Brain
The first vertebrates appeared over 500 million years ago during the Cambrian period, and may have resembled modern jawless fish like hagfish and lampreys. All living multicellular animals are bilaterians, meaning they possess a body plan with left and right sides that act as approximate mirror images of each other. This fundamental bilateral form is a tube containing a hollow gut cavity running from mouth to anus, alongside a nerve cord with segmental enlargements called ganglia. An especially large ganglion at the front end became known as the brain. Some types of worms, such as leeches, also feature an enlarged ganglion at the back end of the nerve cord, which scientists call a tail brain. The simplest brains exist in species like nematode worms, while others like vertebrates possess large and very complex organs. A few existing bilaterians lack a recognizable brain entirely, including echinoderms and tunicates. It remains unproven whether these brainless species indicate that earliest bilaterians lacked a brain or if their ancestors evolved away from having one.
In humans, the cerebral cortex contains approximately 14 billion to 16 billion neurons, and the estimated number of neurons in the cerebellum ranges from 55 billion to 70 billion. Each neuron connects via synapses to several thousand other neurons, communicating through cytoplasmic processes known as dendrites and axons. Axons are usually myelinated and carry trains of rapid micro-electric signal pulses called action potentials to target specific recipient cells. If a pyramidal cell of the cerebral cortex were magnified so its body matched human size, its axon would become a cable extending more than a kilometer. These axons transmit signals as electrochemical pulses lasting less than a thousandth of a second, traveling at speeds between 1 meter per second and 100 meters per second. The human brain has been estimated to contain approximately 100 trillion synapses, while even the fruit fly brain holds several million. A single axon may make as many as several thousand synaptic connections with other cells. When an action potential arrives at a synapse, it causes a chemical neurotransmitter to be released that binds to receptor molecules on the target cell membrane.
Modern reptiles and mammals diverged from a common ancestor around 320 million years ago, yet their brains share high levels of similarity during embryological development. Crocodilians possess the largest telencephalon among reptiles, whereas snakes have the smallest diencephalon relative to body weight. Turtles exhibit the largest diencephalon per body weight, while lizards display the largest mesencephalon. On average, a mammal has a brain roughly twice as large as that of a bird of the same body size, and ten times larger than a reptile of equal mass. Humans maintain an average encephalization quotient in the 7-to-8 range, while most other primates fall within the 2-to-3 range. Dolphins hold values higher than non-human primates, but nearly all other mammals show substantially lower scores. The visual processing network of primates includes at least 30 distinguishable brain areas connected by a complex web of interrelations. Visual processing areas occupy more than half of the total surface of the primate neocortex. In birds, major changes occur in forebrain structure compared to the straightforward geometry seen in sharks.
At about six weeks of human embryonic development, the neural plate folds inward to form the neural groove before merging into a hollow cord called the neural tube. At the front end, ventricles swell to form three vesicles that become the precursors for the forebrain, midbrain, and hindbrain. Neurons are created in special zones containing stem cells, then migrate through tissue to reach their ultimate locations. Axons sprout and navigate through the brain until tips reach targets and form synaptic connections. In many parts of the brain, axons initially overgrow before being pruned by mechanisms dependent on neural activity. New neurons continue to be generated throughout life only in two specific areas: the olfactory bulb and the dentate gyrus of the hippocampus. Animals raised in enriched environments demonstrate thick cerebral cortices indicating high density of synaptic connections compared to those with restricted stimulation. The presence or absence of experience is critical at key periods of development, as genes determine general form while experience refines synaptic matrices.
The suprachiasmatic nucleus serves as the body's central biological clock, located directly above where optic nerves from both eyes cross. Neurons within this tiny region show activity levels rising and falling with a period of about 24 hours known as circadian rhythms. Damage to the reticular formation can produce a permanent state of coma, as these neuron clusters send signals to the thalamus controlling every part of the cortex. Most vertebrate species devote between 2% and 8% of basal metabolism to the brain, though humans rise to 20, 25%. Brain tissue consumes large amounts of energy primarily to sustain the electric charge of neurons. The hypothalamus plays the greatest role in homeostasis, receiving input from sensors lining blood vessels regarding temperature, sodium level, glucose level, and oxygen level. These nuclei send output signals to motor areas that generate actions to rectify deficiencies. Some outputs also go to the pituitary gland, which secretes hormones into the bloodstream circulating throughout the body.
In 1971 Tim Bliss and Terje Lømo published a paper on long-term potentiation showing clear evidence of activity-induced synaptic changes lasting several days. Working memory maintains temporary representations of information through groups of activated neurons constantly stimulating one another. People with severe damage to the hippocampus sometimes show amnesia, an inability to form new long-lasting episodic memories. Semantic memory stores facts and relationships largely within the cerebral cortex via changes in connections representing specific types of information. Instrumental learning modifies behavior based on rewards and punishments through networks centered on the basal ganglia. Motor learning refines movement patterns by practicing or repeating actions involving the premotor cortex and cerebellum. The cerebellum functions as a large memory bank for microadjustments of movement parameters, containing 50% of all neurons despite comprising only 10% of total brain volume. Dopamine plays a central role in the reward mechanism, where addictive drugs like cocaine cause levels to rise or enhance effects inside the brain.
The Human Brain Project is a large scientific research project starting in 2024 that aims to simulate the complete human brain. Investigators studied a large integrated dataset of almost 3 million nuclei from the human prefrontal cortex derived from 388 individuals. Functional imaging techniques such as fMRI require conscious subjects to remain motionless for long periods but offer noninvasive study capabilities. Researchers can use electrodes glued to the scalp for EEG studies or implanted inside brains for extracellular recordings detecting individual neuron action potentials. It is now possible with relative ease to knock out or mutate a wide variety of genes in mice to examine effects on brain function. Using Cre-Lox recombination allows activation or deactivation of genes in specific parts of the brain at specific times. Recent advances in single-cell sequencing technologies leverage cellular heterogeneity to better understand distinct cell types in disease and biology.
Common questions
When did the first vertebrates with brains appear?
The first vertebrates appeared over 500 million years ago during the Cambrian period. These early organisms may have resembled modern jawless fish like hagfish and lampreys.
How many neurons are in the human cerebral cortex and cerebellum?
The human cerebral cortex contains approximately 14 billion to 16 billion neurons. The estimated number of neurons in the cerebellum ranges from 55 billion to 70 billion.
What is the function of the suprachiasmatic nucleus in humans?
The suprachiasmatic nucleus serves as the body's central biological clock located directly above where optic nerves cross. Neurons within this region show activity levels rising and falling with a period of about 24 hours known as circadian rhythms.
Which scientists published on long-term potentiation in 1971?
Tim Bliss and Terje Lømo published a paper on long-term potentiation showing clear evidence of activity-induced synaptic changes lasting several days in 1971. This research demonstrated how synaptic connections can change based on neural activity.
When does the Human Brain Project start its simulation efforts?
The Human Brain Project is a large scientific research project starting in 2024 that aims to simulate the complete human brain. Investigators studied a large integrated dataset of almost 3 million nuclei from the human prefrontal cortex derived from 388 individuals.