Transcript Threshold Levels and the All-or-None Response

Threshold Levels and
the All-or-None
Response
Threshold level is the minimum level of a stimulus required to produce a
response.
Conclusion:
A nerve or muscle fibre responds completely or not at all to a stimulus.
This is called all-or-none response.
Can you differentiate a warm object from a hot
one?
How do we detect the intensity of stimuli if nerve
fibres either fire completely or not at all?
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We need to look at the manner in which brain
interprets nerve impulses b/c the brain
interprets both the number of neurons
excited and the frequency of impulses
A variation with respect to frequency does
occur even though stimuli above threshold
level produce nerve impulses of identical
intensity and speed.
The more intense the stumulus, the greater
the frequency of impulses.
Ex. Let’s consider that we touch a
warm glass and a hot glass.
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The warm glass may cause only a neuron to
reach threshold level, but the hot glass may
cause more than two neurons to reach the
threshold level has a higher threshold level
Synapses
Synapses: The small spaces between neurons, or between
neurons and effectors
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The nerve transmission
slows across synapses
even through the space
between the neurons is
very small (20 nm)
The greater the number
of synapses, the slower
the speed of the
transmission over a
specified distance.
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When an action potential
depolarizes the membrane
of the synaptic terminal it:
Triggers an influx of Ca2+
that:
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Causes synaptic vesicles to
fuse with presynaptic
membrane
Releasing neurotransmitter
molecules into synaptic cleft
which then bound to
receptors on postsynaptic
membrane
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The receptors control
selective ion channels
Binding of neurotransmitter
to its specific receptors
opens the ion channels
Ion movements alter
membrane potential of
postsynaptic membrane.
Excitation or inhibition may
result depending on type of
receptor and the ion
channels they control
Synapse between a Neuron
and a Muscle Fibre
Acetylcholine:
 Acts as:
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An excitatory
neurotransmitter on many
postsynaptic neurons by
opening the sodium
channels
As an inhibitory
neurotransmitter (many
inhibitory
neurotransmitters make
the postsynaptic
membrane more
permeable to K+
hyperpolarization)
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Cholinesterase enzyme
from the postsynaptic
membrane destroys
acetylcholine  Na+
channels are closed 
repolarization.
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Insecticides block the
action of
cholinesterase.
Look at these websites!!
Synapse Animation:
 http://www.lifesci.ucsb.edu/~mcdougal/neurobehavio
r/modules_homework/lect3.dcr
Synapse Tutorial:
 http://www.getbodysmart.com/ap/nervoussystem/ne
urophysiology/synapses/menu/menu.html
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