what is the striatum and how is it related to the fibers of the internal capsule
Basal ganglia
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The basal ganglia, or basal nuclei, are a group of subcortical structures establish deep inside the white matter of the encephalon. They form a part of the extrapyramidal motor organisation and piece of work in tandem with the pyramidal and limbic systems.
The basal ganglia consist of v pairs of nuclei: caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra. These nuclei are grouped into broader clusters;
- Striatum, which further consists of the:
- Dorsal striatum, made by the caudate nucleus and putamen
- Ventral striatum, equanimous of nucleus accumbens and olfactory tubercle (this part of the striatum is considered office of the limbic system)
- Globus pallidus, that consists of an internal segment (GPi) and an external segment (GPe)
- Subthalamic nucleus
- Substantia nigra
The function of the basal ganglia is to fine-melody the voluntary movements. They practise and then by receiving the impulses for the upcoming movement from the cerebral cortex, which they procedure and adjust. They convey their instructions to the thalamus, which and so relays this information back to the cortex. Ultimately, the fine-tuned movement educational activity is sent to the skeletal muscles through the tracts of the pyramidal motor organization. Basal ganglia mediate some and other higher cortical functions also, such every bit planning and modulation of movement, retention, centre movements, reward processing, and motivation.
This article volition discuss the anatomy and function of the basal ganglia.
| Definition | A group of subcortical nuclei that fine-tune the voluntary motor activeness |
| Parts | Striatum Dorsal striatum (caudate nucleus and putamen) Ventral striatum (nucleus accumbens and olfactory tubercle) Globus pallidus |
| Function | Planning and modulation of movement, retentiveness, eye movements, advantage processing, motivation |
Contents
- Overview
- Components
- Striatum
- Subthalamic nucleus
- Substantia nigra
- Connections
- Pathways
- Functions
- Clinical notes
- Sources
+ Prove all
Overview
The basal ganglia are one of the components in the neural concatenation that controls the voluntary motor activity. The supreme component of this chain is the cognitive cortex. Information technology generates the commands that ascertain the motor action of all skeletal muscles in the body. These commands descend through the pathways of the pyramidal system and synapse with the cranial nerve nuclei and motor neurons of the spinal cord. From here, the motor commands travel via the cranial and spinal nerves in order to reach the target muscles.
However, some extent of modulation and refinement of these cortical signals is necessary so that their motor execution at the muscular level happens as smoothly and precisely as planned. These adjustments are performed in the "accessory motor centers", with the about of import one being the basal ganglia. Despite being physically separated from each other, the basal ganglia are interconnected with many pathways making them a strong functional unity. Functionally, the basal ganglia are referred to every bit the extrapyramidal motor organisation although this term nowadays is not used widely. They receive and procedure the inputs from wide areas of the cerebral cortex, later on which they relay information technology back to the thalamus. The thalamus then forwards those refined inputs farther across the brain, mainly back to the cortex, and to the brainstem.
Phylogenetically, the oldest motor centers are the spinal cord and the reticular formation of the brainstem. With the development of the vertebrates, the brain gained new motor centers; the paleostriatum (globus pallidus) and neostriatum (caudate nucleus and putamen), which grew together with the cerebral cortex. Over time, the cerebral cortex and pyramidal system grew larger and developed a myriad of functional backdrop. With this, the extrapyramidal system vicious under the control of the new, pyramidal, motor system, being left with the autonomy to control the nuances of cortical activity, i.due east. to attune the movements.
Components
Striatum
The striatum is a complex nucleus located deep in subcortical structures of the forebrain, inside the insular lobe.
In the introduction, we mentioned that the striatum is equanimous of the dorsal and ventral parts. The ventral striatum is considered part of the limbic system, thus we will non describe it furthermore.
The dorsal striatum on the other hand is a component of the basal ganglia and commonly, information technology is this role that is called "striatum" in the literature, when we describe the basal ganglia. The dorsal striatum (or but the striatum) consists of 2 parts: the caudate nucleus and putamen. The parts of striatum are separated by the internal capsule, whose myelinated fibers radiate through striatum, giving it a characteristic striped appearance. Together with the globus pallidus, the striatum forms a structure called corpus striatum.
The striatum is the main input unit of the basal ganglia. It receives excitatory glutamatergic inputs from the cerebral cortex, whose synapsing pattern reflects the topography of the cortex. This ways that the caudal parts of the cortex project to the caudal part of the striatum, while the rostral parts of the cortex project to the rostral part of the striatum.
The substance of the striatum is mainly (80-95%) equanimous of projection neurons (medium-sized spiny neurons) and minor interneurons. The projection neurons are covered past numerous spines, hence their name. Functionally, they are inhibitory neurons that use GABA equally a neurotransmitter. The axons of these neurons form the direct and indirect pathways of basal ganglia, which project into the globus pallidus and substantia nigra
The interneurons of the striatum lack spines and are classified into four groups:
- Cholinergic big aspiny neurons
- Parvalbumin-containing GABAergic neurons
- Somatostatin/nitric oxide synthase-containing GABAergic neurons
- Calretinin-containing GABAergic
These neurons project to the thalamus, SNc, cerebral cortex, and control the activity of those regions.
Caudate nucleus
The caudate nucleus is an elongated C-shaped nucleus that lies inductive to the thalamus, merely lateral to the lateral ventricles and medial to the internal sheathing.
The caudate nucleus consists of the head, torso and tail. The head of the nucleus contributes to the lateral wall of the lateral ventricle. The tail of the caudate nucleus forms the roof of the junior horn of the lateral ventricle. It arches over the ventral surface of the thalamus, enters the temporal lobe and terminates by connecting with the amygdala. The rostral portion of the caudate nucleus is continuous with the putamen, and inferiorly it's bordered by nucleus accumbens.
The functions of the caudate nucleus lie inside the spectrum of functions that we described previously in the department virtually the striatum. More specifically, the caudate nucleus integrates sensory information nearly the spatial position of the body and according to that, it sends the information about the necessary fine tunes of the motor response to that stimuli to the thalamus. Additionally, it contributes to torso and limb posture and the speed and accurateness of directed movements.
As well motor control, the caudate nucleus is involved in many tasks, such equally retentivity, goal-pursuit, learning, linguistic communication processing, emotions, etc.
Putamen
The putamen is a circular structure situated at the base of operations of the forebrain. It is the most lateral of the basal ganglia on the axial department of the brain. The putamen lies laterally to the globus pallidus and medially to the external capsule, covering it like a shell and extending both rostrally and caudally. Information technology is encircled past the caudate nucleus, from which it is separated by the internal capsule.
The putamen and globus pallidus are separated by a sparse layer of white matter called the medial medullary lamina.
The main part of the putamen is to regulate motor functions and influence various types of learning and information technology employs dopamine to perform its functions.
Nucleus accumbens and olfactory tubercle
Nucleus accumbens and olfactory tubercle are paired structures, situated at the base of the forebrain. They are components of the ventral striatum and component of input nuclei for the ventral tegmental area (VTA).
The nucleus accumbens is found in the rostral forebrain, where the head of the caudate nucleus and putamen see. The olfactory tubercle, notwithstanding, is situated ventral to the nucleus accumbens, between the optic chiasm and olfactory tract.
Both structures are non involved in the movements regulation, rather they play an important role in the "reward circuit" and are referred to as "limbic-motor interface". When nosotros do annihilation rewarding (eastward.thou. food, drugs, sexual practice), dopamine neurons in an area of the brain called the ventral tegmental area (VTA) are activated. These neurons project to the nucleus accumbens and the olfactory tubercle, and when they are activated it results in an increase in dopamine levels.
Globus pallidus
The globus pallidus is a paired subcortical structure, situated medially to the putamen and composed of inhibitory GABAergic projection neurons, which fire spontaneously and irregularly at high frequency. Information technology is divided by a vertically placed sheet of white thing, the medial (internal) medullary lamina, into external (GPe) and internal (GPi) segments.
The superior and medial aspects of the globus pallidus are in contact with the internal sheathing. The capsule separates the caudate nucleus from the globus pallidus. The inferior surface of the globus pallidus is in contact with the subthalamic nucleus and zona incerta, which separate it from the thalamus. Anteriorly, the globus pallidus is closely related to the substantia innominata and the hypothalamus. More caudally, it is in shut proximity to the optic tract. And because the putamen and globus pallidus are in close connection, with their combined shapes resembling a bean, they are referred to as the lenticular nucleus.
Both the GPe and GPi play an essential role in the modulation of the motor program, more than specifically in the direct and indirect pathways.
They both receiveinhibitory GABA-ergic input from the striatum, through striatopallidal fibers, besides known as Wilson's pencils. Fibers that projection from the striatum to the internal part of the globus pallidus are office of the indirect pathway of the motor loop. Meanwhile, fibers that connect the striatum with the external function of the globus pallidus are part of the direct pathway of the motor loop.
The output fibers of the globus pallidus are the pallidothalamic tracts. They dissever into: ansa lenticularis, lenticular fascicles and thalamic fasciculus. Together they are part of the Forel's field. These structures are responsible for connecting the globus pallidus and thalamic nuclei.
The globus pallidus is involved in the constant subtle regulation of movement to create shine and precise motor actions and has a primarily inhibitory action that balances the excitatory action of the cerebellum.
Subthalamic nucleus
The subthalamic nuclei (STN), as well known as Luys' bodies, are pocket-size biconvex paired structures located within the subthalamus. The subthalamic nucleus is not an anatomical part of the basal ganglia. However, given their functional connection, the subthalamus is listed equally a functional part of the basal ganglia.
The subthalamic nucleus lies at the junction of the diencephalon and midbrain, ventral to the thalamus and ventro-lateral to the red nucleus. Anteriorly its bordered by the substantia nigra and medially by the internal capsule. STN is closely related to the Forel's fieldsouthward and the pallidothalamic fibers, which entwine effectually its ventral and medial borders before arching back over its dorsomedial surface equally the thalamic fasciculus. These fibers thus tend to separate the zone incerta from the subthalamic nucleus below and the thalamus higher up.
The subthalamic nuclei are composed of excitatory glutamatergic projection neurons. Information technology receives excitatory inputs from the frontal cortex in a somatotopically organized style. Based on this, the subthalamic nucleus is divided into three parts:
- The dorsal (motor) part, which receives inputs from the master motor cortex
- The ventrolateral (associative) office, which receives inputs from the prefrontal cortex and frontal eye fields, and
- The ventromedial (limbic) part, which receives inputs from the inductive cingulate cortex.
The function of the subthalamic nucleus is unknown, but some theories suggest its crucial role in the hyperdirect pathway in order to attune the planned motor program. Additionally, considering the nucleus firing pattern, the subthalamic nucleus is considered the "pace-maker" of the basal ganglia.
Substantia nigra
The substantia nigra is a small motor nucleus, within the anterior part of the midbrain, between the cerebral peduncle and tegmentum of the midbrain. Despite its location in the midbrain, part-wise information technology is considered office of basal ganglia.
The substantia nigra consists of ii parts with very different connections and functions: the pars compacta (SNc) and the pars reticulata (SNr). It divides the cerebral peduncles from the tegmentum within both sides of the midbrain. Dorso-medially it is bordered by the subthalamic and reddish nuclei, and laterally by the medial lemniscus and the geniculate bodies.
The pars compacta comprises the dorsal portion of the substantia nigra. It consists of numerous closely packed melanin-filled neurons that give the substantia nigra its distinctive dark color. The pars reticulata lie ventral to pars compacta. It is larger than the pars compacta, merely information technology contains fewer cells than it.
Medially to the substantia nigra is a zone chosen the ventral tegmental expanse. It is a pocket-size group of scattered cells that accept like functions to the pars compacta and may actually be considered as an extension of this function.
The pars compacta serve mainly as an output to the basal ganglia circuit, supplying the striatum with dopamine, through specific D1 and D2 neurons within the nigrostriatal pathways. The pars reticulata, though, serves mainly as an input, conveying signals from the basal ganglia to the thalamus.
The loss of dopamine neurons in SNc is believed to exist the reason for the evolution of Parkinson'due south disease and another parkinsonic syndromes.
Struggling with remembering the function of each basal nuclei? Try to sympathise the importance of active recall in learning anatomy.
Connections
The major efferents (outputs) of the basal ganglia consist of the neurons that project towards the thalamus and brainstem from the internal part of globus pallidus and the reticular part of the substantia nigra. These are ansa lenticularis and lenticular fasciculus.
Afferents (inputs) to the basal ganglia include the following:
- From the entire cerebral cortex - through the corticostriatal pathway, which is the largest afferent connexion of the basal ganglia. The fibers are glutamatergic – releasing the neurotransmitter glutamate to excite the striatal neurons.
- From the substantia nigra - fibers arising in the pars compacta of the substantia nigra reach the striatum, forming the nigrostriatal connections. This very important connection of the basal ganglia ensures a continuous supply of dopamine to the striatum, which promotes the regulation of direct, indirect and hyperdirect pathways.
- From the thalamus - fibers from the thalamus to the basal ganglia form the thalamostriatal connections or the thalamostriatal afferents. Those connections or pathways are glutamatergic and responsible for excitatory effects on the cerebral cortex and brainstem.
- From the reticular formation of the brainstem (specifically from the midbrain) - afferents from the reticular formation are noradrenergic and responsible, besides vital functions, for modulation and regulation of flexor and extensor muscles tonus in voluntary movements.
In summary, the basal nuclei tin be grouped functionally into iv categories:
- Input nuclei: striatum and subthalamic nucleus, which receive cortical inputs
- Output nuclei: internal part of globus pallidus and reticular part of substantia nigra, which project outside the basal ganglia to the thalamus and brainstem
- Connecting nucleus: external part of globus pallidus, which connects the input nuclei to the output nuclei.
- Modulatory nucleus: compact office of substantia nigra, which modulates the activity of the basal ganglia.
Pathways
The basal nuclei modulate motor function through diverse pathways in order to initiate, end, or modulate the extent of the movement.
These are the following:
- Direct pathway: which is responsible for the initiation of the movement. In social club to make this happen, the direct pathway funnels the data from the striatum to GPi/SNr via GABAergic inhibitory projections. This inhibition releases the firing from the thalamocortical neurons to initiate the movement.
- Indirect pathway, which has a net excitatory event on the same structures. The neurons from the external part of globus pallidus transport inhibitory fibers to the subthalamic nucleus instead of sending directly to the thalamus (hence its proper noun "indirect"). From the subthalamic nucleus, neurons ship their axons to the internal office of the globus pallidus and reticular part of the substantia nigra and and then go on as the direct pathway with GABAergic inhibitory neurons to the thalamus and glutamate excitatory efferents to the cortex. And so, functionally, the striatum inhibits the external globus pallidus, and that causes disinhibition of the subthalamus.
- Hyperdirect pathway, via which the internal part of globus pallidus and reticular function of the substantia nigra receive stiff excitatory signals from the cortex directly through STN and has a shorter conduction time compared to the directly and indirect pathways. The hyperdirect pathway consists of neurons projecting from the cortex directly to the subthalamic nucleus (STN), skipping the striatum. Therefore, the glutamatergic excitatory neurons of the STN can and so excite the GPi/SNr thus suppressing thalamic activity on the cerebral cortex and increasing inhibitory influences on the upper motor neurons.
Considering the conduction path and time, we can say that the hyperdirect and indirect pathways brand clear initiation and termination of the selected motor program, while at the same time canceling other competing motor programs.
If you want to acquire more about the pathways of the basal ganglia please read the commodity about the direct, indirect and hyperdirect pathways of basal ganglia.
Functions
There is a growing number of studies focused on the functions of the basal ganglia, as its functions are yet to be fully understood. However, the post-obit are functions have been clearly established by now:
- Planning and modulation of movement pathways
- Advantage processing and motivation
- Decision making
- Working memory
- Center movements
Moreover, the basal nuclei use proprioceptive feedback from the periphery to compare the movement patterns generated by the cognitive cortex with the actual movement, so that the movement is subject to ongoing refinement by a continuous servo-control mechanism.
Also, the basal ganglia have been shown to play an important role in motivation. Because that the basal ganglia circuits are influenced heavily past extracellular dopamine, high levels of it accept been linked to satiated "euphoria", medium levels with seeking and depression with aversion. The activation of the basal nuclei pathway that causes the disinhibition of the thalamus, leads to activation of the prefrontal cortex and ventral striatum. At that place is besides evidence that other basal ganglia structures including the globus pallidus, pars medialis and subthalamic nucleus are involved in reward processing.
Regarding memory, the same structures in the prefrontal cortex are shown to be involved in the retention gates and focus. By using the basal ganglia's straight and indirect pathways as a relay betwixt the input data from surroundings to the cerebral structures involved in memory storage.
Now that you learned everything virtually the basal ganglia, test yourself and consolidate your knowledge with our quiz below!
Clinical notes
Degeneration of the basal ganglia and, consequently, its dysfunction tin lead to several neurological weather. The characteristic feature of the basal ganglia lesion is a movement disorder in which there is either too piffling move (hypokinesia), as well much (hyperkinesia), or a combination of both, depending on the location and extent of the affected structure.
Bradykinesia represents a generalized slowness of motion and is the most common hypokinesia. The prototypical hypokinetic motility disorder is Parkinson'due south illness. Parkinson'due south disease results from the degeneration of the dopaminergic nigrostriatal projection. In substantia nigra pars compacta, dopaminergic neurons are decreased, then the dopaminergic output to the striatum is decreased. This leads to the reduction of the inhibition of the indirect (inhibitory) pathway and reduction of the excitation of the direct (excitatory) pathway resulting in bradykinesia, which is the principal symptom of Parkinson'due south disease. The condition is too characterized by resting tremor, rigidity and postural instability.
Parkinsonism is the umbrella term used to draw the symptoms of bradykinesia, tremor, and rigidity. Parkinson'due south affliction is the about common type of parkinsonism, but at that place are as well some rarer types where a specific cause can be identified (ex. drug-induced parkinsonism, progressive supranuclear palsy).
The hyperkinetic movement disorders, unlike Parkinson'due south disease, are characterized by too much movement. The different clinical types of hyperkinesia include dystonia, chorea, ballism, athetosis tremor, myoclonus, tics, and others.
Dystonia is characterized by involuntary, sustained musculus contraction that leads to abnormal postures of the neck, toes, easily, or other parts of the torso. The exact mechanism of dystonia is not completely articulate. Nevertheless, the best prove suggests that in that location is relevant hypoactivity in the indirect (inhibitory) pathway resulting in less inhibition and more than unwanted motion. The clinical types of dystonia classify as either focal, that affects only isolated muscle groups (ex. Spasmodic Torticollis), or generalized, that typically affects muscles in the torso and limbs, and sometimes the cervix and face (ex. DYT1 mutation).
Chorea, ballism, and athetosis are irregular, involuntary, jerky, and purposeless, "dance-like" movements. They are relatively like in physiology. Ballism has a more proximal (shoulder and hip) origin and is slower than chorea. Athetosis, in nature, is slower and more than twitching.
Several disorders are presenting with chorea, and the most mutual is Huntington'southward disease. It is characterized past the degeneration of striatal GABAergic neurons, causing atrophy of the head of the caudate nucleus. Huntington's disease is a genetic, autosomal dominant affliction manifested past chorea, dementia and psychiatric abnormalities, bulbar symptoms, and gait disturbance.
Hemiballismus (ballism on the one side of the body) typically occurs after a lesion (ex. stroke, tumour) adjacent to the subthalamic nucleus.
Tremor is an abnormal involuntary, rhythmic and oscillatory motility of the manus, head, or other parts of the body. Usually, the basal ganglia, cerebellum, and the subthalamic nucleus are involved. Nonetheless, intention tremor is likewise seen in disorders of the cerebellum, in which case, the tremor comes when the individual tries to perform a voluntary move (intention tremor).
Myoclonus is a jerky, involuntary, and usually arrhythmic movement. To imagine how myoclonus looks similar, remember of body jerks equally i is falling comatose, this is physiological myoclonus. A full list of myoclonus-related disorders is very long. Myoclonus tin can present in some hereditary diseases (ex. Juvenile myoclonic epilepsy) and any central nervous syndrome lesions like tumor, hemorrhage, stroke or abscess.
Tics are cursory, stereotyped semi-voluntary movements, which means that dissimilar other motion disorders, they are partially suppressible. Tics can be either motor (motor tics) or sounds (vocal tics). They are common in children and can appear as the effect of directly encephalon injury (ex. head trauma or encephalitis). Nevertheless, most of them are idiopathic and are office of the spectrum of Gilles de la Tourette syndrome or another idiopathic tic disorder.
Most of the diseases typically have a large phenomenology of movement disorders, which includes both hypo- and hyperkinesia. In any case of young-onset parkinsonism, dystonia or other move disorders, Wilson's disease should be considered as this disorder is treatable and the effects of non-recognition may be severe. An autosomal recessive defect causes this disease in copper transport and the neurological manifestations are due to the accumulation of copper in basal ganglia, particularly in the putamen. Because copper also accumulates in other tissues, similar eyes, Kayser–Fleischer rings, a brown-greenish pigmentation of the Descemet membrane of the center are diagnostic signs of Wilson'south disease.
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