Week 3 Motor Pathways Flashcards
Where are the lower motor neurons of the neck, trunk, and limbs found? What about for the head?
- in the spinal cord
- neurons that control the proximal muscles are medial in the ventral horn
- neurons that control distal muscles are lateral - Cranial nerve nuclei in the brain stem
Where are the upper motor neurons located?
motor cortex (primary motor cortex and FEF) and subcortical nuclei -neurons of corticospinal and corticobulbar are largely found in pre central gyrus and paracentral lobule
Where do the planning and programming of motor activity occur?
- supplementary motor area (SMA)- located on medial surface of frontal lobe anterior to paracentral lobule
- Premotor area (PMA)- located lateral to the SMA and anterior to the primary motor cortex
Describe the somatotopic arrangement and course of the corticospinal tract.
-Posterior limb of internal capsule: upper limb anterior to those for lower limb
-midbrain: passes through middle 3/5s of crus cerebri. Upper limb fibers more medial to lower limb
-Pons: passes the ventral part of pons, despised among pontine nuclei.
-medulla: axons in upper medulla form pyramid. 75-90% of axons cross at junction of medulla and spinal cord
-spinal cord:
Lateral corticospinal tract: cross over, they follow this tract in the lateral funiculus
anterior: some cross over and some don’t, they follow the ACST in ventral funiculus and end bilaterally
Where are the cranial nerves with somatomotor components located?
CN III: upper midbrain CN IV: lower midbrain CN V: mid pons CN VI: lower pons CN VII: lower pons CN IX and X: upper medulla CN XI: upper 5-6 segments of spinal cord CN XII: upper medulla
Describe the origin and course of the corticobulbar tracts.
-origin: Primary motor cortex close to lateral sulcus (also from PMA, SMA, general sensory cortex, sensory assoc areas)
-corona radiata: mixed with CST
-internal capsule: at genu of posterior limb, anterior to CST
-midbrain: media to CST fibers
-pons and pyramid: mixed with CST and indistinguishable
Axons leave Corticobulbar tract at level of cranial motor nuclei
Corticobular tracts for which cranial nerve nuclei end bilaterally?
CN V: medially at middle 1/3rd of pons
CN IX and X: nucleus ambiguus
CN XI: C2-C5 levels
Corticobular tracts for which cranial nerve nuclei end contralaterally?
CN XII (actually bilateral but contralateral input predominates) -in the upper medulla
Corticobular tracts for which cranial nerve nuclei end bi- and contra laterally? Describe it.
CN 7 facial nerve
- upper group of neurons innervates upper facial muscles
- Bilaterally: LMNs that innervate upper face
- Contralateral: LMNs that innervate lower face
A UMN lesion anywhere along the corticobulbar tract of CN VII will result in what?
- normal upper face due to bilateral sparing (can wrinkle forehead)
- contralateral lower facial paralysis
What will a LMN lesion along the coriticobulbar tract of CNVII cause?
Bell’s palsy
-paralysis of entire half of face ipsilateral to lesion
What do UMN lesions of the corticobulbar tract of CN XII result in?
contralateral tongue weakness
What do LMN lesions of the corticobulbar tract of CN XII result in?
Ipsilateral tongue weakness
What are corticospinal tracts essential for?
- voluntary control of contralateral skeletal muscles of neck, trunk, and limbs.
- voluntary activity in response to sensory
- rapid finely coordinated movements
What are corticobulbar tracts essential for?
-voluntary contractions of the muscles of the head by V, VII, IX, X, XI, XII
What are UMN pathways from brain stem centers for?
regulate motor activity, mainly of non skilled variety e.g. postural adjustments, locomotion, stabilization of proximal joints
Describe the pathways for saccades in the horizontal plane for conjugate eye movements.
-motor commands from frontal eye fields (FEF) in posterior part of middle frontal gyrus
-ill defined pathway to reach the pons
-axons cross and end on paramedic pontine reticular formation (PPRF)
For CN VI
-axons sent to VI nucleus then to VI nerve to innervate lateral rectus on same side (opposite of origin)
For CN 3
-axons cross to other side to medial longitudinal fascicles (MLF) to reach III nucleus to III nerve to innervate medial rectus (same side as origin)
Unilateral brain stem lesion at the level of the pons that involve the pPRF and the VI nucleus will result in an inability to move the eye to which side?
-side of the lesion
what does the demyelination of MLF, as can occur in MS, result in?
-failure of ipsilateral eye to adduct as contralateral eye abducts
What may a LMN lesion involve?
- anterior horn cells
- brain stem cranial nerve motor nuclei
- dorsal or ventral rami
- peripheral nerves
- cranial nerves
What signs and symptoms characterize a LMN syndrome?
- paresis or paralysis
- flaccidity
- loss of deep tendon reflexes
- muscle atropy
- fasciculations
What are the signs and symptoms of UMN lesions?
- loss of strength and voluntary movements-contralateral side
- small lesions likely to deprive a large # of LMNs of their control–>hemiplegia, paraplegia, monoplegia
- paralysis predominately affecting distal parts
- hyperreflexia- exaggerated DTRs
- increased tone–>spasticity
- absence of profound muscular atrophy
- abnormal babinski sign
- abnormal posturing
What are the two major types of conjugate eye movements?
- saccades: rapid movements that redirect gaze
2. smooth pursuit movements- tracking moving object to allow stable viewing of object in motion
Compare LMN and UMN lesions with respect to weakness, atrophy, fasiculations, reflexes, tone
Weakness: Both atrophy: LMN only fasciculations: LMN only reflexes: increased in UMN, decreased in LMN Tone: increased in UMN, decreased in LMN
Compare LMN and UMN lesions with respect to weakness, atrophy, fasiculations, reflexes, tone
Weakness: Both atrophy: LMN only fasciculations: LMN only reflexes: increased in UMN, decreased in LMN Tone: increased in UMN, decreased in LMN
