Motor Tracts

Question 1

What are lower motor neurons?

Answer 1

Neurons that directly innervate striated or intrafusal muscle cells

Question 2

What are the two types of LMNs?

Answer 2

Alpha motor neurons which innervate extrafusal or skeletal muscle fibers
Gamma motor neurons which innervate modified muscle cells (intrafusal fibers) that form part of the proprioceptive system

Question 3

Lesions of a LMN result in what?

Answer 3

Flaccid paralysis, areflexia, atonia, atrophy and fasciculations

Question 4

What is flaccid paralysis?

Answer 4

Muscle is completely limp and there is no resistance to passive movement

Question 5

What is areflexia?

Answer 5

The loss of efferent components of the reflex arc to a muscle results in the absence of the associated muscle reflex

Question 6

What is atonia?

Answer 6

Destruction of gamma motor neurons or their axons results in the absence of muscle tone

Question 7

What is atrophy?

Answer 7

Denervated muscle atrophies due to the loss of stimulation from the motor neurons

Question 8

What are fasciculations?

Answer 8

Twitching of the denervated muscles probably due to hypersensitivity of the motor end plate

Question 9

What are upper motor neurons?

Answer 9

Carry information down to activate interneurons and LMNs (send messages between cerebral cortex and the spine)

Question 10

UMN paralysis is commonly due to what?

Answer 10

Interruption of the motor cortex, corticospinal and/or corticobulbar tracts
Referred to as spastic paralysis of the antigravity muscles

Question 11

What are some signs and sx of UMN lesions?

Answer 11

Varying degrees of spastic paresis of the axial and proximal limb musculature + some degree of spastic paralysis of the distal limb musculature/UE
Hypertonia and hyperreflexia, Babinksi sign, clonus, rigidity and diffuse atrophy

Question 12

The corticospinal tract arises from what?

Answer 12

Large pyramidally shaped neurons known as the cells of Betz

Question 13

What is the pathway of the CST?

Answer 13

Descends through the corona radiata, internal capsule, cerebral peduncles, pons and upper medulla
Decussation occurs in the lower medulla at the pyramidal decussation —> forms the LCST
Remaining uncrossed fibers continue as the ACST
Terminate in lower motor neuronal pools

Question 14

The LMN pools in which the CST terminates plays an important role in what?

Answer 14

Processing and integrating descending motor info with the incoming current status sensory info

Question 15

Describe the lateral corticospinal tract

Answer 15

Descends in the lateral funiculus
Fibers terminate in neuronal pools at all levels of the SC
Involved in the control of distal limb musculature especially precise individualized movements of the digits (fractionation of finger movements)

Question 16

A unilateral lesion of the LCST results in what?

Answer 16

Ipsilateral paralysis or paresis of the distal limb musculature innervated by those spinal segments below the level of the lesion

Question 17

Describe the anterior corticospinal tract

Answer 17

Descends uncrossed in the anterior funiculus of the cervical and upper thoracic SC
Bilaterally influences axial and proximal limb musculature
Unilateral lesions have minimal clinical effect

Question 18

Unilateral lesions of the CST result in what?

Answer 18

Contralateral spastic hemiplegia or spastic hemiparesis

Question 19

The corticobulbar (corticonuclear) tract influences what?

Answer 19

All somatic motor nuclei of the brainstem

Question 20

What is the pathway for the corticobulbar (corticonuclear) tract?

Answer 20

Descends from head region of the primary motor cortex
Courses through the genu of the internal capsule* and cerebral peduncles
Most fibers decussate in the lower pons b/w levels of entry of CN V and VI
Direct fibers terminate directly upon LMNs of the trigeminal, facial and hypoglossal motor nuclei

Question 21

What is the result of unilateral lesions of the corticobulbar (corticonuclear) tract?

Answer 21

Above the decussation result in some contralateral CN palsies (supranuclear facial palsy)
Below the decussation cause ipsilateral CN palsies

Question 22

Describe indirect corticobulbar fibers (corticoreticulobulbar fibers)

Answer 22

Bilaterally course and terminate in the reticular formation surrounding the brainstem motor nuclei
Descending influences are related to the respective LMNs
Play a role in the recovery of lesions involving the direct corticobulbar fibers

Question 23

Describe the facial, LMCC nuclei

Answer 23

Contralateral cortical representations
Muscle groups innervated: 1/2 facial muscles (anterior portion lower quadrant and posterior portion upper quadrant of face) and limb musculature

Question 24

What are the most common sites for lesions causing supranuclear fascial palsy?

Answer 24

Facial region of the motor cortex and the genu of the internal capsule

Question 25

Unilateral lesions above the level of the facial nucelus result in what?

Answer 25

Supranuclear facial palsy
Contralateral muscles on the lower quadrant of the face are paralyzed
Upper quadrant facial muscles are unaffected due to bilateral innervation of the facial nucleus

Question 26

What is Bell’s palsy?

Answer 26

An ipsilateral paralysis of the mimetic (facial) muscles

Caused by central lesion of the facial nerve

Question 27

What is apraxia?

Answer 27

The inability to perform certain complex learned motor acts in the absence of any paralysis, ataxia, sensory changes or deficiencies of understanding (confusion)
Usually due to a lesion in the superior parietal lobe of the dominant hemisphere
Can be ideomotor or ideational

Question 28

What is ideomotor apraxia?

Answer 28

Inability to perform a given act correctly although old, habitual motor acts can be performed spontaneously or repetitiously (often with perseveration)
Associated with lesions of the supramarginal gyrus of the dominant hemisphere

Question 29

What is ideational apraxia?

Answer 29

Loss of the ability to formulate the ideational concepts necessary to perform a motor activity

Question 30

Describe acute anterior poliomyelitis

Answer 30

Involves the motor neurons of the anterior horns and CN motor nuclei
Initially there is inflammation, vasodilation, edema and macrophagic activity
Then these neurons die causing significant astrocytic gliosis
Onset lasts between 2-4 days with sx such as pyrexia, HA, vomiting, neck stiffness and pain of the back and limbs

Question 31

Describe amyotrophic lateral sclerosis (ALS)

Answer 31

May be due to a defect in glutamate mechanism
Most common form involves a combo of LMN and UMN
Leads to LMN paralysis and atrophy of the intrinsic muscles of the hands followed by the arm and shoulder muscles
Involvement of the CST leads to spastic paralysis, hyperreflexia and Babinksi sign
No sensory deficits