Graded potentials and action potentials, Study notes of Medicine

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GRADED POTENTIALS AND

ACTION POTENTIALS

Near East University

Faculty of Medicine

Department of Biophysics

Dr. Aslı AYKAÇ

Nervous System

Information travels in one direction Dendrite → soma → axon

• Glia
  • Not specialized for information transfer
  • Support neurons
• Neurons (Nerve Cells)
  • Receive, process, and transmit information

Nervous system cells are comprised of glia and neurons.

Neurons are responsible for receive, process, and transmit

information in nervous system.

• Transient changes in the membrane potential of

its resting level produce electrical signals.

• Such changes are the most important way that

nerve cells process and transmit information.

These signals occur in two forms:

1. graded potentials
2. action potentials

Graded potentials are important in short distances.

Action potentials are the long distance signals of

nerve and muscle membranes.

Membrane potential (mV

)

Time

The terms

 depolarize  repolarize  hyperpolarize

are used to describe

the direction of changes in the membrane potential relative

to the resting potential.

Changes in Membrane Potential

The resting membrane potential (at - 70 mV) is polarized.

“Polarized” means that the outside and inside of a cell have

a different net charge.

• The membrane is said to be depolarized when its

potential is less negative than the resting level.

• The membrane is repolarized when the potential returns

toward the resting value.

• The membrane is hyperpolarized when the potential is

more negative than the resting level.

• 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

Graded Potentials

Graded Potentials

• Can only travel short distances
• Voltage changes in graded potentials are

gradual

• Current quickly spreads and disappears due to

the leaky plasma membrane

Terms Describing the Membrane Potential

Potential = potential

difference

The voltage difference between two points.

Membrane potential

=transmembrane potential

The voltage difference between the inside and outside of a cell.

Equilibrium potential The voltage difference across a

membrane that produces a flux of a given ion species that is equal but opposite to the flux due to the concentration gradient of that same ion species.

• A small region of a membrane has been

depolarized by a stimulus,

• Opens membrane channels
• produces a potential less negative than adjacent areas.
• inside the cell, positive charge will flow through the intracellular fluid away from the depolarized region and toward the more negative, resting regions of the membrane.
• outside the cell, positive charge will flow from the more positive region of the resting membrane toward the less positive regions just created by the depolarization.

• Thus, it produces a decrease in the amount of

charge separation (i.e., depolarization) in the

membrane sites adjacent to the originally

depolarized region, and the signal is moved along

the membrane.

• Depending upon the initiating event, graded

potentials can occur in either a depolarizing or

a hyperpolarizing direction.

Such experiments show that graded potentials (a) can be depolarizing or hyperpolarizing, (b) can vary in size.

• The resting membrane potential is - 70 mV.

Membrane potential (mV)

• Charge is lost across the membrane because the

membrane is permeable to ions through open membrane channels.

• As a result, the membrane potential changes

decreases by the distance from the initial site.