Human brain and skull
|System||Central nervous system
|Artery||Internal carotid arteries, vertebral arteries|
|Vein||Internal jugular vein, internal cerebral veins, external veins: (superior and inferior cerebral veins, and middle cerebral veins), basal vein, terminal vein, choroid vein, cerebellar veins|
The human brain and the spinal cord make up the central nervous system. The brain is contained in, and protected by the skull bones of the head. The largest part of the brain is the cerebrum, and other structures include the cerebellum and brainstem. The cerebrum is divided into two halves, the cerebral hemispheres, which are covered by the cerebral cortex. The cortex is an outer layer of gray matter that accounts for much of the size of the brain. The cortex consists of the main part, the neocortex, and the much smaller part of allocortex. The neocortex is made up of six neuronal layers, and the allocortex has three or four such layers. The cortex is folded in a way that increases the amount of surface area that can fit into the volume available. Each hemisphere is conventionally divided into four lobes – the frontal lobe, temporal lobe, parietal lobe, and occipital lobe. The frontal lobe is associated with executive functions including self-control, planning, reasoning, and abstract thought and the occipital lobe is dedicated to vision. Within each lobe there are also cortical areas associated with specific functions, such as the motor cortex, and sensorimotor cortex. Although the left and right hemispheres are broadly similar in shape and function, some functions are associated with a particular side of the brain, such as language (in the left) and visual-spatial ability (in the right). The hemispheres are connected by tracts known as commissures, the largest being the corpus callosum.
The cerebrum is connected by the brainstem to the spinal cord. The brainstem consists of the midbrain, pons, and medulla. The cerebellum is connected to the brainstem by pairs of peduncles. Within the cerebrum is the ventricular system of the brain. This consists of four ventricles in which cerebrospinal fluid is produced and circulated. Underneath the cerebral cortex several important structures are located, including the thalamus, epithalamus (and pineal gland), hypothalamus (and pituitary gland), and subthalamus; the limbic structures including the amygdala and hippocampus; the various nuclei of the basal ganglia; the basal forebrain structures, and the three circumventricular organs.
The brain is protected by the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier. However, the brain is still susceptible to damage, disease, and infection. Damage can be caused by trauma, or a loss of blood supply known as stroke. The brain is also susceptible to degenerative disorders, such as Parkinson's disease, forms of dementia including Alzheimer's disease, and multiple sclerosis. A number of psychiatric conditions, including schizophrenia and clinical depression, are thought to be associated with brain dysfunctions, although the nature of these is not well understood. The brain can also be the site of tumors, both benign and malignant. Malignant tumors mostly originate from sites outside the brain.
The study of the anatomy of the brain is neuroanatomy and the study of the function is neuroscience. A number of different techniques are used to study the brain. Brain specimens from other animals, which may be examined microscopically have been a traditional source of much information. Medical imaging technologies such as functional neuroimaging and EEG recordings are important techniques in studying the brain. Examining the history of people with brain injury has also provided great insight into the function of each part of the brain.
- 1 Structure
- 2 Development
- 3 Function
- 4 Physiology
- 5 Research
- 6 Clinical significance
- 7 Society and culture
- 8 History
- 9 Comparative anatomy
- 10 See also
- 11 References
- 12 Notes
- 13 External links
The adult human brain weighs on average about 1.2–1.4 kg (2.6–3.1 lb), or about 2% of total body weight, with a volume of around 1260 cm3 in men and 1130 cm3 in women, although there is substantial individual variation. Neurological differences between the sexes have not been shown to correlate in any simple way with IQ or other measures of cognitive performance.
The cerebrum consisting of the cerebral hemispheres forms the largest part of the brain and is situated above other brain structures. The outer region of the hemispheres, the cerebral cortex is gray matter consisting of cortical layers. Each hemisphere is divided into four main lobes.
The brainstem, resembling a stalk, attaches to and leaves the cerebrum at the start of the midbrain area. The brainstem includes the midbrain, pons, and medulla oblongata. Behind the brainstem is the cerebellum (little brain), the cerebellar cortex is narrowly furrowed horizontally and this distinguishes the cerebellum from any other brain structure.
The cerebrum, brainstem, cerebellum and spinal cord are covered by three membranes called meninges. The membranes are the tough dura mater; the middle arachnoid mater and the delicate inner pia mater. Between the arachnoid mater and the pia mater is the subarachnoid space, which contains the cerebrospinal fluid. In the cerebral cortex close to the pial basement membrane is a limiting membrane called the glia limitans and this is the outermost layer of the cortex. The living brain is very soft, having a gel-like consistency similar to soft tofu. The neural layers of the cortex are much of the brain's grey matter and the deeper subcortical regions of the brain made up of myelinated axons are the white matter.
The cerebrum is the largest part of the human brain, and is divided into nearly symmetrical left and right hemispheres by a deep groove, the longitudinal fissure. The outer part of the cerebrum is the cerebral cortex, made up of grey matter arranged in layers. It is 2 to 4 millimetres (0.079 to 0.157 in) thick, and deeply folded to give a convoluted appearance. Beneath the cortex is the white matter of the brain. The largest part of the cerebral cortex is the neocortex, which has six neuronal layers. The rest of the cortex is of allocortex, which has three or four layers. The hemispheres are connected by five commissures that span the longitudinal fissure, the largest of these is the corpus callosum. The surface of the brain is folded into ridges (gyri) and grooves (sulci), many of which are named, usually according to their position, such as frontal gyrus of the frontal lobe or the central sulcus separating the central regions of the hemispheres. There are many small variations in the secondary and tertiary folds.
Each hemisphere is conventionally divided into four lobes; the frontal lobe, parietal lobe, temporal lobe, and occipital lobe, named according to the skull bones that overlie them. Each lobe is associated with one or two specialised functions though there is some functional overlap between them. The occipital lobe is the location of the visual cortex and is dedicated to receiving and processing visual information.
The cortex is also divided into two main functional areas – a motor cortex and a sensory cortex. The primary sensory areas receive signals from the sensory nerves and tracts by way of relay nuclei in the thalamus. Primary sensory areas include the visual cortex of the occipital lobe, the auditory area in parts of the temporal lobe and insular cortex, and the somatosensory cortex in the parietal lobe. The primary motor cortex, which sends axons down to motor neurons in the brainstem and spinal cord. This area occupies the rear portion of the frontal lobe, directly in front of the somatosensory area. The remaining parts of the cortex, are called the association areas. These areas receive input from the sensory areas and lower parts of the brain and are involved in the complex processes of perception, thought, and decision-making. The main functions of the frontal lobe are to control attention, abstract thinking, behavior, problem solving tasks, and physical reactions and personality. The occipital lobe is the smallest lobe; its main functions are visual reception, visual-spatial processing, movement, and color recognition. There is a smaller occipital lobule in the lobe known as the cuneus. The temporal lobe controls auditory and visual memories, language, and some hearing and speech.
The cerebrum also contains the ventricles where the cerebrospinal fluid is produced and circulated. Below the corpus callosum is the septum pellucidum, a membrane that separates the lateral ventricles. Beneath the lateral ventricles is the thalamus and to the front and below this is the hypothalamus. The hypothalamus leads on to the pituitary gland. At the back of the thalamus is the brainstem.
The basal ganglia also called basal nuclei, are a set of structures deep within the hemispheres involved in behaviour and movement regulation. The largest component is the striatum, others are the globus pallidus, the substantia nigra and the subthalamic nucleus. Part of the dorsal striatum, the putamen, and the globus pallidus, lie separated from the lateral ventricles and thalamus by the internal capsule, whereas the caudate nucleus stretches around and abuts the lateral ventricles on their outer sides.
Below and in front of the striatum are a number of basal forebrain structures. These include the nucleus accumbens, nucleus basalis, diagonal band of Broca, substantia innominata, and the medial septal nucleus. These structures are important in the production of the neurotransmitter, acetylcholine, which is then distributed widely throughout the brain. The basal forebrain is considered to be the major cholinergic output of the central nervous system.
The cerebellum (Latin: little brain) is divided into an anterior lobe, a posterior lobe, and the flocculonodular lobe. The anterior and posterior lobes are connected in the middle by the vermis. The cerebellum has a much thinner outer cortex. Viewed from underneath between the two lobes is the third lobe the flocculonodular lobe. The cerebellum rests at the back of the cranial cavity, lying beneath the occipital lobes, and is separated from these by the cerebellar tentorium, a sheet of fibre.
It is connected to the midbrain of the brainstem by the superior cerebellar peduncles, to the pons by the middle cerebellar peduncles, and to the medulla by the inferior cerebellar peduncles. Enclosed between the cerebellum and the brainstem lies a space and a tube containing cerebrospinal fluid - the cerebral aqueduct and fourth ventricle. The cerebellum consists of an inner medulla of white matter and an outer cortex of grey matter. The surface of the cerebellum is richly folded. The cerebellum's anterior and posterior lobes appears to play a role in the coordination and smoothing of complex motor movements, and the flocculonodular lobe in the maintenance of balance although debate exists as to its cognitive, behavioral and motor functions.
The brainstem lies beneath the cerebrum and consists of the midbrain, pons and medulla. It lies in the back part of the skull, resting on the part of the base known as the clivus, and ends at the foramen magnum, a large opening in the occipital bone. The brainstem continues below this as the spinal cord.
Many nerve tracts, which transmit information to and from the cerebral cortex to the rest of the body, pass through the brainstem. Many of the cranial nerves[a] emerge from the brainstem. The brainstem also contains nuclei of many cranial and peripheral nerves, as well as nuclei involved in the regulation of many essential processes including breathing, control of eye movements and balance. The reticular formation, a network of nerves of ill-defined formation, is present within and along the length of the brainstem.
The human brain is composed of neurons, glial cells, neural stem cells and blood vessels. Types of neuron include interneurons, pyramidal cells including Betz cells, motor neurons, upper motor neurons, and lower motor neurons, and cerebellar Purkinje cells. Betz cells are the largest cells (by size of cell body) in the nervous system.
Types of glial cell are astrocytes (including Bergmann glia), oligodendrocytes, ependymal cells (including tanycytes), radial glial cells and microglia. Astrocytes are the largest of the glial cells. They are stellate cells with many processes radiating from their cell bodies. Some of these processes end as perivascular end-feet on capillary walls. The glia limitans of the cortex is made up of astrocyte foot processes that serve in part to contain the cells of the brain.
The adult human brain is estimated to contain 86±8 billion neurons, with a roughly equal number (85±10 billion) of non-neuronal cells. Out of these neurons, 16 billion (19%) are located in the cerebral cortex (including subcortical white matter), 69 billion (80%) are in the cerebellum.
Cerebrospinal fluid is clear, colourless fluid that circulates around the brain, within the brain in its ventricular system, and in the central canal of the spinal cord in the subarachnoid space, as well as within some spaces within the brain. There are four ventricles, two lateral ventricles, a third ventricle, and a fourth ventricle, which all produce cerebrospinal fluid. Most cerebrospinal fluid is created in the choroid plexuses of the ventricles. The third ventricle lies in the midline and is connnected to the two ventricles. A single tube, the cerebral aqueduct connects this ventricle to the fourth ventricle, found between the pons and cerebrum, and three separate tubes (one in the middle and two at the sides). From here, cerebrospinal fluid circulates around the brain and spinal cord in the subarachnoid space, between the arachnoid mater and pia mater.
At any one time, there is about 150mL of cerebrospinal fluid - most within the subarachnoid space. It is constantly being created and reabsorbed, and replaces about once every 5–6 hours. In other parts of the body, circulation in the lymphatic vasculature clears extracellular waste products from the cell tissue. For the tissue of the brain, such a system has not yet been identified. However, the presence of a glymphatic pathway has been proposed.
- Blood supply
Arteries bring oxygenated blood to the brain. The brain receives blood from two major sources: at the front, from the left and right internal carotid arteries, and from the back, the vertebral arteries. These two circulations join together (anastamose) in the Circle of Willis, a ring of connected arteries that lies in the interpeduncular cistern between the midbrain and pons.
- The internal carotid arteries are branches of the common carotid arteries. They enter the cranium through the carotid canal, travel through the cavernous sinus and enter the subarachnoid space. They then enter the Circle of Willis, with two branches, the anterior cerebral arteries emerging. These branches travel forward and then upward along the great longitudinal fissure, and supply the front and midline parts of the brain. One or more small anterior communicating arteries join the two anterior cerebral arteries shortly after they emerge as branches. The internal carotid arteries continue forward as the middle cerebral arteries. They travel sideways along part of the skull called the sphenoid bone, then upwards through the insula cortex, where final branches arise. The middle cerebral arteries send branches along their length.
- The vertebral arteries emerge as branches of the left and right subclavian arteries. They travel upward through transverse foramina – spaces in the cervical vertebrae and then emerge as two vessels, one on the left and one on the right of the medulla. They join in front of the middle part of the medulla to form the larger basilar artery, which sends multiple branches to supply the medulla, pons, and two to the cerebellum.[b] Finally, the basilar artery divides into two posterior cerebral arteries. These travel outwards, around the superior cerebellar peduncles, and along the top of the tentorium cerebelli, where it sends branches to supply the temporal and occipital lobes. Each posterior cerebral artery sends a small posterior communicating artery to join with the internal carotid arteries.
- Blood drainage
Veins drain deoxygenated blood from the brain. The brain has two main networks of veins: an exterior network, which sits on the cerebrum and has three braches, and an interior network. These two networks communicate via joining (anastamosing) veins. The veins of the brain drain into larger cavities called sinuses usually situated between the dura mater and the covering of the skull. Blood from the cerebellum and midbrain drains into the great cerebral vein, whereas from the medulla and pons of the brainstem have a variable pattern of drainage, either into the spinal veins or into adjacent cerebral veins.
The blood in the deep part of the brain drains, through veins and networks of veins (plexi) into the cavernous sinus at the front, and the superior and inferior petrosal sinuses at the sides, and the inferior sagittal sinus at the back. Blood drains from the outer brain into the large superior saggital sinus, which rests in the midline on top of the brain. Blood from here joins with blood from the straight sinus at confluence of sinuses.
Blood from here drains into the left and right transverse sinuses. These then drain into the sigmoid sinuses, which receive blood from the cavernous sinus and superior and inferior petrosal sinuses. The sigmoid drains into the large internal jugular veins.
- The blood-brain barrier
The larger arteries throughout the brain supply blood to smaller capillaries. These blood vessels are joined by tight junctions and so do not let fluids seep in and leak out to the same degree as other parts of the body, creating the blood-brain barrier (BBB). The barrier is less permeable to larger molecules, but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (including anaesthetics and alcohol). There is a similar blood–cerebrospinal fluid barrier. The BBB is not present in areas of the brain that may respond to changes in body fluids, such as the pineal gland, area postrema, and some areas of the hypothalamus.
At the beginning of the third week of development, the embryonic ectoderm forms a thickened strip called the neural plate. By the fourth week of development the neural plate has widened to give a broad cephalic end, a less broad middle part and a narrow caudal end. These swellings represent the beginnings of the forebrain, midbrain and hindbrain. Neural crest cells (derived from the ectoderm) populate the lateral edges of the plate at the neural folds. In the fourth week in the neurulation stage the neural plate folds and closes to form the neural tube, bringing together the neural crest cells at the neural crest. The neural crest runs the length of the tube with cranial neural crest cells at the cephalic end and caudal neural crest cells at the tail. Cells detach from the crest and migrate in a craniocaudal (head to tail) wave inside the tube. Cells at the cephalic end will give rise to the brain, and cells at the caudal end will give rise to the spinal cord.
The tube flexes as it grows, forming the crescent-shaped cerebral hemispheres at the head. The cerebral hemispheres first appear on day 32. Early in the fourth week the cephalic part bends sharply forward in a cephalic flexure. This flexed part becomes the forebrain (prosencephalon); the adjoining curving part becomes the midbrain (mesencephalon) and the part caudal to the flexure will become the hindbrain (rhombencephalon). In the fifth week of developmement five brain vesicles have formed. The forebrain separates into two vesicles an anterior telencephalon and a posterior diencephalon. The telencephalon will give rise to the cerebral cortex, basal ganglia, and related structures. The diencephalon will give rise to the thalamus and hypothalamus. The hindbrain also splits into two areas - the metencephalon and the mylencephalon. The metencephalon will give rise to the cerebellum and pons. The myelencephalon will give rise to the medulla oblongata.
A characteristic of the brain is gyrification, or wrinkling of the cortex. In the womb, the cortex starts off as smooth but starts to form fissures that begin to mark out the different lobes of the brain. These fissures form as a result of the growing hemispheres that increase in size due to a sudden growth in cells of the gray matter. The underlying white matter does not grow at the same rate and the hemispheres are crowded into the small cranial vault. The first cleft to appear in the fourth month is the lateral cerebral fossa. The expanding caudal end of the hemisphere has to curve over in a forward direction to fit into the restricted space. This covers the fossa and turns it into a much deeper ridge known as the lateral sulcus and this marks out the temporal lobe. By the sixth month other sulci have formed that demarcate the frontal, parietal, and occipital lobes. A gene present in the human genome (ArhGAP11B) may play a major role in gyrification and encephalisation.
The motor system of the brain is responsible for the generation and control of movement. Generated movements pass from the brain through nerves to motor neurons in the body, which control the action of muscles. The corticospinal tract carries movements from the brain, through the spinal cord, to the torso and limbs. The cranial nerves carry movements related to the eyes, mouth and face.
Gross movement - such as locomotion and the movement of arms and legs - is generated in the motor cortex, divided into three parts: the primary motor cortex, found in the prefrontal gyrus and has sections dedicated to the movement of different body parts. These movements are supported and regulated by other two areas, lying anterior to the primary motor cortex: the premotor area and the supplementary motor area. The hands and mouth have a much larger area dedicated to them than other body parts, allowing finer movement; this has been visualised in a motor cortical homunculus. Impulses generated from the motor cortex travel along the corticospinal tract along the front side of the medulla, with crossing over (decussating) at the pyramids of the medulla. These then travel down the spinal cord, with most connecting to interneurons, in turn connecting to lower motor neurons within the grey matter that then transmit the impulse to move to muscles themselves. The cerebellum and basal ganglia, play a role in fine, complex and coordinated muscle movements. Connections between the cortex and the basal ganglia control muscle tone, posture and movement initiation, and are referred to as the extrapyramidal system.
The sensory nervous system is involved with the reception and processing of sensory information. This information is received through the cranial nerves, through tracts in the spinal cord, and directly at centers of the brain exposed to the blood. The brain also receives and interprets information from the special senses (vision, smell, hearing, and taste). Mixed motor and sensory signals are also integrated.
From the skin, the brain receives information about fine touch, pressure, pain, vibration and temperature. From the joints, the brain receives information about joint position. The sensory cortex is found just near the motor cortex, and, like the motor cortex, has areas related to sensation from different body parts. Sensation collected by a sensory receptor on the skin is changed to a nerve signal, that is passed up a series of neurons through tracts in the spinal cord. The posterior column–medial lemniscus pathway contains information about fine touch, vibration and position of joints. Neurons travels up the back part of the spinal cord to the back part of the medulla, where they connect with "second order" neurons that immediately swap sides. These neurons then travel upwards into the ventrobasal complex in the thalamus where they connect with "third order" neurons, and travel up to the sensory cortex. The spinothalamic tract carries information about pain, temperature, and gross touch. Neurons travel up the spinal cord and connect with second-order neurons in the reticular formation of the brainstem for pain and temperature, and also at the ventrobasal complex of the medulla for gross touch.
Vision is generated by light that hits the retina of the eye, which is transformed into a nerve signal by receptors and sent ultimately to the visual cortex in the occipital lobe. Vision from the left visual field lands on the right side of each retina (and vice versa) and passes through the optic nerve until some information changes sides, so that all information about one side of the visual field passes through tracts in the opposite side of the brain. The nerves reach the brain at the lateral geniculate nucleus, and travel through the optic radiation to reach the optic cortex.
Hearing and balance are both generated in the inner ear. The movement of liquids within the inner ear is generated by motion (for balance) and transmitted vibrations generated by the ossicle bones (for sound). This creates a nerve signal that passes through the vestibulocochlear nerve. From here, it passes through to the cochlear nuclei, the superior olivary nucleus, the medial geniculate nucleus, and finally the auditory radiation to the auditory cortex.
Smell is generated by receptor cells in the epithelium of the olfactory mucosa in the nasal cavity. This information passes through a relatively permeable part of the skull to the olfactory nerve. This nerve transmits to the neural circuitry of the olfactory bulb from where information is passed to the olfactory cortex.
Taste is generated from receptors on the tongue and passed along the facial and glossopharyngeal nerves into the solitary tract in the brainstem. Some taste information is also passed from the pharynx into this area via the vagus nerve. Information is then passed from here through the thalamus into the gustatory cortex.
The brain influences blood pressure and controls the rate of breathing, mainly by centres in the medulla.and pons. The vasomotor center of the medulla causes arteries and veins to be somewhat constricted at rest, as well as influencing the heart rate. It does this by influencing the sympathetic nervous system and parasympathetic via the vagus nerve. Information about blood pressure is generated by stretch receptors in the arch of aorta and passed to the brain along the vagus nerve, and about the stretch of the carotid sinus via the a nerve joining with the glossopharyngeal nerve. This information travels up to the solitary nucleus. Signals from here influence the vasomotor nucleus to adjust vein and artery constriction accordingly.
The respiratory centres of the medulla and pons control respiration, by generating a signal that is passed down the spinal cord, along the phrenic nerve to the diaphragm and other muscles. The respiratory centre is split into three parts. A dorsal respiratory group causes the desire to breath in (inhalation) and receives sensory information directly and from the body. The pneumotaxic center influences the duration of each breath. A ventral respiratory influences breathing during periods of heavy exertion (expiration). The respiratory center directly senses blood carbon dioxide and pH. Information about blood oxygen, carbon dioxide and pH levels are also sensed on the walls of arteries in the aortic arch and carotid bodies. This information stimulates chemoreceptors in the arteries, which pass information again via the vagus nerve and glossopharyngeal nerves to the respiratory nucleus. High carbon dioxide, an acidic pH, or low oxygen stimulate the respiratory centre. The desire to breathe in is also affected by stretch receptors in the lungs which, when activated, prevent the lungs from overinflating by transmitting information to the respiratory centres via the vagus nerve.
The brain, especially the hypothalamus, is heavily involved in regulating multiple bodily functions. The diencephalon include the neuroendocrine regulation, regulation of the circadian rhythm, control of the autonomic nervous system, regulation of fluid homeostasis, and food intake. The circadian rhythm is controlled by two main cell groups in the rostral and caudal hypothalamus. The rostral hypothalamus includes the suprachiasmatic nucleus, which through gene expression cycles generates roughly 24 long clock, and Ventrolateral preoptic nucleus. The caudal hypothalamus contains orexinergic neurons that control arousal through their projections to the ascending reticular activating system. The hypothalamus controls the pituitary gland through the release of peptides such as oxytocin, and vasopressin, as well as dopamine into the median eminence. The hypothalamus influences the autonomic nervous system, through ascending projections into autonomic cell groups in the brain stem. Through the autonomic projections, the hypothalamus is involved in regulating functions such as blood pressure, heart rate, breathing, sweating, and other homeostatic mechanisms. The hypothalamus also plays a role in thermal regulation, and when stimulated by the immune system, is capable of generating a fever. The hypothalamus is influenced by the kidneys. When blood pressure falls, the renin released by the kidneys stimulate the hypothalamus to elicit drinking behavior. The hypothalamus also regulated food intake through autonomic signals, and hormone release by the digestive system.
Language functions are generally localized to Wernicke's area and Broca's area. Wernicke's area is at the posterior part of the superior temporal gyrus of the dominant half of the brain, and seems to be responsible for creation and interpretation of spoken thought. Broca's area is located in the prefrontal cortex and prefrontal cortex, most commonly on the left side of the brain, and is responsible for the creation of motor activity responsible for speaking. These two areas are connected by the arcuate fasciculus. Areas of the cerebellum, basal ganglia and areas of the motor cortex related to the face and larynx also play a role in coordinating and regulating muscle movements during speech. There has been substantial debate over these pathways.
The study of how language is represented, processed, and acquired by the brain is neurolinguistics, which is a large multidisciplinary field drawing from cognitive neuroscience, cognitive linguistics, and psycholinguistics. This field originated from the 19th-century discovery that damage to different parts of the brain appeared to cause different symptoms: physicians noticed that individuals with damage to a portion of the left inferior frontal gyrus now known as Broca's area had difficulty in producing language (Broca's aphasia) whereas those with damage to a region in the left superior temporal gyrus, now known as Wernicke's area, had difficulty in understanding it.
Each hemisphere of the brain interacts primarily with one half of the body: the left side of the brain interacts with the right side of the body, and vice versa. The developmental cause for this is uncertain. Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both cross sides in the brainstem. Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called the optic chiasm, and half of the fibers from each nerve split off to join the other. The result is that connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain. Because each half of the retina receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa. Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field.
The left and right sides of the brain appear symmetrical, but they function asymmetrically. For example, the counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling the left hand. There are, however, several important exceptions, involving language and spatial cognition. The left frontal lobe is dominant for language. If a key language area in the left hemisphere is damaged, it can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere would cause only minor impairment to language skills.
A substantial part of current understanding of the interactions between the two hemispheres has come from the study of "split-brain patients"—people who underwent surgical transection of the corpus callosum in an attempt to reduce the severity of epileptic seizures. These patients do not show unusual behavior that is immediately obvious, but in some cases can behave almost like two different people in the same body, with the right hand taking an action and then the left hand undoing it. These patients, when briefly shown a picture on the right side of the point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable to describe it, but may be able to give an indication with the left hand of the nature of the object shown.
Emotions are generally defined as two step multicomponent processes involving elicitation, followed by psychological feelings, appraisal, expression, autonomic responses, and action tendencies. Attempts to localize basic emotions to certain brain regions have been controversial, with some research finding no evidence for specific locations corresponding to emotions, and instead circuitry involved in general emotional processes. The amygdala, orbitofrontal cortex, mid and anterior insula cortex and lateral prefrontal cortex, appeared to be involved approach related emotions, while weaker evidence was found for the ventral tegmental area, ventral pallidum and nucleus accumbens in incentive salience. Others, however, have found evidence of activation of specific regions, such as the basal ganglia in happiness, the subcallosal cingulate cortex in sadness, and amygdala in fear.
Executive functions is an umbrella term for various cognitive processes and sub-processes, that allow for the control of thought and behavior. These functions include the ability to filter information, or attention, the ability to manipulate working memory, the ability to switch tasks, response inhibition, and the ability to determine the relevance of information. The prefrontal cortex plays a significant role in executive functions. Neuroimaging during tasks testing executive function, such as stroop task and memory tasks, have found that cortical maturation of the prefrontal cortex correlates with executive function in children. Future planning involves activation of the dorsolateral prefrontal cortex, anterior cingulate cortex, angular prefrontal cortex, right prefrontal cortex, and supramarginal gyrus. Working memory manipulation involves the DLPFC, inferior frontal gyrus, and areas of the parietal cortex. Response inhibition involves multiple areas of the cortex, including the inferior frontal gyrus, and ventrolateral prefrontal cortex. Task shifting doesn't involve specific regions of the brain, but instead involves multiple regions of the prefrontal cortex and parietal lobe.
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The brain consumes up to twenty percent of the energy used by the human body, more than any other organ. Brain metabolism normally relies upon blood glucose as an energy source, but during times of low glucose (such as fasting, exercise, or limited carbohydrate intake), the brain will use ketone bodies for fuel with a smaller need for glucose. The brain can also utilize lactate during exercise. Long-chain fatty acids cannot cross the blood–brain barrier, but the liver can break these down to produce ketones. However, the medium-chain fatty acids, octanoic and heptanoic acids, can cross the barrier and be used by the brain. The brain stores glucose in the form of glycogen, albeit in significantly smaller amounts than that found in the liver or skeletal muscle.
Although the human brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization. The brain mostly uses glucose for energy, and deprivation of glucose, as can happen in hypoglycemia, can result in loss of consciousness. The energy consumption of the brain does not vary greatly over time, but active regions of the cortex consume somewhat more energy than inactive regions: this fact forms the basis for the functional brain imaging methods PET and fMRI. These functional imaging techniques produce a three-dimensional image of metabolic activity.
The brain is not fully understood, and research is ongoing. Neuroscientists, along with researchers from allied disciplines, study how the human brain works. The boundaries between the specialties of neuroscience, neurology and other disciplines such as psychiatry have faded as they are all influenced by basic research in neuroscience.
Neuroscience research has expanded considerably in recent decades. The "Decade of the Brain", an initiative of the United States Government in the 1990s, is considered to have marked much of this increase in research, and was followed in 2013 by the BRAIN Initiative. The Human Connectome Project was a five-year study launched in 2009 to analyse the anatomical and functional connections of parts of the brain, and has produced much data.
Information about the structure and function of the human brain comes from a variety of experimental methods, including animals and humans. Information about brain trauma and stroke has provided information about the function of parts of the brain and the effects of brain damage. Neuroimaging is used to visualise the brain and record brain activity. electrophysiology is used to measure, record and monitor the electrical activity of the cortex. This can be measuring local field potentials of cortical areas, or measuring the activity of a single neuron. An electroencephalogram can record the electrical activity of the cortex using electrodes placed non-invasively on the scalp.
Invasive measures include electrocorticography, which uses electrodes placed directly on the exposed surface of the brain. This method is used in cortical stimulation mapping, used in the study of the relationship between cortical areas and their systemic function. By using much smaller microelectrodes, single-unit recordings can be made from a single neuron that give a high spatial resolution and high temporal resolution. This has enabled the linking of brain activity to behaviour, and the creation of neuronal maps.
Functional neuroimaging techniques provide imaging that shows changes in brain activity. Functional magnetic resonance imaging has advantages over earlier methods of SPECT and positron emission tomography. Functional MRI offers higher resolutions and needs no radioactive materials. Another imaging method is functional near-infrared spectroscopy. These methods rely on the hemodynamic response. Changes in brain activity are closely coupled with changes in blood flow in those areas, and this has proved useful in mapping brain functions. The measurement of hemodynamic response, can be used to create images of the brain in which especially active and inactive regions are shown as distinct from one another. Functional MRI and PET are the most common techniques that use this response to map brain function. Any electrical current will generate a magnetic field and neural oscillations will induce weak magnetic fields – a functional neuroimaging method that looks at this is magnetoencephalography. Tractography uses MRI and image analysis to create 3D images of the nerve tracts of the brain. Connectograms give graphical representation of the connectomes (maps of neural connections) of the brain.
There have been reported differences in brain area volumes seen in some disorders, notably schizophrenia and dementia. This area of research is known as brain morphometry. Different biological approaches using imaging have given more insight for example into the disorders of depression and obsessive-compulsive disorder. A key source of information about the function of brain regions is the effects of damage to them.
Advances in neuroimaging have enabled objective and biological insights to be made into mental disorders, which can lead to a faster diagnosis, more accurate prognosis, and help monitor progress of the disorders.
Damage or disease of the brain can manifest in a wide variety of ways. When the head undergoes trauma, for example in contact sport, after a fall, or in traffic or work accidents, the brain may be affected, particularly if there is bleeding within the skull that compresses the brain tissue or damages its blood supply. Concussion, characterised by a confused or dazed state, may result from a traumatic injury. In addition to the site of injury, the opposite side of the brain may be affected, termed a contrecoup injury.
Neurodegenerative diseases result in progressive damage to different parts of the brain's function. Common examples include dementia such as Alzheimer's disease, alcoholic dementia or vascular dementia; Parkinson's disease; and other rarer infectious, genetic, or metabolic causes such as Huntington's disease, motor neuron diseases, HIV dementia, syphilis-related dementia and Wilson's disease. Neurodegenerative diseases can affect different parts of the brain, and can affect movement, memory, and cognition.
The brain, although protected by the blood-brain barrier, can be affected by infections including viruses, bacteria and fungi. Infection may be of the meninges (meningitis), the brain matter (encephalitis), or within the brain matter (such as a cerebral abscess). Creutzfeldt–Jakob disease and its variant (sometimes called human mad cow disease) are linked to prions. Kuru is a similar prion-borne degenerative brain disease affecting humans, (endemic only to Papua New Guinea tribes). There are also demyelinating and dysmyelinating diseases that damage the myelin sheath of neurons, that can have an infectious cause.
The most common cancers in the brains come from elsewhere in the body - most commonly the lung, breast and melanoma. Cancers of brain tissue can also occur, and originate from any tissue in and around the brain. Meningioma, cancer of the meninges around the brain, is more common than cancers of brain tissue. Cancers within the brain may cause symptoms related to their size or position, with symptoms including headache and nausea, or the gradual development of focal symptoms such as gradual difficulty seeing, swallowing, talking, or as a change of mood. Cancers are in general investigated through the use of CT scans and MRI scans. A variety of other tests including blood tests and lumbar puncture may be used to investigate for the cause of the cancer and evaluate the type and stage of the cancer. The corticosteroid dexamethasone is often given to decrease the swelling of brain tissue around a tumour. Surgery may be considered, however given the complex nature of many tumours or based on tumour stage or type, radiotherapy or chemotherapy may be considered more suitable.
Mental disorders, such as clinical depression, schizophrenia, bipolar disorder and post-traumatic stress disorder, may involve particular patterns of neuropsychological functioning related to various aspects of mental and somatic function. These disorders may be treated by psychotherapy, psychiatry, social intervention and personal recovery work or cognitive behavioural therapy; the underlying issues and associated prognoses vary significantly between individuals.
Seizures are thought to relate to abnormal electrical activity. Seizure activity can manifest as absence, focal effects such as limb movement or impediments of speech, or be generalized in nature. Status epilepticus refers to a seizure that has not terminated in a short timespan. A large number of factors can cause seizures, however many seizures occur without a definitive cause being found. In a person with epilepsy, risk factors for further seizures may include sleeplessness, drug and alcohol intake, and stress. Seizures may be assessed using EEG and various medical imaging techniques. Seizures can sometimes be managed with anticonvulsant drugs in addition to other treatments.
Many brain disorders are congenital. Tay-Sachs disease, fragile X syndrome, and Down syndrome are all linked to genetic and chromosomal errors. Normal development of the brain can be also be affected during pregnancy by nutritional deficiencies, teratogens, infectious diseases and by the use of recreational drugs and alcohol. Age is a major risk factor for many neurological and neurodegenerative disorders. Basal ganglia diseases can cause a variety of movement disorders.
A stroke is a decrease in blood supply to an area of the brain causing cell death and brain injury. This can lead to a wide range of stroke symptoms, including the "FAST" symptoms of facial droop, arm weakness, and speech difficulties (including difficulties speaking and difficulties finding words or forming sentences). Symptoms relate to the function of the affected area of the brain and can point to the likely site and cause of the stroke. Difficulties with movement, speech, or sight usually relate to the cerebrum, whereas imbalance, double vision, vertigo and symptoms affecting more than one side of the body usually relate to the brainstem or cerebellum.
Most strokes result from loss of blood supply, typically because of an embolus, rupture of a fatty plaque or narrowing of small arteries. Strokes can also result from bleeding within the brain. Transient ischemic attacks (TIAs) are strokes in which symptoms resolve within 24 hours. Investigation into the stroke will involve a medical examination (including a neurological examination) and the taking of a medical history, focusing on the duration of the symptoms and risk factors (including hypertension, atrial fibrillation, and smoking). Further investigation is needed in younger patients. An ECG and telemetry may be conducted to identify atrial fibrillation, an ultrasound to investigate for narrowing of the carotid arteries, and an echocardiogram to investigate for clots within the heart, diseases of the heart valves or a patent foramen ovale that may point to a cardiac cause. Blood tests are routinely done as part of the workup including diabetes tests and a lipid profile.
Some treatments for stroke are time-critical. These include clot dissolution or surgical removal of a clot for ischaemic strokes, and decompression for haemorrhagic strokes. As stroke is time critical, hospitals and even pre-hospital care of stroke involves expedited investigations - usually a CT scan to investigate for a haemorrhagic stroke and an CT or MR angiogram to evaluate arteries that supply the brain. MRI scans, not as widely available, may be able to demonstrate the affected area of the brain more accurately, particularly with ischaemic stroke.
Having experienced a stroke, a person may be admitted to a stroke unit, and treatments may be directed as preventing future strokes, including ongoing anticoagulation (such as aspirin or clopidogrel), antihypertensives, and lipid-lowering drugs. A multidisciplinary team including speech pathologists, physiotherapists, occupational therapists, psychologists play a large role in supporting a person affected by a stroke and their rehabilitation.
Brain death refers to an irreversible total loss of brain function. This is characterised by coma, loss of reflexes, and apnoea, however, the declaration of brain death varies geographically and is not always accepted. In some countries there is also a defined syndrome of brainstem death. Declaration of brain death can have profound implications as the declaration, under the principle of medical futility, will be associated with the withdrawal of life support, and as those with brain death often have organs suitable for organ donation. The process is often complicated by problems, including those related to poor communication and preparation of families.
When brain death is suspected, reversible differential diagnosis such as hypothermia-induced coma, electrolyte, neurological and drug-related cognitive suppression are first excluded. Testing for reflexes[c] can be of help in the decision, as can the absence of response and breathing. Clinical observations, including a total lack of responsiveness, a known diagnosis, and neural imaging evidence, may all play a role in the decision to pronounce brain death.
Society and culture
Understanding the mind–body problem, which is the relationship between the brain and the mind, is a significant challenge both philosophically and scientifically. This is because of the difficulty reconciling how mental activities, such as thoughts and emotions, can be implemented by physical structures such as neurons and synapses, or by any other type of physical mechanism. This difficulty was expressed by Gottfried Leibniz in an analogy known as Leibniz's Mill:
One is obliged to admit that perception and what depends upon it is inexplicable on mechanical principles, that is, by figures and motions. In imagining that there is a machine whose construction would enable it to think, to sense, and to have perception, one could conceive it enlarged while retaining the same proportions, so that one could enter into it, just like into a windmill. Supposing this, one should, when visiting within it, find only parts pushing one another, and never anything by which to explain a perception.
Doubt about the possibility of a mechanistic explanation of thought drove René Descartes, and most of humankind along with him, to dualism: the belief that the mind is to some degree independent of the brain. There has always, however, been a strong argument in the opposite direction. There is clear empirical evidence that physical manipulations of, or injuries to, the brain (for example by drugs or by lesions, respectively) can affect the mind in potent and intimate ways. For example, a person suffering from Alzheimer's disease – a condition that causes physical damage to the brain – also experiences a compromised mind. Similarly, someone who has taken a psychedelic drug may temporarily lose their sense of personal identity (ego death) or experience profound changes to their perception and thought processes. Likewise, a patient with epilepsy who undergoes cortical stimulation mapping with electrical brain stimulation would also, upon stimulation of his or her brain, experience various complex feelings, hallucinations, memory flashbacks, and other complex cognitive, emotional, or behavioral phenomena. Following this line of thinking, a large body of empirical evidence for a close relationship between brain activity and mental activity has led most neuroscientists and contemporary philosophers to be materialists, believing that mental phenomena are ultimately the result of, or reducible to, physical phenomena.
The size of the brain and a person's intelligence are not strongly related. Studies tend to indicate small to moderate correlations (averaging around 0.3 to 0.4) between brain volume and IQ. The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampi, and the cerebellum, but these only account for a relatively small amount of variance in IQ, which itself has only a partial relationship to general intelligence and real-world performance.
Other animals have larger brains than humans, including whales and elephants. When the ratio of body size to brain is taken into account, when measured using a measure of relative brain size, the human brain is almost twice as large as that of a bottlenose dolphin, and three times as large as that of a chimpanzee—though the quotient for a treeshrew's brain is larger than that of a human's.
Research has disproved some common misconceptions. These include the belief that neurons are not replaced after the age of two – neurogenesis has been shown to take place in some areas of the brain; the belief that vaccines cause autism, and that only ten per cent of the brain is used.
The Edwin Smith Surgical Papyrus, an ancient Egyptian treatise written in the 17th century BC, contains the earliest recorded reference to the brain. The hieroglyph for brain, occurring eight times in this papyrus, describes the symptoms, diagnosis, and prognosis of two traumatic injury to the head. References include to the external surface of the brain, the effects of injury (including seizures and aphasia), the meninges, and cerebrospinal fluid.
In the sixth and fifth centuries BC, Ancient Greek Pythagorean Alcmaeon of Croton first considered the brain to be the place where the mind was located. In the fourth century BC Hippocrates, believed the brain to be the seat of intelligence, and Aristotle, although mistaken about the function of the brain,[d] described the meninges and distinguished between the cerebrum and cerebellum. Herophilus of Chalcedon in the third and second centuries BC distinguished the cerebrum and the cerebellum, and provided the first clear description of the ventricles; and Erasistratus of Ceos experimented on living brains. Their works are now mostly lost, and we know about their achievements due mostly to secondary sources. Some of their discoveries had to be re-discovered a millennium after their death. Anatomist physician Galen in the second century AD, during the time of the Roman Empire, dissected the brains of sheep, monkeys, dogs, and pigs. He concluded that, as the cerebellum was denser than the brain, it must control the muscles, while as the cerebrum was soft, it must be where the senses were processed. Galen further theorized that the brain functioned by movement of animal spirits through the ventricles.
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A revolution took place in both neurology in particular and in anatomy in general when Andreas Vesalius published his De humani corporis fabrica in 1543. It includes detailed images depicting the ventricles, cranial nerves, pituitary gland, meninges, structures of the eye, the vascular supply to the brain and spinal cord, and an image of the peripheral nerves. Vesalius rejected work, such as the rete mirabile.[e] Vesalius rejected the common belief that the ventricles were responsible for brain function, arguing that many animals have similar ventricular system to humans, but no true intelligence. This followed work by Niccolò Massa who in 1536 discovered that the ventricles were in fact filled with fluid. In addition to recording many anatomical features such as the putamen and corpus collusum, Vesalius proposed that the brain was made up of seven pairs of 'brain nerves', each with a specialized function. Anatomist Archiangelo Piccolomini of Rome was first to distinguish anatomically between the cerebrum and cerebral cortex.
René Descartes proposed the theory of dualism to tackle the issue of the brain's relation to the mind. He suggested that the pineal gland was where the mind interacted with the body after recording the brain mechanisms responsible for circulating cerebrospinal fluid. This dualism likely provided impetus for later anatomists to further explore the relationship between the anatomical and functional aspects of brain anatomy.
Thomas Willis is considered a second pioneer in the study of neurology and brain science. In 1664 in Cerebri Anatome (Latin: Anatomy of the brain),[f] followed by Cerebral Pathology in 1667. In these he described the structure of the cerebellum, the ventricles, the cerebral hemispheres, the brainstem, and the cranial nerves, studied its blood supply; and proposed functions associated with different areas of the brain. The circle of Willis was named after his investigations into the blood supply of the brain, and he was the first to use the word "neurology." Willis removed the brain from the body when examining it, and rejected the commonly held view that the cortex only consisted of blood vessels and the view of the last two millenia that the cortex was only incidentally important.
In the late 19th century, Emil du Bois-Reymond, Johannes Peter Müller, and Hermann von Helmholtz showed neurons were electrically excitable and that their activity predictably affected the electrical state of adjacent neurons. Richard Caton presented his findings in 1875 about electrical phenomena of the cerebral hemispheres of rabbits and monkeys. In the 1820s, Jean Pierre Flourens pioneered the experimental method of damaging specific parts of animal brains describing the effects on movement and behavior.
Studies of the brain became more sophisticated after the invention of the microscope and the development of the a staining procedure by Camillo Golgi during the late 1890s that used a silver chromate salt to reveal the intricate structures of single neurons. This was used by Santiago Ramón y Cajal and led to the formation of the neuron doctrine, a revolutionary hypothesis that the neuron is the functional unit of the brain that is now of critical importance in our understanding of the modern brain. He used histology to uncover many cell types and proposed functions for the cells he saw. For this, Golgi and Cajal are considered the founders of twentieth century neuroscience, both sharing the Nobel prize in 1906 for their studies and discoveries in this field.
Charles Sherrington published his influential 1906 work The Integrative Action of the Nervous System examining the function of reflexes, evolutionary development of the nervous system, functional specialisation of the brain, and layout and cellular function of the central nervous system. John Farquhar Fulton, founded the Journal of Neurophysiology and published the first comprehensive textbook on the physiology of the nervous system during 1938. Neuroscience during the twentieth century began to be recognized as a distinct unified academic discipline, with David Rioch, Francis O. Schmitt, and Stephen Kuffler playing critical roles in establishing the field. Rioch originated the integration of basic anatomical and physiological research with clinical psychiatry at the Walter Reed Army Institute of Research, starting in the 1950s. During the same period, Schmitt established a neuroscience research program within the Biology Department at the Massachusetts Institute of Technology, bringing together biology, chemistry, physics, and mathematics. The first freestanding neuroscience department (then called Psychobiology) was founded in 1964 at the University of California, Irvine by James L. McGaugh. Kuffler started the Department of Neuroscience at Harvard Medical School in 1966. The word neuroscience itself arose through the The Neuroscience Research program, an inter-university and international organisation founded in 1962 by Francis Schmitt.
Paul Broca associated regions of the brain with specific functions following work on brain-damaged patients. John Hughlings Jackson, described the function of the motor cortex by watching the progression of seizures through the body in patients with epilepsy. Carl Wernicke described regions associated with language comprehension and production. Korbinian Brodmann's divided regions of the brain based on the appearance of cells. Sherrington, Papez and MacLean had identified many of the brainstem and limbic system functions mainly due to preparations from laboratory animals by 1950. The capacity of the brain to re-organisation and change with age age, and a critical development period to which plasticity of the brain was pioneered by Margaret Kennard, who experimented on monkeys during the 1930-40s.
Harvey Cushing is recognised as the first proficient brain surgeon within the entire world. Walter Dandy began the practice of vascular neurosurgery in the modern understanding during 1937, when he performed the very first surgical clipping of an intracranial aneurysm.
The human brain has many properties that are common to all vertebrate brains, including a basic division into three parts called the forebrain, midbrain, and hindbrain, with interconnected fluid-filled ventricles, and a set of generic vertebrate brain structures including the medulla oblongata and pons of the brainstem, the cerebellum, optic tectum, thalamus, hypothalamus, basal ganglia, olfactory bulb, and many others.
As a mammalian brain, the human brain has special features that are common to all mammalian brains, most notably a six-layered cerebral cortex and a set of associated structures, including the hippocampus and amygdala. The upper surface of the forebrain of other vertebrates is covered in a layer of neural tissue called the pallium. The pallium is a relatively simple three-layered cell structure. The hippocampus and the amygdala originate from the pallium but in mammals they are much more complex. Humans have more association cortex, sensory and motor parts, relative to simpler mammals, such as the rat and the cat.
As a hominid brain, the human brain is substantially enlarged even in comparison to the brain of a typical monkey. The sequence of evolution from Australopithecus (four million years ago) to Homo sapiens (modern man) was marked by a steady increase in brain size, particularly in the frontal lobes, which are associated with a variety of high-level cognitive functions. Differences in DNA, gene expression and gene–environment interactions help explain the differences between the function of the human brain and primates. The brain size has increased over the course of human evolution, altering the size and shape of the skull, from about 600 cm3 in Homo habilis to about 1500 cm3 in Homo neanderthalensis.
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As human's position changed and the manner in which the skull balanced on the spinal column pivoted, the brain expanded, altering the shape of the cranium.
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- Specifically the oculomotor, trochlear nerve, trigeminal nerve, abducens nerve, facial nerve, vestibulocochlear nerve, glossopharyngeal nerve, vagus nerve, accessory nerve and hypoglossal nerves.
- the anterior inferior cerebellar artery and superior cerebellar artery.
- Including the vestibulo-ocular reflex, corneal reflex, gag reflex and dilation of the pupils in response to light ,
- a thought that, while the heart was the seat of intelligence, the brain was a cooling mechanism for the blood. He reasoned that humans are more rational than the beasts because, among other reasons, they have a larger brain to cool their hot-bloodedness.
- This may be explained by Galen's dissections were all on animals – in particular, the rete mirabile is only well developed in ungulates
- Illustrated by architect Christopher Wren
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