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Origine dei segnali elettrici neuronali
Neuronal excitability is based on the principles of electrochemistry that regulate the balance of ionic solutions separated by permeable membranes containing ion channels.
When K⁺ diffuses across a membrane that is permeable only to it, the diffusion continues until the concentration gradient is counterbalanced by the electric potential generated due to charge separation.
When the concentrations of K⁺ are equal inside and outside the cell, the membrane potential (Vm) is 0 mV.
Neurons consist of a dendritic apparatus, a soma, an axon, and one or more synaptic terminals. The soma contains the nucleus and is involved in the synthesis of molecules.
Dendrites are excitable extensions of the neuron that contain channels for Na⁺, Ca²⁺, and K⁺, playing a crucial role in receiving signals.
The giant axon of the squid, with a diameter of about 0.5–1.0 μm, has historically been used to study action potentials and has led to the development of specific devices for measuring these potentials.
The action potential is an all-or-nothing electrical event limited to the cell membrane, occurring in multiple phases regulated by the opening, closing, and inactivation of Na⁺ and K⁺ channels.
K⁺ channels exhibit a closed-open gating mechanism, while Na⁺ channels have a closed-open-inactivated gating mechanism, which temporarily prevents ionic conduction.
The patch-clamp technique allows for the measurement of ionic currents through individual ion channels or membrane patches using glass microelectrodes called patch pipettes.
In a laboratory, an action potential is generated by using a pulse generator to pass current through the membrane while a voltmeter measures the changes in membrane potential.
The Nernst equation is used to calculate the equilibrium potential for a specific ion across a membrane permeable to that ion, based on its concentration gradient.
The equilibrium potential for Na⁺ is +63 mV, while for K⁺ it is -92 mV, indicating that they have opposite signs due to their concentration gradients.
The Goldman equation is used to calculate the membrane potential (Vm) when the membrane is permeable to multiple ionic species, taking into account their relative permeabilities.
The axon conducts electrical signals toward the synaptic terminal and is often myelinated, featuring a high density of Na⁺ and K⁺ channels, especially at the nodes of Ranvier.
The resting potential of neurons is the electrical potential difference across the membrane when the neuron is not actively transmitting signals, primarily determined by the distribution of ions.
The membrane potential is measured by comparing the potential of a recording electrode inside the cell with that of a reference electrode outside, with the difference amplified and displayed.
The S4 segments in Na⁺ channels contain positively charged amino acids that act as voltage sensors, allowing the channel to respond to changes in membrane potential.
When intracellular K⁺ concentration is greater than extracellular concentration, the membrane potential initially is 0 mV but stabilizes to a negative value over time.
At the synaptic terminal, there is a high density of Ca²⁺ channels, which are crucial for the release of neurotransmitters through exocytosis.
The membrane potential is influenced by the permeability of the membrane to different ions; if the membrane is more permeable to K⁺, the potential will tend to be closer to EK.
The P-loop in Na⁺ channels controls the selectivity of the channel, determining which ions can pass through based on their size and charge.
Ion concentration gradients across the neuronal membrane are essential for generating action potentials, as they create the driving force for ion movement during depolarization and repolarization.