central nervous system

Central nervous system:

Three connective tissue membranes protect the central nervous system and projections of it's structures, called meninges. The outermost is the dura mater, or 'hard mother'. It is a double layered membrane where it surrounds the brain (the more superficial is the periosteal dura mater, and the deeper is the meningeal dura mater). The second meningeal layer is the arachnoid. The name comes from 'spider' because it looks a great deal like spider webs. It's thread-like processes span the subarachnoid space to attach to the pia mater, 'gentle mother'. The pia mater clings closely to the tissue of the spinal cord and brain, hugging every surface. Specialized protrusions of the arachnoid extend beyond the dura mater, called arachnoid villi. The cerebrospinal fluid is drawn into the veins through the arachnoid villi

Spinal cord:

White matter refers to the aggregation of myelinated neurons and supporting glial cells. Grey matter is used to describe cell bodies, dendrite and unmyelated axons and neuroglia. Nerve cell bodies that lie outside the central nervous system are called ganglia, primarily masses of grey matter. A tract is a bundle of fibers in the central nervous system. The primary spinal tracts that conduct impulses up the spinal cord, concerned with sensory transport, are ascending tracts. The descending is concerned with the motor responses. The main tracts are myelinated, white matter. A nucleus is a mass of nerve cell bodies and dendrites in the central nervous system, all of which have similar functions. It forms the grey matter. Horns, or columns, are the chief areas in the spinal cord of grey matter. 'Horn' describes the two dimensional appearance (grey matter), and 'column', the three dimensional (white matter)
The spinal cord is a roughly cylindrical structure, slightly flattened posterior to anterior. It begins as a continuation of the medulla oblongata, the inferior part of the brain stem, and extend out the foramen magnum of the occipital bone. The spinal cord in an adult is about forty-three centimeters in length, about two and one half in diameter in the midthoracic region, and larger in the lower cervical area. When viewed externally, the spinal cord can be seen to have two major enlargements. The superior of these is the cervical enlargement, from which the nerves of the upper extremities arise and join. The lower is the lumbar enlargement, where the same is true of the nerves of the lower extremities. From the lumbar enlargement, the spinal cord tapers to a conical portion, the conus medullaris, which ends between the first and second lumbar vertebrae. The filum terminale arises from the conus medullaris, a nonnervous fibrous tissue which attaches to the coccyx, consisting mostly of pia mater. From the conus medullaris, the spinal cord flares from the spine, angling downward. This arrangement of the inferior spinal nerves it the cauda equina, or the horse's tail. The spinal cord is considered to be in thirty-one segments, each giving rise to a pair if spinal nerves, each of these sections being called as spinal segment. Two grooves divide the cord into right and left halves by the anterior and posterior median fissures. The latter is more shallow and narrow
The grey matter of the spinal cord forms an H-shapped darkened area. The center center bar of the H is formed by the grey commissure. In the center of the grey commissure is the small central canal which runs the length of the spinal cord, continuous from the fourth ventricle of the medulla. The wider, more rounded and anterior horns of the grey matter are the anterior grey horns. The smaller, more actually horn-shaped are the posterior grey horns. Between them are small lateral grey horns, present in the thoracic, upper lumbar and sacral segments of the spinal cord. The anterior and lateral grey horns make up the motor part of the grey matter. The posterior represent the sensory section of the grey matter
Anterior to the grey commissure is the anterior white commissure, which connects the symmetrical halves of the white matter of the spinal cord. The horns of the grey matter divide the white into three columns. They are the anterior white columns, the posterior columns, and the lateral columns. Each column (funiculus), in turn, is divided into tracts, or fasciculi. The ascending tracts are made up of sensory fibers. The descending tracts consist of motor neurons
The spinal cord has two major functions. One is to to convey information to and from the brain via the spinal tracts. The second is to serve as the center for reflexes. The posterior or dorsal root contains sensory neurons only. Each dorsal root also has a swelling, the posterior root ganglion, containing the cellular bodies of the neurons of the dorsal root. The other root is the anterior, or ventral, root. If the cell bodies of autonomic motor neurons are located in the lateral grey horn. The somatic are in the anterior grey horn
The path an impulse follows from its origin to termination is known as its conduction pathway. One such pathway is a reflex arc. A reflect arc contains at least two neurons which conduct an impulse from a receptor, through an afferent neuron, to the spinal cord or brain, to an efferent neuron and ending at an effector. Most often, association neurons are involved. Some somatic reflexes:

stretch reflexes-- are based on the two neuron arc model (monosynaptic reflex arc), and it the only arc to do so. The impulse enters the spinal cord on the same side as it exits, thus called an ipsilateral reflex arc. All monosynaptic arcs and deep tendon arcs are ipsilateral. One stretch reflex is the patellar or knee-jerk reflex. These reflexes result in the contraction of a muscle the nervous system believes it stretched. They are tested by tapping on the tendon of the muscle to see if the reflex takes place
tendon reflexes-- are polysynaptic arcs. The receptors used are the Golgi tendon organs. When the tension in a tendon becomes too strong, this reflex causes the relaxation of the muscle
flexor reflexes-- also called a withdrawal reflex. An ipsilateral arc. One example would be stepping on a sharp object and immediately lifting your foot away
crossed extensor reflexes-- an extension of muscles in in one limb occurs as a result of contractions of muscles of the opposite limb

Spinal nerves:

The thirty-one pairs of spinal nerves are named for the vertebra from which the emerge. There is one extra set of cervical nerves between the occipital bone and the atlas. Total, there are eight cervical, twelve thoracic, five lumbar, five sacral and one one pair of coccygeal. Each spinal nerve has two points of attachment, the posterior and anterior roots describes above. Spinal nerves are described as being mixed nerves because they contain both sensory and motor fibers. The individual nerve fibers are enclosed in the endoneurium. Gathered together into fascicles, they are them wrapped in a perineurium. The nerve is then enclosed in the epineurium
Shortly after the spinal nerves leave the vertebrae, they divide into the branches known as rami.

dorsal ramus-- innervates the deep back muscles and skin
ventral ramus-- innervates superficial back muscles and all of the structures of the extremities and lateral and ventral trunk
meningeal branch-- reenters the spinal canal through the intervertebral foramen to supply nerves to the vertebrae, vertebral ligaments, blood vessels and the meninges
rami communicantes-- components of the autonomic nervous system

Except for the second through eleventh thoracic nerves, the spinal nerves do not go directly to their designated structures. They instead form a network called a plexus

cervical plexus-- formed by the ventral rami of the first five cervical nerves. The cervical plexus also connects to the eleventh and twelfth cranial nerves
brachial plexus-- made up of the ventral rami of the fifth through eighth cervical and first thoracic nerves, with contribution from the fourth cervical and second thoracic. In the axillary area, the plexus forms three main cords: the lateral, posterior and medial cords. The axillary nerve is a short nerve which runs behind the humerus and to the outer axillary area. The radial nerve runs behind the humerus and then down the lateral surface of the arm. The median nerve runs the center of the entire length of the arm. On the medial surface is the ulnar nerve
lumbar plexus-- contains the ventral rami of the first through fourth lumbar nerves. The large nerve which runs through the pelvis and along the posterior medial aspect of the superior leg is the sciatic nerve. The shorter femoral nerve runs anterior to the sciatic. The sciatic nerve splits about two thirds of the way down the femur into the more medial tibial nerve, which, in the lower lag, runs over the medial ankle. The other, more lateral branch is the common peroneal nerve. The common peroneal, in turn, branches into the deep and superficial peroneal nerves
sacral plexus-- involves the fourth and fifth lumbar nerves and all four sacral nerves. Situated mostly anterior to the sacrum

The second through eleventh thoracic nerves do not enter a plexus and are known as the intercostal or thoracic nerves

Brain:

The human brain looks, in many ways, a great deal like a mushroom. The stalk is the brain stem, which is inferior to the inner structure called the diencephalon. The large 'hat' of our brain mushroom is the cerebrum. To the posterior and inferior of the cerebrum, behind the brain stem, is the cerebellum
There are four ventricles in the brain. The cerebrospinal fluid washes through the subarachnoid space and through these ventricles:

lateral ventricles (2)-- located in either hemisphere of the brain, at the 'ends' of the corpus callosum
third ventricle (1)-- a vertical slit inferior to the longitudinal fissure. This ventricle communicates with the lateral ventricles via a narrow oval opening, the interventricular foramen
fourth ventricle (1)-- lies between the inferior brain stem and the cerebellum, posterior to the pons and medulla oblongata. Communicates with the third ventricle by way of the cerebral aqueduct, which passes through the midbrain. The roof of the fourth ventricle has three openings through which it communicates with the subarachnoid space of the brain and spinal cord. They are the median aperture and lateral apertures

The ventricles also contain the choroid plexuses, which are the producers of the cerebrospinal fluid

The medulla oblongata, or simply medulla, is a continuation of the superior end of the spinal cord and is the most inferior part of the brain. It lies just superior to the foramen magnum and extends up to the pons, about three centimeters in length. The medulla oblongata contains both descending and ascending tracts which begins the spinal cord and constitutes the white matter of the medulla. One the ventral aspect of the medulla oblongata are two roughly triangular pyramids, composed of motor neurons; the largest tracts that pass from the outer cerebrum to the spinal cord. Just above the foramen magnum, the tracts cross over to exchange side. The left pyramid fibers to the right side and the other way around. This crossing is called the decussation of pyramids. The purpose is as yet unknown. As a result, the right side of the brain controls the left side of the body, and the left brain, the right body. The dorsal side of the medulla oblongata contains two prominent sets of nuclei. They are the nucleus gracilis and nucleus cuneatus. These nuclei receive sensory tracts and relay them to the opposite side of the brain. The medulla also disperses grey matter containing some white. This area is the reticular formation, and may extend down into the superior spinal cord. Within the medulla are three vital reflex centers:

cardiac center-- regulates heartbeat and the force of contractions
medullary rhythmicity area-- adjusts basic breathing rates
vasomotor or vasoconstrictor center-- used to regulate the diameter of blood vessels

Also in the medulla are centers controlling swallowing, vomiting, coughing, sneezing and hiccuping. The origin of the eighth, ninth, tenth, eleventh and twelfth cranial nerves are here. The greater part of the vestibular nuclear complex is associated with the medulla oblongata, helping in our sense of equilibrium

The pons is located anteriorly to the medulla oblongata and superiorly, just inferior and posterior to the frontal lobes of the cerebrum. 'Pons' means bridge, which aptly describes it's purpose. The pons serves as a bridge from the spinal cord to the brain. These connections are provided by the the middle cerebellar peduncles. Also contained or originating from the pons are the fifth, sixth, seventh, sections of the eighth cranial nerves, the pneumotaxic area and the apneustic area. The last two aid in the control of respiration
The midbrain, also called the mesencephalon, extends from the pons to the inferior portions of the diencephalon. The cerebral aqueduct runs through it to connect with the fourth ventricle from the third. The ventral portion of the midbrain contains a pair of fiber bundles called the cerebral peduncles which contain many motor fibers that connect to the pons and spinal cord, as well as sensory leading the other way. The dorsal aspect of the mesencephalon is called the tectum and contains four rounded portions, the corpora quadrigemina. Two of the corpora quadrigemina are the superior colliculi, reflex centers for the eyes and head for visual stimuli. The midbrain also contains the substantia nigra, a large, heavily pigmented nucleus near the cerebral peduncles. Another major nucleus is the red nucleus, a termination point for many fibers from the cerebral cortex. The red nucleus is also the origin for fibers of the descending rubrospinal tract. Cranial nerves of the mesencephalon are the third and fourth cranial nerves. A common feature of the midbrain is the medial lemniscus, a band of white fibers
The diencephalon is made up of the thalamus and hypothalamus
The thalamus is an oval structure located superiorly to the midbrain, about three centimeters in size. It makes up about eighty percent of the diencephalon. The thalamus is made up by two oval masses made up of mostly grey matter that has been organized into nuclei to form part of the wall of the third ventricle. The masses are bridged by grey matter in the form of the intermediate mass. Each mass is in one hemisphere of the cerebrum, in an internal capsule. Although primarily made up of grey matter, there are several areas of white matter in the thalamus:

stratum zonale-- on the dorsal surface
external medullary lamina-- covering the lateral surface
internal medullary lamina-- divides the grey masses into and anterior nuclear group, a medial nuclear group and a lateral nuclear group

Within each group mentioned above are specific nuclei, one in each mass:

medial geniculate nuclei-- a relay station for hearing sensory impulses
lateral geniculate nuclei-- relay for vision
ventral posterior nuclei (2)-- relay for general sensation and taste
ventral lateral nuclei-- relay for the somatic impulses of voluntary motor actions
ventral anterior nuclei-- relay for voluntary motor actions and arousal

The thalamus is the primary relay for sensory information reaching the cerebral cortex. It also serves as an interpretation area for pain, temperature, light touch and pressure. In it's reticular formation, the thalamus contains a reticular nucleus and an anterior nucleus in the floor of the lateral ventricle. The anterior nucleus of the thalamus is part of the limbic system

The hypothalamus makes up the other small portion of the diencephalon. It is partially protected by the sella turcica of the sphenoid bone. It forms the floor and part of the lateral walls of the third ventricle. The hypothalamus also contains the small mammillary bodies, part of the limbic system. Sensory impulses from peripheral sensory organs bringing such information as sound, taste, smell and somatic sensory input. The hypothalamus has ten major functions:

controlling and integrating the autonomic system
reception and integration of sensory input
the major intermediary between the nervous and endocrine system, and producing the hormones antidiuretic hormone (ADH) and oxytocin (OT). Both of these are taken to the pituitary for storage and release
the center for 'mind over body' actions, such as in panic, the rapid beating of the heart. Many psychosomatic disorders reside here
the hypothalamus is associated with feelings of aggression and rage
control of body temperature
houses the feeding center and satiety center
contains a thirst center
controls sleeping patters and wakefulness
a sort of pacemaker for and drives most biological rhythms

The cerebrum makes up the bulk of the brain. The surface of the cerebrum is composed of grey matter about two to four millimeters deep, referred to as the cerebral cortex. The cerebral cortex contains six layers of cell bodies in most areas. Beneath the cortex is the cerebral white matter. During development, the cortex grows faster and larger than the white matter underneath, creating the wrinkled look of the brain. The folds are gyri or convolutions, and the grooves are fissures or sulci. The cerebrum is split in the center by the deep longitudinal fissure which nearly splits the brain and separates it into right and left hemispheres. A bundle of transverse fibers connect the hemispheres are the corpus callosum. Between the hemispheres is an extension of the cranial dura mater called the falx cerebri or the cerebral fold and encloses the superior and inferior cerebral sinuses. The cerebrum is then divided into four principle lobes. The central sulcus, or fissure, separates the frontal and parietal lobes. In the frontal lobe there is a prominent prefrontal gyrus, a primary motor area. In the parietal is a postcentral gyrus, a major sensory area. The lateral cerebral sulcus is the boundary between the frontal and temporal lobes. The parietooccipital sulcus separates the parietal and occipital lobes. The transverse fissure is the division between the cerebrum and the cerebellum. A 'fifth' lobe is buried deep within the the lateral cerebral sulcus; the insula. The basal ganglia or cerebral nuclei are paired masses of grey matter in the cerebrum. The largest of the basal ganglia are the corpus striatum. The corpus striatum is made up of two parts. These are the lentiform and caudate nuclei. The lentiform nucleus, in turn, is made up two more parts. The more medial of these is the globus pallidus. Lateral to the globus pallidus is the portion named the putamen. The portion of the internal capsule that runs between the lentiform, caudate and thalamus is sometimes considered to be a part of the corpus striatum. The internal capsule is a group of sensory and motor white fibers that connects the cerebrum to the brain stem and spinal cord, located in the tail of the caudate nucleus. Other structures associated with the basal ganglia are the claustrum, the amygdaloid nucleus at the end of the caudate nucleus tail, substantia nigra, subthalamic nucleus and red nucleus. The caudate nucleus and the putamen control large subconscious movements of the skeletal muscles
The limbic system combines parts of the diencephalon and the cerebrum into a wish-bone shaped group that functions in the emotional aspect of behavior related to survival. Some of the regions of grey matter involved in the limbic system are:

anterior nucleus of the thalamus-- located in the floor of the lateral ventricle
limbic lobe-- formed by two gyri of the hemispheres (the cingulate and the hippocampal)
mammillary bodies-- parts of the hypothalamus near cerebral peduncles, small and round in appearance
amygdaloid nucleus
hippocampus-- an extension of the hippocampal gyrus that extends to the floor of the lateral ventricle

Some memory impairments result from lesions on the limbic system. Also associated with the limbic system are pleasure and pain. Because of it's role in pain, anger, pleasure, fear, sorrow, sexual feeling, docility, and affection, the limbic system is sometimes called the emotional brain

The cerebellum is the second largest part of the brain. It is located posteriorly to the brain stem and inferior to the occipital lobe. It is separated from the cerebrum by the transverse fissure and an extension of the dura mater called the tentorium cerebelli. The wrinkles in the surface of the cerebellum are called folia. The center area is slightly constricted, called the vermis. The posterior and anterior lobes are concerned with the conscious and subconscious movement of skeletal muscles. The flocculnodular lobe is associated with our maintenance of equilibrium. The surface of grey matter of the cerebellum is also called a cortex. The white matter of the cerebellum is arranged in a tree-like structure, the arbor vitae. The cerebellum is connected to the brain stem by three pairs of cerebellar peduncles. The cerebellum helps us know where one part of the body is in relation to another, called proprioception

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