brain death

brain

[brān] that part of the central nervous system contained within the cranium, comprising the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon); it develops from the embryonic neural tube. The brain is a mass of soft, spongy, pinkish gray nerve tissue that weighs about 1.2 kg in a human being. It is connected at its base with the spinal cord, which is also part of the central nervous system. Called also encephalon. (See also color plates.)
The brain is made up of billions of nerve cells, intricately connected with each other. It contains centers" >nerve centers (groups of neurons and their connections) which control many involuntary functions, such as circulation, temperature regulation, and respiration, and interpret sensory impressions received from the eyes, ears, and other sense organs. Consciousness, emotion, thought, and reasoning are functions of the brain. It also contains centers or areas for associative memory which allow for recording, recalling, and making use of past experiences.Cerebrum. The largest and main portion of the brain, the cerebrum is made up of an outer coating, or cortex" >cerebral cortex, consisting of matter" >gray matter, several cell layers deep, covering the hemispheres" >cerebral hemispheres. The cortex is the thinking and reasoning brain, the intellect, as well as the part of the brain that receives information from the senses and directs the conscious movements of the body.
In appearance the cortex is rather like a relief map, with one very deep valley (longitudinal fissure) dividing it lengthwise into symmetrical halves, and each of the halves again divided by two major valleys and many shallower folds. The longitudinal fissure runs from the brow to the back of the head, and deep within it is a bed of matted white fibers, the corpus callosum, which connects the left and right hemispheres" >cerebral hemispheres.
The major folds of the cortex divide each hemisphere into four sections or lobes: the lobe" >occipital lobe at the back of the skull, the lobe" >parietal lobe at the side, the lobe" >frontal lobe at the forehead, and the lobe" >temporal lobe at the temple. The Senses. The major senses of vision" >vision and hearing" >hearing have been well mapped in the cortex; the center for vision is at the back, in the occipital lobe, and the center for hearing is at the side, in the temporal lobe. Two other areas have been carefully explored, the sensory and motor areas for the body, which parallel each other along the fissure of Rolando. In the sensory strip are the brain cells that register all sensations, and in the motor strip are the nerves that control the voluntary muscles. In both, the parts of the body are represented in an orderly way.
It is in the sensory areas of the brain that all perception takes place. Here sweet and sour, hot and cold, and the form of an object held in the hand are recognized. Here are sorted out the sizes, colors, depth, and space relationships of what the eye sees, and the timbre, pitch, intensity, and harmony of what the ear hears. The significance of these perceptions is interpreted in the cortex and other parts of the brain. A face is not merely seen; it is recognized as familiar or interesting or attractive. Remembering takes place at the same time as perception, so that other faces seen in the past, or experiences linked to that face are called up. Emotions may also be stirred. For this type of association the cortex draws on other parts of the brain by way of the communicating network of nerves. Memory. In the temporal lobe, near the auditory area, is a center for memory" >memory. This center appears to be a storehouse where memories are filed. When this area alone is stimulated, a particular event, a piece of music, or an experience long forgotten or deeply buried is brought to the individual's mind, complete in every detail. This is a very mechanical type of memory; when the stimulation is removed the memory ends. When it is applied again, the memory begins again, not where it left off, but from the beginning. Brainstem. This is the stemlike portion of the brain connecting the cerebral hemispheres with the spinal cord, and comprising midbrain, pons, and medulla oblongata. Some consider it to include the diencephalon. Thalamus. This organ lies beneath the cortex, deep within the cerebral hemispheres. It is a relay station for body sensations, and integrates these sensations on their way to the cortex. The thalamus is an organ of crude consciousness and of sensations of rough contact and extreme temperatures, either hot or cold. It is principally here that pain is felt. In the thalamus, responses are of the all-or-nothing sort; even mild stimuli would be felt as acutely painful if they were not graded and modified by the cortex.Hypothalamus. This organ lies below the thalamus, at the base of the cerebrum. It is small (no larger than a lump of sugar), but takes part in such vital activities as the ebb and flow of the body's fluids and the regulation of metabolism, blood sugar levels, and body temperature. It directs the body's many rhythms, including those of activity and rest, appetite and digestion, sexual desire, and menstrual and reproductive cycles. The hypothalamus is also the body's emotional brain. It is the integrating center of the autonomic nervous system, with its sympathetic and parasympathetic branches, and is located close to the pituitary gland. Midbrain. Just below the thalamus is the short narrow pillar of the midbrain. This contains a center for visual reflexes, such as moving the head and eyes, as well as a sound-activated center, obsolete in humans, for pricking up the ears. Medulla Oblongata. Below the midbrain is the medulla oblongata, the continuation upward of the spinal cord. In the medulla, the great trunk nerves, both motor and sensory, cross over, left to right and right to left, producing the puzzling phenomenon by which the left cerebral hemisphere controls the right half of the body, while the right hemisphere controls the left half of the body. This portion of the brain also contains the centers that activate the heart, blood vessels, and respiratory system. Cerebellum. The cerebellum (Latin for “little brain”) is attached to the back of the brainstem, under the curve of the cerebrum. It is connected, by way of the midbrain, with the motor area of the cortex and with the spinal cord, as well as with the semicircular canals, the organs of balance. The function of the cerebellum is apparently to blend and coordinate motion of the various muscles involved in voluntary movements. It does not direct these movements; that is the function of the cortex. The cortex, however, operates in terms of movements, not of muscles. As a conscious function the cortex may, for example, direct the arm to pick up a glass of water; the cerebellum, which operates entirely below the level of consciousness, then translates this instruction into detailed actions by the 32 different muscles in the hand, plus several more in the arm and shoulder. When the cerebellum is injured, the patient's movements are jerky and uncoordinated.Cranial Nerves. These are twelve nerves that arise within the skull. All but the olfactory nerve emerge from the brainstem. Most, with the important exception of the vagus nerve, serve the head and neck. See also cranial nerves.Protection of the Brain. The brain is protected by the bony skull and by three layers of membranes, the meninges. Between the middle and inner layer is a space filled with cerebrospinal fluid, which serves as a shock absorber. The same system of membranes and fluid protects the spinal cord. The brain is protected from harmful substances in the bloodstream by a barrier called the blood-brain barrier, which keeps some of the substances out of the brain entirely and delays the entry of others for hours or even days after they have penetrated the rest of the body.

Projection areas of the brain.brain abscess a localized suppurative lesion within the intracranial cavity; most cases are secondary to middle ear infections. Other causes include compound fracture of the skull with contamination of brain tissue, sinusitis, and infections of the face, lung or heart. Symptoms include fever, malaise, irritability, severe headache, convulsions, vomiting, and other signs of intracranial hypertension. Treatment consists of surgical removal of the infected area and administration of antibiotics.brain death the irreversible cessation of all brain activity for an appropriate observation period, at least 24 hours, so that cardiopulmonary functions must be artificially maintained. A presidential commission in the USA accepted criteria for such a diagnosis, including cessation of all brain functions, including cerebral functions and brainstem (reflex) functions; irreversibility of the cessation; establishment of the cause of coma, sufficient to explain the loss of brain function; exclusion of possibility of recovery of brain function; and persistence of the cessation for an appropriate period of observation or trial of therapy. Complicating conditions must also be excluded. Called also irreversible coma.brain scanning a nuclear medicine procedure for the detection of brain tumors, areas of stroke syndrome, abscesses, hematomas, and other intracranial lesions. A radiopharmaceutical, such as ^99mTc-pertechnetate, is injected intravenously and is carried to the brain, where it localizes around any lesion that alters the blood-brain barrier. A scintillation camera makes an image of the distribution of radioactivity in which a lesion appears as a region of increased activity. tomography" >Computed tomography brain scanning is an alternative procedure, which is more effective than radionuclide scans for the detection of some lesions.brain tumor a neoplasm of the intracranial portion of the central nervous system. Any abnormal growth within the skull creates a special problem because it is in a confined space and will press on normal brain tissue and interfere with the functions of the body controlled by the affected parts. This is true whether the tumor itself is benign or malignant. Fortunately, the functions of certain areas of the brain are well known, and a disturbance of some specific function guides the clinician to the affected area. If diagnosed early, a benign tumor often can be removed surgically with a good chance of recovery. Malignant tumors are more difficult to remove. The causes of brain tumors are not known. They are not common, but they can occur at any age and in any part of the brain. Some originate in the brain itself, while others metastasize from a tumor in another part of the body.
The symptoms of brain tumor vary and depend on the location and size of the tumor. Headache together with nausea is sometimes the first sign. The headache can be generalized or localized in one part of the head, and the pain is usually intense. Vomiting can be significant if it is sudden and without nausea. Disturbances of vision, loss of coordination in movement, weakness, and stiffness on one side of the body are also possible symptoms. Loss of sight, hearing, taste, or smell may result from brain tumor. A tumor can also cause a distortion of any of these senses, such as seeing flashes at the sides of the field of vision, or smelling odors or hearing sounds that do not exist. It can affect the ability to speak clearly or to understand the speech of others. Varying degrees of weakness or paralysis in the arms or legs may appear. A tumor may cause convulsions. Changes in personality or mental ability are rare in cases of brain tumor. When such changes occur they may take the form of lapses of memory or absentmindedness, mental sluggishness, or loss of initiative.wet brain brain edema.

death

[deth] the cessation of all physical and chemical processes that invariably occurs in all living organisms. (See also dying.) There is at present no standardized diagnosis of clinical death or precise definition of human death. The most widely known and commonly accepted means of determining death evolved from several medical conferences held in the late 1960s for the purpose of defining coma" >irreversible coma or nonfunctioning brain as a new criterion for death. The indications of deep irreversible coma (or death" >brain death) are (1) absolute unresponsiveness to externally applied stimuli; (2) cessation of movement and breathing, including no spontaneous breathing for three minutes after an artificial respirator has been turned off; and (3) complete absence of cephalic reflexes. The pupils of the eyes must be dilated and unresponsive to direct light.
Use of the electroencephalogram is also recommended as being of value in confirmation of irreversible coma or death. If there is a flat electroencephalographic reading at the time of apparent death and a second flat reading 24 hours later, then the patient may be declared dead.
There are two exceptions to the above criteria. These are in regard to patients exhibiting marked hypothermia (body temperature below 32.2°C), and those suffering from severe central nervous system depression as a result of drug overdose.
It is recognized that the above criteria are limited in that the notion of irreversibility is not readily agreed upon and may take on new meaning as medical technology advances. The criteria are especially helpful as complements to the traditional criteria of absence of heart beat and lack of spontaneous respiration as indications of death.
In 1981, a Presidential Commission for the Study of Ethical Problems in Medicine and Biomedical and Behavioral Research strongly recommended that all of the United States recognize the cessation of brain function as a definition of death, even in cases in which life-support systems could maintain respiratory and circulatory functions by artificial means.activation-induced cell death (AICD) recognition and deletion of T lymphocytes that have been activated and so induced to proliferate. T lymphocytes are activated when a foreign agent is perceived, and AICD thereby prevents them from overgrowth. It is particularly important for regulation of lymphocytes that recognize self antigens.black death bubonic plague; see plague.brain death (cerebral death) see brain death.clinical death the absence of heart beat (no pulse can be felt) and cessation of breathing.cot death (crib death) sudden infant death syndrome (SIDS).programmed cell death the theory that particular cells are programmed to die at specific sites and at specific stages of development.

ce·re·bral death

a clinical syndrome characterized by the permanent loss of cerebral and brainstem function, manifested by absence of responsiveness to external stimuli, absence of cephalic reflexes, and apnea. An isoelectric electroencephalogram for at least 30 minutes in the absence of hypothermia and poisoning by central nervous system depressants supports the diagnosis. Synonym(s): brain death

brain death

n. Irreversible brain damage and loss of brain function, as evidenced by cessation of breathing and other vital reflexes, unresponsiveness to stimuli, absence of muscle activity, and a flat electroencephalogram for a specific length of time.

The irreversible cessation of all functions of the entire brain, including the brain stem

brain death

End of life The irreversible cessation of all functions of the entire brain, including the brainstem. See Appropriate period of observation, Attending physician, Corroborating physician, Harvard criteria, Multiorgan donation.

Harvard criteria for brain death

Unreceptivity and unresponsiveness
No movement or breathing
No reflexes
Flat electroencephalogram (confirmatory)

In addition, the following must be present

Body temperature ≥ to 32º C
Absence of CNS depressants

brain death

(brān deth) Loss of brain function.

brain death

The absence of any signs of brain function. The state in which there are no reflex responses above the neck, there is no spontaneous breathing, and the ELECTROENCEPHALOGRAM shows no sign of electrical activity in the brain. This definition does not apply unless it can also be shown that a cause such as deep intoxication, or paralysing drugs, can be completely ruled out, and the effect of lowered body temperature (HYPOTHERMIA) eliminated.

ce·re·bral death

(ser'ĕ-brăl deth) Clinical syndrome characterized by permanent loss of cerebral and brainstem function, manifested by absence of responsiveness to external stimuli, absence of cephalic reflexes, and apnea.
Synonym(s): brain death.

brain death

brain death

brain death

brain′ death`

brain death

brain death

brain death

brain

death

ce·re·bral death

brain death

brain death

Harvard criteria for brain death

brain death

brain death

ce·re·bral death

brain death

Synonyms for brain death

noun death when respiration and other reflexes are absent

Synonyms

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