Neurophysiology (2)

Electrical Signals in Neurons

  • Neurons are electrically excitable due to voltage differences across their membranes.

  • Communication occurs via two types of electric signals:

    • Action Potentials: can travel long distances.

    • Graded Potentials: local changes only.

  • A flow of ions through ion channels occurs in living cells.

Types of Ion Channels

Leakage (Nongated) Channels

  • Remain always open.

  • Nerve cells have more K+ channels than Na+ channels.

  • Higher membrane permeability to K+, resulting in a resting membrane potential of about -70mV.

Gated Channels

  • Open or close in response to stimuli, leading to neuron excitability.

    • Voltage-Gated Channels: open due to voltage changes.

    • Ligand-Gated Channels: open in response to specific chemicals (e.g., hormones, neurotransmitters).

    • Mechanically-Gated Channels: open with mechanical stimulation.

Gated Ion Channels

  • Voltage-gated K+ channels open when extracellular voltage changes.

  • Ligand-gated channels respond to specific chemical stimuli to alter their state (open/close).

Resting Membrane Potential

  • Inside of cell membrane: negative ions.

  • Outside: positive ions, resulting in a potential difference of -70mV.

  • The cell is considered "polarized" due to:

    • Different ion concentrations between inside (e.g., K+) and outside (e.g., Na+, Cl-).

    • Higher permeability for K+ than Na+ (50-100 times greater).

    • Na+/K+ pump helps maintain resting potential by removing Na+ that leaks in.

Action Potentials

Definition

  • Action potentials are rapid changes in membrane potential decreasing and eventually reversing (depolarization), followed by a return to resting state (repolarization).

Graded Potentials

  • Small deviations from -70mV:

    • Hyperpolarization: membrane becomes more negative.

    • Depolarization: membrane becomes more positive.

Phases of Action Potential

Depolarizing Phase

  • Initiated by a stimulus causing graded potential to reach threshold (-55mV).

  • Voltage-gated Na+ channels open, allowing Na+ to rush into the cell.

    • The influx of Na+ changes membrane potential up to +30mV.

Repolarizing Phase

  • K+ channels open, but slower than Na+, allowing K+ to exit the cell.

  • Membrane potential returns to -70mV, may reach -90mV (after-hyperpolarization).

Refractory Period

  • No new action potential can be generated.

    • Absolute Refractory Period: strong stimulus won’t trigger another AP.

    • Relative Refractory Period: A strong enough stimulus may trigger an AP as K+ channels are still open.

Propagation of Action Potential

  • Action potentials propagate across the membrane:

    • The influx of Na+ affects adjacent sections, opening more voltage-gated channels.

    • This creates a self-propagating wave along the membrane.

Local Anesthetics

  • Prevent the opening of voltage-gated Na+ channels, halting nerve impulse conduction in the area.

Continuous vs Saltatory Conduction

  • Continuous Conduction: occurs in unmyelinated fibers with step-by-step depolarization.

  • Saltatory Conduction: occurs in myelinated fibers, where depolarization happens at Nodes of Ranvier, allowing impulses to jump from node to node.

Factors Affecting Propagation Speed

  • Axon diameter, amount of myelination, and temperature contribute to the speed of impulse transmission.

Speed of Impulse Propagation

  • Larger, myelinated fibers conduct impulses faster. Fiber types include:

    • A fibers: largest, myelinated, fastest (130 m/sec).

    • B fibers: medium, myelinated (15 m/sec).

    • C fibers: smallest, unmyelinated, slowest (2 m/sec).

Action Potentials in Nerve and Muscle

  • Muscle cell membranes behave differently than neuron axons; resting potentials:

    • Nerve: -70mV

    • Skeletal/ Cardiac Muscle: closer to -90mV.

Chemical Synapses

  • Action potentials trigger voltage-gated Ca2+ channels to open, causing neurotransmitter release across synaptic clefts.

  • More neurotransmitter leads to greater change in potential in postsynaptic cells.

Excitatory and Inhibitory Potentials

  • EPSP (excitatory): caused by opening ligand-gated Na+ channels, makes it easier for postsynaptic cell to reach threshold.

  • IPSP (inhibitory): caused by opening ligand-gated Cl- or K+ channels, making postsynaptic cell less likely to reach threshold.

Removal of Neurotransmitter

  • Neurotransmitters are removed by:

    • Diffusion

    • Enzymatic degradation (e.g., acetylcholinesterase)

    • Uptake by neurons or glia cells (neurotransmitter transporters).

Summation of Postsynaptic Potentials

  • Spatial Summation: effects from several presynaptic neurons onto a single postsynaptic neuron.

  • Temporal Summation: effects from rapid successive firings of a single presynaptic neuron.

Neurotransmitter Effects & Modification

  • Neurotransmitter effects can be modified through synthesis regulation, release manipulation, and receptor site activation or blocking.

  • Agonist: enhances transmitter effects.

  • Antagonist: blocks neurotransmitter actions.

Neurotransmitters

  • Examples include:

    • Small Molecule Neurotransmitters:

      • Acetylcholine

      • Amino Acids (e.g., GABA, Glutamate)

      • Biogenic Amines (e.g., Norepinephrine, Dopamine, Serotonin)

    • Neuropeptides:

      • Substance P

      • Enkephalins

      • Endorphins

  • Neuropeptides: longer than small molecules, play role in pain regulation and other functions.

Neuronal Regeneration & Plasticity

  • Plasticity allows new dendrites to sprout and new proteins to synthesize, although regeneration is limited (better in PNS than CNS).

Multiple Sclerosis (MS)

  • An autoimmune disorder destroying myelin sheaths in CNS, leading to muscle weakness and sensory alterations, occurring mostly in young adulthood.

Epilepsy

  • Characterized by recurrent, short attacks due to abnormal brain electrical discharges, with various causes, including brain damage and infections.

Neuronal Structure & Function Overview

  • Dendrites: receive stimuli, activated by ion channels.

  • Cell Body: integrates signals and produces potentials.

  • Axon: propagates impulses to terminals where neurotransmitter release occurs.