10 - Motor Systems 1

steps in voluntary goal-directed movements?

decision to move

identification and location of target

knowledge of position of limbs, body

formation of plan of action

execution of plan and action - control of both proximal and distal segments

(unconscious after the task is learned)

hierarchal vs parallel distributed processing in motor system

once thought as hierarchical, now considered as parallel descending systems

numerous regions of the central nervous system are required to plan and execute motor skills - there are series of parallel systems linking various motor areas of cortex more directly with spinal motor circuits

where are the motoneurons located in the spinal cord and brainstem?

spinal cord - predominantly L IX

brainstem - cranial nerve motor nuclei are in abducens, trochlear, oculomotor; hypoglossal, trigeminal motor, facial motor, nucleus ambiguus, spinal accessory

what does a motor unit innervate?

group of muscle fibers called a muscle unit - all contract together

review: muscle innervations, myotome levels and peripheral nerve innervation

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why is lower motor neuron an inaccurate term?

much descending input goes to interneurons which help with coordinated function of motor units/muscles

there are motor neuron pools - distal/proximal, flexor/extensor

what are the central pattern generators? what do they do?

interneurons

create coordinated, multisegmental rhythmic cyclical motor activities - basic rhythm patterns

where do interneurons receive input from?

supraspinal centers for modulation to environmental conditions and sensory receptors that give feedback back to spinal cord

what else provides input to motor related interneurons and motoneurons (to a lesser extent)?

sensory, descending pathways, collaterals from motoneurons - agonists, antagonist

PMC vs SMA

PMC: more important for learning, planning, control of movements requiring visual or other sensory input (external info)

SMA: more important for initiation, planning, control of movements based on learned patterns, activity of SMA decreased as movement patterns become more automatic (internal info)

what are the primary motor tracts? where do they terminate?

corticospinal

vestibulospinal

reticulospinal

rubrospinal

tectospinal

*terminate primarily on interneurons

what part of the cerebrum controls motor activities?

M1, M2 (PMC, SMA), PPAC, cingulate gyrus

the cerebral cx interacts with the cerebellum and basal nuclei. what are the pathways?

M1: cerebral cx → pontine nuclei → cerebellum via middle cerebellar peduncle → thalamus (VL/VPL) → cerebral cx

SMA: cerebral cx → neostriatum (basal nuclei) → globus pallidus → thalamus (VA/VL) → cerebral cx

what brodmann’s area is the SMA?

area 6

what are the general afferents to motor areas?

intracortical communication of motor areas within and between hemispheres

sensory and association cortical areas, ascending sensory pathways

what sensory input are the the cerebellum and basal nuclei responsible for?

cerebellum - motor planning, smoothing ongoing movements, assist in motor learning, posture, balance, equilibrium

basal nuclei - motor planning and execution, scaling amplitude and velocity of movements

what does M1 do?

executes specific well defined motor responses - stimulation of individual cells or small groups of cells. simple and stereotypical, discrete movement of individual muscles especially distal muscles

populations of neurons code for force of contraction and movement of direction

where is the source of projection fibers and corticospinal tract in M1?

layer V

preMC and SMA function

involved in planning of movements especially more complex movements, possible role in motor learning

goal oriented movements, movements planned based on memories, movements requiring interlimb coordination

initial phase of movements

control of proximal and axial muscles

what do secondary motor areas send major efferents to?

M1, reticular formation, spinal cord

what lobe is responsible for random finger movements? planned and execute in specific order? movement that is planned but not executed?

M1

M1 and SMA

SMA

M1 isolated lesion results in?

paresis of contralateral voluntary movements, usually starts as flaccid limbs and may become spastic - often regain some movement of proximal limb segments but distal muscles may remain paralyzed

synergistic contractions - lack independent control

lesion to the secondary motor areas results in?

apraxia - difficulty appropriately using the limb during purposeful tasks despite ability to move limbs

automatic limb movement preserved but purposeful movement usually impaired

what does the PPAC do?

motor planning functions - preparation for movement guided by sensory stimuli, collate input from sensory systems (visual, somatosensory cx, vestibular), create map of space, computation of limb and body trajectory

initiation of movement

motor related efferents are primarily to secondary motor areas

PPAC sensory functions?

integrates somatosensory, visual, auditory, and vestibular systems

interpretation and use of sensory info, planning and guiding movements based on sensory inputs, use of tools, ability to attend to sensory stimuli

PPAC deficit?

sensory deficits - various agnosias, inability to associate tactile and visual images

difficulty attending to contralateral world

motor deficits - inability to perform simple skilled tasks and difficulty with visually and tactilely guided movements

non-dominant hemisphere lesion has greater consequences than dominant

what does the cingulate gyrus do?

in monkeys has a small contribution to the corticospinal tract, projection to M1, possible role in movements that have a motivational or emotional component

what parts of the cortex are responsible for eye fields? what are their functions in relation to eye movement?

frontal, supplemental, parietal

coordination of eye movements and visual tracking, voluntary override of visual reflexes (rectus lateralis)

example of pathways

what are descending fiber paths from motor areas? (6)

corticospinal, corticonuclear, corticostriatal, corticorubral, corticoreticular, corticopontine

(many of these pathways also have fibers originating in non-motor regions of the cortex)

lateral vs medial system of descending motor control pathways, what pathways?

lateral: primary effect on motoneurons of more distal muscles - includes lateral corticospinal and rubrospinal

medial: primary effect on motoneurons of more proximal and axial muscles often bilaterally - includes anterior corticospinal, vestibulospinal, reticulospinal, tectospinal. control/modulation of balance, proximal stability head position, etc

origin of corticospinal tract

pyramidal cells

30% from M1

30% from secondary motor areas

40% from parietal regions (SI, PPAC, SII, cingulate gyrus)

course of corticospinal tract?

lateral CSp Tr: layer V of motor cortices → posterior limb internal capsule → crus cerebri → basilar pons → pyramidal decussation → anterior horn of SC (LVII, VIII, XI)

anterior CSp Tr: same as lateral, but does not decussate in pyramid, some may decussate farther down tract

somatotopically arranged

does the corticospinal tract synapse on interneurons or direct motorneurons?

more synapses to interneurons but some direct to motoneurons esp distal muscles

review: anterior and lateral corticospinal tract terminate on the _____ and _____ muscles respectively

proximal, distal

review: the anterior corticospinal tract is bilateral/unilateral. It does/doesn’t decussate in the pyramidal decussation.

bilateral, doesn’t

motor functions of the corticospinal tract?

controlled, integrated, coordinated movements, esp distal muscles in extremities (lateral corticospinal), initiation/modulation of central pattern generators

T/F the corticospinal tract carries sensory fibers from SI and SII.

T, some fibers travel in the corticospinal tract to the spinal cord posterior horn and intermediate region; modulates sensory transmission

where do collaterals of the corticospinal tract go to?

reticular formation - pons, medulla

inferior olivary nucleus - cerebellar inputs

vascular supply of internal capsule, midbrain, medulla, spinal cord

internal capsule - lenticulostriate a. off MCA

midbrain - post cerebral a.

medulla - anterior spinal a.

spinal cord - arterial vasocorona

where does the red nucleus receive input from?

ipsilateral motor areas of cerebral cortex, cerebellum (4, 5, some 5, 7)

pathway and function of the rubrospinal tract?

ipsilateral motor areas of cerebral cortex and cerebellum → red nucleus and immediately crossover in midbrain→ synapse on axons that travel to the contralateral cervical spinal cord → synapse primarily on MN of flexors

primarily influences contralateral UE flexor motoneurons, supplement the corticospinal

function of medial motor systems?

control of proximal and axial muscles

balance

orientation of head

vestibular nuclei parts

superior, lateral medial, inferior

what are reflex adjustments of the vestibular system and what are the inputs to vestibular nuclei?

axial and LE muscles respond to changes in head position relative to gravity

vestibular apparatus (CN VIII), cerebellum

pathway and function of the lateral vestibulospinal tract

lateral vestibular nucleus → lateral vestibulospinal tract → ipsilateral anterior spinal cord gray area → synapse on motoneurons

postural adjustment (trunk and LE in response to vestibular apparatus input), maintain balance

generally, the lateral vestibulospinal tract produces (EPSP/IPSP) on motoneurons that control ____ and _______ on the spinal cord.

EPSP, axial, LE extensors

pathway and function of medial vestibulospinal tract

medial vestibular nucleus → descends bilaterally via the medial longitudinal fasciculus → terminates in cervical and upper thoracic spinal cord only (LVII and VIII)

influences neck musculature, reflex adjustments like head position in response to activity in vestibular apparatus

is the lateral vestibulospinal tract ipsilateral or bilateral? medial vestibulospinal tract?

ipsilateral, bilateral

pathway of reticulospinal tract

pontine reticular nucleus → pontine reticulospinal tract → terminate intermediate and anterior SC (LVII and VIII) mostly ipsilaterally (some bilaterally)

reticulospinal tract functions

control and modulate motor activity, influence motoneurons of paravertebral muscles and limb extensors

• involved in providing background for primary movements - posture (stabilizes scapula and spine for UE movement), balance

• alter γ-MN activity (alter muscle spindle sensitivity) - modulate muscle tone and maintain posture

modulation of sensory info, especially noxious

types of reticulospinal tracts and where they terminate

pontine (medial)

medullary (lateral)

both terminate in intermediate and anterior spinal cord (LVII and VIII), influence neurons supplying paravertebral and limb extensor muscles

T/F the reticulospinal tract travels the entire length of the spinal cord.

T

function of pontine reticulospinal tract

predominantly excitatory effect on motoneurons

function of medullary reticulospinal tract

predominantly inhibitory effect on motoneurons

what does the reticular formation do? where does it receive input from?

control and modulate motor activities, modulate sensory information

receives from corticoreticular (motor regions of cerebral cortex), cerebellum, collaterals of sensory paths, spinoreticular tract of ALS

pathway and function of tectospinal tract

superior colliculus → axons to the contralateral ventral cervical spinal cord → terminate in LVI, VII of cervical levels

involved in reflex movements of head and neck in response to visual, auditory, and painful stimuli

review: superior colliculus function

integration of visual, somatosensory info

pathway of corticonuclear tract

M1 layer V of face motor cx → genu of internal capsule → crus cerebri medial to corticospinal tr → fibers project bilaterally in pons and medulla → nuclei of motor cranial nerves

*similar to corticospinal tract (parallel)

what are the cranial nerve motor nuclei?

V, VII, XII, Nucleus Ambiguus (IX and X), accessory nucleus

what are the terminations of the corticonuclear tract? what are the functions?

trigeminal: bilateral equally; muscles of mastication

facial: upper face is bilateral, lower face is contralateral; muscles of facial expression

nucleus ambiguus: uvula and soft palate is mostly contralateral; larynx, pharynx, upper esophagus is bilateral

hypoglossal: primarily contralateral to genioglossus; tongue

spinal accessory: primarily ipsilateral; trapezius, SCM

oculomotor, trochlear, abducens: from frontal and parietal eye fields, not face motor cortex

what happens if there is a lesion in the UMN facial motor cortex? LMN (facial n.)?

UMN - central seven, only lower face affected, contralateral

LMN - Bell palsy, both upper and lower face affected, ipsilateral

what happens if there is a lesion to facial motor cortex that affects pathways to nucleus ambiguus and hypoglossal nucleus?

nucleus ambiguus - uvula moves to side of lesion

hypoglossal nucleus - tongue moves opposite side of lesion

typical signs with lesion of corticospinal tract and other descending tracts

paralysis or paresis - initially flaccid but often develop spasticity/hypertonicity

hyperreflexia and possibly clonus

Babinski sign

Synergistic movements patterns of groups of muscles

what is spasticity/hypertonicity?

increased resistance to passive stretch, velocity dependent, clasped knife effect