Energy requirements of brain tissue

The energy demand of the nervous tissue is very high and therefore sufficient blood supply to the brain must be maintained consistently. A normal adult male's brain containing approx. 130 billion neurons (21.5 billion in the neocortex) [49] comprises only 2% of total body mass, yet consumes at rest approximately 20% of the body's total basal oxygen consumption supplied by 16% of the cardiac blood output. The brain's oxygen consumption is almost entirely for the oxidative metabolism of glucose, which in normal physiological conditions is the almost exclusive substrate for the brain's energy metabolism [50] (Table 1.1). It must be kept in mind that the glucose metabolized in neuronal cell bodies is mainly to support cellular vegetative and house-keeping functions, e.g. axonal transport, biosynthesis of nucleic acids, proteins, lipids, as well as other energy-consuming processes not related directly to action potentials. Therefore the rate of glucose consumption of neuronal cell bodies is essentially unaffected by neuronal functional activation. Increases in glucose consumption (and regional blood flow) evoked by functional activation are confined to synapse-rich regions, i.e. neuropil which contains axonal terminals, dendritic processes, and also the astrocytic processes that envelop the synapses. The magnitudes of these increases are linearly related to the frequency of action potentials in the afferent pathways, and increases in the projection zones occur regardless of whether the pathway is excitatory or inhibitory. Energy metabolism by functional activation is due mostly to stimulation of the Na+K+-ATPase

Glutamate-releasing Astrocyte Capillary presynaptic terminal

Glutamate-releasing Astrocyte Capillary presynaptic terminal

Figure 1.5. (a) Schematic representation of the mechanism for glutamate-induced glycolysis in astrocytes during physiological 12 activation [95]. (b) Distribution of energy expenditure in rat cortex at a mean spike rate of 4 Hz: most energy is required for activity, only 13% _ is used for maintenance of resting potentialfor both neurons and glialcells [53, 96].

activity to restore the ionic gradients across the cell membrane and the membrane potentials that were degraded by the spike activity and is rather high compared to the demand of neuronal cell bodies [51] (Figure 1.5).

In excitatory glutamanergic neurons, which account for 80% of the neurons in the mammalian cortex, glucose utilization during activation is mediated by astrocytes which by anaerobic glycolysis provide lactate to the neurons where lactate is further oxidatively phosphorylated [52]. Overall, 87% of the total energy consumed is required by signaling, mainly action potential propagation and postsynaptic ion fluxes, and only 13% is expended in maintaining membrane resting potential [53] (Figure 1.5).

The mechanisms by which neurotransmitters other than glutamate influence blood flow and energy metabolism in the brain are still not understood [54].

A normal adult male's brain comprises only 2% of total body mass, yet consumes at rest approximately 20% of the body's total basal oxygen consumption. Glucose is the almost exclusive substrate for the brain's energy metabolism;87% of the total energy consumed is required by signaling, mainly action potential propagation and postsynaptic ion fluxes.

Table 1.1. Cerebralblood flow, oxygen utilization and metabolic rates of glucose in man (approximate values).

Cortex

White matter

Global

(ml/100 g/min)

65

21

(mol/100 g/min)

230

80

(mol/100 g/min)

40

20

32

Figure 1.5. (a) Schematic representation of the mechanism for glutamate-induced glycolysis in astrocytes during physiological 12 activation [95]. (b) Distribution of energy expenditure in rat cortex at a mean spike rate of 4 Hz: most energy is required for activity, only 13% _ is used for maintenance of resting potentialfor both neurons and glialcells [53, 96].

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