Voluntary Motor Pathways- Wilson Flashcards
What are the voluntary motor pathways?
- corticospinal
- corticobulbar
involved in skilled movement
What are the basic types of movements?
- reflexive movements: ie. monosynaptic reflexes in Unit 1
- voluntary movements: purposeful and goal directed
- rhythmic motor patterns: the in between reflexive and voluntary movements; such as walking, running, and chewing
cortex can initiate a movement but the reflexive circuits in the SC and brainstem will take over
Describe reflexive movements.
autonomic: the motor response is stimulus dependent
stereotypic: there is no learning involved
fast: fastest movement going from stimulus to response
Describe circuits for voluntary movements
- response to stimulus is context dependent: voluntary movement is not necessarily stimulus dependent;
- sensory stimuli are not necessary (may be internally evoked)
- improved by learning and experience: initially voluntary movements are slow and clumsy but as you practice you can get to skilled movements (learning to play a piano)
- flexibility of strategy: motor equivalence ( ie. i can write letters using my right or left hand, different arms performing the same movement
What are the four major components of the motor system?
- Spinal cord
- Primary motor and premotor cortex
- Cerebellum
- Basal ganglia
What is the difference between upper and lower motor neurons?
UPPER motor neurons as all signals coming from cortex cannot reach skeletal muscles directly and instead give commands to lower motor neurons to execute:
- primary motor cortex
- premotor cortex
LOWER motor neurons work directly on the muscle to contract them :
- alpha: go to extrafusal fibers
- gamma: go to muscle spindles to regulate the threshold and sensitivity of the muscle
What is the function of the cerebellum?
- cerebellum coordinates voluntary movements such as posture, balance, coordination, and speech, resulting in smooth and balanced muscular activity
- relays their motor commands through the thalamus and then to cortex
What is the function of the basal ganglia?
-is involved particularly in the gate-keeper of movements and decide which movements to depress and which ones to execute
information is relayed through the thalamus then to cortex
Why are lower motor neurons also known as the final common pathway?
regardless of where a motor signal comes from all that information have to go to the lower motor neuron and all motor signal commands must converge at the lower motor neuron
Many of the basic circuits of movement are located where?
in the spinal cord
If you lesion the spinal cord below the brachial plexus and above S4 what happens?
result in paraplegia
-brachial plexus C5-T1
If you lesion the spinal cord at the origin of the brachial plexus what happens?
results in quadriplegia (paralysis of both lower and motor limbs)
If you lesion the spinal cord at the origin of the phrenic nerve or above what happens?
results in asphyxiation and death (without medical intervention) ‘’
-you interrupt the commands for breathing and get negative motor negative signs: cannot walk, breathe, etc.
Lesions of the supraspinal motor structures can produce positive symptoms. What does this mean?
upper motor neurons in the cerebellum or cortex can suppress certain movements that should not normally occur
-our nervous system not only produces the movements we want to make but it also suppress movements that are not wanted resulting in positive sign
Describe the pathway of corticospinal tract from the cortex to the spinal cord.
- the cell bodies are located in motor cortex which have very long axons that will go through the internal capsule, crus cerebri of midbrain, basilar pons, then to the medulla where they will form the pyramids
- once at the intersection between medulla and spinal cord 90% of the fibers will decussate to the other side
What is the difference between the lateral and medial corticospinal tract?
once the fibers reach the spinal cord the descending corticospinal tract is made up of two parts:
- the lateral corticospinal tract are those that decussated
- anterior corticospinal tract does not decussate and remains on the same side
If there is lesion of the lateral corticospinal tract in the cortex prior to decussation, what are the resulting symptoms?
contralateral symptoms
Where is the primary motor cortex?
- in the precentral gyrus AKA Brodmann’s area 4
- also had portions on the medial side of the precentral gyrus
-this area is most important in initiating and controlling skilled movements
What is the primary sensory cortex?
in the postcentral gyrus AKA Brodmann’s area 312
What is found immediately rostral to the primary motor cortex?
the premotor region AKA area 6
What are the 2 subregions of the premotor region and what are their directionality?
- premotor cortex is found more laterally in area 6
- supplementary cortex is found more medially in area 6
There are 6 cortical layers. The thickness of each layers changes depending upon what?
whether they are sensory or motor or in between associational
Compare area 4 and area 6 in terms of cortical layer content.
- area 4 and 6 lack layer 4 which is the inner granular cortex and thus they are often called the primary and secondary agranular cortex
- area 4 and 6 are THICK with layer 5 that is loaded with pyramidal projection cells; area 4 in particular has very large pyramidal cell bodies
Remember area 4 and 6 make up the precentral gyrus and the portions rostral to it respectively
The cell bodies of the corticospinal tract are found in what layer of the cortex?
layer 5
Which cortical layer does the corticospinal tract originate from?
layer 5
The corticospinal tract consist of 1 million axons. About 30,000 axons are contributed from what cells?
very large pyramidal cells called Betz cells that contribute approximately 30,000 to the 1 million axons
Where are the large Betz neurons found?
in the layer 5 ONLY in the primary motor cortex
Where do axons of the corticospinal tract originate? List the abundance of each origin.
- 1/2 of the axons originate in the primary motor cortex (Brodmann’s area 4)
- most of the other axons come from the medial supplementary motor cortex
- smaller portion from the lateral premotor cortex
- small number of axons come from the area 5 and area 3, 2, 1 (these target the dorsal horn to modulate sensory activity) in the parietal cortex
What is the evidence that the precentral gyrus is the “primary motor cortex”?
- electrical stimulation of the motor cortex (area 4) evokes movements of:
- individual or a few muscles
- the lowest threshold (in the nanoamp range) for producing movement is in the motor cortex
- short latency: time of stimulation to movement of muscle is short
How does latency relate to the number of synapses in a pathway?
the more synapses in a pathway the longer the latency is between the stimulus and response
The primary motor cortex contains a motor map of the body AKA it is somatotopically organized.
The medial bank of the primary cortex controls movements of what parts of the body?
-legs: hip, knee, ankle, toes
gluteal region is the transition between the medial and lateral bank of the primary motor cortex
The primary motor cortex contains a motor map of the body AKA it is somatotopically organized.
The lateral bank of the primary cortex controls movements of what parts of the body?
- trunk
- upper limbs
- hands
- face
- oral cavity
Which regions of the body have the highest cortical representation? What does this mean?
hands and mouth
they have highest motor control (both involve intricate movements) whether its speech, swallowing, holding things, writing
the regions of the body which have the highest motor control have the largest representation
What supplies the cortical region supplying the lower limb?
anterior cerebral artery
-monoplegia will result if there is stroke
What supplies the cortical region supplying the hand and face?
middle cerebral artery
What artery is most often infarcted in the brain when there is a clot, etc.?
middle cerebral artery
Monoplegia may result from stroke of what artery?
anterior or middle cerebral
How does the activity of the corticospinal neurons correlate with flexor and extensor movements????
corticospinal neurons codes force NOT change of position
eccentric contraction has no
The upper motor neurons in the primary motor cortex (area 4) central in the production of precision, skilled movements. What aspects of movement do they control?
- direction: extension or flexion
- force: the faster UMN fire the more force is generated meaning more force it can take
- speed
How does electrical stimulation of area 6 differ with stimulation of area 4 in evoking movements ?
-in contrast to single muscles, stimulation of supplementary cortex produces complex movements involving several joints (ie. opening and closing the hands), often bilaterally
-lesion of area 6 results in apraxia,
difficulty carrying out complex purposeful movement such as dressing
What is ideomotor apraxia?
- can’t carry out complex movement such as pointing to the nose when asked but can touch the nose when it itches
- lesion to apraxic center near Broca’s results in deficits of for example, mimicking hand gestures or tool use????
What is the difference in cerebral blood flow during repeated flexing of finger against a spring and during a complex sequence of finger movements? What about cerebral blood flow during mental rehearsal of finger movements? What do these findings suggest?
- only blood flow in the primary motor cortex for repeated flexing finger against spring
- blood flow in both primary motor cortex and supplementary cortex for complex sequence of finger movements
- only blood flow to supplementary cortex for mental rehearsal of finger movements
This all proves that the supplementary cortex of area 6 is involved in PLANNING of movements and execution of complex movements.
What does area 6 plan?
plans a strategy for movements requiring sequences of muscles contractions or a planned strategy (such as brushing teeth)
What is the supplementary motor cortex particularly important for?
important in programming motor sequences and in coordinating bilateral movements and postural responses
What muscles does the supplementary motor cortex control through the corticospinal tract? Through area 4?
- postural muscles through the corticospinal tract
- distal muscles (such as in the hand) through area 4
What is the lateral premotor cortex important for?
- plays a role in interjoint coordination
- controlling proximal limb movements that project or orient the arm to targets
What are the 3 complex process of voluntary movements and what areas of the cortex control those processes?
- Identification of target: posterior parietal cortex (area 5 and 7)
- Plan movement: supplementary motor cortex (area 6)
- Execute movement: primary motor cortex (area 4)
The movement is involving the upper limb (area 6) and you do postural adjustments in order to do that movement. Explain.
- postural movements have to occur in concert and be coordinated with fine movements
- the supplementary motor cortex would first organize for the postural adjustments which would make possible the desired movement
What is the pathway that corticospinal fibers take to go from the cortex to the spinal cord? How are the fibers organized in the pathway?
- corticospinal fibers pass through the posterior limb of the internal capsule
- the projections are somatotopically organized (upper limb, trunk, lower limb in that order)
Corticobulbar fibers controlling the motor nuclei of the brainstem are found where in the internal capsule?
genu of the internal capsule
-controlling muscles of the face???
What occurs if there is lesion in the posterior limb of the internal capsule?
Monoplegia (lesion is in a particular area): paralysis of 1 limb
Hemiplegia (if lesion is larger): paralysis of 1 side of the body
notice there is no paralysis of the face (fibers controlling it are in the genu of the internal capsule)
Capsular lesions are common due to obstruction or hemorrhage of what blood vessels?
a) deep branches of the middle cerebral
b) anterior choroidal artery
usually the face is not affected by a lesion of the posterior limb of the internal capsule
Why are symptoms of capsular lesions seldom restricted to the “pyramidal syndrome”?
because other projections pass through the internal capsule including:
- thalamic radiations
- corticothalamic fibers
- corticopontine fibers
- corticostriate fibers
- corticoreticular fibers
From the internal capsule, where does the corticospinal tract pass?
it forms part of the cerebral peduncle (ventral part of midbrain) go through pons form the pyramids and 90% of fibers will decussate
A lesion to the corticospinal tract proximal to pyramidal decussation (on the right side in the area of the cerebral peduncle and pons) will produce what kind of symptoms?
contralateral (left) hemiplegia
A lesion of the corticospinal tract distal to the pyramidal decussation will produce what kind of symptoms?
ipsilateral hemiplegia
What does the cerebral peduncle consist of?
- consists of crus cerebri: descending cortical axons part of which are forming the CST
- there’s a topography with the axons going to the face (corticobulbar), upper limb, trunk, and lower limb come together but are still organized in that order
- the other axons are going to the brainstem largely carrying information about the cerebellum
If there is a lesion in the medial part of the midbrain, what occurs?
medial midbrain (Weber/Claude) syndrome: occlusion of the first part of the posterior cerebral artery
you disrupt the long tracts:
- corticospinal
- corticobulbar
- corticopontine
you also get oculomotor signs and so you know that the lesion is in the medial midbrain
What happens to the fibers of the corticospinal tract in the basilar pons?
the corticospinal tract separates into fascicles in the ventral (basilar) pons
What happens to the fibers of the corticospinal tract after the basilar pons?
they come back together to form the pyramids in the medulla
the tract is topographically organized
What is the medial medullary syndrome?
- affects the anterior spinal and medial branches of the vertebral arteries
- lesion affecting the pyramids so you get a contralateral hemiplegia due to disruption of the corticospinal tract
- you would also get hemisensory loss related to the medial lemniscus
- the region will disrupt axons of the hypoglossal nerve thus plegia of the ipsilateral tongue muscles (ipsilateral loss of tongue movements)
What percentage of pyramidal fibers decussate at the junction of the medulla and spinal cord?
90%
If there is a lesion right in the middle of the decussation of the pyramids, will the symptoms be contralateral, ipsilateral, or both?
bilateral because there are fibers going from left to right and fibers going from right to left
Lesions of the corticospinal tract prior to the decussation of the pyramids produce contralateral signs which can include what signs?
- monoplegia (paralysis of a single limb)
2. hemiplegia (paralysis of 1/2 of the body)
The 90% of the pyramidal fibers that cross will form what in the lateral funiculus? What about the uncrossed fibers?
crossed: lateral corticospinal tract
uncrossed: anterior corticospinal tract
What is the difference in innervation of the lateral and anterior corticospinal tract?
- lateral corticospinal tract innervates lateral motor nuclei of rexed lamina 9 in the ventral horn (controls muscles of the limb)
- anterior corticospinal tract innervated the medial motor nuclei of the ventral horn (muscles of the body trunk)
You cannot move your axial musculature unilaterally. Explain in terms of the course of the anterior corticospinal tract.
some axons of the anterior/ventral corticospinal tract cross at the segmental level and thus the projection is BILATERAL
- thus the lateral CST is a unilateral pathway
- anterior CST is a bilateral pathway
What is the somatotopic organization of the proximal, distal, flexor, and extensor muscles in the ventral horn?
medial: proximal
lateral: trunk
ventral: extensors
dorsal: flexors
What is the oldest system: lateral or ventral corticospinal tract?
ventral corticospinal tract
-think of fishes who came before us
What are the function of the ventral corticospinal pathway?
- innervates proximal limb and axial musculature
- controls balance and posture on which highly differentiated movements of distal limbs can be executed
Corticospinal axons monosynaptically synapse on alpha and gamma motor neurons. Why is this important?
- this is new (phylogenetically recently acquired); these are DIRECT connections
- this only occurs in higher level mammals
cortical control of alpha motor neurons is a recent achievement: seen in cats, rhesus, chimpanzees, humans
more direct control by corticospinal neurons enable better dexterity
What are the primary synaptic targets of the corticospinal tract?
intrasegmental interneurons: neurons related to a single myotatic unit
intersegmental interneurons: coordinate different myotatic units
CBT
- projection from motor areas of cortex to the cranial nerve nuclei that innervates skeletal muscles
- unlike CST, CBT is bilateral
- so if you have one stroke affecting one side of the CBT you don’t get a true palsy but a psuedobulbar palsy
facial nerve:
- Facial neurons innervating the lower 1/2 of the face receive only contralateral CB input.
- Facial neurons innervating the upper 1/2 of the face receive bilateral CB input.
hypoglossal nerve:
the tongue will NOT deviate toward side of lesion as axons have crossed but to the intact side
-CBT terminates in the reticular formation
Two descending tracts originate from the reticular formation:
1) pontine reticulospinal tract
2) medullary reticulospinal tract
- these are other routes by which the cortex can control movement
Cortical area 8 (rostral to area 6) is the frontal eye fields for voluntary horizontal gaze. They project indirectly to CNs III, IV, and VI.
Right way eye involves area 8 and area 4. Looking towards the part of the body that is not lesioned???
Wrong way eyes if there is lesion of PPRF and corticospinal tract; Eyes deviate toward the side of the lesion.
How does dexterity relate to the connections of the corticospinal neurons?
more direct control by corticospinal neurons (monosynaptic connections to the alpha motor neurons) enable better dexterity
Describe the effect of the corticospinal tract on flexor and extensor motor neurons.
corticospinal tract:
1) has facilitatory effect on flexor motor neurons
2) inhibitory effect on extensor motor neurons
What is the role of the corticospinal tract in the Babinski sign?
CST normally suppresses the enhanced withdrawal reflex (get flexion of toes) to the the noxious stimulus after the first year of life
so a CNS lesion will release the inhibition of normal interneuron circuits resulting in a return of the withdrawal reflex AKA absence of this suppression leads to extension of toes (positive Babinski sign)
What is the Babinski sign?
it’s a sign that can be evoked clinically to show that CST is no longer suppressing extensors and thus showing lesion to the CST (due to flexion response after stimulus)
One of the functions of the CST is to turn on or turn off flexor/extensor reflexes
Explain how activity of gamma motor neurons is modulated by the corticospinal tract.
CST has the ability through interneurons to regulate or modulate reflex activity
the dynamic and static ????
In lower mammals and man, the corticospinal axons terminate where?
-dorsal horn
????
- fibers originate in somatosensory areas 3, 1, 2
- projections regulate ascending sensory systems
The sensitivity of sensory systems is modulated by what?
circumstances
wrestler is not worried about the caterpillar on his arm but he is focused on how to defeat his opponent.
What are the signs of the upper motor neurons in terms of tone, reflexes, Babinski, clonus, muscle bulk, and fasciculations?
-spastic paralysis (muscle have tension in them and can still do reflexes)
-hyperreflexia
-Babinski present
clonus is frequently present
-not much muscle atrophy only slight atrophy
-no fasciculations
What are the signs of the lower motor neurons in terms of tone, reflexes, Babinski, clonus, muscle bulk, and fasciculations?
- flaccid paralysis (muscle do not contract and there is no tension in them)
- hyporeflexia
- Babinski absent
- clonus is absent
- muscle is atrophied
- fasciculations are present
When you have spinal cord injury, immediately after, often times the injury the muscles of the body trunk will be flaccid. What does this mean?
- the patient seems to have extensive lower motor neuron injury; wait a week to see if it is really flaccid or spastic paralysis due to lower or upper motor neuron injury
- very shortly after injury to the brain or spinal cord, upper motor signs may be lacking, and, therefore, may even be falsely interpreted as lower motor neuron signs
- this transient state is known as ‘cerebral’ or ‘spinal shock’ (not to be confused with “shock” due to hypotension)
What is the corticobulbar pathway?
they are cortical projections that terminate in the brainstem particularly directly on motor neurons of CN’s V, VI, and XII
How does the corticobulbar pathway project?
project bilaterally (with a contralateral predominance)
lower face and tongue is contralateral (unilateral) for CN VII and CN XII
What results after a lesion to the corticobulbar pathway?
paresis (not paralysis) or pseudobulbar palsy results if lesioned because of the bilateral input
What is the major exception to paresis if lesioned?
corticobulbar projections to the lower half of the face
the projections to the lower 1/2 of the face (and tongue) are contralateral
Lesions anywhere along the corticospinal tract will produce what kind of signs?
Upper motor neuron signs
The facial neurons innervating the upper half of the face receive what type of corticobulbar input?
bilateral corticobulbar input
The facial neurons innervating the lower half of the face receive what type of corticobulbar input?
only contralateral corticobulbar input
Following cerebral injury, 90% of motor function in the upper half of the face will be retained. Why?
because of bilateral corticobulbar innervation
Following cerebral injury, 90% of motor function in the lower half of the face will be lost. Why?
you get what looks like Bell’s palsy
and that is because of contralateral (unilateral) corticobulbar innervation
Where do the corticobulbar tract terminate?
reticular formation
cranial nerve nuclei also receive bilateral cortical inputs through the reticular formation
What are the two descending tracts that originate from the reticular formation?
1) pontine reticulospinal tract
2) medullary reticulospinal tract
The frontal eye fields (area 8) project indirectly to what cranial nerve nuclei?
Cortical area 8 (rostral to area 6) is the frontal eye fields for voluntary horizontal gaze. They project indirectly to CNs III, IV, and VI.
If there is a lesion in the frontal eye fields and corticospinal tract, what condition results?
right way eyes
Looking towards the part of the body that is not lesioned
If there is a lesion in the PPRF and corticospinal tract, what condition results?
wrong way eyes
Eyes deviate toward the side of the lesion.
All descending input going to the brainstem and spinal cord are located where?
located in midbrain
If you have a stroke involving the basilar artery and the first branches of it (posterior cerebral arteries), what happens?
- result in bilateral infarct affecting the corticospinal and corticobulbar system eventually into a locked-in syndrome (pseudocoma or Monte Cristo syndrome)
- all voluntary movement controls is destroyed here but the ascending sensory systems are intact
Corticoreticulobulbar pathway
parts of the CB tract go through the reticular formation
-CBT terminates in the reticular formation
Two descending tracts originate from the reticular formation:
1) pontine reticulospinal tract
2) medullary reticulospinal tract
-these are other routes by which the cortex can control movement
Red Nucleus (rubrospinal tract makes a stop here)
magnocellular: cortex
crosses the ventral tegmentum
parvocellular: cerebellum
