Physiopathological Bases and Dynamic Mechanisms of Action

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1. Physiology of Cortical Excitability and Ion Channels

Epileptic activity originates from a sustained imbalance between excitatory neurotransmission mediated by glutamate and inhibitory neurotransmission coordinated by gamma-aminobutyric acid (GABA). Cell membrane stabilization in pyramidal neurons of the cerebral cortex is achieved by manipulating the biophysical states of voltage-gated sodium channels (Na_v 1.1, Na_v 1.2, Na_v 1.6).

Sodium Channel Inactivation Equation (Equilibrium Inactive Fraction) h_{\infty}(V) = \frac{1}{1 + \exp\left(\frac{V - V_{1/2}}{k}\right)} Where V_{1/2} is the half-inactivation potential and k represents the slope factor of the Boltzmann curve. Drugs such as phenytoin and carbamazepine shift this balance towards more hyperpolarized voltages, stabilizing the inactive state of the channel.

Allosteric modulation of the GABA_A receptor, inhibition of GABA reuptake through the GAT-1 transporter, or enzymatic destruction of GABA transaminase (GABA-T) drastically decrease cortical excitability by increasing the transmembrane flux of Chlorine ions (Cl^-), hyperpolarizing the neuron and decreasing the probability of firing action potentials. repetitive.

2. Pathophysiology of the Extrapyramidal Circuits

Parkinson's disease is characterized neurobiologically by the progressive and selective loss of pigmented dopaminergic neurons in the *pars clavata* of the substantia nigra (SNpc). This triggers a massive alteration of the motor feedback circuits of the basal ganglia, characterized by hyperactivity of the indirect pathway and hypoactivity of the direct pathway, resulting in bradykinesia, "cogwheel" muscle rigidity, and resting tremor.

Dynamics of the Direct Way vs. Indirect Way

The striatum receives nigrostriatal dopaminergic projections that differentially modulate two cell populations: the projection neurons of the **direct pathway** (which express excitatory D₁ receptors coupled to G_{α s} proteins and promote voluntary movement) and the neurons of the **indirect pathway** (which express inhibitory D₂ receptors coupled to G_{α i} proteins and inhibit movement unwanted). When the basal dopaminergic tone is lost, the movement facilitation effect of the direct pathway is reduced and the indirect pathway is disinhibited, causing the akinetic state characteristic of the disease.