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

First of all, the importance of spinal reflexes was first discovered by Sherrington.  Basically, he believed that simple reflexes were activated by receptors in the skins and muscles, and these were the basic units of movement.  In addition, complex sequences of movements were basically just some combinations of simple reflexes.  This was the dominant view for at least the last 100 years.  However, newer data basically shows you can complete coordinated movements without sensory information, but despite this new information, Sherrington’s role in understanding reflexes cannot be understated, it was incredible. 

First of all, stimuli for reflexes, as we see in slide three, come from a variety of different receptors located in different areas.  As we know, we have receptors in the skin, but we also have receptors in joints and muscles.  These will also be important for us in relation to reflexes.

Now, in the past, we’ve said that reflexes were automatic and were basically stereotyped.  That is, they were very specific about what they did and they occurred in response to some particular stimulus being applied to some peripheral receptor.  As we can see in slide five, we know today that reflexes can be modified, they can adapt to different types of tasks, and ultimately, they can be smoothly incorporated with movements that are initiated by the cortex.  So, we know today that reflexes just don’t work alone; they can work in combinations with cortical structures.

Now, there are three major principles of reflexes.  The first of these is that the transmission of the reflex basically is set according to a particular motor task.  This is what we call a functional set.  Secondly, sensory information from a localized single source produces reflex responses in many muscles, not just one muscle.  However, many muscles and responses can be far away from where the original stimulus occurred.

And third, supraspinal centers modulate and help spinal reflexes adapt.  So, we’re going to have structures within the spinal cord (all of those nuclei that we talked about last time) that are going to help us in the adaptation of the spinal reflex.

Now spinal reflexes and reflexes in general can be of two types.  These are shown in slide seven:  Monosynaptic and polysynaptic.  So, let’s talk about monosynaptic reflexes first on slide eight.  Basically a monosynaptic reflex is a synapse between one sensory fiber from a muscle and an alpha motor neuron.  So, you’re getting a sensory neuron from fiber coming from a muscle that’s going to an alpha motor neuron.  The classic example of this is what we call the monosynaptic stretch reflex (which is related to posture), or the patellar reflex which is where you smack the knee and it moves. 

The stretch reflex is basically a contraction of the muscle that occurs when a particular muscle is lengthened.  As you begin to stretch out a particular muscle, it begins to actually contract.  In addition to that, you also get a relaxation of muscles on the opposite group.  The stretch reflexes cause excitation of some motor neurons and inhibition in others. 

How do they do that; well as we can see in slide 10, what happens is that sensory receptors in the muscle determine or sense that the muscle is beginning to stretch.  As a result, it sends signals from those neurons to the spinal cord.  What the spinal cord neurons basically do is tell other neurons on the opposite side to relax and help to contract the muscles that begins to stretch.   This allows is a particular feedback loop to occur.  As a result, you are able to get information about where the muscle is in time and space. 

In addition to stretch reflexes, we also have other types of reflexes.  The next group of these is what we call the polysynaptic reflexes.  As we see in slide 11, it involves multiple synapses (ala poly or many) between sensory neurons, interneurons, and motor neurons.  Basically, it causes lots and lots of things to happen.  As we can see in slide 11, the agonist muscle spindles send alpha motor neurons connected to agonist muscles, or inhibitory interneurons connected to antagonist muscles to go to muscles spindles which activate agonist and antagonist muscles.  So, they do multiple things at one time.  The classic example occurs with what was called the withdrawal reflex.  This relates to the concept of that we call reflex arcs.  This occurs from the interaction between an afferent neuron (which is sensory) an internuncial neuron, and efferent neuron (which is a motor neuron).  In addition you’re going to send information to cortical structures as well. 

Why are they important?  Well, let’s take a classic example.  DON’T READ THE NEXT TWO SLIDES UNTIL WE HAVE WALKED THROUGH THIS

 Let’s say you are out walking on the beach,  our life is good, God is great,  and you’re having a good time enjoying the ocean, blah, blah, blah.  Well, let’s say that you start to step on a nail.  What happens?  Well you’re going to get a receptor in your skin that’s going to say (Hey, you’re starting to step on a nail.  That’s going to send information via afferent pathways to the dorsal horn and the spinal cord.  The spinal cord pathways then send information to the thalamus via the gracile and cuneate fasiculus.  The thalamus, then, sends information to area 3, 1, 2.  Area 3, 1, 2 then sends information to the supplementary, pre-motor and motor areas, plus other structures.   These motor neurons fire and those signals go down the lateral and ventral corticospinal tracks plus other pathways.  They synapse at the ventral horn, they go on the final common pathway, the muscle contracts, and you lift your foot.

Well, if you did all of that, what would be the problem?  Well the answer is (as we see on slide 14) that it takes a long time to do that in relation to damage you receive.  So, by the time that you went through that entire process, the nail would be completely through your foot.  So what we have is another system to reduce the amount of damage that’s going to be occurring from the nail.  The way you do that is have an alternative pathway or an alternative system.

The alternative is that you have the stimulus (you start to step on the nail). That information goes via afferent neurons to the dorsal horn of the spinal cord, synapses on the internuncial neuron, and then it’s going to an efferent neuron.  The efferent neuron is basically sending information via the final common pathway to the muscle and it begins to contract. 

Well, what’s the advantage about this kind of system?  Well, here you have a lot less of processing that’s going to be going on, and (as you can see on slide 16) you’re going to get significantly less damage to the tissue.  But, it’s a very gross system, it’s not very refined movement, and it doesn’t in essence pick your foot up all the way.  You need other pathways.   In essence, we have is an all or nothing system, it is a true reflex or what we call a reflex arc.  So, what ultimately happens in this system as we do that?  Well as we kind of started off, you’re going to step on the nail.  And as we talked about earlier, the afferent neuron is going to go to the dorsal horn and the spinal cord, it’s going to synapse on the internuncial neuron, but the internuncial neuron is what we call a unipolar neuron, and it’s shaped like a T.  That is why it’s called a T cell.  It sends a message to two pathways. The first pathway goes to motor neurons and it’s going to connect to an efferent neuron in the ventral horn.  The efferent neuron is going to go through the final common pathway to the muscle and you begin the development of a contraction (to take the pressure off the nail).  At the same time all this is going on, that information is also going to be going to contralateral muscles. Contralateral muscles are situated to provide support and help you with your limb withdrawal in relation to body balance.  You also have inhibitory interneurons that are going to shut down antagonistic muscles, so you don’t have muscles competing with each other. 

Finally we’re going to be sending information to the cortical loop, and that’s basically what we talked about when we first started.  So you’re going to send information up the gracile and cuneate fasciculus to the thalamus.  The thalamus into areas 3, 1, 2.  Areas 3, 1, 2 to the motor areas, motor fibers going down the lateral and ventral horn to the spinal tracks, (other spinal tracks as well; rubro, tecto and other spinal pathways).  Ultimately, it all comes down to the ventral horn, goes to the final common pathway, and then you contract your muscle and lift your foot.  However, it just doesn’t stop there.  Those stimuli are also going to go from the thalamus to other areas.  They’re going to go to association areas, memory areas, and speech integration areas such as Wernicke’s area.  That information is all going to go from Wernicke’s area to Broca’s area and then you say ouch or some other things. 

Also, as you see in slide 23, as you’re lifting your foot, you’re getting opposite muscles relaxing.  You’re going to have a balance system that’s going helping maintain your balance so you don’t fall down; you’re going to develop memory traces so you never walk on the beach again without shoes, and other types of things.  So again, multiple systems are going to be activated in relation to this one little simple aspect of stepping on a nail with your bare foot.

In conclusion, as we see in slide 24, reflexes have phases, but especially in relation to reflex arcs. You have an initial removal of particular amounts of pressure, and then you have complete removal from the damaging object.  Again, as we see here, many systems are involved, depending upon the stimulus event.  Sometimes it can be overridden by cognitive pathways, so you train yourself to not pull away.  For example, something is stinging you, or you stick a fish hook in you skin.  But if you pull away, it’s even going to hurt even more.  So, you train yourself not to do that.   Essentially cognitive motor control can sometimes override a lot of reflexes that we commonly have. 

Well, as we have seen here, spinal reflexes are not as simple as what we commonly thought of when we first started this course.  In the next section, we’re going to continue on talking about the spinal cord and specifically spinal cord damage.  So until then, we hope you have a great day.