Introduction and organization of nervous system The nervous system is divided into two main parts, Central N.S which consists of brain and spinal cord and peripheral N.S which consists of cranial and spinal nerves and their associated ganglia. In the central nervous system CNS, the brain and spinal cord are the main centers where correlation and integration of nervous information occur. Both the brain and spinal cord are covered with membranes called the meninges, and are suspended in the cerebrospinal fluid CSF; they are further protected by the bones of the skull and the vertebral column.
The CNS is composed of large numbers of excitable nerve cells and their processes called Neuron which are supported by specialized tissue called Neuroglia .The long process of a nerve cell are called axons or nerve fibers. The interior of the CNS is organized into gray and white matter. Gray matter consists of nerve cells embedded in neuroglia ,it has a gray color . White matter consists of nerve fibers embedded in neuroglia it has white color due to the presence of lipid materials in the myeline sheaths of many of the nerve fibers. In the peripheral nervous system PNS the cranial and spinal nerves which consist of bundles of nerve fibers or axons, conduct information to and from the CNS, they are unprotected and are commonly damaged by trauma.
Major division of central and peripheral NS CNS consists of Brain which consists of -Forebrain (cerebrum and diencephalon) -Mid brain -Hind brain (Medulla oblongata, pons and cerebellum)
PNS consists of Cranial nerves and their ganglia they are 12 pairs that exit in the skull through the foramina. Spinal nerves and their ganglia they are 31 pairs that exit the vertebral column through the intervertebral foramina.(8 cervical,12 thoracic, 5 lumber, 5 sacral and 1 coccygeal).
Autonomic nervous system Is the part of the nervous system concerned with the innervations of the of involuntary structures such as the heart, smooth muscle, and glands within the body.It is distributed throughout the central and peripheral nervous systems.It is divided to sympathetic and parasympathetic and in both parts there are afferent and efferent nerve fibers. The activities of the sympathetic part prepare the body for emergency.That of parasympathetic aimed to conserving and restoring energy.
Neuron Is the name given to the nerve cell and all its processes, neurons are excitable cells that are specialized for the reception of stimuli and the conduction of the nerve impulse. They vary considerably in size and shape, each has a cell body from those surface project one or more processes called neurites .Those neurites responsible for receiving information and conducting it toward the cell body are called dendrites .The single long tubular neurite that conducts impulses away from the cell body is called the axon.The dendrites and axons are often referred to as nerve fibers. Neurons are found in the brain and spinal cord and in ganglia.Unlike most other cells in the body normal neurons in the mature individual do not undergo division and replication.
Varieties of neurons The Number, length and mode of branching of the neuritis provide a morphological method for classifying neurons as followings: a. Unipolar neurons : are those in which the cell body has a single neurite that divides a short distance from the cell body into two branches ,one proceeding to some peripheral structure and the other entering the central nervous system.Example of this form of neuron are found in posterior root ganglia. b. Bipolar neurons: possess an elongated cell body ,from each end of which a single neuriteemerges.Example of this form of neuron are found in the retinal bipolar cells and the cells of the sensory cochlear and vestibular ganglia. c. Multipolar neurons: have a number of neurites arising from the cell body .with the exception of the long process ,the axon ,the remainder of the neurites are dendrites .Most neurons of the brain and spinal cord are of this type.
Neurons may also be classified according to size to: a. Golgi type I neurons: have a long axon that may be 1 meter or more in length ,examples of this form are the pyramidal cells of the cerebral cortex, the purkinje cells of the cerebellar cortex and the motor cells of the spinal cord. b. Golgi type II neurons:have a short axon that terminates in the neighborhood of the cell body or is entirely absent ,these neurons are very numerous in the cerebral and cerebellar cortex and are often inhibitory in function.
Neuroglia The neurons of the CNS are supported by several varieties of nonexcitable cells which together are called Neuroglia ,they are generally smaller than neurons ,they comprise about half the total volume of the brain and spinal cord.
Types of neuroglial cells are : 1.Astrocytes:have small cell bodies with branching processes that extend in all directions.there are 2 types Fibrous astrocyte which is found mainly in white matter ,and protoplasmic astrocyte which are found mainly in the gray matter. Astrocytes with their branching processes form a supporting framework for the nerve cells and nerve fibers. 2.Oligodendrocytes: have small cell bodies and a few delicate processes ,they are found in rows along myelinated nerve fibers and surround nerve cell bodies.these cells are responsible for the formation of the myelin sheath of nerve fibers in the CNS. 3. Microglia:they are the smallest of the neuroglial cells scattered throughout the CNS,they are macrophages of the CNS. 4. Ependyma:are cells line the cavities of the brain and the central canal of spinal cord they possess microvilli and cilia.
Cerebrum Is the largest part of brain, consists of two cerebral hemisphere , which are connected by a mass of white matter called the Corpus callosum. Each hemisphere extends from the frontal to the occipital bones, superior to the anterior and middle cranial fossae , posteriorly the cerebrum lies above the tentoriumcerebelli. The hemispheres are separated by a deep cleft, the longitudinal fissure, into which projects the falxcerebri.
The surface layer of each hemisphere, the cortex is composed of gray matter. The cerebral cortex is thrown in to folds or gyri, separated by fissures, or sulci .The surface area of the cortex is greatly increased by this means .Numbers of the large sulci are conventionally used to subdivide the surface of each hemisphere in to lobes. The lobes are named from the bones of the cranium under which they lie.
Within the hemisphere is a central core of white matter containing several large masses of gray matter called the basal nuclei or ganglia. A fan shaped collection of nerve fibers called corona radiate, passes in the white matter to and from the cerebral cortex to the brain stem. The corona radiate converges on the basal nuclei and passes between them as the internal capsule. The tailed nucleus situated on the medial side of the internal capsule is called caudate nucleus and the lens shaped nucleus on the lateral side of the internal capsule is called the lentiform nucleus. The cavity presented in each cerebral hemisphere is called lateral ventricle .The lateral ventricles communicate with the third ventricle through the interventricular foramina.
Major divisions of PNS Cranial and spinal nerves They are made up of bundles of nerve fibers supported by connective tissue . There are 12 pairs of cranial n. which leave the brain and pass through the foramina in the skull. There are 31 pairs of spinal nerves which leave the spinal cord and pass through the intervertebral foramina in the vertebral column.The spinal n. are named according to the region of the vertebral column with which they are associated : 8 cervical, 12 thoracic, 5 lumber, 5 sacral and 1 coccygeal. Note that there are 8 cervical nerves and only 7 cervical vertebra and that there is 1 coccygeal n. and there are 4 coccygeal vertebra.
Each spinal nerve is connected to the spinal cord by two roots the anterior and posterior root. The anterior root consists of bundles of nerve fibers carrying nerve impulses away from the CNS. Such nerve fibers are called efferent fibers. Those efferent fibers that go to skeletal muscles and cause them to contract are called Motor fibers. Their cells of origin lie in the anterior gray horn of the spinal cord.
Because of the disproportionate growth in the length of the vertebral column during development, compared with that of the spinal cord, the length of the roots increase progressively from above downward. In the upper cervical region the spinal nerve roots are short and run almost horizontally, but the roots of the lumber and sacral nerves below the level of the termination of the cord ( lower border of the 1st lumber vertebra in adult) form a vertical leash of nerves around the filumterminalis. Together these lower nerves are called the cauda equine.
General topography of CNS The brain and spinal cord are consisting of grey matter (nerve cell bodies and mostly non –myelinated fibers) and white matter (mainly muelinated fibers) with neuroglial cells. The CNS is divided in to forebrain (cerebrum and diencephalon) ,midbrain , hind brain (pons,medulla oblongata, and cerebellum) and spinal cord .The midbrain ,pons and medulla collectively form brain stem.
The forebrain a. Superolateral surface The cerebrum consists of paired hemisphere which occupies the anterior and middle cranial fossae.Each has an external grey cortex and a white centrum in which there are deep grey masses known as basal nuclei (ganglia). The two hemispheres are partly separated by deep median longitudinal fissure, crossed by a massive commissure, the corpus callosum whose fibers interconnect corresponding cortical areas. The cerebral cortex is convoluted by gyri and furrowed by sulci , the insular cortex is submerged in the lateral fissure (of sylvius).
The central sulcus (Fissure of Rolando) extends from the superiomedial border of the hemisphere, slightly posterior to its mid- point ,down and forewards towards the lateral fissure .The precentral and post central gyri lie infront of and behind it,they contain the primary motor and sensory cortical area. Sulci partially divide the hemispheres into lobes named after the cranial bones adjacent to them. The frontal is anterior to the central sulcus and above the lateral fissure, parietal lobe is posterior to the central sulcus and above the lateral fissure, the occipital lobe is behind a line from the parieto occipital sulcus to the pre occipital notch, temporal lobe is below the lateral fissure and in front of the pre occipital notch.
b. Medial surface It is the sagittal section that show corpus callosum ,roof and floor of third ventricle,the medial surface of the hemisphere above the corpus callosum forms the cingulated gyrus ,above which is the cingulated sulcus. At the posterior part of the hemisphere the oblique parieto –occipital sulcus separates the parietal from the occipital lobe and extends over the superior border. The medial surface of the occipital lobe is wedge-shaped and is named cuneus.Betweenparietooccipitalsulcus and the paracentral lobule is the precuneus.Thecuneus id limited by calcarinesulcus which runs foreword from the occipital pole to the medial surface of the temporal lobe.The visual area of the cortex occupies the lips of the sulcus.
The parieto-occipital sulcus and calcarinesulci form a pattern like the letter Y on its side the common stem of the Y is the anterior part of the calcarinesulcus ,and the two limbs are the parieti0occipital sulcus and and the posterior part of the calcarinesulcus. The lingual gyrus lies below the posterior part of the calcarinesulcus and is limited at the border between the medial and inferior surfaces of the occipital lobe by the collateral sulcus.
c. Inferior surface The orbital surface of the frontal lobe has the straight gyrus rectus along its medial margin.The olfactory bulb and the olfactory tract lie on the olfactory sulcus along side gyrus rectus. The temporal pole is boldly convex ;much of the medial surface of the temporal lobe can be seen from the inferior view.it is characterized by two long parallel sulci ,the occipitotemporalsulcus laterally and the collateral sulcus medially ,medial to collateral sulcus is the parahippocampalgyrus and it is anterior end known as uncus.
d. Internal structure The interior of the forebrain is characterized by the presence of large masses of grey matter and also by cavities which contain the CSF,The largest of these masses is the thalamus ;it belongs to the diencephalon. Other cell groups lie lateral to the thalamus within the cerebral hemisphere and some of them constitute the basal nuclei (also called basal ganglia).They are classified to. Caudate nucleus,lentiform n. (its outer part is putamen and inner part the globuspalledus),amygdaloid body and claustrum. The caudate and the lentiform nucleus are separated by the internal capsule . The caudate n. and lentiform n. both referred to as the corpus striatum.
1.The caudate nucleus: * has the shape of a highly curved comma. * has head,body,and tail. * The body curved back around the thalamus . * The long thin tail joins the amygdaloid body. * The whole length of its convexity projects into the lateral ventricle.
3. The amygdaloid body (amygdala) * connects to the tip of the tail of the caudate n. * functionally it is part of limbic system. * It has connection with frontal, temporal lobes including the olfactory cortex. * An efferent bundle of axons the atria terminalis , runs posteriorly following the curvature of the tail of the caudate nucleus, in the roof of the inferior horn and the floor of the body of the lateral ventricle to the septal area and anterior hypothalamus.
4. The claustrum * Is a thin lamina, circular in outline and curved into saucer shape . * It lies lateral to the putamen , identify in the horizontal or coronal sections. * Its significance is unknown
Functionally the basal nuclei exert a supraspinal control over skeletal muscle movements by influencing their rate, range and coordination. The corpus striatum receives fibers mainly from the cerebral cortex, thalamus, and substanitianigra.Theglobuspallidus is the main efferent pathway from the corpua striatum and sends fibers to the thalamus, subthalamic nucleus, substantianigra and reticular formation. The most common disease of the basal nuclei is Parkinsonism characterized by tremor, rigidity , and abnormal slowness of movement (bradykinesia).
The white matter of the cerebral hemisphere Is made up of fibers belonging to three main groups: 1. Commissural fibers: * These types of fibers join the cortices of the two hemispheres. *Most of them are gathered together in the corpus callosum (thickened body) , a few lie in the anterior and posterior commissures. * They radiate widely and symmetrically through the white matter of the hemisphere.
2.Association (arcuate) fibers : are confined to their own hemisphere , in which they connect different parts of the cortex. 3. projection fibers: are those which join the grey matter of the hemisphere with subcortical nuclei in the hemispheres and with nuclei in the brain stem and spinal cord.In the base of the hemisphere is a major collection of projection fibers lies lateral to the thalamus and the head of the caudate nucleus, forming the Internal capsule .
The internal capsule The lentiform nucleus lies lateral to the internal capsule . The tail of the caudate nucleus curls around the capsule and runs down to lie lateral to it. From the internal capsule the fibers radiate upwards and outwards in the shape of a curved fan to reach the cortex and similarly pass from the cortex down to the capsule, their fan shape arrangement is called corona radiata , the fibers of the corpus callosum intersect it. The internal capsule consists of afferent fibers passing up to the cortex from cell bodies in the thalamus ,and of efferent fibers passing down from cell bodies in the cortex to the cerebral peduncle of the midbrain.
It lies in the within the concavity of the C-shaped caudate nucleus . The internal capsule is seen in a typical horizontal section through the hemisphere.It has anterior limb, genu, and posterior limb. The anterior limb lies between the head of caudate nucleus medially and the lentiform nucleus laterally.It contain the fronto-pontine fibers from the cell bodies of the frontal cortex. The genu is the region of the bend in the capsule,at the apex of globuspallidus .It contain the corticonucleir fibers which pass from the cerebral cortex to the motor nuclei of cranial nerves in the brain stem. The posterior limb lies between the thalamus medially and the lentiform nucleus laterally.it contain the corticospinal fibers.
Corpus callosum The corpus callosum consists of a mass of 100 million commissural fibers. The corpus callosum can be seen by separating the two hemispheres and its cut surface is exposed in a midline sagittal section through the brain. It has 4 parts: rostrum, genu, body and splenium. The fibers of the corpus callosum extends to all parts of the cerebral cortex, in horizontal section all fibers of the genu are seen arching forewords on each side of the frontal cortex this appearance gives them the name of forceps minor. Similarly the fibers of the splenium curve backward to the occipital cortex called forceps major.
Between forceps minor and forceps major the fibers of corpus callosum spread out to the cortex on the lateral surface of the hemisphere.They pass over the anterior horn and body of the lateral ventricle, for each of which they form the roof. As they turn down into the temporal lobe they form the lateral wall of the inferior and posterior horns of the lateral ventricle, where they are known as the tapetum
Diencephalon The cavity of diencephalon is the third ventricle, the diencephalon enclosing this cavity, has two side walls, a floor and a roof. The floor and roof converges towards each other posteriorly,where they join in the midbrain and the cavity of the third ventricle is continue through the midbrain as the narrow aqueduct.
The anterior wall of diencephalon is the lamina terminalis ,a thin sheet which extend between the two hemispheres from the rostrum of corpus callosum to the top of optic chiasma.
The floor is seen as the floor of the third ventricle, extends from optic chiasma, tuber cinereum and infundibulum and mamillary bodies to the posterior perforated substance.