The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head close to the sensory organs for senses such as vision; the brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. Physiologically, the function of the brain is to exert centralized control over the other organs of the body; the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, processes it in a variety of ways; this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar; the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context.
The most important is brain disease and the effects of brain damage, that are covered in the human brain article. The shape and size of the brain varies between species, identifying common features is difficult. There are a number of principles of brain architecture that apply across a wide range of species; some aspects of brain structure are common to the entire range of animal species. The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations.
It is possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.
British Journal of Anaesthesia
The British Journal of Anaesthesia is a monthly peer-reviewed international medical journal published by Elsevier on behalf of the Royal College of Anaesthetists. The journal covers all aspects of anaesthesia, perioperative medicine, intensive care medicine and pain management; the current editor-in-chief is Hugh C. Hemmings; the BJA was founded in 1923, one year after the first anaesthetic journal was published by the International Anaesthesia Research Society. The first Editor-in-Chief of the journal was H. M. Cohen, he was based in Manchester, UK, but born in New York City, he edited the journal from 1923 to 1928. Recent Editors-in-Chief include Charles Reilly and Jennifer Hunter; the BJA has a sister journal entitled BJA Education, first established in 2001 as BJA CEPD Reviews. This publication is a commission-only reviews journal, published monthly. BJA Education aims to strengthen the educational platform provided by the flagship journal, BJA; the mission statement of BJA Education is "Improving patient care by supporting continuing professional development in anaesthesia, critical care and perioperative medicine."
The current Editor-in-Chief is Jonathan P. Thompson; the most recent Editor-in-Chief prior to this was Jeremy Langton. The journal is abstracted and indexed in: According to the Journal Citation Reports, the journal has a 2017 Impact Factor of 6.499, ranking it second out of 31 journals in the Anesthesiology category. List of medical journals Official website
Cerebrospinal fluid is a clear, colorless body fluid found in the brain and spinal cord. It is produced by the specialised ependymal cells in the choroid plexuses of the ventricles of the brain, absorbed in the arachnoid granulations. There is about 125mL of CSF at any one time, about 500 mL is generated every day. CSF acts as a cushion or buffer for the brain, providing basic mechanical and immunological protection to the brain inside the skull. CSF serves a vital function in cerebral autoregulation of cerebral blood flow. CSF occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord, it fills the ventricles of the brain and sulci, as well as the central canal of the spinal cord. There is a connection from the subarachnoid space to the bony labyrinth of the inner ear via the perilymphatic duct where the perilymph is continuous with the cerebrospinal fluid. A sample of CSF can be taken via lumbar puncture; this can reveal the intracranial pressure, as well as indicate diseases including infections of the brain or its surrounding meninges.
Although noted by Hippocrates, it was only in the 18th century that Emanuel Swedenborg is credited with its rediscovery, as late as 1914 that Harvey W. Cushing demonstrated CSF was secreted by the choroid plexus. There is about 125–150 mL of CSF at any one time; this CSF circulates within the ventricular system of the brain. The ventricles are a series of cavities filled with CSF; the majority of CSF is produced from within the two lateral ventricles. From here, CSF passes through the interventricular foramina to the third ventricle the cerebral aqueduct to the fourth ventricle. From the fourth ventricle, the fluid passes into the subarachnoid space through four openings – the central canal of the spinal cord, the median aperture, the two lateral apertures. CSF is present within the subarachnoid space, which covers the brain, spinal cord, stretches below the end of the spinal cord to the sacrum. There is a connection from the subarachnoid space to the bony labyrinth of the inner ear making the cerebrospinal fluid continuous with the perilymph in 93% of people.
CSF moves in a single outward direction from the ventricles, but multidirectionally in the subarachnoid space. Fluid movement is pulsatile, matching the pressure waves generated in blood vessels by the beating of the heart; some authors dispute this, posing that there is no unidirectional CSF circulation, but cardiac cycle-dependent bi-directional systolic-diastolic to-and-fro cranio-spinal CSF movements. CSF is derived from blood plasma and is similar to it, except that CSF is nearly protein-free compared with plasma and has some different electrolyte levels. Due to the way it is produced, CSF has a higher chloride level than plasma, an equivalent sodium level. CSF contains 0.3% plasma proteins, or 15 to 40 mg/dL, depending on sampling site. In general, globular proteins and albumin are in lower concentration in ventricular CSF compared to lumbar or cisternal fluid; this continuous flow into the venous system dilutes the concentration of larger, lipid-insoluble molecules penetrating the brain and CSF.
CSF is free of red blood cells, at most contains only a few white blood cells. Any white blood cell count higher. At around the third week of development, the embryo is a three-layered disc, covered with ectoderm and endoderm. A tube-like formation develops in the midline, called the notochord; the notochord releases extracellular molecules that affect the transformation of the overlying ectoderm into nervous tissue. The neural tube, forming from the ectoderm, contains CSF prior to the development of the choroid plexuses; the open neuropores of the neural tube close after the first month of development, CSF pressure increases. As the brain develops, by the fourth week of embryological development three swellings have formed within the embryo around the canal, near where the head will develop; these swellings represent different components of the central nervous system: the prosencephalon and rhombencephalon. Subarachnoid spaces are first evident around the 32nd day of development near the rhombencephalon.
At this time, the first choroid plexus can be seen, found in the fourth ventricle, although the time at which they first secrete CSF is not yet known. The developing forebrain surrounds the neural cord; as the forebrain develops, the neural cord within it becomes a ventricle forming the lateral ventricles. Along the inner surface of both ventricles, the ventricular wall remains thin, a choroid plexus develops and releasing CSF. CSF fills the neural canal. Arachnoid villi are formed around the 35th week of development, with aracnhoid granulations noted around the 39th, continuing developing until 18 months of age; the subcommissural organ secretes SCO-spondin, which forms Reissner's fiber within CSF assisting movement through the cerebral aqueduct. It disappears during early development. CSF serves several purposes: Buoyancy: The actual mass of the human brain is about 1400–1500 grams; the brain therefore exists in neutral buoyancy, which allows the brain to maintain its density without being impaired by its own weight, which would cut off blood supply and kill neurons in the lower sections without CSF.
Protection: CSF protects the brain tissue from injury when jolted or hit, by providing a fluid buffer that acts as a shock absorber from some forms of mechanical injury. Prevention of brain ischemia: The prevention of brai
Cerebral circulation is the movement of blood through the network of cerebral arteries and veins supplying the brain. The rate of the cerebral blood flow in the adult is 750 milliliters per minute, representing 15% of the cardiac output; the arteries deliver oxygenated blood and other nutrients to the brain, the veins carry deoxygenated blood back to the heart, removing carbon dioxide, lactic acid, other metabolic products. Since the brain is vulnerable to compromises in its blood supply, the cerebral circulatory system has many safeguards including autoregulation of the blood vessels and the failure of these safeguards can result in a stroke; the amount of blood that the cerebral circulation carries is known as cerebral blood flow. The presence of gravitational fields or accelerations determine variations in the movement and distribution of blood in the brain, such as when suspended upside-down; the following description is based on idealized human cerebral circulation. The pattern of circulation and its nomenclature vary between organisms.
Blood supply to the brain is divided into anterior and posterior segments, relating to the different arteries that supply the brain. The two main pairs of arteries are the Internal carotid arteries and vertebral arteries; the anterior and posterior cerebral circulations are interconnected via bilateral posterior communicating arteries. They are part of the Circle of Willis. In case one of the supply arteries is occluded, the Circle of Willis provides interconnections between the anterior and the posterior cerebral circulation along the floor of the cerebral vault, providing blood to tissues that would otherwise become ischemic; the anterior cerebral circulation is the blood supply to the anterior portion of the brain. It is supplied by the following arteries: Internal carotid arteries: These large arteries are the medial branches of the common carotid arteries which enter the skull, as opposed to the external carotid branches which supply the facial tissues. Anterior cerebral artery Anterior communicating artery: Connects both anterior cerebral arteries and along the floor of the cerebral vault.
Middle cerebral artery The posterior cerebral circulation is the blood supply to the posterior portion of the brain, including the occipital lobes and brainstem. It is supplied by the following arteries: Vertebral arteries: These smaller arteries branch from the subclavian arteries which supply the shoulders, lateral chest and arms. Within the cranium the two vertebral arteries fuse into the basilar artery. Posterior inferior cerebellar artery Basilar artery: Supplies the midbrain and branches into the posterior cerebral artery Anterior inferior cerebellar artery Pontine branches Superior cerebellar artery Posterior cerebral artery Posterior communicating artery The venous drainage of the cerebrum can be separated into two subdivisions: superficial and deep; the superficial system is composed of dural venous sinuses, which have wall composed of dura mater as opposed to a traditional vein. The dural sinuses are therefore located on the surface of the cerebrum; the most prominent of these sinuses is the superior sagittal sinus which flows in the sagittal plane under the midline of the cerebral vault and inferiorly to the confluence of sinuses, where the superficial drainage joins with the sinus that drains the deep venous system.
From here, two transverse sinuses bifurcate and travel laterally and inferiorly in an S-shaped curve that form the sigmoid sinuses which go on to form the two jugular veins. In the neck, the jugular veins parallel the upward course of the carotid arteries and drain blood into the superior vena cava; the deep venous drainage is composed of traditional veins inside the deep structures of the brain, which join behind the midbrain to form the vein of Galen. This vein merges with the inferior sagittal sinus to form the straight sinus which joins the superficial venous system mentioned above at the confluence of sinuses. Cerebral blood flow is the blood supply to the brain in a given period of time. In an adult, CBF is 750 millilitres per minute or 15% of the cardiac output; this equates to an average perfusion of 50 to 54 millilitres of blood per 100 grams of brain tissue per minute. CBF is regulated to meet the brain's metabolic demands. Too much blood can raise intracranial pressure, which can damage delicate brain tissue.
Too little blood flow results if blood flow to the brain is below 18 to 20 ml per 100 g per minute, tissue death occurs if flow dips below 8 to 10 ml per 100 g per minute. In brain tissue, a biochemical cascade known as the ischemic cascade is triggered when the tissue becomes ischemic resulting in damage to and the death of brain cells. Medical professionals must take steps to maintain proper CBF in patients who have conditions like shock, cerebral edema, traumatic brain injury. Cerebral blood flow is determined by a number of factors, such as viscosity of blood, how dilated blood vessels are, the net pressure of the flow of blood into the brain, known as cerebral perfusion pressure, determined by the body's blood pressure. Cerebral perfusion pressure is defined as the mean arterial pressure minus the intracranial pressure. In normal individuals it should be above 50 mm Hg. Intracranial pressure should not be above 15 mm Hg ( ICP of 20 mm Hg is considered as Int
Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ or a tissue referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as the rate at which blood is delivered to tissue, or volume of blood per unit time per unit tissue mass; the SI unit is m3/, although for human organs perfusion is reported in ml/min/g. The word is derived from the French verb "perfuser" meaning to "pour over or through". All animal tissues require an adequate blood supply for life. Poor perfusion, that is, causes health problems, as seen in cardiovascular disease, including coronary artery disease, cerebrovascular disease, peripheral artery disease, many other conditions. Tests verifying that adequate perfusion exists are a part of a patient's assessment process that are performed by medical or emergency personnel; the most common methods include evaluating a body's skin color, temperature and capillary refill. During major surgery cardiothoracic surgery, perfusion must be maintained and managed by the health professionals involved, rather than left to the body's homeostasis alone.
As the lead surgeons are too busy to handle all hemodynamic control by themselves, specialists called perfusionists manage this aspect. There are more than one hundred thousand perfusion procedures annually. In 1920, August Krogh was awarded the Nobel Prize in Physiology or Medicine for his discovering the mechanism of regulation of capillaries in skeletal muscle. Krogh was the first to describe the adaptation of blood perfusion in muscle and other organs according to demands through the opening and closing of arterioles and capillaries. Malperfusion can refer to any type of incorrect perfusion though it refers to hypoperfusion; the meaning of the terms "overperfusion" and "underperfusion" is relative to the average level of perfusion that exists across all the tissues in an individual body. Perfusion levels differ from person to person depending on metabolic demand. Examples follow: Heart tissues are considered overperfused because they are receiving more blood than the rest of tissues in the organism.
In the case of skin cells, extra blood flow in them is used for thermoregulation of a body. In addition to delivering oxygen, blood flow helps to dissipate heat in a physical body by redirecting warm blood closer to its surface where it can help to cool a body through sweating and thermal dissipation. Many types of tumors, certain types, have been described as "hot and bloody" because of their overperfusion relative to the body overall. Overperfuson and underperfusion should not be confused with hypoperfusion and hyperperfusion, which relate to the perfusion level relative to a tissue's current need to meet its metabolic needs. For example, hypoperfusion can be caused when an artery or arteriole that supplies blood to a volume of tissue becomes blocked by an embolus, causing either no blood or at least not enough blood to reach the tissue. Hyperperfusion can be caused by producing hyperemia of a body part. Malperfusion called poor perfusion, is any type of incorrect perfusion. There is no official or formal dividing line between ischemia.
In equations, the symbol Q is sometimes used to represent perfusion when referring to cardiac output. However, this terminology can be a source of confusion since both cardiac output and the symbol Q refer to flow, whereas perfusion is measured as flow per unit tissue mass. Microspheres that are labeled with radioactive isotopes have been used since the 1960s. Radioactively labeled particles are injected into the test subject and a radiation detector measures radioactivity in tissues of interest. Application of this process is used to develop radionuclide angiography, a method of diagnosing heart problems. In the 1990s, methods for using fluorescent microspheres became a common substitute for radioactive particles. Perfusion of various tissues can be measured in vivo with nuclear medicine methods which are positron emission tomography and single photon emission computed tomography. Various radiopharmaceuticals targeted at specific organs are available, some of the most common are 99mTc labelled HMPAO and ECD for brain perfusion studied with SPECT 99mTc labelled Tetrofosmin and Sestamibi for myocardial perfusion imaging with SPECT 133Xe-gas for absolute quantification of brain perfusion with SPECT 15O-labeled water for brain perfusion with PET 82Rb-chloride for measuring myocardial perfusion with PET Two main categories of magnetic resonance imaging techniques can be used to measure tissue perfusion in vivo.
The first is based on the use of an injected contrast agent that changes the magnetic susceptibility of blood and thereby the MR signal, measured during bolus passage. The other category is based on arterial spin labelling, where arterial blood is magnetically tagged before it enters into the tissue being examined and the amount of labelling, measured and compared to a control recording obtained without spin labelling. Brain perfusion can be estimated with contrast-enhanced computed tomography. Perfusion can be determined by measuring the total thermal diffusion and separating it into thermal conductivity and perf
Jugular venous pressure
The jugular venous pressure is the indirectly observed pressure over the venous system via visualization of the internal jugular vein. It can be useful in the differentiation of different forms of lung disease. Classically three upward deflections and two downward deflections have been described; the upward deflections are the "a", "c" and "v" = venous filling The downward deflections of the wave are the "x" and the "y" descent. The patient is positioned at a 45° incline, the filling level of the external jugular vein determined. Visualize the internal jugular vein when looking for the pulsation. In healthy people, the filling level of the jugular vein should be less than 4 centimetres vertical height above the sternal angle. A pen-light can aid in discerning the jugular filling level by providing tangential light; the JVP is easiest to observe if one looks along the surface of the sternocleidomastoid muscle, as it is easier to appreciate the movement relative to the neck when looking from the side.
Like judging the movement of an automobile from a distance, it is easier to see the movement of an automobile when it is crossing one's path at 90 degrees, as opposed to coming toward one. Pulses in the JVP are rather hard to observe, but trained cardiologists do try to discern these as signs of the state of the right atrium; the JVP and carotid pulse can be differentiated several ways: multiphasic – the JVP "beats" twice in the cardiac cycle. In other words, there are two waves in the JVP for each contraction-relaxation cycle by the heart; the first beat represents that atrial contraction and second beat represents venous filling of the right atrium against a closed tricuspid valve and not the mistaken'ventricular contraction'. These wave forms may be altered by certain medical conditions; the carotid artery only has one beat in the cardiac cycle. Non-palpable – the JVP cannot be palpated. If one feels a pulse in the neck, it is the common carotid artery. Occludable – the JVP can be stopped by occluding the internal jugular vein by pressing against the neck.
It will fill from above. The jugular venous pulsation has a biphasic waveform; the a wave corresponds to right atrial contraction and ends synchronously with the carotid artery pulse. The peak of the'a' wave demarcates the end of atrial systole; the x descent follows the'a' wave and corresponds to atrial relaxation and rapid atrial filling due to low pressure. The c wave corresponds to right ventricular contraction causing the tricuspid valve to bulge towards the right atrium during RV isovolumetric contraction; the x' descent follows the'c' wave and occurs as a result of the right ventricle pulling the tricuspid valve downward during ventricular systole.. The x' descent can be used as a measure of right ventricle contractility; the v wave corresponds to venous filling when the tricuspid valve is closed and venous pressure increases from venous return – this occurs during and following the carotid pulse. The y descent corresponds to the rapid emptying of the atrium into the ventricle following the opening of the tricuspid valve.
A classical method for quantifying the JVP was described by Borst & Molhuysen in 1952. It has since been modified in various ways. A venous arch may be used to measure the JVP more accurately; this sign is used to determine. Feel the radial pulse while watching the JVP; the waveform, seen after the arterial pulsation is felt is the'v wave' of the JVP. The term "hepatojugular reflux" was used as it was thought that compression of the liver resulted in "reflux" of blood out of the hepatic sinusoids into the inferior vena cava, thereby elevating right atrial pressure and visualized as jugular venous distention; the exact physiologic mechanism of jugular venous distention with a positive test is much more complex and the accepted term is now "abdominojugular test". In a prospective randomized study involving 86 patients who underwent right and left cardiac catheterization, the abdominojugular test was shown to correlate best with the pulmonary arterial wedge pressure. Furthermore, patients with a positive response had lower left ventricular ejection fractions and stroke volumes, higher left ventricular filling pressure, higher mean pulmonary arterial, higher right atrial pressures.
The abdominojugular test, when done in a standardized fashion, correlates best with the pulmonary arterial wedge pressure, therefore, is a reflection of an increased central blood volume. In the absence of isolated right ventricular failure, seen in some patients with right ventricular infarction, a positive abdominojugular test suggests a pulmonary artery wedge pressure of 15 mm Hg or greater. Certain wave form abnormalities, include cannon a-waves, or increased amplitude'a' waves, are associated with AV dissociation, when the atrium is contracting against a closed tricuspid valve, or in ventricular tachycardia. Another abnormality, "c-v waves", can be a sign of tricuspid regurgitation; the absence of'a' waves may be seen in atrial fibrillation. An elevated J
The Starling resistor was invented by English physiologist Ernest Starling and used in an isolated-heart preparation during work which would lead to the "Frank–Starling law of the heart". The device consisted of an elastic fluid-filled collapsible-tube mounted inside a chamber filled with air; the static pressure inside the chamber was used to control the degree of collapse of the tube, so providing a variable resistor. This resistance was used to total peripheral resistance. Starling resistors have been used both as an instrument in the study of interesting physiological phenomena and as a rich source of physical phenomena in their own right. Two non-linear behaviours are characteristic: 1) the “waterfall effect” wherein, subsequent to collapse, the flow through the tube becomes independent of the downstream pressure and 2) self-excited oscillations. Expiratory flow-limitation in the disease COPD is an example of the former behaviour and snoring an example of the latter. Knowlton, F. P. Starling, E.
H.. "The influence of variations in temperature and blood pressure on the performance of the isolated mammalian heart". Journal of Physiology. 44: 206–219. Doi:10.1113/jphysiol.1912.sp001511. PMC 1512817. PMID 16993122. CS1 maint: Multiple names: authors list Levick, J. R.. An Introduction to Cardiovascular Physiology. Hodder Arnold. ISBN 0-340-80921-3. Conrad, W. A.. "Pressure-flow relationships in collapsible tubes". IEEE Trans. Biomed. Eng. 16: 284–295. Doi:10.1109/TBME.1969.4502660. Bertram C. D.. "The dynamics of collapsible tubes". Symp. Soc. Exp. Biol. 49: 253–64. Armitstead J. P.. D.. E.. "A study of the bifurcation behaviour of a model of flow through a collapsible tube". Bull. Math. Biol. 58: 611–41. Doi:10.1007/BF02459476