Biomedical Engineering Reference
In-Depth Information
FIGURE A.13 (Left) A subsystem for the pulse input from the previous example. (Right) Double-clicking the
subsystem block opens a new window that displays the blocks within the subsystem.
using a subsystem for the pulse. Double-clicking the subsystem block opens a new win-
dow with the blocks in the subsystem with the output from the subsystem indicated by
the Out1 block.
Two blocks that are useful in simulations are the “Math Function” and “Fcn”. The “Math
Function” from the “Math Operations” library is convenient for entering mathematical func-
tions such as the exponential, logarithm, and power functions from a drop down list. For
example, when the input variable name is “u” and the function “exp”, the output is “exp
(u)”. The label within the block is the math function. The “Fcn” from the “User-Defined Func-
tions” library is convenient for entering general expressions. Once again, the input variable
name is “u”. Any arithmetic operator (
þ * /) and mathematical function (cos, sin, tan,
exp, power,
) can be used in the block, as well as any parameter from MATLAB. For
instance, in writing the potassium conductance for a membrane, the expression
...
g k n
4
is
g k
needed, where
is a voltage and time dependent rate constant. In SIMU-
LINK, this expression is written using the “Fnc” block with the parameter “Gkbar * u 4”
where “Gkbar” is defined in MATLAB and “u” the input to the block representing
is a constant and
n
.
In modeling systems, some parameters may change based on threshold conditions. For
example, in the saccadic eye movement system, the input to the agonist muscle,
n
F ag
,isa
low-pass filtered plus-step waveform given by the following equation.
E ag ¼ N ag F ag
t ag
where
N ag
is the neural control input (pulse-step waveform), and
t ag ¼
t ac
(
u
(
t Þ u
(
t t
ÞÞ þ
t de u
(
t t
Þ
1
1
where t ag is the time-varying time constant made up of t ac and t de , the activation and deac-
tivation time constants. To simulate this subsystem we use the “Switch” block in the “Signal
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