Phylogenetically, the vestibulocerebellum is the oldest part of the cerebellum. As its name implies, it is involved in vestibular reflexes such as the vestibuloocular reflex; see below and in postural maintenance. The spinocerebellum comprises the vermis and the intermediate zones of the cerebellar cortex, as well as the fastigial and interposed nuclei.
As its name implies, it receives major inputs from the spinocerebellar tract. Its output projects to rubrospinal, vestibulospinal, and reticulospinal tracts. It is involved in the integration of sensory input with motor commands to produce adaptive motor coordination. The cerebrocerebellum is the largest functional subdivision of the human cerebellum, comprising the lateral hemispheres and the dentate nuclei. Its name derives from its extensive connections with the cerebral cortex, via the pontine nuclei afferents and the VL thalamus efferents.
It is involved in the planning and timing of movements. In addition, the cerebrocerebellum is involved in the cognitive functions of the cerebellum. The cerebellar cortex is divided into three layers Figure 5. The innermost layer, the granule cell layer, is made of 5 x 10 10 small, tightly packed granule cells. The middle layer, the Purkinje cell layer, is only 1-cell thick. The outer layer, the molecular layer, is made of the axons of granule cells and the dendrites of Purkinje cells, as well as a few other cell types.
The Purkinje cell layer forms the border between the granule and molecular layers. This basic pattern is repeated throughout all regions of the cerebellum. Granule cells. Granule cells are very small, densely packed neurons that account for the huge majority of neurons in the cerebellum.
Indeed, cerebellar granule cells account for more than half of the neurons in the entire brain. These cells receive input from mossy fibers and project to the Purkinje cells.
This view shows that the cell is virtually flat in this dimension. Note the parallel fibers of the granule cells that run perpendicularly to the Purkinje cell. Purkinje cells. The Purkinje cell is one of the most striking cell types in the mammalian brain. Its apical dendrites form a large fan of finely branched processes Figure 5. Remarkably, this dendritic tree is almost two-dimensional; looked at from the side, the dendritic tree is flat click PLAY on Figure 5.
Moreover, all Purkinje cells are oriented in parallel. This arrangement has important functional considerations, as we shall see below. Other cell types. In addition to the major cell types granule cells and Purkinje cells , the cerebellar cortex also contains various interneuron types, including the Golgi cell, the basket cell, and the stellate cell. The cerebellar cortex has a relatively simple, stereotyped connectivity pattern that is identical throughout the whole structure.
Figure 6 illustrates a simplified diagram of the connectivity of the cerebellum. Cerebellar input can be divided into two distinct classes. The Purkinje cell is the sole source of output from the cerebellar cortex.
It is important to note that Purkinje cells make inhibitory connections onto the cerebellar nuclei. Note the distinction between the Purkinje cells, which constitute the sole output of the cerebellar cortex, and the cerebellar nuclei, which constitute the sole output of the entire cerebellum.
Almost all of the spikes generated by the Purkinje cell are caused by its parallel-fiber inputs. The Purkinje cell spikes that are generated by climbing fibers are calcium-spikes, however, which allow the climbing fibers to initiate a number of calcium-dependent changes in the Purkinje cell. As described below, one important change appears to be a long-lasting change in the strength of the parallel-fiber inputs to the Purkinje cell.
Much of what is known about cerebellar function comes from studies of patients with cerebellar damage. In general, such patients display uncoordinated voluntary movements and problems maintaining balance and posture.
The following are some symptoms of cerebellar damage we will discuss more symptoms in the next chapter :. Click PLAY to begin demonstration. Under normal conditions, when a human or animal subject rotates the head back and forth, the eyes rotate in an equal and opposite direction in order to keep the image stable on the retina. The vestibular system provides the input regarding the head movement, and the motor system has to learn the precise output commands in order to keep the image stable. Over time, however, the motor system learns to move the eyes faster e.
When the goggles are removed, the eyes now move too quickly, causing retinal slip in the same direction as head movement. With time, the system will learn to calibrate the VOR again. Patients and experimental animals with damage to the vestibulocerebellum are not able to adapt their VOR to the addition and removal of the goggles, demonstrating the role of the cerebellum in this form of motor learning.
A second example of cerebellum-dependent motor learning involves the execution of accurate, coordinated movements. Subjects wore prism goggles that shifted the visual image to the right, and they were asked to then throw balls at a target on the wall. Because of the prisms, the accuracy of the subjects was initially quite low, as the balls consistently hit to the left of the target. With repeated practice, however, the subjects became more and more accurate at hitting the target.
When the goggles were removed, the subject now began to throw the balls to the right of the target, because their motor programs had been recalibrated to use the shifted visual input. Over time, once again, they gradually increased their accuracy. Patients with cerebellar damage never learned to compensate for the prism, as their balls always landed to the left of the target when the goggles were worn.
When the goggles were removed, they were immediately accurate at hitting the target, because they never made compensations for the earlier prism trials.
A third example involves the Pavlovian classical conditioning of the eye blink reflex. In this task, a neutral stimulus such as a tone is paired with a noxious stimulus such as a puff of air to the eye that causes a reflexive eye blink.
Over time, experimental animals will learn to close their eye when the tone occurs, in anticipation of the air puff. This learned eyelid closure is remarkably well-timed to peak at the expected time of the puff. Animals with cerebellar damage do not learn to produce the eyelid closure in response to the tone.
What do the various symptoms of cerebellar damage have in common that reveal the function of the cerebellum? A number of different theories have been proposed. Recall the discussion in Chapter 1 of the ubiquitous use of sensory information in motor control. The cerebellum receives extensive sensory input, and it appears to use this input to guide movements in both a feedback and feedforward control manner. In a feedback controller, a desired output is compared continuously with the actual output, and adjustments are made during the execution of the movement until the actual movement matches the desired movement.
A common example of a feedback control system is the thermostat in your home Figure 5. The myotatic reflex is an example of a feedback control system in the spinal cord. The thermostat is set to a desired temperature e. 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. All sensory receptors rely on one of these four capacities to detect changes in the environment, but may be tuned to detect specific characteristics of each to perform a specific sensory function.
In some cases, the mechanism of action for a receptor is not clear. For example, hygroreceptors that respond to changes in humidity and osmoreceptors that respond to the osmolarity of fluids may do so via a mechanosensory mechanism or may detect a chemical characteristic of the environment.
Sensory receptors perform countless functions in our bodies. During vision, rod and cone photoreceptors respond to light intensity and color. During hearing, mechanoreceptors in hair cells of the inner ear detect vibrations conducted from the eardrum. During taste, sensory neurons in our taste buds detect chemical qualities of our foods including sweetness, bitterness, sourness, saltiness, and umami savory taste.
During smell, olfactory receptors recognize molecular features of wafting odors. During touch, mechanoreceptors in the skin and other tissues respond to variations in pressure. Adequate stimulus can be used to classify sensory receptors. Somatic sensory receptors near the surface of the skin can usually be divided into two groups based on morphology:. A tonic receptor is a sensory receptor that adapts slowly to a stimulus, while a phasic receptor is a sensory receptor that adapts rapidly to a stimulus.
As we exist in the world, our bodies are tasked with receiving, integrating, and interpreting environmental inputs that provide information about our internal and external environments. Our brains commonly receive sensory stimuli from our visual, auditory, olfactory, gustatory, and somatosensory systems. Remarkably, specialized receptors have evolved to transmit sensory inputs from each of these sensory systems.
Sensory receptors code four aspects of a stimulus:. Receptors are sensitive to discrete stimuli and are often classified by both the systemic function and the location of the receptor. Sensory receptors are found throughout our bodies, and sensory receptors that share a common location often share a common function.
For example, sensory receptors in the retina are almost entirely photoreceptors. Our skin includes touch and temperature receptors, and our inner ears contain sensory mechanoreceptors designed for detecting vibrations caused by sound or used to maintain balance. Force -sensitive mechanoreceptors provide an example of how the placement of a sensory receptor plays a role in how our brains process sensory inputs. While the cutaneous touch receptors found in the dermis and epidermis of our skin and the muscle spindles that detect stretch in skeletal muscle are both mechanoreceptors, they serve discrete functions.
In both cases, the mechanoreceptors detect physical forces that result from the movement of the local tissue, cutaneous touch receptors provide information to our brain about the external environment, while muscle spindle receptors provide information about our internal environment. Muscle spindle : Mammalian muscle spindle showing typical position in a muscle left , neuronal connections in spinal cord middle , and an expanded schematic right.
The spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibers. The sensory endings of a primary group Ia afferent and a secondary group II afferent coil around the non-contractile central portions of the intrafusal fibers.
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