Discovered over 100 years ago by Bayliss, the myogenic response is the intrinsic property of smooth muscle to respond to changes in mechanical load or intravascular pressure . It is a critical component of resistance artery function and is more prominent in the cerebral circulation than many other vascular beds. The smooth muscle of both large arteries and small arterioles constrict in response to increased pressure and dilate in response to decreased pressure (known as the “Bayliss effect”), thus contributing to autoregulation of blood flow [68,69]. This innate myogenic activity is also crucial for normal hemodynamic function and for maintaining vascular resistance, which serves to protect smaller downstream arterioles and capillaries from damage in the face of changing perfusion pressures and to maintain tissue perfusion during periods of decreased blood pressure [68,69]. The myogenic response arises from smooth muscle and exists in arteries and arterioles denuded of endothelium and in sympathetically denervated animals . Thus, the myogenic response is truly myogenic in nature. However, release of vasoactive factors from both endothelium and perivascular nerves and local metabolites can increase or decrease the level of myogenic tone and thus affect vascular resistance.
The myogenic behavior of resistance arteries and arterioles involves two phenomenon: myogenic tone, which is a state of partial constriction at a constant pressure, and myogenic reactivity, which is the alteration in tone in response to a change in pressure  (Figure 11). A third phase also occurs at excessively high arterial pressure beyond the autoregulatory pressure range and involves marked increases in vessel diameter and loss of tone that occurs during autoregulatory breakthrough, known as forced dilatation . Although the term “forced dilatation” implies this phenomenon is a mechanical event, it likely is an active vasodilation possibly involving KCa channels and/or reactivity oxygen species, invoked to protect the arterial wall from damage . Mechanisms underlying both the development of myogenic tone and the more physiological myogenic reactivity are not completely understood and appear to vary between species and vascular beds. However, for the sake of simplicity, it is worth considering the phases of the response as having different triggers and mechanisms of regulation, although it is likely that they are highly interactive and interdependent.
Mechanisms of Myogenic Response Initiation
The initiation or development of the myogenic response occurs via ionic and enzymatic mechanisms, both of which increase intracellular calcium . Increased pressure causes depolarization of the smooth muscle cell membrane and calcium influx via opening of voltage-operated calcium channels (Cav) [75,76]. In particular Cav1.2 are the prominent calcium channel involved, but in some segments of the vasculature such as small parenchymal arterioles, other Cav channels may participate . The rise in intracellular calcium increases myosin light-chain (MLC) phosphorylation and promotes vasoconstriction. Removal of extracellular calcium abolishes the myogenic response,
suggesting a prominent role for calcium influx in myogenic response initiation [74,75]. Although a role for Cav channels in the initiation of the myogenic response is well-established, the primary stimulus or sensor that transduces the change in pressure (a mechanical event) into depolarization and vasoconstriction are not clear. Wall tension has been proposed as a stimulus for initiation of the myogenic response because it has been shown to correlate with changes in cell calcium and MLC phosphorylation, a relationship not seen with diameter [77,78]. Stretch-activated cation channels, including transient receptor potential (TRP) channels, are thought to be the sensor by which pressure causes depolarization [79,80], although Cav channels may also be activated directly by transmural pressure . In addition, chloride channels may also participate in pressure-induced smooth muscle depolarization because their activation increases an inward current, causing depolarization . In addition to ionic mechanisms, there is evidence for other factors involved in mechanotransduction during the development of myogenic tone including integrins and actin cytoskeletal dynamics [83,84]. Both integrins and TRP channels are linked to the actin cytoskeleton [85,86], providing a connection by which all these process can interact to transduce pressure or stretch into a depolarization and contractile response.
Mechanisms of Myogenic Reactivity
The vasoconstriction associated with the development of tone is different in many aspects than the response of the vessel to pressure once tone is present. During this phase, an increase in intravascular pressure does not change vessel diameter significantly and can cause further constriction [71,84]. The vessel wall stiffens due to enhanced MLC phosphorylation and contraction that is further reinforced by actin polymerization [77,84,87]. It is unlikely that tension or stretch is a stimulus for constriction during this phase because wall tension is actually increased due to elevated pressure, not decreased as the model predicts with tone development. Another fundamental difference from tone initiation is that there is little change in smooth muscle calcium, but calcium sensitivity is enhanced during myogenic reactivity [71,73,88]. When calcium is clamped by exposure to high extracellular calcium or in permeabilized artery preparations, myogenic reactivity is still present [71,89]. Several mechanisms exists by which pressure induces calcium sensitivity in smooth muscle, including activation of protein kinase C (PKC) and RhoA-Rho kinase pathways [71,73,89,90] (Figure 12).
At least one major negative feedback mechanism exists that limits the myogenic vasoconstriction. Calcium-activated potassium channels (Kca), in particular large-conductance KCa or BKCa channels, expressed on cerebral artery smooth muscle, are activated by intracellular calcium release events or “calcium sparks” whose frequency is regulated by transmural pressure [91–93]. Activation of BKCa channels by calcium sparks causes hyperpolarization and attenuation of the myogenic vasoconstriction [91,92]. This negative feedback mechanism serves to put the brakes on the constriction induced by pressure-induced depolarization and increased intracellular calcium [73,93].
Effect of Disease States on Myogenic Tone and Reactivity
The myogenic response has a prominent role in normal hemodynamic processes in the brain. The basal constriction due to myogenic mechanisms provides a state from which an artery or arteriole can increase or decrease diameter on demand, thereby modulating cerebrovascular resistance and contributing to local and global blood flow regulation [68,69,71,72]. Conducted or flow-mediated vasodilation of upstream vessels associated with functional hyperemia may involve myogenic vasodilation in response to decreased intravascular pressure . The importance of the myogenic response in the brain is demonstrated by numerous disease states in which myogenic mechanisms are dysregulated, causing secondary brain injury such as ischemia and vasogenic edema [95,96]. For example, during focal ischemia when a thrombus or emboli occludes a cerebral vessel, there is both a decrease in flow and pressure that both contribute to autoregulation. Decreased flow causes hypoxia that when severe can promote vasodilation by metabolic mechanisms , whereas decreased pressure causes myogenic vasodilation. The decreased tone, due to both metabolic and myogenic involvement, diminishes cerebrovascular resistance, which can cause vasogenic edema formation due to significantly elevated hydrostatic pressure on the microcirculation  (see Vasogenic Edema Formation).
Endothelial Regulation of Tone
The endothelium is a highly specialized cell type in the brain [12,23]. Similar to peripheral organs, it is involved in numerous physiological processes, including regulation of inflammatory and immune responses, thrombosis, adhesion, angiogenesis, and permeability  (see Barriers of the CNS). The importance of the endothelium is demonstrated by the fact that endothelial dysfunction has a central role in the pathogenesis of several cerebrovascular diseases such as Alzheimer’s disease, epilepsy, and stroke [12,23,98–100]. The endothelium can produce several vasoactive mediators, including nitric oxide (NO), prostacyclin, and endothelium-derived hyperpolarizing factor (EDHF) that have a significant influence on vascular tone and thereby influence cerebral blood flow  (Figure 13).
NO has some of the most prominent effects in the brain and is one of the most studied vasodilators. Under resting conditions, basal NO production by the cerebral endothelium inhibits resting tone in both large and small pial arteries and parenchymal arterioles and thus affects CBF . NO synthase (NOS) is the enzyme responsible for the oxygen-dependent conversion of L-arginine to NO and L-citrulline . Three isoforms of NOS exist in brain with neuronal (nNOS) and endothelial (eNOS) constitutively expressed in neurons and cerebral endothelium, respectively . Inducible (iNOS) is not normally expressed in brain, but its expression can be induced under certain pathological conditions . The significant influence of NO on basal tone is demonstrated by effects of compounds that inhibit NOS, such as L-nitro-arginine (L-NNA), that cause significant endothelium-dependent vasoconstriction of cerebral arteries and arterioles [101,102,105].
NO production by the cerebral endothelium can be induced by stimuli that increase endothelial cell intracellular calcium (e.g., acetylcholine, ACH, shear stress) since NOS is calcium-dependent . Production of NO by eNOS is principally activated by calcium-dependent binding of calmodulin, making eNOS activation calcium-dependent. Thus, many factors that increase intracellular calcium activate eNOS. Whether NO is produced basally or is induced, NO diffuses to VSM where it causes vasodilation primarily by activation of soluble guanylyl cyclase (sGC) . Activation of sGC increases the levels of cyclic guanine monophosphate (cGMP) that in turn activates protein kinase G, causing relaxation of VSM in part by opening BKCa channels and reducing intracellular calcium .
A critical determinant of eNOS activity is the availability of the cofactor tetrahydrobiopterin (BH4) (108). Under conditions of limited BH4 availability (due to oxidation or reduced formation), eNOS can function in an uncoupled state so that NAD(P)H-derived electrons are added to molecular oxygen instead of L-arginine, leading to the production of superoxide . Uncoupling of eNOS has been implicated in a number of diseases associated with decreased BH4 levels, including atherosclerosis, diabetes, and hypertension [110–113]. eNOS activity is also regulated by phosphorylation of the enzyme on specific amino acid residues [114,115]. Following Ser1177 phosphorylation, NO production is increased. In contrast, eNOS activity is inhibited by Thr495 phosphorylation .
Endothelium-Derived Hyperpolarizing Factor
Experimentally, EDHF is defined as the residual vasodilator mechanism that remains in the presence of inhibition of NOS and cyclooxygenases (COX). EDHF-mediated vasodilation potentially occurs under basal conditions, but also in response to agonist-induced increase in endothelial cell calcium and activation of small- and intermediate-conductance calcium-activated potassium channels or SKCa and IKCa channels (also known as KCa2.3 and KCa3.1, respectively) that hyperpolarizes endothelial cells [116,117]. Endothelial cell hyperpolarization is then transferred to adjacent smooth muscle causing closure of Cav channels and relaxation (Figure 14). How hyperpolarization is transferred from endothelium to smooth muscle is not clear, but may involve gap junctions, which communicate through direct electrical coupling or by passage of chemical mediators [118,119]. Proposed chemical mediators include K+ ions, cytochrome P450 metabolites, or epoxyeicosatrienoic acid (EETs) [120–122].
The hyperpolarizing influence of EDHF appears to depend on the activity of SKCa and IKCa channels that are expressed in cerebral endothelium, but not smooth muscle [105,116,117]. Combined and specific blockade of these channels abrogates EDHF responses [105,116,117]. However, while activation of SKCa and IKCa channels by endothelial cell calcium is a defining feature of EDHF production, the precise contribution of each channel in cerebral vessels to the response is less clear. Agonist-induced increase in endothelial cell calcium increases the open probability of both channels, but inhibitor sensitivity and genetic deletion of IKCa channels indicate that the opening of IKCa channels plays a greater role in mediating hyperpolarization and EDHF-dependent relaxation [123,124]. Although inhibition of SKCa and IKCa channels abolishes stimulated EDHF (i.e., agonist-induced in the presence of NOS and COX inhibition), it does not have direct effects on tone of larger cerebral arteries. In contrast, parenchymal arterioles constrict to both SKCa and IKCa channel inhibition , suggesting that in these small arterioles EDHF influences tone under basal conditions and thus influences resting cerebral blood flow.
The transfer of hyperpolarization from endothelium is thought to occur at structures called myoendothelial junctions (MEJ) . MEJ are holes in the internal elastic lamina where there is close (10–30 nm) contact between endothelium and smooth muscle cell membranes  (Figure 15). Endothelium and smooth muscle cells communicate at these projections that act as pathways for diffusion of vasoactive substances between endothelium and smooth muscle or vice versa. Some communication at MEJ occurs through gap junctions known as myoendothelial gap junctions (MEGJ);however, not all MEJ contain gap junctions and the prevalence of MEJ increases with decreasing size of vessels [126,127].
In endothelial cells, arachidonic acid metabolism produces vasoactive products that are vasoconstricting and vasodilating in nature, although the involvement of this pathway in cerebral artery vasoactivity appears to be considerably less than in peripheral endothelium [127,128]. Activation of the calcium-dependent enzyme phospholipase A2 hydrolyzes cellular lipid membranes to produce arachidonic acid lipid precursors that are substrates for cyclooxygenase (COX), lipoxygenases, and cytochrome P450 monooxygenases . Products of the COX pathway can be vasodilating (prostaglandin I2 (PGI2), prostaglandin E2 (PGE2), prostaglandin D2 (PGD2)) or vasoconstricting (prostaglandin F2α (PGF2α) and thromboxane A2 (TXA2)) in nature . PGI2 is synthesized from PGH2 by PGI2 synthase, which can activate adenylate cyclase and increase cyclic AMP and protein kinase A in smooth muscle, causing vasodilation . Some prostaglandins are elevated in response to injury. TXA2 causes vasoconstriction in response to hemorrhage [130,131]. Products of the lipoxygenase pathway are thought to be EDHFs in peripheral vessels, but not cerebral .