Download - Neurophysiology Made Easy
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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