Astrocytes and extracellular Ca2

Calcium concentration in small extracellular compartments and particularly in perisynaptic compartments may fluctuate rather substantially, as Ca2+ is accumulated by neurones when the invading action potential activates Ca2+ channels. The actual [Ca2+]ocan decrease below 1 mM, which may affect generation of Ca2+ signals in the terminal, and hence neurotransmission. The lowering of extracellular Ca2+ concentration to ~0.5 mM triggers Ca2+ signalling in astrocytes, which originates from InsP3-driven intracellular Ca2+ release from the ER stores. This may, in principle, help restore [Ca2+]o, as Ca2+ can leave the astrocyte through either plasmalemmal Ca2+ pump or sodium-calcium exchanger. Extracellular Ca2+ concentration may plunge much deeper (to 0.01-0.1 mM) under ischaemic conditions, which in turn can initiate seizures (see Chapter 10).

Muller Cell

Figure 7.9 'Potassium siphoning' in retinal Müller glial cell. Potassium buffering in the retina is provided by Müller glial cells: K+ ions enter the cytosol of the Müller cell in the inner plexiform layer; K+ then equilibrates within the glial cytosol and excess K+ is expelled through Kir channels located in the endfoot into the vitreous humour or through Kir channels located in perivascular processes into the perivascular space. Some of the K+ ions may be also released through apical processes, where light induces a decrease in [K+]o in the subretinal space. The same process of K+ siphoning can occur in astrocytes, taking up K+ via their perisynaptic or perinodal processes and releasing K+ via their perivascular endfeet, where it could help regulate blood flow

Figure 7.9 'Potassium siphoning' in retinal Müller glial cell. Potassium buffering in the retina is provided by Müller glial cells: K+ ions enter the cytosol of the Müller cell in the inner plexiform layer; K+ then equilibrates within the glial cytosol and excess K+ is expelled through Kir channels located in the endfoot into the vitreous humour or through Kir channels located in perivascular processes into the perivascular space. Some of the K+ ions may be also released through apical processes, where light induces a decrease in [K+]o in the subretinal space. The same process of K+ siphoning can occur in astrocytes, taking up K+ via their perisynaptic or perinodal processes and releasing K+ via their perivascular endfeet, where it could help regulate blood flow

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