The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head, usually close to the sensory organs for senses such as vision. The brain is the most complex organ in a vertebrate’s body. In a human, the cerebral cortex contains approximately 15–33 billion neurons, each connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.
Physiologically, the function of the brain is to exert centralized control over the other organs of the body. The brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment. Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain.
The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer, very different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, and processes it in a variety of ways.
This article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar as it shares the properties of other brains. The ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context. The most important is brain disease and the effects of brain damage, that are covered in the human brain article.
a blob with a blue patch in the center, surrounded by a white area, surrounded by a thin strip of dark-colored material
Cross section of the olfactory bulb of a rat, stained in two different ways at the same time: one stain shows neuron cell bodies, the other shows receptors for the neurotransmitter GABA.
The shape and size of the brain varies greatly between species, and identifying common features is often difficult. Nevertheless, there are a number of principles of brain architecture that apply across a wide range of species. Some aspects of brain structure are common to almost the entire range of animal species; others distinguish “advanced” brains from more primitive ones, or distinguish vertebrates from invertebrates.
The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, and then sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations. It is also possible to examine the microstructure of brain tissue using a microscope, and to trace the pattern of connections from one brain area to another.
All vertebrates have a blood–brain barrier that allows metabolism inside the brain to operate differently from metabolism in other parts of the body. Glial cells play a major role in brain metabolism by controlling the chemical composition of the fluid that surrounds neurons, including levels of ions and nutrients.
Brain tissue consumes a large amount of energy in proportion to its volume, so large brains place severe metabolic demands on animals. The need to limit body weight in order, for example, to fly, has apparently led to selection for a reduction of brain size in some species, such as bats. Most of the brain’s energy consumption goes into sustaining the electric charge (membrane potential) of neurons. Most vertebrate species devote between 2% and 8% of basal metabolism to the brain. In primates, however, the percentage is much higher—in humans it rises to 20–25%. The energy consumption of the brain does not vary greatly over time, but active regions of the cerebral cortex consume somewhat more energy than inactive regions; this forms the basis for the functional brain imaging methods PET, fMRI, and NIRS. The brain typically gets most of its energy from oxygen-dependent metabolism of glucose (i.e., blood sugar), but ketones provide a major alternative source, together with contributions from medium chain fatty acids (caprylic and heptanoic acids), lactate, acetate, and possibly amino acids.
Amazing !!! Unfixed Brain Surgery (Video Inside)
The fixed brain is so much worse – with some brains, no amount of rinsing can remove the stench of formalin that makes your eyes and throat sting.
WARNING: I AM ABOUT TO BE SLIGHTLY DISGUSTING BELOW, but if you’re in this thread, you’re probably fine with what I am about to describe.
Of all the brains I had to cut up when I was learning neuropathology, there are two that still strike me because they were so unique and informative. (Of note, pathologists use food analogies, so preserved brains were like firm tofu or mushrooms, and the cauda equina at the end of the spine legitimately DOES look like ramen noodles stretched out. So if they’re your teachers, you pick up that bad habit.)
The first one was the brain of someone who had been declared brain-dead for three or four days before the family could bring themselves to withdraw care. Most brains that have been pickled in formaldehyde for the required number of weeks are sort of greyish, firm, easy to handle. This brain was purplish, soft, rotting, falling apart. You could tell that it had started to “decompose” prior to the person’s physical death.
The neuropathologist commented that once you’re braindead, your brain might as well be on the shelf next to your body for all the good it’s doing you. And you could *see* this. I know I say the brain is dead, but you could see how very dead the brain was!
The second memorable brain came from a small plane crash. I was hanging out in the lab, waiting for the neuropathologist to show, and one of the pathology fellows saw me and excitedly called me in to the next room to see “something cool.” (Note: never trust a pathologist’s definition of “cool”.) WATCH VIDEO