Pubmed 2. Nolte's The Human Brain 7th edition. Standring S. Gray's Anatomy 40th edition. Crossman AR, Neary D.
Neuroanatomy 6th edition. Related articles: Anatomy: Brain. Promoted articles advertising. Figure 1: Gray's anatomy illustration Figure 1: Gray's anatomy illustration. Figure 2: Gray's anatomy illustration Figure 2: Gray's anatomy illustration. Figure 3: coronal brainstem section Figure 3: coronal brainstem section.
MS is predominantly a disease of the white matter in the CNS. Gray Matter. The gray matter regions of the CNS, the brain and spinal cord, contrast with the white matter regions. The gray matter is the areas where the actual "processing" is done whereas the white matter provides the communication between different gray matter areas and between the gray matter and the rest of the body.
The neurons in the gray matter consist of neuronal cell bodies and their dendrites. The dendrites are short protrusions that communicate with immediately neighboring neurons in the CNS. In contrast with the neurons of the white matter, gray matter neurons don't contain long axons that transmit the nerve impulses to more distant regions of the CNS. Gray matter is so-called because in section it has a gray color due to all the gray nuclei in the cells that make it up.
Gray matter involvement is detected even in the earliest stages of MS, and gray matter atrophy occurs at a faster rate than white matter atrophy early in the disease course. Gray matter involvement and in particular cortical demyelination can be extensive in MS.
They're largely hidden by the lower parts of the cerebral hemispheres, the temporal lobes. To see the cerebral peduncles better, we'll look at a brain in which the temporal lobe, and the cerebellum have been removed. Here are the cerebral peduncles again.
Here on the outside of the cerebral peduncle are the medial geniculate body, and the lateral geniculate body, which gives rise to the optic tract. Between the cerebral peduncles the third cranial nerve, the oculomotor, emerges. We'll return to the intact brain. Here are the two oculomotor nerves. Here are the two optic tracts. They meet at the optic chiasm. From the optic chiasm the two optic nerves emerge. If the thermostat the comparator detects that the room is cooler than the desired temperature, it sends an error signal that turns on the furnace.
If the comparator detects that the room is warmer than the desired setting, its sends an error signal that turns on the air conditioner. Feedback control systems can produce very accurate outputs; however, in general they are slow. In order to change the output, the effector must wait until information is transmitted from the sensor to the comparator and then to the effector. At this point, another comparison is made, and the process continues. Consider further the thermostat example.
It reads the new room temperature, and, if it is still too cool, it instructs the furnace to deliver more heat, and so on.
Although this will eventually produce an accurate room temperature at the desired point, it takes a number of cycles to reach that point. One possible solution for quicker results would be to turn an enormous furnace on full-blast, such that is heats the room very quickly. This solution, however, can generate another problem. It will tend to cause the system to oscillate if the feedback pathways are slow.
In order for a feedback system to work well, the transmission time of sensory information through the comparator to the effector must be rapid compared to the time of the action. Feedback control systems work well only when the sensory feedback about the actual output is fast relative to the actual output. Thus, a feedback controller is useful for slow movements, like postural adjustments. The role of the myotatic reflex in posture maintenance is an example of a feedback controller in the spinal cord, and the cerebellum plays a role in coordinating these postural adjustments.
Feedback control is not effective for most of the fast movements we make routinely such as an eye movement or reaching out for a cup. For these movements, a feedforward controller is needed. In a feedforward control system , when a desired output is sent to the controller, the controller evaluates sensory information about the environment and about the system itself before the output commands are generated. It uses the sensory information to program the best set of instructions to accomplish the desired output.
However, in a pure feedforward system, once the commands are sent, there is no way to alter them i. The advantage of a feedforward system is that it can produce the precise set of commands for the effector without needing to constantly check the output and make corrections during the movement itself. The main disadvantage, however, is that the feedforward controller requires a period of trial-and-error learning before it can function properly.
In most biological systems, it is hard perhaps impossible to pre-program all of the possible sensory conditions that the controller may encounter during the life of the organism. Furthermore, the environment and conditions under which actions are made are constantly changing, and the feedforward controller must be able to adapt its output commands to account for these changes.
The controller would use diverse sensory information about the environment before sending its command to the furnace Figure 5. For example, it would read the current temperature, the current humidity level, the size of the room, the number of people in the room, and so forth. There would be no need to continually compare the current temperature with the desired setting, as the system has predetermined how long the furnace needs to be working in order to achieve the desired temperature.
How did the controller obtain this information? A feedforward controller requires a large amount of experience in order to learn the appropriate actions needed for each set of environmental conditions.
If on one trial it turns the furnace off too soon and the room does not reach the desired temperature, it adjusts its programming such that the next time it encounters the same environmental conditions, it turns the furnace on for a longer period of time. The key distinction between a feedback and feedforward system is that the feedback system uses sensory information to generate an error signal during the control of a movement, whereas a feedforward system uses sensory information in advance of a movement.
Any error signal about the final output is used by the feedforward system only to change its programming of future movements. The cerebellum may be a feedforward control system. The cerebellar involvement in the VOR may be explained in terms of the learning requirements of a feedforward controller.
When the head moves, a compensatory eye movement must be made to maintain a stable gaze. The cerebellum receives sensory input from the vestibular system informing it that the head is moving. It also receives input from eye muscle proprioceptors and other relevant sources of information about current conditions in order to make an accurate compensatory eye movement. It evaluates all of this advance sensory information and calculates the proper eye movement to exactly counterbalance the head movement.
What if the eye movement does not match the head movement, however, and the visual image moves across the retina such as in the experimental condition in which a prism was worn, or in a real-life situation in which an individual wears new prescription eyeglasses?
The retinal slip constitutes an error signal to tell the cerebellum that next time these conditions are met, adjust the eye movement to decrease the retinal slip.
This trial and error sequence will be repeated until the movement is properly calibrated; moreover, these mechanisms will ensure that the movements stay calibrated. As another example, the coordination of movements requires that muscle groups be activated in precise temporal sequence.
Not only do the different joints need to be coordinated temporally, but even antagonist muscles that control the same joint need precise temporal coordination. For example, an extensor muscle needs to be activated to start a reaching movement, and the corresponding flexor muscle needs to be activated at the end of the movement to stop the movement appropriately. The precise timing of muscle contractions and the force necessary for each contraction varies with the amount of load placed on a muscle, as well as on the inherent properties of the muscle itself e.
Moreover, a similar movement will require different patterns of motor activity depending on the weight being born by the muscle for example, if an extended hand is empty or holding a heavy weight.
The cerebellum appears necessary for the proper timing and coordination of muscle groups, very likely through a trial-and-error learning mechanism discussed previously.
Such a role helps explain the deficits seen in dysdiadochokinesia, in which patients cannot perform rapidly alternating sequences of movements. It is believed that the mossy fiber inputs to the cerebellum convey the sensory information used to evaluate the overall sensory context of the movement.
Mossy fibers are known to respond to sensory stimuli; they are also correlated with different movements Figure 5.
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