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The control of voluntary movements is extremely complex. Many different systems acrossnumerous brain areas need to work together to ensure proper motor control. We will start a journeythrough these areas, beginning at the spinal cord and progressing up the brain stem and eventuallyreaching the cerebral cortex. Then to complete the picture we’ll add two “side loops,” the basalganglia and the cerebellum. This is relatively DIFFICULT MATERIAL because our understandingof the nervous system decreases as we move up to higher CNS structures. It is EXTREMELYIMPORTANT TO COMPLETE ALL OF THE PRACTICE QUESTIONS. Please KEEP UPand you will really enjoy this section. Get behind and you will NOT! Remember, this material isextremely important for you as future physicians, because many of your patients will exhibit signsand symptoms associated with motor diseases and an understanding of the underlying mechanismspresented here is essential.

SPINAL LOWER MOTOR NEURONS

Lower motor neurons (LMNs; also called alpha motor neurons), are directly responsible forgeneration of force by the muscles they innervate. A motor unit is a single LMN and all of themuscle fibers it innervates. The collection of LMNs that innervate a single muscle (triceps) is calleda motor neuron pool. LMNs respond to, and are therefore controlled by, inputs from three sources,dorsal root ganglia, interneurons (cells that do not project out of the area), and projections fromhigher centers such as the brain stem and cerebral cortex.

You should remember from the spinal cord and brain stem module that LMNs that controlproximal and distal muscles are found mainly in the cervical and lumbosacral segments of the spinalcord. At these levels, the LMNs are distributed such that neurons controlling axial muscles liemedial to those LMNs controlling distal muscles and motor neurons controlling flexors lie dorsal tothose controlling extensors. In the thoracic cord below T1, the LMNs are associated only with axialmusculature.

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Dorsal Root Inputs to LMNs

Stretch reflexes work to resist lengthening of a muscle. They are functionally efficientbecause they allow weight-bearing muscles to adjust to a changing load at the level of the spinalcord, without the information having to go all the way up to cortex for a decision. You should recallthe Ia and II fibers in the dorsal roots. Ia fibers are associated mainly with the nuclear bag intrafusalfibers and carry information regarding the length and change in length of the muscle. The II fibersare primarily concerned with the constant length of the muscle and thus bring information into thespinal cord from the nuclear chain fibers. These fibers are important in stretch reflexes. Stretch ofthe intrafusal muscle fibers results in contraction of the extrafusal muscle fibers in the same(homonymous) muscle. This is a monosynaptic reflex, in that there is only one synapse between theincoming information and the outgoing signal to contract the muscle. (It has been shown that thismonosynaptic reflex takes about 1 ms; the synaptic delay between the sensory input and the LMN isbetween 0.5 and 0.9 msec). In the cat, a single 1a fiber makes excitatory connections with allhomonymous motor neurons. This Ia divergent signal produces a very strong excitatory drive to themuscle within which it originates. This is called autogenic excitation. The Ia fiber also makesmonosynaptic connections with LMNs that innervate muscles that are synergistic to thehomonynous muscle.

The incoming Ia fiber also synapses on a Ia inhibitory interneuron that in turn inhibitsLMNs that innervate the antagonist muscles (stretch biceps——contract biceps——inhibit triceps;notice this is NOT a monosynaptic reflex). This is called reciprocal innervation. Ia inhibitoryinterneurons are also important for coordinating voluntary movements, as corticospinal axons makedirect excitatory connections with LMNs and also send a collateral to the Ia inhibitory neuron. Thus,the cortex does not have to send separate messages to the opposing muscles.

The intrafusal muscle fibers can be controlled by the small gamma efferent neurons that lie inthe ventral horn. These gamma efferent cells, which are controlled by descending inputs, bias theintrafusal muscle fibers in order to ensure that the intrafusal muscle fibers are always signalinginformation to the brain. For instance, contraction/shortening of the extrafusal muscle will put slackon the intrafusal fibers, and they will stop telling the brain about the length of the muscles. This is

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bad! To get around this, the cortical input tells the gamma efferents to fire and tighten up theintrafusal muscle fibers (to keep them firing) when the extrafusal fibers contract. The gammaefferents that innervate the bags are called gamma dynamics while those targeting the chains aregamma statics.

Inverse Stretch (myotatic) Reflex

The Ib fibers are carrying information from Golgi tendon organs (GTOs; Dr. Royce used todrive a ’68). Each GTO is comprised of collagen fibers intertwined with a Ib fiber. Thus, stretchingof the tendon “squeezes” the Ib fiber and it begins to fire. While muscle spindles are sensitive tochanges in length of a muscle, GTOs are most sensitive to changes in muscle tension and thus signalthe force in a muscle. GTOs provide the nervous system with precise information about the state ofcontraction of the muscle.

The inverse stretchreflex is also known as theinverse myotatic reflex orGolgi tendon reflex. Thisreflex involves Ib afferents, Ibinhibitory interneurons, andLMNs. Thus, increased firingof the Ib results in theinhibition of thehomonymous muscle(autogenic inhibition).

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The inverse stretch reflex is polysynaptic, meaning that it involves more than one synapse.This reflex is therefore slower than the stretch reflex. However, the inverse stretch reflex can over-ride the stretch reflex. If there is a very large stimulus, such as a strong blow to the patella tendonwith a hammer, the quadriceps muscles will contract due to the stretch reflex. To prevent damage tothe tendon due to the muscle pulling too hard on it, the inverse stretch reflex is initiated by increasingtension in the tendon, and the contraction of the muscle is inhibited. The inverse stretch reflex istherefore damping down the effect of the stretch reflex.

Flexion/withdrawal reflex

Pain and temperature fibers in the dorsal roots (Cs and deltas) play a role in the flexion reflex,also known as the withdrawal reflex (you already know the pathway over which this pain andtemperature information reaches cortex via the ALS etc). At the spinal level, this reflex responds tonoxious stimuli, such as a hot object on the skin. The result is a rapid flexion that confers a protectivemechanism against damage by the stimulus. Another example of this reflex is stepping on a pin. Thepin will be sensed by delta and C fibers, which synapse with a series of excitatory and inhibitoryinterneurons to produce a flexion response. The excitatory interneurons excite LMNs to thehamstring, while the inhibitory interneurons inhibit LMNs to the quadriceps (reciprocal inhibition).

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The flexion reflex is often accompanied by a crossed extension reflex acting on thecontralateral limb. Using the example of stepping on a hot match, the crossed extension reflexwould brace the other leg, helping to maintain balance. In this case, excitatory interneurons exciteLMNs innervating the quadriceps, while inhibitory interneurons inhibit LMNs that project to thehamstrings.

You can see from the flexion/crossed extensor reflex that flexion withdrawal is a complete,albeit simple, motor act. While this reflex is reasonably stereotyped, the spatial extent and force ofmuscle contraction is dependent upon stimulus intensity. For instance, a moderately painful stimuluswill result in the production of a moderately fast withdrawal of your finger and wrist, while a realpainful stimulus results in the forceful withdrawal of the entire limb. Thus reflexes are not simplyrepetitions of a stereotyped movement pattern, but instead are modulated by the properties of thestimulus.

It is important to understand that reflexes are adaptable and control movements in apurposeful manner. For example, a perturbation (a disturbance of motion, course, arrangement, orstate of equilibrium) of the left arm can cause contraction of the opposite elbow extensor in onesituation (opposite arm used to prevent the body from being pulled forward) but not the other, wherethe opposite arm holds a cup and the perturbation causes an inhibition of opposite elbow extensor.Also, don’t forget that spinal reflexes are functionally efficient because they enable adjustments ofmovements to be made at the level of the spinal cord, without having to involve (and wait for)decisions from higher levels of the motor systems.

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SPINAL CORD NEURONAL NETWORKS (CENTRAL PATTERN GENERATORS)

Hopefully you now understand some basic reflexes involving the LMNs. Let’s move “up”slightly in the motor “hierarchy,” and in particular, to neuronal networks in the spinal cord thatgenerate rhythmic alternating activity. A central pattern generator (CPG) is a neuronal networkcapable of generating a rhythmic pattern of motor activity. Normally, these CPGs are controlled byhigher centers in the brain stem and cortex. A simple spinal cord circuit is shown below and to theright. This circuitry underlies alternating flexion and extension because when some cells are activethe others are inhibited. These cells lie in the ventral horn on the same side of the spinal cord andinclude flexor and extensor motor neurons, together with their associated interneurons. Descendinginputs from higher levels provide continuousinput to the excitatory interneurons. However,because of random fluctuations in excitability orother inputs, one “half-center” initiallydominates and inhibits the other. Let’s say forexample that excitatory interneuron #1 turns onfirst. It will not only excite the flexor LMN andcause flexion, but it will also turn on inhibitoryinterneuron #2, which inhibits excitatoryinterneuron #2 so that the extensor LMN isinhibited. Inhibitory interneuron #2 also inhibitsitself, so the flexion will switch to extensionwhen the inhibitory interneuron #2 stops firing.Remember, the tonic inputs are excitingexcitatory neuron #2. This will mean that nowthe excitatory interneuron #2 will fire and exciteinhibitory interneuron #1. You do not have tounderstand any of the details of such circuits.What you need to know is that there are groupsof cells in the spinal cord, both LMNs andinterneurons, which generate basic patterns oflocomotion.

Sometimes these CPGs can be activatedbelow the level of a spinal cord transection. Forinstance, if a quadriplegic’s hips are extended,spontaneous uncontrollable rhythmic movements(flexion and extension) of the legs occur.Moreover, if babies are held erect and moved over a horizontal surface, rhythmic steppingmovements take place. Granted, these stepping rhythmic activities in babies involve sensory input,but the circuitry for the rhythmicity of the movements is in the spinal cord. These two examplesindicate that basic neuronal circuitry for locomotion is established genetically.

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Practice Questions

1. Which of the following statements is TRUE?

A. LMNs innervating axial muscles lie laterally in the spinal cord ventral hornB. LMNs innervating distal flexors are present at all levels of the spinal cord ventral hornC. LMNs innervating extensors lie dorsal to those innervating flexors in the spinal cord ventral hornD. LMNs innervating distal muscles lie lateral to those innervating axial muscles in the spinal cord

ventral hornE. a motor neuron pool is a LMN and all of the muscle fibers it innervates

2. Which of the following is monosynaptic?

A. crossed extensor reflexB. flexor reflexC. inverse myotatic reflexD. stretch reflex involving agonist muscleE. stretch reflex involving inhibition of antagonist muscle

3. Which of the following statements about gamma efferents is TRUE?

A. they innervate extrafusal muscle fibersB. stimulation of dynamics causes shortening of the chain fibersC. are controlled by descending inputsD gamma dynamic stimulation leads to a “pause” of Ia firing during contraction/shorteningE. control GTOs

4. Which of the following statements is TRUE?

A. basic patterns of locomotion can only be accomplished via cortical involvementB. a quadraplegic is incapable of any rhythmic movementC the basic neuronal circuitry for locomotion is established geneticallyD. Ib fibers that are involved in the inverse myotatic reflex directly inhibit LMNs innervating the

muscle that is attached to the tendon/GTO innervated by the Ib fiberE. when you step on a tack, you activate ipsilateral extensors and contralateral flexors

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5. Which of the following statements is TRUE?

A. reciprocal innervation is excitation of groups of LMNs that innervate muscles withopposing actionsB. Ib fibers synapse upon Ia excitatory interneuronsC. autogenic excitation is associated with Ia fibersD. autogenic inhibition is associated with Ia fibersE. Ia fibers synapse upon Ia excitatory interneurons

6. Which of the following associations is TRUE?

A. B = excitatory; B. A = inhibitory; C. C = increase in firing; D. D = decrease in firing;E. E = alpha-beta

Answers1. D. 2. D 3. C 4. C 5. C 6. D

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INFLUENCE OF “HIGHER” CENTERS UPON LMNS

At this point in our examination of the motor systems we know that there is reflex and centralpattern program circuitry within the spinal cord and that certain reflexes and movements can occurusing this intrinsic circuitry of the spinal cord.

The next level of organization in the motor systems “hierarchy” includes descending inputs tothe spinal cord circuitry from a number of different nuclei in the brain stem and primary motorcortex. Descending pathways to the spinal cord can be divided into ventromedial and dorsolateralsystems based upon which spinal cord funiculus the pathway travels in and the termination pattern ofits axons in the spinal cord grey.

Ventromedial system—control of axial and proximal musculature

You already know that the tecto- and vestibulospinal tracts (MVST and LVST) travel in theventral funiculus and terminate on LMNs that control axial and proximal musculature. Thus, theykeep the head balanced on the shoulders as the body navigates through space, and the head turns inresponse to new sensory stimuli.

The pontine and medullary reticulospinal tracts (PRST and MRST) also travel in theventral funiculus. (These pathways were not presented in the spinal cord and brain stem lectures, soyou have not forgotten them). The PRST, which lies medial to the MRST in the ventral funiculus,arises from cells in the pons that lie around and near the PPRF (paramedian pontine reticularformation at the level of the abducens nucleus and motor VII; level 5). In contrast, the MRST has itsorigin from cells dorsal to the inferior olive (level 3). The reticular formation consists of those cellsin the brain stem that do not comprise the sensory and motor nuclei that you so carefully learnedearlier in the course.

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The PRST enhances the anti-gravity reflexes of the spinal cord. Thus, this pathway excitesupper limb flexors and lower limb extensors. This is an important component of motor control,since most of the time the activity of ventral horn cells maintains, rather than changes, muscle lengthand tension. PRST cells are spontaneously active but they also receive excitatory input from thecerebellum (soon to be discussed) and the cortex

The MRST has the opposite effect of the PRST, in that it inhibits anti-gravity reflexes ofspinal cord (inhibits upper limb flexors and lower limb extensors). MRST cells, like those in thePRST also receive excitatory input from the cerebellum (soon to be discussed) and the cortex.

All of these ventromedial pathways (MVST, LVST, TST, MRST, PRST) innervate LMNsand interneurons that innervateaxial and proximal limb muscles.Moreover, the axons in theventromedial pathway distributecollaterals to many segmentsalong their distribution in thespinal cord (for the coordinationof intersegmental movements).You can see that such adistribution is not well suited forthe discrete control of a fewmuscles but rather for control ofgroups of muscles involved inposture and balance.

Lesions of theventromedial pathways in animalsresult in difficulty in righting to asitting or standing position andimmobility of the body axis andproximal parts of the extremities.However, the hands and distalparts of the extremities are stillactive. Thus, descendingpathways controlling LMNs thatinnervate these more distalmuscles must not travel in theventromedial part of the spinalcord.

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Dorsolateral system—control of distal musculature

You already know about the lateral corticospinal tract (LCST), and this is one of the twodorsolateral pathways. This pathway is especially important for the control of distal musculature andfor steering extremities and manipulating the environment. The other pathway in the dorsolateralsystem is the rubrospinal tract (RST), which arises from our old friend the “ruber-duber.” Both ofthese pathways course within the lateral funiculus and are important for the control of distal limbmusculature. It should not come as a surprise that axons of the dorsolateral system terminate at onlyone or two segments (in contrast to the more extensive distribution of ventromedial axons) andreach the LMNs that lie more laterally in the ventral horn. Thus these pathways are involved in thefiner motor control of the distal musculature (finger movements for example).

Lesions involving both dorsolateral pathways result in the inability to make fractionated(independent) movements of the wrist or fingers. Moreover, voluntary movements are slower andless accurate, however, the patient still has the ability to sit upright, and stand with relatively normalposture (via functioning pathways in the ventromedial system).

A lesion of only the LCST initially resembles the combined lesion involving the LCST andthe RST, but after a while the main deficit is that there is weakness of the distal flexors and theinability to move the fingers independently (fractionated movements). Evidently, the functioningRST accounts for the recovered function of the wrist.

Practice Questions

1. Which of the following statements is TRUE?

A. pathways in the ventromedial system are functionally related to the control of the distal musclesB. pathways in the dorsolateral system are functionally related to the control of axial and proximal musclesC. the rubrospinal tract is part of the dorsolateral pathwayD. the PRST is part of the dorsolateral system systemE. following a lesion of the ventromedial pathway, there would be atrophy of the axial and proximal

musculature

2. Which of the following statements regarding the dorsolateral pathway is TRUE?

A. lesions result in the inability to control the distal musculatureB. it would be easy to button a shirt/blouseC. contains the pontine and medullary reticulospinal tractsD. contains the lateral vestibulospinal tractE. terminates in the more medial parts of the spinal cord grey

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3. Which of the following statements regarding the pontine reticulospinal tract (PRST) is TRUE?

A. inhibits the quadricepsB. excites flexors in the armsC. courses within the lateral funiculusD. is functionally associated with the ruber-duberE. lesion would result in atrophy of interosseous muscles

4. Which of the following statements regarding the medullary reticulospinal tract (MRST) isTRUE?

A lies in the rostral brain stemB. excites antigravity muscles (flexors in the arms and extensors in the legs)C. courses within the lateral funiculusD. is functionally associated with the corticospinal tractE. inhibits antigravity muscles (flexors in the arms and extensors in the legs)

5. Which of the following is/are polysynaptic?

A. crossed extensor reflexB. flexor reflexC. inverse myotatic reflexD. stretch reflex involving inhibition of antagonist muscle (reciprocal innervation)E. all of the above are polysynaptic

Answers1. C2. A3. B4. E5. E

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PRIMARY MOTOR CORTEX: EXECUTION OF MOVEMENTS

Now we are up in the motor cortex. Pretty far from those lowly grunts the LMNs, and thoseslightly more intelligent CPGs and those pathways in the ventromedial system.

The primary motor cortex is also called area 4 of Brodmann and motor I (MI).Corticospinal cells in area 4 project directly to the lower motor neurons (LMNs) in the spinal cordand brain stem and tell these LMNs, and in turn the muscles they innervate, what to do. They arethe classic upper motor neurons (UMNs). MI is located in the precentral gyrus, which lies in front(rostral) of the central sulcus. MI occupies most of this gyrus along both its lateral and medialsurfaces and is bound caudally by the central sulcus.

Left: Human motor map in a coronal or frontal section through area 4 or MI. The entire lateralsurface of the right hemisphere is seen while only the dorsal and medial aspect of the lefthemisphere is visible. Right: The map of the body (homunculus) as represented on the precentralgyrus. Medial is to the left, lateral to the right.

Somatotopic organization of MI

In the late 1950s Wilder Penfield studied the somatotopic organization of MI in patients bystimulating MI with relatively large electrodes (the patients gave their permission to do this as part oftheir surgery). He found a topographically organized representation of the entire head and body,which is schematically shown above and to the right as the classic homunculus (little man or person).The head and forelimbs are represented laterally, and the hind limb and feet medially. The corticalareas allotted to different bodily regions are of unequal size, with the degree of representationproportional to the amount of skilled movement that is required of the respective part of the body.Thus, in humans the face (particularly the tongue and lips used in speaking), thumb, and handreceive a disproportionate representation, giving these structures a correspondingly exaggeratedappearance. Movements of limbs evoked by stimulation of MI are strictly contralateral, butmovements of the face (remember corticobulbars) may be bilateral.

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The main outputs of MI are the now familiar corticospinal and corticobulbar pathways, whicharise from cells that are pyramidal in shape and lie in layer 5 of MI (most of the cerebral cortex has 6layers, 1 being most superficial and 6 the deepest). The largest corticospinal cells are called Betzcells.

Several corticospinal axons are shown in the drawing above as they course through theposterior limb of the internal capsule, the cerebral peduncle, pyramids of the medulla and pyramidaldecussation. Once in the spinal cord, these corticospinal axons comprise the LCST. You will learnmore about the internal capsule in a future lecture. For now, all you need to know is that it is 1) alarge fiber bundle that contains axons going to and coming from the cerebral cortex, 2) that it has ananterior and posterior limb separated by a genu (bend or knee) and 3) that the corticospinal tractaxons course through the posterior limb, while the corticobulbars are found in the genu.

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The experimental setup for recording from MI neurons in a monkey trained to move a lever to the rightor left. Weight can be added to resist or assist flexion or extension by attaching a weight to the lever andplacing the line over the left or right pulley.

Physiology of corticospinal meurons

Much of what we know about the physiology of cells that project into the corticospinal tractcomes from single neuron recordings in awake monkeys trained to perform specific tasks. Sincecorticospinal neurons course within the pyramids when they are in the medulla, they are also calledpyramidal tract neurons (PTNs). If a monkey is trained to move a handle by flexing and extending thewrist, then the PTNs can be studied. In the experiment shown above, a light comes on to signal to themonkey that he/she should either flex or extend their wrist. It takes a while for the brain to get the visualinformation (the light signal) to MI before the PTN will begin to fire. There are PTNs related to eitherflexion or extension of the wrist and the ones that fire depend on the light signal. If the light signal is acue to flex, the PTNs involved in flexion will fire and send information down the corticospinal tract tothe level of the medulla where the pathway decussates. The lateral corticospinal tract then continues tothe LMNs in the ventral horn that innervate either the extensors or flexors of the wrist.

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Trace A below shows the activity of PTNs recorded with a microelectrode placed into MI.Each vertical line, or spike, represents an action potential, with spikes of similar sizes coming from asingle neuron. (Note that there were 2 neurons recorded in this series, one with large spikes, andanother with smaller potentials). Trace B shows the electromyographic (EMG) activity of the wristflexor muscle related to the task, and trace C indicates the time during which the lever is moved.Note the timing relationship between the discharge of PTNs, the EMG activity of the muscle, and theactual movement of the limb. As mentioned earlier, the PTN response always precedes the EMGactivity, as expected since it is producing the motor command for that movement. Moreover, thesame neuronal activity is seen even when the movement is done repeatedly many hundreds of timeseach day (i.e., there is no adaptation or habituation of the activity).

The response of a wrist flexor PTN during flexion of the wrist with no load. Note how the cell firesbefore the muscles start to contract. Also notice that the muscles start to contract before the leveractually moves.

Functions of MI

Recent studies have shown that corticospinal cells are telling LMNs in the spinal cord whatto do. In fact, corticospinal cells are involved in conveying a lot of details about amplitude, forceand direction to the LMN. In other words, MI neurons are NOT leaving these decisions up to theLMNs, as they are too dumb. Thus, MI cells tell the LMNs (and in turn the muscles) what directionto move, how far to move (amplitude) and how much force to use. The corticospinals in MI arespecifying some pretty basic stuff instead of just sitting back and saying “grab that glass of water,arm and hand” and leaving the rest of the work to the poor LMNs. Remember, MI’s contributions tomotor control include controlling the number of muscles and movement forces and trajectories. MIcells are mainly concerned with the actual execution of the movements.

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Effects of lesions of MI

In general, with lesions of MI there will be the usual constellation of signs associated withUMN lesions. With larger lesions there will be hemiplegia that is especially noticeable for voluntarymovements of the distal extremities. Also a Babinski sign would be present if the lesion involvesUMNs of the lower limb. Remember thatskilled, independent movements of thedistal extremities (fingers) are mostaffected. The rubrospinal tract (whichalong with the corticospinal tract is acomponent of the dorsolateral system) canhelp carry out some of its more distal(wrist) limb movement tasks but isn’tmuch help when it comes to the morerefined movements of the fingers). Also,recall that the ventromedial system isfunctioning and thus the axial and moreproximal muscles are able to function.

Blood Supply of MI

Area 4 or MI receives its bloodsupply from the Middle Cerebral Arteryand Anterior Cerebral Artery. Note thatthe anterior cerebral artery feeds the partof MI that controls the lower limbs, whilethe middle cerebral artery feeds the upperlimb and head portions of MI.

Table 1. Relationship of Injured Artery and Resulting UMN Deficit

AFFECTED ARTERY AREA AFFECTED SIDE

Ant. Cerebral Lower limb Contra

Mid. Cerebral Upper limb and Head Contra

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Practice Questions

1. Which of the following statements is TRUE regarding primary motor cortex (MI)?

A. lies in the precentral gyrusB. contains corticospinal or pyramidal tract neuronsC. is found in Brodmann’s area 4D. is supplied in part by the anterior cerebral arteryE. all of the above are TRUE

2. Which of the following statements is TRUE regarding primary motor cortex (MI)?

A. head representation is lateral to that of the upper limbB. complete occlusion or rupture of the left anterior cerebral artery near the Circle of Willis willresult in an upper motor neuron (UMN) syndrome in the left legC. contains cells that fire following contralateral movementsD. supplied only by the middle cerebral arteryE. is the same as area 6

3. Which of the following statements is TRUE?

A. a lesion of MI results in hemiplegia of the ipsilateral distal extremity musclesB. the largest corticospinal neurons in area 4 are called Betz cellsC. there is considerable muscle atrophy associated with lesions of the corticospinal/LCSTD. the inverse myotatic reflex involves Ia excitatory interneuronsE. autogenic excitation is functionally associated with GTOs

4. Which of the following statements is TRUE?

A. circuits for basic patterns of locomotion are found only in the motor cortexB. the lateral corticospinal tract (LCST) is part of the ventromedial systemC. LMNs innervating axial muscles lie medially in the spinal cord ventral hornD. when you step on a tack, you activate ipsilateral extensors and contralateral flexorsE. gamma efferents innervate extrafusal muscle fibers

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5. Which of the following statements is TRUE?

A. there is a greater representation of the big toe in MI than the lipB. the lip representation lies near the lateral fissure in MIC. gamma efferents control GTOsD. pontine reticulospinal tract (PRST) fibers inhibit antigravity muscles (flexors in the arms andextensors in the legs)E. the MRST travels in the lateral funiculus

6. Which of the following statements is TRUE?

A. the key word that reflects the function of MI is “execution”B. a motor neuron pool is all of the muscle fibers innervated by a single LMNC. Betz cells are the smallest corticospinal neuronsD. a motor unit is all of the LMNs that project to a muscleE. the ruber-duber is associated with the ventromedial system

Answers1. E2. A3. B4. C5. B6. A

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MOTOR ASSOCIATION/PREMOTOR CORTICAL AREAS

The primary motor cortex (MI; area 4) was the first motor cortical area discovered. It takesrelatively little stimulating current of this cortical area to evoke a movement. This is because thecorticospinal neurons that lie in MI are directly connected to the LMNs in the brain stem and spinalcord. Movements also result from stimulation of other areas of cortex that lie immediately rostral toMI, and we call these motor association areas (supplementary motor area and lateral premotor areaon the drawing below). Since these motor association areas influence LMNs primarily via theirinputs to MI corticospinalcells, more current is neededto evoke movements.Premotor areas tell MI whatmovements to execute. Thatis, they are involved with theplanning of a movement.The total size of thesepremotor areas isconsiderably larger than area4, and thus these areas proveto be particularly importantin the control of humanvoluntary movement and areoften compromised indiseases. The two premotorareas we will talk about arethe supplementary motorarea and the lateralpremotor area. Remember,both are part of area 6.

Lateral premotor area

The lateral premotor area (PMl) lies on the lateral convexity of the hemisphere. (This samearea has also been called the dorsal and ventral premotor cortex but let’s use PMl). Many motoractions are often responses to visual or auditory cues, and cells in PMl are active during suchexternally cued movements. For instance, you see an apple (external cue) and the apple cues amovement to reach out and grasp it. The pathways involved in this sensorimotor transformation(seeing it is “sensory” and reaching for and grasping it is “motor”) are seen on the next page. Thevisual information (apple) falls first on the retina, and the retinal signal eventually reaches theprimary visual cortex in the back of the brain (area 17). From area 17, the retinal information isconveyed rostrally and bilaterally to what is termed the posterior parietal cortex (areas 5 and 7 orjust call it PPC). The PPC helps to transform visual information about the location of the apple inextrapersonal space into the direction of a reaching movement (moving a body part relative toanother body part is an example of a movement in intrapersonal space, while movements inextrapersonal space are movements executed in a three dimensional coordinate system fixed bypoints in the environment—whew!). Information from PPC is conveyed to PMl. PMl projectsbilaterally to another premotor area called the supplementary motor area (SMA). The SMAcontralateral to the apple then turns on MI to execute the reaching movement.

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Lesions of the PMl result in apraxia, which is an inability to perform a purposefulmovement (especially skilled) in the absence of any significant paralysis, sensory loss, or deficitin comprehension. For example, monkeys can be trained to turn or pull a handle depending uponwhether the handle is red or blue. If there is a lesion of PMl, the monkey can still turn and pull thehandle (MI is OK), but they can not learn to associate the correct movement with the correctexternal visual cue (colors). That is, the monkey’s deficit lies in “finding” that particularmovement when they are presented with a particular cue.

Supplementary motor cortex

The SMA is located immediately rostral to area 4, and includes the caudal one-third of themedial, dorsal, and lateral surfaces of the superior frontal gyrus. The SMA is involved in theprogramming of all movement sequences. Thus, it will be active when reaching for the apple. Thereaching movement needs to be seqeunced by the SMAs (both sides are active) after the PMl givesthe SMA the proper coordinates in extrapersonal space.

The SMAs are also involved in internally generated movements. For instance, if yousuddenly have the “internal need” to reach out and grab something in front of you (this is NOTprecipitated by an external cue), the memory/motor patterns necessary for this movement areencoded in cells within the SMA. Cells in the SMA will start to fire and, in turn, send theinstructions for the movement to the workhorse, MI, so that the right muscles are moved with thecorrect amount of force and speed (execution of the movement). Interestingly, the two SMAs areinterconnected by cortico-cortical fibers, and, unlike cells in MI, the activity in the SMA is usuallybilateral. So, during the movement of the right arm/hand, cells in both SMAs start firing, but onlycells in the left MI fire. Cells in SMA fire before those in MI but both areas will be firing duringthe movement. Interestingly, if you just imagine reaching out and grabbing something in front ofyou, cells in SMA will increase firing (bilaterally).

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Monkeys with lesions of SMA have impaired ability to carry out a motor act that they hadlearned earlier, i.e., apraxia. There is NO paralysis, only problems in planning. For instance,monkeys who had learned how to open a latch box by opening three different latches in a particularsequence are greatly impaired following either uni- or bilateral lesions of SMA. Remember, the twoSMAs are interconnected across the midline. A lesion of one affects the proper functioning of theother. For instance, a lesion of the right SMA means the right MI is not getting proper planninginformation. This affects the left arm/hand. In addition, the left SMA has lost its input from theright SMA so the left SMA input to the left MI is not normal. In the end, bimanual coordination isaffected. So, SMA=bimanual coordination.

The role of the SMA in sequencing of movements is further exemplified by data fromexperiments in which monkeys were trained to do three movements; push, pull or turn amanipulandum (a joystick like you use with video games) in four different orders (e.g. push, turn,pull, would be one choice). Cells were found that increased their firing to a particular sequence(turn, push, pull but NOT turn, push, turn). So, SMA is big on sequencing and especially internallyor memory-guided movements. Remember, in the above task with the manipulandum, theappearance (an external cue) of the apparatus gave no clue as to which movement to make first, oreven the order of the movements.

You can see that “planning” premotor areas are more specialized than the “execution” cellsin MI and are important for selecting and controlling movements made in particular behavioralcontexts, such as when the direction of a movement that needs to be made must be remembered(internally generated) or selected on the basis of an available external cue (externallyguided). The movement is the same but the areas that activateand tell MI which muscles to turn on (execution) differ.

STUDIES OF REGIONAL CEREBRAL BLOOD FLOW

Additional information about the activation of corticalareas during different types of movements can be gathered bylooking at the regional cerebral blood flow (rCBF) duringparticular tasks. In these studies an increase in rCBF means thecortical area is active. For example, it has been shown that fastflexions of the index finger against a spring loaded movable

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cylinder increases rCBF in the finger area of the contralateral MI and SI (primary somatosensoryarea). The increase in rCBF in the somatosensory cortex reflects the activation of peripheralreceptors caused by the rapid finger flexions. Most important, no higher cortical motor areaexhibited an increase in rCBF in this task. Thus, MI (and its associated LMNs), is the only motorcortical area involved in the control of simple repetitive movements.

When a person is asked to carry out from memory more sophisticated and complexmovements, then other motor cortical area(s) besides MI exhibit an increase in rCBL. Such acomplex movement is where the thumb, in quicksuccession, must touch the index finger twice, themiddle finger once, the ring finger three times,and the little finger two times. Then, with thethumb opposed to the little finger, the wholesequence is reversed!!! This is a complex seriesof movements guided by memory and carried outin intrapersonal space (moving a body partrelative to another body part). Again, there isincreased rCBF in the contralateral MI and SI (abigger area since more fingers are involved), butin addition there is increased rCBF within SMAbilaterally. Thus, a complex task involving asequence of remembered movements requiresthe activity of both SMAs and the MIand SIcontralateral to the active fingers. Of course, MIis needed for the execution. Interestingly, if youjust imagine the finger sequence movement, SMAlights up bilaterally, but not MI.

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Practice Questions

1. Which of the following statements is TRUE regarding premotor association areas?

A. include the lateral premotor (PMl) area and supplementary motor area (SMA), both of which arein Brodmann’s area 4B. lesions result in contralateral paralysisC. thought to be involved in the execution rather than the planning of a movementD. the SMA is partly on the medial wall of the hemisphereE. SMA is supplied entirely by the middle cerebral artery

2. Which of the following statements is TRUE regarding the shaded area shown below?

A. a lesion would result in weaknessB. a lesion would result in atrophyC. SMA is damagedD. the lesion involves areas 3, 1, 2E. there is apraxia

3. Which of the following statements is TRUE regarding the shaded area shown below?

A. blood supply is predominantly fromthe anterior cerebral arteryB. a lesion here will result in spasticityand paresis only in the contralateral lowerlimbC. area contains Betz cellsD. axons of these cells course through thecontralateral pyramidE. lesion would result in apraxia

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4. Which of the following statements is TRUE regarding the shaded area shown below?

A. includes SMA and area 4B. lateral part is involved in movements basedupon externally generated cuesC. lateral part of this area will show increasedrCBF when imaging a complex sequence ofmovementsD. this area is involved in motor execution andis at a relatively low level in the hierarchy ofmotor controlE. lesions result in an increase in CK in blood

5. Which of the following statements is TRUE regarding the shaded area shown below?

A. this is the supplementary motor area andcorresponds to Brodmann’s area 6B. is supplied by anterior cerebral arteryC. receives input from posterior parietal cortex(PPC)D. cells fire when repeated flexing fingeragainst a springE. is involved in internally generatedmovements

6. Which of the following statements is TRUE?

A. LMNs innervating extensors lie dorsal to those innervating flexors in the spinal cord ventral hornB. the part of the stretch reflex involving inhibition of antagonist muscles is monosynaptic?C. gamma dynamic stimulation leads to a “pause” of Ia firing during contraction/shorteningD. a part of SMA receives its blood supply from the anterior cerebral arteryE. Ib fibers directly inhibit LMNs innervating the muscle that is attached to the tendon/GTOinnervated by the Ib fiber

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7. Which of the following statements is TRUE?

A. fibers of cells in MI travel in the ventromedial pathwayB. the medullary reticulospinal tract (MRST) excites antigravity musclesC. the pontine reticulospinal tract (PRST) terminates in the lateral parts of the spinal cord greyD. buttoning a shirt button would be possible following a lesion of the dorsolateral systemE. MI cells are like the cab dispatcher, while LMNs are like the cab driver (think!!)

8. Which of the following is TRUE?

A. apraxia can result from blockage of the anterior cerebral arteryB. area 4 is larger than area 6C. autogenic excitation is associated with the inverse myotatic reflexD. MI is higher in the motor hierarchy than SMAE. autogenic inhibition is associated with the stretch reflex

9. Which of the following associations is TRUE? (the big integrator!)

A. LMNs—spinal cord, brain stem, input from contralateral MI, input from Ia inhibitory interneurons,input from Ib inhibitory neurons, input from interneurons in contralateral spinal cord, “workers” insteadof “planners,” ventral=extensors, lateral=distal muscles, some fire during fast flexions of the index fingeragainst a spring loaded movable cylinder, lesion=apraxiaB. ventromedial system—vestibular nuclei, PRST, MRST, medial spinal cord grey, ventral funiculus,balance and posture, axons innervate many spinal levels, digital dexterityC. dorsolateral system—ruber-duber, CST, lateral ventral horn, lateral funiculus, digital dexterity, axonsinnervate single or limited spinal levels, MVSTD. MI—force, fires before LMNs, lip area larger than shoulder area, input from SMA, blood supply fromanterior and middle cerebrals, area 4, lesion=atrophyE. premotor association areas—area 6, PML, SMA, anterior and middle cerebral arteries, project to MI,planning, lesion=apraxia

Answers 1. D 2. A 3. C 4. B 5. C 6. D 7. E 8. A 9. E.

SELF LEARNING Monday, March 1, 11AM-12

This is “MOTOR SYSTEMS WEEK” AND IT IS VERY TOUGH SLEDDING! Use this critical hour to watch the reviewCD-ROM on Motor Systems and try to get ahead on the practice questions. FINISH ALL OF THEM THROUGH“MOTOR ASSOCIATION PREMOTOR CORTICAL AREAS” BEFORE LUNCH!

DON’T GET BEHIND THIS WEEK!!!!!Also, if you have time, go to neuroanatomy.wisc.edu and look over the self-learning www sites for Parkinson’s, TouretteSyndrome and Deep Brain Stimulation. Tonight, look over “old stuff” cases: 1) muscular dystrophy, 2) myasthenia gravis,3) Guillain-Barre, 4) S1 radiculopathy, 5) amyotrophic lateral sclerosis (ALS), 6) Brown Sequard syndrome (spinal cordhemisection), 7) facial colliculus-vestibulo-cochlear, 8) lateral medullary (Wallenberg’s) syndrome, 9) acoustic neuroma, 10)Weber Syndrome, 11) syringomyelia and 12) subacute combined systems disease. Begin to look over those quirky and neverending power points listed under “Motor Systems power point for quiz #6.”

PLEASE—DON’T WASTE THIS PRECIOUS HOUR OF SELF STUDY