Physiology sheet # 36
Date of lec.: 20-4
Neural Processing
This lec. We will talk about the neuronal circuits for
processing information .
-these neural circuits sometimes used to:
 Amplify the impulse
 Prolong the impulse
 Sharp the impulse ( tow point discrimination )
Neuronal Pools :
-the degree of arborization is great in the center due
to the presence of many axon terminals.
-The central area because of many convergence fibers
is going to get stimulated otherwise the lateral area is
get facilitated that means there MP becoming closer to
threshold.
Type of circuit:
1. Divergence of signal:
 Started with one neuron
Ended with multiple neurons
Sometimes called "cascade" as in biochemistry
 often it is important for amplifying weak signals
entering a neuronal pool to excit far greater
numbers of nerve fibers leaving the pool.
2. Convergence the impulse:
 Signals from multiple inputs uniting to excite a
single neurons
 The importance of this is that neurons are almost
never excited by an action potential from a single
input terminal but action potentials converging on
the neurons from multiple terminals provide
enough spatial summation to bring the neuron to
the threshold required for discharge.
 Convergence can also result from multiple sources,
in this case we might lose localization.
 Such convergence allows summation information
from different sources, and the resulting
response is a summated affect of all the different
types of information.
 REMEMBER :
AP is ALL or NON so it will stimulating ALL neurons
with the same amplitude without any spreading.
3. Sharping the impulse:
 The lateral area make the center more sharp (
contrast )
 The lateral inhibition is user for two point
discrimination
Prolong the stimulus:
By lasting a few millisecond to as long as many minutes
after the incoming signal is over
The most important mechanism :
1) Reverberatory circuit :are one kind of circuits
that is used to prolong the timing of the impulse
 caused by positive feedback within the neuronal
circuit that feeds back to re-excite the input of
the same circuit
 Feedback stimuli could keep the neuron
discharging for a protracted time thereafter
 Fatigue beyond a certain critical level lowers the
stimulation of the next neuron in the circuit
below threshold level so that the circuit
feedback is suddenly broken.
2) parallel circuit :
prolong the circuit by a delay that happened before
reach the post synaptic neuron
it takes 0.5 msec to reach it or so. And yet the output
many millisecond or even minuites
3) synaptic after discaharge:
-EPSP (15-20 msec)it will give impulses although it is
one stimulus
-AP ( 0.1-10 msec )
Then more number of AP per one EPSP.
Ex. :Light is one stimulus ,
if you close your eyes, the excitatory post synaptic
potential continue and the AP also goes as if there
stimulus
so the brain receives AP and takes it as stimulus.
Stability of Neuronal Circuit
1.synaptic fatigue means simply that synaptic
transmission becomes progressively weaker the more
prolonged and more intense the period of excitation.
2.Gross inhibitory circuit:
Many of basal ganglia exert inhibitory influences
When the impulse descend from the cortex through
descending fibers,the receptor potential of
corticofugal fibers decreased.so the area become more
sensitive at the lower range of stimulus
High stimulus less sensitive
this called feed back inhibition
importance of corticofugal fibers when it descend to
back to the sensory system. It lower the receptor
potential back to sensitivity area.
3.down regulation:
If there stimulus is repetitive after awhile ,the
stimulus is no more stimulating the circuit because the
receptor of neurotransmitter is down regulated due to
internalization( enter inside the cell )  the cell will
become less sensitive because of decreasing the
number of receptor
Note:the internalization decreses the number of
receptor through phosphorylation
4.up regulation: increasing the number of receptor at
the synapse site.
SO any simple amount of neurotransmitter will exit the
circuit ( the circuit will be more sensitive ).
Done by:
Aya Mahfouz