Fundamentals of the nervous system II

Electrophysiology – Some basics 1. Opposite charges are attracted to each other 2. Energy is required to keep opposite charges separated across a membrane 3. Energy is liberated when the charges move toward one another 4. When opposite charges are separated, the system has potential energy 5. Greater the charge difference between points = higher voltage

Opening and closing of ion-channels in the membrane allows ions to cross the membrane which results in a change in charge – a change in voltage (potential energy generated) • Change of Membrane Potential Ion-channels open and close in response to: • Changes in membrane potential – voltage-gated ion channels • Binding of molecules to membrane – ligand-gated ion channels

Opening and closing of ion-channels in the membrane allows ions to cross the membrane which results in a change in charge – a change in voltage (potential energy generated) • Change of Membrane Potential Ion-channels open and close in response to: • Changes in membrane potential – voltage-gated ion channels • Binding of molecules to membrane – ligand-gated ion channels Like most cells, neurons have a resting membrane potential, BUT unlike most other cells, neurons can rapidly change their resting membrane potential Two main types • Action potential • Graded potential

A voltmeter can measure potential (charge) difference across the membrane of a resting cell. • The cytoplasmic side of a cell is negatively charged compared to the outside, in this state, the membrane is said to be polarized

Na+/K+ gradients are maintained by active transport • Na+/K+ pumps in membranes use ATP (energy) to move 3 Na+ out of the cell and 2 K+ in the cell

Membrane potential changes when: • Concentrations of ions across membrane change • Membrane permeability to ions changes Changes in membrane potential are used as signals to receive, integrate, and send information Changes produce two types of signals • Graded potentials • Incoming signals operating over short distances • Action potentials • Long-distance signals of axons

Short-lived, localized changes in membrane potential • The stronger the stimulus, the more voltage changes and the farther current flows Triggered by stimulus that opens ion-gated channels • Results in depolarization or sometimes hyperpolarization Named according to location and function • Receptor potential (generator potential): graded potentials in receptors of sensory neurons • Postsynaptic potential: neuron graded potential

Once gated ion channel opens, depolarization spreads from one area of membrane to next

Principal way neurons send signals • Means of long-distance neural communication • Occur only in muscle cells and axons of neurons Brief reversal of membrane potential with a change in voltage of ~100 mV Action potentials (APs) do not decay over distance as graded potentials do • Also referred to as a nerve impulse • Involves opening of specific voltage-gated channels

Action potentials can be broken down into distinct phases: - At Membrane Resting Potential - Initial Membrane Depolarization result of passive response - Fast Rising Depolarizing Phase, Membrane Potential Approaches - Fast Falling Repolarizing Phase, Membrane Potential Approaches - Repolarization to Membrane Resting Potential, setting conditions for the next action potential What happens on membrane level?

All action potentials are alike and are independent of stimulus intensity • The nervous system tells the difference between a weak stimulus and a strong one by the frequency of impulses – high frequency = stronger impulse

Occur only in axons, not other cell areas and conduction velocities (how fast a signal travels) in axons vary widely Propagation depends on two factors: 1. Axon diameter • Larger-diameter fibers have less resistance, so have faster impulse conduction 2. Degree of myelination • Two types of conduction depending on presence or absence of myelin • Continuous conduction – nonmyelinated axons (slow conduction) • Saltatory conduction – myelinated axons (fast conduction)