Mrs. Kaser's Science Page



Chapter 7 The Nervous Systemright0You are driving down the freeway, and a horn blares on your right. You swerve to your left. Charlie leaves a note on the kitchen table: "See you later-have the stuff ready at 6." You know that the "stuff" is chili with taco chips. You are dozing, and your infant son makes a soft cry. Instantly, you awaken. What do all these events have in common? They are all everyday examples of the functioning of your nervous system, which has your body cells humming with activity nearly all the time.The nervous system is the master controlling and communicating system of the body. Every thought, action, and emotion reflects its activity. Its signaling device, or means of communicating with body cells, is electrical impulses, which are rapid and specific and cause almost immediate anization of the Nervous System List the general functions of the nervous system. Master controlling and communicating system of the body Three overlapping functionsUses million of sensory receptors to monitor changes occurring both inside and outside the body – changes are called stimuli and the gathered information is called sensory inputIt processes and interprets the sensory input and makes decisions about what should be done at each moment, a process called integrationIt then effects a response by activating muscles or glands – the response is called motor output Explain the structural and functional classifications of the nervous system. Structural classificationCentral nervous system (CNS) – brain and spinal cordPeripheral nervous system (PNS) – part of nervous system outside the CNSSpinal nerves – carry impulses to and from the spinal cordCranial nerves – carry impulses to and from the brain Functional classification Sensory or afferent division – nerve fibers that convey impulses to the CNS from sensory receptors Somatic sensory fibers – deliver impulses from the skin, skeletal muscles, and joints Visceral sensory fibers (AKA visceral afferents) – transmit impulses from visceral organs (internal organs)Motor or efferent division – carries impulses from the CNS to effector organs, the muscles and glands to bring about a motor response Somatic nervous system – conscious, voluntary control of most (not all) skeletal muscles – skeletal muscle reflexes such as stretch reflex are initiated involuntarily – often called voluntary nervous systemAutonomic nervous system – automatic, involuntary activities such as smooth and cardiac muscles and glands – often called involuntary nervous system Sympathetic – mobilizes the body during extreme situations such as fear, exercise, and rage – the fight-or-flight response Parasympathetic – allows us to unwind and conserve energy – the resting-and-digesting system Define central nervous system and peripheral nervous system and list the major parts of each. See above objective Nervous Tissue: Structure and Function State the function of neurons and neuroglia.Neurons – aka nerve cells – highly specialized to transmit messages in the form of nerve impulses from one part of the body to another – all types have two common features:All have a cell body, which contains the nucleus and is the metabolic center of the cellOne or more slender processes extending from the cell body Neuroglia – supporting cells – includes many types of cells that generally support, insulate, and protect the neurons – are not able to transmit nerve impulses – never lose their ability to divide Astrocytes – star-shaped cells whose numerous projections have swollen ends that cling to neurons, bracing them and anchoring them to their nutrient supply lines, the capillaries – form a living barrier between capillaries and the neurons and make exchanges between the two to help protect the neurons from harmful substances in the blood Microglia – spiderlike phagocytes that dispose of debris such as dead cells and bacteria Ependymal cells – glial cells line cavities o the brain and spinal cord – beat cilia to help circulate cerebrospinal fluid forming a protective cushion around the CNSOligodendrocytes – glia that wrap their flat extensions tightly around the nerve fibers, producing fatty insulating covering called myelin sheaths Describe the general structure of a neuron and name its important anatomical regions. Cell body – metabolic center of the cell with the usual organelles except centrioles, rough ER (Nissl substance), neurofibrils, intermediate filaments for maintaining cell shape Cell bodies found in the CNS are in clusters called nuclei Small collections of cell bodies called ganglia are found in a few sites outside the CNS in the PNS Bundles of nerve fibers (neuron processes) running through the CNS are called tracts whereas in the PNS they are called nerves Arm-like processes or fibers – vary in length Dendrites – convey incoming messages toward the cell body – may be present in the hundreds depending on neuron typeAxons – convey nerve impulses away from the cell body – each neuron only has one, which arises from a cone-like region of the cell body called the axon hillock Each axon may have a collateral branch along its length but all axons branch profusely at their terminal end forming hundreds of axon terminals each with hundreds of tiny vesicles that contain chemicals called neurotransmitters Each axonal terminal is separated from the next neuron by a tiny gap called the synaptic cleft – a functional junction is called a synapse Myelin – whitish, fatty material with a waxy appearance that covers most long nerve fibers – protects and insulates the fibers and increases transmission rate of nerve impulses Axons outside the CNS are myelinated by Schwann cells, support cells that wrap themselves tightly around the axon in a jelly-roll fashion resulting in a tight coiled of wrapped membranes called the myelin sheath encloses the axon The external part of the Schwann cell is called the neurilemmaGaps at regular intervals in the myelin sheath that result from the individual Schwann cells are called nodes of Ranvier Describe the composition of gray matter and white matter. Gray matter – contains mostly unmyelinated fibers and cell bodiesWhite matter – consists of dense collections of myelinated fibers (tracts) Both refer to regions of the CNSList the two major functional properties of neurons. Irritability – the ability to respond to a stimulus and convert it into a nerve impulseConductivity – the ability to transmit the impulse to other neurons, muscles, or glandsClassify neurons according to structure and function. Functional classification – groups neurons according to the direction the nerve impulse is traveling relative to the CNSNeurons carrying impulses from sensory receptors to the CNS are sensory, or afferent, neurons – the cell bodies of sensory neurons are always found in ganglion outside the CNS – keep us informed about what is happening both inside and outside the body Neurons carrying impulses from the CNS to the viscera and /or muscles and glands are motor, or efferent, neurons – cell bodies are always located in the CNS Association neurons or interneurons connect the motor and sensory neurons in neural pathways – cell bodies are always located in the CNS Structural classification – based on the number of processes extending from the cell bodyMultipolar neuron – several processes extending from the cell bodyBipolar neurons – two processes – an axon and a dendrite – rare in adults and found only in special senses organs such as eyes and ears Unipolar neurons – have a single process emerging from the cell body but it is short and divides almost immediately into proximal (central) and distal (peripheral) fibers List the types of general sensory receptors and describe their functions. Cutaneous sense organs – skin – pain receptors are least specialized and the most numerous because pain warns us of body damage – strong stimulation by searing heat, extreme cold, or excessive pressure is also detected as pain Proprioceptor – detect the amount of stretch, or tension, in skeletal muscles, their tendons, and joints – they send information to the brain to the body can maintain balance and normal posture Describe the events that lead to the generation of a nerve impulse and its conduction from one neuron to another. – for an unmyelinated nerve Read figure 7.9 Cell membrane is polarized, at rest – fewer positive ions sitting on the inner face of the neuron’s plasma membrane than there are on its outer face in the tissue fluid that surrounds it – positive ions inside the cell are potassium (K+) and the positive ions outside the cell are sodium (Na+) A stimulus excites the neuron to become active and generate an impulse – could be from a sensory receptor (light, sound, taste, pain, pressure) or neurotransmitters released by other neurons – this causes the sodium gates to open in the membrane Depolarization – there is an inward rush of sodium ions that changes the polarity of the neuron’s membrane – the inside is now more positive and the outside less positive – activates the cell to initiate and transmit an action potential (nerve impulse) – an all-or-none-responseRepolarization – almost immediately after sodium ions rush into the neuron, the membrane becomes impermeable to sodium ions and permeable to potassium ions allowing potassium ions to diffuse out of the neuron restoring the electrical conditions at the membrane to the polarized, or resting, state – until this occurs, a neuron cannot conduct another impulse After repolarization – the initial concentrations of sodium and potassium ions inside and outside are restored by the activation of the sodium-potassium pump that uses ATP to move the ions back into placeIn nerves that have myelin sheaths, the nerve impulse must jump from node to node along the length of the never because no current can flow across the axonal membrane where there is fatty myelin insulation – this is a much faster impulse propagation called salutatory conduction Define reflex arc and list its elements. Reflexes – rapid, predictable, and involuntary responses to stimuli that occur over neural pathways called reflex arcs – classified as either autonomic or somatic reflexesAutonomic – regulate the activity of smooth muscles, the heart, and glands – digestion, elimination, blood pressure, sweating, pupillary (eye), salivary secretionsSomatic – include all reflexes that stimulate the skeletal muscles such as when you pull your hand away from a hot objectHave a minimum of five elements Sensory receptor – reacts to stimulusEffector organ – muscle or gland eventually stimulatedAfferent and efferent neurons to connect the two The synapse between the afferent and efferent neurons represents the central element – the CNS integration center Central Nervous System Identify and indicate the functions of the major regions of the cerebral hemispheres, diencephalon, brain stem, and cerebellum on a human brain model or diagram. See diagram page 215Cerebral hemispheres – speech, memory, logical and emotional responses, consciousness, interpretation of sensation, and voluntary movementsDiencephalon Thalamus – relay station for sensory impulses passing upward to the sensory cortex – gives us a crude idea of whether the sensation is pleasant or unpleasant Hypothalamus – regulates body temperature, water balance, and metabolism, also the center for many drives and emotions making it part of the limbic system (emotional-visceral brain) Epithalamus – contains the pineal body (part of the endocrine system) and the choroid plexus (forms the cerebrospinal fluid) Brain stem – pathway for ascending and descending tracts, gray matter which control vital activities such as breathing and blood pressureMidbrain – parts are involved as reflex centers involved with vision and hearing Pons – control of breathingMedulla oblongata – regulate vital visceral activities such as heart rate, blood pressure, breathing, swallowing, and vomiting Reticular formation – involved in motor control of the visceral organs with the reticular activating system (RAS) involved in consciousness and the awake/sleep cycles – damage to this area can result in permanent unconsciousness (coma) Cerebellum – provides precise timing for skeletal muscle activity and controls our balance and equilibrium – automatic pilot that compares the brain’s intentions with actual body performance by monitoring body position and amount of tension in various body parts Name the three meningeal layers and state their functions. Dura matter – outermost layer – tough/hard double-layered membrane that surrounds the brain Periosteal layer – inner layer attached to the inner surface of the skull forming the Periosteum Meningeal layer – outer layer that forms the outer-most covering of the brain and continues as the dura mater of the spinal cord Arachnoid matter – middle cobweb-like layer whose threadlike extensions span the subarachnoid space to attach it to the innermost membrane, the pia materPia matter – clings tightly to the surface of the brain and spinal cord, following every fold Discuss the formation and function of cerebrospinal fluid and the blood-brain barrier. Cerebrospinal fluid – watery broth similar to blood plasma from which it forms but it contains less protein, more vitamin C, and different ion concentrations – continually formed from blood by the choroid plexuses (clusters of capillaries) and is constantly flowing until it returns to the blood in the dural sinuses through the arachnoid villi Blood-brain barrier – keeps the brain neurons separate from bloodborne substances – composed of the least permeable capillaries found in the body and of the water-soluble substances, allows only water glucose, and essential amino acids to pass through easily keeping out both nonessential amino acids and potassium, which are actively pumped out of the brain – metabolic wastes (urea, toxins, proteins, and most drugs) are also prevented from entering the brain – virtually useless against fats, respiratory gases, and other fat-soluble molecules, which is why alcohol, nicotine, and anesthetics can affect the brain Compare the signs of a CVA with those of Alzheimer's disease; of a contusion with those of a concussion. Cerebrovascular accidents (CVA) – strokes – occur when blood circulation to a brain area is blocked, by a blood clot or a ruptured blood vessel leading to death of vital brain tissue – it is possible to determine the area of brain damage by observing the patient’s symptoms such as visual impairment, paralysis, and aphasias (speech problems) Alzheimer’s disease – a degenerative brain disease in which abnormal protein deposits and other structural changes appear resulting in slow, progressive loss of memory and motor control plus increasing dementia Contusion – is the result of marked tissue destruction – may result in coma lasting for hours or longer Concussion – occurs when brain injury is slight – patient is dizzy or may lose consciousness briefly but no permanent brain damage occursDefine EEG and explain how it evaluates neural functioning. Electroencephalogram – recording of the electrical impulses by neurons of the brain taken by placing electrodes at various points on the scalp and connecting these to a recording device – the patters of electrical activity of the neurons are called brain waves – each person’s pattern is unique Interference with the function of the cerebral cortex is suggested by brain waves that are too fast or too slow – unconsciousness occurs at both extremes, sleep and coma result in slower than normal brain waves while fright, epileptic seizures, drug overdoses result in abnormally fast brain waves – the absence of brain waves is evidence of clinical death List two important functions of the spinal cord. Provides a two-way conduction pathway to and from the brainIt is a major reflex center – the spinal reflexes are completed at this level Describe spinal cord structure. Spinal cord structure~17 inches long continuation of the brain stem enclosed within the vertebral column that ends just below the ribs - ~ the size of a thumb for most of its length and enlarged in the cervical and lumbar regions where nerves serving the upper and lower limbs arise and leave the cord Cushioned and protected by meninges, which do not end at L2 but extend beyond the end of the spinal cord in the vertebral canal 31 pairs of spinal nerves arise from the cord and exit from the vertebral column to serve the body area close by Collection of spinal nerves at the inferior end of the vertebral canal is called the cauda equine because it looks like a horse’s tail Gray matter of the spinal cord – the two posterior projections are the posterior, or dorsal, horns; the two anterior projections are the anterior, or ventral, horns – the gray matter surrounds the central canal of the cord, which contains CSF White matter of the spinal cord – composed of myelinated fiber tracts – divided into three regions – posterior, lateral, anterior columns – each column contains a number of fiber tracts made up of axons with the same destination and functionPeripheral Nervous System Describe the general structure of a nerve. A bundle of neuron fibers found outside the CNS – within a nerve, neurons fibers, or processes, are wrapped in protective connective tissue coveringsEach fiber is surrounded by a delicate connective tissue sheath, an endoneurium Groups of fibers are found by a coarser connective tissue wrapping, the perineuruim to form fiber bundles, or fascicles Fascicles are bound together by a tough fibrous sheath, the epineurium, to form the cordlike nerve Identify the cranial nerves by number and by name, and list the major functions of each. See table 7.1 on page 231 and 23212 pairs of cranial nerves are numbered in order, and in most cases, their names reveal the most important structures they control Describe the origin and fiber composition of (a) ventral and dorsal roots, (b) the spinal nerve proper, and (c) ventral and dorsal rami. Ventral root – of the spinal cord – where the cell bodies of the sensory neurons, whose fibers enter the cord by the dorsal root, and are found in an enlarged area called the dorsal root ganglionDorsal root – of the spinal cord – where the ventral horns of the gray matter contain cell bodies of motor neurons of the somatic (voluntary) nervous system, which send their axons out the ventral root of the cord – the dorsal and ventral roots fuse to form the spinal nerves Spinal nerve proper – very short and splits into dorsal and ventral rami Ventral rami –supply the muscles between the ribs and the skin and muscles of the anterior and lateral trunk Dorsal rami – serve the posterior body trunk both rami, like the spinal nerves, contain both motor and sensory fibers Discuss the distribution of the dorsal and ventral rami of spinal nerves. Dorsal rami – serve the skin and muscles of the posterior body trunkCentral rami – form the intercostal nerves, which supply the muscles between the ribs and the skin and muscles of the anterior and lateral trunk The ventral rami of all other spinal nerves form complex networks of nerves called plexuses, which serve the motor and sensory needs of the limbsName the four major nerve plexuses, give the major nerves of each, and describe their distribution. See table 7.2 on pages 235-236 for more detail Cervical – C1 – C5Brachial – C5 – C8 and T1Lumbar – L1 – L4 Sacral – L4 – L5 and S1 – S4Identify the site of origin and explain the function of the sympathetic and parasympathetic divisions of the autonomic nervous system. Parasympathetic division – the first neurons are located in brain nuclei of several cranial nerves – III, VII, IX, X and in the S2 – S4 level of the spinal cord – is active when the body is at rest and not threatened in any way – the resting and digesting system is chiefly concerned with promoting normal digestion and elimination of feces and urine, and with conserving body energy, and decreasing demands on the cardiovascular systemSympathetic division – the first neurons are in the gray matter of the spinal cord from T1 – L2 – the fight-or-flight system – works at full speed when you are emotionally upset and when you are physically stresses – effects of sympathetic nervous system activation continue for several minutes until its hormones are destroyed by the liver explaining why we need a calm down time even though the effects of the nerve impulses may act only briefly Contrast the effect of the parasympathetic and sympathetic divisions on the following organs: heart, lungs, digestive system, blood vessels. The sympathetic nervous system increases heart rate, blood pressure, and blood glucose levels, dilates the bronchioles of the lungs, dilated blood vessels in skeletal muscles and withdraws blood from the digestive organs and shunts blood to the heart, brain, and skeletal muscles The parasympathetic nervous system reverses the affects of the sympathetic nervous system – decreases the demands on the cardiovascular system to low normal levels, the digestive tract is actively digesting food and the skin is warm, the eye pupils are constricted to protect the retinas from excessive light and lenses are set for close vision Developmental Aspects of the Nervous System List several factors that may have harmful effects on brain development. Maternal measles (rubella), lack of oxygen, smoking, radiation, various drugs such as alcohol, opiate, and cocaineBriefly describe the cause, signs, and consequences of the following congenital disorders: spina bifida, anencephaly, cerebral palsy. Spina bifida – results when the vertebrae form incompletely (typically in the lumbosacral region) – in the most severe cases, meninges, nerve roots, and even parts of the spinal cord protrude from the spine, rendering the lower part of the spinal cord functionless and the child is unable to control the bowels or bladder, and the lower limbs are paralyzed Anencephaly – a failure of the cerebrum to develop, results in a child who cannot hear, see, or process sensory inputsCerebral palsy – neuromuscular disability in which the voluntary muscles are poorly controlled and spastic due to brain damage - may result from the lack of oxygen during delivery – half the victims have seizures, are mentally retarded, and/or have impaired hearing or visionExplain the decline in brain size and weight that occurs with age. The brain reaches its maximum weight in the young adult but over the next ~60 years, neurons are damages and die, and since they cannot replace themselves, our store of neurons continually decreases – however, an unlimited number of neural pathways are always available and ready to be developed Eventual shrinking of the brain is normal – however, certain activities can accelerate the process such as boxing and chronic alcoholism Define senility and list some possible causes. Results from a gradual lack of oxygen due to the aging process characterized by forgetfulness, irritability, difficulty in concentrating and thinking clearly and confusion Low blood pressure can be one cause – orthostatic hypotension is a type of low blood pressure resulting from quick changes in body position such as standing up too fast after sitting for a long period and the sympathetic nervous system does not have time to act quickly enough to counteract the pull of gravity by activating the vasoconstrictor fibers causing blood to pool in the feet resulting in the person becoming lightheaded or fainting ................
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