Neurophysiology

Cellular Electrical Difference and Membrane Potential

• The electrical difference (voltage) across the cell membrane is mainly due to ion concentration gradients, primarily between K⁺ and Na⁺.
• The resting membrane potential is primarily influenced by the permeability of the cell membrane to potassium ions (K⁺) compared to sodium ions (Na⁺).
• At rest, the cell membrane exhibits higher permeability to K⁺ due to more open potassium channels.

Typical Resting Membrane Potential

• The typical resting membrane potential in mammalian neurons is approximately -70 mV.

Equilibrium Potential

• The equilibrium potential for an ion species is determined by the Nernst equation, which factors in ion concentration across the membrane.

Functions of Glial Cells

• Neuroglia or glial cells primarily support, protect, and maintain the function of neurons in the nervous system.

Neuron Types

• Unipolar neurons have a single stem axonal process that branches to form two processes, typically found in sensory pathways.
• A major functional unit of the nervous system is the neuron.

Axon Functions

• The axon hillock or trigger zone in a neuron serves as the site of action potential initiation.
• Efferent neurons transmit information from the brain or spinal cord to muscles and glands.

Myelin Sheath

• The myelin sheath surrounding some neurons increases the speed of action potential conduction along the axons by providing insulation.

Sodium-Potassium Pump

• The Na⁺/K⁺ pump (ATPase) maintains resting membrane potential by actively transporting Na⁺ out of the cell and K⁺ into the cell, countering diffusion forces.

Synaptic Signals

• Synaptic signals alter the membrane potential of a neuron or muscle cell through graded potentials.
• EPSP (excitatory postsynaptic potential) depolarizes the membrane, while IPSP (inhibitory postsynaptic potential) hyperpolarizes it.
• The transient nature of EPSP/IPSP is due to the quick inactivation of receptors and reuptake of neurotransmitters.

Action Potential and Conduction

• The generation of an action potential depends on the resting membrane potential; a sufficient depolarization must occur.
• Action potential propagation depends on the opening of voltage-gated Na⁺ and K⁺ channels.

Propagation Speed Factors

• Factors influencing action potential conduction speed include axon diameter and myelination.
• Myelinated axons conduct action potentials faster due to saltatory conduction, jumping between nodes of Ranvier.

Calcium Ions and Neurotransmitter Release

• During the opening of voltage-gated Ca2+ channels at the axon terminal, Ca²⁺ enters the cell, triggering neurotransmitter release via exocytosis.

Neurotransmitter Classes and Functions

• GABA and glutamate are classified as amino acid neurotransmitters.
• Synaptic vesicles in the presynaptic terminal are responsible for storing and releasing neurotransmitters.

Mitochondrial Function

• Mitochondria are the primary organelle for ATP production in neurons, providing energy for cellular functions.

Dendritic Spines and Axon Functions

• Dendritic spines enhance synaptic strength and connectivity, facilitating communication between neurons.
• The axon hillock and initial axon segment are crucial regions for action potential initiation and integration of synaptic signals.

Conductivity of Myelinated Axons

• Myelinated axons achieve faster conduction velocity due to reduced capacitance and increased resistance, facilitating rapid signal transmission.
• In unmyelinated axons, conduction velocity is primarily determined by the properties of the axon membrane and internal resistance.

Postsynaptic Potentials and Drugs

• If a drug inhibits the postsynaptic potential of a sensory neuron, it would disrupt normal signal transmission to the brain.
• Voltage-gated Na⁺ channels play a critical role in generating action potentials through rapid depolarization.

Role of Ion Channels in Action Potential

• Voltage-gated K⁺ channels help repolarize the membrane during action potentials, restoring resting conditions after depolarization.
• Hyperpolarization helps stabilize the membrane potential, preventing excessive neuronal firing.

Summary on Myelin Sheath Function

• The myelin sheath primarily functions to insulate axons, enhance conduction speed of electrical impulses, and improve signal integrity.

Learn about the major classes of neurotransmitters and categories of cells in the nervous system. Explore the functions of neurons and neuroglia, the helper cells in the nervous system.