Chapter 1 Organization of the Nervous System

Cervical disc herniations

Are less common than in the lumbar region. The discs most susceptible to this condition are those between the fifth and sixth and the sixth and seventh cervical vertebrae. Lateral protrusions cause pressure on a spinal nerve or its roots. Each spinal nerve emerges above the corresponding vertebra; thus, the protrusion of the disc between the fifth and sixth cervical vertebrae may compress the C6 spinal nerve or its roots. Pain is felt near the lower part of the back of the neck and shoulder and along the area in the distribution of the spinal nerve involved. Central protrusions may press on the spinal cord and the anterior spinal artery and involve the various spinal tracts.

Lumbar disc herniations

Are more common than cervical disc herniations. The discs usually affected are those between the fourth and fifth lumbar vertebrae and between the fifth lumbar vertebra and the sacrum. In the lumbar region, the roots of the cauda equina run posteriorly over a number of intervertebral discs . A lateral herniation may press on one or two roots and often involves the nerve root going to the intervertebral foramen just below. The nucleus pulposus occasionally herniates directly backward, and if it is a large herniation, the whole cauda equina may be compressed, producing paraplegia. In lumbar disc herniations, pain is referred down the leg and foot in the distribution of the affected nerve. Because the sensory posterior roots most commonly pressed on are the fifth lumbar and first sacral, pain is usually felt down the back and lateral side of the leg, radiating to the sole of the foot. This condition is often called sciatica. In severe cases, paresthesia or actual sensory loss may occur. Pressure on the anterior motor roots causes muscle weakness. Involvement of the fifth lumbar motor root weakens dorsiflexion of the ankle, whereas pressure on the first sacral motor root causes weakness of plantar flexion. The ankle jerk reflex may be diminished or absent A large, centrally placed protrusion may give rise to bilateral pain and muscle weakness in both legs. Acute retention of urine may also occur.

Early Development of the Nervous System

Before the formation of the nervous system in the embryo, three main cell layers become differentiated. The innermost layer, the entoderm, gives rise to the gastrointestinal tract, the lungs, and the liver. The mesoderm gives rise to the muscle, connective tissues, and the vascular system. The third and outermost layer, the ectoderm, formed of columnar epithelium, gives rise to the entire nervous system. During the third week of development, the ectoderm on the dorsal surface of the embryo between the primitive knot and the buccopharyngeal membrane becomes thickened to form the neural plate. The plate, which is pear shaped and wider cranially, develops a longitudinal neural groove. The groove now deepens so that it is bounded on either side by neural folds. With further development, the neural folds fuse, converting the neural groove into a neural tube. Fusion starts at about the midpoint along the groove and extends cranially and caudally so that in the earliest stage, the cavity of the tube remains in communication with the amniotic cavity through the anterior and posterior neuropores. The anterior neuropore closes first, and 2 days later, the posterior neuropore closes. Thus, normally, the neural tube closure is complete within 28 days. Meanwhile, theneural tube has sunk beneath the surface ectoderm.

Central Nervous System

Brain Forebrain Cerebrum Diencephalon- (between brain) Midbrain HindbrainMedulla oblongata Pons Cerebellum Spinal cord Cervical segments Thoracic segments Lumbar segments Sacral segments Coccygeal segments

Brain Forebrain Cerebrum Diencephalon (between Brain)

CNS

Hindbrain Medulla Oblongata Pons Cerebellum

CNS

Midbrain

CNS

Spinal cord Cervical segments Thoracic segments Lumbar segments Sacral segments Coccygeal segments

CNS

Proliferation of cells at the cephalic end of the neural tube

Causes it to dilate and form three primary brain vesicles: the forebrain vesicle, the midbrain vesicle, and the hindbrain vesicle. The rest of the tube elongates and remains smaller in diameter; it will form the spinal cord. The subsequent differentiation of cells in the neural tube is brought about by the inductive interactions of one group of cells with another. The inducing factors influence the control of the gene expression in the target cells. Ultimately, the simplest progenitor cell will differentiate into neurons and neuroglial cells. It is interesting to note that excessive numbers of neurons and neuroglial cells are developed, and many (nearly half of the developing neurons) will be programmed to die by a process known as programmed cell death. Research into the identification of neurotrophic factors that promote the development and survival of neurons is of great importance, as the results could possibly be applied to the problem of regeneration of the spinal cord neurons following trauma or the inhibition of degenerative diseases, such as Alzheimer disease.

Peripheral Nervous System

Cranial nerves and their gangliaâ!" 12 pairs that exit the skull through the foraminaSpinal nerves and their gangliaâ!" 31 pairs that exit the vertebral column through the intervertebral foramina 8 Cervical 12 Thoracic 5 Lumbar 5 Sacral 1 Coccygeal

The Shaken-Baby Syndrome

Inflicted head injury is the most common cause of traumatic death in infancy. It is believed that sudden deceleration, which occurs when an infant is held by the arms or trunk and shaken or the head is forcefully struck against a hard surface, is responsible for the brain injuries. Biomechanical studies have shown that the rotation of the floating brain about its center of gravity causes diffuse brain injuries, including diffuse axonal injury and subdural hematoma. In shaken-baby syndrome, major rotational forces have to occur that clearly exceed those encountered in normal child play activities. Most cases of shaken-baby syndrome take place during the first year of life, and they are usually restricted to infants under 3 years of age. Common symptoms include lethargy, irritability, seizures, altered muscle tone, and symptoms indicating raised intracranial pressure, such as impaired consciousness, vomiting, breathing abnormalities, and apnea. In severe cases, the baby may be unresponsive, the fontanelles are bulging, and the child may have retinal hemorrhages. Spinal tap may reveal blood in the cerebrospinal fluid. Subdural or subarachnoid hemorrhages can be readily detected on CT or MRI scans. Autopsy findings commonly include localized subdural hemorrhage in the parietal-occipital region and subarachnoid blood, associated with massive cerebral swelling and widespread neuronal loss

Epidural (extradural) hemorrhage

Results from injuries to the meningeal arteries or veins. The anterior division of the middle meningeal artery is the common artery to be damaged. A comparatively minor blow to the side of the head, resulting in fracture of the skull in the region of the anterior inferior portion of the parietal bone, may sever the artery. Arterial or venous injury is especially likely to occur if the vessels enter a bony canal in this region. Bleeding occurs and strips the meningeal layer of dura from the internal surface of the skull. The intracranial pressure rises, and the enlarging blood clot exerts local pressure on the underlying precentral gyrus (motor area). Blood may also pass laterally through the fracture line to form a soft swelling on the side of the head. To stop the hemorrhage, the torn artery must be ligated or plugged. The burr hole through the skull wall should be placed about 1-1/2 inches (4 cm) above the midpoint of the zygomatic arch.

Subarachnoid hemorrhage

Results from nontraumatic leakage or rupture of a congenital aneurysm on the cerebral arterial circle (circle of Willis) or, less commonly, from an arteriovenous malformation. The symptoms, which are sudden in onset, will include severe headache, stiffness of the neck, and loss of consciousness. The diagnosis is established by performing CT or MRI or by withdrawing heavily blood- stained cerebrospinal fluid through a lumbar puncture. With regard to cerebral hemorrhage, spontaneous intracerebral hemorrhage is most common in patients with hypertension. It is generally due to rupture of the thin-walled lenticulostriate artery , a branch of the middle cerebral artery. The hemorrhage involves important descending nerve fibers in the internal capsule and produces hemiplegia on the opposite side of the body. The patient immediately loses consciousness, and the paralysis is evident when consciousness is regained. The diagnosis is established by performing brain CT or MRI.

Subdural hemorrhage

Results from tearing of the superior cerebral veins where they enter the superior sagittal sinus. The cause is usually a blow to the front or back of the head, resulting in excessive anteroposterior displacement of the brain within the skull. This condition, which is much more common than middle meningeal hemorrhage, can be produced by a sudden minor blow. Once the vein is torn, blood under low pressure begins to accumulate in the potential space between the dura and the arachnoid. In a few patients, the condition is bilateral. Acute and chronic forms of the clinical condition occur, depending on the speed of accumulation of fluid in the subdural space. For example, if the patient starts to vomit, the venous pressure will rise as the result of a rise in the intrathoracic pressure. Under these circumstances, the subdural blood clot will rapidly increase in size and produce acute symptoms. In the chronic form, over a course of several months, the small blood clot will attract fluid by osmosis, in which case a hemorrhagic cyst forms and gradually expands and produces pressure symptoms. In both forms, the blood clot must be removed through burr holes in the skull.

Sensory Ganglia

Sensory ganglia are fusiform swellings. situated on the posterior root of each spinal nerve just proximal to the root's junction with a corresponding anterior root. They are referred to as posterior root ganglia. Similar ganglia that are also found along Posterior view of the spinal cord showing the origins of the roots of the spinal nerves and their relationship to the different vertebrae. On the right, the laminae have been removed to expose the right half of the spinal cord and the nerve roots. The course of cranial nerves V, VII, VIII, IX, and X are called sensory ganglia of these nerves.

Autonomic nervous system

The autonomic nervous system is the part of the nervous system concerned with the innervation of involuntary structures, such as the heart, smooth muscle, and glands within the body. It is distributed throughout the central and peripheral nervous systems. The autonomic system may be divided into two parts, the sympathetic and the parasympathetic, and in both parts, there are afferent and efferent nerve fibers. The activities of the sympathetic part of the autonomic system prepare the body for an emergency.

Brain

The brain lies in the cranial cavity and is continuous with the spinal cord through the foramen magnum. It is surrounded by three 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. The brain is conventionally divided into three major divisions. These are, in ascending order from the spinal cord, the hindbrain, the midbrain, and the forebrain. The hindbrain may be subdivided into the medulla oblongata, the pons, and the cerebellum. The forebrain may also be subdivided into the diencephalon (between brain), which is the central part of the forebrain, and the cerebrum. The brainstem (a collective term for the medulla oblongata, pons, and midbrain) is that part of the brain that remains after the cerebral hemispheres and cerebellum are removed.

Invagination of the neural plate to form the neural groove

The cells forming the lateral margin of the plate do not become incorporated in the neural tube but instead form a strip of ectodermal cells that lie between the neural tube and the covering ectoderm. This strip of ectoderm is called the neural crest ; subsequently, this group of cells will migrate ventrolaterally on each side around the neural tube. Ultimately, the neural crest cells will differentiate into the cells of the posterior root ganglia, the sensory ganglia of the cranial nerves, autonomic ganglia, the cells of the suprarenal medulla, and the melanocytes. It is also believed that these cells give rise to mesenchymal cells in the head and neck.

Hindbrain - Cerebellum

The cerebellum lies within the posterior cranial fossa of the skull, posterior to the pons and the medulla oblongata. It consists of two 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.

Hindbrain - Cerebrum

The cerebrum, the largest part of the brain, consists of two cerebral hemispheres, which are connected by a mass of white matter called the corpus callosum. Each hemisphere extends from the frontal to the occipital bones in the skull, superior to the anterior and middle cranial fossae; posteriorly, the cerebrum lies above the tentorium cerebelli. The hemispheres are separated by a deep cleft, the longitudinal fissure, into which projects the falx cerebri

Interior of the CNS

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.

Hindbrain - Medulla Oblongata

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.

Hindbrain - Midbrain

The midbrain is the narrow part of the brain that connects the forebrain to the hindbrain. The narrow cavity of the midbrain is the cerebral aqueduct, which connects the third and fourth ventricles. The midbrain contains many nuclei and bundles of ascending and descending nerve fibers.

Hindbrain - Pons

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 two cerebellar hemispheres. It also contains many nuclei and ascending and descending nerve fibers.

Structure of the Spinal Cord

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

Spinal Cord

The spinal cord is situated within the vertebral canal of the vertebral column and is surrounded by three 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. Along the entire length of the spinal cord are attached 31 pairs of spinal nerves by the anterior or motor roots and the posterior or sensory roots Eachroot is attached to the cord by a series of rootlets, which extend the whole length of the corresponding segment of the cord. posterior nerve root possesses a posterior root ganglion, the cells of which give rise to peripheral and central nerve fibers.

Head Injuries

A blow to the head may cause the scalp to be merely bruised; severe blows may cause the scalp to be torn or split. Even if the head is protected by a crash helmet, the brain may be severely damaged without clinical evidence of scalp injury.

Intracranial Hemorrhage

Although the brain is cushioned by the surrounding cerebrospinal fluid in the subarachnoid space, any severe hemorrhage within the relatively rigid skull will ultimately exert pressure on the brain. Intracranial hemorrhage may result from trauma or cerebral vascular lesions. Four varieties are considered here: (1) epidural, (2) subdural, (3) subarachnoid, and (4) cerebral.

Autonomic Ganglia

Autonomic ganglia, which are often irregular in shape, are situated along the course of efferent nerve fibers of the autonomic nervous system. They are found in the paravertebral sympathetic chains. Around the roots of the great visceral arteries in the abdomen and close to, or embedded within, the walls of various viscera.

Meninges of the brain

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

Ganglia

Ganglia may be divided into sensory ganglia of spinal nerves (posterior root ganglia) and cranial nerves and autonomic ganglia.

Herniated Intervertebral Discs

Herniation of the intervertebral discs occurs most commonly in those areas of the vertebral column where a mobile part joins a relatively immobile partâl "for example, the cervicothoracic junction and the lumbosacral junction. In these areas, the posterior part of the anulus fibrosus of the disc ruptures, and the central nucleus pulposus is forced posteriorly like toothpaste out of a tube. This herniation of the nucleus pulposus may result either in a central protrusion in the midline under the posterior longitudinal ligament of the vertebrae or in a lateral protrusion at the side of the posterior ligament close to the intervertebral foramen

PNS

Peripheral nervous system, which consists of the cranial and spinal nerves and their associated ganglia. The cranial and spinal nerves, which consist of bundles of nerve fibers or axons, conduct information to and from the central nervous system. Although the nerves are surrounded by fibrous sheaths as they run to different parts of the body, they are relatively unprotected and are commonly damaged by trauma.

Parasympathetic

The activities of the parasympathetic part of the autonomic system are aimed at conserving and restoring energy.

Hindbrain - Diencephalon

The diencephalon is almost completely hidden from the surface of the brain. It consists of a dorsal thalamus and a ventral hypothalamus. The thalamus is a large, egg-shaped mass of gray matter that lies on either side of the third ventricle. The anterior end of the thalamus forms the posterior boundary of the interventricular foramen, the opening between the third and lateral ventricles. The hypothalamus forms the lower part of the lateral wall and floor of the third ventricle.

CNS

central nervous system which consists of the brain and spinal cord, In the central nervous system, the brain and spinal cord are the main centers where correlation and integration of nervous information occur. 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.