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Nervous System

Nervous System. Chapter 7. I. Overview. Three overlapping functions Monitor changes inside and outside of the body (sensory input) Processes sensory input and makes decisions about what to do (integration) Then effects a response called a motor output

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Nervous System

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  1. Nervous System Chapter 7

  2. I. Overview • Three overlapping functions • Monitor changes inside and outside of the body (sensory input) • Processes sensory input and makes decisions about what to do (integration) • Then effects a response called a motor output • Works with the endocrine system to maintain homeostasis

  3. II. Organization of the Nervous System • Structural Classification • Central nervous system – brain and spinal cord • Interpret incoming sensory information and issues instructions based on past experiences and current conditions • Peripheral nervous system – outside of CNS • Spinal nerves – impulse to and from spinal cord • Cranial nerves – impulse to and from brain

  4. II. Organization of the Nervous System • Functional Classification • Concerned only with PNS • Sensory (afferent) division – convey impulses to the CNS from sensory receptors • Somatic sensory fibers – from skin, skeletal, and joints • Visceral sensory fibers – from visceral organs • Motor (efferent) division – carries impulses from the CNS to effector organs, muscles, and glands • Somatic nervous system (voluntary nervous system) – allows us to consciously, or voluntarily, control our skeletal muscles (except for those reflex muscles) • Autonomic nervous system (involuntary nervous system) – regulates events that are automatic or involuntary • sympathetic and parasympathetic

  5. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Supporting Cells Neuroglia (glia) – literally “nerve glue”, generally support, insulate, and protect Resemble neurons, but not able to transmit nerve impulses Never lose ability to divide, thus most brain tumors are gliomas or formed from neuroglia

  6. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Astrocytes – abundant, star-shaped, nearly half of neural tissue Numerous swollen projections that cling to neurons and anchor them to capillaries for nutrients Form a barrier between capillaries to protect the neurons from substances in the blood Control chemical environment by picking up excess ions and released neurotransmitters

  7. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Microglia – spider-like phagocytes that dispose of debris, including dead brain cells and bacteria Ependymal cells – line cavities of brain and spinal cord, cilia help to circulate cerebrospinal fluid and forms a protective cushion around the CNS

  8. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Oligodendrocytes – wrap flat extensions tightly around nerve fibers, producing fatty insulating coverings called myelin sheath PNS supporting cells Schwann cells – form myelin sheaths Satellite cells – act as protective, cushioning cells

  9. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Neurons Anatomy Neurons (nerve cells) – highly specialized to transmit messages from one part of the body to another All include a cell body and one or more processes Cell body is metabolic center of neuron containing organelles except for centrioles Particularly abundant are rough ER (Nissl substance), neurofibrils, and intermediate filaments for cell shape

  10. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Arm-like processes vary from microscopic to 3-4 feet Longest from lumbar region to big toe Processes conveying incoming messages are dendrites, contains hundreds per neuron Processes generating nerve impulses are axons, only one per neuron, arises from axon hillock Most long nerve fibers are covered with whitish, fatty material, with a waxy material called myelin – protects, insulates, and increases transmission rates

  11. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Axons outside CNS are surrounded by Schwann cells which wrap their membranes tightly around the axon, forming a myelin sheath; the cytoplasm ends up on the outside and the outer layer is called the neurilemma There are gaps between the Schwann cells called nodes of Ranvier at regular intervals

  12. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Myelinated fibers also found in CNS, but formed from oligodendrocytes, which can coil around as many as 60 fibers with their flat extensions, but lack a neurilemma which means they are not protected and cannot regenerate Axons occasionally give off collateral branches along its length, but branch profusely at their terminal end to form axonal terminals which contain hundreds of tiny vesicles, or membranous sacs, containing neurotransmitters which are released when stimulated; they are separated from next neuron by a tiny gap called synaptic cleft (synapse)

  13. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Clustered neuron cell bodies in the CNS are called nuclei, a few small clusters outside the CNS in the PNS are called ganglia Bundles of nerve fibers running through CNS are called tracts and in PNS are called nerves Cell body carries out most of the metabolic functions, so if it is damaged or dies it is not replaced Regions of the CNS • White matter is dense collections of myelinated fibers • Gray matter is mostly unmyelinated fibers and cell bodies

  14. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Homeostatic Imbalance Importance of myelin sheaths is seen in people with multiple sclerosis (MS) Myelin sheaths are gradually destroyed, converted to hardened sheaths (scleroses) and when this happen the electrical current is short-circuited, which causes loss of muscle control and they become increasingly disabled Classification Functional Classification Groups neurons according to the direction the nerve impulse is traveling

  15. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Sensory neurons – cell body found outside the CNS in a ganglion, dendrite endings have specialized receptors that are activated by changes nearby Complex receptors of special sense organs (vision, hearing, equilibrium, taste, and smell) Skin – cutaneous sense organs, extreme heat, cold or excessive pressure can be interpreted as pain Muscles and tendons – proprioceptors, detect amount of stretch or tension so proper adjustments can be made to maintain balance and normal posture Pain – bare dendrite endings, most numerous Motor neurons – carry nerve impulses from CNS to viscera, muscles, and/or glands, cell bodies are always located in CNS Associations neurons (interneurons) – connect motor and sensory neurons, cell bodies always located in CNS

  16. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Structural classification – based on number of processes extending from the cell body Multipolar neuron – several processes, most common structural type, include all motor and association neurons Bipolar neurons – two processes (axon and dendrite), rare in adult, only in special sense organs (eye, ear)

  17. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Unipolar neurons – single process, very short and divides almost immediately into proximal (central) and distal (peripheral) fibers; only the small branches at the end of the peripheral process are dendrites, remainder act as axons Conduct nerve impulses toward and away from the cell body, included are sensory neurons in the PNS ganglia

  18. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION • Physiology • Nerve impulses – two major functions • Irritability - the ability to respond to a stimulus and convert it into a nerve impulse • A neuron is polarized when it is resting, or inactive; this also means that there are fewer positive ions inside the plasma membrane than out • Ions inside are K+ and ions outside are Na+, and as long as the inside remains more negative the neuron stays inactive

  19. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION • Nerves are excited by many different types of stimuli, but most of the neurons in the body are excited by neurotransmitters released by other neurons • Nerve impulses are an all-or-none response and either happens over the whole axon or not at all • When a neuron is adequately stimulated, the “sodium gates” open and Na+ quickly diffuses into the neuron, changing the polarity of the membrane; this process is called depolarization

  20. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION • If this stimulus is strong enough, and there’s a large enough Na+ in-rush, then depolarization activates the neuron to initiate and transmit an action potential (nerve impulse) • Once Na+ ions rush in the membrane permeability changes again and is no longer permeable to Na+, but permeable to K+, which diffuse out and the electrical charge is restored; this is called repolarization

  21. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Until repolarization occurs there cannot be another nerve impulse conducted The initial concentrations must still be restored, so the Na-K pump is activated, which uses ATP This nerve impulse is along unmyelinated fibers Nerve impulses occur much faster though down myelinated fibers because it leaps from node to node since the current cannot flow across myelin insulation – this is called saltatory conduction Nerve Impulses

  22. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION • Conductivity – the ability to transmit the nerve impulse to other neurons, muscles, or glands • Impulses from one neuron travel across the synapse to another by means of a neurotransmitter • Dendrite of the next neuron receives the neurotransmitter and an action potential is then started

  23. Homeostatic Imbalance – impact on conduction of impulses alcohol, sedatives, and anesthetics all reduce membrane permeability Cold and continuous pressure interrupt blood circulation, so after warming up or pressure removed then prickly feeling comes when impulses are transmitted again III. NERVOUS TISSUE: STRUCTURE AND FUNCTION

  24. III. NERVOUS TISSUE: STRUCTURE AND FUNCTION Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector Reflexes are rapid, predictable, and involuntary responses to stimuli that always travel in the same direction Autonomic reflexes regulate the activity of smooth muscles, the heart, and glands, saliva, eye pupils, digestion, elimination, blood pressure, sweating Somatic reflexes are all reflexes that stimulate skeletal muscles

  25. IV. CENTRAL NERVOUS SYSTEM Composed of brain and spinal cord During embryonic development, appears as a simple tube (neural tube) that extends down the dorsal median plane, and by the fourth week the anterior end begins to expand and brain formation begins

  26. IV. CENTRAL NERVOUS SYSTEM Functional Anatomy of the Brain Opening of the neural tube becomes the four chambers of the brain called ventricles, filled with cerebrospinal fluid (CSF) Cerebral hemispheres – paired, superior part of the brain, larger than all other regions combined

  27. IV. CENTRAL NERVOUS SYSTEM Surface has ridges called gyri seperated by shallow grooves called sulci, and few deeper grooves called fissures that separate large regions of the brain these all serve as landmarks Single, deep fissure (longitudinal fissure) separates hemispheres Other fissures or sulci divide hemispheres into lobes named for cranial bones that lie over them

  28. IV. CENTRAL NERVOUS SYSTEM

  29. IV. CENTRAL NERVOUS SYSTEM Parietal lobe lies posterior to the central sulcus and is home to the somatic sensory area all of the body’s sensory receptors (except special senses) are localized and interpreted here, and all of the pathways are upside-down and backwards

  30. IV. CENTRAL NERVOUS SYSTEM Frontal lobe lies anterior to the central sulcus and is home to the primary motor area Axons of these motor neurons form major voluntary motor tract – the pyramidal, or corticospinal tract, which descends to the cord Like the somatic sensory cortex, all of the pathways are upside-down and backwards

  31. IV. CENTRAL NERVOUS SYSTEM Broca’s area is involved in the ability to speak, damage causes inability to say words properly Higher intellectual reasoning located in anterior of frontal lobe, and language comprehension Speech area is at the junction of temporal, parietal, and occipital lobes, it allows you to sound out words Cell bodies of neurons involved in cerebral hemisphere functions are found only in the outermost gray matter – the cerebral cortex

  32. IV. CENTRAL NERVOUS SYSTEM Most of the remaining cerebral hemisphere – cerebral white matter – is composed of fiber tracts carrying impulses to/from the cortex Corpus callosum – large fiber tract connecting cerebral hemispheres Basal nuclei – islands of gray matter deep in white matter, help regulate voluntary motor activities by modifying instructions sent to the skeletal muscles by the primary motor cortex

  33. IV. CENTRAL NERVOUS SYSTEM

  34. IV. CENTRAL NERVOUS SYSTEM Diencephalon (interbrain) – sits atop brain stem and enclosed by cerebral hemispheres Thalamus – encloses shallow third ventricle, relay station for sensory impulses passing to sensory cortex and recognizes whether the impulse in pleasant or unpleasant

  35. IV. CENTRAL NERVOUS SYSTEM Hypothalamus makes up floor of diencephalon, plays role in regulation of body temperature, water balance, and metabolism Also center for many drives and emotions, called limbic system – thirst, appetite, sex, pain, and pleasure centers Regulates pituitary gland or hypophysis (endocrine organ) Mammillary bodies involved in olfaction (smell) bulge from floor

  36. IV. CENTRAL NERVOUS SYSTEM Epithalamus – roof of third ventricle, includes pineal body (endocrine) and choroid plexus (knots of capillaries in each ventricle that form the CSF) Brain Stem – about the size of a thumb in diameter and about 3 inches long Pathway for tracts, many small gray matter areas, control vital activities such as breathing and blood pressure

  37. IV. CENTRAL NERVOUS SYSTEM Midbrain – small part of brain stem Cerebral aqueduct is a tiny canal that travels through the midbrain and connects the third and fourth ventricle Cerebral peduncles – two anteriorly bulging fiber tracts which convey ascending and descending impulses Corpora quadrigemina – four rounded dorsal protrusions, these are reflex centers involved with vision and hearing

  38. IV. CENTRAL NERVOUS SYSTEM Pons – means bridge, rounded structure protruding just below midbrain, mostly fiber tracts, nuclei for breathing Medulla oblongata – most inferior of brain stem, merges into spinal cord, fiber tract area, control center for heart rate, blood pressure, breathing, swallowing, vomiting, and others Reticular formation – extends length of brain stem, diffuses mass of gray matter, involved in motor control of visceral organs Reticular activating system (RAS) plays a role in consciousness and the awake/sleep cycles and damage to this area can result in coma

  39. IV. CENTRAL NERVOUS SYSTEM Cerebellum – large, cauliflower-like projection dorsally from under occipital lobe of cerebrum Much like cerebrum – two hemispheres, convoluted surface, outer cortex of gray matter and inner white matter Provides precise timing for skeletal muscle activity and controls balance and equilibrium Fibers come here from the equilibrium apparatus of the inner ear, the eye, the proprioceptors, and other areas

  40. IV. CENTRAL NERVOUS SYSTEM Protection of the Central Nervous System Meninges – three connective tissue membranes covering and protecting the CNS Dura mater – leathery, outermost layer, double-layered membrane surrounding the brain One layer is attached to the inner surface of the skull forming the periosteum Other layer is outermost covering of the brain that continues as the dura mater of the spinal cord, called meningeal layer Fused together except where they separate to enclose dural sinuses (collects venous blood) Dural sinuses fold inward in several places to attach brain to cranial cavity – falx cerebri and tentorium cerebelli

  41. IV. CENTRAL NERVOUS SYSTEM

  42. IV. CENTRAL NERVOUS SYSTEM Arachnoid mater – web-like middle meningeal layer Thread-like extensions span subarachnoid space, which is filled with CSF, and attach to pia mater Arachnoid villi – protrude through dura mater, CSF absorbed into venous blood in dural sinuses Pia mater – delicate, innermost membrane, clings tightly to surface of brain following every fold

  43. IV. CENTRAL NERVOUS SYSTEM Pneumococcal meningitis in an alcoholic patient. Head opened at autopsy revealing purulent inflammation of leptomeninges beneath reflected dura mater.

  44. IV. CENTRAL NERVOUS SYSTEM Cerebrospinal Fluid – similar to blood plasma, but less protein, more vitamin C, and different ion composition Continually formed from blood by choroid plexuses, which are capillaries hanging from the top in each ventricle Continually moving by circulating through lateral ventricles in cerebral hemispheres, third ventricle (diencephalon), and fourth ventricle (brain stem) Some fluid reaching fourth ventricle travels down central canal of spinal cord, but most stays in subarachnoid space which leaves through holes in fourth ventricle Fluid is recycled through arachnoid villi into blood in the dural sinuses Normal volume is about 150 mL or ½ cup

  45. IV. CENTRAL NERVOUS SYSTEM Any changes in CSF can indicate a problem in brain or spinal cord, and a sample can be obtained through a lumbar (spinal) tap, and checked for blood cells Since the fluid is decreased with this test, patient must remain horizontal for 6 – 12 hours to prevent a headache

  46. IV. CENTRAL NERVOUS SYSTEM

  47. IV. CENTRAL NERVOUS SYSTEM • Hydrocephalus • If the CSF cannot drain it accumulates and causes pressure on the brain, this is called hydrocephalus, which literally means water on the brain • In babies the head enlarges to allow for the extra spinal fluid because of the soft bones • In adults though the condition can lead to brain damage because of hardened skull • A shunt to drain excess fluid is then placed

  48. IV. CENTRAL NERVOUS SYSTEM • The Blood-Brain Barrier • Separates neurons from blood-borne substances, composed of least permeable capillaries in the whole body which provide most of the protection and the astrocytes help with this • Only water, glucose, and essential amino acids pass easily, but urea, toxins, proteins, and most drugs are prevented from entering • Useless against fats, respiratory gases, and other fat-soluble molecule (alcohol, nicotine, anesthetics)

  49. Brain Dysfunctions Traumatic Brain Injuries Leading cause of accidental death in US Concussion – slight brain injury, cause dizziness and brief loss of consciousness but no permanent brain damage Contusion – result of marked tissue destruction Hemorrhage or cerebral edema – individuals usually alert then deteriorate neurologically, caused from compression of vital brain tissue IV. CENTRAL NERVOUS SYSTEM

  50. IV. CENTRAL NERVOUS SYSTEM Cerebrovascular Accident (CVAs) – strokes Third leading cause of death in the US Occur when blood circulation to a brain area is blocked by clot, rupture, or tissue dies Area of brain damage can be determined by patient’s symptoms Fewer than 1/3 of those that survive are alive three years later, but not hopeless, undamaged neurons take over some of the lost function • Not all strokes are “completed”, result in temporary restriction of blood flow called a transient ischemic attack (TIA) that last from 5 to 50 minutes, these are red flags though for impending more serious CVAs

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