Biomedical Engineering Reference
In-Depth Information
Environmental
temperature
Fingertip
temperature
Sensory nerves
Blood perfusion
in ngertips
ermal receptors
in hypothalamus
Arteriole
smooth muscle
Central nervous
system (CNS)
Sympathetic
vasoconstrictors
FIgure 23.4
Flow chart of the thermoregulation pathway. (From Zhang HD et al.,
Computers in Biology
and Medicine
, 40, 2010. With permission.)
external stimulation, and this frequency is a function of the current. Therefore, the frequency in the
afferent fibers can be expressed as a function of temperature as follows:
f
+
f
exp[(
T
−
T
) /]
K
af
,min
af
,max
ftip t
,
Tcri
(23.1)
f
=
af
1 xp[(
+
T
−
T
) /]
K
ftip t
,
Tcri
Here,
f
af
is the frequency of spikes in the afferent fibers, and
f
faf,min
and
f
faf,max
are the minimum
and maximum values, respectively, of the frequency.
K
is the parameter that controls the slope
between temperature
T
Tftip,t
and afferent frequency
f
af
. If
K
is equal to 3,
f
af
will reach the maximum
value (15 Hz) when the fingertip temperature is 43°C. The threshold value of the tissue is 43°C (Xu
et al. 2008). Thus, it is reasonable to choose
K
as being 3.
T
Tcri
was considered to be 30°C.
After the thermal receptor receives the frequency in the hypothalamus, it delivers these signals
to the CNS. The CNS is instrumental in gathering input information from the receptors and sending
the lowest signal to the effectors. The expression for this process is as follows:
f
=
min(
f
,
T
)
(23.2)
ef in
,
ef in
,
,max
af
Here,
f
fef,in,max
is the maximum constant frequency of discharge from the sympathetic nerves.
The control signal from the CNS is subsequently transferred to the effectors. For thermoregulation
during cold-water stimulation, a cutaneous vasoconstrictor nerve may be activated as the effector
to allow the arteriole smooth muscle to constrict, thereby causing changes in blood perfusion. This
variation is expressed as follows:
σ
()
t
=
G
log[
f
(
t
−
Df
)
−
+
1]
(23.3)
A
efin
,
A
efin
,min
f
ef in
is the threshold of the sympathetic nerve,
G
A
is the gain factor, and
D
A
is the latency
time of the nerves. Therefore, blood perfusion after cold-water stimulation can be expressed as
follows:
Here,
, min
dt
dt
∆
ω
()
1
[
]
=
∆+
ωσ
()
t
()
t
τ
A
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