© 2010
  University of Idaho
  All rights reserved.
  Psychology Dept.
  University of Idaho
  Design - P&D  CTI

Within motor control systems you also have (as we see in slide three) three levels of control.  The first level of control is conducted by the cortex and it is the highest level of control. Here you have three major systems, the primary motor cortex which is called Broadman’s area four, the lateral pre-motor area which is Broadman’s area six, and supplementary motor areas which are also part of area six and also part of area eight.  In addition, we have other systems.  We have the brain stem (Medulla, Pons, etc) which are going to be extremely important in motor control, cerebellar structures, and we have the system within the spinal cord related to the dorsal horn and internuncial neurons, and reflex arcs that we will talk about a little bit later.

Finally, there’s a third level of control.  These relate to other structures that are going to have major impacts.  The first of these is the cerebellum which is extremely important in recall of motor systems.  It’s also involved with (as we talked about earlier), some of the rapid movements that we have.  Finally, we have structures such as basal ganglia structures which are extremely important and also helping to smooth out things, and are very, very important in slower movements.  For example, if you were taking notes today, the basal ganglia would be extremely important in helping you do that.

So let’s talk about these structures in more detail.  The first of these structures is called the cortex, and we show this on slide four.  The cortex has several major structure areas that are extremely important in voluntary motor movement, and in motor control.  The first of these structures, as we see in slide four is the precentral gyrus.  This is also called your primary motor cortex and it’s full of the major structures and neurons called Pyramidal neurons.  Pyramidal neurons have very long axons and extend into the spinal cord.  Anterior to the precentral gyrus, we have what is called the pre-motor area that contains superior and middle frontal gyrus.  Finally, we have supplementary motor areas which include structures within the superior frontal gyrus, and tertiary motor areas which include both the middle and inferior frontal gyrus.  Ultimately, all of these systems are going to work together, act on systems, and send information to systems that are going to be controlling movement at lower levels.

On slide five, you see a representation of where these structures are located.  As you can see, the precentral gyrus which is kind of a lavender color and the pre-motor areas and supplementary motor areas are anterior to that.  We also have other structures and other systems that are going to be important as well.  For example, planning of motor movements is done in association areas of the cortex and other areas.

Let’s talk about some of these structures in detail.  The first of these structures is what is called the precentral gyrus or what is also called Broadman’s area four.  Broadman’s area four is the primary motor cortex.  It controls all of major voluntary motor movements that we have in our system.  This cortex is synaptotopically organized and called (as seen in Carlson and other figures) the motor homunculus.  If we look at this structure, most of the homunculus in Broadman’s area four is taken up with four structures--the neck, the mouth, the face and the hands--and when you put all these structures together, it makes up approximately three-quarters of all the primary motor area that you have within your system.  So, if you damage one part of this, you develop paralysis in that particular structure.  So, an individual having a stroke would have major problems if they damage a severe amount of this structure across the system.

The precentral gyrus, as we see in slide eight, receives input from a variety of different structures.  It receives information from premotor, supplementary motor cortex areas, and also from frontal association cortex.  It also is going to receive a lot of information from postcentral gyrus structures which is the primary sensory structure that we have in our system.  Ultimately, movement involves using all of these structures to do it well.

The next major structures (as we see in slide nine) are pre-motor cortex and supplementary motor cortex, or what we also call Broadman’s areas six and eight.  These structures help coordinate and plan complex sequences of movement.  They don’t really do the movement themselves, but they set up the muscle systems so that when voluntary motor movement neurons signals come down, the muscle will contract.  These structures also receive information from posterior and prefrontal association areas.

The next motor areas that are important are called tertiary motor areas, or what are called Broadman’s areas nine, ten and eleven, forty-five, forty-six and forty-seven.  These are involved with planning and thinking of movement.  Forty-five, forty-six and forty-seven are extremely, extremely, extremely, extremely important in speech, and are areas that are highly related to what we call Broca’s area within the precentral gyrus.

Slide 11 shows an example of all these systems working together.  As you can see, Broadman’s motor areas are going to be sending signals to the brain stem, into the spinal cord, and into thalamic structures.  Ultimately, the spinal cord will be sending signals to muscles which will then contract.  Those signals are then picked up by receptor system via feedback, will send information back to the spinal cord.  Ultimately, this information goes to the brain stem, cerebellar structures, thalamus, and into cortical structures as well.

So, once we have this information within the system and we have a little bit of an overview, let’s talk about how this information gets to the muscle groups and where it goes.  In essence, the system sends information down what we call motor pathways.  So, axons from area four go to the spinal cord via two major groups: the lateral group and the ventromedial group.

The lateral group, as we see on slide 14, generally controls any kind of limb movements, and there are lots and lots of different pathways involved.  The first of these are what we call the corticospinal track.  The corticospinal track is going to be extremely important for hand, finger movements and other particular structures. 

The second major pathway that we have is the corticobulbar track.  These are going to innervate movements of the neck and face, tongue, eye, and other structures as well.

And finally we also have what we call is the rubrospinal track, which is going to include fore and hind limb structures, and structures that are going to be in lower motor pathways.  We also have corticospinal track systems that are going to be working with that as well.

In addition to the lateral pathway and its materials, we also have a ventromedial group and a variety of structures within that. The ventromedial pathways control gross motor movements that we have, and there’s a variety of different structures (as we see in slide 14).  The vestibulospinal track controls structures related to posture, while tectospinal track controls things related to eye and head and trunk movements.  Finally the reticulospinal track involves walking, sneezing, muscle tone and other structures within the ventral corticospinal track are related to muscles of the upper leg and trunk.  Again, each of these work together with other structures.  And when one damages each one of these, it causes major problems.

Let’s talk about a specific structure that we have within our system and that is what is called the corticospinal track.  The corticospinal track basically consists of neurons that terminate on motor neurons within gray matter of the spinal cord, specifically the ventral dorsal horn.  It begins in layer five of the primary motor cortex.  Axons from area four are going to pass down through the cerebral peduncles of the mid brain.  Of these, 80% cross over to the opposite of the body (decussate) at a structure called the pyramidal decussation in the medulla.  These 80% are going to become the lateral cortical spinal track.  The remaining 20% of all these neurons remain on the same side; they remain what is called ipsilateral.  These pathways are going to become the ventrocortical spinal track.  So, now we have two pathways, we have a lateral corticospinal track and we have a ventrocortical spinal track.

Ultimately, these neurons from both the lateral and ventral corticospinal track are going to terminate on internuncial neurons or alpha-motor neurons in the ventral horn.  Ultimately the connection from those pathways will make up what is called the final common pathway.  These efferent neurons are going to some particular muscle group.  The function of these structures and these systems are (as we see on slide 17) are to control fine motor movements.  When you damage these structures, you develop a loss of muscle strength, reduced dexterity, etc.  However, there’s no effect of corticospinal lesions on posture or using limbs for reaching.  These are going to use different brain structures that come from the basal ganglia and cerebellum.

Figure 18 shows a picture of the final common pathway.  So, we’ve had information coming down the lateral and ventral corticospinal track, they’re going into the ventral horn, and these fibers are going to muscles which then cause a flexion and a relaxation of a particular muscle group.

Now there’s a structure that we call the neuromuscular junction.  This is shown in slide 19.  The neuromuscular junction is a synapse.  It’s formed between the alpha motor neuron and a muscle fiber.  Each axon is going to have a lot of connections to these muscle fibers.  They form what is called a motor movement.  The precision of this movement is related to the motor movement signals.  For example, you might have a small movement where you get very precise movements of the hands and fingers, or very large movements such as leg movements or some other type of structure.

Now there’s one neuro-transmitter that’s extremely, extremely important at the neuromuscular junction and this is shown in slide 20 and called Acetylcholine.  Acetylcholine release produces large end-plate potentials.  These cause calcium channels to open.  When those calcium channels open, calcium enters to the muscle group and basically causes a myosin-actin interaction, and we get a rowing action.  This basically causes the muscle to shorten.  As it shortens, it causes a contraction to occur.  Now we need to remember there is a muscle on the opposite side which t the same time needs to not fire and remain relaxed so you don’t have two muscles opposing each other.

Within these muscles, we also have Golgi Tendon Organ receptors.  Their function is to prevent over-contraction of some kind of striated muscles.  They are located within tendons and test the degree of stretch on the muscle.  They also inhibit the agonist muscle.  So, what we have is one muscle contracting, having receptors to detect that, and when it does, causing the muscles on the opposite side to relax.

Now there are a variety of other motor structures that are involved with movement.  Three of these are listed here and include the basal ganglia, the cerebellum, and the spinal cord. We will begin talking about these structures in more detail within their own particular sections.  So, what we’ve started with are basic structures involved with movement.  That is the primary motor areas and other motor cortical areas of the brain.  Next we’re going to start to talk about other structures that are going to contribute information to help us with motor movement.  So, until then, I hope you have a good day.