Neuronal and Cellular Signalling Lecture 4 Notes: The Role of Calcium in the Nervous System Part 2 Voltage-Gated Ion Channels - There are four main types of gated ion channels: (a) Ligand-gated channels (ionotropic receptors, open/close in response to binding of a chemical messenger/ligand, usually the ligand is a neurotransmitter) (b) Mechanical-gated ion channels, sense and respond to mechanical stimuli e.g. touch, mainly found in the skin and inner ear) (c) Voltage-gated ion channels (open/close in response to voltage changes across the membrane) (d) Metabotropic receptors (G-protein coupled receptors, do not form an ion channel pore but use signal transduction mechanism to activate intracellular events). - The main voltage-gated ion channels are: (a) Voltage-dependent Na+ channels (required for action potentials) (b) Voltage-dependent K+ channels (required for APs, slow- and fast-activating, very diverse functions inside and out of the nervous system) (c) Voltage-dependent Ca2+ channels (transduce membrane potential changes into intracellular calcium transients that initiate physiological events) (d) Voltage-dependent Cl channels (e.g. in skeletal muscle) (e) Voltage-dependent H+ channels (proton extrusion) - Voltage-dependent ion channels are ion-specific transmembrane proteins found in all excitable cells. Their conductance changes as a function of the membrane potential (i.e. they depend on the voltage changes across the membrane potential) and are closed at the RMP. - Voltage-gated Na+ and K+ channels have a primary structure of four domains with six transmembrane segments each. - The pore/activation gate is between transmembrane domains 5-6. The voltage sensors are present at transmembrane domains 1-4 (No. 4 is charged positively due to a large Arg residue). The inactivation gate is an intracellular, short linker-loop between transmembrane domains III and IV. Finally, the wide range of B-subunits can bind and interact with the a- subunit, as once they are synthesised, they are targeted to the cell membrane and have a modulator role for other molecules. - APs are propagated unidirectionally along the membrane. This begins (i.e. at site 'A') with the opening of voltage-gated Na+ channels, leading to membrane depolarisation. The subsequent site (i.e. site 'B') is in a pre-depolarisation phase and is about to become depolarised. Site 'Z' (before site A) is in the refractory period (i.e. voltage-gated Na + channels inactivated, K+ channels activated) which prevents retrograde AP propagation. - Patch clamp electrophysiology enables measurements of electric current/voltage which are created as a result of ionic flow through the membrane. This can also be used to monitor ionic flow through a small number of ion channels (sometimes through a single channel). An electrode and micropipette are inserted into the cell to measure and generate currents. - The depolarising pulses are applied to a small patch of the membrane (i.e. single Na+/K+ channel) and the patch clamp measures the microscopic current. Macroscopic Na+/K+
currents measured using a voltage clamp correlate with the microscopic currents, therefore these microscopic currents must give rise to the macroscopic current. - The probability of Na+ and K+ channels opening depends on the membrane potential, increasing as the membrane potential is depolarised - Measurements reveal