(Transcribed from Dr. Glasser’s lecture, 30 May 2000 by Brian Buschman)
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The tectum controls the muscle reflex that deals with the auditory or visual reflex. When you hear/see something unexpected it is natural to turn to look at it. That is really a very complicated process given that you need to coordinate the turning of your body, head and eyes all in the same direction. The tectum coordinates these three actions. It does it through two tracts:
1) The tectospinal tract crosses almost immediately in the dorsal tegmental decussation and descends contralaterally and goes to the spinal nerves to turn the trunk and head (remember that head turning is by the spinal accessory nerve so it’s fibers will come from the tectospinal tract).
2) The MLF (medial longitudinal fasciculus) is a bilateral pathway that connects the tectum with the cranial nerve nucli which move the eyes. It also connects with control centers that coordinate eye movement with the spinal cord. The MFL is very high yield and we will have an entire lecture just on this.
It was once found that if you sever a cat’s spinal cord where it connects to the brainstem and sever all of the dorsal roots (sensory) and put it on a treadmill it will walk. Because of the severing of the brainstem it cannot add sensory input from the eyes and with the dorsal roots severed it cannot receive such sensory information yet it still walked. It also was able to speed up or slow down based with the treadmill. The only explanation is that there are some motor afferents in the ventral roots.
The motor homunculus is the map of what part of area 4 innervates which muscles, area 4 is the primary motor cortex. It shows a large area for the hands and small areas for the lower body. Area 6 also has a homunculus which runs from the lateral part to a point partway up on area 6. This region is called the premotor cortex. The superior part of are 6 is the supplementary motor cortex which is where motor functions are initiated. Area 6 will stimulate the basal ganglia (BG) to tell the cerebellum to initiate the program for the given action.
The cerebellum will make up a program for the desired motor function and will coordinate the movement. As the program is running the cerebellum is able to make changes as needed.
The cerebral cortex also works through the BG which works through the thalamus to work back on the cerebral cortex. In the process the BG functions to eliminate unwanted motor behavior.
The cerebral cortex also works on the cerebellar cortex which works through the thalamus and back to the cerebral cortex. The cerebellum will also project directly to give some UMNs.
Area 4 initiates a program.
Area 6, the cerebellum and BG make the program.
The data is sent to area 4 which sends the program to the corticospinal tract to carry it out.
Orientation is the action that must take place to put you in position to carry out a specific task. If you want to pick up a pen orientation would be taking the necessary step or two to get to the table where the pen is located. Supplementary motor function would be the act of actually picking up the pen.
Paralysis involves a completely destroyed motor nerve with complete loss of function.
Peresis involves a partially destroyed motor nerve with a weakness of the area innervated by the nerve.
|
UMN
Legion |
LMN
Legion |
|
Spastic paralysis |
Flaccid paralysis |
|
Hyperreflexia |
Hyporeflexia |
|
- spactity |
Atrophy |
|
- clonus |
Fasculations |
Normally the ACh receptors will only be located at the myoneural junction. With denervation you will get ACh receptors all over the place as the muscle fiber is looking for ACh. You will also get atrophy due to the lack of trophic factors and flaccid paralysis due to the loss of all reflexes.
Fasculations are associated twitches which come with a temporary atrophy of a specific point.
The spastic paralysis is also known as the clasp knife reflex. If you pull on the flexed muscle it will stick and then all of a sudden it will give.
Hyperreflexia includes spactity which is:
1) Unidirectional (When flexed it will resist extension but will not add resistance to further flexion.)
2) It is inversely proportional to length. The shorter the muscle the more tension will result from the spactity.
3) The faster you pull on it the more tension you encounter.
The other part of UMN Hyperreflexia is clonus which is the rhythmic contraction of muscles when passively stretched. If you elevate the flexed leg you will see a twitch in the gastroc.
Babinski sign is a way of identifying damage to the corticospinal tract. If you stroke the side of an adult’s foot the normal response is to curl the toes. If you have UMN damage you will get a Babinski sign which is an abduction of the toes. In children younger then two years of age the Babinski sign is the normal condition. This is because the corticospinal tract is not fully developed (myelinated) until two years of age.
This is a legion of ½ of the spinal cord. At the level of the legion you will have LMN damage. For all segments below the legion you will have an UMN legion. Don’t get confused that the LMN damage is located superior to the UMN damage.
A quick and dirty way to remember the spinal segment of reflexes you will test clinically is:
|
|
Reflex |
Segment |
|
1,2 |
Achilles |
S1,2 |
|
3,4 |
Patellar |
L3,4 |
|
5,6 |
Forearm flexor |
C5,6 |
|
7,8 |
Forearm extensor |
C7,8 |
Immediately following spinal damage or surgery there will be some degree of spinal shock which will appear as LMN damage. In a matter of days it will either change to signs of UMN damage or will clear up completely.
(HE WARNED US THAT ONE OF THESE TWO WILL BE ON MINI II)
A midcollicular legion will leave the lateral vestibulospinal tract as the highest order motor tract. Because of this you will get rigid extension.
Decerebrate rigidity is a lower brainstem legion where all of the limbs are extended.
A legion at the level of the cerebral peduncle will leave the red nucleus as the highest order control center of the motor system. This will cause the rubriospinal tract to dominate and there will be UL flexion. The antigravity reflex (lateral vestibular nucli) will still control the LL function as it’s the primary LL function if all functions are intact.
UMNs project onto lower motor neurons of the brainstem as well as onto lamina IX. It is the corticobulbar tract that carries the UMN, through the internal capsule, to these nucli. The corticospinal tract is bilateral except for the fibers of CN VII which innervate the lower face which are contralateral. Therefore all CNs with a motor function (CN III, IV, VI, V, VII, XII and ambiguus to IX, X, XI) receive corticobulbar input from both sides except for part of VII.
A legion of the right corticobulbar tract will present with the ability to wrinkle the brow bilaterally but the patient will be unable to smile contralaterally to the side of the damage. This can be differentiated from a legion of VII or it’s nucli because such a legion would present with the loss of the ability to smile or wrinkle the brow ipsilaterally to the damage.
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