With the subthalamic neurons free to fire, the GPint neurons inhibit the thalamus, thereby producing a net inhibition on the motor cortex. Thus, as a result of the complex sequences of excitation, inhibition, and disinhibition, the net effect of the cortex exciting the direct pathway is to further excite the cortex positive feedback loop , whereas the net effect of cortex exciting the indirect pathway is to inhibit the cortex negative feedback loop.
Presumably, the function of the basal ganglia is related to a proper balance between these two pathways. Motor cortex neurons have to excite the proper direct pathway neurons to further increase their own firing, and they have to excite the proper indirect pathways neurons that will inhibit other motor cortex neurons that are not adaptive for the task at hand see below.
An important pathway in the modulation of the direct and indirect pathways is the dopaminergic, nigrostriatal projection from the substantia nigra pars compacta to the striatum Figure 4. Direct pathway striatal neurons have D1 dopamine receptors, which depolarize the cell in response to dopamine. In contrast, indirect pathway striatal neurons have D2 dopamine receptors, which hyperpolarize the cell in response to dopamine.
The nigrostriatal pathway thus has the dual effect of exciting the direct pathway while simultaneously inhibiting the indirect pathway. Because of this dual effect, excitation of the nigrostriatal pathway has the net effect of exciting cortex by two routes, by exciting the direct pathway which itself has a net excitatory effect on cortex and inhibiting the indirect pathway thereby disinhibiting the net inhibitory effect of the indirect pathway on cortex. The function of the basal ganglia in motor control is not understood in detail.
It appears that the basal ganglia is involved in the enabling of practiced motor acts and in gating the initiation of voluntary movements by modulating motor programs stored in the motor cortex and elsewhere in the motor hierarchy Figure 4.
Thus, voluntary movements are not initiated in the basal ganglia they are initiated in the cortex ; however, proper functioning of the basal ganglia appears to be necessary in order for the motor cortex to relay the appropriate motor commands to the lower levels of the hierarchy.
The proper motor programs are selected based on the desired motor output relayed from cortex. Note that the complex circuits of the direct and indirect pathways are schematically diagramed as single neurons for clarity of illustration.
Recall that the major output from the basal ganglia is an inhibitory connection from the GPint or SNr to the thalamus or superior colliculus. Studies of eye movements in monkeys have shed light on the function of the basal ganglia loop.
Normally, the SNr neurons are tonically active, suppressing the output of the collicular neurons that control saccadic eye movements. When the direct pathway striatal neurons are excited by the cortical frontal eye fields, the SNr neurons are momentarily inhibited, releasing the collicular neurons from inhibition.
This allows the appropriate collicular neurons to signal the target of the eye movement, allowing the monkey to change its gaze to a new location. The movement was initiated in the frontal eye fields; however, the proper activation of the eye movement required that collicular neurons be released from the inhibition of the basal ganglia. What is the function of the tonic inhibitory output of the basal ganglia? Recall from the Motor Cortex chapter that stimulating the motor cortex of monkeys at various locations results in stereotyped sequences of movements, such as bringing the hand to the mouth or adopting a defensive posture.
It is important that only one motor program be active at a given time, however, such that one motor act e. It is thought that the basal ganglia is normally active in suppressing inappropriate motor programs, and that activation of the direct pathway temporarily releases one motor program from inhibition, enabling it to be executed by the organism.
Thus, the basal ganglia act as a gate that enables the execution of automatic programs in the hierarchy. A If a reward occurs unexpectedly, the dopaminergic neuron fires briskly. This may strengthen the cortical motor programs that led up to the reward. B If a reward occurs that the monkey previously learned was predicted by a stimulus, the neuronal firing is not altered, thus signaling that all is proceeding normally.
C If the reward-predicting stimulus does not produce a reward, the neuronal firing is inhibited. This inhibition may weaken the cortical motor programs that did not produce the expected reward. Which motor programs should be released from inhibition at a given moment?
The basal ganglia may have a major role in learning what motor acts result in rewards for the organism. This information is provided by the dopaminergic neurons of the SNc and ventral tegmental nucleus. Recordings from these neurons in monkeys have shown that they tend to respond when the monkey receives an unexpected reward, and they tend to be inhibited when the monkey fails to receive an expected reward Figure 4.
In this way, motor habits can be constructed that tend to reward the animal. As mentioned earlier, there are a number of cortical loops through the basal ganglia that involve prefrontal association cortex and limbic cortex. Through these loops, the basal ganglia are thought to play a role in cognitive function that is similar to their role in motor control. That is, the basal ganglia are involved in selecting and enabling various cognitive, executive, or emotional programs that are stored in these other cortical areas.
Moreover, the basal ganglia appear to be involved in certain types of learning. Jones, J. Corpus striatum. Reference article, Radiopaedia. Central Nervous System. URL of Article. History and etymology The term originates from the Latin "striatus", meaning "striped", referring to the caudatolenticar bridges of grey matter crossing the internal capsule from the putamen to the caudate nucleus 2.
Churchill Livingstone. Read it at Google Books - Find it at Amazon 2. Finger S. Origins of Neuroscience. The striatum primarily the dorsal striatum is one of the main input areas for the basal ganglia. It receives the bulk of its incoming fibers from the cerebral cortex, but it also receives afferent fibers from the substantia nigra and thalamus.
The fibers from the cerebral cortex i. However, it should be noted that the fibers that travel to the striatum from the cortex are not only movement-related. Indeed, the striatum and more generally the basal ganglia is thought to be involved in many aspects of cortical function and thus many aspects of cognition , and so it receives input not just from motor areas but also from areas throughout the cortex.
The afferents from the substantia nigra , collectively known as the nigrostriatal pathway , seem to play an especially important role in movement as they are severely affected by neurodegeneration in patients suffering from Parkinson's disease.
The role of the fibers from the thalamus , known as thalamostriate fibers, is not very well understood in humans. Fibers that leave the striatum mostly travel to the main output nuclei of the basal ganglia: the globus pallidus and substantia nigra.
From there, the fibers extend to the thalamus and other areas; projections from the thalamus carry the information back to the cortex. The ventral striatum contains the nucleus accumbens , a nucleus that has been extensively studied for its role in rewarding experiences. Corpus striatum is considered to be the largest structure present in basal ganglia. It is divided into two parts, dorsal striatum and ventral striatum.
A brief detail of these parts is discussed below. It is further divided by the internal capsule into two parts, caudate nucleus, and putamen. Caudate Nucleus is a large C-shaped mass of grey matter a head, body, and tail. The head of the caudate nucleus is large and round and forms the anterior horn of the fourth ventricle. It is also continuous with the putamen of the lentiform nucleus. The head is continuous with a linear and narrow-body which in turn tapers into a tail.
The tail follows the contour of the lateral ventricle. The tail of the caudate nucleus terminates as the amygdaloid nucleus. Putamen is a round nucleus located at the base of the forebrain.
It is continuous with the head of the caudate nucleus. Nucleus accumbens is located in basal forebrain in the preoptic area. It has an outer shell and an inner core.
The inner core is a part of the ventral striatum. Most of the neurons in nucleus accumbens are GABAergic neurons. Olfactory tubercle is a processing center that is common to both the olfactory cortex and ventral striatum.
It is also located in the basal area of the forebrain. Corpus striatum receives a number of connections from other areas of the brain. These connections can be divided into the afferent fibers entering the striatum and the efferent fibers leaving it.
A brief detail is given below. The most important input fibers come to striatum from the cortex. It receives projection fibers arising from the pyramidal neurons located in the fifth layer of cortex.
0コメント