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 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 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is 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 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 different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, 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; 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. The shape and size of the brain varies between species, identifying common features is difficult. 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 the entire range of animal species. 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, 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 possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.
Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems, their neuroanatomy is therefore better understood. In vertebrates, the nervous system is segregated into the internal structure of the brain and spinal cord and the routes of the nerves that connect to the rest of the body; the delineation of distinct structures and regions of the nervous system has been critical in investigating how it works. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affects behavior or other neural functions. For information about the composition of non-human animal nervous systems, see nervous system. For information about the typical structure of the Homo sapiens nervous system, see human brain or peripheral nervous system; this article discusses information pertinent to the study of neuroanatomy.
The first known written record of a study of the anatomy of the human brain is the ancient Egyptian document the Edwin Smith Papyrus. The next major development in neuroanatomy came from the Greek Alcmaeon, who determined that the brain and not the heart ruled the body and that the senses were dependent on the brain. After Alcmaeon’s findings, many scientists and physicians from around the world continued to contribute to the understanding of neuroanatomy, notably: Galen, Herophilus and Erasistratus. Herophilus and Erasistratus of Alexandria were the most influential Greek neuroscientists with their studies involving dissecting the brains. For several hundred years afterward, with the cultural taboo of dissection, no major progress occurred in neuroscience. However, Pope Sixtus IV revitalized the study of neuroanatomy by altering the papal policy and allowing human dissection; this resulted in a boom of research in neuroanatomy by scientists of the Renaissance. In 1664, Thomas Willis, a physician and professor at Oxford University, coined the term neurology when he published his text Cerebri anatome, considered the foundation of neuroanatomy.
The subsequent three hundred and fifty some years has produced a great deal of documentation and study of the neural systems. At the tissue level, the nervous system is composed of neurons, glial cells, extracellular matrix. Both neurons and glial cells come in many types. Neurons are the information-processing cells of the nervous system: they sense our environment, communicate with each other via electrical signals and chemicals called neurotransmitters across synapses, produce our memories and movements. Glial cells maintain homeostasis, produce myelin, provide support and protection for the brain's neurons; some glial cells can propagate intercellular calcium waves over long distances in response to stimulation, release gliotransmitters in response to changes in calcium concentration. The extracellular matrix provides support on the molecular level for the brain's cells. At the organ level, the nervous system is composed of brain regions, such as the hippocampus in mammals or the mushroom bodies of the fruit fly.
These regions are modular and serve a particular role within the general pathways of the nervous system. For example, the hippocampus is critical for forming memories; the nervous system contains nerves, which are bundles of fibers that originate from the brain and spinal cord, branch to innervate every part of the body. Nerves are made of the axons of neurons, along with a variety of membranes that wrap around and segregate them into nerve fascicles; the vertebrate nervous system is divided into the peripheral nervous systems. The central nervous system consists of the brain and spinal cord, while the peripheral nervous system is made up of all the nerves outside of the CNS that connect it to the rest of the body; the PNS is further subdivided into the autonomic nervous systems. The somatic nervous system is made up of "afferent" neurons, which bring sensory information from the sense organs to the CNS, "efferent" neurons, which carry motor instructions out to the muscles; the autonomic nervous system has two subdivisions, the sympathetic and the parasympathetic, which are important for regulating the body's basic internal organ functions such as heartbeat, breathing and salivation.
Autonomic nerves, like somatic nerves, contain efferent fibers. In anatomy in general and neuroanatomy in particular, several sets of topographic terms are used to denote orientation and location, which are referred to the body or brain axis; the pairs of terms used most in neuroanatomy are: Dorsal and ventral: dorsal loosely refers to the top or upper side, ventral to the bottom or lower side. These descriptors referred to dorsum and ventrum – back and belly – of the body; the case of the head and the brain is peculiar, since the belly does not properly extend into the head, unless we assume that the mouth represents an extended belly element. Therefore, in common use, those brain parts that lie close to the base of the cranium, through it to the mouth cavity, are called ventral – i.e. at its bottom or lower side, as defined above – whereas
Anatomical terms of neuroanatomy
This article describes anatomical terminology, used to describe the central and peripheral nervous systems - including the brain, spinal cord, nerves. Neuroanatomy, like other aspects of anatomy, uses specific terminology to describe anatomical structures; this terminology helps ensure that a structure is described with minimal ambiguity. Terms help ensure that structures are described depending on their structure or function. Terms are derived from Latin and Greek, like other areas of anatomy are standardised based on internationally accepted lexicons such as Terminologia Anatomica. To help with consistency and other species are assumed when described to be in standard anatomical position, with the body standing erect and facing observer, arms at sides, palms forward. Anatomical terms of location depend on the location and species, being described. To understand the terms used for anatomical localisation, consider an animal with a straight CNS, such as a fish or lizard. In such animals the terms "rostral", "caudal", "ventral" and "dorsal" mean towards the rostrum, towards the tail, towards the belly and towards the back.
For a full discussion of those terms, see anatomical terms of location. For many purposes of anatomical description and directions are relative to the standard anatomical planes and axes; such reference to the anatomical planes and axes is called the stereotactic approach. Standard terms used throughout anatomy include anterior / posterior for the front and back of a structure, superior / inferior for above and below, medial / lateral for structures close to and away from the midline and proximal / distal for structures close to and far away from a set point; some terms are used more in neuroanatomy, particularly: Rostral and caudal: In animals with linear nervous systems, the term rostral is synonymous with anterior and the term caudal is synonymous with posterior. Due to humans having an upright posture, their nervous system is considered to bend about 90°; this is considered to occur at the junction of the diencephalon. Thus, the terminology changes at either side of the midbrain-diencephalic junction.
Superior to the junction, the terminology is the same as in animals with linear nervous systems. Inferior to the midbrain-diencephalic junction the term rostral is synonymous with superior and caudal is synonymous with inferior. Dorsal and ventral: In animals with linear nervous systems, the term dorsal is synonymous with superior and the term ventral is synonymous with inferior. In humans, however the terminology differs on either side of the midbrain-diencephalic junction. Superior to the junction, the terminology is the same as in animals with linear nervous systems. However, inferior to the midbrain-diencephalic junction the term dorsal is synonymous with posterior and ventral is synonymous with anterior. Contralateral and ipsilateral referring to a corresponding position on the opposite left or right side and on the same side respectively. Standard anatomical planes and anatomical axes are used to describe structures in animals. In humans and most other primates the axis of the central nervous system is not bent.
This means that there are certain major differences that reflect the distortion of the brains of the Hominidae. For example, to describe the human brain, "rostral" still means "towards the face", or at any rate, the interior of the cranial cavity just behind the face. However, in the brain "caudal" means not "towards the tail", but "towards the back of the cranial cavity". Alternative terms for this rostro-caudal axis of the brain include antero-posterior axis. "Dorsal" means "in the direction away from the spinal cord i.e. in the direction of the roof of the cranial cavity". "Ventral" means downwards towards floor of the cranial cavity and thence to the body. They lie on the superior-inferior or Dorsoventral axis; the third axis passes through the ears, is called the left-right, or lateral axis. These three axes of the human brain match the three planes within which they lie though the terms for the planes have not been changed from the terms for the bodily planes; the most used reference planes are: Axial, the plane, horizontal and parallel to the axial plane of the body in the standard anatomical position.
It contains the medial axes of the brain. Coronal, a vertical plane that passes through both ears, contains the lateral and dorsoventral axes. Sagittal, a vertical plane that passes from between the nostrils, between the cerebral hemispheres, dividing the brain into left and right halves, it contains the medial axes of the brain. A parasagittal plane is any plane parallel to the sagittal plane. Specific terms are used for peripheral nerves. An afferent nerve fiber is a fibre originating at the present point. For example, a striatal afferent is an afferent originating at the striatum. An efferent nerve fiber is one. For example, a cortical efferent is a fibre coming from elsewhere, arriving to the cortex. Note that, the opposite of the direction in which the nerve fibre conducts signals. Specific terms are used to describe the route of a nerve or nerve fibre: A chiasm i
Middle frontal gyrus
The middle frontal gyrus makes up about one-third of the frontal lobe of the human brain. The middle frontal gyrus, like the inferior frontal gyrus and the superior frontal gyrus, is more of a region in the frontal gyrus than a true gyrus; the borders of the middle frontal gyrus are the inferior frontal sulcus below.
Broca's area or the Broca area or is a region in the frontal lobe of the dominant hemisphere the left, of the brain with functions linked to speech production. Language processing has been linked to Broca's area since Pierre Paul Broca reported impairments in two patients, they had lost the ability to speak after injury to the posterior inferior frontal gyrus of the brain. Since the approximate region he identified has become known as Broca's area, the deficit in language production as Broca's aphasia called expressive aphasia. Broca's area is now defined in terms of the pars opercularis and pars triangularis of the inferior frontal gyrus, represented in Brodmann's cytoarchitectonic map as Brodmann area 44 and Brodmann area 45 of the dominant hemisphere. Functional magnetic resonance imaging has shown language processing to involve the third part of the inferior frontal gyrus the pars orbitalis, as well as the ventral part of BA6 and these are now included in a larger area called Broca's region.
Studies of chronic aphasia have implicated an essential role of Broca's area in various speech and language functions. Further, fMRI studies have identified activation patterns in Broca's area associated with various language tasks. However, slow destruction of the Broca's area by brain tumors can leave speech intact, suggesting its functions can shift to nearby areas in the brain. Broca's area is identified by visual inspection of the topography of the brain either by macrostructural landmarks such as sulci or by the specification of coordinates in a particular reference space; the used Talairach and Tournoux atlas projects Brodmann's cytoarchitectonic map onto a template brain. Because Brodmann's parcelation was based on subjective visual inspection of cytoarchitectonic borders and Brodmann analyzed only one hemisphere of one brain, the result is imprecise. Further, because of considerable variability across brains in terms of shape and position relative to sulcal and gyral structure, a resulting localization precision is limited.
Broca's area in the left hemisphere and its homologue in the right hemisphere are designations used to refer to the triangular part of inferior frontal gyrus and the opercular part of inferior frontal gyrus. The PTr and POp are defined by structural landmarks that only probabilistically divide the inferior frontal gyrus into anterior and posterior cytoarchitectonic areas of 45 and 44 by Brodmann's classification scheme. Area 45 receives more afferent connections from the prefrontal cortex, the superior temporal gyrus, the superior temporal sulcus, compared to area 44, which tends to receive more afferent connections from motor and inferior parietal regions; the differences between area 45 and 44 in cytoarchitecture and in connectivity suggest that these areas might perform different functions. Indeed, recent neuroimaging studies have shown that the PTr and Pop, corresponding to areas 45 and 44 play different functional roles in the human with respect to language comprehension and action recognition/understanding.
For a long time, it was assumed that the role of Broca's area was more devoted to language production than language comprehension. However, there is evidence to demonstrate that Broca's area plays a significant role in language comprehension. Patients with lesions in Broca's area who exhibit agrammatical speech production show inability to use syntactic information to determine the meaning of sentences. A number of neuroimaging studies have implicated an involvement of Broca's area of the pars opercularis of the left inferior frontal gyrus, during the processing of complex sentences. Further, it has been found in functional magnetic resonance imaging experiments involving ambiguous sentences result in a more activated inferior frontal gyrus. Therefore, the activity level in the inferior frontal gyrus and the level of lexical ambiguity are directly proportional to each other, because of the increased retrieval demands associated with ambiguous content. There is specialisation for particular aspects of comprehension within Broca's area.
Work by Devlin et al. showed in a repetitive transcranial magnetic stimulation study that there was an increase in reaction times when performing a semantic task under rTMS aimed at the pars triangularis. The increase in reaction times is indicative that that particular area is responsible for processing that cognitive function. Disrupting these areas via TMS disrupts computations performed in the areas leading to an increase in time needed to perform the computations. Work by Nixon et al. showed that when the pars opercularis was stimulated under rTMS there was an increase in reaction times in a phonological task. Gough et al. performed an experiment combining elements of these previous works in which both phonological and semantic tasks were performed with rTMS stimulation directed at either the anterior or the posterior part of Broca's area. The results from this experiment conclusively distinguished anatomical specialisation within Broca's area for different components of language comprehension.
Here the results showed that under rTMS stimulation: Semantic tasks only showed a decrease in reaction times when stimulation was aimed at the anterior part of Broca's area Phonological tasks showed a decrease in reaction times when stimulation was aimed at the posterior part of Broca's area (where a decrease of 6% was seen compared t
Superior frontal sulcus
The superior frontal sulcus is a sulcus between the superior frontal gyrus and the middle frontal gyrus. Inferior frontal sulcus
Orbital part of frontal bone
The orbital or horizontal part of the frontal bone consists of two thin triangular plates, the orbital plates, which form the vaults of the orbits, are separated from one another by a median gap, the ethmoidal notch. The inferior surface of each orbital plate is smooth and concave, presents, under cover of the zygomatic process, a shallow depression, the lacrimal fossa, for the lacrimal gland; the superior surface is convex, marked by depressions for the convolutions of the frontal lobes of the brain, faint grooves for the meningeal branches of the ethmoidal vessels. The ethmoidal notch separates the two orbital plates; the margins of the notch present several half-cells which, when united with corresponding half-cells on the upper surface of the ethmoid, complete the ethmoidal air cells. Two grooves cross these edges transversely; the anterior canal transmits the nasociliary nerve and anterior ethmoidal vessels, the posterior, the posterior ethmoidal nerve and vessels. In front of the ethmoidal notch, on either side of the frontal spine, are the openings of the frontal air sinuses.
These are two irregular cavities, which extend backward and lateralward for a variable distance between the two tables of the skull. Absent at birth, they are fairly well-developed between the seventh and eighth years, but only reach their full size after puberty, they vary in size in different persons, are larger in men than in women. They are lined by mucous membrane, each communicates with the corresponding nasal cavity by means of a passage called the frontonasal duct; this article incorporates text in the public domain from page 137 of the 20th edition of Gray's Anatomy Anatomy photo:29:st-0202 at the SUNY Downstate Medical Center "Anatomy diagram: 34256.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01