Descending motor system Flashcards
List the functions of the descending tract/ (4)
- produce movement
- modify tone
- control autonomic function
- regulate sensory transmission
Where do most descending tracts synapse?
interneurons; small part directly on motor neuron (alpha motor neurons, esp corticospinal tract predominately)
What are the three tracts in MLF (medial longitudinal funiculus)?
-include the tectospinal, medical vestibulospinal, and ponine reticulospinal tracts
What does medial motor system include?
Descending MLF, Lateral vestibulospinal, and medullary reticulospinal tract.
What are the common characteristics of MLF?
- run in anterior funiculus
- all end above upper thoracic cord
- all terminate on axial motor neurons
- all terminate bilaterally
- mainly control axial muscles
What is the function of lateral vestibulospinal tract?
facilitating extensor
What is the function of medullary reticulospinal tract?
inhibits cord motor neuron excitability
What is the general definition of “corticobulbar” or “corticalspinal”
everything from cerebral cortex and going to brain stem or spinal cord.
Need to know the effect of corticobubar tract damage on face, tongue, jaw, and palate movement
yes, damage to cortical efferents produce UMN signs
What are the two aspects of UMN (upper motor neuron) signs?
- hyperactive reflexes, spasticity, pathological reflexes (e.g. Babinski)
- Motor neuron damage (LMN) -decreased tone & reflexes, atrophy
How to tell the difference in gross structure between the cervical and thoracic spinal cord?
Since there is not much lateral motor neurons -> thoracic spinal cord has a smaller ventral horn (lower extremities considered axial)
How does the DMT INDIRECTLY affect alpha motor neurons?
- affect reflex excitability - primarily by affecting interneurons
- initiate movement through activation of the gamma loop - activate gamma neurons, producing reflex contraction of muscles to new length “set point”
How do you explain the fact that interneuron to limb muscles have more focused distribution compared to the ones controlling axial muscle motor neurons pools
You can control your individual finger - certain dexterity; versus spinal/back/lower extermities muscle turn to move in groups/as a whole
What does “older system’ mean in explaining that there are multiple motor pathway?
- there is a phylogenetic hierarchy (primitive -> sophisticated)
- more “recent” tracts utilize the “older system”
- “older system” have more general effects and often influence reflex response
- “older systems” are primarily directed at postures, tone, and proximal limb or trunk motion.
Compare the differences between medial and lateral motor systems.
- medial motor system run in ventral orventrolateral funiculus, mainly regulate interneurons pools going to axial and proxiamal limb muscles
- Lateral motor system run in lateral funiculus, regulating the interneurons going to limb muscles.
What about superior colliculus?
- has a precise map of the entire sensory world
- esp the visual map
- if stimulate, eyes move immediately
What is the PPRF (paramedial pontine reticular formation)?
has to do with horizontal gaze; prevent neck from moving while the eye gaze at somewhere.
What is medial vestibulospinal tract?
- from medial vestibular nucleus
- runs in the descending MLF
- reflex adjustment of head position to vestibular stimuli
Tell the pathway of the tectospinal tract.
- Arises from the deep layers of the superior colliculus.
- Crosses in the midbrain (dorsal tegmental decussation) and descends in the MLF and anterior funiculus to upper thoracic levels.
- Terminates in the medial part of the intermediate gray
What kind of inputs does superior colliculus receives?
receives input from the cortex, visual systems, auditory systems and somatic sensory systems (multisensory).
What is the result of stimulating tectospinal tract?
Reflex head & arm movement to stimuli (esp. visual and auditory)
What is (pontine) reticulospinal tracts?
- mostly from pontine lateral gaze center
- promote head movement to follow eye movement
- run in MLF
What about the other medial motor tract (lateral vestibulospinal tract and medullary reticulospinal tract)?
- run near the cap of the ventral horn
- extend the length of the spinal cord
- involved in regulating muscles tone and producing crude movement
Give me more details about the lateral vestibulospinal tract.
- from lateral vestibular nucleus
- runs down the length of the spinal cord in ventral lateral funiculus
- powerfully excites extensor motor neurons: vestibular righting reflex (in order to maintain balance of the people preventing falls). This should be suppressed in 3-4 months during infant development.
What about (medullary) reticulospinal tract?
- predominantly activates inhibitory interneruons in spinal cord
- activity in this tract can be regulated by corticobular tract: indirect cortico reflex
- control generalized, gross movements
- regulate gamma motor neuron activity
- contains descending autonomic fibers and respiratory control fibers
What are the two main lateral motor descening tract?
Rubrospinal and corticospinal tract
Tell me about the rubrospinl tract.
- arise from the red nucleus of midbrain
- crosses in the midbrain, at the ventral tegmental decussation
- descends in the lateral funiculus
- reaches all spinal leves (esp. cervical)
- small tracts in humans; in lower animals, important in control of limbs.
- terminates: intermediate gray matter interneuronsa dn in dorsal horn
Where do red nuclues receive input from and what is the result of stimulus transported by rubrospinal tract?
- input from cortex and cerebellum
- control movements of more proximal limb flexor muslces (crawling)
- can be an indirect corticospinal tract
What is the function of corticospinal system?
- voluntary command for movement
- regulate sensory transmission through dorsal horn
What is the function of corticobulbar system?
- affect cranial nerve nuclei -voluntary movement of head and face
- affect sensory transmission nuclei
- activate brainstem nuclei involved in movement -indirect corticospinal projection.
- project to pons for relay to cerebellum
If the distal limb function is limited, where do you suspect first?
Corticospinal tract
Corticospinal Tract
- arise from the motor cortex-precentral gyrus (specifically, from pyramidal neurons -largest ones called “betz” cells)
- axons transverse the posterior limb of internal capsule
- enter the cerebral peduncles of midbrain
- run through the basal pons
- comprise the pyramids of the medulla
Pre-motor cortex is about:
- in front of motor cortex
- lot of planning, synthesis, and patterning of movement
supplementary motor area (cortex)
- decision whether to move or not.
- active even if the movement is not executed!
What do motor cortical neurons encode during movement initiation?
direction of movement, not a specific muscle (compared to anterior horn neurons - going to single muscles)
What is damage to the supplementary motor area called? What is the symptom?
abulia - difficulty/inability in initiation of movement
Where is the internal capsule?
Lateral to the thalamus
Where are the H,A, L corticofugal fibers?
The head (H) fibers are in the genu and the rest are in the posterior limb of the internal capsule (all for voluntary movements!)
Talk about the crossing of the corticospinal tract
90 percent of fibers cross at the lowest part of the medulla and 10% uncrossed
- crossed fibers make up the lateral corticpinal tract in lateral funiculus; some go directly to motor neurons to limb muscles; particular important in control of independent digits (e.g. in case of massive stroke: can still move axial muscles, but lose voluntary hand function)
- run in anterior funiculus; go to axial muscles -crude movement
Where does the corticospinal tract erminates?
in the dorsal horn where the tract may influence sensory transmission and reflexes
What is the overall function of the corticospinal tract?
Inhibits reflexes at the cord
List the function of the corticobulbar tracts (6)
- cranial nerve nuclei (esp. facial)
- pontine neuclei (replay to cerebellum)
- indirect corticospinal tracts
- eye movement control
- sensory nuclei (regulate transmission)
- visceral centers (respiration, etc)
Described the location of the facial nucleus.
From the face part of somatosensory cortx and anterior cingulate
- crossed projection to the part that innervates the lower face (e.g. tongue muscle)
- the part innervates the upper face is bilaterally innervated (frown, close eyes,..)
- cingulate connection involved in producting emotion facial expression
Does the upper or lower face muscles produce more delicated movement?
Lower
What about other tracts strctures in the corticobular tracts?
- pontine neuclei -major target of corticobulbar tract: for relay to cerebellum and comes from all parts of the cortex
- sensory neuclei: modify sensory transmission
- Reticular formation: affect muscle tone, autonomic response and level of alertness
- brains stem areas giving rise to descending motor tracts (e.g red nucleus, reticular formation, superior colliculus)
Compare of lesion of premotor cortex and motor cortex
- motor cortex: weakness in general, loss of muscle tone
- premotor cortex: increase reflex
What about anticipatory maintenance of body posture?
Activity in many supporting and stabilizing muscles is produced by indirectly corticospinal projection
-.e.g. using bicep to full an lever: the stimulus sending to gastrocnemius muscle is prior to that of biceps.
What are the clinical aspects o corticospinal tract lesions?
- weakness in contralateral movements (when lesion above the pyramidal decussation)
- most severe distally, with preservation of axial muscles
- severe deficits in fine motor control (especially hand) contralaterally
- hyperactive myotatic and withdrawal reflexes
- spasticity is common due to lack of inhibition of indirect corticospinal tracts (such as those involving corticoreticular => reticulospinal tracts)
Babinski sign
on the lateral side of the foot -> normal: toes down; Babinski sign: externsor plantar response -> up and fanning of toes (in pt and infant)
Compare the upper versus lower motor neuron weakness
Lower motor neuron: ++ atrophy; flaccid, decreased reflexes; no pathological reflexes
Upper motor neuron: little atrophy, spasticity, increased reflexes, and pathological reflexes-Babinski
What is lower motor neurons (LMN)?
the anterior horn cells (alpha) and its axons
What is upper motor neurons (UMN)?
descending motor tracts
What causes decorticate posturing
lesion above the red nucleus
What causes the decerebrate posturing
lesion between red and vestibular nuclei
