Transcript of the moving action potential. - Lectures For UG-5

Nerve Impulse Generation &
Conduction
31.1.13
Two types of physicochemical disturbances
1. Local, non-propagated potential (Graded potentials)
2. Propagated potentials, Action potentials or nerve
impulse
The size of a
graded potential
(here, graded
depolarizations)
is proportionate
to the intensity
of the stimulus.
The duration of a
graded potential
(here, graded
depolarizations)
is proportionate
to the duration of
the triggering event
Graded
potential spread
by passive
current flow
Electrical Signal Generation
• The graded responses produced throughout the
dendrites or cell body is summed spatially and
temporally, and if the summed response is large
enough to pass the threshold by the time it reaches
axon hillock, an action potential will be generated at
axon hillock.
• The axon hillock has the lowest threshold in the
neuron because this region has a much higher
density of voltage gated Na+ channels than
anywhere else in the neurons
• Action potential originates at axon hillock
Panel 1:
Panel 2:
Panel 3:
Panel 4:
Panel 5:
Two distinct, non-overlapping, graded depolarizations.
Two overlapping graded depolarizations demonstrate temporal
summation.
Distinct actions of stimulating neurons A and B demonstrate spatial
summation.
A and B are stimulated enough to cause a suprathreshold graded
depolarization, so an action potential results.
Neuron C causes a graded hyperpolarization; A and C effects add, cancel
each other out.
All-or-None Principle
• The all or none feature of action potential implies
that stimulus less than certain threshold level of
depolarization results in a graded response which
would not be transferred. However a stimulus big
enough to move the membrane potential beyond the
threshold will generate action potential that can
propagate to distant regions of the cells
• Threshold potential of-55mV corresponds to the potential to
which an excitable membrane must be depolarized in order to
initiate an action potential
•
•
•
Throughout depolarisation, the Na+ continues to rush inside
until the action potential reaches its peak and the sodium
gates close.
If the depolarisation is not great enough to reach threshold,
then an action potential and hence an impulse are not
produced.
This is called the All-or-None Principle.
One–way propagation of the AP
One–way propagation of the AP
One–way propagation of the AP
AP in neurons is unidirectional
• After generating an axon potential at trigger zone, it
begins to propagate to neighboring segments of the
membrane and depolarize them to threshold
triggering action potentials in the next neighboring
area and so on
• This propagation is unidirectional from axon hillock
to axon terminal because in the case of the neuron,
the proximal segment just traversed by the action
potential just enters a refractory period and thus
becomes inexcitable
Absolute Refractory Period
• The period when a recently activated patch
of a membrane is completely refractory
(meaning “stubborn” or unresponsive) to
further stimulus is known as “absolute
refractory period”
• Once the voltage gated Na+ channels are
triggered to open at threshold, they cannot
open again in response to another stimulus
no matter how strong until they pass
through their “closed and not capable of
opening” conformation and they are reset to
their “closed and capable of opening”
conformation
• Absolute refractory period lasts the entire
time from threshold to depolarization peak
and to hyperpolarization peak
Relative Refractory Period
• Following the absolute refractory period is a “relative
refractory period”, during which a second action potential can
be produced only by a triggering event considerably stronger
than usual
• The voltage gated K+ channels that opened at the peak of
action potential are slow to close. During this time, the
resultant less than normal Na+ entry in response to another
triggering event is opposed by K+ still leaving through its slow
to close channels during hyperpolarization
• Thus a greater depolarizing triggering event than normal is
needed to offset the persistent hyperpolarizing outward
movement of K+ and bring the membrane to threshold during
the relative refractory period
The propagation of the action potential from the dendritic
to the axon-terminal end is typically one-way because the
absolute refractory period follows along in the “wake”
of the moving action potential.
Nerve Impulse Conduction
• Once initiated action potential are conducted
throughout a nerve fiber
• Two types of conduction
1. Contiguous conduction
2. Saltatory conduction
Contiguous conduction
Contiguous conduction
Saltatory Conduction
• In myelinated neurons, charge carriers cannot cross
myelinated parts of the axonal membrane
• Only at nodes of Ranvier, the axonal membrane is
bare and exposed to ECF
• Voltage gated Na+ and K+ channels are concentrated
at the nodes , whereas the myelin covered regions
are devoid of these special passageways
• Therefore, action potentials can be generated only at
the nodes
Saltatory
Conduction
Saltatory Conduction
Saltatory Conduction: Action potentials jump from one node to the
next as they propagate along a myelinated axon.
Myelin acts as an insulating
sheath
allowing
an
action
potential to spread along the axon
until it gets to a node of Ranvier
which is a bare portion of axon
without mylin. As a result action
potential jumps from one node to
the next and so on. This is called
saltatory conduction
Enhancing Speed of Electrical Conduction
in Neurons
Speed of electrical conduction will be high if
• Internal diameter of axon is increased:
Increase in internal diameter of axon decreases the internal
resistance to ion flow and thus increases conduction speed
• By myelinating axons:
Myelination increases the resistance of the
plasma membrane to charge flow by acting
as an insulator. The presence of a myelin
sheath greatly increases the velocity at
which impulses are conducted along the
axon of a neuron. In unmyelinated fibres,
the entire axon membrane is exposed and
impulse conduction is slower
Importance of Myelination
• Large myelinated fibers such as those supplying skeletal
muscles can conduct action potentials at a velocity of up to
120m/s compared with a conduction velocity of 0.7m/s in
small unmyelinated fibers supplying the digestive tract
• Speed related to urgency of information being conveyed
• Invertebrates have axons with large diameters
• In humans, the optic nerve leading from the eye to the brain
is only 3mm in diameter but is packed with more than a
million myelinated axons. If these axons were unmyelinated,
each would have to be about 100 times thicker to conduct
impulses at the same velocity, resulting in an optic nerve
about 300mm in diameter
Properties of Graded vs Action Potential
• Graded Potential
• Action Potential
1.
1.
2.
3.
4.
5.
6.
7.
8.
Amplitude varies with size of
initiating event
Can be summed
Has no threshold
Has no refractory period
Is conducted decrementally i.e.
amplitude decreases with distance
Can be a depolarization or a
hyperpolarization
It begins with a stimulus (chemical,
electrical, mechanical) or a synapse
Mechanism depends on ligand gated
channels, or other physical or
chemical changes
2.
3.
4.
5.
6.
7.
8.
All or None. Once the membrane is
depolarized to threshold, amplitude
is independent of size of initiating
event
Cannot be summed
Has a threshold which is usually
15mV depolarized relative to resting
potential
Has a refractory period
Is conducted without decrement
Is a depolarization initially than a
hyperpolarization
Initiated only by a graded potential
Mechanism depends on voltage
gated channels