Neural Control of Movement Part IV: Reciprocal Inhibition

When we think about movement, we often focus on the central nervous system sending excitatory signals (turning things on) through the alpha motoneurons to the muscles so they will contract.  However, in order to control the motion and prevent unwanted movements, the nervous system also has to inhibit (or turn off) certain neurons to keep them from firing in order to prevented unwanted muscles from contracting.  This process is called inhibition.
There are two main types of inhibition: recurrent inhibition, and reciprocal inhibition.  Recurrent inhibition turns off alpha motoneurons that connect to the same muscle fibers that are contracting, and reciprocal inhibition turns off alpha motoneurons that control the opposing muscle group.  On the surface, reciprocal inhibition is the easiest to understand.  For example, if you are trying to actively flex your elbow (think about a dumbbell curl), you would use the elbow flexors (brachialis and biceps brachii).  You do not want your elbow extensors (triceps brachii) to turn on, because that would work against the desired movement.  So, through reciprocal inhibition, the nervous system will inhibit the alpha motoneurons that connect to the muscle fibers of the elbow extensors, in essence shutting them off. 

Another example of reciprocal inhibition is through the muscle spindles.  If a muscle is lengthening to much and too fast, the muscle spindles will send signals to the CNS.  In order to prevent the muscle from any further lengthening, the CNS will inhibit the alpha motoneurons of the opposing muscle group (that is causing the stretch to occur), and excite the alpha motoneurons of the muscle that is lengthening, so that the muscle will contract and shorten.  This inhibition of the opposing muscle group is know as reciprocal inhibition.  I will talk about recurrent inhibition during the next post.

Neural Control of Movement Part III: Golgi Tendon Organs

A couple of weeks ago, we talked about a special type of proprioceptive receptor called muscle spindles.  Muscle spindles detect changes in muscle length and velocity of lengthening, and send signals to the CNS.  This helps the nervous system know about changes in joint angles and muscle length, and can help protect the muscle from lengthening too much and too fast.

Another type of proprioceptive receptor found in muscle (actually between the muscle and tendon) is the Golgi Tendon Organ (GTO).  These receptors are sensitive to changes in muscular force.  Whenever a muscle contracts (shortens), tension is developed within the muscle and tendon, which activates the GTO, causing it to send signals to the CNS.  Thus, the GTO provides feedback to the CNS about the amount of force a muscle is producing.  If a muscle is producing too much force, and is at risk of injury, the CNS can send inhibitory signals back down to the muscle so it will stop contracting and relax, thus reducing the amount of force.  Unlike the muscle spindles, which are are sensitive to changes in muscle length and the rate of change, GTOs are only sensitive to changes in muscle force, not the rate of change.

So, the muscle spindles send information to the CNS about  muscle length and the velocity of lengthening, and GTOs send information about muscular force.  This information allows the nervous system to make quick adjustments so we can move more efficiently and safely.   

Neck Injuries

It seems that this football season, especially last weekend, has seen a very high number of neck injuries.  Devon Walker, a defensive back for Tulane, sustained a cervical spine fracture this past Saturday when he attempted to make a tackle and collided helmet to helmet with a teammate.  Walker had surgery but the extent of the damage is not yet known.  Hopefully he will make a full recovery.

The question is, why are we seeing so many head/neck injuries in football?  Obviously football is a contact sport and there are hundreds of violent collisions every game.  These players are very massive and move at high velocities, meaning they generate a large amount of momentum that is transferred between the players during a collision.  Injuries are going to happen.  Now, I have never played or coached football, I've only worked with football teams as an athletic trainer, and I watch a lot of football.  Through my observations, it seems that many football players attempt to make tackles, or attempt to "run into" a tackler with their necks in a flexed position (think about looking down).  This is the worst possible position for the neck to be in during a collision.  Cervical (neck) flexion removes the natural curvature from the cervical spine, and places the vertebrae in direct alignment.  When the head makes contact with another person, the force is transferred from the head straight down the vertebrae, essentially creating a domino effect.  If there is enough force, an injury such as a cervical vertebrae fracture can occur, which can potentially damage the spinal cord.

I do not think there is a simple solution to this problem.  The best way to avoid this injury would be to tackle with the head up, or to teach the defender to be able to see the person they are tackling.  I've seen several examples of defenders "launching" themselves headfirst into the offensive player.  However, I think a lot of these players have been tackling with their heads down for so long, that it is a difficult habit to break, especially in heat of the game when they have to make a play.  Hopefully improvements will continue to be made to equipment and more research will be conducted to help answer these questions.

Remembering 9/11

It is hard to believe that today is the 11th anniversary of the terrorist attacks of 9/11.  I thought I would do something different with the blog today due to the 11 year anniversary of the day the terrorists attacked our country and many brave men and women lost their lives.  I was a sophomore at Southern Miss on 9/11/2001, and had just finished a morning workout at the Payne Center when the news broke.  It seemed surreal at the time, and it still does.  I was working as a student athletic trainer with the football team, and it was very difficult for anyone to focus on football for a few days, and all the games for that weekend were canceled.  Looking back, it was definitely the right thing to do, although later on I do believe sports played an important part in helping our country heal.  Amy and I had a chance to visit ground zero in New York a couple of years ago, and even though the rebuilding process is underway, you could still sense that something terrible had happened there.  Let's just remember everyone that lost their lives that day or later due to the attacks, and all the brave men and women that are fighting for our freedom.

Neural Control of Movement Part II: Muscle Spindles

When we think about muscles, we often think about the contractile components, actin and myosin, that attach and slide past each other, causing a muscular contraction.  But, there is another component of the muscle that is critical for coordinated movement, and that is the muscle spindle.  In the picture above, the extrafusal muscle fibers are the ones that contract and develop force, while the muscle spindle contains the intrafusal muscle fibers, afferent neurons, and gamma motor neurons.

There are three types of intrafusal fibers: dynamic bag fibers, static bag fibers, and chain fibers.  When muscle lengthens (think about when a muscle is stretched), the intrafusal fibers send signals to the spinal cord through the Group Ia and Group II afferent neurons, which relays information about how much the muscle is lengthening and how fast the muscle is lengthening.  The greater the lengthening or speed of lengthening, the more signals will be sent.  The gamma motor neurons send signals to the muscle spindles from the CNS (central nervous system) that can increase or decrease the sensitivity of the muscle spindle.  The gamma motor neurons help the CNS control the gain of the muscle spindles.

Why are muscle spindles important?  There are two big reasons.  1) The muscle spindles send information to the CNS about muscle length, which helps the nervous system know how joint angles are changing and where the different body parts are located in space.  For example, if you extend (straighten) your elbow, this lengthens the biceps brachii muscle.  The muscle spindles in the biceps will send signals to the CNS, indicating that the muscle is lengthening.  If the biceps is lengthening, then the elbow has to be moving into an extended (more straight position).  Also, if you were to flex (bend) your knee, this would lengthen the quadriceps, which activate the muscle spindles, indicating that the muscle is lengthening and the knee if flexing.  2) Muscle spindles also help protect the muscle from injury due to the muscle lengthening too much and too fast.  If a muscle is lengthening too much and too fast, the CNS can send signals to the muscle for it to contract and shorten.

So, muscle spindles play a crucial role in providing feedback to the CNS about muscle length and speed of lengthening.  This information helps the body know how joint angles are changing, and it can serve to help protect the muscle against injury.

Neural Control of Movement Part I: Please Do Not Say Muscle Memory

This semester, I am teaching a class called "Neural Control of Human Movement."  This is a very challenging course for both myself and the students, because the nervous system is very complex.  To me, it is the most complex and difficult system in the body to understand.  I am going to do a series of blog posts discussing how the nervous system works with the muscular system to produce coordinated movement.

When we think of voluntary movement, such as walking, running, hitting or catching a baseball, etc., we often focus primarily on the muscles and the bones involved in the movement.  What we fail to consider is that none of this motion would be possible without the nervous system.  This simplified view of movement has given rise to a very commonly misused term called "muscle memory."  I hear sportscasters, coaches, and even so called scientists use this term often, and every time I hear it I cringe.  The ESPN segment called
"Sports Science" was airing the other day and the host used the term "muscle memory" to describe how a baseball player caught a ball. 

Why is "muscle memory" not correct?  The biggest reason is that there is no memory structure in the muscle.  A skeletal muscle cannot contract unless it is stimulated by the nervous system.  Now, it is true that through practice and experience, movements become more coordinated, efficient, and require less attentional demands, and many people want to label this as "muscle memory."  The next few blog posts will discuss the interaction between the nervous system and the skeletal system, and the actual processes that occur that lead to an improvement in performance that involves both the nervous and muscular systems.