Action Potential 3

7/29/2019 Action Potential 3

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Ginus Partadiredja

Department of Physiology

Faculty of Medicine GMU

THE GRADED POTENTIAL &THE ACTION POTENTIAL

http://thecomplementarynature.com/wordpress/all-posts/tcn-book


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Learning Objectives

1. To understand basic concepts related to graded potentials

2. To understand basic concepts related to action potentials

3. To understand the differences between graded potentials

and action potentials


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Neuron: Functional unit of nervous system, with

excitability and conductivity characteristics

The number of neurons in the CNS = 1011 = 10 billion

Glial cells (neuroglia): Non conductive cells which protect,

maintain, and support the nervous system

The number of glial cells = 10 50 x of neurons


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THE HISTOLOGY OF NEURON

Dendrite Cell body/ soma

Axon hillock

Axon Myelin sheath

Synaptic knobs/

terminal buttons/axon telodendria


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(pseudounipolar)

Pyramidal


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Dendrites & somareceptive segment

Axon closest to axon hillock initial

segment

Axonconductive segment

Axon terminaltransmissive segment


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Stimulus: Any change in the environment that is strong

enough to initiate an action potential

Action potential: An electrical signal that propagates alongthe surface of the membrane of a neuron

Graded potential: A small deviation from the resting

membrane potential that occurs because ligand-gated or

mechanically gated channels open or close

hyperpolarizing ordepolarizing graded potential

Receptor potential (sensory receptors)

Post-synaptic potential (mainly in dendrites & soma):

Excitatory post-synaptic potential (EPSP)

Inhibitory post-synaptic potential (IPSP)


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Action potential generator potential/ receptor potential

Receptor - sensory receptor

- proteins bind to hormones/ neurotransmitters

Sensory receptors: Transducers which alter various energy

in the environment into action potentials in neurons

Sensory organs = receptor + non neural cells


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Mechanism:

Stimulus receptor/ generator potential (EPSP like;

does not spread, graded, local) reach firing level/

neuronal threshold action potential


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THE PHYSIOLOGY OF NEURON

Recording with an electrode inside an axon resting

membrane potential/ polarization typically -70 mV

(the potential difference between the inside and outside of the

axon, the inside being more negative than the extra-cellular

fluid)


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Resting membrane potential small build-up of negative

ions along the inside of membrane, and positive ions along

the outside

Neurons range: -40 to -90 mV (ranges of membrane

potential of cells: +5mV to -100 mV)

Resting

membrane

potential


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ECF Na+ and Cl-

ICF

K+

and phosphates (attached to ATP and aminoacids)

Factors causing the negativity inside neurons:

1. Leakage of K+

to ECF (K+

channels > Na+

channels)2. Negative ions inside neurons cannot leave cells

(attached to ATP, proteins, or larger molecules)

3. Na+/K+ ATPase pumps (3 Na+ out for 2 K+ in)

contributes only -3 mV


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Threshold stimulus: a stimulus which is strong enough to

depolarize the membrane to threshold

Subthreshold stimulus; suprathreshold stimulus

Firing level/ threshold: The point where depolarization

accelerates (following the increase by 15 mV)


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Na+ channels open Na+ enters the cell


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Depolarization: The reversal of potential membrane +

inside and outside


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Overshoots exceeds the isopotential + 35 mV

Spike potential: The sharp upward and downward curve of

action potential


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Repolarization: The potential returns K+ channels open

(slower than Na+ channels) K+ exits

K+


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After-hyperpolarization: Under the level of polarization


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Refractory periods: Unresponsive period to adequate

stimulus

Absolute: firing level 1/3 repolarization

Relative: 1/3 repolarization the beginning of after-

depolarization (neurons can be stimulated by a larger-

than-normal stimulus)


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All or none law

Saltatory conduction: The

jump of depolarization from one

Ranvier node to the next Ranvier

node

many voltage-gatedchannels present in Ranvier

nodes ionic currents flow

through cytosol & ECF

energy-efficient mode of

conduction (less ATP for Na+/K+

pumps)


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Factors affecting the speed of conduction:

1. The amount of myelination

2. The diameter of axon (the larger the diameter of an axon,

the faster the propagation of impulses larger surface

areas)

3. Temperature (slower conduction at lower temperature)

Encoding stimulus intensity

Frequency of impulses

Number of sensory neurons activated


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The Types of Nerve Fibers in Mammalian Nerves

Type Function Diameter(m)

Conduction(m/sec)

Duration(msec)

Absoluterefractory

period

A Proprioceptive,

somatic motor

12 -20 70 - 120

A Touch,

pressure

5 - 12 30 - 70 0,4 0,5 0,4 - 1

A Motor in

muscle spindle

3 - 6 15 - 30

A Pain, cold,touch

2 - 5 12 - 30


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Type Function Diameter

(m)

Conduction

(m/sec)

Duration

(msec)

Absolut

refractory

periodB Autonomic pre-

ganglion


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Synapses

Axodendritic

Axosomatic

Axoaxonic

Synapses

Electrical synapses gap junctions connexons

Chemical synapses neurotransmitters


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Once the action potential reaches the axon terminal:

Voltage-gated Ca+2 channels open Ca+2 enters the cells

exocytosis of synaptic vesicles neurotransmitters

released


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Synaptic delay: The interval for the transmitters to traverse

the synaptic cleft neurotransmitters receptors in ligand-

gated channels

One-way conduction: transmitters are only in pre-synaptic

cells

Orthodromic conduction

Antidromic conduction

Post-synaptic potentials graded local potentials

spread around local cells membrane


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Excitatory Post-Synaptic Potentials (EPSP)

Partial depolarization which decreases membrane potential/

increases neuronal excitability

Cation channels open (Na+, K+, Ca+2)

Na+ enters cells > Ca+2 inflow or K+ outflow

Local depolarization action potential, but facilitating action

potential


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I hibit P t S ti P t ti l (IPSP)


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Inhibitory Post-Synaptic Potentials (IPSP)

The increase of negative potential inside cells -90 mV

(hyperpolarizing post-synaptic potential)

Opening of Cl- or K+ channels (Cl- enter to the cells and K+

exit from the cells), or

Na+ and Ca+2 channels are closed

Cells body/ soma integrates EPSP and IPSP

An example of excitatory and inhibitory system skeletal

muscles motor neuron Examples of inhibitory system organization:

Negative feedback (Renshaw cell), spinal motor neuron

Cerebral cortex, limbic system, cerebellum


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Spatial summation Temporal summation

Repeated stimulation of one

pre-synaptic neuron on a

post-synaptic neuronSimultaneous stimulation

of many pre-synaptic

neurons on one post-

synaptic neurons


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C G


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Characteristics Graded Potential Action Potential

Origin Dendrites/ Soma Trigger zone of an

axonChannels Ligand-gated/

mechanically gated

Voltage-gated (Na+

and K+)

Conduction Local, not

propagated

Propagated

Amplitude Stimulus intensity(1 mV 50 mV)

All-or-none (100

mV)

Duration Longer (msec

min)

Shorter (0.5 2

msec)

Polarity Hyperpolarizing/Depolarizing

Depolarizing

Polarizing

Refractory period Not present Present


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Ion Channels

1. Leakage channels K+ leakage channels > Na+ leakage

channels2. Voltage-gated channels open/ close in response to a

change in membrane potential Na+, K+, Ca+

3. Ligand-gated channels open/ close in response to a

specific chemical stimulus (neurotransmitter, hormones,

ions) directly or indirectly (second messenger system)

Na+, Ca+ inward, K+ outward

4. Mechanically gated channel

open/ close in response tomechanical stimulation (vibration, pressure, stretching)

auditory receptors, stretch receptors of internal organs,

touch receptors of skin


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References

1. Barrett KE, Barman SM, Boitano S, Brooks HL (2010).

Ganongs Review of Medical Physiology. 23rd ed.

Chapter 4, Pages: 79-89; Chapter 6, Pages: 115-123.

2. Carola R, Harley JP, Noback CR (1990). Human

Anatomy & Physiology. Chapter 11, Pages: 309-327

3. Guyton AC, Hall JE (2006). Textbook of Medical

Physiology, 11th ed. Chapter 5, Pages: 57-71; Chapter

45, Pages: 555-571

4. Tortora GJ, Derrickson BD (2009). Principles of

Anatomy and Physiology. 12th ed. Chapter 12, Pages:

417-447

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