Motor Systems Flashcards
what is general flow of motor systems?
frontal lobes (motivation) —> premotor cortex (motor plan) –> primary motor cortex (action)
Frontal lobes
provide “motivation” or “ideation of motor plan”
- input comes from sensory stimuli in the parietal motor area as well as emotions/memory from the limbic system
premotor cortex? BA?
- motor plan
- provides “blueprint” or “module” for movement
- premotor cortex (lateral BA6)
- supplementary cortex (medial BA6 & BA8)
what are the two UMN system?
- primary motor cortex
- brainstem nuclei
- have action of LMN in spinal cord and cranial motor nuclei
- all systems converge on LMNs to produce movement, its the final common pathway.
alpha-LMNs
synapse on and innervate SKELETAL MUSCLE
- “final common pathway”
- always excitatory and release NT => contraction
- target is ipsilateral to soma
- very large
- found in spinal cord ventral rami and cranial n. motor nuclei
- have baseline activity, changed by excitatory and inhibitory synapses, to adjust firing rate
what is a motor unit
1 LMN and all of the m. fibers that it innervates
- 10 fibers/neuron = fine control, extraocular mm.
- 1000 fibers/neuron = gross postural control mm.
how do we regulate the strength of muscle contraction?
- recruitment- “size principle” = # motor units firing = more strength
(smallest motor units are recruited first) - AP frequency- increased AP frequency results in increased strength
general UMN characteristics
- locations: cerebral cx and brainstem nuclei
- UMN = axon from brain that influences a LMN, mostly indirectly through interneurons
- interneurons can be excitatory or inhibitory
- never synapse directly on muscle
- 8 different UMN pathways (2 from primary motor cx)
Lateral motor systems
influence distal limb mm.
- precise, fractionated mvmts
medial motor systems
axial/proximal limb mm.
- affects postural mvmts.
flexors/extensors? distal vs. proximal muscles? - orientation in spinal cord?
flexors are posterior in gray matter, extensors are more anterior
proximal musculature is more medial, distal limb musculature for fine mvmts are more lateral
important features of lateral corticospinal tract
LCST:
- largest motor tract of humans
- contralateral projection
- some UMNs synapse directly on LMNs
- necessary for critical and complex fractionated mvmts of hands
- think “volition”, voluntary
anatomy of LCST?
comes from more than just BA4
- 1/3 from primary motor cortex
- 2/3 from frontal lobes and somatosensory cortex (“posterior parietal motor area” links motor activity, especially of hands to sensory input)
where is premotor cx of LCST located?
in lateral BA6 and 8
- active in motor planning at the start of voluntary unit (involved in blueprint and timing)
where is supplmenetary motor cx of LCST?
medial BA6
- involved in motor planning and interconnected with contralteral side = bimanual mvmt
- active right before movement takes place
- will instruct proper UMN to carry out movement
Corticobulbar tract
similar to corticospinal but it terminates in brainstem
- influences:
- cranial nerve motor nuclei (LMNs) of facial expression/chewing
* influences nuclei bilaterally - UMN nuclei: gives voluntary control of proximal limb voluntary muscles
- vestibular nuclei
- reticular nuclei
- red nucleus
LMN damage?
- hypo- to areflexia
- hypo- to atonia
- paralysis/paresis (varying)
- flaccidity (limp)
- RAPID/SEVERE ATROPHY (due to loss of trophic relationship with motor neuron- “neurogenic atrophy”)
- EMG changes: see spontaneous activity in mm.
paralysis vs paresis
paralysis = complete loss of mvmt paresis = partial loss of movement
UMN damage?
hyperreflexia
- increased stretch reflex –> hypertonia
- appearance of pathological reflexes
- paresis/paralysis (spastic, esp. in antigravity mm.)
- disuse atrophy
- Few EMG changes
how are reflexes increased with UMN damage?
- loss of normal inhibitory background
- results in sprouting of local affarents (to increase input)
- also results in increased expression of receptors on surface of LMNs and interneurons (denervation hypersensitivity)
-plegia
literally means “stroke” = paralysis from stroke
paralysis
complete loss of voluntary mvmt
paresis
partial loss of voluntary mvmt
hemi, para, quadra
hemi = half
para = pair of limbs
quadra (or tetra) = four limbs
rubrospinal tract
(technically a part of LMS, but is non-cortical tract)
- functions in posture and projection bilaterally
- cell bodies are located in red nucleus
- axons decussate in midbrain and descend in contralateral lateral funiculus to reach LMN of distal mm of arms (cervical cord only), primarily to arm/forearm flexors
vestibulospinal tracts
- Medial and Lateral: describes location in cord and nucleus of origin
- MVST cell bodies are medial/inferior (projects bilaterally in cervical and upper thoracic)
- LVST cell bodies are lateral (projects ipsilaterally throughout cord)
MVST
cell bodies are medial and inferior in medial VS nucleus
- projects bilaterally in cervical and upper thoracic
- acts on neck mm
- makes changes in head/neck posture
LVST
cell bodies are found more laterally in lateral VS nucleus
- projects ipsilaterally throughout cord
- makes + changes to axial/antigravity mm.
- makes changes in trunk posture
medial longitudinal fasciculus “ascending portion”
commonly called “MLF”
- cell bodies located in medial/inferior vestibular nucleus
- ascends to innervate motor nuclei of III, IV, VI (LMNs of extraoccular eye mm.)
- bilateral with +/- effects
(located dorsally and medially)
Medial reticulospinal tract
- located medially in the cord
- MRST = pontine RST
- cell bodies: located in PONTINE reticular formation
- projects ipsilaterally
- main back up pathway for voluntary mvmt with injury to corticospinal
Lateral reticulospinal tract
- found laterally in cord
LRST = medullary RST - cell bodies found in MEDULLARY reticular formation
- projects BILATERALLY
- main back up pathway for voluntary mvmt with injury to corticospinal
Tectospinal tract
cell bodies: located in superior colliculus
- decussates in midbrain (contralaterally)
- provides +/- to neck muscles (tract terminates in cervical cord)
- aids in the visual grasp reflex
Look at where these are in cross section.
do it NOW!
decorticate
- “flexed arms, extended legs” - mummy position
- loss of LCST- no flexion of legs
- unopposed rubrospinal (flexed arms)
- shows that brainstem is working, but there is damage in cerebral hemispheres or internal capsule or thalamus
decerebrate
- extension of arms and legs
- loss of rubrospinal tract (loss of flexors)
- unopposed RST (backup pathway)
- indicates brainstem damage, only RST is working which originates in pons and medulla.
Lateral corticospinal tract
- UMN’s originate in cortex (1/3 in primary motor cortex, 2/3 in frontal lobe/ SS cx)
- the fibers descend ipsilaterally down the IC, through cerebral peduncles, through longitudinal fibers, through pyramids of medulla.
- will decussate at the pyramidal decussation in lower medulla
- descend contralaterally in the LCST of the spinal cord
- will synapse on LMN of the arm and leg
- will control contralateral complex and fractionated mvmt of the distal limb musculature
Rubrospinal tract Path
cell bodies located in red nucleus in midbrain
- decussate in midbrain and descend contralaterally in the contralateral lateral funiculus
- terminates in cervical spinal cord where they innervate LMN responsible for arm/forearm flexors
- Note: this is technically in the lateral motor system, but it is still a noncortical tract
Vestibulospinal tract pathway
Cell bodies are located in the Medial/Lateral Vestibulospinal Nucleus
- the MVST descend bilaterally and have changes in head/neck posture
- the LVST descend ipsilaterally and have changes in trunk posture
- The medial longitudinal fasciculus is the ascending portion of motor nuclei of CN III, IV, VI and control extraoccular eye movment, they synapse in MVNu as well
