The use of the word ganglia , however, is a bit of a misnomer according to contemporary neuroscience conventions. The word nucleus is generally used to describe clusters of neurons found in the central nervous system.
Thus, the basal ganglia might more accurately be considered nuclei. The separate nuclei of the basal ganglia all have extensive roles of their own in the brain, but they also are interconnected with one another to form a network that is thought to be involved in a variety of cognitive, emotional, and movement-related functions. The basal ganglia are best-known, however, for their role in movement. The contributions of the basal ganglia to movement are complex and still not completely understood.
In fact, the basal ganglia probably have multiple movement-related functions, ranging from choosing actions that are likely to lead to positive consequences to avoiding things that might be aversive. But the basal ganglia are most often linked to the initiation and execution of movements. To understand how this might work, think about the action of reaching out to pick up a pencil. Although it might seem like there would be very little movement-related activity going on in the brain at this point because you are sitting still , your brain is actually constantly at work to inhibit unwanted movements like jerking your hand involuntarily up in the air or suddenly turning your head to one side.
The basal ganglia are hypothesized to play a critical role in this type of movement inhibition, as well as in the release of that inhibition when you do have a movement that you want to make reaching for the pencil in this case.
These thalamic neurons in turn project to the motor cortex an area of the brain where many voluntary movements originate and can stimulate movement via these connections. The basal ganglia, however, continuously inhibit the thalamic neurons, which stops them from communicating with the motor cortex—inhibiting movement in the process. Then, the signal follows a circuit in the basal ganglia known as the direct pathway , which leads to the silencing of neurons in the globus pallidus and substantia nigra.
This frees the thalamus from the inhibitory effects of the basal ganglia and allows movement to occur. There is also a circuit within the basal ganglia called the indirect pathway , which involves the subthalamic nucleus and leads to the increased suppression of unwanted movements. It is thought that a balance between activity in these two pathways may facilitate smooth movement. Again, this is just one perspective on basal ganglia function, and despite the importance the basal ganglia are thought to have in movement, there is still much we need to learn to fully understand their contribution to it.
We can see the importance of the basal ganglia to movement, however, when we look at cases where the basal ganglia have been damaged. In Parkinson's disease , for example, dopaminergic neurons of the substantia nigra degenerate. A Location of basal ganglia components in idealized brain section. B Cell bridges between the caudate and putamen give a striated appearance.
The subthalamic nucleus is part of the diencephalon; as its name implies, it is located just below the thalamus. The substantia nigra is a midbrain structure, composed of two distinct parts: the pars compacta and the pars reticulata. The substantia nigra is located between the red nucleus and the crus cerebri cerebral peduncle on the ventral part of the midbrain. An area that is functionally analogous to the substantia nigra pars compacta is the ventral tegmental area, which is located nearby and makes a dopaminergic projection to the nucleus accumbens.
Historically, the amygdaloid complex and the claustrum were considered parts of the basal ganglia. However, modern usage usually restricts the term to those structures that cause the motor impairments characteristic of the extrapyramidal syndrome caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra.
For diagram simplicity, in this and subsequent figures, the caudate and putamen are represented by the putamen only, as the two regions have similar connections. The striatum is the main recipient of afferents to the basal ganglia Figure 4. These excitatory afferents arise from the entire cerebral cortex and from the intralaminar nuclei of the thalamus primarily the centromedian nucleus and parafascicularis nucleus.
The projections from different cortical areas are segregated, such that the frontal lobe projects predominantly to the caudate head and the putamen; the parietal and occipital lobes project to the caudate body; and the temporal lobe projects to the caudate tail. The primary motor cortex and the primary somatosensory cortex project mainly to the putamen, whereas the premotor cortex and supplementary motor areas project to the caudate head.
Other cortical areas project primarily to the caudate. Thus, along the C-shaped extent of the caudate nucleus, the caudate cells receive their input from the cortical regions that are close by. The enlarged head of the caudate reflects the large projection from the frontal cortex to the caudate. In addition, the nucleus accumbens ventral striatum receives a large input from limbic cortex. In the motor regions of the basal ganglia, there is a motor homunculus similar to that seen in the primary motor cortex.
Thus, the projections from the medial wall of the anterior paracentral lobule the part of M1 that contains a representation of the legs and torso innervate regions of the striatum that are next to the recipient zones from the dorsal surface of the precentral gyrus the part of M1 that contains a representation of the arms and hands. Similarly, the projections from the lateral surface of the precentral gyrus the part of M1 that contains a representation of the face innervate regions that are next to the arm and hand representation.
This topography of projections is maintained in the intrinsic circuitry of the basal ganglia. The major output structures of the basal ganglia are the globus pallidus internal segment GPint and the substantia nigra pars reticulata SNr Figure 4. Both of these structures make GABAergic, inhibitory connections on their targets. The GPint projects to a number of thalamic structures by way of two fiber tracts: the ansa lenticularis and the lenticular fasciculus.
The loop that processes sensorimotor information from the motor cortex and the somatosensory cortex projects to the ventral anterior VA and ventral lateral VL nuclei.
The loop that processes other neocortical information projects to the dorsomedial nucleus DM , intralaminar nuclei, and parts of the VA nucleus. There are two distinct pathways that process signals through the basal ganglia: the direct pathway and the indirect pathway.
These two pathways have opposite net effects on thalamic target structures. Excitation of the direct pathway has the net effect of exciting thalamic neurons which in turn make excitatory connections onto cortical neurons. Excitation of the indirect pathway has the net effect of inhibiting thalamic neurons rendering them unable to excite motor cortex neurons. The normal functioning of the basal ganglia apparently involves a proper balance between the activity of these two pathways.
One hypothesis is that the direct pathway selectively facilitates certain motor or cognitive programs in the cerebral cortex that are adaptive for the present task, whereas the indirect pathway simultaneously inhibits the execution of competing motor programs. An upset of the balance between the direct and indirect pathways results in the motor dysfunctions that characterize the extrapyramidal syndrome see below. Direct pathway.
Although the connectivity patterns of the direct and indirect pathways are relatively straightforward, the predominance of inhibitory connections in the system can make an understanding of the functional circuitry complicated and non-intuitive Figure 4. Solid lines represent direct pathway and dashed lines represent indirect pathway. The output from GPi is common to both pathways.
Green lines represent excitatory connections and red lines represent inhibitory connections. Click on individual pathway names to view each pathway in isolation. The direct pathway starts with cells in the striatum that make inhibitory connections with cells in the GPint. The GPint cells in turn make inhibitory connections on cells in the thalamus. Thus, the firing of GPint neurons inhibits the thalamus, making the thalamus less likely to excite the neocortex. When the direct pathway striatal neurons fire, however, they inhibit the activity of the GPint neurons.
This inhibition releases the thalamic neurons from inhibition i. Indirect pathway. The indirect pathway starts with a different set of cells in the striatum. These neurons make inhibitory connections to the external segment of the globus pallidus GPext. The GPext neurons make inhibitory connections to cells in the subthalamic nucleus, which in turn make excitatory connections to cells in the GPint. Remember that the subthalamic-GPint pathway is the only purely excitatory pathway within the intrinsic basal ganglia circuitry.
As we saw before, the GPint neurons make inhibitory connections on the thalamic neurons. To see the net effects of activation of the indirect pathway, let us work backwards from the GPint. When the GPint cells are active, they inhibit thalamic neurons, thus making cortex less active. When the subthalamic neurons are firing, they increase the firing rate of GPint neurons, thus increasing the net inhibition on cortex.
Firing of the GPext neurons inhibits the subthalamic neurons, thus making the GPint neurons less active and disinhibiting the thalamus. However, when the indirect pathway striatal neurons are active, they inhibit the GPext neurons, thus disinhibiting the subthalamic neurons.
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.
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