Babinski's Sign, Hoffmann's Sign, posturing
Term
What is important to remember about ACUTE upper motor neuron lesions?
Definition
Initially they can present with flaccid paralysis (like lower motor neuron) but after one week this develops into spasticity- seen in spinal shock
Term
What is innervated by the corticonuclear (corticobulbar) tract?
Definition
Cranial nerves
Term
What is unique about the innervation of the cranial nerves by the corticonuclear tract, and what are the exceptions?
Definition
Innervation is bilateral, except for:
Facial Nuc- LOWER FACE is contralateral
Hypoglossal Nuc- GENIOGLOSSUS is contralateral
Nuc Ambiguus- SOFT PALATE AND UVULA are contralateral
Spinal Accessory- SCM and TRAP are ipsilateral
Term
If there is a stroke in the genu of the internal capsule on one side, what do we see?
Definition
In this UPPER MOTOR NEURON lesion:
-the upper face and forehead is spared
but the patient can't smile on the OPPOSITE side because there will be lower facial paralysis
Term
What happens if there is a stroke in the midbrain at the level of the occulomotor nucleus on one side?
Definition
Should be ok because innervation to the occulomotor nucleus is bilateral from each corticonuclear tract
Term
What happens to the tongue if the corticonuclear tract is damaged on the RIGHT side?
Definition
Tongue deviates OPPOSITE
(Unlike if the nerve or nucleus is damaged, in which case tongue deviates same side)
Term
Where does the corticonuclear tract cross?
Definition
I tricked you- it doesn't "cross" in the traditional sense- most everything is innervated bilaterally, except for 4 things
Term
Where does the rubrospinal tract originate, and where does it cross?
Definition
Red Nucleus- crosses right away at the Ventral Tegmental Decussation
-inputs from the cortex and cerebellum
Term
What tract does the rubrospinal tract descend close to?
Definition
Anterolateral System
Term
What is innervated by the Rubrospinal tract? What types of muscles are handled mostly by this tract?
Definition
Only cervical levels are innervated by this tract- upper extremity
This tract is FLEXOR BIASED and influences mostly flexor tone
Term
What does the rubrospinal tract travel with in the spinal cord?
Definition
In the lateral funiculus with the Lateral Corticospinal Tract
Term
Where does the reticulospinal tract cross?
Definition
It doesn't
:b
Term
What two tracts are flexor biased?
Definition
Corticospinal and Rubrospinal
Term
What are the two divisions of the reticulospinal tract?
Definition
Pontine reticulospinal (medial)
Medullary reticulospinal (lateral)
Term
Describe the course and innervation of the pontine (medial) reticulospinal tract:
Definition
Descends uncrossed to facilitate EXTENSOR tone
-high level of spontanteous activity, with input from the deep cerebellar and vestibular nuclei
ALWAYS ACTIVE
Term
Describe the course and innervation of the medullary (lateral) reticulospinal tract:
Definition
Descends uncrossed to INHIBIT EXTENSOR TONE
-ANTAGONIZES PONTINE(MEDIAL)RETICULOSPINAL TRACT
-inputs from cerebral cortex, red nucleus, and spinoreticular tract
Term
Why do painful stimuli influence muscle tone via the reticulospinal tracts?
Definition
Both (medial and lateral) receive inputs from the spinoreticular tract, so painful stimuli influences muscle tone
Term
What are the two descending vestibulospinal tracts?
Definition
Lateral vestibulospinal tract- all levels of the cord EXTENSORS
Medial vestibulospinal tract- cervical and upper thoracic levels, neck muscles
Term
What is the function of the lateral vestibulospinal tract?
Definition
Balance- keeps you from falling over
Term
What is the function of the medial vestibulospinal tract?
Definition
Head position- keeps eyes level
Term
Where do the vestibulospinal tracts cross?
Definition
They don't
:b
Term
Where does the medial vestibulospinal tract travel?
Definition
It runs in the MLF-
Term
Where does the tectospinal tract cross?
Definition
Dorsal Tegmental Decussation
Term
Where does the tectospinal tract originate?
Definition
Superior Colliculus
Term
Where does the tectospinal tract run?
Definition
Very close to the MLF, down the brainstem and into the spinal cord
Term
What does the tectospinal tract do?
Definition
Helps with visual direction/head orientation toward a visual or auditory stimuli
This tract is less about muscle tone
Term
If you see the thalamus in a coronal slice, what part of the internal capsule are you looking at?
Definition
Posterior Limb
Term
What is a pure motor stroke?
Definition
A stroke that takes out just the posterior limb of the internal capsule- no somatosensory deficits (not near VPM, VPL)
***If the stroke is higher, there will be motor AND sensory loss***
Term
What tract innervates alpha motor neurons that control distal muscles and delicate, precise movements?
Definition
Corticospinal tract (lateral)
Term
What happens to the neurons in the spinal cord with a corticospinal tract lesion? (UMN)
Definition
Lose alpha motor neuron control and drive to the inhibitory interneurons (which inhibit alpha motor neurons)
Loss of inhibition= alpha motor neurons more likely to fire
Term
Why does an upper motor neuron lesion cause spasticity
Definition
Loss of inhibition of alpha motor neurons by interneurons, but reticulospinal tract and vestibulospinal tract are still innervating/activating alpha motor neurons
Term
When testing muscles for spasticity in an upper motor neuron (CST) lesion, what do we find?
Definition
Spasticity is velocity dependent
Testing slowly shows normal tone
Testing quickly there is a stress response to stretch and joints/muscles become stiff
Term
What is the difference between spasticity and rigidity?
Definition
Spasticity is velocity dependent
Rigidity is velocity INDEPENDENT- stiff all the time- think basal nuclei
Term
What happens in a lesion of the thalamus, internal capsule, or cortex ABOVE the red nucleus?
Definition
Loss of corticospinal descending control, but the rubrospinal, reticulospinal, and vestibulospinal tracts are ok-
DECORTICATE POSTURING
Term
What is decorticate posturing and why does it happen?
Definition
Loss of CST control, but the rubrospinal tract is causing a flexor bias on the upper limbs so it dominates and the arms are flexed inward
Reticulospinal tract is causing an unopposed extensor bias on the lower limb, so the legs are extended
Term
What is the posture called where the arms are flexed and the legs are extended?
Definition
DECORTICATE
Term
What happens if there is a lesion below the red nucleus, etc. in the lower brainstem?
Definition
Loss of corticospinal tract control and rubrospinal tract
Reticular formation/reticulospinal tract dominates with EXTENSOR BIAS of all 4 limbs
DECEREBRATE POSTURING
Term
What is decerebrate posturing?
Definition
With no opposition from the corticospinal and rubrospinal tracts, the reticulospinal tract dominates with EXTENSOR BIAS on all 4 limbs- the patient is completely extended
Term
What 3 elements compose the "motor cortex"?
Definition
M1, Supplementary motor area, premotor area
Term
What motor function gets lost in a stroke of the Anterior Cerebral Artery?
Definition
Lower Extremities
Term
What motor function gets lost in a stroke of the Middle Cerebral Artery?
Definition
Face, Upper Extremities, and Trunk
Term
What do we see in lesions of M1 (primary motor cortex)?
Definition
Weakness and apraxia, but not paralysis
-hard to hold a pencil
Term
What do we see in lesions of the premotor area?
Definition
No paralysis, but slow complex limb movements- no facilitation of the M1 neurons- crappy planning and initiation
Term
What effect does the premotor area have on the M1 primary motor cortex neurons?
Definition
The PMA gets M1 neurons closer to threshold
Term
What do we see in lesions of the supplemental motor area?
Definition
Motor apraxia, but not paralysis-
Term
What is the main role of the premotor area?
Definition
Planning and initiation
Term
What is the main role of the supplemental motor area?
Definition
Organizing- for movement of an entire limb
Term
What two parts of the brain are required for translating strategy into action?
Definition
Premotor area and supplemental motor area
Term
What is the role of the posterior parietal area?
Definition
Motivation or intent- active during motor acts
"I see a pencil, I grab the pencil"
Bigger Picture
Term
What do we see in a lesion of the posterior parietal area?
Definition
CONTRALATERAL HEMINEGLECT
Term
What regions of the thalamus make feedback reciprocal connections to the motor cortex?
Definition
Ventrolateral and ventral anterior nuclei
Term
Which of the Rexed lamina carry lower motor neuron innervation to skeletal muscle?
Definition
Anterior Horn Lamina IX
Term
What is the Babinski sign?
Definition
Normally, stimulation of the bottom of the foot causes toes to curl
A positive Babinski sign is when they fan out and extend
UPPER MOTOR NEURON- CST DAMAGE
Term
What are the signs and symptoms of spinal shock, and why is it tricky?
Definition
Flaccid paralysis, and muscle stretch reflexes are not hyperactive right away but become so
The only sure thing is a positive Babinski sign right away
Term
What happens if you damage the anterior horn of the spinal cord at that level?
Definition
Lower motor neuron lesion- loss of a myotome
Term
What happens below the level of damage to the anterior horn of the spinal cord?
Definition
IPSILATERAL Upper Motor Neuron Signs -killing the tract
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