<s>
A	O
neural	B-General_Concept
circuit	I-General_Concept
(	O
also	O
known	O
as	O
a	O
biological	B-General_Concept
neural	I-General_Concept
network	I-General_Concept
)	O
is	O
a	O
population	O
of	O
neurons	O
interconnected	O
by	O
synapses	B-Application
to	O
carry	O
out	O
a	O
specific	O
function	O
when	O
activated	O
.	O
</s>
<s>
Multiple	O
neural	B-General_Concept
circuits	I-General_Concept
interconnect	O
with	O
one	O
another	O
to	O
form	O
large	O
scale	O
brain	O
networks	O
.	O
</s>
<s>
Neural	B-General_Concept
circuits	I-General_Concept
have	O
inspired	O
the	O
design	O
of	O
artificial	B-Architecture
neural	I-Architecture
networks	I-Architecture
,	O
though	O
there	O
are	O
significant	O
differences	O
.	O
</s>
<s>
Early	O
treatments	O
of	O
neural	B-Architecture
networks	I-Architecture
can	O
be	O
found	O
in	O
Herbert	O
Spencer	O
's	O
Principles	O
of	O
Psychology	O
,	O
3rd	O
edition	O
(	O
1872	O
)	O
,	O
Theodor	O
Meynert	O
's	O
Psychiatry	O
(	O
1884	O
)	O
,	O
William	O
James	O
 '	O
Principles	O
of	O
Psychology	O
(	O
1890	O
)	O
,	O
and	O
Sigmund	O
Freud	O
's	O
Project	O
for	O
a	O
Scientific	O
Psychology	O
(	O
composed	O
1895	O
)	O
.	O
</s>
<s>
In	O
1959	O
,	O
the	O
neuroscientists	O
,	O
Warren	O
Sturgis	O
McCulloch	O
and	O
Walter	O
Pitts	O
published	O
the	O
first	O
works	O
on	O
the	O
processing	O
of	O
neural	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
They	O
showed	O
theoretically	O
that	O
networks	O
of	O
artificial	B-Algorithm
neurons	I-Algorithm
could	O
implement	O
logical	O
,	O
arithmetic	O
,	O
and	O
symbolic	O
functions	O
.	O
</s>
<s>
Simplified	O
models	O
of	O
biological	O
neurons	O
were	O
set	O
up	O
,	O
now	O
usually	O
called	O
perceptrons	B-Algorithm
or	O
artificial	B-Algorithm
neurons	I-Algorithm
.	O
</s>
<s>
These	O
simple	O
models	O
accounted	O
for	O
neural	O
summation	O
(	O
i.e.	O
,	O
potentials	B-Algorithm
at	O
the	O
post-synaptic	O
membrane	O
will	O
summate	O
in	O
the	O
cell	O
body	O
)	O
.	O
</s>
<s>
The	O
connections	O
between	O
neurons	O
in	O
the	O
brain	O
are	O
much	O
more	O
complex	O
than	O
those	O
of	O
the	O
artificial	B-Algorithm
neurons	I-Algorithm
used	O
in	O
the	O
connectionist	O
neural	B-Architecture
computing	I-Architecture
models	O
of	O
artificial	B-Architecture
neural	I-Architecture
networks	I-Architecture
.	O
</s>
<s>
The	O
basic	O
kinds	O
of	O
connections	O
between	O
neurons	O
are	O
synapses	B-Application
:	O
both	O
chemical	O
and	O
electrical	O
synapses	B-Application
.	O
</s>
<s>
The	O
establishment	O
of	O
synapses	B-Application
enables	O
the	O
connection	O
of	O
neurons	O
into	O
millions	O
of	O
overlapping	O
,	O
and	O
interlinking	O
neural	B-General_Concept
circuits	I-General_Concept
.	O
</s>
<s>
One	O
principle	O
by	O
which	O
neurons	O
work	O
is	O
neural	O
summation	O
–	O
potentials	B-Algorithm
at	O
the	O
postsynaptic	O
membrane	O
will	O
sum	O
up	O
in	O
the	O
cell	O
body	O
.	O
</s>
<s>
If	O
the	O
depolarization	O
of	O
the	O
neuron	O
at	O
the	O
axon	B-Algorithm
hillock	O
goes	O
above	O
threshold	O
an	O
action	O
potential	O
will	O
occur	O
that	O
travels	O
down	O
the	O
axon	B-Algorithm
to	O
the	O
terminal	O
endings	O
to	O
transmit	O
a	O
signal	O
to	O
other	O
neurons	O
.	O
</s>
<s>
Excitatory	O
and	O
inhibitory	O
synaptic	O
transmission	O
is	O
realized	O
mostly	O
by	O
excitatory	O
postsynaptic	B-Algorithm
potentials	I-Algorithm
(	O
EPSPs	O
)	O
,	O
and	O
inhibitory	O
postsynaptic	B-Algorithm
potentials	I-Algorithm
(	O
IPSPs	O
)	O
.	O
</s>
<s>
On	O
the	O
electrophysiological	B-Application
level	O
,	O
there	O
are	O
various	O
phenomena	O
which	O
alter	O
the	O
response	O
characteristics	O
of	O
individual	O
synapses	B-Application
(	O
called	O
synaptic	O
plasticity	O
)	O
and	O
individual	O
neurons	O
(	O
intrinsic	O
plasticity	O
)	O
.	O
</s>
<s>
The	O
induction	O
of	O
long-term	O
changes	O
in	O
synaptic	O
efficacy	O
,	O
by	O
long-term	O
potentiation	O
(	O
LTP	O
)	O
or	O
depression	O
(	O
LTD	O
)	O
,	O
depends	O
strongly	O
on	O
the	O
relative	O
timing	O
of	O
the	O
onset	O
of	O
the	O
excitatory	O
postsynaptic	B-Algorithm
potential	I-Algorithm
and	O
the	O
postsynaptic	O
action	O
potential	O
.	O
</s>
<s>
LTP	O
is	O
induced	O
by	O
a	O
series	O
of	O
action	O
potentials	B-Algorithm
which	O
cause	O
a	O
variety	O
of	O
biochemical	O
responses	O
.	O
</s>
<s>
Backpropagating	O
action	O
potentials	B-Algorithm
cannot	O
occur	O
because	O
after	O
an	O
action	O
potential	O
travels	O
down	O
a	O
given	O
segment	O
of	O
the	O
axon	B-Algorithm
,	O
the	O
m	O
gates	O
on	O
voltage-gated	O
sodium	O
channels	O
close	O
,	O
thus	O
blocking	O
any	O
transient	O
opening	O
of	O
the	O
h	O
gate	O
from	O
causing	O
a	O
change	O
in	O
the	O
intracellular	O
sodium	O
ion	O
(	O
Na+	O
)	O
concentration	O
,	O
and	O
preventing	O
the	O
generation	O
of	O
an	O
action	O
potential	O
back	O
towards	O
the	O
cell	O
body	O
.	O
</s>
<s>
While	O
in	O
synapses	B-Application
in	O
the	O
developing	O
brain	O
synaptic	O
depression	O
has	O
been	O
particularly	O
widely	O
observed	O
it	O
has	O
been	O
speculated	O
that	O
it	O
changes	O
to	O
facilitation	O
in	O
adult	O
brains	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
neural	B-General_Concept
circuit	I-General_Concept
is	O
the	O
trisynaptic	O
circuit	O
in	O
the	O
hippocampus	O
.	O
</s>
<s>
There	O
are	O
several	O
neural	B-General_Concept
circuits	I-General_Concept
in	O
the	O
cortico-basal	O
ganglia-thalamo-cortical	O
loop	O
.	O
</s>
<s>
Neural	B-General_Concept
circuits	I-General_Concept
in	O
the	O
spinal	O
cord	O
called	O
central	O
pattern	O
generators	O
are	O
responsible	O
for	O
controlling	O
motor	O
instructions	O
involved	O
in	O
rhythmic	O
behaviours	O
.	O
</s>
<s>
There	O
are	O
four	O
principal	O
types	O
of	O
neural	B-General_Concept
circuits	I-General_Concept
that	O
are	O
responsible	O
for	O
a	O
broad	O
scope	O
of	O
neural	O
functions	O
.	O
</s>
<s>
In	O
a	O
diverging	O
circuit	O
,	O
one	O
neuron	O
synapses	B-Application
with	O
a	O
number	O
of	O
postsynaptic	O
cells	O
.	O
</s>
<s>
may	O
synapse	B-Application
with	O
many	O
more	O
making	O
it	O
possible	O
for	O
one	O
neuron	O
to	O
stimulate	O
up	O
to	O
thousands	O
of	O
cells	O
.	O
</s>
<s>
A	O
resulting	O
repetitive	O
pattern	O
is	O
the	O
outcome	O
that	O
only	O
stops	O
if	O
one	O
or	O
more	O
of	O
the	O
synapses	B-Application
fail	O
,	O
or	O
if	O
an	O
inhibitory	O
feed	O
from	O
another	O
source	O
causes	O
it	O
to	O
stop	O
.	O
</s>
<s>
Each	O
synapse	B-Application
in	O
the	O
circuit	O
acts	O
to	O
delay	O
the	O
signal	O
by	O
about	O
0.5	O
msec	O
so	O
that	O
the	O
more	O
synapses	B-Application
there	O
are	O
will	O
produce	O
a	O
longer	O
delay	O
to	O
the	O
output	O
neuron	O
.	O
</s>
<s>
This	O
circuit	O
type	O
is	O
found	O
in	O
the	O
reflex	B-Algorithm
arcs	O
of	O
certain	O
reflexes	B-Algorithm
.	O
</s>
<s>
Different	O
neuroimaging	B-Algorithm
techniques	O
have	O
been	O
developed	O
to	O
investigate	O
the	O
activity	O
of	O
neural	B-General_Concept
circuits	I-General_Concept
and	O
networks	O
.	O
</s>
<s>
The	O
use	O
of	O
"	O
brain	O
scanners	O
"	O
or	O
functional	B-Algorithm
neuroimaging	I-Algorithm
to	O
investigate	O
the	O
structure	O
or	O
function	O
of	O
the	O
brain	O
is	O
common	O
,	O
either	O
as	O
simply	O
a	O
way	O
of	O
better	O
assessing	O
brain	O
injury	O
with	O
high-resolution	O
pictures	O
,	O
or	O
by	O
examining	O
the	O
relative	O
activations	O
of	O
different	O
brain	O
areas	O
.	O
</s>
<s>
Such	O
technologies	O
may	O
include	O
functional	B-Algorithm
magnetic	I-Algorithm
resonance	I-Algorithm
imaging	I-Algorithm
(	O
fMRI	B-Algorithm
)	O
,	O
brain	B-Algorithm
positron	I-Algorithm
emission	I-Algorithm
tomography	I-Algorithm
(	O
brain	B-Algorithm
PET	I-Algorithm
)	O
,	O
and	O
computed	O
axial	O
tomography	O
(	O
CAT	O
)	O
scans	O
.	O
</s>
<s>
Functional	B-Algorithm
neuroimaging	I-Algorithm
uses	O
specific	O
brain	B-Algorithm
imaging	I-Algorithm
technologies	O
to	O
take	O
scans	O
from	O
the	O
brain	O
,	O
usually	O
when	O
a	O
person	O
is	O
doing	O
a	O
particular	O
task	O
,	O
in	O
an	O
attempt	O
to	O
understand	O
how	O
the	O
activation	O
of	O
particular	O
brain	O
areas	O
is	O
related	O
to	O
the	O
task	O
.	O
</s>
<s>
In	O
functional	B-Algorithm
neuroimaging	I-Algorithm
,	O
especially	O
fMRI	B-Algorithm
,	O
which	O
measures	O
hemodynamic	O
activity	O
(	O
using	O
BOLD-contrast	B-Algorithm
imaging	I-Algorithm
)	O
which	O
is	O
closely	O
linked	O
to	O
neural	O
activity	O
,	O
PET	O
,	O
and	O
electroencephalography	B-Application
(	O
EEG	B-Application
)	O
is	O
used	O
.	O
</s>
<s>
artificial	B-Architecture
neural	I-Architecture
networks	I-Architecture
,	O
where	O
parts	O
of	O
the	O
nodes	O
are	O
deliberately	O
destroyed	O
to	O
see	O
how	O
the	O
network	O
performs	O
,	O
can	O
also	O
yield	O
important	O
insights	O
in	O
the	O
working	O
of	O
several	O
cell	O
assemblies	O
.	O
</s>
<s>
Modern	O
development	O
of	O
concentration	O
of	O
measure	O
theory	O
(	O
stochastic	O
separation	O
theorems	O
)	O
with	O
applications	O
to	O
artificial	B-Architecture
neural	I-Architecture
networks	I-Architecture
give	O
mathematical	O
background	O
to	O
unexpected	O
effectiveness	O
of	O
small	O
neural	O
ensembles	O
in	O
high-dimensional	O
brain	O
.	O
</s>
<s>
Sometimes	O
neural	B-Architecture
circuitries	I-Architecture
can	O
become	O
pathological	O
and	O
cause	O
problems	O
such	O
as	O
in	O
Parkinson	O
's	O
disease	O
when	O
the	O
basal	O
ganglia	O
are	O
involved	O
.	O
</s>
