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Brain

Synonyms: brain, cerebrum (lat.), encephalon (Greek.)
German: Gehirn

1 Definition

Brain refers to the part of the central nervous system located in the head or in the skull (CNS) of vertebrates and represents the central control centre of the body.

2 Evolution

The vertebrate brain processes highly-centralised sensory input and coordinates complex behaviours. It is the main location for integration of all the vital information which is processed in an organism and, together with the spinal cord, it constitutes the central nervous system.

It is not only the vertebrates which have highly complex brains that enable them to perform specific activities. Squid have them too, for example. In a broader sense, brain therefore also refers to the central point of the nervous system of various invertebrates, such as the segmented worms or insects. Depending on the brain type, it is possible to talk of the cerebral ganglion, supraoesophageal ganglion etc.

The human brain is (next to the simple nervous systems of certain worms) the most widely studied brain in the animal kingdom.

3 Embryology

3.1 General Principles

Brain development begins with the closure of the neural canal. At the cranial end of the neural canal, three cerebral vesicles are formed.

The wall of the neural canal is formed from undifferentiated neural epithelium. The neuroepithelial cells divide, forming two new cell types: the primitive nerve cells, so-called geuroblasts, and the glioblasts (primitive supporting cells of the neural canal).

From the neuroblasts, the motorised neurons of the ventral horn and the sensory neurons of the dorsal horn are formed.

Neuroblasts enter into the mantle layer, resulting in the formation of ventral and dorsal thickening, i.e. of the motoric basal plate and the sensory alar plate. In between is located the sulcus limiting membrane. This supplies the neurons of the autonomic nervous system.

Between the two alar plates we find the cover plate; between the two base plates is found the floor plate. Around the basal and alar plates, the marginal zone is located.

If the formation of neuroblasts is complete, the glioblasts start to develop. These migrate into the intermediate zone and are differentiated into protoplasmic and fibrillar astrocytes.

If the neuroepithelial cells have ceased to form neuroblasts and glioblasts, they become ependymocyte cells.

3.2 Special Embryology

3.2.1 General

The neural canal consists of the cranial part of the brain vesicles and the caudal portion.

Forebrain, midbrain and hindbrain: From the brain vesicles, the three parts of the brain develop. From the caudal portion of the neural tube, the spinal cord comes into being.

The development of the individual parts of the brain always proceeds from the basal and alar plate, the cover plate, the floor plate and the marginal zone. In each section, along the neural canal, these differentiate themselves in various ways.

In the area of ​​the caudal neural canal (spinal cord), the ependymal lines the central channel which forms there. In the cranial neural canal (the area of ​​the brain vesicles), the ventricular system is formed from the central channel. This is correspondingly lined by ependymal cells, which produce cerebrospinal fluid.

3.2.1.1 Prosencephalon

From the prosencephalon (forebrain), the diencephalon and telencephalon are formed. This occurs via numerous differentiation processes and by the rotation of the hemispheres: during development, the hemisphere vesica (brain vesicles are located on both sides) is stretched in a way that is not uniform in all directions; rather, it expands mainly in caudal and basal directions. This is how the temporal lobe comes into being. Movements occur from back to front and up to down.

3.2.1.2 Mesencephalon

The mesencephalon forms the later crurae cerebri, the tegmentum and the tectum.

3.2.1.3 Rhombencephalon

From the rhombencephalon is formed the later metencephalon (with cerebellum and pons) and the myelencephalon (= medulla oblongata).

4 Anatomy

4.1 Macroanatomy

Observed from above, the human brain has a roughly oval shape. The brain tissue is gray-yellowish. On the undivided brain can be externally distinguished 3 sections: the powerfully bulging cerebrum, the cerebellum and hindbrain with its transition to the spinal cord.

The cerebrum is separated by a central groove (longitudinal fissure), into two equal halves, the hemispheres. On the surface of the cerebrum are found the cerebral gyrus (gyri cerebri) and the intervening brain furrows (sulci cerebri). They form a characteristic relief. In the gyri and sulci are found the various centres of the brain such as the auditory cortex in the temporal lobe or visual cortex in the occipital lobe.

The average weight of the organ is between 1300 and 1600 grams.

4.1.1 Brain Parts

4.2 Microanatomy

Each of the sections of the brain that arises from the respective cerebral vesicles has its own structure.

However, since the cerebral cortex makes up the largest portion of the brain in terms of volume, two special structural divisions are highlighted here: the isocortex and the allocortex.

These are not demarcated areas of the brain, but are rather divisions within the structure of the cerebral cortex.

5 Physiology

5.1 Cerebral Blood Flow

Cerebral blood flow is the basis for the supply of oxygen and nutrients to the neurons of the brain. In healthy adults, about 15% of cardiac output passes through the brain and its surrounding tissues - about 700ml of blood per minute.

5.2 Cerebral Metabolism

For metabolism under normal conditions, the brain almost exclusively uses glucose and oxygen. In the event of high concentration of ketone bodies (fasting, ketoacidosis) in the plasma, these may be used for energy production, being limited however to about 50% of energy requirements.

5.3 Brain Functions

Not all information reaches the cortex and thus the consciousness. Peripherally located plexuses (plexus) and above all centres in the brainstem provide unconscious preprocessing of signals, reflex arcs, taking on tasks at full speed and without registering in the consciousness. In humans, there is a so-called autonomic nervous system which serves to coordinate vegetative functions (respiration, circulation, ingestion, digestion and dispensing, fluid intake and excretion, and reproduction). The regulation of all these processes would completely overwhelm and thus block those structures of the brain that are involved in conscious perception.

The structure and - to a lesser extent - the size of the brain can be taken as an indication of the learning skills and intelligence of an animal. Again, it is not only the brain which is capable of learning neural plasticity - these abilities can be found on virtually all hierarchical levels of the nervous system.

6 History of Brain Research

  • Egypt: Textbook of Surgery with description of brain cleavage, recognising brain injuries (deviating eye position, drawing a foot backwards, loss of speech)
  • Hippocrates: Epilepsy can be triggered by stimuli
  • Claudius Galen: First neurophysiological experiments (cuts, lesions)
  • Andreas Vesalius: Brain anatomy
  • Descartes: Separation of the body and the soul
  • Willis: Grey/white matter
  • Gall: Phrenology (cranial mapping)
  • Paul Broca: Localised motoric speech centre in the left frontal lobe
  • Wernicke: Localised sensory speech centre in the left temporal lobe
  • Brodmann: Classification of the cerebral cortex into 52 areas
  • Ramon y Cajal: Neuroscience

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