Fundamentals of the nervous

system II

David Awde Lecture 14

Fall, 2024

Last class

•        General break down of the nervous system

•        Neurons and Glia cells

•        Physiology of synapses

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

Extracellular

[Na⁺]

[K⁺]

[Ca²⁺]

[Mg²⁺]

[Cl^-]

+

-          high extracellular [Na⁺]

-          low intracellular [Na⁺]

-          high intracellular [K⁺]

-          low extracellular [K⁺]

How is the Na⁺/K⁺ gradient created and maintained?

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

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

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

-65 mV

-50 mV

-50 mV

to

+40 mV

+40 mV

to

-80 mV

-         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)