(4) Action Potential

7/28/2019 (4) Action Potential

1/53

MEPS01 (Physiology)

Action potential


2/53

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


3/53

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


4/53

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


5/53


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.


6/53


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


7/53

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


8/53

Oligodendrocytes


9/53

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


10/53

Non-myelinated nerve fibers

Not all axons of peripheral nerves are

myelinated. But that doesn't mean they'reuncovered; neurons are never left naked.


11/53

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.


12/53

Other support cells (CNS)


13/53


14/53


15/53

A synapse is a junction between two neurones across which electrical signals pass

presynaptic cell

postsynaptic cellsynapticcleft

Transmission of impulses


16/53

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



17/53

The neurotransmitters diffuse across the synaptic cleftand bind with receptors on

the next neurone, triggering another impulse.

nerveimpulse

receptor

synaptic

cleft



18/53


Neurons send messageselectrochemically.

Chemicals cause an

electrical signal.

Chemicals in the body are

"electrically-charged" - when

they have an electricalcharge, they are called ions.


19/53


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.


20/53

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).


21/53


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.


22/53

The K+ channel and Na+/K+ pump


23/53


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.


24/53


25/53

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


26/53

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.


27/53

Action Potential


28/53

What happens when a

nerve is stimulated?


29/53

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


30/53

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!


31/53

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)


32/53


33/53


34/53

1) D l i ti


35/53

1) Depolarisation

1 D l i ti


36/53

1. Depolarisation

Th th h ld


37/53

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.


38/53

Time

-70

-55

0

+35

Threshold

mV

Resting potential Action potential

More Na+

channels open

Na+ floods

into neurone

Na+

voltage-gated

channels open


39/53

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


40/53

2)Repolarisation


41/53

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



42/53




43/53


M h i f A ti P t ti l


44/53

Mechanism of Action Potential


45/53


46/53

N l j ti (NMJ)


47/53

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 &


48/53

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 &


49/53

Synapses &


Synapses &


50/53

Synapses &


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


51/53


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.



52/53


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.



53/53


(4) Action Potential

Documents

Transcript of (4) Action Potential