AE

Excitable Cells, Nernst, Action Potenial, Synapses

Excitable Cells: Electrical Potentials

Principles of Electrostatics and Chemical Gradients

  1. Electrostatic Attraction: Governed by Coulomb's Law, ions with opposite charges attract, while like charges repel. This force influences ion movement across membranes.

  2. Chemical (Diffusional) Gradients: Ions move from areas of high concentration to low concentration. This movement, known as flux, contributes to potential differences across the membrane.

  3. Relating Gradients to Ion Behavior: The behavior of ions is influenced by both the chemical (concentration) gradient and the electrical gradient (charge difference across the membrane). These forces can act in the same or opposite directions.

  4. Principle of Equilibrium: An ion is at equilibrium when the electrical force acting on it is equal and opposite to the chemical force. At this point, there is no net movement of the ion across the membrane even though the membrane may be permeable to it. The electrical potential at which an ion is at equilibrium is its equilibrium potential (E_x).

Measurement of Electrical Potentials (V_m)

  • Electrical potentials, such as membrane voltage (V_m), are measured using intracellular microelectrodes placed inside cells, with reference to an external ground electrode, which is set to "0" mV.

Nernst Equation

  • The Nernst equation allows for the prediction of the sign and rough value of the equilibrium potentialE_{x}=\frac{RT}{zF}\ln\frac{\left\lbrack X_{out}\right\rbrack}{\left\lbrack X_{IN}\right\rbrack}

  • R = Gas constant

  • T = Absolute temperature (Kelvin)

  • z = Valence of the ion

  • F = Faraday's constant

  • [X]_{out} = Concentration of ion X outside the cell

  • [X]_{in} = Concentration of ion X inside the cell

  • Changes in Ex: the terms that can alter the equilibrium potential (E) are temperature (T), the number of electrons transferred (n), and the reaction quotient (Q).

  • Logarithm Rules: Note that if y = 1, \ln y = 0; if y < 1, \ln y < 0 (negative); if y > 1, \ln y > 0 (positive). Also, if \ln x = V, then \ln (1/x) = -V.

  • Specificity of Ex: Ex applies solely to one ion under consideration and does not define the overall membrane voltage (V_m), which is influenced by multiple ions.

  • Typical Mammalian Nernst Values (in mV):

  • E_K = -93 mV

  • E_{Cl} = -75 mV

  • E_{Na} = +61 mV

  • E_{Ca} = +132 mV

  • These values can change based on different physiological conditions.

Goldman-Hodgkin-Katz (GHK) Equation

  • The GHK equation can be used to predict the membrane voltage (V_m) at different ion permeabilities and concentrations. It accounts for the contributions of multiple ions (typically Na+, K+, Cl-) to the membrane potential, weighted by their relative permeabilities.

  • Understanding the equation's function is key; actual calculations are not required.

Synapses General Outline

  • I. Electrical vs. Chemical Synapses

  • II. Presynaptic Mechanisms: Release and Recycling

  • III. Postsynaptic Mechanisms

  • IV. Neurotransmitter Inactivation

  • V. Synaptic Potentials: Summation and Integration

Summary of Transmission Types: Electrical vs. Chemical Synapses

  • Electrical Synapse:

  • Involve direct coupling between neurons via gap junctions, which form channels connecting the cytoplasm of adjacent cells.

  • Feature direct electrical signal transmission, allowing for bi-directional current flow.

  • Exhibit no synaptic delay between pre- and post-synaptic electrical signaling.

  • The activities of coupled cells often "mirror" each other due to direct electrical continuity.

  • Chemical Synapse:

  • Involve a synaptic cleft between the pre- and post-synaptic neurons.

  • Utilize neurotransmitters released from the presynaptic terminal, which diffuse across the cleft.

  • Neurotransmitters bind to specific postsynaptic receptors, leading to a response.

  • Characterized by a short synaptic delay due to the neurotransmitter diffusion and receptor binding process.

  • Signaling is predominantly unidirectional, from presynaptic to postsynaptic neuron.

  • Key mechanisms involve changes in potential driving synaptic activity, with membrane potential established by ionic distributions.

Action Potentials: Overview and Graded Potentials

  • Local current flow causes action potentials in neurons, involving sodium and potassium permeability changes.

  • All-or-None Principle: Action potentials are generated when a threshold is reached and propagate without decrement. They are largely depolarizing events.

  • Graded Potentials (GPs):

  • Most relevant to dendrites, under myelin, and at the neuromuscular junction (NMJ).

  • Differ from action potentials by being variable in value (amplitude), width (duration), and sign (de- or hyperpolarizing).

  • Decrease in size with distance traveled (decrementally).

  • Can summate (spatially or temporally) to affect the membrane threshold.

  • EPSPs at the NMJ in skeletal muscle are specifically called "EPPs" (End Plate Potentials).

  • Action Potentials (APs):

  • Key in axons and muscle cells.

  • Once initiated, APs travel all the way to the end of an axon without diminishing.

Specific Phases of Action Potentials: Detailed Channel Functions

  • Action potential generation involves a precise sequence of opening and closing voltage-gated ion channels. Depolarization opens (activates) all voltage-gated channel types discussed, while re- or hyperpolarization closes (deactivates) them.

  1. Resting Membrane Potential: Maintained by leak K+ channels (high permeability to K+) and to a lesser extent leak Na+ channels and leak Cl- channels.

  2. Rising Phase (Depolarization):

  • Action potential is initiated when a threshold potential is reached.

  • Voltage-gated Na+ channels (VGNCs) rapidly activate (open) in response to depolarization.

  • Massive Na+ influx occurs, leading to rapid depolarization and the rising phase of the action potential.

  • VGNCs activate relatively fast.

  1. Peak: All Na+ channels are reluctant to open at the same time, leading to rapid depolarization.

  2. Falling Phase (Repolarization):

  • VGNCs inactivate (block) slowly over time in response to sustained depolarization. This stops Na+ influx.

  • Voltage-gated K+ channels (VGKCs) activate (open) slowly in response to depolarization.

  • K+ efflux (out of the cell) re-polarizes the membrane.

  • VGKCs activate and deactivate relatively slowly compared to VGNCs.

  • Re- or hyperpolarization unblocks (un-inactivates) VG Na+ channels over time, preparing them for another action potential.

  1. Undershoot (Hyperpolarization):

  • VGKCs close slowly, leading to a transient period where too much K+ leaves the cell, causing the membrane potential to become more negative than the resting potential.

  • Voltage-gated Calcium Channels (VGCCs): Similar to VGNCs, depolarization opens and blocks them, but the opening and blocking kinetics are slower.

  • Predicting Changes: Alterations in the function of these channels (e.g., faster/slower activation, altered inactivation, changes in density) would predictably change the shape, amplitude, duration, and refractory period of the action potential.

Fast and Slow Potentials

  • Fast potentials ( < 100 ms) involve quick ion channel responses.

  • Slow potentials ( > 100 ms) involve second messenger pathways and longer signal outcomes.

Presynaptic Mechanisms: Release and Recycling

  • Action Potential Coupling: An action potential arriving at the presynaptic terminal is coupled to vesicular release via a calcium signal.

  • Depolarization from the action potential opens voltage-gated calcium channels (VGCCs) in the presynaptic terminal.

  • This leads to a rapid Ca2+ influx.

  • Vesicle Cycle (Exocytosis and Endocytosis):

  1. Filling: Neurotransmitters are synthesized and transported into synaptic vesicles.

  2. Trafficking: Vesicles move towards the active zone of the presynaptic terminal.

  3. Docking/Priming: Vesicles become physically attached to the presynaptic membrane and prepared for fusion.

  • This process involves SNARE proteins (e.g., synaptobrevin on the vesicle, syntaxin and SNAP-25 on the plasma membrane) that mediate vesicle fusion.

  1. Fusion (Exocytosis): Upon Ca2+ influx, synaptotagmin (the calcium sensor protein on the vesicle) binds calcium, triggering the fusion of the vesicle membrane with the presynaptic membrane.

  • Neurotransmitters are released into the synaptic cleft.

  1. Membrane Translocation & Protein Clustering: After fusion, portions of the vesicle membrane are retrieved.

  2. Clathrin Coating: The retrieved membrane is coated by clathrin, forming clathrin-coated pits.

  3. Endocytosis/Pinching Off: The clathrin-coated pit pinches off from the presynaptic membrane, a process facilitated by the protein dynamin, forming a new vesicle.

  4. De-coating & Refilling/Recycling: The newly formed vesicle sheds its clathrin coat and is either refilled with neurotransmitter or sent for degradation and new vesicle synthesis.

Postsynaptic Mechanisms: Receptors and Response Dynamics

  • Neurotransmitter Binding: The binding of neurotransmitter to a receptor on the postsynaptic membrane is solely responsible for initiating the postsynaptic action.

Receptor Classes in Postsynaptic Mechanisms

  • Ionotropic Receptors (Ligand-Gated Ion Channels):

  • Responsible for fast synaptic transmission ( < 100 ms).

  • The receptor and ion channel are part of the same protein complex.

  • Neurotransmitter binding directly opens the ion channel, allowing ion flow and causing a rapid change in membrane potential.

  • Exhibit little amplification; one or two neurotransmitters typically open one channel.

  • Example: Nicotinic Acetylcholine (nACh) receptors at the NMJ, which are ligand-gated cation channels.

  • Metabotropic Receptors:

  • Responsible for slow synaptic transmission ( > 100 ms).

  • The receptor and the ion channel (if involved) are separate proteins.

  • Neurotransmitter binding activates a G protein, which then initiates intracellular second messenger pathways.

  • These pathways can lead to various actions, including opening or closing ion channels, modifying existing proteins, or regulating protein synthesis.

  • Exhibit significant amplification; one neurotransmitter may affect many channels and downstream effectors.

  • Example: Muscarinic Acetylcholine (mACh) receptors at the heart, which indirectly open K+ channels via G protein signaling.

Postsynaptic Response Dynamics

  • Coordinated intracellular responses occur upon neurotransmitter binding to receptors.

  • Rapid short-acting potentials lead to influences on mood and cognitive function.

  • Modifications of existing proteins and synthesis of new proteins are regulated, contributing to long-term modifications and synaptic plasticity.

Synaptic Transmission: POST-synaptic Steps

  • Neurotransmitter binding causes ion channels in the postsynaptic cell to open.

  • EPSP (Excitatory Postsynaptic Potential): Occurs when the membrane becomes more positive relative to rest (depolarizes).

  • This typically involves an increase in Na+ permeability, leading to Na+ influx (e.g., glutamate activating non-selective cation channels).

  • IPSP (Inhibitory Postsynaptic Potential): Occurs when the membrane becomes more negative relative to rest (hyperpolarizes).

  • This typically involves an increase in K+ out or Cl– in (e.g., GABA activating chloride channels).

  • Key Components:

  • Presynaptic axon terminal

  • Neurocrine signal (neurotransmitter)

  • G protein–Coupled receptor (for metabotropic actions)

  • Chemically gated ion channel (for ionotropic actions)

  • Nature of Potentials:

  • Rapid, short-acting fast synaptic potential

Detailed Steps of Synaptic Transmission (Figure 8-23, Steps 1-5)

  • Step 1: Ion channels in the postsynaptic cell open, increasing ion permeability (e.g., Na+).

  • Resulting in EPSP or IPSP depending on the ions involved.

  • Step 2: The altered open state of ion channels can activate second messenger pathways, particularly with metabotropic receptors.

  • Step 3: Ion channels can close, decreasing ion flow (e.g., decreasing Na+ in and K+ out).

  • Step 4: Changes are made in the postsynaptic cell in response to neurotransmitter action, including immediate electrical responses and biochemical cascades.

  • Step 5: Long-term modifications and protein synthesis regulation potentially occur, contributing to synaptic plasticity.

Types of Neurotransmitters and Their General Functions

  • Excitatory Neurotransmitters:

  • Glutamate (Glu): The major excitatory neurotransmitter in the CNS (found everywhere). It activates non-selective cation channels at fast chemical synapses, leading to neuronal firing (EPSP).

  • Acetylcholine (ACh):

  • At nerve-muscle connections (Neuromuscular Junction), it stimulates muscle contraction by binding to nicotinic ACh receptors (nAChRs), which are ligand-gated cation channels.

  • At the heart, ACh binds to muscarinic ACh receptors (mAChRs), which are metabotropic and indirectly open K+ channels, causing hyperpolarization and slowing heart rate.

  • Norepinephrine (NE): Facilitates “fight or flight” situations, often through metabotropic receptors.

  • Inhibitory Neurotransmitters:

  • GABA: The major inhibitory neurotransmitter in the CNS (found everywhere). It activates chloride channels at fast chemical synapses, leading to inhibition of neuronal firing (IPSP).

  • Glycine: Provides inhibition throughout the central nervous system, also often by activating chloride channels.

  • How to get rid of neurotransmitters:

    • diffusion away from the synapse,

    • enzymatic degradation by specific enzyme

    • reuptake back into the presynaptic terminal or glial cells by transporter proteins

Synaptic Transmission Mechanisms: Termination of Neurotransmitter Action

  1. Diffusion: Neurotransmitters diffuse away from the synaptic cleft.

  2. Reuptake: Neurotransmitters are actively transported back into the presynaptic terminal or glial cells by specific pumps and transporters.

  3. Cleavage by Enzymes: Neurotransmitters are broken down into inactive metabolites by enzymes in the synaptic cleft (e.g., Acetylcholinesterase breaking down ACh).

Membrane Permeability and Resting Potential

  • Nernst and Goldman Equations: Determine resting and action potentials based on ionic permeability and concentration gradients.

  • Changes in permeability influence voltage and drive signal propagation.

Key Concepts

a) Comparison of Graded Potentials and Action Potentials

  • Graded potentials diminish over distance, whereas action potentials do not.

  • Graded potentials can be summed spatially or temporally; action potentials are all-or-nothing and do not summate.

b) Ion channel activity highlights the essential physiological dynamics in neuronal signaling.