neurotransmitters cerebral cortex colliculli hemisphere parietal lobe CNS white matter skull myelin sheath midbrain action potential PNS glía oligodendrocyte fire trigger temporal lobe spectrum wire multitasker corpus callosum cerebrum groove SNS amygdala forebrain Schwann cell thalamus occipital lobe hypothalamus hindbrain pons basal ganglia arousal medulla oblongata synapse grey matter dendrite ANS axon nucleus cell body neuromodulator cytoplasm frontal lobe