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Nervous System, text 4. <small>Brief description of the main parts.</small>

Peripheral nervous system

The signal traffic between the CNS and the external sensory and effector organs is mediated by the peripheral nervous system (PNS). The peripheral nervous system (PNS) consists of the nerves and ganglia, the nerve ganglia.

A nerve can generally be divided into different sections. It is said that a nerve has
* "roots", i.e. the parts closest to the CNS,
* "stem parts"
* "branches". Three main types of nerve branches are distinguished: skin, muscle, joint and visceral branches

A nerve consists of 100's, in some cases 10,000's, of tightly packed longitudinal nerve fibers (nerve tissue). The thickness, diameter, of the nerve fibers varies from 0.2 μm (1 μm = 1/1000 mm) in the thinnest to approx. 20 μm in the thickest.

Nerves

The majority of nerve fibers in PNS are sensory, afferent, i.e. transmit signals from the body's various sensory organs into the CNS. The rest of the nerve fibers in the PNS, the outwardly conductive, motor/efferent nerve fibers, are partly those that transmit command signals from the CNS either to the body's skeletal muscle cells or to the autonomic ganglia, and partly those that send signals from the autonomic ganglia to muscle cells in different viscera or to different types of glandular cells

The ganglia, the ganglia.

A nerve ganglion consists of a small lump of tightly packed nerve cell bodies (nerve tissue).

Some ganglia, the sensory/afferent ganglia, contain cell bodies (nerve cells) that give rise to the sensory/inward/afferent nerve fibers. These ganglia are found in connection with the roots of the nerves.

Other ganglia, the visceral/efferent ganglia, which belong to the autonomic nervous system, contain cell bodies that send nerve fibers to smooth muscle, heart muscle and glandular cells. These ganglia are found, among other things, in the posterior wall of the thoracic and abdominal cavity and in connection with various viscera.

The peripheral nerves.

The peripheral nerves are divided into two main groups: cranial nerves and spinal nerves.

The cranial nerves (12 pairs) connect to the underside of the brain. They are symmetrically distributed: 12 pieces on the right and 12 on the left (cranial nerves I, II, .... up to and including XII). It is debatable whether the other cranial nerve, the optic nerve (nervus II), should be counted as peripheral nerve; it consists of CNS tissue.

Some cranial nerves (the fifth, seventh, eighth, ninth and tenth) are provided with afferent/sensory ganglia, so-called cranial nerve ganglia. Some (the third, seventh, ninth and tenth) are provided with efferent/visceral/autonomic ganglia. The cranial nerves lack a common principal branching pattern and their course in the head is often very complex.

The spinal/spinal nerves (31 pairs) are connected with their roots to the spinal cord. They are symmetrically arranged with 31 on the right side and 31 on the left side of the vertebral column.

The part of the spinal cord that is connected to a pair of spinal nerves is called a spinal cord segment.

The spinal nerves follow a common basically simple branching pattern. Each spinal nerve connects to "its" half spinal cord segments with two bundles of wire: one posterior, posterior, and one front, anterior, The hind root contains inward/sensory/afferent nerve fibers. The foreroot contains outward conductive/motor/efferent nerve fibers.

The fore and hind roots merge into the stem part of the spinal nerve at the place where the collected filamentous mass leaves the vertebral canal through the intervertebral hole (vertebral column). From here to the target areas of the nerve fibers out in the body, motor and sensory nerve fibers run together. In the place where the hind and front roots merge, one finds, connected to the hind root, the sensory ganglia, the spinal ganglia, for the spinal nerve in question.

Once outside the vertebral canal, each spinal nerve divides into a thinner posterior branch and a coarser anterior branch. The posterior branches seek out the muscles and skin of the neck and back. The anterior ones pull to the muscles and skin of the neck, trunk wall and extremities. Added to this are the complex branches of the intestinal nerves of the autonomic nervous system.

Central nervous system.

The central nervous system (CNS) consists of the spinal cord and brain. Both of these body parts are well packaged inside walls of bone and connective tissue. Both are surrounded by membranes and float/float more or less weightless in a water-clear liquid: cerebrospinal fluid or cerebrospinal fluid.

Spinal cord.

The spinal cord is 40-50 cm long and is located in the midline of the body. The spinal cord begins at the level of the large neck hole, where it disappears into the cranial cavity and passes into the brain. It ends at the height of the first or second lumbar vertebra.

The spinal cord can be divided into 5 different contiguous parts: neck, chest, lumbar, sacral and tail

The different parts are in turn divided into spinal cord segments: 8 neck segments, 12 thoracic segments, 5 lumbar segments, 5 pelvic segments and 1 tail segment; in all 31 pcs.

Each spinal cord segment* is connected to a pair of spinal nerves. The segment receives incoming nerve signals at the rear through its two hind roots and sends signals through its two front roots at the front.* Halssegm.1 usually lacks hind roots.

The spinal cord has, on the whole, the same cross-sectional appearance throughout its extent. The nerve cell bodies in the spinal cord form a central H-shaped gray figure surrounded by shiny white nerve fiber masses.

The upward bars on the H form the back horns of the spinal cord, which receive the signals coming in with the hind roots. The descending bars form the anterior horns of the spinal cord, which emit the signals from the spinal cord into the front roots. In the middle of the transverse spine lies the hairline central canal (canalis centralis)

The gray matter is especially abundant at the segmental levels connected to the extremities. Here the spinal cord is markedly thickened, one speaks of the spinal cord's neck and lumbar swellings (intumescientia cervicalis and lumbalis).

The white matter of the spinal cord is divided into strands, 3 on each side of the mean line:
* the hind cord located between the midline and the entry of the hind roots into the spinal cord
* the side string located between the entry of the hind roots and the exit of the front roots
* the front strand located between the exit of the front roots and the middle furrow.

The white matter gradually decreases in quantity in a descending direction. The explanation is simple: At the top of the dorsal wall, all upward and downward threads are gathered in the cross section. Further down, some of the downward threads have already turned into the grey matter where they end, and upward threads from above-ground segments are of course not included.

Along the entire anterior part of the spinal cord runs in the midline a deep narrow furrow: the anterior median anterior slit of the spinal cord (fissure mediana anterior). Between the bottom of the slit and the crossbar of the H-et is a section of white matter containing nerve fibers which cross over the mean line: the anterior white transverse connection of the spinal cord (commissura alba anterior). The junction enables nerve fibers to change sides during their course up or down the spinal cord.

Brain.

The brain (encephalon) is the part of the central nervous system that is enclosed in the skull (cranium) cavity. In an adult, the brain is 16-17 cm long, approx. 12 cm high, approx. 12 cm wide and weighing about 1400 grams.

The brain consists of 3 main parts:

1/ Large brain (cerebrum) consisting of the end brain (telencephalon) and the interbrain enclosed therein (diencephalon).

2/ Cerebellum attached like a backpack to the back of the brain stem

3/ The brainstem/bulb (truncus encephali) which is the direct continuation of the spinal cord up into the head

The cerebrum is angled forward-downward almost 90 degrees relative to the brainstem, parts of which are located more or less on a straight line one after another.

Viewed from above, the brain is completely dominated by the cerebrum hemispheres.

If you separate the hemispheres a little, you notice that they are connected to each other across the middle line. The two halves are connected by a powerful bundle of nerve fibres called the corpus callosum. The brain beam consists of 300 - 400 million nerve fibers that coordinate the activities in the two hemispheres. You also see more of the cerebellum.

The cerebellum is the size of a lemon, weighs approx. 150 grams and is attached to the back of the brain stem like a transverse backpack. The cerebellum ultimately consists of a millimeter-thick layer of bark (cortex cerebellaris) and inside it the white matter of the cerebellum with the cerebellar nuclei.
The cerebellum is usually counted to the motor (movement-controlling) system and more specifically to its extrapyramidal part.
The cerebellum is very important when it comes to our ability to keep balance, perform well-coordinated movements and store rehearsed movement programs such as cycling. It is said about certain movements that 'they are in the spinal cord'. More accurate is probably to say that 'they are in the little brain'.

The brainstem is divided into three consecutive sections:

* Elongated medulla oblongata, which is the direct continuation of the spinal cord up into the brain and contains a series of vital centers for respiration and blood circulation.

* The bridge (pons) with centers for balance, hearing processing and switching centers (the pons cores) for signal traffic between the cerebrum and cerebellum.

* The midbrain (mesencephalon), which at the back is equipped with 4 humps, the so-called four-pile plate. The upper four-mound pair (hay. + left. colliculus superior) is an important switching station for visual impulses. The lower four-mound pair (hay + left colliculus inferior) is an equally important switching station for auditory impulses. The midbrain also contains centers of great importance for our ability to move (motor system)

Sandwiched between the cerebellum and the medulla bridge lies the 4th. cerebral ventricle. Downwards turn 4de. The ventricle in the central canal of the spinal cord, and upwards it continues in the midbrain as the "water pipe of the brain" (aqueductus cerebri).

The midbrain transitions into the midbrain. The midbrain is angled forward-downward almost 90 degrees relative to the brainstem, the parts of which are located more or less in a straight line one after another.

The midbrain is divided into two halves by a narrow spring-shaped cavity: the 3rd. cerebral ventricle. A midbrain hemisphere consists of a larger upper egg-shaped part: the thalamus (thalamus = visual high), and a smaller lower part: the hypothalamus. Parts of the hypothalamus form the floor of the third ventricle.

The thalamus can be called "the key to the cortex of the cerebrum" because almost all nerve signals that are going to the cortex of the cerebrum must first be switched in the thalamus. The thalamus "selects" what can be passed on to the cerebral cortex and what cannot be released all the way up there.

The hypothalamus is directly responsible for ensuring that the metabolism is in the right balance (that homeostasis is maintained) and reproduction functions normally. There are centers that, among other things, control hunger, thirst, sex drive and the hormone game in the body.

The connection between the main parts of the brain is best seen if you look at the underside of the brain.

Here the transition between spinal cord and extended marrow is clear. The extended pith has on both sides of the middle line a longitudinal ridge - the pyramid - and on either side a well-marked "hill" - the olive.

The bridge appears as a very strong, upwards and downwards well-marked transverse thickening, which disappears to the sides into the cerebellum (middle cerebellar arm).

The midbrain is on the underside provided with two powerful longitudinal elevations - hay. and we. Brain shanks (hay. and Vä. crus cerebri) - and between them a deep pit. The cerebral hemlocks emerge from the cerebrum, sandwiched between the end brain (telencephalon) and the midbrain (diencephalon), and disappear downwards into the bridge. The brain arms contain several million nerve fibers that send signals from the cerebrum down to the brainstem, cerebellum and spinal cord.

The underside of the midbrain (diencephalon) is formed by the hypothalamus. Here the two optic nerves meet in the optic nerve cross (chiasma opticum) and some threads change sides so that those coming from the left eye continue in the right hemisphere and vice versa. Just behind the optic nerve cross, the pituitary gland hangs down in a thin stalk and behind this are the two wart bodies (weasel and hay. corpus mamillare).

At the front, on the underside of the cerebellum, are the right and left olfactory tubercles (bulbus olfactorius). The odorous tubers rest in separate holes on the bone that forms the roof of the nasal cavity. The bone is provided with a large number of channels through which bundles of thin (unmyelinated) nerve fibers travel from the olfactory mucosa in the roof of the nasal cavity into the cranial cavity and end in the olfactory lump. It is the combined amount of these filaments - several million - that make up the first pair of cranial nerves - the olfactory nerves.

All 12 cranial nerve pairs are visible on the underside of the brain.
Nervus I: olfactory nerve (nervus olfactorius)
Nervus II: optic nerve (n. opticus)
Nervus III: eye muscle nerve (n.oculomotorius. Eye movements
Nervus IV: the rolling nerve. Eye movements.
Nervus V: triplet nerve (n.trigeminus). Sensation in the face and in the eyes, nose and mouth cavity/teeth. Chewing.
Nervus VI: "side-viewing nerve" (n.abducens). Eye movements.
Nervus VII: facial nerve (n.facialis). Mimicry, taste.
Nervus VIII: balance-auditory nerve (n.vestibulocochlearis).
Nervus IX: tongue-pharyngeal nerve (n.glosso-pharyngeus). Taste, swallowing.
Nerve X: the wandering nerve (n.vagus). The muscles of the larynx, viscera in the throat, in the chest cavity and in the abdominal cavity.
Nervus XI: "extranerven" n.accessorius). Neck o neck muscles.
Nervus XII. tongue nerve (n.hypoglossus). The mobility of the tongue.

In a splitting section (mean line section) through the brain, it is clearly visible how the midbrain together with the solid hemisphere, ie the cerebrum, are tipped forward in relation to the brain stem.

The corpus callosum, which connects the right and left hemispheres of the cerebrum, forms like a roof some distance above the midbrain.

The cavities inside the brain - the ventricular system of the brain - can be followed from the 4th ventricle through the water pipe up to the 3rd ventricle. The roof of the 3rd ventricle consists of special cells that form cerebrospinal fluid, liquor. At the back of the ceiling hangs the pineal gland (corpus pineale, epiphysis) that participates in the regulation of our circadian rhythm.

The floor of the 3rd ventricle is formed by the hypothalamus. The anterior wall of the third ventricle (lamina terminalis) is formed by a thin layer of tissue, which turns upward-forward into the cerebral cortex. Lamina terminalis contains at the top the anterior transverse connection between the cerebral hemispheres, (commissura anterior).

Behind the fornix, just where the mass of wire disappears into the ventricular wall, there is a hole out into the hemisphere. This hole, the "ventricular interventriculare", one on the right and one on the left side, connects the 3rd ventricle to the lateral ventricle out in the hemisphere.

The cerebrum, the cerebrum, constitutes approx. 75% of the brain's total weight of approx, 1400 grams. It is therefore not surprising that the cerebrum is the part of the brain that dominates the image when you look from the outside at the brain. Particularly remarkable is the strongly folded surface of the cerebrum. It is reminiscent of a collection of tightly packed winding sausages. What looks like sausages, it is the brain windings themselves (winding = gyrus) The gaps between the sausages, it is the brain furrows (furrow = sulcus).

Practically all windings and furrows have their own names, despite the fact that there are quite large variations in the surface pattern both between the two hemispheres in the same brain and between the brains of different individuals. Some furrows and windings must be known in order to understand the hemisphere's division into 6 so-called brain lobes.

Another distinct furrow - the central furrow (sulcus centralis) - divides, on the outside, the upper part of the hemisphere into an anterior and a posterior part.

The cerebral winding in front of the central furrow is called the "anterior central winding" (gyrus precentralis) and the behind the central furrow the "posterior central winding" (gyrus postcentralis).

Both windings are significant! The anterior central cortex sends out the signals that are responsible for the conscious control of the musculature on the opposite half of the body. The posterior central cortex receives all sensations from the skin and musculoskeletal system on the opposite half of the body.

At the rear, at the top of the outside of the hemisphere, is the end of a furrow - the crown-neck furrow - which runs mainly on the inside of the hemisphere. At the back, in the lower edge of the outside of the hemisphere there is a small notch - the neck indentation.

The frontal lobe (lobus frontalis) is located in front of the central furrow and above the lateral furrow. This is where the highest thought functions are located and here we have our ability to concentrate our thinking activities around a certain task. Here, the continued behavior is planned, here its possible consequences are analyzed, and from here our conscious movements are controlled. The frontal lobe has particularly strong growth after birth from the age of 3 up to the age of 6 (Thompson et al. NATURE, 404, p.190-3, 2000)

The parietal lobe (lobus parietalis) is located behind the central furrow, above the lateral furrow and in front of a line between the parietal and neck furrow and the neck indentation. In the parietal lobe, sensory impressions from the eye, ear, skin and musculoskeletal system are coordinated with each other and the brain creates an "inner picture" of the outside world and the position and movements of one's own body in this surrounding world.

The temporal lobe (lobus temporalis) is located below the lateral furrow and in front of the line between the crown-neck furrow and the neck-indentation. The temporal lobe cooperates closely with the adjacent crown of the head and occipital lobes. The temporal lobe is important for hearing and here language and music-carrying sound signals are interpreted. The temporal lobe is also crucial for how visual impressions are to be interpreted and for learning, memory storage and memory management. The parietal temporal lobe area has a particularly strong growth after birth in the period 6 - 13 years (Thompson et al. NATURE, 404, p.190-3, 2000)

The occipitalis lobe is separated from the rest of the hemisphere by the line between the crown-neck furrow and the neck indentation. The occipital lobe processes above all vision information.

The island-like lobe (insular lobe) is covered and "overgrown" by parts of the forehead, crown and temporal lobes and lies hidden in the depths of the lateral furrow. The lobe can only be observed if the covering parts, the "lids" (operculars) are moved aside. The insular lobe receives taste sensations and sensory impressions from the digestive organs. It is also important in connection with our reactions to pain stimuli.

The border lobe (limbic lobe, limbicus lobe) - the "fifth cerebral lobe" - is located on the inside of the hemisphere and appears best in a cleavage section through the mean line of the brain

A concept of the limbic lobe is given when you have removed the brainstem and cerebellum from the split brain." Limbus" means border and the limbic lobe, partly lined with a special kind of "archaic" cerebral cortex, lies like a horseshoe-shaped boundary wall between the central part of the cerebrum - the midbrain + the brain beam - and the outside more "modern" part of the end brain.

The upper part of the "limbic horseshoe", the one located in front of and above the brain beam, is called the cinguli winding (gyrus cinguli). The lower part, which is located on the inside of the temporal lobe (and is often also included in the temporal lobe), is called the parahippocampal winding (gyrus parahippo-campalis).

Along and as a deeper subdivision of the parahippocampus winding is the hippocampus itself (=seahorse). The hippocampus and its immediate surroundings (entorhinal cortex) are necessary for the brain's ability to learn and thus for our ability to store memories. The hippocampus belongs to the cerebellum and comprises the part of the cortex of the cerebrum that first matures during fetal development.

The hippocampus contains a very special kind of primitive cerebral cortex. From the hippocampus emanates a spaghetti thin bundle of wire; The vault (fornix ) that contains 5-6 million nerve fibers, and which connects the hippocampus with e.g. the hypothalamus.
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