Download action potential

Physiology of Synapses in the
CNS- L4
Faisal I. Mohammed, MD, PhD
University of Jordan
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Objectives
Students should be able to:
 Define synapse and list the types of synapse
Describe the mechanism of neurotransmitter release
List the major types of neurotransmitters (NT)
Compare the small molecules NT and Neuropeptides
Describe the resting membrane potential and Nernst Equation
Determine the how EPSP, IPSP and Presynaptic inhibition
develops
Describe summation of EPSP and IPSP
Describe the characteristics of synapse (Fatigue and Delay)
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Characteristics of Postsynaptic
Potentials (EPSP and IPSP)
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It is a local potential, propagates for a short distance
It is graded potential so it can be summated
It takes 1-2 msec to develop and stays for 15-20
msec
Its amplitude is directly proportional to the strength
of the stimulus (amount of NT)
It is decremental potential (decreases as it travels)
It is due to a change in the permeability of ligandgated (chemically) channels
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Presynaptic Inhibition
Activation of presynaptic synapses decreases
ability of Ca+ channels to open on the
presynaptic terminals.
inhibition of Ca+ influx results in reduced neuronal
excitation
Presynaptic inhibition occurs in many of the
sensory pathways in the nervous system.
The neurotransmitter is usually GABA.
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Comparison between Presynaptic and
Postsynaptic inhibition
IPSP need 1-2 msec to be formed and lasts only
15-20 msec in contrast to presynaptic inhibition
that need 15-20 msec to develop and last longer
than 100 msec sometimes
IPSP leads to changes in the postsynaptic
membrane potential in contrast to presynaptic
membrane potential where it leads to decrease NT
release
Besides all these the site where both work is
different
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Graded Potentials
Small deviations from resting potential of -70mV
hyperpolarization = membrane has become more
negative
depolarization = membrane has become more positive
The signals are graded, meaning they vary in
amplitude (size), depending on the strength of the
stimulus and localized.
Graded potentials occur most often in the
dendrites and cell body of a neuron.
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Graded Potentials
Short-lived, local changes in membrane potential
Decrease in intensity with distance
Their magnitude varies directly with the strength
of the stimulus
Sufficiently strong graded potentials can initiate
action potentials
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Graded Potentials
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Graded Potentials
Voltage changes in graded potentials are
decremental
Current is quickly dissipated due to the leaky
plasma membrane
Can only travel over short distances
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Summation of
Postsynaptic
Potentials
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Summation
If several presynaptic end bulbs release their
neurotransmitter at about the same time, the
combined effect may generate a nerve impulse
due to summation
Summation may be spatial or temporal.
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Summation of Postsynaptic Potentials
Spatial Summation
Excitation of a single presynaptic neuron on a
dendrite will almost never induce an action
potential in the neuron.
Each terminal on the dendrite accounts for about a
0.5 - 1.0 mV EPSP.
When multiple terminals are excited simultaneously
the EPSP generated may exceed the threshold for
firing and induce an action potential.
The rate of firing is directly proportional to the
amplitude of the EPSP
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Summation of Postsynaptic Potentials
Temporal Summation
A neurotransmitter opens a membrane channel for
about 1 msec but a postsynaptic potential lasts for
about 15 msec.
A second opening of the same membrane channel can
increase the postsynaptic potential to a greater level.
Therefore, the more rapid the rate of terminal
stimulation the greater the postsynaptic potential.
Rapidly repeating firings of a small number of
terminals can summate to reach the threshold for
firing.
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Graded potentials in response to opening
mechanically-gated channels or ligandgated channels
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Stimulus strength and graded potentials
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Summation
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Generation of Action Potentials
 An action potential (AP) or impulse is a sequence of rapidly
occurring events that decrease and eventually reverse the
membrane potential (depolarization) and then restore it to
the resting state (repolarization).
During an action potential, voltage-gated Na+ and K+
channels open in sequence (Na+ then K+ )
According to the all-or-none principle, if a stimulus
reaches threshold, the action potential is always the same.
A stronger stimulus will not cause a larger impulse.
An action potential is generated mostly at the axon hillock
since it has the lowest threshold compared to the soma or
dendrites.
The axon hillock has the highest density of voltage gated
Na+ channels
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Action Potentials
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Facilitation of Neurons
 Often the summated postsynaptic potential is
excitatory in nature but has not reached threshold
levels.
 This neuron is said to be facilitated because the
potential is nearer the threshold for firing than normal
but not yet to the firing level.
 It is easy to stimulate this neuron with subsequent
input.
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Function of Dendrites in Stimulating
Neurons
Dendrites spaced in all directions from neuronal
soma.
allows signal reception from a large spatial area
providing the opportunity for summation of
signals from many presynaptic neurons
Dendrites do not transmit action potentials.
they have few voltage gated Na+ channels
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Dendrite Function Cont.
Dendrites transmit signals by electrotonic
conduction.
transmission of current by conduction in the fluids
of the dendrites
no generation of action potentials in the dendrites
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Special Characteristics of Synaptic
Transmission
 Fatigue
exhaustion of the stores of transmitter in synaptic
terminals.
excitatory synapses are repetitively stimulated at a
rapid rate until rate of postsynaptic discharge
becomes progressively less.
causes areas of nervous system to lose excitability
after a while.
development of fatigue is a protective mechanism
against excess neuronal activity.
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Special Characteristics of Synaptic
Transmission cont.
Post-tetanic facilitation
enhanced responsiveness following repetitive
stimulation
mechanism thought to be build-up of calcium ions in
the presynaptic terminals
build-up of calcium causes more vesicular release of
transmitter
Synaptic delay
the process of neurotransmission takes time, from the
delay can calculate the number of neurons in a circuit
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Synaptic Delay
Neurotransmitter must be released, diffuse across
the synapse cleft, bind to receptor, leads to
membrane potential changes that reaches the
threshold then the postsynaptic membrane
discharges.
Synaptic delay – time needed to do this (0.3-0.5
msec)
Synaptic delay is the rate-limiting step of neural
transmission
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Environmental Changes and Synaptic
Transmission
Effect of acidosis
depresses neuronal activity
pH change from 7.4 to 7.0 usually will induce coma
Effect of alkalosis
increases neuronal excitability
pH change from 7.4 to 8.0 usually will induce seizures
Effect of hypoxia
brain highly dependent on oxygen
interruption of brain blood flow for 3 to 7 sec can lead to
unconsciousness
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