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Overview of Glutamate and its Role

  • Glutamate is identified as a stimulatory neurotransmitter.

  • Its action stimulates neurons within the striatum, leading to:

    • Increased excitatory postsynaptic potentials (EPSPs).

    • Increased action potentials traveling down the axons from the striatum to the globus pallidus externus.

Action of GABA in the Pathway

  • Neurons in the globus pallidus externus release GABA.

  • GABA is classified as an inhibitory neurotransmitter.

  • Consequences of increased action potentials:

    • Increased GABA release inhibits the following neuron:

    • Leads to hyperpolarization.

    • The inhibited neuron sends fewer action potentials from the globus pallidus externus to the subthalamic nucleus.

    • Resulting decrease in action potentials affects the following pathway.

  • Decreased action potentials result in:

    • Release of decreased amounts of GABA under the subthalamic nucleus.

    • GABA's inhibition is diminished, leading to disinhibition of the neuron.

Consequences of Disinhibition in the Indirect Pathway

  • Stimulation of subthalamic nucleus neurons occurs due to decreased GABA inhibition.

  • Increased stimulation results in:

    • Enhanced activation of neurons in globus pallidus internus.

    • Increased action potentials traveling down neurons from the globus pallidus internus to the thalamus.

    • Increased release of GABA onto thalamic nuclei from globus pallidus internus.

  • Increased GABA means stronger inhibition of the thalamic nuclei.

  • Resulting effect on the cortex:

    • Decreased action potentials from thalamus to motor cortex.

    • This ultimately reduces motor activity for specific body parts.

Modulation of Direct and Indirect Pathways by Basal Ganglia

  • The last function of the basal ganglia involves pathway modulation.

  • The nigrostriatal pathway plays a crucial role in this modulation.

    • Its function is described as amplifying motor activity.

Influence of Nigrostriatal Pathway on Direct and Indirect Pathways

Overview of Nigrostriatal Pathway Impact

  • The nigrostriatal pathway influences:

    • Direct pathway (amplifies motor activity).

    • Indirect pathway (modulates motor activity).

Influence on the Direct Pathway

  • The direct pathway leads to communication:

    • From cortex to striatum, then to globus pallidus internus, and finally to thalamus.

  • In this pathway:

    • Dopamine from substantia nigra influences striatum via D1 receptors (stimulatory).

    • Glutamate from the cortex enhances neuron activation in striatum, resulting in elevated action potentials toward globus pallidus internus.

  • Consequence of stimulation:

    • Increased GABA release from globus pallidus internus results in significant inhibition.

    • Underactive inhibition leads to increased action potentials back from thalamic nuclei to the cortex, thereby heightening motor activity.

  • Importance:

    • Damage to the direct pathway (especially involving substantia nigra) can lead to Parkinson's disease with symptoms including difficulty initiating and maintaining movement.

Influence on the Indirect Pathway

  • The indirect pathway connects:

    • From cortex to striatum, globus pallidus externus, subthalamic nucleus, and globus pallidus internus.

  • This pathway additionally involves D2 receptors (inhibitory) on striatum neurons:

    • Glutamate stimulates striatum.

    • Dopamine released onto D2 receptors inhibits the neurons in this pathway, modulating the thalamic nuclei’s activity.

  • Increased motor activity correlated to:

    • Suppression of unwanted motor movements via the indirect pathway.

Clinical Relevance of Basal Ganglia Disorders

  • Understanding the pathways contributes to understanding disorders:

    • Parkinson's Disease: Associated with damage to the direct pathway resulting in reduced motor activity initiation.

    • Huntington's disease: Damage to the indirect pathway can cause unwanted motor movements.

    • Wilson's disease: Effects from copper buildup leading to degeneration of the central nervous system affect movement.

    • Sydenham’s chorea: Resulting from antibodies attacking basal ganglia post-rheumatic fever.

    • Drug-induced movement disorders, including extrapyramidal symptoms (e.g., tardive dyskinesia, akathisia, dystonic reactions) often arising from first-generation antipsychotics affecting the D2 receptors.