Tag Archives: basal ganglia

The Striatum

The striatum is the largest collection of neurons in the basal ganglia. Composed of the caudate nucleus and putamen, the basal ganglia, as the name suggests, sits at the base of the cerebrum. It receives input from regions of the cerebral cortex, the limbic system, and the sensorimotor and motivational systems via the thalamus. In addition to the cerebrum, the striatum receives input from the brainstem including the substantia nigra and the raphe nuclei of the reticular formation. The dopamine and serotonin of these two structures serve a modulatory function. Anatomists organise the striatum on the “basis of differential connectivity and distribution of neurochemical markers” (Redgrave, 2007). Processing strong excitatory input, the striatal neural circuits generate a strong inhibitory output, which controls the output of basal ganglia further along in the motor loop.

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The major cytology of the striatum is GABAergic medium spiny neurons (MSN), making up about 95% of the total cellular structure. MSNs are organised into two groups based on the peptide they contain, substance P and enkephalin and the proportion of dopamine receptors (D1 or D2) they contain. MSNs create dense networks of axon collaterals. As projection neurons, the MSNs create this dense network by forming axon collaterals with one another. Tunstall et al, 2002 found that almost 30% form an axon collateral with a neighbouring MSN. Research has shown that the function of these collaterals is in cellular recognition and “classification of cortical patterns” (Blomeley, et al. , 2009).

The striatum is a vital part of the basal ganglia, and all pathways run through it. From the striatum onwards, the pathway either becomes direct or indirect. As shown in the figure below.

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Citations:

Blomeley, C. P., Kehoe, L. A., & Bracci, E. (2009). Substance P mediates excitatory interactions between striatal projection neurons. The Journal of Neuroscience29(15), 4953-4963.

Redgrave, P. (2007). Basal ganglia. Scholarpedia, 2(6): 1825.

Basal Ganglia: Substance P

As mentioned in my last post on basal ganglia, the majority of the striatum consists of medium spiny neurons. These medium spiny neurons are GABAergic and organised based on the peptide they contain as well the dopamine receptors they contain. One these peptides is called substance P (SP). As a neuropeptide, SP functions as a neurotransmitter as well as a neuromodulator. Other than GABA, SP functions as a neurotransmitter in MSNs. Specifically, SP-releasing neurons mediate “synaptic communication between MSNs” (Blomeley, et al. , 2009).

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Previously it was thought that striatal projection neurons like MSNs only inhibit each other; however, a study by Blomeley, et al. , 2009 has proven that they can also interact in an excitatory manner. Studies have shown the synaptic NK1 receptors, whose major receptor molecule is SP are present in a glutamteric terminals in the stiratum (Jakab et al., 1996). In the study by Blomeley, the importance of these NK1 receptors was investigated. The results suggest that SP plays a crucial role in facilitating the release of glutamate between medium spiny neurons. In other words, communication between the neurons is increased by SP attaching NK1 receptors found on the terminals of glutamate releasing MSNs.

Citations:

Blomeley, C. P., Kehoe, L. A., & Bracci, E. (2009). Substance P mediates excitatory interactions between striatal projection neurons. The Journal of Neuroscience29(15), 4953-4963.

Introduction to Basal Ganglia: Anatomy and the Motor Loop

To begin with…

Anatomy 

Studies have shown that the motor loop through the basal ganglia helps initiate conscious movement. One model has shown that furthered inhibition of the thalamus via the basal ganglia underlies what is known as hypokinesia or the reduction of movement. Contrarily, decreased output by the basal ganglia leads to hyperkinesia or the excess of movement.

Now the basal ganglia consists of several structures ,which includes the caudate nucleus, the putamen, the globus pallidus and the subthalamic nigra. Some neuroscientists also include the substantia nigra as part of the basal ganglia even though technically it is part of the midbrain. This is because the substantia nigra plays a quintessential role in the control of movement. Together with the putatmen, the caudate nucleus makes up the striatum, which is the target of cortical input to the basal ganglia. The globus pallidus controls output to the thalamus, which helps create a loop of information from the cortex back to the cortex.

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Motor Loop 

A simplified version of the motor loop:

Cortex -> Striatum -> Globus pallidus -> Thalamus -> Cortex

The impulse that drives the motor loop originates from the cortex (the frontal, parietal and prefrontal) and forms an excitatory connection with the putamen. The putamen cells then form an inhibitory connection with neurons in the globus pallidus, which then forms an inhibitory connection with the thalamus. More specifically, a part of the thalamus known as the ventral lateral nucleus or VLo. The VLo then forms the thalamocortical connection with the supplementary motor area or SMA, which is a medial region of cortical area 6 that directly sends axons to motor units.

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This may seem counter-intuitive. Why would an inhibitory signal cause the activation of a motor unit? Well, basically at rest, neurons in the globus pallidus are active. Because the neurons globus pallidus are active, this inhibits the activity of the neurons in the thalamus, specifically in the VLo. So, when the impulse from the cortex excites the putamen, the neurons globus pallidus are inhibited. The inhibition of these neurons allows the VLo to become active or excited. The activation of the neurons in the VLo sends neural activity via the thalamocortical connection to the SMA.

References:

Bear, Mark F., Barry W. Connors, and Michael A. Paradiso. Neuroscience: Exploring the Brain. Philadelphia, PA: Lippincott Williams & Wilkins, 2007. Print.