1/23 Motor Pathways Flashcards
What do upper motor neurons do?
carry motor outputs from cerebral cortex/brainstem to lower motor neurons in brainstem/ spinal cord
Where are lower motor neurons, what do they do?
cell bodies in brainstem or spinal cord; innervate muscles in the periphery
What are the types of lower motor neurons?
alpha, gamma, beta
What are alpha motor neurons, what do they innervate?
large cell bodies; axons innervate skeletal muscle
What are gamma motor neurons, what do they innervate?
smaller axon fiber diameter; innervate muscle spindles to help control stretch reflexes
What are beta motor neurons, what do they innervate?
least abundant; innervate muscle fibers both in and out of muscle spindles
What are the symptoms of a lower motor neuron lesion?
Muscle weakness and atrophy
Fasciculations
Decreased tone
Hyporeflexia
What are the symptoms of a upper motor neuron lesion?
Muscle weakness
Increased tone
Hyperreflexia
Abnormal reflexes/signs
*acute UMN lesions may initially present with flaccid paralysis, decreased tone, and hyporeflexia
What are the origins of the major pathways?
cortical, brainstem
lateral, medial
What do lateral pathways control?
appendicular muscles (limbs)
What do medial pathways control?
axial/girdle postural muscles
How do medial pathways descend? What kind of influence do they have?
descend ipsilaterally
bilateral influence because they synapse in on interneurons in the intermediate zone as well as LMNs in the ventral horn
What are the major pathways with cortical origin? Are they lateral or medial?
Lateral corticospinal (lateral)
Ventral corticospinal (medial)
Corticobulbar (lateral)
What are the major pathways with brainstem origin? Are they lateral or medial?
rubrospinal (lateral)
vestibulospinal (medial)
tectospinal (medial)
reticulospinal (medial)
What does the lateral corticospinal tract control?
Rapid, dexterous movements at individual joints, particularly the limbs.
Where does the lateral corticospinal tract originate?
Most fibers originate in primary motor cortex, precentral gyrus. Other neurons originate in premotor, supplementary motor, or parietal lobe. Cortical layer 5.
Where/how do the axons descend in the lateral corticospinal tract?
Axons descend as part of the corona radiata, through the posterior limb of the internal capsule.
Continue into the cerebral peduncles of midbrain – middle 1⁄3 of crus cerebri, then through the ventral pons.
Ventral surface of the medulla as the pyramids.
Axons cross in the caudal medulla as the pyramidal decussation.
Descend in the lateral funiculus of the spinal cord and synapse on cell bodies of LMNs in the ventral horn
What does the corticobulbar tract control?
Movement of the facial, tongue, jaw, laryngeal, and pharyngeal muscles. NOT extraocular muscles.
Where does the corticobulbar tract originate?
Cell bodies in the lateral aspect of precentral gyrus project descending axons through the genu of the internal capsule.
Where/how do the axons descend in the corticobulbar tract?
descending axons through the genu of the internal capsule.
Projections through the brainstem to the facial motor nucleus, trigeminal motor nucleus, nucleus ambiguus, and hypoglossal nucleus.
• Projections to facial motor nucleus (pons): bilateral to the portion of the nucleus controlling upper face, contralateral to the lower face.
What are the symptoms of lesions of the corticobulbar tract?
weakness of contralateral lower face
upper face spared because bilaterally innervated
What are the symptoms of lesions of the facial motor nucleus?
ipsilateral whole face paralysis
What are the symptoms of lesions of the facial nerve?
ipsilateral whole face paralysis
Where does the corticobulbar tract have projections to?
trigeminal motor nucleus
nucleus ambiguus
hypoglossal nucleus
spinal accessory nucleus
Where is the trigeminal motor nucleus? What laterality are the projections?
mid-pons
bilateral projections
Where is the nucleus ambiguus? What laterality are the projections?
rostral medulla
bilateral projections
innervates laryngeal and pharyngeal muscles
Where is the hypoglossal nucleus? What laterality are the projections?
rostral dorsal medulla
most often bilateral projections
Where is the spinal accessory nucleus? What laterality are the projections?
caudal medulla
bilateral projections but not symmetrical
What does the rubrospinal tract control?
Influences movements of the upper limbs; exact function unclear in humans.
Where does the rubrospinal tract originate?
neurons whose cell bodies are in the red nucleus
Where/how do the axons descend in the rubrospinal tract?
Axons decussate immediately in the midbrain and descends in the dorsolateral aspect of pons and medulla.
Descends in the lateral funiculus of the spinal cord then synapse in the ventral horn of the cervical cord.
In comatose patients, what happens in response to painful stimulus with corticospinal tract damage above the red nucleus?
Upper limb flexor posturing
In comatose patients, what happens in response to painful stimulus with brainstem damage below the red nucleus?
upper limb extensor posturing