An Extended Computational Framework to Study Arterial Vasomotion and Its Links to Vascular Disease - Biomedical Technology - page 127

that translates and amplifies the electrochemical signalling between the lumen-side
and the SM, where the arterial wall contractile apparatus is located.
1.2.1 Vasomotion
The vascular SM is responsible for spontaneous fluctuations in vessel calibre that are
not attributable to heart rate and blood pressure, and are referred to as 'vasomotion'.
Vasomotion was first described more than a century ago, but its physiological signif-
icance remains the subject of ongoing debate. The nonlinear nature of the contractile
dynamics associated with vasomotion is thought to confer specific hæmodynamic
advantage over regular or steady-state flow [ 5 ]. At the simplest level, it can be inter-
preted as a mechanism that continuously redistributes flow to tissue, in order to
maintain adequate perfusion. More subtle effects, such as the enhancement of lym-
phatic drainage and microcirculatory mass transport, may reflect the critical role of
smaller arteries and arterioles in regulating interstitial tissue pressure, delivery of
oxygen and nutrients and washout of metabolites [ 6 ]. Vasomotion becomes particu-
larly prominent in pathological states such as hypoxia and hæmorrhagic shock and
may then represent an adaptive dynamic response that maintains or re-establishes
flow [ 7 ]. Oscillations in the diameter of larger arteries have also been observed in
vitro, such as human coronary [ 8 ] and pial arteries [ 9 ], rabbit small ear [ 10 ] and
mesenteric arteries [ 11 ] or carotid arteries from rats and dogs [ 12 , 13 ], suggesting
a similar origin as that seen in the microcirculation. The interested reader is also
referred to the excellent review by Nilsson and Aalkjaer [ 14 ].
Vasomotion represents an emergent behaviour associated with a complex dynam-
ical system, whereby coordination of calcium ion Ca 2 + oscillations in individual
SMCs give rise to 'higher-level' phenomena. In isolated vessels, vasomotion exhibits
specific patterns of behaviour that are generic to nonlinear physico-chemical sys-
tems [ 15 - 18 ]. An important characteristic of nonlinear systems is the co-existence
of multiple operating modes with the potential for transitions between them, con-
trolled by single system parameters. State responses of such systems often exhibit
completely unpredictable behaviour in response tominute perturbations, allowing the
selection of patterns that may confer biological advantage with minimal expenditure
of energy.
1.2.2 Characterisation of the Vascular Smooth Muscle
From a dynamical point of view, SMCs are regulators of Ca 2 + , which is the cat-
alyst of the cells' contractile machinery. Intracellular Ca 2 + regulation is predomi-
nantly controlled by endothelium-mediated polarization and the operating point of
ionic transport mechanisms across the cell membrane. For vasomotion to occur,
an oscillator must be present; and in order to get macroscopic oscillations of a
blood vessel, SMCs' individual oscillations must be synchronized. Although it is
well-established that synchronization of Ca 2 + oscillations in SMCs depends on gap