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Transcript
ELECTRICAL PROPERTIES OF NEURON
LEARNING OBJECTIVES
 At the end of the lecture, student should be able to:
 know the basic structure and function of neuron
 understand its basic physical and electrical properties of neuron
 to know the factors on which the resting membrane potential
depends
 to appreciate the mechanics of action potential initiation and
measurements of action potentials.
 Know the basics of patch-clamp technique
NERVOUS TISSUE
 Nervous Tissue comprises of
o Neurons,
o Neuroglia,
o and blood vessels
 Neuron is the basic structural and functional unit of nervous
tissue
 Neuroglia is the supporting or glial tissue that is 10-times more
abundant in mammalian brain than neurons
NEURON
 Basic structural and functional
unit of the nervous system
 Specialized cells conduct
electrical impulses along the
plasma membrane
 Composed of:
Body:
 Soma/Perikaryon
 Body Contains;




Nucleus:
Cytoplasmic organelles
Inclusions
Cytoskeletal components
Processes:
 Axon and
 Dendrites
MORPHOLOGY OF NERVE CELLS (THE NEURON)
A NEURON
Nucleus
Axon
Dendrites
Myelin
• Dendrites -- Input
• Cell body (soma) -- Integration
• Axon -- Output
STRUCTURE OF NEURONS DENDRITES
At dendrites, the
neurons recieve input
via axons of other
neurons at synapses
dendritic
spine
STRUCTURE OF NEURONS AXON
The axon transmits the information electrically from the soma to
the synapses –
it is surrounded by myelin that insulate the axon, provided by
oligodendrocytes (glial cells)
NEURON: MORPHOLOGICAL
CLASSIFICATION
Unipolar
•
• Bipolar
• Multipolar
•Single axon & multiple
dendrites
•Most common type in men, e.g
•Motor cortex, interneurons, …
• Pseudounipolar
•Single process arises from
body
•Branches into an axon and
dendrite, e.g
•Present in spinal and cranial
ganglia
ELECTRICAL PROPERTIES OF NEURON
• ALL CELLS (NOT JUST EXCITABLE CELLS) HAVE A
RESTING POTENTIAL (MEMBRANE POTENTIAL) : AN
ELECTRICAL CHARGE ACROSS THE PLASMA
MEMBRANE
 THE INTERIOR OF THE CELL NEGATIVE
 TYPICALLY -70MV ( VARIES -40 TO -90 MV DEPENDS
UPON THE TYPE OF NEURON)
RESTING MEMBRANE POTENTIAL RMP
 All cells (not just excitable cells) have a resting potential
(membrane potential) : an electrical charge across the plasma
membrane
 The interior of the cell negative
 Typically -70mV ( varies -40 to -90 mV depends upon the type of
neuron)
ELECTRICAL PROPERTIES OF NEURON RESTING
MEMBRANE POTENTIAL RMP
 Neurons are enclosed by a membrane separating interior from
extra cellular space
 The concentration of ions inside is different (more –ve) to that in
the surrounding liquid
 -ve ions therefore build up on the inside surface of the
membrane and an equal amount of +ve ions build up on the
outside
 The difference in concentration generates an electrical potential
(membrane potential) which plays an important role in neuronal
dynamics.
 Cell membrane: 2-3 nm thick and is impermeable to most
charged molecules and so acts as a capacitor by separating the
charges lying on either side of the membrane.
 NB Capacitors, store charge across an insulating medium. Don’t
allow current to flow across, but charge can be redistributed on
each side leading to current flow.
 The ion channels are proteins in the membrane, which lower the
effective membrane resistance by a factor of 10,000 (depending
on density, type etc)
RESTING MEMBRANE POTENTIAL RMP
 In order for a potential difference to be present across a membrane
lipid bilayer, two conditions must be met.
 1.unequal distribution of ions of one or more species across the
membrane (ie, a concentration gradient).
 2.Two, the membrane must be permeable to one or more of these ion
species. The permeability is provided by the existence of channels or
pores in the bilayer; these channels are usually permeable to a single
species of ions.
 represents an equilibrium situation at which the driving force for the
membrane-permeant ions down their concentration gradients across the
membrane is equal and opposite to the driving force for these ions
down their electrical gradients.
EXCITATION AND CONDUCTION
 Most biological neurons communicate by short electrical pulses
called action potentials or spikes or nerve impulses
 These action potentials are generated by means of influx and
out flux of ions through the ion channels embedded in
membrane
 Suitable electrical probe (electrode) and measurement
instrumentation (amplifier and read-out) can measure these tiny
potentials on the order of few milli-volts
EXCITATION AND CONDUCTION
 Nerve cells have a low threshold for excitation – may easily be
excited by electrical, chemical or mechanical stimuli
 Two types of physiochemical disturbances are produced as a
result
 Local or Non-Propagated Potentials
such as Synaptic, Generator or Electrotonic
Potentials
 Propagated Disturbances such as Action Potentials or Nerve
Impulses
ELECTRICAL PROPERTIES OF NEURONS
extracellular
intracellular
The cell membrane isolates the intracellular from extracellular space
ELECTRICAL PROPERTIES OF
NEURONSThe membrane potential
extracellular
difference of -70 mV
intracellular
In the resting state, the intracellular space contains more negative ions
than the extracellular space
ION CHANNELS CONNECT THE INTRA- AND
EXTRACELLULAR SPACE
Opening of ion channels lead to a flux of ions through the
membrane and to a change of the membrane potential
THE ACTION POTENTIAL
The action potential is generated by ion flux through voltage gated channels
All or none
principle!!
PROPAGATION OF THE ACTION
POTENTIAL
SYNAPSE – COMMUNICATION BETWEEN
NEURONS
SYNAPSE – COMMUNICATION BETWEEN
NEURONS
Presynaptic vesicles
with
neurotransmitter
Released
transmitter
TransmitterResorption
from synaptic
cleft
Transmitter
binds to
receptor
Na+
SYNAPSE – COMMUNICATION BETWEEN
NEURONS
THE MORPHOLOGY OF AN ACTION POTENTIAL
 Action potential is a rapid, reversible, and
conductive change of the membrane
potential after the cell is stimulated.
 Nerve signals are transmitted by action
potentials.
 Reduction in membrane potential
(depolarization) to "threshold" level leads
to opening of Na+ channels, allowing Na+
to enter the cell
 Interior becomes positive
 The Na+ channels then close automatically
followed by a period of inactivation.
 K+ channels open, K+ leaves the cell and
the interior again becomes negative.
 Process lasts about 1/1000th of a second.
PROPERTIES OF THE ACTION
POTENTIAL
 “All or none” phenomenon
 A threshold or suprathreshold stimulus applied to a single nerve fiber
always initiate the same action potential with constant amplitude,
time course and propagation velocity.

Propagation
 Transmitted in both direction in a nerve fiber
THE PATCH-CLAMP TECHNIQUE
 This is a novel technique in which physiological currents flowing
through the cells can be detected without disrupting the cell or
its contents
 A micropipette (diameter in microns) filled with a buffer solution
and carrying a metal electrode is gently touched to the cell
membrane and the membrane contents are sucked in. This is
called a patch-clamp.
THE PATCH CLAMP TECHNIQUE
THANK YOU.