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MEPS01 (Physiology)
Action potential
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Learning objective
Describe briefly the division of nervous system
Describe the parts of neurons in terms of structure
and functions
Describe the type ofchannel proteins involved intransmission of signals in the cell membrane based
on electrical and chemical gradient
Brief explanation on the mechanism of both resting
and action potential
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The Nervous System (recall)
Has two distinct parts:1. The central nervous system (the brain and
spinal cord)
2. The peripheral nervous system (the nerves
outside the brain and spinal cord)
The basic unit of the nervous system is the nerve
cell (neuron)
Neurons come in several forms, which can beclassified by theirstructure, their function, or
both
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Histology of the nervous system
A "nerve" is a visible structure, a bundle of axonal
processes from many different neurons, wrapped in aconnective tissue sheath.
A "neuron" is a cell, one part of which is included in a
nerve.
Analogy : A telephone cable carr ies messages back and
for th along w ires
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Histology of the nervous system
Basic functional unit of the nervous system is neuron
formed of cell body / soma and cell processes.
Cell body usually multipolarsurrounded by cell
membrane and contains nucleus with many
cytoplasmic organelles.
Cell processes have two types:
1. Dendrites short , branched which receive ongoing
impulses to the cell body.2. Axon / nerve fiber a long process of the cell
carries impulses from the cell body.
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Histology of the nervous system
Nerve formed of large number of nerve fibers and
maybe of:
1. Myelinated axon covered with myelin sheath
2. Non-myelinated myelin sheath absent
Impulses transmitted from one nerve to another cell at
synapses
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Myelinated Nerve Fibers
A very elaborate multi-layered covering of plasma
membrane, a thick and efficient barrier to chargeleakage, called the myelin sheath made by:
Schwann cells in the peripheral nervous system
Oligodendrocytes in the central nervous system
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Oligodendrocytes
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Function of myelination
In axons of equal size the rate of conduction of a signal is
much stable when there's a myelin sheath.
Myelination prevents leakage of membrane charge into
the surrounding intercellular space.
It also lessens the strain on the neuron's sodium
potassium pump by restricting ion release to specific sites
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Non-myelinated nerve fibers
Not all axons of peripheral nerves are
myelinated. But that doesn't mean they'reuncovered; neurons are never left naked.
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Other support cells (CNS)
The brain and spinal cord also contain support cells
called glial cells of many types:
1. As t rocy tes: help provide nutrients to nerve cells and
control the chemical composition of fluids around nerve
cells, enabling them to thrive.
2. Oligodendrocytes: make myelin, a fatty substance
that insulates nerve axons and speeds the conduction
of impulses along nerve fibers.
3. Mic rog lia: help protect the brain against infection and
help remove debris from dead cells.
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Other support cells (CNS)
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A synapse is a junction between two neurones across which electrical signals pass
presynaptic cell
postsynaptic cellsynapticcleft
Transmission of impulses
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When a nerve impulse arrives at the end of one neurone it triggers the release
ofneurotransmittermolecules from synaptic vesicles.
synaptic
vesicle
neurotransmitter
molecules
eg: acetylcholine
Transmission of impulses
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The neurotransmitters diffuse across the synaptic cleftand bind with receptors on
the next neurone, triggering another impulse.
nerveimpulse
receptor
synaptic
cleft
Transmission of impulses
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Transmission of impulses
Neurons send messageselectrochemically.
Chemicals cause an
electrical signal.
Chemicals in the body are
"electrically-charged" - when
they have an electricalcharge, they are called ions.
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Transmission of impulses
Important ions in the nervous systemare sodium, Na+ and potassium,
K+ (both have 1 positive charge, +),
calcium, Ca2+ (has 2 positive
charges, ++) and chloride, Cl- (has
a negative charge, -)
There are also some negatively
charged protein molecules.
** Nerve cells are surrounded by a semi-
permeablemembrane that allows some ions to
pass through and blocks the passage of other
ions.
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Resting Membrane Potential
When a neuron is not sending a signal, it is "at rest."
At rest, the inside of the neuron is -ve relative to the
outside.
Although the concentrations of the different ions
attempt to balance out on both sides of the
membrane, they cannot because the cell membrane
allows only some ions to pass through channels
(ion channels).
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Resting Membrane Potential
At rest, K+ ions can cross through the membrane
easily but not for Cl- ions and Na+ ions.
Presence of a pump that uses energy to move 3 Na+
ions out of the neuron for every 2 K+ ions it puts in.
When all these forces balance out, and the difference
in the voltage between the inside and outside of the
neuron is measured as the resting potential.
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The K+ channel and Na+/K+ pump
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Resting Membrane Potential
The resting membrane potential of a neuron is about -70
mV (mV=millivolt) - the inside of the neuron is 70 mV lessthan the outside.
At rest, there are relatively more Na+ ions outside the
neuron and more K+ ions inside that neuron.
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Action Potential
A stimulus causes the resting potential to move toward 0
mV
When the depolarization reaches about -55 mV a neuron
will fire an action potential. This is the threshold
If the neuron does not reach this critical threshold level,then no action potential will fire
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Action Potential
When the threshold level is reached, an action potential of
a fixed sized will always fire...for any given neuron, thesize of the action potential is always the same.
There are no big or small action potentials in one nerve cell
- all action potentials are the same size. The neuron either does not reach the threshold or a full
action potential is fired - this is the "ALL OR NONE"
principle.
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Action Potential
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What happens when a
nerve is stimulated?
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Getting excited!
As the neurones membrane at rest is more -ve inside
than outside, it is said to be polarised
Neurones are excitable cells
If a stimuli above a threshold level is applied to the
membrane, it causes a massive change in the potential
difference
The cells are excited when their membranes become
depolarised; making the inside of the axon +ve & theoutside -ve
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The potential difference becomes +40mV; lasting about
3ms, before returning to the resting state (why it is
important to return the membrane to the resting state
a.s.a.p?)
This return to a resting potential of -70mV;repolarisation
The large change in the voltage across the membrane;
action potential
Getting excited!
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What causes an Action Potential?
When the cell membranesare stimulated, there is achange in the permeability ofthe membrane to Na+
The membrane becomes
more permeable to Na+andK+
Due to the opening & closingof voltage-dependent Na+ &
K+ channels (at rest, thesechannels are blocked bygates)
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1) D l i ti
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1) Depolarisation
1 D l i ti
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1. Depolarisation
Th th h ld
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The threshold
55mV represents the threshold potential
Beyond this we get a full action potential
The membrane potential rises to +35mV this is the
peak of the action potential
The cells are almost at the equilibrium for Na+ ions
In order for the neuron to generate
an action potential the membrane
potential must reach the threshold
of excitation.
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Time
-70
-55
0
+35
Threshold
mV
Resting potential Action potential
More Na+
channels open
Na+ floods
into neurone
Na+
voltage-gated
channels open
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2)Repolarisation
After about 0.5ms, the V-D Na+ channels close &
permeability of the membrane to Na+ returns tonormal
V-D K+ channels open due to depolarisation of the
membrane; K+ move out of the axon down the
electrochemical gradient As K+ flow out of the cell, the inside of the cell once
again becomes more ve than outside
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2)Repolarisation
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3) Hyperpolarisation
The membrane is now highly permeable to K+ & more
ions move out than occurs at resting potential, making thepotential diff. more ve than the normal resting potential
The resting potential is re-established by closing of the V-
D K+ channels & K+ diffusion into the axon
3) Hyperpolarisation
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3) Hyperpolarisation
3) Hyperpolarisation
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3) Hyperpolarisation
M h i f A ti P t ti l
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Mechanism of Action Potential
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N l j ti (NMJ)
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Neuromuscular junction (NMJ)
A neuromuscular junction (NMJ) is a contact between a
nerve and a muscle - it is like a synapse, the actionpotential stops and the signal is carried by a chemical.
There is a delay at synapses- chemical transmission is
slower than electrical transmission
Synapses &
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Synapses &
Neuromuscular Junctions
Chemical transmitters are made and stored in thepresynaptic terminal
Calcium is required for transmitter release
Transmitter diffuses across the synaptic gap and binds toa receptor
When transmitter binds to a receptor, it produces an
EPSP(excitatory postsynaptic potential) or an IPSP
(inhibitory postsynaptic potential)
If there are enough EPSPs, an action potential will be
produced in the postsynaptic membrane
Synapses &
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Synapses &
Neuromuscular Junctions
Synapses &
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Synapses &
Neuromuscular Junctions
The transmitter is broken down and/orrecycled In the CNS, nerves make synapses with thousands of
other nerves
There are dozens of transmitters in the nervous system Synapses are believed to be the sites of Learning and
Memory
Many toxins and diseases affect neuromuscularjunction & synaptic transmission
Nervous system disorder
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Nervous system disorder
The nervous system is an extraordinarily complex
communication system that can send and receivevoluminous amounts of information simultaneously.
However, the system is vulnerable to diseases and
injuries.
Nervous system disorder
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Nervous system disorder
Degeneration of nerve cells can cause Alzheimer's,
Hunt ington's, orParkin son's disease.
Inflammation ofOligodendrocytesmay cause multiple
sclerosis.
Infection of bacteria or viruses on the brain or spinal cordcan causing encephal i t isormeningi t is .
A blockage in the blood supply to the brain can cause a
stroke.
Injuries or tumors can cause structural damage to the
brain or spinal cord.
Nervous system disorder
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Nervous system disorder