Chapter 12 Nervous SystemThe nervous system is the bodys information gatherer, storage center and control system. Its overallfunctions are to collect information about the bodys external or internal states and transfer this informationto the brain, to analyze this information, and to send impulses out to initiate appropriate motor responses tomeet the bodys needs.The system is composed of specialized cells, termed nerve cells or neurons, that communicate with eachother and with other cells in the body.A neuron has three parts:The cell body, containing the nucleusThe dendrites, hair-like structures surrounding the cell body, which conduct incoming signals.The axon or nerve fiber, varying in length from a millimeter to a meter, which conduct outgoing signalsemitted by the neuron. Axons are encased in a fat-like sheath, called myelin, which acts like an insulator and,along with the Nodes of Ranvier, speeds impulse transmission.Major Divisions of the Nervous SystemThe nerves of the body are organized into two major systems:The central nervous system (CNS), consisting of of the brain and spinal cord,The peripheral nervous system (PNS), the vast network of spinal and cranial nerves linking the body to thebrain and spinal cord. The PNS is subdivided into:The autonomic nervous system involuntary control of internal organs, blood vessels, smooth and cardiacmuscles, consisting of the sympathetic NS and parasympathetic NSThe somatic nevous system (voluntary control of skin, bones, joints, and skeletal muscle).Central Nervous System (CNS)The Central Nervous System consists of the Brain and Spinal Cord. It contains millions of neurons callnerve cells. If you slice through some fresh brain or spinal cord you will find some areas appear grey whilst
other areas appear rather white. The white matter consists of axons, it appears white because it contains a lotof fatty material called myelin. The myelin sheath insulates an axon from its neighbors. This means thatnerve cells can conduct electrical messages without interfering with one another. The grey matter consists ofcell bodies and the branched dendrites which effectively connect them together. So this area is mainlycytoplasm of nerve cells which is why it appears white.Different areas of the brain are concerned with different functions. If I drilled a hole in your head with myBlack & Decker, and then put a piece of copper wire in and wiggled it about, I could give your brain a littleelectric shock; not enough to kill you of course, but enough to make something happen. So if the electrodewas put into your taste centre you might taste something even though there was nothing in your mouth. Weknow exactly where to put the wires to make different things happen. So an electric shock in another areamight make you wiggle your toes. That explains why you "see stars" when you bang your head and stir upthe visual centre which is at the back of your brain. There are areas of the brain which deal with speech,hearing, smell, sight, movements, salivating, and so on. Some of these centers are concerned with theinformation coming into the brain (sensory areas) and others are concerned with making something happen(motor centers).If your brain is anything like mine, the sensory areas and motor areas are connected up so that when you arestimulated, you do something sensible. What do you do when you bite into a ripe apple? Do you wiggle yourtoes or salivate? Some responses are very simple and everyone makes the same response: e.g. we all sneezewhen our noses are tickled. Other stimuli are much more complicated and we do not all react or respond inthe same way. Do you run away or go and stroke a lion when you see one in the playground? Well it alldepends on whether you know the lion and if you thought he was hungry. Some people make a big fusswhen they see a fly because they think that it is a wasp and it will sting them to death. Other people havelearnt the difference between a wasp and a fly.PNSThe Peripheral Nervous System consists of all the sensory nerves (these feed information into the spinal cordand brain) and the motor nerves (these carry messages to other parts of the body from the brain and spinalcord). Sensory nerves contain sensory neurons. Motor nerves contain motor neurons. Mixed nerves containboth sensory and motor neurons. Sensory neurons are usually connected to motor neurons by intermediateneurons (sometimes called inter neurons). Sensory, intermediate and motor nerves have gaps between themcalled synapses.The nerve cell bodies are generally located in groups. Within the brain and spinal cord, the collections ofneurons are called nuclei and constitute the gray matter, so-called because of their color. Outside the brainand spinal cord the groups are called ganglia. The remaining areas of the nervous system are tracts of axons,the white matter, so-called because of white myelin sheath. Tracts carrying information of a specific type,such as pain or vision, generally have specific names.Somatosensory ReceptorsInput (afferent system) to the nervous system is in the form of our five senses: touch or pain, vision, taste,smell, and hearing. Pain, temperature, and pressure are known as somatic senses. Sensory receptors areclassified according to the type of energy they can detect and respond to.Mechanoreceptors: hearing and balance, stretching.Photoreceptors: light.
Chemoreceptors: smell and taste mainly, as well as internal sensors in the digestive and circulatory systems.Thermoreceptors: changes in temperature.Electroreceptors: detect electrical currents in the surrounding environment.Neurons: a neuron is a nerve cell; it has a cell body, a very long axon sheathed in myelin, and many tinybranches called dendrites. There are three kinds of neurons: sensory, intermediate and motor neurons.Axons: these are long cytoplasmic tubes, they carry electric impulses from one part of the body to another.They are insulated from each other by their myelin sheathes.Dendrites: these are tiny branches on the cell body and at the ends of all neurons. The dendrites of one celldo not actually touch the dendrites of any other cell. There are very tiny gaps between them called synapses.Synapses: these are the gaps between the dendrites of one neuron and the cell body of another one. There isno electrical connection between nerve cells. when one neuron stimulates another it does so by secreting achemical into the synapse. Many drugs work by interfering with these chemical transmitters.Grey Matter: this is the material in the brain and spinal cord which contains the cell bodies and dendrite ofnerve cells. It is mainly cytoplasm. It appears grey to the naked eye. So if someone says you have not gotmuch grey matter, they are being very rude.White Matter: this is the material in the brain and spinal cord which contains the axons and myelin sheathesof nerve cells. It is mainly myelin which is a fat, so it appears white to the naked eye.A central nervous system disease can affect either the spinal cord (myelopathy) or brain (encephalopathy),both part of the central nervous system. The central nervous system controls behaviors in the human body,
so this can be a fatal illness.Types of diseaseEncephalitis: is an inflammation of the brain. It is usually caused by a foreign substance or a viral infection.Symptoms for this disease include: headache, neck pain, drowsiness, nausea, and fever. If caused by the WestNile virus, it may be lethal to humans, as well as birds and horses.Meningitis is an inflammation of the meanings (membranes) of the brain and spinal cord. It is most oftencaused by a bacterial or viral infection. Fever, vomiting, and a stiff neck are all symptoms of meningitis.Troby a virus that can also cause leukemia, a disease of the bone marrow.Arachnoid cysts are cerebrospinal fluid covered by arachnoidal cells that may develop on the brain or spinalcord. They are a congenital disorder and in some cases may not show symptoms. However, if there is a largecyst, symptoms may include headache, seizures, ataxia (lack of muscle control), hemiparesis, and severalothers. Macrocephaly and ADHD are common among children, while pre-senile dementia, hydrocephaluswhich an abnormality of the dynamics of the cerebrospinal fluid, and urinary incontinence are symptoms forelderly patients 65 and older.Huntingtons disease is a rare neurological disorder that is inherited. Degeneration of neuronal cells in thefrontal lobe of the brain occurs. There is a progressive decline which results in abnormal movements.Statistics show that Huntington’s disease may affect 10 per 100,000 people of Western European descent.Alzheimer’s is a neurodegenerative disease typically found in people over the age of 65 years. Worldwide,approximately 24 million people have dementia; 60% of these cases are due to Alzheimer’s. The ultimatecause is unknown. The clinical sign of Alzheimer’s is progressive cognition deterioration.Locked-in syndrome is due to a lesion on the brain stem, damaging the pons. It is a condition where thepatient is awake, but suffers from paralysis of all or nearly all voluntary muscles of the body and cannotcommunicate or move. Causes of locked-in syndrome may be: traumatic brain injury, circulatory systemdisease, nerve cell damage, and overdose of medication.Parkinson’s affects the motor skills and speech. Symptoms may include bradykinesia (slow physicalmovement), muscle rigidity, and tremors. Behavior, thinking, and sensation disorders are non-motorsymptoms.Tourettes syndrome is an inherited neurological disorder. Early onset may be during childhood, and ischaracterized by physical tics and verbal tics. The exact cause of Tourettes, other than genetic factors, isunknown.Multiple sclerosis (MS) is a chronic, inflammatory demyelinating disease, meaning that the myelin sheath ofneurons is damaged. Symptoms of MS include: visual and sensation problems, muscle weakness, anddepression.