Human Brain


Image Caption : Brain and Head Anatomy : The limbic system is composed of structures within and below the cortex, such as the hypothalamus, amygdala, hippocampus, and cingulate gyrus. It deals with the interpretation of emotions, motivation, the process of learning, and the storage and retrieval of memory.

Brain

The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. It includes the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, and cerebellum. There are three different ways that a brain can be sectioned in order to view internal structures: a sagittal section cuts the brain left to right, as shown in Figureb, a coronal section cuts the brain front to back, as shown in Figurea, and a horizontal section cuts the brain top to bottom.

Cerebral Cortex

The outermost part of the brain is a thick piece of nervous system tissue called the cerebral cortex, which is folded into hills calledgyri (singular: gyrus) and valleys called sulci (singular: sulcus). The cortex is made up of two hemispheres-right and left-which are separated by a large sulcus. A thick fiber bundle called the corpus callosum (Latin: "tough body") connects the two hemispheres and allows information to be passed from one side to the other. Although there are some brain functions that are localized more to one hemisphere than the other, the functions of the two hemispheres are largely redundant. In fact, sometimes (very rarely) an entire hemisphere is removed to treat severe epilepsy. While patients do suffer some deficits following the surgery, they can have surprisingly few problems, especially when the surgery is performed on children who have very immature nervous systems.


Illustration shows coronal (front) and sagittal (side) sections of a human brain. In the coronal section, the large upper part of the brain, called the cerebral cortex, is divided into left and right hemispheres. A cavity resembling butterfly wings exists between the left and right cortex. The corpus callosum is a band that connects the two hemispheres together, just above this cavity. The surface of the cerebral cortex contains bumpy protrusions called gyri. The cerebral cortex is anchored by the brain stem, which connects with the spinal cord. On either side of the brainstem tucked beneath the cerebral cortex is the cerebellum. The surface of the cerebellum is bumpy, but not as bumpy as the cerebral cortex. The sagittal section reveals that the cerebral cortex makes up the front and top part of the brain, while the brainstem and cerebellum make up the lower back part. The oval thalamus sits in the cavity in the middle of the cerebral cortex. The corpus callosum wraps around the top part thalamus. The basal ganglia wraps around the corpus callosum, starting at the lower front part of the brain and continuing three-quarters of the way around so the back end almost meets the front end. The basal ganglia is separated into segments that are connected along the top and bottom. The amygdala is a spherical structure at the end of the basal ganglia.

These illustrations show the (a) coronal and (b) sagittal sections of the human brain.

In other surgeries to treat severe epilepsy, the corpus callosum is cut instead of removing an entire hemisphere. This causes a condition called split-brain, which gives insights into unique functions of the two hemispheres. For example, when an object is presented to patients' left visual field, they may be unable to verbally name the object (and may claim to not have seen an object at all). This is because the visual input from the left visual field crosses and enters the right hemisphere and cannot then signal to the speech center, which generally is found in the left side of the brain. Remarkably, if a split-brain patient is asked to pick up a specific object out of a group of objects with the left hand, the patient will be able to do so but will still be unable to vocally identify it.

Each cortical hemisphere contains regions called lobes that are involved in different functions. Scientists use various techniques to determine what brain areas are involved in different functions: they examine patients who have had injuries or diseases that affect specific areas and see how those areas are related to functional deficits. They also conduct animal studies where they stimulate brain areas and see if there are any behavioral changes. They use a technique called transmagnetic stimulation (TMS) to temporarily deactivate specific parts of the cortex using strong magnets placed outside the head; and they use functional magnetic resonance imaging (fMRI) to look at changes in oxygenated blood flow in particular brain regions that correlate with specific behavioral tasks. These techniques, and others, have given great insight into the functions of different brain regions but have also showed that any given brain area can be involved in more than one behavior or process, and any given behavior or process generally involves neurons in multiple brain areas. That being said, each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital. Figure illustrates these four lobes of the human cerebral cortex.

Sagittal, or side view of the human brain shows the different lobes of the cerebral cortex. The frontal lobe is at the front center of the brain. The parietal lobe is at the top back part of the brain. The occipital lobe is at the back of the brain, and the temporal lobe is at the bottom center of the brain. The motor cortex is the back of the frontal lobe, and the olfactory bulb is the bottom part. The somatosensory cortex is the front part of the parietal lobe. The brainstem is beneath the temporal lobe, and the cerebellum is beneath the occipital lobe.

The human cerebral cortex includes the frontal, parietal, temporal, and occipital lobes.

The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which processes smells. The frontal lobe also contains the motor cortex, which is important for planning and implementing movement. Areas within the motor cortex map to different muscle groups, and there is some organization to this map, as shown in Figure. For example, the neurons that control movement of the fingers are next to the neurons that control movement of the hand. Neurons in the frontal lobe also control cognitive functions like maintaining attention, speech, and decision-making. Studies of humans who have damaged their frontal lobes show that parts of this area are involved in personality, socialization, and assessing risk.

Diagram shows the location of motor control for various muscle groups on the right hemisphere cerebral cortex. From the top middle of the motor cortex to the bottom right, the order of areas controlled is toes, ankles, knees, hips, trunk, shoulders, elbows, wrists, hands, fingers, thumbs, neck, eyebrows and eyelids, eyeballs, face, lips, jaw, tongue, salivation, chewing and swallowing.

Different parts of the motor cortex control different muscle groups. Muscle groups that are neighbors in the body are generally controlled by neighboring regions of the motor cortex as well. For example, the neurons that control finger movement are near the neurons that control hand movement.

The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and also reading. Two of the parietal lobe's main functions are processing somatosensation-touch sensations like pressure, pain, heat, cold-and processing proprioception-the sense of how parts of the body are oriented in space. The parietal lobe contains a somatosensory map of the body similar to the motor cortex.

The occipital lobe is located at the back of the brain. It is primarily involved in vision-seeing, recognizing, and identifying the visual world.

The temporal lobe is located at the base of the brain by your ears and is primarily involved in processing and interpreting sounds. It also contains the hippocampus (Greek for "seahorse")-a structure that processes memory formation. The hippocampus is illustrated in Figure. The role of the hippocampus in memory was partially determined by studying one famous epileptic patient, HM, who had both sides of his hippocampus removed in an attempt to cure his epilepsy. His seizures went away, but he could no longer form new memories (although he could remember some facts from before his surgery and could learn new motor tasks).

Glossary

amygdala

structure within the limbic system that processes fear

arachnoid mater

spiderweb-like middle layer of the meninges that cover the central nervous system

basal ganglia

interconnected collections of cells in the brain that are involved in movement and motivation; also known as basal nuclei

basal nuclei

see basal ganglia

brainstem

portion of the brain that connects with the spinal cord; controls basic nervous system functions like breathing, heart rate, and swallowing

cerebellum

brain structure involved in posture, motor coordination, and learning new motor actions

cerebral cortex

outermost sheet of brain tissue; involved in many higher-order functions

choroid plexus

spongy tissue within ventricles that produces cerebrospinal fluid

cingulate gyrus

helps regulate emotions and pain; thought to directly drive the body's conscious response to unpleasant experiences

corpus callosum

thick fiber bundle that connects the cerebral hemispheres

cerebrospinal fluid (CSF)

clear liquid that surrounds the brain and spinal cord and fills the ventricles and central canal; acts as a shock absorber and circulates material throughout the brain and spinal cord.

dura mater

tough outermost layer that covers the central nervous system

frontal lobe

part of the cerebral cortex that contains the motor cortex and areas involved in planning, attention, and language

gyrus

(plural: gyri) ridged protrusions in the cortex

hippocampus

brain structure in the temporal lobe involved in processing memories

hypothalamus

brain structure that controls hormone release and body homeostasis

limbic system

connected brain areas that process emotion and motivation

meninge

membrane that covers and protects the central nervous system

occipital lobe

part of the cerebral cortex that contains visual cortex and processes visual stimuli

parietal lobe

part of the cerebral cortex involved in processing touch and the sense of the body in space

pia mater

thin membrane layer directly covering the brain and spinal cord

proprioception

sense about how parts of the body are oriented in space

somatosensation

sense of touch

spinal cord

thick fiber bundle that connects the brain with peripheral nerves; transmits sensory and motor information; contains neurons that control motor reflexes

sulcus

(plural: sulci) indents or "valleys" in the cortex

temporal lobe

part of the cerebral cortex that processes auditory input; parts of the temporal lobe are involved in speech, memory, and emotion processing

thalamus

brain area that relays sensory information to the cortex

ventricle

cavity within brain that contains cerebrospinal fluid

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BRAIN

Cerebrum

The cerebrum is the part of the brain that receives and processes conscious sensation, generates thought, and controls conscious activity. It is the uppermost and largest part of the brain and is divided into left and right hemispheres, which are joined by and communicated through the corpus callosum.

Each cerebral hemisphere is divided into five lobes, four of which have the same name as the bone over them: the frontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. A fifth lobe, the insula or Island of Reil, lies deep within the lateralsulcus.

Illustration of the lobes of the cerebrum

Cerebellum

The Cerebellum is a cauliflower-shaped section of the brain located in the hindbrain, at the bottom rear of the head, directly behind the pons. The cerebellum is a complex system mostly dedicated to the intricate coordination of voluntary movement, including walking and balance. Damage to the cerebellum leaves the sufferer with a gait that appears drunken and is difficult to control.

Ventricles and Cerebrospinal Fluid

A series of interconnected, fluid-filled cavities called ventricles lie within the brain. The fluid is cerebrospinal fluid (CSF), which also circulates over the outside of the brain and spinal cord.

Brain Stem

The brain stem is the part of the brain continuous with the spinal cord and comprising the medulla oblongata, pons, midbrain, and parts of the hypothalamus.

Tentorium

The tentorium is a fold of the dura mater, which separates the cerebellum from the cerebrum, and often encloses a process or plate of the skull called the bony tentorium.

NCI / NIH

The human brain is the main organ of the human central nervous system. It is located in the head, protected by the skull. It has the same general structure as the brains of other mammals, but with a more developed cerebral cortex. Large animals such as whales and elephants have larger brains in absolute terms, but when measured using a measure of relative brain size, which compensates for body size, the quotient for 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. Much of the size of the human brain comes from the cerebral cortex, especially the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. The area of the cerebral cortex devoted to vision, the visual cortex, is also greatly enlarged in humans compared to other animals.

The human cerebral cortex is a thick layer of neural tissue that covers the two cerebral hemispheres that make up most of the brain. This layer is folded in a way that increases the amount of surface area that can fit into the volume available. The pattern of folds is similar across individuals but shows many small variations. The cortex is divided into four lobes – the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. (Some classification systems also include a limbic lobe and treat the insular cortex as a lobe.) Within each lobe are numerous cortical areas, each associated with a particular function, including vision, motor control, and language. The left and right hemispheres are broadly similar in shape, and most cortical areas are replicated on both sides. Some areas, though, show strong lateralization, particularly areas that are involved in language. In most people, the left hemisphere is dominant for language, with the right hemisphere playing only a minor role. There are other functions, such as visual-spatial ability, for which the right hemisphere is usually dominant.

Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier, the human brain is susceptible to damage and disease. The most common forms of physical damage are closed head injuries such as a blow to the head or other trauma, a stroke, or poisoning by a number of chemicals that can act as neurotoxins, such as alcohol. Infection of the brain, though serious, is rare because of the protective blood-to brain and blood-to cerebral fluid barriers. The human brain is also susceptible to degenerative disorders, such as Parkinson's disease, forms of dementia including Alzheimer's disease, (mostly as the result of aging) and multiple sclerosis. A number of psychiatric conditions, such as 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 brain tumors and these can be benign or malignant.

There are some techniques for studying the brain that are used in other animals that are not suitable for use in humans and vice versa; it is easier to obtain individual brain cells taken from other animals, for study. It is also possible to use invasive techniques in other animals such as inserting electrodes into the brain or disabling certains parts of the brain in order to examine the effects on behaviour – techniques that are not possible to be used in humans. However, only humans can respond to complex verbal instructions or be of use in the study of important brain functions such as language and other complex cognitive tasks, but studies from humans and from other animals, can be of mutual help. Medical imaging technologies such as functional neuroimaging and EEG recordings are important techniques in studying the brain. The complete functional understanding of the human brain is an ongoing challenge for neuroscience.



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