Brain

Axons are usually myelinated and carry trains of rapid micro-electric signal pulses called action potentials to target specific recipient cells in other areas of the brain or distant parts of the body.

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.

[20] There are several invertebrate species whose brains have been studied intensively because they have properties that make them convenient for experimental work: The first vertebrates appeared over 500 million years ago (Mya) during the Cambrian period, and may have resembled the modern jawless fish (hagfish and lamprey) in form.

All of these brains contain the same set of basic anatomical structures, but many are rudimentary in the hagfish, whereas in mammals the foremost part (forebrain, especially the telencephalon) is greatly developed and expanded.

[36] As a result of the osmotic restriction by the blood-brain barrier, the metabolites within the brain are cleared mostly by bulk flow of the cerebrospinal fluid within the glymphatic system instead of via venules like other parts of the body.

[38] Here is a list of some of the most important vertebrate brain components, along with a brief description of their functions as currently understood: Modern reptiles and mammals diverged from a common ancestor around 320 million years ago.

[55][56][57] Vertebrates share the highest levels of similarities during embryological development, controlled by conserved transcription factors and signaling centers, including gene expression, morphological and cell type differentiation.

Birds possess large, complex brains, which process, integrate, and coordinate information received from the environment and make decisions on how to respond with the rest of the body.

[71] As the embryo transforms from a round blob of cells into a wormlike structure, a narrow strip of ectoderm running along the midline of the back is induced to become the neural plate, the precursor of the nervous system.

The result of this pathfinding process is that the growth cone navigates through the brain until it reaches its destination area, where other chemical cues cause it to begin generating synapses.

The retina, before birth, contains special mechanisms that cause it to generate waves of activity that originate spontaneously at a random point and then propagate slowly across the retinal layer.

The two areas for which adult neurogenesis is well established are the olfactory bulb, which is involved in the sense of smell, and the dentate gyrus of the hippocampus, where there is evidence that the new neurons play a role in storing newly acquired memories.

The electrical properties of neurons are controlled by a wide variety of biochemical and metabolic processes, most notably the interactions between neurotransmitters and receptors that take place at synapses.

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.

[8] A key component of the sleep system is the suprachiasmatic nucleus (SCN), a tiny part of the hypothalamus located directly above the point at which the optic nerves from the two eyes cross.

The SCN continues to keep time even if it is excised from the brain and placed in a dish of warm nutrient solution, but it ordinarily receives input from the optic nerves, through the retinohypothalamic tract (RHT), that allows daily light-dark cycles to calibrate the clock.

In vertebrates, the part of the brain that plays the greatest role is the hypothalamus, a small region at the base of the forebrain whose size does not reflect its complexity or the importance of its function.

Already in the late 19th century theorists like Santiago Ramón y Cajal argued that the most plausible explanation is that learning and memory are expressed as changes in the synaptic connections between neurons.

Even though it is protected by the skull and meninges, surrounded by cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier, the delicate nature of the brain makes it vulnerable to numerous diseases and several types of damage.

In animal studies, most commonly involving rats, it is possible to use electrodes or locally injected chemicals to produce precise patterns of damage and then examine the consequences for behavior.

On the other hand, it is possible to study algorithms for neural computation by simulating, or mathematically analyzing, the operations of simplified "units" that have some of the properties of neurons but abstract out much of their biological complexity.

Recent years have seen increasing applications of genetic and genomic techniques to the study of the brain [129] and a focus on the roles of neurotrophic factors and physical activity in neuroplasticity.

... And by the same organ we become mad and delirious, and fears and terrors assail us, some by night, and some by day, and dreams and untimely wanderings, and cares that are not suitable, and ignorance of present circumstances, desuetude, and unskillfulness.

[132] Galen's ideas were widely known during the Middle Ages, but not much further progress came until the Renaissance, when detailed anatomical study resumed, combined with the theoretical speculations of René Descartes and those who followed him.

[132] The first real progress toward a modern understanding of nervous function, though, came from the investigations of Luigi Galvani (1737–1798), who discovered that a shock of static electricity applied to an exposed nerve of a dead frog could cause its leg to contract.

In the hands of Camillo Golgi, and especially of the Spanish neuroanatomist Santiago Ramón y Cajal, the new stain revealed hundreds of distinct types of neurons, each with its own unique dendritic structure and pattern of connectivity.

Reflecting the new understanding, in 1942 Charles Sherrington visualized the workings of the brain waking from sleep: The great topmost sheet of the mass, that where hardly a light had twinkled or moved, becomes now a sparkling field of rhythmic flashing points with trains of traveling sparks hurrying hither and thither.

[139] Over the years, though, accumulating information about the electrical responses of brain cells recorded from behaving animals has steadily moved theoretical concepts in the direction of increasing realism.

[140] A few years later David Hubel and Torsten Wiesel discovered cells in the primary visual cortex of monkeys that become active when sharp edges move across specific points in the field of view—a discovery for which they won a Nobel Prize.

[142] Other investigations of brain areas unrelated to vision have revealed cells with a wide variety of response correlates, some related to memory, some to abstract types of cognition such as space.

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 neuronal cell bodies , the other shows receptors for the neurotransmitter GABA .
drawing showing a neuron with a fiber emanating from it labeled "axon" and making contact with another cell. An inset shows an enlargement of the contact zone.
Neurons generate electrical signals that travel along their axons . When an electrical impulse reaches a junction called a synapse , it causes a neurotransmitter to be released, which binds to receptors on other cells and thereby alters their electrical activity.
A rod-shaped body contains a digestive system running from the mouth at one end to the anus at the other. Alongside the digestive system is a nerve cord with a brain at the end, near to the mouth.
Nervous system of a generic bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front
A fly resting on a reflective surface. A large, red eye faces the camera. The body appears transparent, apart from black pigment at the end of its abdomen.
Fruit flies ( Drosophila ) have been extensively studied to gain insight into the role of genes in brain development.
A T-shaped object is made up of the cord at the bottom which feeds into a lower central mass. This is topped by a larger central mass with an arm extending from either side.
The brain of a shark
The nervous system is shown as a rod with protrusions along its length. The spinal cord at the bottom connects to the hindbrain which widens out before narrowing again. This is connected to the midbrain, which again bulges, and which finally connects to the forebrain which has two large protrusions.
The main subdivisions of the embryonic vertebrate brain (left), which later differentiate into structures of the adult brain (right)
Corresponding regions of human and shark brain are shown. The shark brain is splayed out, while the human brain is more compact. The shark brain starts with the medulla, which is surrounded by various structures, and ends with the telencephalon. The cross-section of the human brain shows the medulla at the bottom surrounded by the same structures, with the telencephalon thickly coating the top of the brain.
The main anatomical regions of the vertebrate brain, shown for shark and human. The same parts are present, but they differ greatly in size and shape.
Anatomical comparison between the brain of a lizard (A and C) and the brain of a turkey (B and D). Abbreviations: Olf, olfactory lobes; Hmp, cerebral hemispheres; Pn, pineal gland ; Mb, optic lobes of the middle brain ; Cb, cerebellum; MO, medulla oblongata; ii, optic nerves; iv and vi, nerves for the muscles of the eye; Py, pituitary body.
Comparison of Vertebrate Brains: Mammalian, Reptilian, Amphibian, Teleost, and Ammocoetes. CB., cerebellum; PT., pituitary body; PN., pineal body; C. STR., corpus striatum; G.H.R., right ganglion habenulæ. I., olfactory; II., optic nerves.
Brains of an emu , a kiwi , a barn owl , and a pigeon , with visual processing areas labelled
Very simple drawing of the front end of a human embryo, showing each vesicle of the developing brain in a different color.
Brain of a human embryo in the sixth week of development
Graph showing 16 voltage traces going across the page from left to right, each showing a different signal. At the middle of the page all of the traces abruptly begin to show sharp jerky spikes, which continue to the end of the plot.
Brain electrical activity recorded from a human patient during an epileptic seizure
Model of a neural circuit in the cerebellum, as proposed by James S. Albus
Drawing showing the ear, inner ear, and brain areas involved in hearing. A series of light blue arrows shows the flow of signals through the system.
Diagram of signal processing in the auditory system
Cross-section of a human head, showing location of the hypothalamus
Components of the basal ganglia, shown in two cross-sections of the human brain. Blue: caudate nucleus and putamen . Green: globus pallidus . Red: subthalamic nucleus . Black: substantia nigra .
The Human Brain Project is a large scientific research project, starting in 2013, which aims to simulate the complete human brain.
Drawing showing a monkey in a restraint chair, a computer monitor, a rototic arm, and three pieces of computer equipment, with arrows between them to show the flow of information.
Design of an experiment in which brain activity from a monkey was used to control a robotic arm [ 126 ]
Illustration by René Descartes of how the brain implements a reflex response
Andreas Vesalius ' Fabrica , published in 1543, showing the base of the human brain, including optic chiasma , cerebellum, olfactory bulbs , etc.
A drawing on yellowing paper with an archiving stamp in the corner. A spidery tree branch structure connects to the top of a mass. A few narrow processes follow away from the bottom of the mass.
Drawing by Santiago Ramón y Cajal of two types of Golgi-stained neurons from the cerebellum of a pigeon
Gulai otak , beef brain curry from Indonesia