nervous SYSTEM

The central nervous system consists of the brain and spinal cord. The brain masters many of the functions including sensation, movement, awareness, thought and memory (Fig. 1). The spinal cord is a continuation of brainstem, and it runs down to L2 (lumbar vertebra) through the spinal canal to be protected by body the vertebrae (Fig.2). Spinal cord carries information from various parts of the body to and from the brain through ascending (afferent) and descending (efferent) tracts. 

The cerebrum, sometimes known as the forebrain, is the largest part of the brain which is derived embryologically from the telencephalon. It is enveloped inside thin but protective membranes called meninges; one of which includes the subarachnoid space which is full of the cerebrospinal fluid (Fig. 20). The cerebrum contains a torrent of grooves and ridges running in every direction. They are called sulci and gyri, respectively (Fig. 3). Their role is to increase surface area, and consequently the number of neurons, within the cerebrum. This helps in conducting larger processing and getting bigger cognitive abilities within the cerebral hemispheres. 

The main components of the cerebrum are two cerebral hemispheres, the right and the left one, which are separated by a deep longitudinal fissure and connected by the corpus callosum (Fig. 4). Also, they are detached by the falx cerebri of the dura mater (Fig. 5).

According to the name of the corresponding cranial bone that approximately overlies each part, the cerebral cortex is classified into five lobes each of which contains various cortical association areas where information from different modalities is arranged for processing (Fig. 6). Together, these areas function to give us a meaningful perceptual interpretation and experience of our surrounding environment.

First, the frontal lobe is the most anterior part of the cerebrum. Its functions are related to different activities including muscle control, personality, higher intellect, social behaviour, mood, and language (Fig. 7). Posteriorly, the frontal lobe is separated from the parietal lobe by the central sulcus (of Rolando), and inferiorly it is detached from the temporal lobe by the lateral sulcus (of Sylvius) (Fig. 8). The precentral, superior, middle, inferior, and orbital coils are considered as the most important convolutions of the frontal lobe. There are two cerebral arteries, the anterior and the middle ones, supplying the entire frontal lobe; they are branches of the internal carotid artery (Fig. 9).

Second, the parietal lobe contains two parts called lobules (superior and inferior) separated by an intraparietal sulcus. The lobe is located between the frontal and occipital lobes, and discrete from them by the central and parieto-occipital sulci, respectively. Its functions are latent in language and calculation, in addition to the cognition of various sensations such as touch, pain and pressure (Fig. 7). Other significant milestones include the postcentral sulcus together with the postcentral, angular, and supramarginal gyri. There are branches of various cerebral arteries supplying the parietal lobe: the anterior, middle, and posterior ones. The latter emerges from the basilar artery (Fig. 9).

Third, the temporal lobe is involved in tasks of memory, language and hearing. It is situated underneath the previous two lobes and disconnected from them by the lateral sulcus (Fig. 8). The superior, middle, and inferior temporal gyri are the main components of the temporal lobe, and they are demarcated by the superior and inferior sulci (Fig. 10). The middle and posterior cerebral arteries have a great role in feeding the temporal lobe (Fig. 9).

Fourth, the occipital lobe is the most posterior portion of the cerebrum, and it has a basic role in processing visual stimuli. It is separated from the cerebellum by the tentorium cerebelli, a fold of dura mater on which the occipital lobe rests (Fig. 5). The parieto-occipital and calcarine sulci detaches the occipital lobe from the parietal and temporal lobes, respectively (Fig. 8). Additional important features and landmarks include the superior and inferior occipital gyri (divided by the lateral occipital sulcus), lingual gyrus and the cuneus. The posterior cerebral artery presents the vascular supply of the occipital lobe. 

Fifth, the insular lobe which is hidden beneath the frontal, parietal and temporal lobes is responsible for handling various sensations such as taste, visceral, pain and vestibular function. The surface of the insula is divided into short and long gyri though the central sulcus (Fig. 11). The branches of the middle cerebral artery present the main supply of the insular lobe (Fig. 9).

In addition to the five lobes highlighted above, one important anatomical structure seen on a sagittal view of the brain is the limbic lobe (Fig. 12) which plays a basic role in the modulation of visceral, hormonal and autonomic functions, as well as emotions, learning and memory. The limbic lobe is, in fact, a misnomer because it is considered as a region of the cerebrum rather than a lobe. Located superior to the corpus callosum, it consists of subcallosal, cingulate, and parahippocampal gyri, together with the hippocampal formation (a subcortical structure).

The cerebellum is located at the back of the brain, immediately inferior to the occipital and temporal lobes, and within the posterior cranial fossa (Fig. 12). It is separated from these lobes by the tentorium cerebelli, a tough layer of dura mater (Fig. 5). It lies at the same level of and posterior to the pons, from which it is separated by the fourth ventricle.

Cerebellum has an important role in motor control, with cerebellar dysfunction that often associate with motor signs. In particular, cerebellum is responsible for coordination, balance, precision, timing of movements and motor learning.

There are three main lobes of cerebellum, nine lobules along the vermis, and five hemispheric lobules. The anterior lobe extends from the level of the cerebellar peduncles anteriorly and includes the anterior two-thirds of the superior vermis, along with the anterior third of each hemisphere. This lobe terminates at the primary fissure. From this point posteriorly and laterally and continuing along the inferior surface to the posterolateral fissure is the larger posterior lobe. The smallest of the lobes is the flocculonodular lobe. It is a flattened lobe that lies between the posterolateral fissure (inferiorly) and the inferior medullary velum and the cerebellar peduncles (superiorly) (Fig. 13).

There are three-foot processes that not only anchor the cerebellum to the brainstem, but also provide a pathway for neuronal tracts to travel to and from the cerebellum. These structures are the superior, middle and inferior cerebellar peduncles that are connected to the three parts of brainstem: midbrain, pons and medulla oblongata respectively (Fig. 14).

The vertebrate neural tube has a region that is responsible for forming the posterior forebrain structures. This region is called the diencephalon (interbrain). However, the most anterior vesicle of the neural tube which is called the prosencephalon gives rise first to the forebrain and later to both the diencephalon and the telencephalon. In adults, the diencephalon is located between the cerebrum and the brain stem, and it is seen at the upper end of the brain stem (Fig. 4). The thalamus, the hypothalamus, the epithalamus and the subthalamus are the four main components of the diencephalon.

The brainstem is the distal part of the brain that is made up of the midbrain, pons, and medulla oblongata (Fig. 4). Each of the three components has its own unique structure and function. Together, they help to regulate breathing, heart rate, blood pressure, and several other important functions. All of these brainstem functions are enabled because of its unique anatomy. The brainstem is a stalk-like projection extending caudally from the base of the cerebrum. It facilitates communication between the cerebrum, cerebellum, and spinal cord. The brainstem begins at the level of the cerebral peduncles (anteriorly) and the corpora quadrigemina or quadrigeminal plate (posteriorly) or tectal plate. It continues along a slight posteroinferior course until it ends at the decussation of the pyramids (at the level of the foramen magnum of the skull) (Fig. 15). 

The spinal cord appears as a cylinder-shaped, long tube sited within the vertebral canal. Its length ranges from 42-45 cm. It is a conduction passageway with two ways to the brain. The medulla oblongata of brainstem is considered as the starting point of the spinal cord which extends down to terminate with what is called the conus medullaris, a caudal tapering end (Fig. 16). From the tip of the conus medullaris, the filum terminale, a thin thread, extends to reach the 1st coccygeal vertebra; it acts as an anchor for the spinal cord in place (Fig. 16). Foramen magnum is the point through which the spinal cord exits the skull to reach the level of L2 vertebra. The cord has a major reflex center (Fig. 17).

The spinal cord is split into five parts: cervical, thoracic, lumbar, sacral, and coccygeal. Several pairs of spinal nerves are provided by each part. There are 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair of spinal nerves (Fig. 18). There are also two points of enlargements along the spinal cord, unlike the rest of its length which has a constant pattern in diameter. The increased size of the gray matter for the purpose of responding to the massive demands of the upper and lower limbs stands behind the cervical and lumbosacral enlargements of the spinal cord (Fig. 19). 

The meninges are membranous sheets that cover both the brain and spinal cord. There are three layers of meninges called dura mater, arachnoid mater and pia mater (Fig. 20). Meninges have two primary functions: (1) to provide a framework to support the cerebral and cranial structures. (2) To house the cerebrospinal fluid to protect the CNS from mechanical damage.

Dura mater is the outermost layer of the three layers of meninges, and it is located directly underneath the skull and vertebral column. This layer is thick, tough and inextensible. The dural venous sinuses are located withing dura mater, and they are responsible for the venous drainage of the cranium and take it back to the heart via internal jugular veins. This layer receives blood supply from middle meningeal artery (Fig. 21).

Arachnoid mater is the middle layer, and it lies directly underneath the dura mater. It consists of layers of connective tissue, and is considered avascular, which means no blood supply is provided. This layer has arachnoid space called sub-arachnoid space, and it contains cerebrospinal fluid, which acts as a protection of the brain (Fig. 21).  

Pia mater is the innermost layer located underneath the sub-arachnoid space. It is very thin and connected directly to the surface of the brain and spinal cord. This layer is rich with blood vessels perforating through the membrane to reach internal neural tissue (Fig. 21).

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The peripheral nervous system consists of all nerves and ganglions outside the central nervous system. It includes the cranial nerves, spinal nerves and all roots and branches. In peripheral nervous system, the nerve fibers or axons conduct information to and from the central nervous system (Fig. 22).  

There are 12 cranial nerves found in the nervous system and have a direct link with the brain without passing through the spinal cord. Therefore, cranial nerves allow sensory information to move from the head organs such as the ears and eyes directly to the brain, while allowing those nerves to transmit motor information from the brain to these organs (Fig. 23).

Despite the first two, the olfactory (I) and optic (II) which originate from nuclei in the cerebrum, the rest of the cranial nerves (III-XII) projects from nuclei in the brainstem. They can project from a specific portion of the brainstem (midbrain, pons, or medulla oblongata), or from the junction between two portions. For example, the trochlear nerve (IV) is the longest intracranial length of all the cranial nerves, and it projects from the posterior aspect of the midbrain. The midbrain-pontine junction is the place where the oculomotor nerve (III) originates, whilst the pontine-medulla junction produces several cranial nerves including abducens (VI), facial (VII), vestibulocochlear (VIII). Pons is where the largest cranial nerve, trigeminal nerve (V), arises and medulla oblongata, specifically posterior to the olive, gives glossopharyngeal (IX), vagus (X) and accessory (XI) However, that anterior side to the olive in medulla oblongata gives the hypoglossal nerve (XII) (Fig 24).

Every cranial nerve has a specific function and role as being sensory, motor or mixed. Cranial nerves provide both sensory and motor information to regions in the head and neck. The sensory are involved with the sight, smell, hearing and touch. The motor nerves, however, are involved in controlling the movements and functions of muscles and glands (Visit Cranial Nerves for more details).

There are 31 pairs of spinal nerves are produced by the spinal cord which is attached to each spinal nerve by two roots: dorsal (sensory) and ventral (motor). The dorsal root is known with a sensory ganglion sited immediately after the protrusion of the root from spinal cord. Through the intervertebral foramen, each spinal nerve comes out from vertebral column with two divisions: dorsal ramus and ventral ramus which together function as a container for both sensory and motor fibers. While the individual distribution of the dorsal rami works to innervate only the skin and the muscles of the back, the ventral rami compose several plexuses for the innervation of the anterior part of the body except the thoracic region which is innervated by single intercostal nerves (Fig. 25).

Four main plexuses are composed by the ventral rami. The first plexus is called cervical plexus; it includes ventral rami from spinal nerves C1-C4 (Fig. 26). One of the branches of the cervical plexus is phrenic nerve, which is responsible for the innervation of the muscles of neck, diaphragm, and skin of the neck and upper chest. The second plexus is known as brachial plexus; it includes ventral rami from spinal nerves C5-T1, and it is responsible for the innervation of upper limbs including pectoral girdle (Fig. 27). The third contains ventral rami from spinal nerves L1-L4 and it is called lumbar plexus (Fig. 28). The last has ventral rami from spinal nerves L4-S4 and it is called sacral plexus (Fig. 29). The last two plexuses, the lumbar and sacral, work to innervate the lower limbs including pelvic girdle. There is a miner plexus called coccygeal plexus that originates from S4 & S5 plus the ventral ramus of the coccygeal nerve and serves a small area in tailbone (Fig. 30).

At subconscious level, involuntary responses occur in reply to sensory stimuli; they are referred to as reflexes. The reflex pathway is composed of two types of neurons: the afferent neurons which transfer sensory information from sensory receptors to the CNS, and efferent neurons which transfer/convey the motor stimulus back to the effector muscle or gland. Afferent and efferent neurons are connected together via interneurons within the spinal cord (Fig. 31).

The dermatome is a part of skin nourished by one spinal nerve each of which is in charge of transferring sensation from a particular region of skin to the brain. The skin of the neck is supplied by the nerves from the upper cervical spine, the arms are supplied by nerves of C5 to T1, the chest and abdomen by nerves of T2 to L2, the skin of the legs by nerves of L3 to S1, and finally the groin by nerves of S1-C1. Testing dermatomes has a considerable role in determining the pathological disc level for being a part of the neurological examination that is looking for sensation changes within a specific dermatome (Fig. 32).

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The nervous system consists of two types of cells: neurons and glial cells. Neurons are the functional neuronal cells of the nervous systems responsible to detect changes and to communicate with other neurons. Glial cells are known as the supportive cells to nourish, protect neurons and to remove waste products. Human body contains between millions to billions of neurons and glial cells.

They are the functional cells located within the nervous system that transmit information to other neuronal, muscular, or glandular cells. Most neurons are composed of cell bodies, dendrites, and axons (Fig. 33). The cell bodies contain the nucleus and cytoplasm. Dendrites project from cell bodies to receive messages from other neurons. Axons are thin fibers extend from cell bodies and often give smaller branches before ending at nerve terminals. Synapses are the points where several neurons communicate together (Fig. 34). When neurons send electrical signals to other cells through the axons, chemical substances called neurotransmitters are released at the junction named synapses.

The transmission of information between neurons almost occurs by chemical rather than electrical means. Action potential causes the release of specific chemical that are stored in synaptic vesicles in the presynaptic ending. These chemicals (neurotransmitters) diffuse across the narrow gap between presynaptic and postsynaptic membranes to bind to receptors on the postsynaptic cell (Fig. 35).

They are derived from the Greek word for glue, and they are specialized cells that support, protect, and nourish neurons. Removing waste is additional function of glial cells to ensure safety area for neurons to maintain their functions. There are different types of glial cells, and the following are the most dominant cells:

Astrocytes

Astrocytes shape like star and located within the central nervous system. They make up approximately 30-40% of all glial cells. Astrocytes provides metabolic support where they store glycogen which can be then broken down to glucose to supply neurons. Astrocytes also are able to store lactate to be released during the periods of high energy consumption. As additional functions, Astrocytes remove excess potassium ions and neurotransmitters from the extracellular space to maintain normal function of neurons (Fig. 36).

Oligodendrocytes

Oligodendrocyte are glial cells located within the central nervous system and are responsible for producing myelin sheath which wraps the axons to accelerate the signal conduction (Fig. 36).

Microglia

Microglial cells make up approximately 10-15% of glial cells within the central nervous system. These cells apply the phagocytic functions of the nervous system. They are activated in response to tissue damage and have the ability to recognize foreign antigens and start the process of phagocytosis to remove all foreign materials (Fig. 36).

Ependymal Cells

Ependymal cells are found in the ventricular system of the brain and spinal cord. The main function of ependymal cells is the production of cerebrospinal fluid (CSF) as a part of the choroid plexus. They are covered with cilia to facilitate the circulation and absorption of CSF (Fig. 37).

Schwann Cells 

Schwann cells are named after the German physiologist Theodor Schwann who discovered them in the 19th century. They are located in the peripheral nervous system (PNS), and they equivalent to the Oligodendrocyte in the central nervous system where they apply myelination to axons in the peripheral nervous system. They also have phagocytotic function to clear cellular debris that help the regrowth of PNS neurons (Fig. 37).

Satellite Cells 

Satellite cells are small cells and also found in peripheral nervous system. They surround neurons in sensory, sympathetic, and parasympathetic ganglia. They help regulating the external chemical environment, and they act like Astrocytes where they are interconnected by gap junctions and respond to ATP by raising the intracellular calcium concentration (Fig. 38). 

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A stroke occurs when there is a blockage or leakage of blood flow to part of the brain. This condition is caused by a sudden occlusion or hemorrhage to cerebral blood vessels, in which that part of brain will stop working. A stroke also is called a cerebrovascular accident or brain attack. The stroke is considered a medical emergency and required immediate treatment. Early action can prevent serious damage and complications to the brain.  Symptoms depend on the location of the stroke, but motor dysfunction, paralysis, disturbance in speaking and comprehension, severe headache and blurred or blackened vision are common symptoms associated with stroke (Fig. 39). 

Dysfunction of the cerebellum occurs as a result of stroke, physical trauma or tumours. Symptoms depend on the functional area of the cerebellum that is affected. Motor learning disturbance, planned movement, and skilled movements, abnormal walking gait with balance loss are common to be seen with cerebellar dysfunction. The manifestations include ataxia, nystagmus, intention tremor, scanning speech and hypotonia (Fig. 40).

Paralysis is loss of the ability to move one or more muscles partially or completely, and it could be temporary or permanent as well. Loss of feeling and other bodily functions are potential to be involved in the case of paralysis. It indicates that there is a problem with the spinal cord or nerves that innervate muscles. A person with paralysis will usually have some form of nerve damage. Most paralysis results from spinal cord injuries or cerebrovascular damages. Other causes of paralysis include Bell’s palsy and multiple sclerosis (Fig. 41).

It is a progressive degenerative neurological disease with scattered patches of demyelination of nerve fibers of both brain and spinal cord. Common symptoms include tingling, numbness, muscle weakness or spasm, ataxia, dysarthria, dysphagia, diplopia, fatigue, pain and bowel incontinence (Fig. 42). 

It is an inflammation in the meninges due to a bacterial or viral infection. It is characterized by a severe headache, stiffness of the neck, fever, nausea, irritability, vomiting and delirium. Meningitis can threaten our lives because of the proximity to the brain and spinal cord; therefore, the condition is classified as a medical emergency. A lumbar puncture is a performed procedure to diagnose the condition. Specific antibiotics are common treatment (Fig. 43).

A sheaf of nerve fibers arising from the distal end of the spinal cord is defined as the cauda equina. Cauda equina syndrome is a case occurring as a result of pressure on these nerves which produces a range of signs and symptoms. Intervertebral discs prolapse, spinal stenosis, primary cord tumours, abscess formation and trauma are the causes behind this pressure. Potential cauda equina patients should pass by full lower limb neurological assessment. With the existence of sufficient clinical evidence, MRI should be immediately required for further diagnosis. In case the first manifestation of the symptoms is confirmed, surgical decompression is highly recommended (Fig. 44).

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  a. Controls and organizes all body activities

  b. Sends signals from one cell to the other or part of the body to another

  c. Controls and organizes all body activities

  d. All the above

  a. Synapse

  b. Grey Mater

  c. Nerve

  d. White Mater

  a. Goiter

  b. Cancer

  c. Multiple sclerosis

  d. Arthritis

  a. Foramen of Monro

  b. Foramen Magnum

  c. Jugular Foramen

  d. Foramen Spinosum

  a. Choroid plexus

  b. Ependymal cells

  c. Neuroglial cell

  d. Neurons