Transcript Conduction of a Nerve Impulse

Conduction of a Nerve
Impulse
Function of a Neuron
Primary function is the generation of a
nerve impulse
 Similar in many ways to the flow of
electricity through an insulated wire

Resting Membrane Potential
A potential difference occurs when there is
a separation of charge between two points
(ex. Battery – potential difference
between + and – poles; measured in
volts)
 Occurs in cells due to an unequal
distribution of ions across membrane
 The primary ions that we are concerned
with are potassium ions and sodium ions

At rest, there are approximately 10x more
Na+ outside than inside
 At rest, there are approximately 10x more
K+ inside than outside
 The resting membrane potential is
maintained by the action of a particular
ion channel referred to as the sodiumpotassium pump which forces Na out
while forcing K in to establish a
concentration gradient
 Neurilemma Animation

If conditions were ideal, there would be
no difference in the concentration of ions
inside and outside the cell, therefore no
potential difference (no difference in net
charge)
 Two condition make diffusion conditions
less than ideal

– Na-K pump transports uneven ratio of ions:
 3 Na out for each 2 K in
– Plasma membranes are more permeable to K
ions than Na ions:
 there is a tendency for K ions to leak out faster
than Na ions leak in
The result of these two conditions is that
there are more positively charged ions on
the outside of the plasma membrane than
the inside; the potential difference can be
measured at – 70 millivolts
 This condition of the cell is referred to as
being “polarized”
 Neurilemma Animation

Action Potential
•Neurons can respond to changes in the
environment, a property called excitability
•When the change becomes great enough,
an action potential in the cell can be
generated by increasing the membrane’s
permeability to sodium
This is done by opening proteins referred
to as sodium ion channels
 Resultantly, sodium ions rush into the cell
 This results in the cell becoming
depolarized (a reversal of the distribution
of charges by allowing the concentration
of Na ions to return to equilibrium)
 Neurilemma Animation

Repolarization
The cell can quickly re-establish potential
difference between inside and outside by
opening potassium ion channels, while
closing sodium ion channels
 Positively charged K ions flow out of the
cell, restoring the distribution of charges,
but not the concentration of the particular
ions
 Neurilemma Animation

The depolarization / repolarization of
membranes can occur very quickly (in less
than 1 / 1000th of a second)
 During this process, the number of ions
moving is relatively small; the potential
difference of charges in more important
than the concentration of ions
 Several action potentials can be generated
before concentrations of ions need to be
restored to original levels
 Neurilemma Animation

Nerve Impulse Conduction
Neurilemma Animation
 Action Potential Movement Animation

Continuous Conduction

Continuous Conduction – nerve impulse
travels through entire neurilemma
(membrane) in one direction
– Occurs only in unmyelinated neurons
– Moves at about 10 m/s
Saltatory Conduction
Nerve impulse travels down portions of
the neurilemma that are not covered by
Schwann cells in one direction
 Skips large spaces due to presence of
insulating myelin, from node of Ranvier to
node of Ranvier

– Much faster, up to 130 m/s
– Adaptation for split-second responses to
emergency situations
All-or-None Response
Like muscle cells, if a stimulus is strong
enough to cause an action potential, the
nerve impulse travels the entire length of
a neuron at full strength
 Minimum strength of stimulus required to
initiate an action potential is called the
threshold stimulus
 Stimuli weaker than threshold, called subthreshold, cannot initiate an impulse

Transmission of Impulses from
Cell to Cell
Occurs at the junction between adjacent
neurons, called a synapse
 Impulse must pass this gap to move to
next neuron
 Presynaptic neuron sends impulse;
postsynaptic neuron receives impulse
 Axon of pre-synaptic neuron ends at a
round bulb, called the synaptic end bulb
and contains synaptic vesicles filled with
neurotransmitters

About 50 different types of
neurotransmitters
 Postsynaptic neuron has a concave
surface that forms a gap, called the
synaptic cleft, across which
neurotransmitters must diffuse in order to
contact the postsynaptic neuron
 Transmission of Impulse Across a Synapse

When a nerve impulse arrives at the
synaptic end bulb of a presynaptic neuron,
calcium ion channels in the plasma
membrane open briefly, and calcium ions
flow into the end bulb from the
surrounding interstitial fluid
 Ca ions presence cause synaptic vesicles
to fuse to the plasma membrane and the
neurotransmitters are released into the
synapse by exocytosis

These chemicals then diffuse across the
gap until they contact the plasma
membrane of the postsynaptic neuron
 When contact is made the result is either
excitatory or inhibitory, depending on the
nature of the neurotransmitter and its
receptor
 The NT is then deactivated by an enzyme
or transported away by other enzymes
 This limits the effect of neurotransmitters
to fractions of a second and readies the
synapse for the next transmission


Transmission of Impulse Across a Synapse
Excitatory vs. Inhibitory
Transmissions
Both depend upon altering the
permeability of the postsynaptic neuron to
certain ions
 Excitatory neurotransmitters, such as
acetylcholine (Ach) or norepinephrine
increase the permeability to Na ions,
increasing the likelihood of generating an
action potential

Inhibitory neurotransmitters, such as
GABA, endorphins, and enkephalins,
increase the permeability to potassium
ions, decreasing the likelihood of the
generation of an action potential
 Some inhibitory ions also open chloride ion
channels which allow negative charges to
flow into the cell, cause hyperpolarization

Processing at the Synapse
The postsynaptic membrane is located at
the dendrites and cell body of a single
neuron
 May receive thousands of presynaptic end
bulbs from thousands of different neurons
 Some may send excitatory impulses where
others may send inhibitory impulses
 The overall effect is determined by the
sum of these individual effects


This provides a means of gathering
information from a bunch of different
receptors (stimuli) and coming up with a
response according to the individual
neuron’s threshold of stimulus