Lec 28 Motor System Neuroanatomy Flashcards Preview

Brain and Behavior Exam 2 > Lec 28 Motor System Neuroanatomy > Flashcards

Flashcards in Lec 28 Motor System Neuroanatomy Deck (73):

What composes the pyramidal tract? Where do these fibers go?

- descending cortical fibers from sensory + motor areas of cerebral cortex
- fraction of these extend into spinal cord via cotical spinal tracts
- the rest terminate in brainstem nuclei in mid/pons/med


What is the difference between anterior [ventral] and lateral corticospinal tracts?

- lateral = contralateral fibers that crossed in the pyramidal decussation
- anterior = ipsilateral, smaller tract of fibers


What motor pathways make up the lateral group? function?

lateral and anterior corticospinal tracts
rubrospinal tract

- these tracts target motoneuronal pools important for fine motor control of distal muscles


Where does lateral corticospinal tract [LCST] originate? path? terminate? role?

originate: motor and sensory cortex
path: crosses at medulla, descends contralaterally
terminate: contralateral spinal interneurons, LMNs
role: fine motor control of distal muscles


Where does rubrospinal tract originate? path? terminate? role?

originate: red nucleus [midbrain]
path: crosses, descends contralaterally
terminate: contralateral spinal interneurons
role: activate flexor aMNs, inhibit extensor aMNs


Where does tectospinal tract originate? path? terminate? role?

originate: superior colliculus [midbrain]
path: crosses, ends at cervical spinal cord
terminate: interneurons, CN 3, 4, 6 nuclei
role: postural reflexes of head, neck rotation


Where does lateral lateral vestibulospinal tract originate? path? terminate? role?

originate: lateral vestibular nucleus [pons]
path: ipsilateral
terminate: ipsilateral γMNs, interneurons
role: maintain posture, tonic excitation of axial extensors


What motor pathways make up the medial group? function?

- tectospinal tract
- lateral vestibulospinal tract
- medial reticulospinal tract
- lateral reticulospinal tract

function: mostly target motoneuronal pools that control axial and proximal muscles. control body posture and reflex adjustments to posture [act against gravity]


Where does medial reticulospinal tract originate? path? terminate? role?

originate: pons reticular nuclei [pons]
path: ipsilateral tract
terminate: ipsilateral γMNs, interneurons
role: maintain posture, facilitate axial extensors


Where does lateral reticulospinal tract originate? path? terminate? role?

originate: medullary reticular nuclei [medulla]
path: bilateral tract
terminate: interneurons
role: maintain posture, inhibit axial extensors


Where in spinal cord does anterior [ventral] corticospinal tract terminate?

extends only to upper thoracic levels


What types of neurons do corticospinal axons terminate on?

- mostly spinal interneurons
- some monosynaptic connections directly to alpha motoneurons


What is the medial vestibulospinal tract? function?

origin: medial vestibular nucleus
path: descend in MLF
terminate: terminate on spinal interneurons in cervical spinal cord
function: neck posture, facilitates neck extensors


What is fasciculus proprious?

fiber tract adjacent to central grey at all levels
- arises from intrinsic interneurons of spinal cord
- forms ipsilateral and contralaterl ascending and descending intersegmental connections

function: integrate motor activity at different spinal levels [eg in reflexes]


What is organization within lateral corticospinal and spinothalamic tracts?

legs [lumbosacral] are lateral --> cervical is medial


what is organization within dorsal column tracts?

organized as you are --> arms outside, legs inside


Which descending motor tract is responsible for head/neck rotation, begins at the superior colliculus in midbrain, and ends at cervical spinal cord?

tectospinal tract [medial group]


Match these tracts and the sites of their origin:
1. medial reticulospinal
2. lateral reticulospinal

A. medulla
B. pons

medial = pons
lateral = medulla


Which descending motor tract is bilateral and inhibits axial extensors?

lateral reticulospinal tract [LRST]


What is mech of anti-gravity muscle control of static posture? which 5 structures involved?

- VST, lateral RST, medial RST, cerebral cortex, cerebellum

lateral VST and medial RST --> tonic excitation of antigravity extensors]

purkinje cells of cerebellum --> inhibit lateral VST

cerebral cortex --> stimulates lateral RST --> inhibits extensors


What are 2 types of inhibitory input to balance out tonic excitation of anti-gravity extensors by lateral VST and medial RST?

purkinje cells of cerebellum --> inhibit lateral VST

cerebral cortex --> stimulates lateral RST --> inhibits extensors


Are vestibulospinal tracts under cortical or cerebellar control? how?

- under cerebellar control


What is decerebrate rigidity model?

- brainstem transected cat at intercollicular level has excessive hypertonia of extensor anti-gravity muscles

- lesion knocks out cortical inhibitory input to lateral RST; have unbalanced γMNs [from lateral VST and medial RST]

also called gamma rigidty


How do you alleviate decerebrate/gamma rigidity?

- section the dorsal root [DRG] input from extensor muscles --> breaking the 1a/2 afferent arc


What is hypertonia?

excessive muscle tone = increased resistance to passive limb manipulation


What are signs of spasticity?

- hyperactive stretch reflexes
- hypertonia of anti-gravity muscles [arm flexors, axial extensors]
- clasp-knife resistance [initial strong resistance to passive manipulation that decreases as manipulation maintained


What causes spasticity?

- damage to upper motor neurons [UMNs], motor cortex, cortical tracts [internal capsule, corticospinal tract]


What causes rigidity?

- parkinsons
- damage to basal ganglia


What are signs of rigidity?

- hypoactive muscle reflexes
- hypertonia in both extensors and flexors
- cog-wheel resistance [limb moves like a ratchet when manipulated]


Where is primary motor cortex? function?

M1 area 4
location: precentral gyrus
function: motor execution


Where is premotor cortex? function?

PM, area 6
location: anterior to M1
function: sensory guided movement planning


Where is supplemental motor area? function?

SMA or M2, area 6
location: above cingulate gyrus
function: internally generated movement planning


What are some features of all motor cortical areas?

- neurons active prior to movement onset
- low threshold of excitability
- granular: lack prominent layer 4
- receive inputs from thalamic nuclei
- contribute to pyramidal tract either directly on spinal MNs via corticospinal tract or via indirect descending brainstem projections


What does it mean that motor cortical areas are granular?

- lack prominent layer 4 [major somatosensory layer]


What are brodmann's areas corresponding to S1 [primary somatosensory cortex]?

areas 3a, 3b, 1, 2


What is brodman's area corresponding to M1?

area 4


what is brodmann's area corresponding to PM [premotor cortex]?

area 6 [lateral]


what is brodmann's area corresponding to SMC/SMA/M2?

area 6 [medial]


Do motor cortical neurons fire after or before movement onset?



Do motor cortical neurons have high or low threshold for activation?

low threshold [because direct connect with spinal cord]


What thalamic inputs does M1 receive? where does that thalamic nucleus get it's own input?

cerebellum --> VLp --> M1


What thalamic inputs do PM + SMA receive? where does that thalamic nucleus get it's own input?

globus pallidus --> VLa --> PM and SMA


How is M1 organized?

legs = medial; face = lateral
disproportionate: hands, feet, mouth
all contralateral
organized in interconnected micro columns


What is difference small vs large [betz] cells in M1?

small = 90% of neurons, fire proportional to force generation, directionally selective, get sensory feedback,
function: control delicate movement

large [betz] = 10%, all or non firing, no sensory feedback,
function: ballistic/powerful movement/grasping


What is effect of M1 lesions?

- permanent loss of fine motor control
- loss of individual finger movements


What is function of premotor cortex?

- neurons discharge prior to M1 activity
- prepares upcoming motor acts based on sensory-related instructional cue [gets input from parietal/visual/auditory/striatum]

ex. the monkey who has to anticipate when the light goes on he goes


What is effect of PM lesions?

impairment in performing sensory-guided motor tasks


What is function of the SMA/SMC? organization?

- self generated motor planning, bimanual coordination
- bilateral representation


What is effect of SMC/SMA lesion?

- impairment in self-initiated tasks
- impairment in tasks that require bimanual coordination
- forced grasping [hard to let go]


Where does PM get input from in the thalamus??



What happens in UMN lesion?

- spastic paralysis
- hyperrefflexia
- hypertonia
- weakness of upper extensors + lower flexors
- positive babinskiy
- arm swing in gait
- clonus may be present


What happens in LMN lesion?

- falccid paralysis
- hyporeflexia
- hypotonia
- muscle atrophy
- distribution in one nerve, root, or plexus
- fasciculations/fibrillations = abnormal firing LMNs
- absent clonus


What is positive babinski sign of UMN damage?

- when do babinski --> fanning of toes, get dorsiflexion

normal = toes down


How can facial weakness tell you upper vs lower motor neuron?

UMN lesion = only inferior half of one side of face
LMN = all [inferior + superior] of one side of face


What is pseudobulbar palsy?

- inability to control facial movements, swallowing, spastic speech

due to damage to corticobulbar tract


What is function of spinal cord ventral horn motor neurons in the descending motor system?

they are the final common pathway neurons [LMNs]
- site of integration of all descending and reflex paths


What part of brain is responsible for concept of extra personal space?

areas 5 and 7 of parietal lobe


What is function of extrapyramidal structures?

initiate movement, smooth movement, motor learning


what is function of cerebellum in extrapyramidal path?

- integrate movements into smooth sequence


what is function of basal ganglia in extrapyramidal path?

- involved in rapid adjustments of movement [ex sudden turning]


What is the difference extrafusal vs intrafusal muscle fibers?

extrafusal: force generating
intrafusal: control and adjust sensitivity of muscle spindle


What do alpha motorneurons innervate?

extrafusal muscle
- large aMNs innervate fast/fatigable extrafusals
- small aMNs innervate slow/fatigue resistant extrafusals


What do gamma motor neurons innervate?

intrafusal muscle, control sensitivty of muscle spindle in detecting changes in muscle length


What is a motor unit?

one motor neuron and all the muscle fibers it innervates


What is ration of motor neurons to muscle fibers?

varies from 1:10 to 1:2000


What are the first type of motor neurons to respond to low-level afferent inputs?

small aMNs and gamma MNs


What are the last type of motor neurons to respond to afferent input?

large MNs require strong stimulus


What information to muscle spindles record?

they are stretch receptors
- record muscle length and rate of change


What are the two functional regions of muscle spindle and what innervates each part?

central region: non-ccontractile, sensory innervation from group 1a and 2 sensory afferents

polar: contractile intrafusal fiber, motor innervation from gamma MNs


What is alpha-gamma motoneurona coactivation?

- needed to detect muscle length

1. surrounding muscle is stretched --> spindles stretched --> 1a and 2 fibers fire
2. 1a/2 fibers stimulate both alpha and gamma
3. aMNs --> contraction of extrafusal --> muscle shortens
4. activated gamma --> contraction of intrafusal --> keeps spindle taught while muscle is shortening so that 1a/2 fibers keep firing


What is function of the golgi tendon organs?

- records tension generated at myotendinous junction during contraction
- composed of group 1b fibers --> fire when muscle contracted --> stimulated spine interneurons that in turn terminate active muscle contraction


What is the path of myotatic [stretch, deep tendon] reflex?

1. afferent: group 1a and group 2 sense some sort of stimulus at muscle spindle [ie. muscle stretched]
2. intraspinal:
- afferents terminate monosynaptically on aMNs innervating the same [homonymous] muscle and on synergist [heteronymous] muscles
- also activate type 1a inhibitory interneuron which inhibits aMNs innervating antagonistic muscles
3. efferent: aMN causes stretched muscle to shorten [contract]


What is path of the inverse myotatic reflex?

- tension feedback reflex to reduce over-contraction

1. afferent: group 1b afferents sense tension on fGTO
2. intraspinal:
- 1b fibers inhibit homonymous a-MNs and synergists
- excite antagonistic aMNs
3. efferent: as muscle contract with increasing forcefulness, increased number of GTOs discharge function to minimize over contraction