(Transcribed from Dr. Glasser’s lecture, 26 May 2000 by Brian Buschman)
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We started with the simplest, myotactic, reflex and are building up in the hierarchy of control. In the reflexes we have all the spinal reflexes working in a complex series. In a given a-motor neuron you have Ia and Ib fibers from both extensors and flexors. It creates a massively complex network of special summation, epically after adding today’s material.
The antigravity reflex is the motor control that primarily works extensors of the L.L. to keep you upright while standing against gravity. It is made of three parts which use three tracts and four nucli. One new term we need is upper motor neuron (UMN) which is any neuron that projects onto a lower motor neuron.
1) The lateral vestibular nucli gives rise to the vestibulospinal tract which is an ipsilateral extensor bias. It descends and stays ipsilateral and for the most part uses interneurons to project onto lower motor neurons (LMN). In some cases in the LL the UMN synapse directly onto the LNM without use of an interneuron.
2) The reticular formation is a group of diffuse nucli in the brainstem of which two function in the antigravity reflex. They are the reticularis pontis oralis and the reticularis pontis caudalis. Both join to give the pontine reticulospinal tract which descends ipsilaterally and has an extensor bias. All fibers of the pontine reticulospinal tract synapse onto interneurons which then synapse onto LMNs.
3) Reticularis gigantocellularis are large cells in the medullary reticular formation which are the strange group of the antigravity reflex. These big fat cells send fibers that descend ipsilaterally but then work to cause a bilateral flexor bias. Just remember that the big fat ones (giganto-) are just strange.
The red nucleus gives fibers that cross almost immediately in the ventral tegmental decussation (in the medulla) and descend contralaterally to the red nucleus in the rubriospinal tract. Most will exit in the cervical spine and are responsible for maintaining muscle tone and for a flexor bias specifically in the UL.
Area 4 (the precentral gyrus) is the primary motor cortex. It’s fifth layer includes some large pyramidal cells called Batz cells which give the corticospinal tract. In reality the corticospinal tract begins somewhat equally from cells in area 4, area 6 and areas 3, 1 and 2. For our sake we will just consider it to come from area 4. The corticospinal tract has a contralateral flexor bias, epically in the distal LL.
The fibers exit from the cortex and go into the corona radiata which bundles them together. The tract then runs in the internal capsule between the thalamus and the lentiform nucleus. The anterior limb of the internal capsule is located between the head of the caudate and the lentiform and the posterior between the lentiform and thalamus. The corticospinal tract fibers then run down into the crus cerebri, the basis pontis and the pyramids of the medulla. This also shows why the corticospinal tract is sometimes called the pyramidal tract.
At the lower medulla about 85% of the fibers will cross over and descend in the lateral faniculis of the white matter. This is the lateral corticospinal tract. The anterior cortical spinal tract descends ipsilaterally but we don’t need to worry about it, he said it’s not clinically significant.
Better then 50% of the corticospinal fibers will exit in the cervical region because of all the fibers needed for the fine motor control of the hands. This also ties in with the vast about of brain area devoted to the hands when looking at the motor homunculus for area 4.
Most of the neurons will synapse on an interneuron but a few UMNs will synapse directly onto LMNs.
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