Substantia nigra | |
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![]() Substantia nigra highlighted in red. | |
![]() Section through superior colliculus showing substantia nigra. | |
Details | |
Part of | Midbrain, basal ganglia |
Identifiers | |
Latin | substantia nigra |
Acronym(s) | SN |
MeSH | D013378 |
NeuroNames | 536 |
NeuroLex ID | birnlex_789 |
TA98 | A14.1.06.111 |
TA2 | 5881 |
FMA | 67947 |
Anatomical terms of neuroanatomy |
The substantia nigra (SN) is a basal ganglia structure located in the midbrain that plays an important role in reward and movement. Substantia nigra is Latin for "black substance", reflecting the fact that parts of the substantia nigra appear darker than neighboring areas due to high levels of neuromelanin in dopaminergic neurons. [1] Parkinson's disease is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta. [2]
Although the substantia nigra appears as a continuous band in brain sections, anatomical studies have found that it actually consists of two parts with very different connections and functions: the pars compacta (SNpc) and the pars reticulata (SNpr). The pars compacta serves mainly as a projection to the basal ganglia circuit, supplying the striatum with dopamine. The pars reticulata conveys signals from the basal ganglia to numerous other brain structures. [3]
The substantia nigra, along with four other nuclei, is part of the basal ganglia. It is the largest nucleus in the midbrain, lying dorsal to the cerebral peduncles. Humans have two substantiae nigrae, one on each side of the midline.
The SN is divided into two parts: the pars reticulata (SNpr) and the pars compacta (SNpc), which lies medial to the pars reticulata. Sometimes, a third region, the pars lateralis, is mentioned, though it is usually classified as part of the pars reticulata. The (SNpr) and the internal globus pallidus (GPi) are separated by the internal capsule. [4]
The pars reticulata bears a strong structural and functional resemblance to the internal part of the globus pallidus. The two are sometimes considered parts of the same structure, separated by the white matter of the internal capsule. Like those of the globus pallidus, the neurons in pars reticulata are mainly GABAergic. [5] [6]
The main input to the SNpr derives from the striatum. It comes by two routes, known as the direct and indirect pathways. The direct pathway consists of axons from medium spiny cells in the striatum that project directly to pars reticulata. The indirect pathway consists of three links: a projection from striatal medium spiny cells to the external part of the globus pallidus; a GABAergic projection from the globus pallidus to the subthalamic nucleus, and a glutamatergic projection from the subthalamic nucleus to the pars reticulata. [6] [7] [ better source needed ] Thus, striatal activity via the direct pathway exerts an inhibitory effect on neurons in the (SNpr) but an excitatory effect via the indirect pathway. The direct and indirect pathways originate from different subsets of striatal medium spiny cells: They are tightly intermingled, but express different types of dopamine receptors, as well as showing other neurochemical differences.
Significant projections occur to the thalamus (ventral lateral and ventral anterior nuclei), superior colliculus, and other caudal nuclei from the pars reticulata (the nigrothalamic pathway), [8] which use GABA as their neurotransmitter. In addition, these neurons form up to five collaterals that branch within both the pars compacta and pars reticulata, likely modulating dopaminergic activity in the pars compacta. [9]
The substantia nigra is an important player in brain function, in particular, in eye movement, motor planning, reward-seeking, learning, and addiction. Many of the substantia nigra's effects are mediated through the striatum. The nigral dopaminergic input to the striatum via the nigrostriatal pathway is intimately linked with the striatum's function. [10] The co-dependence between the striatum and substantia nigra can be seen in this way: when the substantia nigra is electrically stimulated, no movement occurs; however, the symptoms of nigral degeneration due to Parkinson's is a poignant example of the substantia nigra's influence on movement. In addition to striatum-mediated functions, the substantia nigra also serves as a major source of GABAergic inhibition to various brain targets.
The pars reticulata of the substantia nigra is an important processing center in the basal ganglia. The GABAergic neurons in the pars reticulata convey the final processed signals of the basal ganglia to the thalamus and superior colliculus. In addition, the pars reticulata also inhibits dopaminergic activity in the pars compacta via axon collaterals, although the functional organization of these connections remains unclear.
The GABAergic neurons of the pars reticulata spontaneously fire action potentials. In rats, the frequency of action potentials is roughly 25 Hz. [11] The purpose of these spontaneous action potentials is to inhibit targets of the basal ganglia, and decreases in inhibition are associated with movement. [12] The subthalamic nucleus gives excitatory input that modulates the rate of firing of these spontaneous action potentials. However, lesion of the subthalamic nucleus leads to only a 20% decrease in pars reticulata firing rate, suggesting that the generation of action potentials in the pars reticulata is largely autonomous, [13] as exemplified by the pars reticulata's role in saccadic eye movement. A group of GABAergic neurons from the pars reticulata projects to the superior colliculus, exhibiting a high level of sustained inhibitory activity. [14] Projections from the caudate nucleus to the superior colliculus also modulate saccadic eye movement. Altered patterns of pars reticulata firing such as single-spike or burst firing are found in Parkinson's disease [15] and epilepsy. [16]
The most prominent function of the pars compacta is motor control, [17] though the substantia nigra's role in motor control is indirect; electrical stimulation of the substantia nigra does not result in movement, due to mediation of the striatum in the nigral influence of movement. The pars compacta sends excitatory input to the striatum via D1 pathway that excites and activates the striatum, resulting in the release of GABA onto the globus pallidus to inhibit its inhibitory effects on the thalamic nucleus. This causes the thalamocortical pathways to become excited and transmits motor neuron signals to the cerebral cortex to allow the initiation of movement, which is absent in Parkinson's disease. However, lack of pars compacta neurons has a large influence on movement, as evidenced by the symptoms of Parkinson's. The motor role of the pars compacta may involve fine motor control, as has been confirmed in animal models with lesions in that region. [18]
The pars compacta is heavily involved in learned responses to stimuli. In primates, dopaminergic neuron activity increases in the nigrostriatal pathway when a new stimulus is presented. [19] Dopaminergic activity decreases with repeated stimulus presentation. [19] However, behaviorally significant stimulus presentation (i.e. rewards) continues to activate dopaminergic neurons in the substantia nigra pars compacta. Dopaminergic projections from the ventral tegmental area (bottom part of the "midbrain" or mesencephalon) to the prefrontal cortex (mesocortical pathway) and to the nucleus accumbens (mesolimbic pathway – "meso" referring to "from the mesencephalon"... specifically the ventral tegmental area) are implicated in reward, pleasure, and addictive behavior. The pars compacta is also important in spatial learning, the observations about one's environment and location in space. Lesions in the pars compacta lead to learning deficits in repeating identical movements, [20] and some studies point to its involvement in a dorsal striatal-dependent, response-based memory system that functions relatively independent of the hippocampus, which is traditionally believed to subserve spatial or episodic-like memory functions. [21]
The pars compacta also plays a role in temporal processing and is activated during time reproduction. Lesions in the pars compacta leads to temporal deficits. [22] As of late, the pars compacta has been suspected of regulating the sleep-wake cycle, [23] which is consistent with symptoms such as insomnia and REM sleep disturbances that are reported by patients with Parkinson's disease. Even so, partial dopamine deficits that do not affect motor control can lead to disturbances in the sleep-wake cycle, especially REM-like patterns of neural activity while awake, especially in the hippocampus. [24]
The substantia nigra is critical in the development of many diseases and syndromes, including parkinsonism and Parkinson's disease. There exists a study showing that high-frequency stimulation delivery to the left substantia nigra can induce transient acute depression symptoms. [25]
Parkinson's disease is a neurodegenerative disease characterized, in part, by the death of dopaminergic neurons in the SNpc. The major symptoms of Parkinson's disease include tremor, akinesia, bradykinesia, and stiffness. [26] Other symptoms include disturbances to posture, fatigue, sleep abnormalities, and depressed mood. [27]
The cause of death of dopaminergic neurons in the SNpc is unknown. However, some contributions to the unique susceptibility of dopaminergic neurons in the pars compacta have been identified. For one, dopaminergic neurons show abnormalities in mitochondrial complex 1, causing aggregation of alpha-synuclein; this can result in abnormal protein handling and neuron death. [28] Secondly, dopaminergic neurons in the pars compacta contain less calbindin than other dopaminergic neurons. [29] Calbindin is a protein involved in calcium ion transport within cells, and excess calcium in cells is toxic. The calbindin theory would explain the high cytotoxicity of Parkinson's in the substantia nigra compared to the ventral tegmental area. Regardless of the cause of neuronal death, the plasticity of the pars compacta is very robust; Parkinsonian symptoms do not generally appear until at least 30% of pars compacta dopaminergic neurons have died. [30] Most of this plasticity occurs at the neurochemical level; dopamine transport systems are slowed, allowing dopamine to linger for longer periods of time in the chemical synapses in the striatum. [31]
Menke, Jbabdi, Miller, Matthews and Zari (2010) used diffusion tensor imaging, as well as T1 mapping to assess volumetric differences in the SNpc and SNpr, in participants with Parkinson's compared to healthy individuals. These researchers found that participants with Parkinson's consistently had a smaller substantia nigra, specifically in the SNpr. Because the SNpr is connected to the posterior thalamus, ventral thalamus and specifically, the motor cortex, and because participants with Parkinson's disease report having a smaller SNprs (Menke, Jbabdi, Miller, Matthews and Zari, 2010), the small volume of this region may be responsible for motor impairments found in Parkinson's disease patients. This small volume may be responsible for weaker and/or less controlled motor movements, which may result in the tremors often experienced by those with Parkinson's. [32]
Oxidative stress and oxidative damage in the SNpc are likely key drivers in the etiology of Parkinson's disease as individuals age. [33] DNA damages caused by oxidative stress can be repaired by processes modulated by alpha-synuclein. [34] Alpha synuclein is expressed in the substantia nigra, but its DNA repair function appears to be compromised in Lewy body inclusion bearing neurons. [34] This loss may trigger cell death. [35] [36] [37]
Increased levels of dopamine have long been implicated in the development of schizophrenia. [38] However, much debate continues to this day surrounding this dopamine hypothesis of schizophrenia. Despite the controversy, dopamine antagonists remain a standard and successful treatment for schizophrenia. These antagonists include first generation (typical) antipsychotics such as butyrophenones, phenothiazines, and thioxanthenes. These drugs have largely been replaced by second-generation (atypical) antipsychotics such as clozapine and paliperidone. In general, these drugs do not act on dopamine-producing neurons themselves, but on the receptors on the post-synaptic neuron.
Other, non-pharmacological evidence in support of the dopamine hypothesis relating to the substantia nigra include structural changes in the pars compacta, such as reduction in synaptic terminal size. [39] Other changes in the substantia nigra include increased expression of NMDA receptors in the substantia nigra, and reduced dysbindin expression. Increased NMDA receptors may point to the involvement of glutamate-dopamine interactions in schizophrenia. Dysbindin, which has been (controversially) linked to schizophrenia, may regulate dopamine release, and low expression of dysbindin in the substantia nigra may be important in schizophrenia etiology. [40] Due to the changes to the substantia nigra in the schizophrenic brain, it may eventually be possible to use specific imaging techniques (such as neuromelanin-specific imaging) to detect physiological signs of schizophrenia in the substantia nigra. [41]
Wooden chest, also called fentanyl chest wall rigidity syndrome, is a rare side effect of synthetic opioids such as Fentanyl, Sulfentanil, Alfentanil, Remifentanil. It results in a generalised increase in skeletal muscle tone. The mechanism is thought to be via increased dopamine release and decreased GABA release in the nerves of the substantia nigra/striatum. The effect is most pronounced on the chest wall muscles and can lead to impaired ventilation. The condition is most commonly observed in anaesthesia where rapid and high doses of these drugs are given intravenously. [42] [43] [44]
Multiple system atrophy characterized by neuronal degeneration in the striatum and substantia nigra was previously called striatonigral degeneration.
Chemical manipulation and modification of the substantia nigra is important in the fields of neuropharmacology and toxicology. Various compounds such as levodopa and MPTP are used in the treatment and study of Parkinson's disease, and many other drugs have effects on the substantia nigra.
Studies have shown that, in certain brain regions, amphetamine and trace amines increase the concentrations of dopamine in the synaptic cleft, thereby heightening the response of the post-synaptic neuron. [45] The various mechanisms by which amphetamine and trace amines affect dopamine concentrations have been studied extensively, and are known to involve both DAT and VMAT2. [45] [46] [47] Amphetamine is similar in structure to dopamine and trace amines; as a consequence, it can enter the presynaptic neuron via DAT as well as by diffusing through the neural membrane directly. [45] Upon entering the presynaptic neuron, amphetamine and trace amines activate TAAR1, which, through protein kinase signaling, induces dopamine efflux, phosphorylation-dependent DAT internalization, and non-competitive reuptake inhibition. [45] [48] Because of the similarity between amphetamine and trace amines, it is also a substrate for monoamine transporters; as a consequence, it (competitively) inhibits the reuptake of dopamine and other monoamines by competing with them for uptake, as well. [45]
In addition, amphetamine and trace amines are substrates for the neuronal vesicular monoamine transporter, vesicular monoamine transporter 2 (VMAT2). [47] When amphetamine is taken up by VMAT2, the vesicle releases (effluxes) dopamine molecules into the cytosol in exchange. [47]
Cocaine's mechanism of action in the human brain includes the inhibition of dopamine reuptake, [49] which accounts for cocaine's addictive properties, as dopamine is the critical neurotransmitter for reward. However, cocaine is more active in the dopaminergic neurons of the ventral tegmental area than the substantia nigra. Cocaine administration increases metabolism in the substantia nigra, which can explain the altered motor function seen in cocaine-using subjects. [50] The inhibition of dopamine reuptake by cocaine also inhibits the firing of spontaneous action potentials by the pars compacta. [51] The mechanism by which cocaine inhibits dopamine reuptake involves its binding to the dopamine transporter protein. However, studies show that cocaine can also cause a decrease in DAT mRNA levels, [52] most likely due to cocaine blocking dopamine receptors rather than direct interference with transcriptional or translational pathways. [52]
Inactivation of the substantia nigra could prove to be a possible treatment for cocaine addiction. In a study of cocaine-dependent rats, inactivation of the substantia nigra via implanted cannulae greatly reduced cocaine addiction relapse. [53]
The substantia nigra is the target of chemical therapeutics for the treatment of Parkinson's disease. Levodopa (commonly referred to as L-DOPA), the dopamine precursor, is the most commonly prescribed medication for Parkinson's disease, despite controversy concerning the neurotoxicity of dopamine and L-DOPA. [54] The drug is especially effective in treating patients in the early stages of Parkinson's, although it does lose its efficacy over time. [55] Levodopa can cross the blood–brain barrier and increases dopamine levels in the substantia nigra, thus alleviating the symptoms of Parkinson's disease. The drawback of levodopa treatment is that it treats the symptoms of Parkinson's (low dopamine levels), rather than the cause (the death of dopaminergic neurons in the substantia nigra).
MPTP, is a neurotoxin specific to dopaminergic cells in the brain, specifically in the substantia nigra. MPTP was brought to the spotlight in 1982 when heroin users in California displayed Parkinson's-like symptoms after using MPPP contaminated with MPTP. The patients, who were rigid and almost completely immobile, responded to levodopa treatment. No remission of the Parkinson's-like symptoms was reported, suggesting irreversible death of the dopaminergic neurons. [56] The proposed mechanism of MPTP involves disruption of mitochondrial function, including disruption of metabolism and creation of free radicals. [57]
Soon after, MPTP was tested in animal models for its efficacy in inducing Parkinson's disease (with success). MPTP induced akinesia, rigidity, and tremor in primates, and its neurotoxicity was found to be very specific to the substantia nigra pars compacta. [58] In other animals, such as rodents, the induction of Parkinson's by MPTP is incomplete or requires much higher and frequent doses than in primates. Today, MPTP remains the most favored method to induce Parkinson's disease in animal models. [57] [59]
The substantia nigra was discovered in 1784 by Félix Vicq-d'Azyr, [60] and Samuel Thomas von Sömmerring alluded to this structure in 1791. [61] The differentiation between the substantia nigra pars reticulata and compacta was first proposed by Sano in 1910. [62] In 1963, Oleh Hornykiewicz concluded from his observation that "cell loss in the substantia nigra (of Parkinson's disease patients) could well be the cause of the dopamine deficit in the striatum." [63]
The putamen is a subcortical nucleus with a rounded structure, in the basal ganglia nuclear group. It is located at the base of the forebrain and above the midbrain.
The striatum or corpus striatum is a cluster of interconnected nuclei that make up the largest structure of the subcortical basal ganglia. The striatum is a critical component of the motor and reward systems; receives glutamatergic and dopaminergic inputs from different sources; and serves as the primary input to the rest of the basal ganglia.
The basal ganglia (BG) or basal nuclei are a group of subcortical nuclei found in the brains of vertebrates. In humans and other primates, differences exist, primarily in the division of the globus pallidus into external and internal regions, and in the division of the striatum. Positioned at the base of the forebrain and the top of the midbrain, they have strong connections with the cerebral cortex, thalamus, brainstem and other brain areas. The basal ganglia are associated with a variety of functions, including regulating voluntary motor movements, procedural learning, habit formation, conditional learning, eye movements, cognition, and emotion.
MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) is an organic compound. It is classified as a tetrahydropyridine. It is of interest as a precursor to the monoaminergic neurotoxin MPP+, which causes permanent symptoms of Parkinson's disease by destroying dopaminergic neurons in the substantia nigra of the brain. It has been used to study disease models in various animals.
Dopaminergic pathways in the human brain are involved in both physiological and behavioral processes including movement, cognition, executive functions, reward, motivation, and neuroendocrine control. Each pathway is a set of projection neurons, consisting of individual dopaminergic neurons.
The nigrostriatal pathway is a bilateral dopaminergic pathway in the brain that connects the substantia nigra pars compacta (SNc) in the midbrain with the dorsal striatum in the forebrain. It is one of the four major dopamine pathways in the brain, and is critical in the production of movement as part of a system called the basal ganglia motor loop. Dopaminergic neurons of this pathway release dopamine from axon terminals that synapse onto GABAergic medium spiny neurons (MSNs), also known as spiny projection neurons (SPNs), located in the striatum.
The indirect pathway, sometimes known as the indirect pathway of movement, is a neuronal circuit through the basal ganglia and several associated nuclei within the central nervous system (CNS) which helps to prevent unwanted muscle contractions from competing with voluntary movements. It operates in conjunction with the direct pathway.
The subthalamic nucleus (STN) is a small lens-shaped nucleus in the brain where it is, from a functional point of view, part of the basal ganglia system. In terms of anatomy, it is the major part of the subthalamus. As suggested by its name, the subthalamic nucleus is located ventral to the thalamus. It is also dorsal to the substantia nigra and medial to the internal capsule.
Hypokinesia is one of the classifications of movement disorders, and refers to decreased bodily movement. Hypokinesia is characterized by a partial or complete loss of muscle movement due to a disruption in the basal ganglia. Hypokinesia is a symptom of Parkinson's disease shown as muscle rigidity and an inability to produce movement. It is also associated with mental health disorders and prolonged inactivity due to illness, amongst other diseases.
The pars reticulata (SNpr) is a portion of the substantia nigra and is located lateral to the pars compacta. Most of the neurons that project out of the pars reticulata are inhibitory GABAergic neurons.
The basal ganglia form a major brain system in all vertebrates, but in primates there are special differentiating features. The basal ganglia include the striatum, globus pallidus, substantia nigra and subthalamic nucleus. In primates the pallidus is divided into an external and internal globus pallidus, the external globus pallidus is present in other mammals but not the internal globus pallidus. Also in primates, the dorsal striatum is divided by a large nerve tract called the internal capsule into two masses named the caudate nucleus and the putamen. These differences contribute to a complex circuitry of connections between the striatum and cortex that is specific to primates, reflecting different functions in primate cortical areas.
The pars compacta (SNpc) is one of two subdivisions of the substantia nigra of the midbrain ; it is situated medial to the pars reticulata. It is formed by dopaminergic neurons. It projects to the striatum and portions of the cerebral cortex. It is functionally involved in fine motor control.
Medium spiny neurons (MSNs), also known as spiny projection neurons (SPNs), are a special type of inhibitory GABAergic neuron representing approximately 90% of neurons within the human striatum, a basal ganglia structure. Medium spiny neurons have two primary phenotypes : D1-type MSNs of the direct pathway and D2-type MSNs of the indirect pathway. Most striatal MSNs contain only D1-type or D2-type dopamine receptors, but a subpopulation of MSNs exhibit both phenotypes.
The external globus pallidus combines with the internal globus pallidus (GPi) to form the globus pallidus, an anatomical subset of the basal ganglia. Globus pallidus means "pale globe" in Latin, indicating its appearance. The external globus pallidus is the segment of the globus pallidus that is relatively further (lateral) from the midline of the brain.
The internal globus pallidus is one of the two subcortical nuclei that provides inhibitory output in the basal ganglia, the other being the substantia nigra pars reticulata. Together with the external globus pallidus (GPe), it makes up one of the two segments of the globus pallidus, a structure that can decay with certain neurodegenerative disorders and is a target for medical and neurosurgical therapies. The GPi, along with the substantia nigra pars reticulata, comprise the primary output of the basal ganglia, with its outgoing GABAergic neurons having an inhibitory function in the thalamus, the centromedian complex and the pedunculopontine complex.
Oxidopamine, also known as 6-hydroxydopamine (6-OHDA) or 2,4,5-trihydroxyphenethylamine, is a synthetic monoaminergic neurotoxin used by researchers to selectively destroy dopaminergic and noradrenergic neurons in the brain.
Basal ganglia disease is a group of physical problems that occur when the group of nuclei in the brain known as the basal ganglia fail to properly suppress unwanted movements or to properly prime upper motor neuron circuits to initiate motor function. Research indicates that increased output of the basal ganglia inhibits thalamocortical projection neurons. Proper activation or deactivation of these neurons is an integral component for proper movement. If something causes too much basal ganglia output, then the ventral anterior (VA) and ventral lateral (VL) thalamocortical projection neurons become too inhibited, and one cannot initiate voluntary movement. These disorders are known as hypokinetic disorders. However, a disorder leading to abnormally low output of the basal ganglia leads to reduced inhibition, and thus excitation, of the thalamocortical projection neurons which synapse onto the cortex. This situation leads to an inability to suppress unwanted movements. These disorders are known as hyperkinetic disorders.
The ventral pallidum (VP) is a structure within the basal ganglia of the brain. It is an output nucleus whose fibres project to thalamic nuclei, such as the ventral anterior nucleus, the ventral lateral nucleus, and the medial dorsal nucleus. The VP is a core component of the reward system which forms part of the limbic loop of the basal ganglia, a pathway involved in the regulation of motivational salience, behavior, and emotions. It is involved in addiction.
The cortico-basal ganglia-thalamo-cortical loop is a system of neural circuits in the brain. The loop involves connections between the cortex, the basal ganglia, the thalamus, and back to the cortex. It is of particular relevance to hyperkinetic and hypokinetic movement disorders, such as Parkinson's disease and Huntington's disease, as well as to mental disorders of control, such as attention deficit hyperactivity disorder (ADHD), obsessive–compulsive disorder (OCD), and Tourette syndrome.
Animal models of Parkinson's disease are essential in the research field and widely used to study Parkinson's disease. Parkinson's disease is a neurodegenerative disorder, characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). The loss of the dopamine neurons in the brain, results in motor dysfunction, ultimately causing the four cardinal symptoms of PD: tremor, rigidity, postural instability, and bradykinesia. It is the second most prevalent neurodegenerative disease, following Alzheimer's disease. It is estimated that nearly one million people could be living with PD in the United States.
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