| Cerebral hemisphere | |
|---|---|
 Human brain seen from front | |
 Right cerebral hemisphere Left cerebral hemisphere | |
| Details | |
| Identifiers | |
| Latin | hemisphaerium cerebri |
| NeuroNames | 241 |
| NeuroLex ID | birnlex_1796 |
| TA98 | A14.1.09.002 |
| TA2 | 5418 |
| FMA | 61817 |
| Anatomical terms of neuroanatomy | |
Thecerebrum, or the largest part of thevertebratebrain, is made up oftwo cerebral hemispheres. The deep groove known as thelongitudinal fissure divides the cerebrum into the left and right hemispheres, but the hemispheres remain united by thecorpus callosum, a large bundle ofnerve fibers in the middle of the brain whose primary function is to integrate sensory and motor signals between the hemispheres. Ineutherian (placental) mammals, other bundles of nerve fibers like the corpus callosum exist, including theanterior commissure, theposterior commissure, and thefornix, but compared with the corpus callosum, they are much smaller in size.
Broadly, the hemispheres are made up of two types of tissues. The thin outer layer of the cerebral hemispheres is made up ofgray matter, composed ofneuronal cell bodies,dendrites, andsynapses; this outer layer constitutes thecerebral cortex (cortex is Latin for "bark of a tree"). Below that is the larger inner layer ofwhite matter, composed ofaxons andmyelin.
Each hemisphere is further subdivided into a frontal, parietal, occipital, and temporallobe. Thecentral sulcus is a prominent fissure that separates both thefrontal lobe from theparietal lobe and theprimary motor cortex from theprimary somatosensory cortex. Three of the four lobes also have "poles": theoccipital pole, thefrontal pole, and thetemporal pole.
The two cerebral hemispheres are nicelymacroscopic mirror images of each other, with subtle anatomical differences between them, such as theYakovlevian torque that is sometimes seen in thehuman brain. Nevertheless, on a microscopic level, the functions of cells, the quantities ofneurotransmitters, and the types ofreceptors between the hemispheres is markedly asymmetrical.[1][2] While some of these hemispheric distribution differences are consistent across human beings, or even across some species, many observable distribution differences vary from individual to individual within a given species.
Each cerebral hemisphere has an outer layer ofcerebral cortex which is ofgrey matter and in theinterior of the cerebral hemispheres is an inner layer or core ofwhite matter known as thecentrum semiovale.[3] The interior portion of the hemispheres of thecerebrum includes thelateral ventricles, thebasal ganglia, and the white matter.[4]
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There are three poles of the cerebrum: the occipital pole, the frontal pole, and the temporal pole. The occipital pole is the posterior end of eachoccipital lobe in each hemisphere. It is more pointed than the rounder frontal pole. The frontal pole is at the frontmost part of thefrontal lobe in each hemisphere, and is more rounded than the occipital pole. The temporal pole is located between the frontal and occipital poles, and sits in the anterior part ofmiddle cranial fossa in each temporal lobe.[5]
If the upper part of either hemisphere is removed, at a level about 1.25 cm above thecorpus callosum, the central white matter will be exposed as an oval-shaped area, thecentrum semiovale, surrounded by a narrow convoluted margin of gray substance, and studded with numerous minute red dots (puncta vasculosa), produced by the escape of blood from divided blood vessels.[citation needed]
If the remaining portions of the hemispheres be slightly drawn apart a broad band of white substance, the corpus callosum, will be observed, connecting them at the bottom of thelongitudinal fissure; the margins of the hemispheres which overlap the corpus callosum are called thelabia cerebri.[6]
Each labium is part of the cingulate gyrus already described; and the groove between it and the upper surface of the corpus callosum is termed thecallosal sulcus.
If the hemispheres are sliced off to a level with the upper surface of the corpus callosum, the white substance of that structure will be seen connecting the two hemispheres.
The large expanse of medullary matter now exposed, surrounded by the convoluted margin of gray substance, is called the centrum semiovale. The blood supply to the centrum semiovale is from the superficialmiddle cerebral artery.[3] The cortical branches of this artery descend to provide blood to the centrum semiovale.[7]
The cerebral hemispheres are derived from thetelencephalon. They arise five weeks afterconception as bilateralinvaginations of the walls.The hemispheres grow round in a C-shape and then back again, pulling all structures internal to the hemispheres (such as theventricles) with them.Theintraventricular foramina (also called the foramina of Monro) allows communication with thelateral ventricles.Thechoroid plexus is formed fromependymal cells andvascularmesenchyme.[8]
Broad generalizations are often made inpopular psychology about certain functions (e.g. logic, creativity) beinglateralized, that is, located in the right or left side of the brain. These claims are often inaccurate, as most brain functions are actually distributed across both hemispheres. Most scientific evidence for asymmetry relates to low-level perceptual functions rather than the higher-level functions popularly discussed (e.g. subconscious processing of grammar, not "logical thinking" in general).[9] In addition to this lateralization of some functions, the low-level representations also tend to represent thecontralateral side of the body.
The best example of an established lateralization is that of Broca's and Wernicke's Areas (language) where both are often found exclusively on the left hemisphere. These areas frequently correspond to handedness however, meaning the localization of these areas is regularly found on the hemisphere opposite to the dominant hand. Function lateralization, such assemantics,intonation,accentuation, andprosody, has since been called into question and largely been found to have a neuronal basis in both hemispheres.[10][11]
Perceptual information is processed in both hemispheres, but is laterally partitioned: information from each side of the body is sent to the opposite hemisphere (visual information is partitionedsomewhat differently, but still lateralized). Similarly, motor control signals sent out to the body also come from the hemisphere on the opposite side. Thus,hand preference (which hand someone prefers to use) is also related to hemisphere lateralization.[citation needed]
In some aspects, the hemispheres are asymmetrical; the right side is slightly bigger. There are higher levels of theneurotransmitternorepinephrine on the right and higher levels ofdopamine on the left. The right hemisphere is more sensitive totestosterone[citation needed]. There is morewhite matter (longer axons) on the right and moregrey matter (cell bodies) on the left.[12]
Linearreasoning functions oflanguage such as grammar and word production are often lateralized to the left hemisphere of the brain. In contrast,holisticreasoning functions oflanguage such as intonation and emphasis are often lateralized to the right hemisphere of the brain. Other integrative functions such as intuitive orheuristic arithmetic, binaural sound localization, etc. seem to be more bilaterally controlled.[13]
Infarcts of the centrum ovale can occur.[3]
As a treatment forepilepsy the corpus callosum may be severed to cut the major connection between the hemispheres in a procedure known as acorpus callosotomy.
Ahemispherectomy is the removal or disabling of one of the hemispheres of the brain. This is a rareprocedure used in some extreme cases ofseizures which are unresponsive to other treatments.
