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Neuro: 1:00 - 2:00 (Lecture A)
Scribe: Brittney Wise
Monday, February 16, 2009
Proof: Laura Adams
Dr. Gamlin
Spinal Reflexes Lecture
Page 1 of 7
Introduction: This is the lecture is a combination of last Wednesday February 12 from 2-3 and today. Also note that
when he is talking about the 1A neurons he sometimes says interneuron. This may need further clarification.
I.
II.
III.
IV.
V.
VI.
[S1] Spinal Reflexes Lecture (Figure 35-3)
a. We are going to go over basic spinal cord mechanisms, motor muscle recruitment, spinal reflexes, descending
influences, and cortical control.
b. The first part of the lecture will cover the spinal cord mechanisms, how we generate movements and as your
familiar muscles act as agonist/antagonist pairs. You can only actively contract muscles, you can’t actively relax
muscles, so they always work as agonist/antagonist (ex// biceps and triceps).
c. After you contract your muscles, the thing for you to do is to keep track of where the movement is going. It is
pretty clear that if you want to contract you arm, you can close your eyes and contract your arm, but if you don’t
keep track of where your fist is going you might hit yourself.
d. However, you do have sensory receptors in the muscles and the joints which keep track of the sensory
consequences of movement. So, you have ongoing sensory feedback in a dynamic fashion.
e. This forms the basis for spinal cord reflexes. These reflexes allow you to stand somewhere despite the fact that
you might be doing nothing. If you go onto one leg you can typically generate the right reflexes so that you don’t
fall over. If someone were to tap you on your knee and stretch the tendon to your quadriceps, you would show
a knee jerk response. Those same reflexes are used to make sure that you don’t collapse when you are tapped
on the knee. In most motor control, you also want to generate voluntary control of movement and so we will talk
later about how the motor cortex, can emphasize the motor cortical control of voluntary movement, the sorts of
movements that you need to make to take notes and write and things of that nature.
[S2] Basic Functions of descending tracts
a. This is just a hierarchal version of the same picture on slide one, with some of the pathways labeled in a little
more detail (ex// corticospinal, reticulospinal).
[S3] Motor Neuron-Muscle Relationships
a. Now we are going to focus in on spinal cord, muscle contraction, and sensory feedback.
b. The 1st thing to note is that within the spinal cord there is an approximate topography within the ventral horn so
that the motor neurons that innervate the proximal musculature are located medially and those that innervate
the distal musculature tend to be located laterally.
[S4] The Motor Unit
a. An important concept is that of the motor unit. An individual motor neuron innervates a number of muscle
fibers. The combination of motor neurons and the muscle fibers that it innervates is referred to as the motor
unit. So when a motor neuron fires action potentials it will cause contraction in all of the individual fibers that it
innervates and this is again referred to as a motor unit.
[S5] Regulation of Muscle Force: Terminology
a. What is terminology of motor unit activity? If you stimulate a motor unit or the motor neuron with a single action
potential you will generate a single twitch in the muscle.
b. If you stimulate at a low rate you get individual muscle twitches.
c. If you stimulate at a greater rate, those twitches summate. Once you reach a certain critical frequency, this unfused tetanus (called this because you still have some twitches) becomes what you call a fused tetanus. This
term “tetanus” here is not a clinical term; it just refers to tetanic contraction that is smooth and consistent, unlike
the disease.
[S6] Regulation of Muscle Force (Figures 36-9 and 36-10)
a. Mammals including humans have different types of motor units that can be categorized into 3 major categories:
i. Slow twitch or slow motor unit:
1. long contractions times
2. 50-100msec for each twitch
3. generates small amounts of force (maximum tetanic force is in the range of a few grams)
4. great thing about it is that it never fatigues
5. these allow you to stand up for an hour and not have your legs collapse
ii. Fast Fatigue resistant:
1. they will fatigue but will do this slowly over a few minutes
2. advantage over the slow motor unit is that they generate intermediate amounts of force in the range
of about 10-50g of force (each of the motor units)
iii. Fast Fatigable Motor Unit:
1. generated the maximum force of the motors units between about 40-120g of force
2. these fatigue extremely rapidly
b. (These last 2 are both fast twitch motor units which means that twitch time is somewhere around 40msec; these
have a relatively fast twitch)
Neuro: 1:00 - 2:00 (Lecture A)
Scribe: Brittney Wise
Monday, February 16, 2009
Proof: Laura Adams
Dr. Gamlin
Spinal Reflexes Lecture
Page 2 of 7
VII. [S7] Regulation of Muscle Force (Figure 16-5)
a. This is showing a diagram of the 3 different types. It is showing the twitch characteristics and the amount of
force that they can generate.
b. As you can see the fast fatigue resistant have a peak time to twitch of about 50msec. This is fast fatigue and
fast fatigue resistant and the slow ones are slower. Notice these also generate a lot greater force, intermediate
and very little force.
c. If you look at their fatigue rate over time for these 3 different types of motor units, slow, fast fatigue resistant,
and fast fatigable:
ii. Let’s look at the time frame of 50min. The slow fibers will generate force continuously for an hour and will
never fatigue.
i. The fast fatigue resistant ones, they are still generating substantial amounts of forces after 5-6min. After
about an hour they fatigue. They are fatigue resistant but they still fatigue!
ii. The fast fatigable are the ones that let you generate the most force and they fatigue in about 30 seconds.
These are the ones that a power lifter will try and use to put extremely heavy weights above their heads.
d. Metabolism is also appropriate for their different fatigue rates:
i. Slow fibers:
1. rely on aerobic metabolism
2. they are myoglobin rich
3. they have dense capillaries, so they generate no oxygen debt
ii. Fast fatigable:
1. rely on anaerobic metabolism
2. they have very few mitochondria
3. they rely on glycogen stores which convert to lactic acid
4. they build up large oxygen debt
e. This has consequences on motor activity. You might ideally want a very strong muscle fiber that never fatigues,
but metabolically that’s just not feasibly. So the motor system has these different types of motor fibers available
to them: slow, fast fatigue resistant, and fast fatigable.
f. The other thing that you should note is that these motor units are characterized by specific cell body size:
i. The motor neuron of the slow motor units can have small cell bodies
ii. The fast fatigable motor units tend to have large cell bodies
iii. Size matters when it comes to how you activate your motor units in an orderly fashion
g. One of the advantages to having a motor unit that it slow and can only generate a slow amount of force is that
you may only want to generate a few grams of force. For example, if you want to pick up a pencil you don’t
want a lot of force, maybe a few grams, vs. if you want to pick up something heavier.
h. When you want to lift something with a certain weight up, you issue a command to the spinal cord to lift this
weight. At this point your motor units are sort of feeble and you’ve only got the slow ones activated. You have
to actually recruit in the appropriate motor units to lift the chair.
VIII. [S8] Motor Unit Recruitment “Size Principle” (Figure 36-14)
c. The spinal cord actually recruits in based on a size principle.
i. The slow come in first because they have smaller cells bodies, then the fast-fatigue resistant ones come in,
and then if you need them, the fast-fatigable will come in if you need to generate many kilograms of force.
d. The drawing is a logarithmic scale with force going from 1 gram to 1 kilogram. If you only have this number of
motor units active you can generate literally a few grams of force. If you then recruit all of the motor units, you
can generate multiple kilograms of force. So, you can go from generating very small, precise movement in the
gram range up to lifting much heavier weights.
e. You have this way of recruiting in motor units to generate precise forces.
IX. [S9] Muscle Stretch Receptors and Golgi Tendon Organs (Figure 37.1)
c. Assume that you are making these movements and your muscles are contracting, how do you keep track of that
muscle contraction?
d. There are 2 major players we should take into consideration when we are talking about the control of muscle
contraction.
i. The 1st is the so-called muscle stretch receptors.
1. Muscle stretch receptors are modified muscle fibers.
2. These are referred to as the intrafusal fibers as opposed to the muscle fibers themselves which are
referred to as extrafusal fibers.
3. Intrafusal fibers are interesting in the sense that they have a central region which has what is called
an annular spiral sensory ending on it. This is the 1A afferent and this central region is sensitive to
muscle stretch.
Neuro: 1:00 - 2:00 (Lecture A)
Scribe: Brittney Wise
Monday, February 16, 2009
Proof: Laura Adams
Dr. Gamlin
Spinal Reflexes Lecture
Page 3 of 7
4. In addition, they have polar regions which receive innervation from, in humans, gamma motor
neurons. These are separate from the alpha motor neurons that we just talked about which go to the
extrafusal fibers and make up the motor neurons.
e. The gamma motor neurons in the spinal cord, instead innervate the polar regions of the stretch receptor. We
will see how these gamma motor neurons work with this stretch receptors central region to maintain a signal
from the 1A afferent about muscle length.
f. In addition there is a golgi tendon organ which is a specialized receptor in the tendon and that keeps track of the
tension or the force that’s being generated between the muscle in the tendon.
g. The golgi tendon organ tells you about the forces that are going to be exerted on the tendon.
h. The stretch receptor tells you about how much stretch is going on.
i. The 2 play separate roles: one is keeping track of stretch the other if keeping track of the tension or the force at
the tendon.
X. [S10] Figure 37-2) He skipped this slide (but then he said this about it). This slide is just to say that the muscle
stretch receptor is a little more complicated that just a central annular spiral ending, it gives rise to both 1A and 2
afferents. The only thing that we need to emphasize is that there is a polar region where the gamma motor neurons
go. He is not going to ask us to memorize all this stuff about dynamic and static nuclear bag fibers.
XI. [S11] The Spinal Cord Circuitry Underlying Muscle Stretch Reflexes
c. This is a simple example of how the stretch receptor contributes to the normal mono-synaptic stretch reflex.
d. The basic principle is that the blue axon here is supposed to be arising from this muscle spindle, a stretch
receptor running up coming up through the dorsal horn making mono-synaptic connections on the motor
neurons of the homonymous muscle.
e. The 1A afferent feeds back, comes in on the dorsal horn, and makes contact on the motor neurons of the
homonymous neuron and about 70% of the synergistic motor neurons.
f. What happens for example when you are holding a glass of soda, and your arm is deflected downwards? The
biceps are stretched so the muscle stretch receptors detect that the muscle is being stretched and this causes
an increase in firing rate of the 1A afferents.
i. This increased firing rate, then directly relays to the motor neurons of this muscle causing those motor
neurons to increase their firing rate, causing this muscle to contract, and causing the muscle to return to its
original position
g. He used a classmate as an example to show that with unexpected force the arm goes down slightly and then
comes back up to its original position. That was a mono-synaptic stretch receptor reflex and probably with a
latency of about 12 seconds. This is the basic reflex and keeps things in position unless you issue a voluntary
command to change it.
XII. [S12] Role of Gamma Motor Neurons
c. The great thing about stretch receptors are that they are very precise and accurate. The bad thing about them
is that they are only sensitive over a relatively small amount of muscle length change.
d. (People tend to have problems with this cartoon that represents the role of the gamma neurons). The cartoon
assumes that the muscle spindle and the extrafusal muscle are separate, and then it suggests that the idea in
part B is what would happen if we just stimulate the alpha motor neurons and cause the extrafusal fibers, the
main part of the muscle, to contract. This cartoon shows that if you just do that, then the muscle spindle will
become limp and the central region is no longer stretched and it no longer reliably signals muscle stretch to the
spinal cord. You can see here that the muscle has contracted but that the afferent signal has gone away
because the central region does not know the tension on it.
e. Diagram C shows you what happens if you activate alpha motor neurons and gamma motor neurons at the
same time. The alpha motor neurons cause the main extrafusal fibers to contract, but the gamma motor
neurons then cause these polar regions to contract as well and they maintain the central region, this annular
spiral ending, the tension is maintained there so that it can keep up with muscle length even though the muscle
contracts.
f. So, the gamma motor neurons basically contract the polar regions of the muscle stretch receptor so that it
always has a certain amount of stretch so that it always signals to the spinal cord, the correct muscle length.
g. Without the gamma motor neurons, you would not get a very reliable signal from the 1A afferents.
XIII. [S13] Figure 37-11)
a. This is another diagram of the mono-synaptic stretch reflex using the 1A afferent just to re-emphasize with a
slightly better diagram.
b. Here is a muscle spindle (referring to the cartoon) and the muscle spindle is obviously much smaller than this.
The 1A afferent comes back, makes mono-synaptic connections on the homonymous the motor neurons for the
homonymous muscle and it contacts about 70% of the motor neurons for the synergistic muscles. And then
more importantly it also makes a connection between a 1A inhibitory interneuron. This afferent that is activated
will cause the antagonist or the motor neurons of the antagonist to decrease their firing rate.
Neuro: 1:00 - 2:00 (Lecture A)
Scribe: Brittney Wise
Monday, February 16, 2009
Proof: Laura Adams
Dr. Gamlin
Spinal Reflexes Lecture
Page 4 of 7
c. Now let’s look at passive stretch of this muscle and we are going to look at both the excitation to the
homonymous muscle, the synergistic muscle, and the antagonistic muscle.
i. When there is a passive stretch in the muscle the increase in the 1A afferent activity results in increased
activity in the motor neurons to these homonymous muscles and the synergists, which will cause
contraction of these muscles.
d. In addition, there is inhibition of the motor neurons going to the antagonistic muscle, through the 1A inhibitory
neurons causing the innervation to this muscle to be reduced and for this muscle to be relaxed.
e. This is exactly what you want: these 2 muscles contract and the antagonist relaxes it a little bit.
f. This is the di-synaptic inhibitory response to the antagonist.
XIV.
[S14] Figure 38-2)
a. This is just emphasizing the role of the 1A inhibitory inter-neuron.
b. Under normal circumstances the 1A inhibitory interneuron ensures that when this muscle contracts the
antagonist relaxes.
c. There are some cases where you want to co-contract, or contract both the antagonist and the agonist, in which
case the corticospinal pathways can influence of the 1A inhibitory interneuron.
d. Why would you want to contract both you biceps and triceps at the same time? Sometimes you want to do this
so you can hold your arm in a rigid form. You will sometimes co-contract muscles to stabilize the joint.
e. If you are a surgeon doing relatively precise procedures, you will feel sore and tired all over for example your
back muscles because they actually co-contracted to stabilize all the shoulder, elbow, and rist joins.
f. Co-contraction is under voluntary control by these descending pathways.
g. He is not going to go into too many details about the spinal circuitry but it is just to say that the simple reflexes
are fine but you have ways of controlling the relative agonist/antagonist activity using voluntary control of
movement.
XV. [S15] Figure 38-4)
a. In addition to those simple mono-synaptic stretch reflexes, the golgi tendon organ also plays an important role.
He just wants to focus on the circuitry just around the golgi tendon organ circuit. This gives rise to the 1B
afferents.
b. 1B afferents don’t make mono-synaptic connections; instead they make a di-synaptic excitatory connection to
the antagonistic muscles. There is an excitatory interneuron from the 1B afferent to the extensor muscle and
there is a 1B inhibitory interneuron to the motor neurons of the homonymous muscle. This is actually an
inhibitory circuit so that when the golgi tendon organ increases activity the net result is inhibition of the motor
neurons going to that muscle, and excitation motor neurons going to the antagonistic muscle.
c. The result from this is that when the tension increases in the tendon, significant increases in tendon forces will
cause this contraction to be reduced and the agonist to contract. This is a system that prevents you from
exerting too much force on the tendon.
d. In the very simplest case, it’s a reflex for making sure that when the forces on the tendon are too high, this
muscle relaxes. This is the protective role that this plays.
e. When you look in more detail at an animal that is actually making voluntary movements, under those conditions
during voluntary movements, it actually may help this muscle contract. During reflexes, it’s playing a role to
protect the tendon against too much tension.
f. He used an ex// referencing arm wrestling. At some point during arm wrestling the tension on the tendon gets
too great that your arm just collapse. This is called the jack knife response. This is where in theory you are
almost dis-inserting your tendon. This is almost a protective mechanism: at some point the signals from the
golgi tendons organ override your attempt to keep contracting.
g. If you look at these in more sophisticated voluntary control that these reflexes may actually work together to
allow you to use that force information to actually generate meaningful forces in the muscle for voluntary control.
XVI.
[S16] Flexion Withdrawal Reflex / Crossed Extensor Reflex (Figure 38-6)
a. The previous slide was a single reflex. This slide has a slightly more sophisticated reflex: a polysynaptic reflex.
b. This is referred to as the flexion withdrawal and crossed extensor reflex.
c. Imagine you are walking on a beach, barefoot, and you step on a piece of glass with your right foot. Your first
move is to say ouch (a pain pathway, a polysynaptic pain pathway, a nocioceptive pathway). A-delta fibers from
the foot get to the spinal cord very quickly. The first response is a flexion withdrawal (withdrawel of foot from the
pain) and basically involves a flexion of the stimulated leg and the extensor should then relax. Flexor contracts
and extensor relaxes, that is the flexion withdrawal. The other thing is that if you only do that you will collapse
because your other leg is not supporting you. The crossed extensor reflex causes the extensor muscles to
extend and inhibits the flexor muscles. So, one leg will flex and the other will extend and will take your weight
so you don’t fall.
d. This diagram is mainly for legs but it can be used for arms. The stimulated leg will flex and the other leg will
extends.
Neuro: 1:00 - 2:00 (Lecture A)
Scribe: Brittney Wise
Monday, February 16, 2009
Proof: Laura Adams
Dr. Gamlin
Spinal Reflexes Lecture
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XVII. [S17] Basic Functions of descending tracts (same as slide #2)
a. So, what about the descending pathways that control the spinal cord motor neuron/unit system.
b. He’s gonna focus on 1st just on the brain stem descending system which we should know: the reticulospinal,
vestibulospinal, and the rubriospinal tracts.
XVIII. [S18] Descending Motor Pathways
a. Medial brainstem pathways and lateral brainstem pathways. He’s gonna give us a quick sense of the vestibular
spinal tract, the reticular spinal tract, the tectal spinal tract (mainly goes to cervical levels and is involved in head
movements), the vestibulospinal, and then the rubrospinal tract (terminates more laterally).
b. These medial pathways are usually bilateral and occur in postural control.
c. The lateral pathways are usually more medial and have more lateral voluntary control through the rubrospinal
tract.
XIX.
[S19] Figure 39-7
a. Just to reiterate a little bit. We will talk about the vestibular system/imbalance and the lateral and medial
vestibular nuclei a little later on.
XX.
[S20] Figure 39-8
a. The reticulospinal tract coming here from the medullary reticular formation and terminating here basically in this
lateral reticulospinal tract here, again, just remember these pathways.
b. This also shows that the motor cortex, pre-motor areas, project into the reticular area to modulate the
reticulospinal tract.
XXI.
[S21] Basic Functions of descending tracts (same as slide #2 and 17)
a. What about the corticospinal tract? This is intimately involved in voluntary control of movement.
XXII. [S22] Figure 35-7
a. You should be familiar with the fact that there are 2 corticospinal tracts. The one that you are probably the
most familiar with is the lateral corticospinal tract which originates from pre-motor motor cortex and there is
some projection through the pyramidal deccusation. This is a somatosensory projection.
b. Lateral corticospinal tract terminates laterally as well as a small component located here in the dorsal horn that
is related to the somatosensory modulation. It crosses to the levels of the pyramids which is clinically very
important.
c. The ventral corticospinal tract is a motor tract, motor cortex pre-motor cortex, does not cross at the levels of the
pyramids but goes bilaterally to the medial part of the ventral horn. It also gives off collaterals to the medial
brainstem pathways.
d. Again, you should be relatively familiar with these.
e. When we are talking about voluntary movements we will be mainly talking about the lateral corticospinal tract.
XXIII. [S23] 6 Cross sections in a line
a. This is a diagram of the lateral corticospinal track showing its trajectory here and crossing at the pyramidal
deccusation and terminating here in the ventral horn.
b. The first thing that you should note is that the cell body is in the motor cortex and the terminal is in the ventral
horn. Very few terminate on alpha motor neurons. The only cells in the motor cortex that actually project to
motor neurons are motor units for the digits. Those are making mono-synaptic connections for digit control
found in humans and non-human primates. Most of these terminations are not mono-synaptic onto alpha motor
neurons.
XXIV. [S24] Figure 40-1
a. Within the motor cortex there is a somatotopic representation. This is in some ways complimentary to the
somatosensory representation with some slight differences.
b. If you look at the map of the motor cortex, the toes, feet, legs are represented here almost like you hooked your
knees over the cortex right here. The representation is proportional to the density of motor neurons that you
have in the spinal cord. So the representation around the thumb and fingers is very dense and expanded.
There is a large representation of the lips, jaws, tongue, swallowing, and areas of the vocal cords. Humans
have this very large part of the motor cortex given over to vocalization.
c. Damage to certain areas will affect specific motor functions.
XXV. [S25] Figure 17-11
a. The question is what exactly do these cells in the motor cortex do for the control of muscles in individual limbs?
b. This concept is a little difficult to explain so if you don’t get it don’t worry about it. Many experiments have been
done to look at control of limb movement have in trained non-human primates.
c. This is the Resus monkey brain. People will record from individual cells in the motor cortex and then the they
will record electrical activity, the EMG activity, in the muscles that control, say the digits in this case. The
approach is to record an action potential in a cortical neuron, a corticospinal tract neuron. Every time there is an
action potential in this corticospinal tract neuron, you look at the electrical activity here in the muscles. Basically
if you were to average enough of those action potentials, and were to get over 9,000 action potentials, what you
Neuro: 1:00 - 2:00 (Lecture A)
Scribe: Brittney Wise
Monday, February 16, 2009
Proof: Laura Adams
Dr. Gamlin
Spinal Reflexes Lecture
Page 6 of 7
find is that every time this corticospinal tract neuron fires and action potential, there is a very small change in the
electrical activity of the muscle of the hand.
d. So this corticospinal tract neuron is contributing, a small amount yet a significant contraction in this muscle.
e. You can use this approach to see how cells in the motor cortex control the individual muscles in say the wrist or
the hand.
XXVI. [S26] Corticospinal tract neurons (upper motor neurons) have multiple branches within the spinal cord
i.e. not a point-to-point topography (Figure 40-5)
a. Using that approach you can first of all look at a corticospinal tract neuron anatomically using anatomical traces.
b. The take-home message is summarized by saying that the corticospinal tract neurons have …
i. multiple branches within the spinal cord
ii. it’s not a point-to-point topography
iii. they terminate usually in a number of different motor neuron pools
iv. this one terminates in 3 different levels of the spinal cord.
c. The overall concept of corticospinal neurons is that it is recording from a number of different muscles and
showing that it actually correlates with about 4 excitatory muscles controlling the wrist.
d. Take Home Message: A given corticospinal tract motor neuron influences more than one muscle and they are
usually agonists (have the same action) and they are usually at the same joint (such as the wrist or digit).
e. There isn’t a point-to-point relationship between corticospinal tract neurons and a muscle contraction it’s more of
what we would call “population coding”.
f. But the population is organized in a topographic fashion and it tends to excite at muscles that are agonists.
XXVII. [S27] Corticospinal Tract Neurons Code for Force (Figure 40-6)
a. What do these corticospinal tract neurons do? They were thinking years ago that the motor cortex might be like
an executive. They were thinking like 30-40 years ago that the motor cortex would send out a command to
make a movement and it didn’t really care about the outcome of the movement it just sent out the command and
the spinal cord took care of it and the appropriate reflexes in the spinal cord would take care of it and that was
fine and so the motor cortex was just involved in motor planning.
b. Back in the 60’s a series of experiment in non-human primates showed that when you trained a monkey to
make specific movement, the neurons in the motor cortex modulated their firing based on how much force the
monkey had to produce on the movement.
c. There are 3 examples here of the monkey making a movement, and we are going to try and figure out why it
suggests that the corticospinal tract neuron is actually coding for the force.
d. So the first thing was that this monkey was trained to make wrist movements and they would add a weight so
that it was a little harder for the monkey to flex.
e. The 3 categories were that you either had a flexor load, an extensor load, or no load.
i. no load, the corticospinal neuron activity correlated with the flexor activity of the muscle
ii. flexor load so that the flexor increased its force of contraction, the corticospinal tract neuron, increased its
activity
iii. no flexor load, basically when there was an extensor load, so you had to do no work to do any flexion, the
corticospinal tract neuron was not active.
f. Take Home Message: The neurons in the motor cortex modulate their firing rate based on the force you need to
generate. They are not just a passive executive, they really care about how much force you need to generate.
g. This has implications for strokes of the motor cortex. If you have damage to the motor cortex it can have
obviously profound effects on the signals that you can generate for the appropriate force.
XXVIII. [S28] Figure 40-11
a. In addition, if you record from an animal making a normal movement back and forth, and you unexpectedly drop
a load on that movement, when you look at the activity in the motor cortex you see that there is a basic monosynaptic reflex at the spinal cord level.
b. When you actually get that unexpected load presented there is an activity in the motor cortex within about
50msec that is related to the unexpected load and the motor cortex codes for the corrective movements that are
needed to be made to correct that.
c. So basically, the motor cortex is intimately involved in controlling the ongoing movement. It’s not just sending a
passive command to the spinal cord, it’s updated on a msec frame and it keeps track of where the limb is and it
keeps track of making corrective movements.
XXIX. [S29] Figure 17-12
a. Motor cortex codes for force and for the direction of the movement.
b. Corticospinal tract neurons code for which way the movement will go. They don’t code in a precise fashion, it’s a
population coding.
c. If you record from 1 corticospinal tract neuron, when the monkey moves the lever to the left it increases activity,
and when the monkey moves the lever to the right the cell it decreases activity.
Neuro: 1:00 - 2:00 (Lecture A)
Scribe: Brittney Wise
Monday, February 16, 2009
Proof: Laura Adams
Dr. Gamlin
Spinal Reflexes Lecture
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d. If you record from this in 8 different directions of movement, here is the activity of the cell. You can tell that if it
was firing at a high rate the monkey is moving in one direction and if it is firing at a low rate you know he is
moving in the opposite direction.
e. It turns out though that if you record say 70-80 corticospinal tract neurons and average these responses
together, then you can predict from the population average the precise direction of movement.
f. So the argument that individual corticospinal tract neurons do not code for the direction of movement very
precisely (they kind of give you an approximate direction) but the population of neurons in motor cortex does
precisely code for the direction of movement. This again, is called a population coding.
g. So, unlike the visual system where you have a point-to-point topography, this is a population code which in
some ways is useful because if you lose a few of these corticospinal tract neurons, you can still code 99.9%
precisely. This also means that in fact you can probably get more recovery from damage to motor cortex than
say visual cortex damage.
He said the following was to get us read for our cerebellar and basal ganglion lectures:
XXX.
[S30] Figure 40-4
a. In addition to the primary motor cortex and it’s descending influences on the spinal cord, there are also 2 areas
that are involved, the basal ganglia circuit through the supplementary motor cortex and the cerebellum is
involved in motor control.
b. You may notice that in the supplementary motor cortex, pre-motor cortex, and primary motor cortex are
important players here and just to give you a sense of how they might differ in their activity for different
movements, he wants to show us the next slide.
XXXI. [S32] Figure 40-14
a. This is a study in a human, a pet study. They were looking at activation during 3 different types of movement:
i. The 1st movement was a simple finger flexion and the activity was seen in primary motor cortex and
somatosensory cortex
ii. The 2nd movement was a sequence of finger movements and this activates the primary motor cortex,
somatosensory cortex, but in addition because the individual is having to plan a sequence of movements
one of the areas that is activated is supplementary motor area.
iii. The 3rd movement was to mentally practice the movements that they made but don’t use your fingers. You
can then see that the supplementary motor area is just as active for the imaginary movement.
b. So, somehow there is activity there that is involved with motor planning preparing for the movement, but it
doesn’t show up here in the motor cortex of the somatosensory because there really is no movement.
c. Understanding how these different areas, the supplementary motor areas, the pre-motor cortex, and the motor
cortex, interact to plan for movements and execute complicated sequences of movement is a challenge that
people are still working on.
d. You will hear a little more about the sequence of movements when we talk about the basal ganglia lectures and
the cerebellar lecture will tell you more about the specific dynamics and motor plasticity.
[end time = 51:55 min]