Abstract
A cabel model is formulated to estimate the spatial distribution of intracellular electric potential and current, from the cement line to the lumen of an osteon, as the frequency of the loading and the conductance of the gap junction are altered. The model predicts that the characteristic diffusion time for the, spread of current along the membrane of the osteocytic processes, 0.03 sec, is nearly the same as the predicted pore pressure relaxation time in Zenget al. (Annals of Biomedical Engineering. 1994) for the draining of the bone fluid into the osteonal canal. This approximate equality of characteristic times causes the cable to behave as a high-pass, low-pass filter cascade with a maximum in the spectral response for the intracellular potential at approximately 30 Hz. This behavior could be related to the experimetns of Rubin and McLeod (Osteoporosis, Academic Press, 1996) which show that live bone appears to be selectively responsive to mechanical loading in a specific frequency range (15–30 Hz) for several species.
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Abbreviations
- a :
-
radius of the osteocytic process, herea=0.075 μm
- a gi :
-
radius of the open channel of theith connexon within a gap junction
- A p :
-
intermediary parameter defined in Eq. 15; a function of ξ and θ
- A :
-
intermediary parameter defined in Eq. 17; a function of ξ and θ
- B p :
-
intermediary parameter defined in Eq. 15; a function of ξ and θ
- c :
-
diffusion coefficient (m/sec2) of the bone fluid pressure within the canaliculi-lacunae system, evaluated through a poroelastic analysis
- c m :
-
capacitance of the plasma membrane per unit surface areac m=0.01 Farad/m2
- C m :
-
membrane capacitance per unit length of the cable (Farad/cm)
- CmL,CmR:
-
membrane capacitance at the left and right end of the cable (Farad)
- Ci,i=1,4:
-
coefficients used in Eq. 14 to expressV i *; expressions for them can be found in Eqs. 21 and 22
- d :
-
diameter of the osteocytic process,d=2a=0.15 μm
- e :
-
universal constant,e=2.718
- I i :
-
intracellular current (Amp)
- I m :
-
leakage current per unit length through the membrane (Amp/cm)
- I ci :
-
nondimensionalized intracellular current,I ci =I i/(V c (1 Hz)L/R m)I ci =I *i (V c(ω)/V c(1 Hz))
- I cir :
-
nondimensionalized intracellular current at the right (osteoblastic) end,I cir =I ci (x *=1)
- I *i :
-
nondimensionalized intracellular current,I *i =I i/(V c(ω)L/R m)
- I *ir :
-
nondimensionalized intracellular current at the right (osteoblastic) end,I cir =I ci (x *=1)
- L :
-
length of the cable,L=105 μm
- L g :
-
length of a connexon,L g=20 nm
- L cp :
-
typical distance between two gap junctions in the bone cell network,L cp=35 μm
- n :
-
number of open connexons in a gap junction; can be a fractional number to represent partially closed channels
- p :
-
bone fluid pressure
- r o :
-
annular thickness of an osteon,r o=105 μm
- R i :
-
internal, longitudinal, resistance per unit length of the cable (Ω/cm)
- R m :
-
resistance of the enclosing membrane of a unit length of the cylindrical cable (Ω/cm)
- R o :
-
external, longitudinal, resistance per unit length of the cable (Ω/cm)
- RmL,RmR:
-
membrane resistances at the left and right ends of the cable (Ω)
- S,SL,SR:
-
leakage membrane areas along the whole cable, at the left, and right ends
- t :
-
time (sec)
- t e :
-
2π/ω, period of the external loading and its induced mechanical and electrical responses (sec)
- t p :
-
r 2o /c, relaxation time of the bone fluid pressure (sec)
- t * :
-
t/gt, nondimensional time
- V c :
-
Vc(ω), amplitude of the stain generated streaming potential (used as the extracellular driving potential in the paper)
- V i :
-
Vi(x, t), electrical potential inside the cable (volt)
- V m :
-
Vm(x, t), transmembrane potential of the cable,Vm=Vi−Vc
- V o :
-
Vo(x, t), electrical potential outside the cable (volt)
- V ci :
-
V ci (x*, t*)=Vi/Vc (1 Hz), nondimensional intracellular potential normalized with respect toVc (1 Hz)
- V cir :
-
V ci (1,t*), value ofV ci at the right end of the cable,x*=1
- V *i :
-
V *i (x*,t*)=Vi/Vc(ω), nondimensional intracellular potential normalized with respect toVc(ω)
- V *m :
-
V *m (x*,t*)=Vm/Vc(ω), nondimensional transmenbrane potential normalized with respect toVc(ω)
- V *o :
-
V *o (x*,t*)=Vo/Vc(ω), nondimensional extracellular potential normalized with respect toVc(ω)
- V *ir :
-
V *i (1,t*), value ofV *i at the right end of the cable,x*=1
- x :
-
axial coordinate (μm)
- x * :
-
x/L, nondimensional axial coordinate
- Δ:
-
intermediary variable defined in Eq. 16
- ζL, ζR :
-
leakage area ratios at the left and right ends, respectively, relative to the total membrane leakage area along the cable, ζL,R=S L,R/S
- ν:
-
ratio of the resistance of the gap junction to the resistance of a cell process of lengthL cp, defined in Eq. 2
- θ:
-
ωτ=τ/(1/ω), ratio of membrane time constant τ to the time parameter associated with the external loading, 1/ω
- λ:
-
λ=√R m /R i, electrical coupling length or decay length of the cellular cable (μm)
- λmax :
-
coupling length of the cable when the gap junction resistance vanishes
- ξ:
-
λ/L, ratio of the coupling length of the cable λ to the length of the cableL
- ϱ:
-
resistivity of the cytoplasm (110 Ω−cm)
- τ:
-
membrane time constant (sec), τ=R m C m
- ω:
-
angular frequency of the external loading and its induced responses (rad/sec)
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Zhang, D., Cowin, S.C. & Weinbaum, S. Electrical signal transmission and gap junction regulation in a bone cell network: A cable model for an osteon. Ann Biomed Eng 25, 357–374 (1997). https://doi.org/10.1007/BF02648049
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DOI: https://doi.org/10.1007/BF02648049