Brain activity: regions, energy, rest, and sleep
Brain activity continues during ordinary life, rest, and sleep. Different regions contribute to different tasks, with activity changing as demands change. Speech, movement, visual processing, and remembering involve specialized structures working together. A person can carry out several of those functions at once. The brain also controls processes outside conscious awareness, including aspects of breathing and heart rate. Its activity is distributed across many regions rather than confined to a small fixed section, and reduced activity in one setting does not mean that tissue has no function.
The cerebrum's outer layer, the cerebral cortex, has deep folds that increase its surface area. Its principal lobes have characteristic roles. Frontal regions participate in planning, decision-making, speech, and voluntary movement. Parietal regions integrate sensory signals, while occipital regions process vision, including color and shape. Temporal regions contribute to language, memory, and emotion. Deeper structures add further functions: the hippocampus helps form new memories, and the cerebellum coordinates movement and motor learning. Brainstem structures participate in basic functions such as swallowing, breathing, and heart rate.
Maintaining this activity requires substantial energy. In a typical resting adult, the brain consumes about 20% of the body's energy. A large share supports electrical signaling rather than changes in outward behavior. Neurons and supporting cells also need energy simply to maintain themselves. At synapses, where cells communicate, the movement of charged particles contributes to the energy demand. Sodium and potassium gradients are essential to signaling. Even while a person rests, cells continue communicating, sustaining a high baseline demand rather than waiting for a difficult task to begin.
Energy use differs between tissues. Gray matter contains cell bodies, dendrites, and many synapses and generally requires more energy than white matter. White matter contains bundles of axons, many insulated by myelin, which reduces electrical leakage. A specific task can increase demand in the regions particularly involved, but the change is relatively small compared with the brain's continuing baseline consumption. Speaking, for example, increases activity in speech-related regions. The contrast between a task and rest highlights a change in local activity; it does not measure all the ongoing activity as zero.
Quiet wakefulness has its own organized patterns. Regions collectively called the default mode network become more active during rest and less active during many demanding tasks. Imaging with functional MRI and positron emission tomography helped identify this network. It includes parts of the prefrontal and cingulate cortex, lateral parietal cortex, and precuneus. Those regions participate in functions involving memory, emotion, and reflection. Daydreaming is consequently associated with brain activity, even though the person may have temporarily stopped concentrating on an external task.
Different parts of the network show different relationships with mental experience. Dorsomedial prefrontal activity increases during introspection and thoughts about oneself in the past, present, or future. Posterior regions become more active while recalling concrete experiences and connect with the hippocampus. The network's exact role remains under investigation, with links to remembering, emotion, and a sense of oneself over time. Patterns also vary across the day. Both the hippocampus and default mode network can be more active during evening rest than shortly after waking in the morning.
Sleep introduces another set of changing states. A night usually includes four to six cycles, each roughly 80 to 100 minutes long, alternating between non-REM and rapid-eye-movement sleep. Non-REM sleep has three stages distinguished by recorded brain activity. The first is the transition from waking, the second is a period of sleep before deeper sleep, and the third has a characteristic slow-wave pattern. Deep sleep is generally more prominent earlier in the night. Sleep researchers classify these states using signals such as brain activity and eye movement.
During REM sleep, the brain shows activity resembling waking activity. The eyes move rapidly, dreaming commonly occurs, and most muscles become limp. REM periods generally become more prominent later in the night. The proportion of different stages changes with age: newborns spend more time in REM, while slow-wave sleep peaks in early childhood and declines through adolescence and adulthood. Older people may have very little slow-wave sleep. Sleep therefore involves changing forms of activity rather than a uniform shutdown lasting until morning.
Below the cortex, the midbrain coordinates eye movements and responses to sound, while pathways through the pons and medulla connect brain activity with bodily functions. The cerebellum, at the back of the brain, contains more than half its neurons and contributes to precise timing and coordination. Damage there can disrupt walking or accurate movement. The basal ganglia participate in regulating complex movements. These structures continue to have specialized roles even when attention is directed elsewhere, alongside the cortical networks involved in conscious thought.
- BrainFacts/Society for Neuroscience, Identifying Major Brain Landmarks — www.brainfacts.org
- BrainFacts/Society for Neuroscience, How Much Energy Does the Brain Use? — www.brainfacts.org
- BrainFacts/Society for Neuroscience, Rest: The Default Mode Network — www.brainfacts.org
- NHLBI, Sleep Phases and Stages — www.nhlbi.nih.gov