The nervous system is organized into two parts: the central nervous system, which consists of the brain and the spinal cord, and the peripheral nervous system, which connects the central nervous system to the rest of the body.
An image depicting the central nervous system can be seen below.
In the central nervous system, 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 a system of membranes, called meninges, and are suspended in the cerebrospinal fluid; they are further protected by the bones of the skull and the vertebral column.
The central nervous system is composed of large numbers of excitable nerve cells and their processes, called neurons, which are supported by specialized tissue called neuroglia. The long processes of a nerve cell are called axons or nerve fibers. The interior of the central nervous system 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 a white color due to the presence of lipid material in the myelin sheaths of many of the nerve fibers. The billions of neurons in the brain are connected to neurons throughout the body by trillions of synapses.
The brain contains more than 90% of the body's neurons. The brain has been divided into 3 different areas: the hindbrain, the midbrain, and the forebrain.
The hindbrain is found in even the most primitive vertebrates. It is made up of the cerebellum, the pons, and the medulla. The medulla is a narrow structure nearest the spinal cord; it is the point at which many of the nerves from the left part of the body cross to the right side of the brain and vice versa. The medulla controls such functions as breathing, heart rate, and blood pressure. The pons, located just above the medulla, connects the top of the brain to the cerebellum. Chemicals produced in the pons help maintain our sleep-wake cycle. The cerebellum is divided into 2 hemispheres and handles certain reflexes, especially those that have to do with balance. It also coordinates the body's actions. 
The midbrain lies between the hindbrain and forebrain and is crucial for hearing and sight.
The forebrain is supported by the brain stem and buds out above it, drooping somewhat to fit inside the skull. It consists of the thalamus, the hypothalamus, and the cerebral cortex. The thalamus relays and translates incoming messages from the sense receptors—except those for smell. The hypothalamus governs motivation and emotion and appears to play a role in coordinating the responses of the nervous system in times of stress.
The cerebral hemispheres, located above the thalamus and hypothalamus, take up most of the room inside the skull. The outer covering of the cerebral hemispheres is known as the cerebral cortex. The cerebral hemispheres are what most people think of when they think of the brain. They are the most recently evolved portion of the brain, and they regulate the most complex behavior. Each cerebral hemisphere is divided into 4 lobes, delineated by deep fissures on the surface of the brain. 
The occipital lobe of the cortex, located at the back of the head, receives and processes visual information. The temporal lobe, located roughly behind the temples, is important to the sense of smell; it also helps us perform complex visual tasks, such as recognizing faces.
The parietal lobe, which sits on top of the temporal and occipital lobes, receives sensory information, in the sensory projection areas, from all over the body and figures in spatial abilities. The ability to comprehend language is concentrated in 2 areas in the parietal and temporal lobes.
The frontal lobe is the part of the cerebral cortex responsible for voluntary movement and attention as well as goal-directed behavior. The brain starts response messages in the motor projection areas, from which they proceed to the muscles and glands. The frontal lobe may also be linked to emotional temperament.
These 4 lobes are both physically and functionally distinct. Each lobe contains areas for specific motor sensory function as well as association areas. The association areas are areas that are free to process all kinds of information and make up most of the cerebral cortex and enable the brain to produce behaviors requiring the coordination of many brain areas. The brain lies in the cranial cavity and is continuous with the spinal cord through the foramen magnum. It is surrounded by 3 meninges: the dura mater, the arachnoid mater, and the pia mater; these are continuous with the corresponding meninges of the spinal cord. The cerebrospinal fluid surrounds the brain in the subarachnoid space. [3, 4]
The medulla oblongata is conical in shape and connects the pons superiorly to the spinal cord inferiorly. It contains many collections of neurons, called nuclei, and serves as a conduit for ascending and descending nerve fibers. The pons is situated on the anterior surface of the cerebellum, inferior to the midbrain and superior to the medulla oblongata. The pons, or bridge, derives its name from the large number of transverse fibers on its anterior aspect connecting the 2 cerebellar hemispheres. It also contains many nuclei and ascending and descending nerve fibers.
The cerebellum lies within the posterior cranial fossa of the skull, posterior to the pons and the medulla oblongata. It consists of 2 laterally placed hemispheres connected by a median portion, the vermis. The cerebellum is connected to the midbrain by the superior cerebellar peduncles, to the pons by the middle cerebellar peduncles, and to the medulla by the inferior cerebellar peduncles. The peduncles are composed of large bundles of nerve fibers connecting the cerebellum to the remainder of the nervous system. The surface layer of each cerebellar hemisphere is called the cortex and is composed of gray matter. The cerebellar cortex is thrown into folds, or folia, separated by closely set transverse fissures. Certain masses of gray matter are found in the interior of the cerebellum, embedded in the white matter; the largest of these is known as the dentate nucleus.
The medulla oblongata, the pons, and the cerebellum surround a cavity filled with cerebrospinal fluid, called the fourth ventricle. This is connected superiorly to the third ventricle by the cerebral aqueduct; inferiorly, it is continuous with the central canal of the spinal cord. It communicates with the subarachnoid space through three openings in the inferior part of the roof. It is through these openings that the cerebrospinal fluid within the central nervous system can enter the subarachnoid space.
The spinal cord is situated within the vertebral canal of the vertebral column and is surrounded by 3 meninges: the dura mater, the arachnoid mater, and the pia mater. Further protection is provided by the cerebrospinal fluid, which surrounds the spinal cord in the subarachnoid space. The spinal cord is roughly cylindrical and begins superiorly at the foramen magnum in the skull, where it is continuous with the medulla oblongata of the brain. It terminates inferiorly in the lumbar region. Below, the spinal cord tapers off into the conus medullaris, from the apex of which a prolongation of the pia mater, the filum terminale (internum), descends to attach to the back of the coccyx.
Thirty one pairs of spinal nerves are attached by the anterior or motor roots and the posterior or sensory roots along the entire length of the spinal cord. Each root is attached to the cord by a series of rootlets, which extend the whole length of the corresponding segment of the cord. Each posterior nerve root possesses a posterior root or spinal ganglion, the cells of which give rise to peripheral and central nerve fibers.
Structure of the brain
As a general rule, the brain is composed of an inner core of white matter, which is surrounded by an outer covering of gray matter. However, certain important masses of gray matter are situated deeply within the white matter. For example, within the cerebellum are the gray cerebellar nuclei, and within the cerebrum are the gray thalamic, caudate, and lentiform nuclei.
The hemispheres are separated by a deep cleft, the longitudinal fissure, into which projects the falx cerebri. The surface layer of each hemisphere, the cortex, is composed of gray matter. The cerebral cortex is thrown into folds, or gyri, separated by fissures, or sulci. The surface area of the cortex is greatly increased by this means. A number of the large sulci are conveniently used to subdivide the surface of each hemisphere into 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, the basal nuclei or ganglia.
A fan-shaped collection of nerve fibers, termed the corona radiata, passes in the white matter to and from the cerebral cortex to the brainstem. The corona radiata converges on the basal nuclei and passes between the internal capsule. The tailed nucleus situated on the medial side of the internal capsule is referred to as the caudate nucleus, and the lens-shaped nucleus on the lateral side of the internal capsule is called the lentiform nucleus. The cavity present within each cerebral hemisphere is called the lateral ventricle. The lateral ventricles communicate with the third ventricle through the interventricular foramina.
Structure of the spinal cord
Unlike the brain, the spinal cord is composed of an inner core of gray matter, which is surrounded by an outer covering of white matter. The gray matter is seen on cross-section as an H-shaped pillar with anterior and posterior gray columns, or horns, united by a thin gray commissure containing the small central canal. The white matter, for purposes of description, may be divided into anterior, lateral, and posterior white columns.
Nervous system primarily originates from the neural plate, which is an ectodermal thickening in the floor of the amniotic sac. During the third week after fertilization, the plate forms paired neural folds, which unite to create the neural tube and neural canal. Union of the folds commences in the future neck region of the embryo and proceeds rostrally and caudally from there. The open ends of the tube, the neuropores, are closed off before the end of the fourth week. The process of formation of the neural tube from the ectoderm is known as neurulation. Cells at the edge of each neural fold escape from the line of union and form the neural crest alongside the tube. Cell types derived from the neural crest include spinal and autonomic ganglion cells and the Schwann cells of peripheral nerves.
The dorsal part of the neural tube is called the alar plate; the ventral part is the basal plate. Neurons developing in the alar plate are predominantly sensory in function and receive dorsal (or posterior) nerve roots growing in from the spinal ganglia. Neurons in the basal plate are predominantly motor and give rise to ventral or anterior nerve roots. At appropriate levels of the spinal cord, the ventral roots also contain autonomic fibers. The posterior and anterior roots unite to form the spinal nerves, which emerge from the vertebral canal in the interval between the neural arches being formed by the mesenchymal vertebrae. The cells of the spinal (posterior root) ganglia are initially bipolar. They become unipolar by the coalescence of their 2 processes at 1 side of the parent cells.
Late in the fourth week, the rostral part of the neural tube undergoes flexion at the level of the future midbrain. This region is the mesencephalon; slight constrictions mark its junction with the prosencephalon (future forebrain) and rhombencephalon (future hindbrain). The alar plate of the prosencephalon expands on each side to form the telencephalon (cerebral hemispheres).
The basal plate remains in place here as the diencephalon. Finally, an optic outgrowth from the diencephalon is the forerunner of the retina and optic nerve. The diencephalon, mesencephalon, and rhombencephalon constitute the embryonic brainstem. The brainstem buckles as development proceeds. As a result, the mesencephalon is carried to the summit of the brain. The rhombencephalon folds on itself, causing the alar plates to flare and creating the rhomboid (diamond-shaped) fourth ventricle of the brain. The rostral part of the rhombencephalon gives rise to the pons and cerebellum. The caudal part gives rise to the medulla oblongata. The neural canal dilates within the cerebral hemispheres, forming the lateral ventricles; these communicate with the third ventricle contained within the diencephalon.
The third and fourth ventricles communicate through the aqueduct of the midbrain. The thin roofs of the forebrain and hindbrain are invaginated by tufts of capillaries, which form the choroid plexuses of the four ventricles. The choroid plexuses secrete cerebrospinal fluid (CSF), which flows through the ventricular system. The fluid leaves the fourth ventricle through 3 apertures in its roof.
In the telencephalon, mitotic activity takes place in the ventricular zone, just outside the lateral ventricle. Daughter cells migrate to the outer surface of the expanding hemisphere and form the cerebral cortex. Expansion of the cerebral hemispheres is not uniform. A region on the lateral surface, the insula (Latin for "island"), is relatively quiescent and forms a pivot around which the expanding hemisphere rotates. Frontal, parietal, occipital, and temporal lobes can be identified at 14 weeks' gestational age.
On the medial surface of the hemisphere, a patch of cerebral cortex, the hippocampus, belongs to a fifth, limbic lobe of the brain. The hippocampus is drawn into the temporal lobe, leaving in its wake a strand of fibers called the fornix. Within the concavity of this arc is the choroid fissure, through which the choroid plexus invaginates into the lateral ventricle.
The anterior commissure develops as a connection linking olfactory (smell) regions of the left and right sides. Above this, a much larger commissure, the corpus callosum, links matching areas of the cerebral cortex of the 2 sides. It extends backward above the fornix. Coronal sections of the telencephalon reveal a mass of gray matter in the base of each hemisphere, which is the forerunner of the corpus striatum. Beside the third ventricle, the diencephalon gives rise to the thalamus and hypothalamus.
The expanding cerebral hemispheres come into contact with the diencephalon, and they fuse with it. One consequence is that the term "brainstem" is restricted thereafter to the remaining, free parts: midbrain, pons, and medulla oblongata. A second consequence is that the cerebral cortex is able to project fibers direct to the brainstem. Together with fibers projecting from thalamus to cortex, they split the corpus striatum into caudate and lentiform nuclei. By the 28th week of development, several sulci (fissures) have appeared on the surface of the brain, notably the lateral, central, and calcarine sulci.
The development of cranial nerves is provided below:
The olfactory nerve (I) forms from bipolar neurons developing in the epithelium lining the olfactory pit.
The optic nerve (II) is growing centrally from the retina.
The oculomotor (III) and trochlear (IV) nerves arise from the midbrain, and the abducens (VI) nerve arises from the pons; all 3 will supply extrinsic muscles of the eye.
The 3 divisions of the trigeminal (V) nerve will be sensory to the skin of the face and scalp, to the mucous membranes of the oronasal cavity, and to the teeth. A motor root will supply the muscles of mastication.
The facial (VII) nerve will supply the muscles of facial expression. The vestibulocochlear (VIII) nerve will supply the organs of hearing and balance, which develop from the otocyst.
The glossopharyngeal (IX) nerve is composite. Most of its fibers will be sensory to the oropharynx.
The vagus (X) nerve too is composite. It contains a large sensory element for the supply of the mucous membranes of the digestive system and a large motor (parasympathetic) element for the supply of the heart, lungs, and gastrointestinal tract.
The spinal accessory (XIs) nerve will supply the sternocleidomastoid and trapezius muscles. The hypoglossal (XII) nerve will supply the muscles of the tongue.
An approach to patient with CNS versus PNS problems
Step I: Localization
The first step after a neurological history and examination is to localize all the patient’s signs and symptoms to one, single lesion in the nervous system. It may be surprising that various signs and symptoms, at first glance apparently unrelated, can localize accurately to a single lesion. If this approach fails, then consider multiple, separate lesions for the patient’s signs and symptoms. [5, 4]
The tempo or rate at which signs and symptoms develop or occur often suggests the underlying pathological process.
Sudden onset - Favors stroke (ischemia or hemorrhage), seizure, migraine (or other headache syndromes), and trauma
Subacute onset - Favors inflammatory, infectious or immune-mediated disorders
Chronic onset - Favors degenerative disorders, tumors
Step II: The next step is to start thinking about the etiologies to make a differential diagnosis.
Toxic metabolic disorders, potentially treatable and reversible, may mimic lesions in the nervous system and can evolve at variable tempos.
Hereditary conditions may be congenital (present at birth) and nonprogressive or static, or develop later in life, with variable rates of progression. Family members affected by the same genetic disorder may be remarkably similar with regards to onset and clinical severity, while some genetic disorders vary widely regarding when and how severely family members are affected.
In the central nervous system, “positive symptoms or phenomena,” such as flashes of light, or a tingling sensation, suggest “excitation” or increased activity in the nervous system: migraine or seizure. “Negative symptoms or phenomena,” such as blindness or sensorimotor deficits, suggest “inhibition” or decreased activity (or a permanent deficit) in the nervous system: stroke, tumor, or degenerative disorder.
Patients with focal weakness or numbness
Are there signs of upper motor neuron (UMN) or lower motor neuron (LMN) involvement? (If not, consider weakness from a neuromuscular junction disorder or myopathy.) An upper motor neuron (UMN) lesion would include the following:
Hyper-reflexia, spastic tone, Babinski signs
More diffuse weakness, less severe atrophy (disuse atrophy)
Are any other symptoms or signs present to better localize this corticospinal tract lesion (cortical or subcortical brain, brainstem, spinal cord)?
Typical UMN patterns: hemiplegia, quadriplegia, paraplegia (or less severely weak: hemiparesis, quadriparesis, paraparesis)
AA lower motor neuron (LMN) lesion would include the following:
- Hypoflexia or areflexia, flaccid tone, fasciculations
- More focal weakness, more severe atrophy
- Does the weakness involve muscles proximally (typical myopathy) or distally (typical neuropathy)?
- Does the weakness involve muscles innervated by one (or more) specific spinal nerve roots, plexus (brachial or lumbosacral), or peripheral nerves?
Arterial Supply of the Forebrain
The blood supply to the forebrain is derived from the 2 internal carotid arteries and from the basilar artery. Each internal carotid artery enters the subarachnoid space by piercing the roof of the cavernous sinus. In the subarachnoid space, it gives off ophthalmic, posterior communicating, and anterior choroidal arteries before dividing into the anterior and middle cerebral arteries. The basilar artery divides at the upper border of the pons into the 2 posterior cerebral arteries.
The cerebral arterial circle (circle of Willis) is completed by a linkage of the posterior communicating artery with the posterior cerebral on each side and by linkage of the 2 anterior cerebrals by the anterior communicating artery. The choroid plexus of the lateral ventricle is supplied from the anterior choroidal branch of the internal carotid artery and by the posterior choroidal branch from the posterior cerebral artery. Dozens of fine central (perforating) branches are given off by the constituent arteries of the cerebral arterial circle (of Willis). They enter the brain through the anterior perforated substance beside the optic chiasm and through the posterior perforated substance behind the mammillary bodies. They have been classified in various ways but can be conveniently grouped into short and long branches.
Short central branches arise from all the constituent arteries and from the 2 choroidal arteries. They supply the optic nerve, chiasm, and tract, and the hypothalamus. Long central branches arise from the 3 cerebral arteries. They supply the thalamus, corpus striatum, and internal capsule. They include the striate branches of the anterior and middle cerebral arteries.
The brainstem and cerebellum are supplied by the vertebral and basilar arteries and their branches. The 2 vertebral arteries arise from the subclavian arteries and ascend the neck in the foramina transversaria of the upper 6 cervical vertebrae. They enter the skull through the foramen magnum and unite at the lower border of the pons to form the basilar artery. The basilar artery ascends to the upper border of the pons and divides into 2 posterior cerebral arteries.
Venous drainage of the brain
The venous drainage of the brain is of great importance in relation to neurosurgical procedures. This is also important to the professional neurologist, because various clinical syndromes can be produced by venous obstruction, venous thrombosis, and congenital arteriovenous communications. In general medical practice, however, problems (other than subdural hematomas) caused by cerebral veins are rare in comparison with arterial disease. The cerebral hemispheres are drained by superficial and deep cerebral veins. Like the intracranial venous sinuses, they are devoid of valves.