Exam 2- Motor Systems Flashcards
skeletal muscle contraction is initiated by
lower motor neurons in the spinal cord
cell bodies of lmns are located in
the ventral horn of the spinal cord
lmns are also known as
alpha motor neurons
where do lmns send axons
to skeletal muscles
lmns are known as
the final common pathway for transmitting information to the skeletal muscles
what are upper motor neurons
neurons in the brainstem or cortex that descent to synapse with circuit neurons or LMNs directly; the help initiate movement and complex spatiotemporal sequences of skilled movement
what do lower motor neurons receive input from
sensory inputs, local circuit neurons, which then go to skeletal muscles
what do upper motor neurons receive input from
basal ganglia and cerebellum, then go down to local circuit and motor neuron pools to the skeletal muscles
basal ganglia
prevents UMNs from initiating unwanted movement, prepares motor circuit for initiation of movement, regulates transition from one pattern of movement to another
disorders of the basal ganglia
parkinsons, huntingtons
cerebellum
detects and attenuates the difference/motor error between an intended movement and the actual movement, mediating real-time and long term reductions in these errors
cerebellar damage leads to
incoordination with persistent errors in controlling direction and amplitude of ongoing movements
each lower motor neuron innervates
muscle fibers in a single muscle
motor neuron pool
a group of lower motor neurons innervating muscle fibers in a single muscle
organization of lower motor neurons
columnar organization in ipsilateral ventral horn
topographical organization of the ventral horn
cervical- neck/arms
thoracic- abs
lumbar/sacral- legs/pelvis
medial to lateral arrangement with medial being more proximal and lateral being more distal (fingers and toes most lateral)
pathways in medial part of spinal cord
control posture and locomotion and are bilateral
interneurons in medial part
cover many segments vertically
pathways in lateral spinal cord
control fine movement in distal extremities– interneurons here are strictly local and ipsilateral
2 types of lmn
alpha motor neurons and gamma motor neurons
alpha motor neurons
innervate extrafusal, striated, force producing muscles needed for posture and movement
gamma motor neurons
innervate intrafusal muscle fibers (muscle spindles), helping with stretch detection and setting them to an appropriate length
a single alpha motor neuron innervates
multiple extrafusal muscle fibers in the same muscle, which helps spread force across muscle and reduces the chance that loss of one neuron results in loss of muscle function
what generates muscle contraction
an action potential from a motor neuron
motor unit
an alpha motor neuron and the muscle fibers it innervates
sizes of alpha motor neurons
slow, fast fatigue resistant, fast fatigable
fast fatigable neurons
large neurons, high threshold, fastest conduction
the unit is large, easily fatigued, and used for brief force like running and jumping
fast fatigue resistant
neurons are medium size, medium threshold, medium conduction
unit is medium force, fatigue and function is for things like walking
slow
neurons are small, low threshold, slow conduction
unit produces small force and slow fatigue, function is sustained things like standing
recruitment of motor neuron pool
larger recruitment means more force
at low frequencies, every AP
generates a muscle twitch
at higher frequencies, action potentials
summate twitches
higher force requirements
recruit more and more motor units– so with more force, number and rate of firing of active motor units increases
tetanus
prolonged contraction of a muscle caused by repeated stimuli
group 1a and 2 afferents
largest axons in peripheral nerves so also very fast
group 1a afferents
coiled around the middle region of intrafusal fibers, forming annulospiral primary endings, respond phasically (so fire then accommodate) to small stretch
1a activity is dominated by
signals transduced by the dynamic subtype of nuclear bag fiber, which is sensitive to the velocity of fiber stretch
group 2 afferents
innervate static nuclear bag fibers and the nuclear chain fibers, signal the level of sustained fiber stretch, fire tonically (continuously) at a frequency proportional to the degree of stretch with little dynamic sensitivity
solution to stretch reflex
group 2 afferents continuously signal sustained stretch for muscle tone, adding a load stretches the muscle spindle and activates 1a afferents, which excites the alpha motor neuron and inhibits antagonist muscle
myotatic reflex
hammer stretches tendon, stretching sensory receptors (muscle spindles) in extensor- quadriceps
sensory neuron (1a afferent) synapses w/ and excites motor neuron in spinal cord and interneuron, which inhibits flexor muscles
alpha motor neuron conducts AP to synapses on extensor, causing contraction, flexor relaxes due to inhibition, leg extends
motor nuclei of the basal ganglia
striatum (caudate, putamen)
globus pallidus (gp): external and internal segments
substantia nigra: pars reticulate and pars compacta (globus pallidus and DA neurons)
subthalamic nucleus
role of basal ganglia
permission for the initiation of movement by upper motor neurons
corticostriatal pathway
projections from association, temporal, insular, and cingulate cortices
caudate receives input from
multimodal frontal and motor cortices, controlling eye movement
putamen
receives input from higher order sensory cortices, premotor, and motor cortices
projections to caudate and putamen
are parallel and segragated, which is maintained in output too
msns are located
in the striatum
msns in the striatum receive
excitatory glutamatergic synapses from cortical neurons
msns have large
dendritic trees, allowing them to collect and integrate input from local, thalamic, brainstem, dopamine serotonin
firing of msns is associated with
impending movement
caudate fires
before eye movements
putamen
fires before limb and trunk movements
t or f: msns have high spontaneous activity
false– needs many excitatory inputs
msns send out
converging inhibitory GABAergic projections to the globus pallidus and substantia nigra pars reticulata, which send gabaergic projections to superior colliculus and va/vl thalamus
organization of the basal ganglia thalamocortical motor circuit
loops
neurons in sn and gp
tonically inhibit superior collicular cells and va/vl thalamus to prevent unwanted movement
gabaergic projection from msn in striatum
inhibits inhibition of sn and gp, causing disinhibition, causing initiation of movement
excitatory nt of basal ganglia
glutamine– primary and premotor cortices, also prefrontal, parietal, temporal, cingulate, subthalamic
why are y-motor neurons important
allows for fine adjustments in movements, increasing in activity if a movement is more difficult/precise
when alpha motor neurons are stimulated without y motor neurons, 1a fiber decreases as muscle contracts
when both a and y are activated, no decreasing in 1a firing during muscle contraction, showing us that y motor neurons regulate the gain (muscle force generated) of muscle spindles so they can operate efficiently at any length
golgi vs muscle spindle
muscle spindle– muscle length
golgi– muscle force
golgi tendon organs
encapsulated afferent nerve endings at the junction of a muscle and a tendon
gto is innervated by
a single group 1b sensory axon
when a muscle actively contracts
force acts on tendon, leading to tension of collagen in gto, causing compression of nerve endings, the acons contact gabaergic circuit neurons that synapse and innervate the same musccle– this means gto is negative feedback so decreases muscle activation when large forces generated
gto effect
increases contraction of antagonist muscle, relaxes stretched muscle
passive stretch
ms and gto activated, ms to a greater extent
active muscle contraction
ms are unloaded, activity decreases, gto are activated
painful sensory stimuli leads to
flexion reflex (inhibition of extensor, activation of flexor on affected side so leg withdraws, also crossed extension reflex so extensor on other side activates to support leg
local spinal cord circuitry
also contributes to rhythmic movements like locomotion and swimming
local circuits in spinal cord
called central pattern generators– control timing and coordination
transection of thoracic spinal cord
continued locomotion
locomotion is
independent of sensory input and descending projections
lower motor neuron syndrome
paralysis, paresis (weakness), loss of reflexes and muscle tone, fibrilations and fasciculations, atrophy
als
aka lou gherigs disease, slow degenration of alpha motor neurons in ventral horn and brainstem and motor cortex, progressive weakness, wasting
motor cortex neurons
descend via lateral white matter, allowing for voluntary expression of precise, skilled movements of distal parts of limbs
brainstem neurons
descent via anterior medial white matter, helping with posture, balance, orienting mechanisms, and initiation and regulation of stereotyped, rhythmic behavior
premotor and motor cortex
premotor is in front of primary motor
upper motor neurons in cortex
planning and initiation of temporal sequences of voluntary movements , receive input from cerebellum and ganglia via ventrolateral thalamus and somatosensory cortex
umn of primary motor cortex
pyramidal cells of primary motor cortex– descending projections of axons to brainstem and spinal cord via corticobulbar and corticospinal tract
corticospinal and corticobulbar tracts
upper motor neuron axons descend through internal capsule, go thru cerebral peduncle and pontine fibers, form medularry pyramids, corticobulbar innervates cranial nerve nuclei, reticular formation, red nucleus, corticospinal decussates so 90% of lateral corticospinal tract cross midline, 10% of ventral terminate ipsilaterally
lateral corticospinal tract
direct pathway from cortex neurons to local circuitry in ventral horn, some synapse on alpha motor neurons for forearms and hands
lesion of lower motor neurons in facial motor nucleus or axons in facial nerve
affects all muscles of facial expression on side of lesion
umns in cortex lesion
difficulty controlling contralateral mouth muscles, upper facial muscles still function on both sides because of umns in cingulate gyrus that bifurcate and bilaterally innervate the same area
fritch and hitzig
electrical stimulation of motor cortex elicits contralateral muscle contraction
john hughlings jackson
motor cortex contains a complete map of body’s musculature
charles sherrington
spatial representation of body’s muscle in great apes
wilder penfield
400 neurosurgical patients– human motor cortex also topographical, motor homunculus
icms
small currents initiated excitation of several muscles, motor map is less precise than somatotopic map, high overlap
motor maps represent
maps of organized movement
stimulation of arm regions in primary cortex elicits
movements of arm towards central space
upper motor neurons are broadly tuned
discharrges prior to movements in many directions
can individual upper motor neurons specify direction of arm movement
no but directional preferences are present
premotor cortex
complex mosaic of interconnected frontal lobe areas contributing to motor function, receives multisensory input and complex signals related to motivation and intention
direct premotor pathway
axons through corticobulbar and corticospinal pathways to local circuits; 30% of corticospinal axons from premotor cortex
indirect premotor pathway
extensive reciprocal connections with primary motor cortex, premotor cortex uses info from cortical regions to select movements appropriate to context/goal of action
premotor has more or less monosynaptic connections to alpha motor neurons in spinal cord
fewer
m1 action goal encoding
localized to personal space w/in arms length
premotor goal encoding
localized to extrapersonal space
responses of premotor cortex are
linked in time to ocurrence of movemtns, firing most strongly before and during movements in a specific direction
when do umns in premotor fire
before and during movements made in a specific direction, fire at the appearance of a cue and encode intention for a movement– selection of movements based on external events
neurons that fire while monkey performs food grabbing task
also fires when observing similar action by experimenter’s hand , even behind barrier, but not with a tool
supplementary motor area
mediates movement selection, initiating movements by internal self initiated cues, lesion reduces number of self initiated or spontaneous movements, activated when individuals perform motor sequences from memory (autopilot)
brain machine interfaces
restore lost function, acquire brain signals and implement signals to operate systems, sensory feedback generated
brainstem umn pathway
nuclei of vestibular complex, reticular formation, superior colliculus
role of brainstem umns
balance, posture, locomotion, gaxe, work with motor cortex for skilled and reflexive activiries
vestibular nuclei
inputs from 8th cranial nerve, semicircular and otolith organs, descending axons that terminate in medial region of spinal cord
medial vrstibulospinal tract
inputs from semicricular canals, terminates bilaterally in medial ventral horn, regulates head position
lateral vestibulospinal tract
input from otolith organs, terminates ipsilaterally among medial lmn pools governing proximal muscles, activates limb extensor muscles, also control eye movement
reticular formation
network of circuits in core of brainstem, numerous neuronal clusters with many functions, cardiovascular and respiratory control, sensorimotor reflexes, eye movements, sleep/wakefulness
reticulospinal pathway
terminate in medial parts of gray matter, influence local circuit neurons coordinating axial and proximal limb muscles, bilaterally to medial ventral horns, feedforward adjustments for posture
reticulospinal pathway helps with
anticipatory feed-forward mechanism
upper motor neuron syndrome
damage to motor cortex or descending upper motor axons, flacidity of contralateral muscles, hypotonia called spinal shock , babinski sign, spasticity with increased muscle tone, hyperactive stretch, clunus, inability to perform fine movements
striatum at rest
gp is active, inhibiting thalamus, so no excitation of upper motor neuron
active striatum
gp inhibited so thalamus disinhibited, leading to upper motor neuron excitation
eyes fixating on a target
UMNs in SC tonically inhibited by substantia nigra cells
target moves
cortical input excites msn to inhibit inhibitory SN to release UMNs in SC
direct pathway
internal globus pallidus, which tonically inhibits va/vl
indirect pathway
involves external gp, inhibition from msn in striatum to tonically active inhibitory neurons in external segment of gp
disinhibition of subthalamic nucleus to internal gp
excites gpi, leading to less movement
range of direct and indirect pathways
focused in direct, diffuse in indirect, which balance eachother and facilitate selection of motor programs, suppresses competing motor programs – center surround organization
dopaminergic cells in sn pars compacta
msns in striatum receive and project to compacta, which sends projections back to msns
d1 dopamine receptors
excitatory g protein coupled, excites direct pathway
d2 receptors
inhibitory, gpcr, inhibit indirec so both d1 and d2 decrease inhibitory outflow of bg to increase excitability of upper motor neurons
