Medium spiny neuron

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Medium spiny neuron

The medium spiny neurons (medium-sized densely spiny neurons) are a special type of inhibitory cells representing approximately 90% of the neuron

Neuron

Neurons are responsive cell in the nervous system that process and transmit information by electrochemical Signal . They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves....
s within the corpus striatum

Corpus striatum

The corpus striatum is a compound structure consisting of the striatum and the globus pallidus....
of the basal ganglia

Basal ganglia

The basal ganglia are a group of Nucleus in the brain interconnected with the cerebral cortex, thalamus and brainstem. Mammalian basal ganglia are associated with a variety of functions: motor control, cognition, emotions, and learning....
. They play a key role in initiating and controlling movements of the body, limbs and eyes.

Appearance and location
The medium spiny neurons are medium sized neurons with large and extensive dendritic trees.

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The medium spiny neurons (medium-sized densely spiny neurons) are a special type of inhibitory cells representing approximately 90% of the neuron

Neuron

Corpus striatum

The corpus striatum is a compound structure consisting of the striatum and the globus pallidus....
of the basal ganglia

Basal ganglia

Appearance and location

The medium spiny neurons are medium sized neurons with large and extensive dendritic trees. Each branch of the these dendritic trees is packed with numerous small spines which receive synaptic inputs

Synaptic inputs

Synaptic inputs are the area in a neuron where presynaptic neurons synapse. Neurons can have inputs from other neurons or from sensory receptors....
from neurons outside the striatum. The corpus striatum - consisting of nucleus caudatus

Caudate nucleus

The caudate nucleus is a nucleus located within the basal ganglia of the brains of many animal species. The caudate, originally thought to primarily be involved with control of voluntary movement, is now known to be an important part of the brain's learning and memory system....
, putamen, and nucleus accumbens - is the main input station of the basal ganglia. Medium spiny neurons in this structure receive cortical, thalamic

Thalamus

The thalamus is a pair and symmetric part of the brain. It constitutes the main part of the diencephalon....
and brain-stem

Brain stem

The brain stem is the lower part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves....
inputs. In fact, the whole human neocortex except the primary visual and primary auditory cortex

Primary auditory cortex

The primary auditory cortex is the region of the brain that is responsible for processing of auditory system information....
project to the striatum.

Within the striatum, there are at two different types of medium spiny neurons. These types were first distinguished because of the different neuropeptides they contain. About half the spiny cells express Substance P and dopamine D1 receptors and project to the internal globus pallidus and substantia nigra (the direct pathways) whereas the other half express enkephalin and dynorphin and the dopamine D2 receptor and project to the globus pallidus (the "indrect pathway"). With different types of immunocytochemical or histochemical staining one can identify small clusters (150-300 µm in diameter) of medium spiny neurons (called "patches" or "striosomes" - making up about 15% of the volume of the neostriatum) embedded in a surrounding "matrix" (making up about 85% of the volume of the striatum).

Function

The medium spiny neurons are GABAergic

Gamma-aminobutyric acid

γ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays an important role in regulating neuronal excitability throughout the nervous system....
neurons and hence have an inhibitory influence on the neurons they project to. Within the basal ganglia, there are several complex circuits of neuronal loops all of which include the medium spiny neurons (for further information see basal ganglia

Basal ganglia

The basal ganglia are a group of Nucleus in the brain interconnected with the cerebral cortex, thalamus and brainstem. Mammalian basal ganglia are associated with a variety of functions: motor control, cognition, emotions, and learning....
). They send axons to the internal and external segment of the globus pallidus

Globus pallidus

The globus pallidus is a sub-Cerebral cortex structure of the brain. It is a major element of the basal ganglia system. In this system, it is a major constituent of the basal ganglia core, which consists of the striatum and its direct targets: globus pallidus and substantia nigra....
as well as the substantia nigra

Substantia nigra

The substantia nigra is a brain structure located in the mesencephalon that plays an important role in reward, addiction, and movement. Substantia nigra is Latin for "black substance", as parts of the substantia nigra appear darker than neighboring areas due to high levels of melanin in dopaminergic neurons....
pars reticulata.

The cortical, thalamic, and brain-stem inputs that arrive at the medium spiny neurons show a vast divergence in that each incoming axon forms contacts with many spiny neurons and each spiny neuron receives a vast amount of input from different incoming axons. Since these inputs are Glutamatergic they exhibit an excitatory influence on the inhibitory medium spiny neurons.

There are also a large number of interneurons originating in different areas which regulate the excitability of the medium spiny neurons. The synaptic connections between the spiny neurons and the interneurons are typically close to the spiny neurons' cell soma, or body. Recall that excitatory postsynaptic potentials caused by Glutamatergic inputs at the dendrites of the spiny neurons only cause an action potential

Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
when the depolarization wave is strong enough upon entering the cell soma. Since the interneurons' influence is located so closely to this critical gate between the dendrites and the soma, they can readily regulate the generation of an action potential. As a result, the excitatory input coming from cortical etc. neurons has to be very strong, or caused by many simultaneously arriving excitations.

In consequence the medium spiny neurons are usually quiet and do not exhibit any spontaneous activity unless sufficiently activated.

Direct pathway within the basal ganglia

The direct pathway within the basal ganglia makes excitatory inputs coming from e.g. the cortex cause a net excitation of upper motor neuron

Upper motor neuron

Upper motor neurons are motor neurons that originate in the Motor cortex of the cerebral cortex or the brain stem and carry motor information down to the final common pathway, that is, any motor neurons that are not directly responsible for stimulating the target muscle....
s in the motor areas of the cortex. In the direct pathway, the medium spiny neurons project to the internal division of the globus pallidus which in turn sends axons to the substantia nigra pars reticulata (SNpr) and the ventroanterior and ventrolateral thalamus (VTh). The SNpr projects to the deep layer of the superior colliculus

Superior colliculus

The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but even in mammals, the adjective tectal is commonly used....
thus controlling fast eye movements

Eye movements

Eye movement is the Voluntary action or Reflex action movement of the eyes, helping in acquiring, fixating and tracking visual stimuli. In addition, rapid eye movement occurs during REM sleep....
(saccades). The VTh projects to upper motor neurons in the primary motor cortex

Primary motor cortex

The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with Brodmann area 6 areas to plan and execute movements....
(precentral gyrus).

Neurons in the globus pallidus are also inhibitory, thus inhibiting the excitatory neurons in the SNpr and VTh. But in contrast to the medium spiny neurons, globus pallidus neurons are tonically active when not activated. Thus in the absence of cortical stimulation, SNpr and VTh neurons are tonically inhibited thus preventing involuntary spontaneous movements.

Once the medium spiny neurons receive sufficient excitatory cortical input, they are excited and fire a burst of inhibitory action potentials to globus pallidus neurons. These tonically active neurons are then inhibited, causing their inhibitory influence on SNpr and VTh to decline. Thus SNpr and VTh neurons are disinhibited resulting in net excitement causing them to activate upper motor neurons commanding a movement. Cortical activation of the basal ganglia thus eventually results in excitement (disinhibition

Disinhibition

Disinhibition is a term in psychology used to describe a lack of restraint wikt:manifest in several ways, including disregard for Convention , impulsivity, and poor risk assessment....
) of motor neuron

Motor neuron

In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles....
s causing movement to take place.

Indirect pathway

In the indirect pathway, excitatory e.g. cortical input to the basal ganglia results in net inhibition of upper motor neurons. In this pathway the medium spiny neurons in the striatum project to the external segmemt of the globus pallidus. These neurons in turn project to the internal segment of the globus pallidus and to the subthalamic nuclei which form a closed loop

PID controller

A proportional?integral?derivative controller is a generic control loop feedback mechanism widely used in industrial control systems. A PID controller attempts to correct the error between a measured process variable and a desired Setpoint by calculating and then outputting a corrective action that can adjust the process accordingly....
by projecting back to the internal globus pallidus.

Cortical excitement of medium spiny neurons causes them to inhibit external globus pallidus neurons. These tonically inhibiting neurons thus decrease their inhibitory influence on the internal globus pallidus and the subthalamic nuclei.

Let's first look at the internal globus pallidus neurons which are also tonically inhibiting VTh and SNpr neurons. Since the inhibitory influence from the external globus pallidus is now reduced, these neurons show stronger activity thus increasing their inhibition of SNpr and VTh neurons.

The projections of the external globus pallidus to the subthalamic nuclei causes these neurons to increase their firing rate, since the globus pallidus neurons are inhibited by medium spiny neurons. The subthalamic nuclei have excitatory projections to the internal globus pallidus thus causing the internal globus pallidus neurons to increase their inhibititory influence on SNpr and VTh.

Eventually excitatory inputs from the cortex results in net inhibition of upper motor neurons thus preventing them from initiating a movement.