The Central Nervous System consists of the brain and spinal cord, which arelocated in the dorsal body cavity. The brain is surrounded by the cranium, and thespinal cord is protected by the vertebrae. The brain is continuous with the spinalcord at the foramen magnum. In addition to bone, the CNS is surrounded byconnective tissue membranes, called meninges, and by cerebrospinal fluid.MeningesThere are three layers of meninges around the brain and spinal cord. The outerlayer, the dura mater, is tough white fibrous connective tissue. The middle layer ofmeninges is arachnoid, which resembles a cobweb in appearance, is a thin layerwith numerous threadlike strands that attach it to the innermost layer. The spaceunder the arachnoid, the subarachnoid space, is filled with cerebrospinal fluid andcontains blood vessels. The pia mater is the innermost layer of meninges. This thin,delicate membrane is tightly bound to the surface of the brain and spinal cord andcannot be dissected away without damaging the surface.Meningiomas are tumors of the nerve tissue covering the brain and spinal cord.Although meningiomas are usually not likely to spread, physicians often treat themas though they were malignant to treat symptoms that may develop when a tumorapplies pressure to the brain.
BrainThe brain is divided into the cerebrum, diencephalons, brain stem, and cerebellum.CerebrumThe largest and most obvious portion of the brain is the cerebrum, which is divided by adeep longitudinal fissure into two cerebral hemispheres. The two hemispheres are twoseparate entities but are connected by an arching band of white fibers, called the corpuscallosum that provides a communication pathway between the two halves. Each cerebralhemisphere is divided into five lobes, four of which have the same name as the bone overthem: the fontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. A fifthlobe, the insula or Island of Reil, lies deep within the lateral sulcus
DiencephalonThe diencephalons is centrally located and is nearly surrounded by the cerebralhemispheres. It includes the thalamus, hypothalamus, and epithalamus. The thalamus,about 80 percent of the diencephalons, consists of two oval masses of gray matter thatserve as relay stations for sensory impulses, except for the sense of smell, going to thecerebral cortex. The hypothalamus is a small region below the thalamus, which plays akey role in maintaining homeostasis because it regulates many visceral activities. Theepithalamus is the most dorsal portion of the diencephalons. This small gland is involvedwith the onset of puberty and rhythmic cycles in the body. It is like a biological clock.
Brain StemThe brain stem is the region between the diencephalons and the spinal cord. It consistsof three parts: midbrain, pons, and medulla oblongata. The midbrain is the most superiorportion of the brain stem. The pons is the bulging middle portion of the brain stem. Thisregion primarily consists of nerve fibers that form conduction tracts between the higherbrain centers and spinal cord. The medulla oblongata, or simply medulla, extendsinferiorly from the pons. It is continuous with the spinal cord at the foramen magnum. Allthe ascending (sensory) and descending (motor) nerve fibers connecting the brain andspinal cord pass through the medulla.
CerebellumThe cerebellum, the second largest portion of the brain, is located below theoccipital lobes of the cerebrum. Three paired bundles of myelinated nervefibers, called cerebellar peduncles, form communication pathways between thecerebellum and other parts of the central nervous system.Ventricles and Cerebrospinal FluidA series of interconnected, fluid-filled cavities are found within the brain. Thesecavities are the ventricles of the brain, and the fluid is cerebrospinal fluid (CSF).Spinal CordThe spinal cord extends from the foramen magnum at the base of the skull tothe level of the first lumbar vertebra. The cord is continuous with the medullaoblongata at the foramen magnum. Like the brain, the spinal cord is surroundedby bone, meninges, and cerebrospinal fluid.The spinal cord is divided into 31 segments with each segment giving rise to apair of spinal nerves. At the distal end of the cord, many spinal nerves extendbeyond the conus medullaris to form a collection that resembles a horses tail.This is the cauda equina. In cross section, the spinal cord appears oval inshape.
The spinal cord has two main functions:Serving as a conduction pathway for impulses going to and from the brain.Sensory impulses travel to the brain on ascending tracts in the cord. Motorimpulses travel on descending tracts.Serving as a reflex center. The reflex arc is the functional unit of the nervoussystem. Reflexes are responses to stimuli that do not require conscious thoughtand consequently, they occur more quickly than reactions that require thoughtprocesses. For example, with the withdrawal reflex, the reflex action withdrawsthe affected part before you are aware of the pain. Many reflexes are mediatedin the spinal cord without going to the higher brain centers.Brain TumorGlioma refers to tumors that arise from the support cells of the brain. Thesecells are called glial cells. These tumors include the astrocytomas,ependymomas and oligodendrogliomas. These tumors are the most commonprimary brain tumors.
The peripheral nervous system consists of the nerves that branch out from the brainand spinal cord. These nerves form the communication network between the CNS andthe body parts. The peripheral nervous system is further subdivided into the somaticnervous system and the autonomic nervous system. The somatic nervous systemconsists of nerves that go to the skin and muscles and is involved in conscious activities.The autonomic nervous system consists of nerves that connect the CNS to the visceralorgans such as the heart, stomach, and intestines. It mediates unconscious activities.Structure of a NerveA nerve contains bundles of nerve fibers, either axons or dendrites, surrounded byconnective tissue. Sensory nerves contain only afferent fibers, long dendrites of sensoryneurons. Motor nerves have only efferent fibers, long axons of motor neurons. Mixednerves contain both types of fibers.
A connective tissue sheath called the epineurium surrounds each nerve. Each bundle ofnerve fibers is called a fasciculus and is surrounded by a layer of connective tissue calledthe perineurium. Within the fasciculus, each individual nerve fiber, with its myelin andneurilemma, is surrounded by connective tissue called the endoneurium. A nerve mayalso have blood vessels enclosed in its connective tissue wrappings.Cranial NervesTwelve pairs of cranial nerves emerge from the inferior surface of the brain. All of thesenerves, except the vagus nerve , pass through foramina of the skull to innervatestructures in the head, neck, and facial region.The cranial nerves are designated both by name and by Roman numerals, according tothe order in which they appear on the inferior surface of the brain. Most of the nerveshave both sensory and motor components. Three of the nerves are associated with thespecial senses of smell, vision, hearing, and equilibrium and have only sensory fibers.Five other nerves are primarily motor in function but do have some sensory fibers forproprioception. The remaining four nerves consist of significant amounts of both sensoryand motor fibers.
Acoustic neuromas are benign fibrous growths that arise from the balance nerve, alsocalled the eighth cranial nerve or vestibulocochlear nerve. These tumors are non-malignant, meaning that they do not spread or metastasize to other parts of the body.The location of these tumors is deep inside the skull, adjacent to vital brain centers in thebrain stem. As the tumors enlarge, they involve surrounding structures which have to dowith vital functions. In the majority of cases, these tumors grow slowly over a period ofyears. In other cases, the growth rate is more rapid and patients develop symptoms at afaster pace. Usually, the symptoms are mild and many patients are not diagnosed untilsome time after their tumor has developed. Many patients also exhibit no tumor growthover a number of years when followed by yearly MRI scans.Spinal NervesThirty-one pairs of spinal nerves emerge laterally from the spinal cord. Each pair ofnerves corresponds to a segment of the cord and they are named accordingly. Thismeans there are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves,and 1 coccygeal nerve.Each spinal nerve is connected to the spinal cord by a dorsal root and a ventral root. Thecell bodies of the sensory neurons are in the dorsal root ganglion, but the motor neuroncell bodies are in the gray matter. The two roots join to form the spinal nerve just beforethe nerve leaves the vertebral column. Because all spinal nerves have both sensory andmotor components, they are all mixed nerves..
Autonomic Nervous SystemThe autonomic nervous system is a visceral efferent system, whichmeans it sends motor impulses to the visceral organs. It functionsautomatically and continuously, without conscious effort, to innervatesmooth muscle, cardiac muscle, and glands. It is concerned with heartrate, breathing rate, blood pressure, body temperature, and othervisceral activities that work together to maintain homeostasis.The autonomic nervous system has two parts, the sympathetic divisionand the parasympathetic division. Many visceral organs are suppliedwith fibers from both divisions. In this case, one stimulates and theother inhibits. This antagonistic functional relationship serves as abalance to help maintain homeostasis
RESTING MEMBRANE POTENTIAL
Resting PotentialResting Potential At rest the cell membrane is polarized maintaining a negative interiorcharge of -70mv. This is called the electrical gradient. There is about ten times moresodium (Na+) on the outside and twenty times more potassium (K+) on the inside. This iscalled the concentration gradient. So how is this uneven distribution of electricallycharged ions established? In part by properties of the membrane, and in part by thesodium/potassium pump. Remember that the membrane is semipermeable. Oxygen,carbon dioxide, urea, and water cross freely, but larger or electrically charged ions andmolecules usually canÕt cross. Certain ions, potassium (K+), chloride (Cl-), and sodium(Na+) cross the membrane at specialized gates. The gates control the rates at whichthese ions enter. The Na+ gates are closed until the membrane is depolarized, so Na+ isheld out. Therefore Na+ builds up in higher concentrations on the outside of themembrane. The Na+ ions are strongly attracted however to the negative interior of theneuron. The K+ ions, which are in greater abundance on the inside of the membrane, cancross the membrane at a controlled rate. Potassium tends to passively diffuse out of thecell because it is drawn to an area of lesser concentration, but it is partially held in the cellby the negative interior. Also the sodium/potassium pump returns K+ to the interior after ithas diffused out. Chloride, which also crosses at a controlled rate, as well as largenegatively charged proteins, also help to establish the negative interior. So Na+ is heldout, K+ diffuses out, but is brought back in, and this imbalance of positive and negativecharges (the electrical gradient) stops the movement of K+ ions. As a result, anequilibrium is reached at which point there is no net movement of K+ ions across themembrane (as many K+ are brought back in as diffuse out). This uneven distributioncauses the excess of positive charge outside with respect to the -70mv interior. This isthe resting potential and the neuron is ready to fire.
An action potential is caused by positive ions moving in and then out of theneuron at a certain spot on the neuron membrane.An action potential is initiated by a stimulus above a certain intensity orthreshold. Not all stimuli initiate an action potential. The stimulus could be a pinprick, light, heat, sound or an electrical disturbance in another part of theneuron.DepolarizationA stimulus causes a gate in the Na+ Channel to open. Since there is a highconcentration of Na+ outside, Na+ diffuses into the neuron. The electricalpotential changes to ~ +40 mV.RepolarizationDepolarization causes the K+ Channel gate to immediately open. K+ diffusesout of the neuron. This reestablishes the initial electrical potential of ~-60 mV.Refractory PeriodDuring this time (~ 1 msec), the Na+ and K+ Channels cannot be opened by astimulus.The Na+/K+ Pump actively pumps Na+ out of the neuron and K+ into theneuron. This reestablishes the initial ion distribution of the resting neuron.
Signal Transduction Across the Synapse When the wave of Action Potentials reach the end of the axon theelectrical signal is converted into a chemical signal. This chemical orneurotransmitter crosses the space (Synapse) between adjacentneurons and initiates an Action Potential on another neuron.The action potential activates a calcium channel and Ca++ diffuses intothe neuron.This Ca++ causes vesicles to fuse with the cell membrane. Throughexocytosis, neurotransmitters (chemicals) are released into thesynapse.These neurotransmitters diffuse across the synapse and bind toreceptors on another neuron. This causes special Na+ channels toopen and an action potential is initiated in the next neuron.