Ronald Allan Cruz, MDDepartment of Physiology

FEU-NRMF Institute of Medicine

Neuron

Membrane Potential

The difference in electrical potential between the two sides of the plasma membrane

Negative ions – inside the membrane

Positive ions – outside the membrane

The body is electrically neutral

Types of Membrane Potential Resting membrane potential

Local potential Synaptic potential Generator or receptor potential Electrotonic potential

Action potential

Resting Membrane Potential

Nature of the plasma membrane Selectively

permeable

Unequal distribution of ions

Concentration of Ions

Mmol/L of H2O

Inside the cell

Outside the cell

Na+ 15.0 150.0

K+ 150.0 5.5

Cl- 9.0 125.0

Inside thecell

Outside the cell

K+ Proteins-

Na+

Cl-

Negative charge inside the membranePositive charge outside the membrane

-70 mv K+

Proteins-

Ca+

Na+

Cl-

ATP ADP

Na+

Cl-

Ca+

K+

3 Na+

2 K+

Equilibrium potential

Membrane potential when no work is involved in movements of a given ion Inward flow = outward flow

Nernst equation E = 61 log(Ci/Co) Io

n Equilibrium potential

(mV)

NaKCl

+60-90-70

Excitability and Conductivity Neurons have low threshold of

excitation Stimulus may be:

Electrical Chemical Mechanical

Responses produced: Local potential Action potential

Action Potential

Electrical signals or nerve impulses by which information is conducted from one area to another

Production of Action Potential Resting state Depolarization Repolarization After

depolarization After

hyperpolarization

Threshold

Threshold stimulus K+ flow = Na+ flow 15-20 mV fom the

RMP Critical firing level All-or-none

phenomenon

Strength-Duration Relationship Weak stimulus:

longer time to excite a fiber

Strong stimulus: shorter time to excite a fiber

Rheobase

Utilization time

Chronaxie

Excitability Cycle

Refractory period Absolute refractory

period Relative refractory

period

Supernormal period

Subnormal period

Conduction of an Action Potential

Depolarization Na+ influx Electronic potential

Action potential Refractory period

+++++ - - - - - - +++++ +++++ +++++ +++++ +++++ +++++

Local Potential

Transient shift in membrane potential in a localized area of the membrane

Localized change in the ion channels that alter the permeability to one or more ions

Hyperpolarization Hypopolarization Depolarization

Types of Local Potential

Synaptic potential

Generator potential – receptor potential

End-plate potential

Electrotonic potential

Characteristics of Local Potentials

Graded

Summation

Short-distance signals - localized

Production of Synaptic Potential

K+ channels open Na+ channels open Cl- channels open K+ channels close Ca+ channels open

Production of Generator potential

Na+ channels open

Production of Electrotonic Potential

Na+ channels open

Opening or closing of several ion channels by an externally applied voltage Anodal (+) pole – hyperpolarization Cathodal (-) pole – depolarization

Factors Affecting Conduction Axon diameter

Degree of myelinization

Ion Channels in Myelinated Neurons Saltatory

conduction

Na+ channels Initial segment Nodes of Ranvier

Reflex

Functional unit of the nervous system

Somatic reflex arc

Autonomic reflex arc

Components Receptors Afferent nerve Center Efferent nerve Effector

Receptors

Cutaneous receptors

Receptors in special sense organs

Receptors which respond to changes in the internal environment

Telereceptors Exteroreceptors Interoreceptors Proprioreceptors

Properties of a Receptor

Responds to lowest threshold

Transforms different types of energy into nerve impulse

When stimulated, it produces a series of impulses ( stimulus intensity , frequency of discharge)

Properties of a Receptor

stimulus intensity, receptors stimulated recruitment of sensory units

Adaptation Frequency of receptor discharge may

become lower after a certain duration of stimulation

Receptors may be inhibited

Mechanism of Impulse Initiation in Receptors

Sequence of reactions Stimulus produces depolarizaion Generator or receptor potential is created Generator potential is sufficient to

generate an action potential

Mechanism of Impulse Initiation in Receptors

Ionic mechanism Stimulus increases permeability to Na+

Depolarization, creation of a generator potential

Depolarization spreads and reaches the first node of Ranvier

Threshold is reached, action potential is generated

Generation potential is maintained, steady action potentials are evoked at the node Repetitive firing

A stronger stimulus produces a generator potential of greater magnitude Increased frequency of discharge upon

application of a stronger stimulus

Adaptation

Inhibition by other efferent nerve fibers

Synaptic Transmission

Pre-synaptic element

Synaptic cleft

Post-synaptic element

Methods of Synaptic Transmission Electrical

Rare in vertebrates Occurs in tight

junctions Transmission is by

electrical coupling Transmission may

be bidirectional

Chemical Excitatory

transmission

Inhibitory transmission

Neurotransmitter Release

Release of neurotransmitter

Fate of neurotransmitter

Excitatory or inhibitory mechanism Acetylcholine,

epinephrine, dopamine

GABA, glycine

Characteristics pre-synaptic spike,

transmitter release

Ca ions, post-synaptic spike

Mg ions, response

Orthodromic conduction

Characteristics of Synaptic Transmission

Summation

Inhibition Development if IPSP Pre-synaptic inhibition Renshaw cell inhibition

Negative feedback

Convergence Several pre-synaptic neuron : 1 post-

synaptic neuron

Divergence 1 pre-synaptic neuron : several post-

synaptic neurons

Facilitation Successive stimulation of an efferent

neuron may build up and EPSP reaching threshold

Occlusion 2 efferent neurons produce a lesser

response than the sum of the responses produced when the neurons are stimulated separately

Unidirectional transmission Orthodromic transmission

Repetitive discharge

Transmission is depressed by hypoxia

Synaptic delay in the passage of a nerve impulse is accounted for the time it takes for: Release of neurotransmitter Diffusion of neurotransmitter Change in membrane permeability of

post-synaptic ending Depolarization – EPSP

Neuromuscular Transmission

Acetylcholine

End-plate potential

Mechanism of Neuromuscular Transmission

Depolarization of nerve endings Release of neurotransmitters Diffusion of the neurotransmitters Neurotransmitters interact with

receptors ↑ permeability of membrane to Na+

Depolarization Action potential

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