Motor Tracts Flashcards
What are lower motor neurons?
Neurons that directly innervate striated or intrafusal muscle cells
What are the two types of LMNs?
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
Lesions of a LMN result in what?
Flaccid paralysis, areflexia, atonia, atrophy and fasciculations
What is flaccid paralysis?
Muscle is completely limp and there is no resistance to passive movement
What is areflexia?
The loss of efferent components of the reflex arc to a muscle results in the absence of the associated muscle reflex
What is atonia?
Destruction of gamma motor neurons or their axons results in the absence of muscle tone
What is atrophy?
Denervated muscle atrophies due to the loss of stimulation from the motor neurons
What are fasciculations?
Twitching of the denervated muscles probably due to hypersensitivity of the motor end plate
What are upper motor neurons?
Carry information down to activate interneurons and LMNs (send messages between cerebral cortex and the spine)
UMN paralysis is commonly due to what?
Interruption of the motor cortex, corticospinal and/or corticobulbar tracts
Referred to as spastic paralysis of the antigravity muscles
What are some signs and sx of UMN lesions?
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
The corticospinal tract arises from what?
Large pyramidally shaped neurons known as the cells of Betz
What is the pathway of the CST?
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
The LMN pools in which the CST terminates plays an important role in what?
Processing and integrating descending motor info with the incoming current status sensory info
Describe the lateral corticospinal tract
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)
A unilateral lesion of the LCST results in what?
Ipsilateral paralysis or paresis of the distal limb musculature innervated by those spinal segments below the level of the lesion
Describe the anterior corticospinal tract
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
Unilateral lesions of the CST result in what?
Contralateral spastic hemiplegia or spastic hemiparesis
The corticobulbar (corticonuclear) tract influences what?
All somatic motor nuclei of the brainstem
What is the pathway for the corticobulbar (corticonuclear) tract?
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
What is the result of unilateral lesions of the corticobulbar (corticonuclear) tract?
Above the decussation result in some contralateral CN palsies (supranuclear facial palsy)
Below the decussation cause ipsilateral CN palsies
Describe indirect corticobulbar fibers (corticoreticulobulbar fibers)
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
Describe the facial, LMCC nuclei
Contralateral cortical representations
Muscle groups innervated: 1/2 facial muscles (anterior portion lower quadrant and posterior portion upper quadrant of face) and limb musculature
What are the most common sites for lesions causing supranuclear fascial palsy?
Facial region of the motor cortex and the genu of the internal capsule
Unilateral lesions above the level of the facial nucelus result in what?
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
What is Bell’s palsy?
An ipsilateral paralysis of the mimetic (facial) muscles
Caused by central lesion of the facial nerve
What is apraxia?
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
What is ideomotor apraxia?
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
What is ideational apraxia?
Loss of the ability to formulate the ideational concepts necessary to perform a motor activity
Describe acute anterior poliomyelitis
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
Describe amyotrophic lateral sclerosis (ALS)
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
