The brain needs continuous oxygen + sugar, each neuron contains mitochondrias that burn sugar in the presence of O2, producing energy to sustain cell life. After injecting a high dose of insulin without eating anything, blood sugar is immediately bound and injected into all the cells in the body, most obviously into the muscles which have the largest capacity. After a very short time, there is almost no sugar in the blood for the brain or coronary vessels of the heart, only oxygen. Severe overdose - insulin coma and further unpredictable consequences, vegetative state as after cerebral hypoxia, myocardial infarction, including death of course.