Resting Membrane Potential

Definition and Establishment

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell at rest. It is primarily established by the differential distribution of ions, especially potassium (K+) and sodium (Na+), and the selective permeability of the membrane to these ions.

• Open Channels at Rest: In muscle and nerve cells, K+ leak channels are predominantly open at rest, allowing K+ to move out of the cell.

• Equilibrium Potential for K+: The equilibrium potential is the membrane voltage at which there is no net movement of K+ across the membrane. It can be calculated using the Nernst equation:

• Resting Membrane Potential Values: In a typical muscle cell, the resting membrane potential is about -90 mV; in a nerve cell, it is about -70 mV.

• Difference Between Muscle and Nerve Cells: Nerve cells at rest use more energy due to higher activity of the Na+/K+ ATPase pump, which maintains ion gradients.

Electrochemical Equilibrium refers to the state where the electrical and chemical (concentration) gradients for an ion are balanced, resulting in no net movement of that ion across the membrane.

Action Potentials

Generation and Propagation

An action potential is a rapid, transient change in membrane potential that propagates along excitable cells such as neurons and muscle fibers.

• Gated Na+ Channels: These channels open in response to depolarization, allowing Na+ to enter the cell and further depolarize the membrane.

• Channel Opening Sequence:

  1. Depolarization opens voltage-gated Na+ channels.

  2. Na+ influx causes further depolarization.

  3. Channels inactivate after a brief period.

  4. Voltage-gated K+ channels open, allowing K+ efflux and repolarization.

• Absolute and Relative Refractory Periods:

  • Absolute refractory period: No new action potential can be initiated due to inactivation of Na+ channels.

  • Relative refractory period: A stronger stimulus is required to initiate another action potential as K+ channels remain open.

• Propagation Speed: Myelinated axons conduct action potentials more quickly due to saltatory conduction.

Example: In myelinated neurons, action potentials jump from one node of Ranvier to the next, greatly increasing conduction velocity.

Myelination and Nodes of Ranvier

Role in Nerve Conduction

Myelin is a fatty substance that insulates axons, preventing ion leakage and speeding up electrical conduction. Nodes of Ranvier are gaps in the myelin sheath where voltage-gated channels are concentrated, allowing action potentials to be regenerated.

• Multiple Sclerosis (MS): Loss of myelin leads to slower and less efficient nerve conduction.

• Pharmacological Application: Blocking voltage-gated Na+ channels would prevent action potential generation, which could have clinical uses (e.g., local anesthetics).

Synaptic Transmission

Structure and Function

A synapse is the junction between two neurons or between a neuron and another cell (e.g., muscle). It consists of a presynaptic cell (which releases neurotransmitters) and a postsynaptic cell (which receives the signal).

• Neurotransmitters: Chemical messengers released from presynaptic terminals in response to an action potential.

• Release Mechanism: Neurotransmitters are released via exocytosis when Ca2+ enters the presynaptic terminal.

• Postsynaptic Effects: Neurotransmitters bind to receptors on the postsynaptic membrane, causing excitatory or inhibitory responses.

• EPSP and IPSP:

  • Excitatory postsynaptic potential (EPSP): Depolarizes the postsynaptic membrane, increasing the likelihood of an action potential.

  • Inhibitory postsynaptic potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of an action potential.

• Summation: Multiple EPSPs and IPSPs can combine to determine whether the postsynaptic cell reaches threshold for an action potential.

Receptors and Signal Transduction

• G-protein linked receptors: Metabotropic receptors that activate intracellular signaling cascades.

• Ionotropic receptors: Ligand-gated ion channels that mediate fast synaptic transmission.

Table: Comparison of Synaptic Potentials

Additional info:

• Action potentials are all-or-none events; once threshold is reached, the full action potential is generated.

• Neurotransmitter removal from the synaptic cleft occurs via reuptake, enzymatic degradation, or diffusion.

• Synaptic plasticity underlies learning and memory in the nervous system.