Human brain

This article is about features specific to the human brain. For basic information about brains, see Brain.

Human brain
Human brain and skull
Cerebral lobes: the frontal lobe (pink), parietal lobe (green) and occipital lobe (blue)
Latin	Cerebrum
Gray's	subject #184 736
System	Central nervous system
Artery	Anterior communicating artery, middle cerebral artery
Vein	Cerebral veins, external veins, basal vein, terminal vein, choroid vein, cerebellar veins

The human brain is the center of the human nervous system. Enclosed in the cranium, the human brain has the same general structure as that of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size.^[1] Most of the spatial expansion comes from the cerebral cortex, a convoluted layer of neural tissue which covers the surface of the forebrain. Especially expanded are the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. The portion of the brain devoted to vision, the occipital lobe, is also greatly enlarged in human beings.
Brain evolution, from the earliest shrew-like mammals through primates to hominids, is marked by a steady increase in encephalization, or the ratio of brain to body size. Estimates vary for the number of neuronal and non-neuronal cells contained in the brain, ranging from 80 or 90 billion (~85 10⁹) non-neuronal cells (glial cells) and an approximately equal number of (~86 10⁹) neurons,^[2] of which about 10 billion (10¹⁰) are cortical pyramidal cells, to over 120 billion neuronal cells, with an approximately equal number of non-neuronal cells.^[3] These cells pass signals to each other via as many as 1000 trillion (10¹⁵, 1 quadrillion) synaptic connections.^[4] Due to evolution, however, the modern human brain has been shrinking over the past 28,000 years.

Structure

Structure

Further information: Brain size

Bisection of the head of an adult man, showing the cerebral cortex and underlying white matter^[7]

The adult human brain weighs on average about 3 lb (1.5 kg)^[8] with a size (volume) of around 1130 cubic centimetres (cm³) in women and 1260 cm³ in men, although there is substantial individual variation.^[9] Men with the same body height and body surface area as women have on average 100g heavier brains,^[10] although these differences do not correlate in any simple way with gray matter neuron counts or with overall measures of cognitive performance.^[11] Neanderthals, an extinct subspecies of modern humans, had larger brains at adulthood than present-day humans.^[12] The brain is very soft, having a consistency similar to soft gelatin or soft tofu.^[13] Despite being referred to as "grey matter", the live cortex is pinkish-beige in color and slightly off-white in the interior. At the age of 20, a man has around 176,000 km and a woman about 149,000 km of myelinated axons in their brains.^[14]

General features

Drawing of the human brain, showing several important structures

The cerebral hemispheres form the largest part of the human brain and are situated above most other brain structures. They are covered with a cortical layer with a convoluted topography.^[15] Underneath the cerebrum lies the brainstem, resembling a stalk on which the cerebrum is attached. At the rear of the brain, beneath the cerebrum and behind the brainstem, is the cerebellum, a structure with a horizontally furrowed surface that makes it look different from any other brain area. The same structures are present in other mammals, although the cerebellum is not so large relative to the rest of the brain. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is more convoluted than the cortex of a human.

Lateralization

Lateralization

Main article: Lateralization of brain function

Routing of neural signals from the two eyes to the brain

Each hemisphere of the brain interacts primarily with one half of the body, but for reasons that are unclear, the connections are crossed: the left side of the brain interacts with the right side of the body, and vice versa.^{[citation needed]} Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both cross the midline at brainstem levels. Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called the optic chiasm, and half of the fibers from each nerve split off to join the other. The result is that connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain. Because each half of the retina receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa. Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field—an arrangement that presumably is helpful for visuomotor coordination.

The corpus callosum, a nerve bundle connecting the two cerebral hemispheres, with the lateral ventricles directly below

The two cerebral hemispheres are connected by a very large nerve bundle called the corpus callosum, which crosses the midline above the level of the thalamus. There are also two much smaller connections, the anterior commissure and hippocampal commissure, as well as many subcortical connections that cross the midline. The corpus callosum is the main avenue of communication between the two hemispheres, though. It connects each point on the cortex to the mirror-image point in the opposite hemisphere, and also connects to functionally related points in different cortical areas.

Metabolism

Metabolism

The brain consumes up to twenty percent of the energy used by the human body, more than any other organ.^[28] Brain metabolism normally is completely dependent upon blood glucose as an energy source, since fatty acids do not cross the blood-brain barrier.^[29] During times of low glucose (such as fasting), the brain will primarily use ketone bodies for fuel with a smaller requirement for glucose. The brain can also utilize lactate during exercise.^[30] The brain does not store any glucose in the form of glycogen, in contrast, for example, to skeletal muscle.

Additional images

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Structural and functional imaging

Structural and functional imaging

Main article: Neuroimaging

A scan of the brain using fMRI

There are several methods for detecting brain activity changes by three-dimensional imaging of local changes in blood flow. The older methods are SPECT and PET, which depend on injection of radioactive tracers into the bloodstream. The newest method, functional magnetic resonance imaging (fMRI), has considerably better spatial resolution and involves no radioactivity.^[25] Using the most powerful magnets currently available, fMRI can localize brain activity changes to regions as small as one cubic millimeter. The downside is that the temporal resolution is poor: when brain activity increases, the blood flow response is delayed by 1–5 seconds and lasts for at least 10 seconds. Thus, fMRI is a very useful tool for learning which brain regions are involved in a given behavior, but gives little information about the temporal dynamics of their responses. A major advantage for fMRI is that, because it is non-invasive, it can readily be used on human subjects.

Effects of brain damage

Main article: Neuropsychology

A key source of information about the function of brain regions is the effects of damage to them.^[26] In humans, strokes have long provided a "natural laboratory" for studying the effects of brain damage. Most strokes result from a blood clot lodging in the brain and blocking the local blood supply, causing damage or destruction of nearby brain tissue: the range of possible blockages is very wide, leading to a great diversity of stroke symptoms. Analysis of strokes is limited by the fact that damage often crosses into multiple regions of the brain, not along clear-cut borders, making it difficult to draw firm conclusions.

Sources of information

Sources of information

Neuroscientists, along with researchers from allied disciplines, study how the human brain works. Such research has expanded considerably in recent decades. The "Decade of the Brain", an initiative of the United States Government in the 1990s, is considered to have marked much of this increase in research.^[22]
Information about the structure and function of the human brain comes from a variety of experimental methods. Most information about the cellular components of the brain and how they work comes from studies of animal subjects, using techniques described in the brain article. Some techniques, however, are used mainly in humans, and therefore are described here.

Computed tomography of human brain, from base of the skull to top, taken with intravenous contrast medium

EEG

By placing electrodes on the scalp it is possible to record the summed electrical activity of the cortex, in a technique known as electroencephalography (EEG).^[23] EEG measures mass changes in population synaptic activity from the cerebral cortex, but can only detect changes over large areas of the brain, with very little sensitivity for sub-cortical activity. EEG recordings can detect events lasting only a few thousandths of a second. EEG recordings have good temporal resolution, but poor spatial resolution.

MEG

Apart from measuring the electric field around the skull it is possible to measure the magnetic field directly in a technique known as magnetoencephalography (MEG).^[24] This technique has the same temporal resolution as EEG but much better spatial resolution, although not as good as Magnetic Resonance Imaging (MRI). The greatest disadvantage of MEG is that, because the magnetic fields generated by neural activity are very weak, the method is only capable of picking up signals from near the surface of the cortex, and even then, only neurons located in the depths of cortical folds (sulci) have dendrites oriented in a way that gives rise to detectable magnetic fields outside the skull.