Anatomical and Clinical Review The Cerebellum The cerebellum is the largest structure in the posterior fossa see figures Inputs also follow this pattern, so each cerebellar hemisphere receives information about the ipsilateral limbs Cerebellar Input Pathways Input to the Cerebellum Arises From All areas within the CNS Multiple sensory modalities e. Principles for Localizing Cerebellar Lesions Ataxia is ipsilateral to the side of the cerebellar lesion Midline lesions of the cerebellar vermis or flocculonodular lobes mainly cause unsteady gait i.
Hydrocephalus treated by ventriculostomy carries with it the risk of upward transtentorial herniation. Signs and Symptoms of Cerebellar Disorders Nausea Vomiting Vertigo Slurred speech Unsteadiness Uncoordinated limb movements Headache Occurs in the frontal, occipital, or upper cervical regions, and usually occurs on the side of the lesion Most Abnormalities a Combination Of Dysmetria Abnormal undershoot or overshoot i.
May change directions depending upon the direction of gaze unlike peripheral vertigo. Vertical Nystagmus may occur. Otherwise it is hidden from view. Forgot Username? About MyAccess If your institution subscribes to this resource, and you don't have a MyAccess Profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.
Learn More. Sign in via OpenAthens. Sign in via Shibboleth. AccessBiomedical Science. AccessEmergency Medicine. Case Files Collection. Clinical Sports Medicine Collection. Davis AT Collection. Davis PT Collection. Murtagh Collection. About Search. Enable Autosuggest.
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. These fibers convey such information as: Where are the appropriate body parts proprioceptors , what is the current load on the muscle proprioceptors, somatosensory receptors, etc.
The error signal is believed to be conveyed by the climbing fiber inputs. Climbing fibers are known to be especially active when an unexpected event occurs, such as when a greater load than expected is placed on a muscle or when a toe is stubbed. Thus, the large divergence of input from the mossy fibers to the granule cells to the parallel fibers is believed to create complex representations of the entire sensory context at present and the desired motor output.
When the desired output is not achieved, the climbing fibers signal this error and trigger a calcium spike in the Purkinje cell. These findings indicate that both ipsilateral and contra-lateral movements of patients with unilateral cerebellar lesions are slightly impaired. Abstract Limb ataxia is seen as a sign of ipsilateral cerebellar dysfunction. Publication types Research Support, Non-U.
0コメント