수면이 4시간 미만 또는 10시간 이상이면 사망율 증가.
입면 장애. 유지 장애
수면주기는 suprachiasmatic nucleus가 햇빛을 받을 때 결정된다.
Pierre J. Magistretti, Luc Pellerin, and Jean-Luc Martin . Brain Energy Metabolism. Available URL from:http://www.acnp.org/g4/GN401000064/CH064.HTML
Although the brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization. With a global blood flow of 57 ml/100 g·min, the brain extracts approximately 50% of oxygen and 10% of glucose from the arterial blood.
The respiratory quotient (RQ) of the brain is nearly 1, indicating that carbohydrates are the substrates for oxidative metabolism (60). Glucose can produce metabolic intermediates, such as lactate and pyruvate, which do not enter necessarily in the tricarboxylic acid cycle but rather can be released and removed by the circulation. Glucose can be incorporated into lipids, proteins, and glycogen, and it is also the precursor of certain neurotransmitters such as g-aminobutyric acid (GABA), glutamate, and acetylcholine (10, 60).
Lactate and pyruvate can sustain synaptic activity in vitro (36, 55) and could substitute for glucose as an alternative substrate for brain energy metabolism. Because of their limited permeability across the blood–brain barrier, they cannot substitute for plasma glucose to maintain brain function (43). However, if formed inside the brain parenchyma, they are useful metabolic substrates for neural cells (66). Under particular conditions, such as starvation, diabetes, or in breast-fed neonates, plasma levels of the ketone bodies acetoacetate and D-3-hydroxybutyrate increase markedly (41). Under these conditions, acetoacetate and D-3-hydroxybutyrate can be used by the brain as metabolic substrates (41).
At best less than 20% of glucose may eventually be utilized glycolytically. 20% of total utilized glucose is not oxidized completely to CO2 and H2O, and only a portion of it will yield pyruvate and lactate. Synaptic activity in vitro can be maintained when lactate is the only metabolic substrate present.
The increase in glucose utilization following activation of pathways subserving specific modalities, such as visual, auditory, olfactory, or somatosensory stimulations, as well as during motor activity, has been revealed in the pertinent brain structures (10).
Glucose oxidation is already nearly maximal under basal conditions, implying that the activation-induced increases in energy demands are to be met primarily by glycolysis (69). One of the possible roles for activation-induced glycolysis may be to provide ATP to fuel energy-dependent ion transport, in particular the Na+/K+-ATPase, which represents the main energy-consuming process in neural cells (58).
뇌에도 글리코겐이 있다
Neurons contribute at most 50% of cerebral cortical volume (23).
Within the brain, glycogen is primarily stored in astrocytes, although ependymal and choroid plexus cells, as well as certain large neurons in the brainstem contain the polysaccharide (see ref. 34 for review). Glycogen levels in brain are low compared to liver and muscle; however, the glycogen turnover rate is very rapid; its synthesis and breakdown are regulated by the two key enzymes glycogen phosphorylase and synthase (34).
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운동시에 lactate
At rest, brain energy is provided by a balanced oxidation of glucose as MR is close to 6, but activation provokes a 'surplus' uptake of glucose relative to that of O2. Whereas MR remains stable during light exercise, it is reduced by 30% to 40% when exercise becomes demanding. The MR integrates metabolism in brain areas stimulated by sensory input from skeletal muscle, the mental effort to exercise and control of exercising limbs. The MR decreases during prolonged exhaustive exercise where blood lactate remains low, but when vigorous exercise raises blood lactate, the brain takes up lactate in an amount similar to that of glucose. This lactate taken up by the brain is oxidised as it does not accumulate within the brain and such pronounced brain uptake of substrate occurs independently of plasma hormones. The 'surplus' of glucose equivalents taken up by the activated brain may reach approximately 10 mmol, that is, an amount compatible with the global glycogen level. It is suggested that a low MR predicts shortage of energy that ultimately limits motor activation and reflects a biologic background for 'central fatigue'.
Dalsgaard MK. Fuelling cerebral activity in exercising man. J Cereb Blood Flow Metab. 2006 Jun;26(6):731-50.
mental exercise (brain teasers and verbal memory training techniques);
Reduced resting activity in left dorsolateral prefrontal cortex may reflect greater cognitive efficiency of a brain region involved in working memory.
Small GW, et al. Effects of a 14-day healthy longevity lifestyle program on cognition and brain function. Am J Geriatr Psychiatry. 2006 Jun;14(6):538-45
뇌활동이 대사과 무관?
His views were adopted and extended by Hans Berger (1873-1941). F. G. Benedict (1870-1957), drawing upon extensive experience with balance experiments conducted on humans in large-scale respiration calorimeters, concluded that mental effort probably had no effect upon the brain's metabolism. Modern approaches to the problem make use of PET imaging, which detects local changes in glucose utilization by the brain during cognitive activity.
Sourkes TL. On the energy cost of mental effort. J Hist Neurosci. 2006 Mar;15(1):31-47
생각할 때 활성화되는 영역
In the rest condition, glucose metabolism was correlated with the MMSE score primarily within the posterior cingulate and parietal lobes. For the activation condition, additional correlations were within the primary and association audiovisual areas.
Bokde AL, et al. Association between cognitive performance and cortical glucose metabolism in patients with mild Alzheimer's disease. Dement Geriatr Cogn Disord. 2005;20(6):352-7. Epub 2005 Sep 26.
Compared to rest, performing the reflective tasks was associated with increased blood flow in the dorsomedial prefrontal cortex, the left anterior middle temporal gyrus, the temporal pole bilaterally, and the right cerebellum; there was a decrease of blood flow in right prefrontal regions and in medial and right lateral parietal regions. In addition, the ventromedial prefrontal cortex (VMPFC) (1) was more active during the self-referential reflective task than during the other two reflective tasks, (2) showed common activation during rest and the self-referential task, and (3) showed a correlation between cerebral metabolism and the amount of self-referential processing. It is suggested that the VMPFC is crucial for representing knowledge pertaining to the self and that this is an important function of the resting state.
D'Argembeau A, et al. Self-referential reflective activity and its relationship with rest: a PET study. Neuroimage. 2005 Apr 1;25(2):616-24.