(Transcribed from Dr. Glasser’s lecture, 22 May 2000 by Brian Buschman)
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There are two types of fibers in striated muscle as we discussed last time:
1) Extrafusal fibers which are innervated by the a-motor neurons and just contract. They are the normal muscle fibers that we think about.
2) Intrafusal fibers which are like extrafusal fibers except that they contain an equatorial region that contains two types of receptors. The intrafusal fibers are aligned in parallel to the extrafusal ones.
Intrafusal fibers are located in parallel to the extrafusal muscle fibers and have a widened equatorial region. They are sometimes referred to as the muscle spindle and sometimes muscle spindle is the term used only for the equatorial region. The equatorial regions of the intrafusal fibers contain two types of receptors, the nuclear bag and nuclear chain ganglia which have motor afferents attached to them.
The motor afferent fibers can be classified into four main types:
There are two types of motor afferents that are associated with the equatorial region of the intrafusal fibers:
When the muscle is at it’s rest length the afferents will have a given firing rate. When the muscle becomes stretched the afferent firing rate will be increased to a higher level. When the stretch is in the middle of taking place the firing rate will be even faster. The length of the muscle fiber is sensed in the type Ia neurons and the type II afferents are unimportant for our purposes.
a-motor neurons are the neurons that innervate all muscle fibers of a given motor unit.
g-motor neurons innervate the striated portion of the intrafusal fibers.
The a-motor neurons fire and cause contractions of the entire muscle which causes scrunching up of the muscle spindle. When the muscle spindle is squished like this it will be unable to properly sense muscle length and will not be able to properly signal to the Ia afferents. To prevent this when the a-motor neurons are causing contraction of the motor units the g-motor units will cause contraction of the striated part of the muscle spindle which will keep the equatorial region at the proper length to function.
As the intrafusal fibers detect the length of the muscle the myotactic reflex works to keep the muscle at it’s set length in response to sensations from the Ia fibers. When the muscle is stretched by some outside force this reflex kicks in to take compensate. The afferent limb of the reflex is the Ia motor afferent and it has four branches:
1) Autogenic excitation involves direct stimulation of an a-motor neuron by the Ia fiber. In this case the DRG cells go all the way into lamina IX to cause the action.
2) Synergistic excitation is a branch that functions to stimulate a-motor neurons but in this case it functions through an interneuron. In the synergistic excitation there are EPSPs on both the dendrites of the interneuron and on the a-motor neuron.
3) Reciprocal inhibition is the branch that functions to keep the reflex from overcompensating for the stretch. It acts through an interneuron that creates an IPSP on the a-motor neuron.
4) Renshaw inhibition is the final function which functions through an interneuron that exerts IPSPs through presynaptic inhibition on all three of the above processes. It is the part of the reflex that is put in place to stop the reflex and bring everything back to normal. There is a special case known as disinhibition which is when a neuron acts to turn off an inhibitory neuron like the Renshaw neuron does when acting on the interneuron for the reciprocal inhibition.
This myotactic reflex is what a doc is testing when he strikes the patellar reflex. It is an ipsilateral reflex involving only one spinal cord segment.
The type II neuron comes into the spinal cord and branches to two interneurons. One will be excitatory to the flexors and the other inhibitatory to the extensors. It can be summarized by calling it an ipsilateral flexor bias.
In all motor functions there is a final common pathway which is the a-motor neurons. Regardless of the path of the reflex or other CNS motor functions all motor efferent signals will be carried by these lower motor neurons to the muscle units. If you lose the a-motor neurons you will loose all function of the skeletal muscles.
The GTO is found in the tendons of muscles or you can say the GTO is found in series with the extrafusal muscle fibers. They are innervated by Ib motor afferent fibers. The GTO functions to sense the strength of contraction of a muscle fiber and inhibit it enough so that you will not rip your muscles out of your body.
The GTO senses tension and sends it’s info to the Ib reflex. It has a stimulatory interneuron and a pair of inhibitory interneurons. This limits the work that the muscle does to prevent the muscle from ripping itself out of the body. The stimulatory limb is to stimulate the opposing muscle group. This inhibits the stretch of the working muscle group and stimulates the opposing group. Again we are dealing with an ipsilateral, unisegmental reflex.
The fasciculus proprius tract plays a large role in reflexes to protect you such as the example that Dr. Glasser used about when you put your hand on a hot stove.
In this example you will have an ipsilateral flexor bias and a contralateral extensor bias. The information comes into the dorsal horn and it them is passed to the fasciculus proprius tract. The afferents are type III and IV fibers that get the sensations from free nerve endings. The fasciculus proprius tract is a very thin tract located right up against the gray matter in the spinal cord on the ventral side.
In the case of the hand on the stove some of the type III and IV fibers will ascend to around C5 where they will leave the tract and excite ipsilateral flexors and inhibit ipsilateral extensors for the sake of getting the hand away from the stove. Some of the fibers will also descend into the lumbar region where they will cross contralaterally in the anterior white commisure and excite contralateral extensors and inhibit contralateral flexors. This will help you pull your hand away from the stove and lean away for even greater separation from the danger.
As with all reflexes that we discussed you can consciously override the reflex. Higher order thought can take control. You can chose to not kick your leg when your patellar tendon is hit or you can put your hand back on the burning stove and leave it there if you focus on doing it.
The cerebellum senses all information regarding proprioception so that it can coordinate the functions related to motor movement. This allows you to touch the tip of your nose if you have your eyes closed. It takes advantage of both length (Ia) and tension (Ib) information.
The information on proprioception ascend to the cerebellum through four tracts based on what type of fibers they are and were they come from.
1) Ia fibers from the lower body ascend in the dorsal spinocerebellar tract.
2) Ia fibers from the upper body ascend in the cuniocerebellar tract.
3) Ib fibers from the lower body ascend in the ventral spinocerebellar tract.
4) Ib fibers from the upper body ascend in the rostral spinocerebellar tract.
THIS IS THE END OF THE MATERIAL FOR MINI 1.
Muscle fibers need tropic factors to maintain their muscle tone. Without them they will atrophy regardless of how much PT one may do. The tropic factors are received with the innervations that they get so if the muscles is deinervated then they will atrophy. Lower motor neurons are found in lamina IX in the spinal cord and in the midbrain, pons and medulla. Their fibers come from lamina XI and the nuclei of CN 3, 4, 6, 5, 7, 12 and the nucleus ambiguus part of CN 9, 10 and 11.
Stay tuned there is lots more to come before the final. J
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