Ch 8 Nervous System PDF

Lecture Outline C e p a h 8 t The Nervous System 神經系統...

Lecture Outline C e p a h 8 t The Nervous System 神經系統 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Learning Objectives After studying this chapter, students should be able to: o Describe the structure of a neuron and the functions of the three types of neurons. o Explain how a nerve impulse is conducted along a nerve and across a synapse. o Describe the three layers of meninges and state the functions of the meninges. o Describe the location and function of cerebrospinal fluid. o Describe the major parts of the brain and the lobes of the cerebral cortex. o Describe in detail the structure of the spinal cords and state its functions. o Describe the structure of a reflex arc and the function of a reflex action. o Define and describe the autonomic nervous system. o Describe how the nervous system works with other systems of 2 the body to maintain homeostasis. Nervous System o Three specific functions that work to maintain homeostasis 體內平衡: Sensory input Integration Motor output 3 Nervous System o Divisions of the Nervous System Central nervous system (中樞神經系統 CNS) including ❑ Brain ❑ Spinal cord 脊髓 Peripheral nervous system (PNS周邊神經 系統) – all other nervous components aside CNS, including all the cranial and spinal nerves ❑ Afferent (sensory) division ❑ Efferent (motor) division 4 Organization of the Nervous System Nervous System : the tissue o Nervous Tissue consists of: Neurons (神經元 nerve cells) – generate & transmit impulses 脈衝 Neuroglia神經膠質 – support and nourish neurons ❑ They cannot conduct impulse 6 Example of Neuroglia 神經膠質 in CNS 7  Nervous Tissue : Neuroglia 神經膠質 o Neuroglia in the CNS: Microglia小神經膠質-engulf bacterial and cellular debris Astrocytes星形細胞- provide nutrients to neurons & produce a hormone-glia-derived growth factor Oligodendrocytes(少突膠質細胞)- form myelin Ependymal cells(室管膜細胞)- line the internal cavities of the ventricles of the brain and the central canal of the spinal cord， those in choroid plexus help form cerebrospinal fluid(CSF) o Neuroglia in the PNS https://www.youtube.com/watch?v=BoI6mBbdPnw Schwann cells(許旺氏細胞) that form myelin covering of axons in the PNS Satellite glial cells (SGCs) that form sheaths around the neuronal cell body. 8  Nervous Tissue : Neuron -Structure Neuron structure ❑ Cell body –contains nucleus and other organelles ❑ Dendrite – receive signals from sensory receptors or other neurons ❑ Axon ▪ Conducts nerve signals away from cell body ▪ Nerve – bundle of parallel axons in the PNS ▪ Tract – bundle of parallel axons in the CNS ▪ May be covered by myelin (lipid coating) - Formed by Schwann cells or neurolemmocytes in Peripheral Nervous System (PNS) - Formed by oligodendrocytes in Central Nervous System (CNS) ❑ Axonal terminal (form part of Synapse (突觸)) ▪ Communicate with an effector cell ▪ Use either chemical or electrical signals. 9 10 10  Nervous Tissue : Neuron – types Types of neurons (classified by function) ❑ Motor neurons (運動神經元) ▪ Take nerve impulses from the CNS to muscles, organs or glands ▪ Multipolar – have many dendrites ❑ Sensory neurons (感官神經元) ▪ Take nerve impulses from sensory receptors to the CNS ▪ Most are unipolar ❑ Interneurons (association neurons 間神經元) ▪ All are in the CNS ▪ Typically multipolar ▪ Convey nerve impulses between various parts of the CNS ▪ Form complex pathways for thinking, memory and language 11 Neuron: Signal generation and conduction I 2kt A. Neuron at rest outsidecell Resting membrane potential (靜止電位) ▪ Sodium potassium pumps insidecell - pump 3 Na+ ions out for 2 K+ ions into the cell across the plasma membrane. - and generate imbalance of ions concentration across the plasma membrane: more Na+ outside and K+ inside the cell ▪ Plasma membrane is less permeable to Na+ but more permeable to K+ at rest. change of charge in the membrane ▪ The membrane is thus polarized: - More positively charged ions outside the neuron - And more negatively charged inside 12 Ionic movements & Resting Membrane Potential Na+/K+ pump action =>more negatively charged ions inside relative to the outside at rest. Neuronal membrane is polarized due to distribution of ions Resting membrane Potential Video: https://www.youtube.com/watch?v=MplWXZTOk6o 13 Neuron Resting Potential - YouTube Neuron: Signal generation and conduction II B. Nerve signal Animation: Ion channels - YouTube Action potential (AP) Voltage-Gated Sodium Channels in Neurons – YouTube Action Potential in the Neuron - YouTube ▪ It occurs on neural axons. ▪ It is triggered by a sufficiently positive stimulus (ie. membrane potential reaching the threshold). ▪ It is the potential change across a neural membrane, involving movement of Na+ and K+ ions, by opening Na+ and K+ voltage-gated channels sequentially. ▪ It is an ‘all-or-none’ event: may or may not occur, but when it does occur, it will always be of the same size/amplitude (not directly proportional to the size of the stimulus) 14 ▪ Action Potential consists of three phases: Depolarization: Membrane sodium voltage-gated channels are opened but potassium voltage-gated channels closed Na+ ions flow through voltage-gated channels into the neuron Repolarization: Sodium voltage-gated channels are closed but potassium voltage-gated channels open. K+ ions exit the neuron. After-hyperpolarization: action of sodium and potassium pump restores charge distribution on both sides of the neural membrane, returning the membrane potential back to resting level (ie. Resting member potential) and make the neuron ready to generate another impulse. 15 Animation Action Potential in Neurons, Animation. - YouTube 16 16 Action Potentials Phases Resting phase at rest membrane is polarized and Na+ & K+ voltage-gated channels closed Depolarization phase After threshold reached, when sodium voltage-gated channels open & membrane depolarizes Repolarization phase When K+ voltage-gated channels open & Na+ voltage-gated channels close, so potassium ions leave cytoplasm and 17 membrane repolarizes ▪ An Action Potential has Refractory Period absolute refractory period : a time when sodium voltage-gated channels are all open, so can’t respond ‘anymore’ to another stimulus. The neuron is NOT able to respond to another stimulus. relative refractory period : In this period, the potassium voltage-gated channels are still open, but the sodium voltage- gated channels closed. It is possible for the membrane to depolarize again, with a stronger than normal stimulus to generate a second Action Potential. 18 Neuron: Signal generation and conduction III C. Nerve signal conduction/propagation Action potential conduction/propagation ▪ Often occurs on a neuronal axon ▪ When Action potential is generated at one point on an axon, where charges would be different from the adjacent regions. Opposite charges attract, causes a “local current”, which in turn causes the adjacent region to depolarize. If the depolarization is sufficient an action potential will be initiated in the adjacent region. 19 Conduction/propagation of action potentials On unmyelinated axons(無髓鞘) ▪ is called continuous conduction ▪ moves from region to region ▪ is slow (~1 meter/second) On myelinated (髓鞘) axons ▪ is called saltatory conduction ▪ an action potential “jumps” over myelinated portion of axon from node to node ▪ Moves much faster (>100 meters/second) Propagation of Action Potential-video Propagation of action potential in an unmyelinated axon - Animated medical physiology – YouTube How the action potentials are propagated – YouTube 20 Action potential propagation in an unmyelinated axon – YouTube Saltatory conduction - Conduction through Myelinated nerve fiber : Physiology medical animations - YouTube Neuron: Signal Transmission Definitions: ❑ Axon terminal (軸突) – small swelling at tips of the branched end of an axon ❑ Synapse(突觸) ▪ Region of close proximity between two neurons: ✓ Presynaptic membrane – membrane of the first neuron ✓ Postsynaptic membrane – membrane of the next neuron ▪ Synaptic cleft – small gap between the presynaptic and the postsynaptic neuron ❑ Neurotransmitters(神經傳遞介質) – molecules that carries information across a synapse 21 Neuron: Signal transmission How a synapse works - YouTube Information transfer using chemical signals 1. When an action potential, travelling along an axon, reaches an axon terminal, calcium voltage-gated channels open and Ca2+ enters the terminal. 2. Rise in Ca2+ causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitter (NT 神經遞質) molecules by exocytosis into synaptic cleft. 3. Neurotransmitters diffuse to postsynaptic membrane and bind to specific receptors. 4. Ligand-gated channels will open. 5. Depending on the type of neurotransmitter, the postsynaptic membrane will be ‘excited’ or ‘inhibited’. 6. The neurotransmitter will be removed from the synapse eventually. 22  Chemical synapse Types of synapses - electrical & chemical - YouTube Nerve Synapse Animation – YouTube Changes of membrane potential ❑ Graded potentials (梯度電位) and synaptic integration (突觸整合) ▪ Graded potentials – small signals on post-synaptic membrane produced by the neurotransmitter, include ▪ Excitatory graded potential - potential that promotes formation of an Action Potential ▪ Inhibitory graded potential - potential that inhibits formation of an Action Potential ▪ Integration involves summing up the excitatory and 24 inhibitory signals Summation of Postsynaptic Potentials - YouTube ❑ Neurotransmitter Molecules ▪ More than 75 types have been identified ▪ Two well-known neurotransmitters are: - Acetylcholine (ACh) - Norepinephrine (NE) ▪ After a neurotransmitter has initiated a response, it is removed from the synaptic cleft by - reabsorption to the presynaptic terminal to prevent continuous stimulation (/ inhibition) of postsynaptic membranes or - inactivation by enzymes 25 Central Nervous System (CNS): tissue, membranes and fluid Definitions: o Gray matter – contains cell bodies and unmyelinated fibers o White matter – contains myelinated axons o Meninges腦膜 and Cerebrospinal Fluid(CSF) Meninges – protective membranes of the CNS ❑ Dura matter(硬膜) – outer menix composed of tough, fibrous connective tissue ❑ Arachnoid matter(蛛網膜) – middle menix composed of spider-web-like connective tissue ❑ Pia matter(軟腦膜) – deepest menix 26 Meninges Cerebrospinal fluid (腦脊液CSF) Produced mainly by choroid plexus specialized structure produced by ependymal cells created by folds of pia matter, projecting into brain ventricles. Fills the following: ❑ Subarachnoid space – space between the arachnoid matter and the pia matter ❑ Ventricles (腦室) – hollow, interconnecting cavities of the brain ❑ Central canal – hollow, space of the spinal cord Reabsorbed back into dural venous sinuses Blockages can cause hydrocephalus. 28 https://www.youtube.com/watch?v=444vP0Qzg_M Central Nervous System: The spinal cord o Extends from the base of the brain to the lumbar vertebra o Protected by vertebral column (脊柱) o Occupied the vertebral canal o Contain Spinal nerves, connecting periphery and spinal cord through intervertebral foramen, divided into root sections: a. Posterior (dorsal) root – carries sensory fibers into the spinal cord; posterior root ganglion b. Anterior (ventral) root – carries motor fibers out of the spinal cord Structure of the spinal cord ▪ Central canal contains CSF ▪ Centrally located gray matter - H-shaped contains: Posterior (dorsal) horn – contains sensory fibers Anterior (ventral) horn – contains motor fibers interneurons ▪ White matter-surrounds gray matter - Posterior white matter composed of ascending tracts carrying sensory information to the brain - Anterior white matter composed of descending tracts carrying motor information from the brain - Tracts generally cross from one side of the spinal cord to the other https://www.youtube.com/watch?v=K-P_BKOUFXs 31  Functions of the spinal cord a. Provides communication between the brain and the peripheral nerves b. Reflexes(反射) 1) Sensory receptors respond to a stimulus 2) Impulse travels over sensory neurons to the spinal cord 3) Interneurons integrate data and relay a response by way of motor neurons 4) Motor neurons communicate with effectors to cause action * The pathway of the information flow is called a Reflex Arc**  Central Nervous System: The brain ❑ Cerebrum大腦 ❑ Diencephalon間腦 ❑ Cerebellum小腦 ❑ Brain stem腦幹 ❑ Ventricles ▪ Two lateral ventricles (cerebrum) ▪ Third ventricle (diencephalon) ▪ Fourth ventricle (cerebellum) 33 The human brain  The Cerebrum(大腦) ❑ Largest portion of the brain ❑ Voluntary motor responses ❑ Coordinates the activities of the other parts of the brain ❑ Involved in higher thought processes ❑ The cerebral hemispheres(大腦半球) ▪ Longitudinal fissure(縱裂) divides the left and right Channel connecting the left/right brain ▪ Connected by the corpus callosum(胼胝體) ▪ Gyri (腦回ridges) are separated by sulci (溝 shallow grooves) 35 The lobes of a cerebral hemisphere Lobes Frontal lobe Parietal lobes Occipital lobe Temporal lobes Insula (deep to the lateral sulcus)  ☆ exam: point different area of brain & ask the actions The cerebral cortex ❑ Outer layer of gray matter ❑ Accounts for sensation, voluntary movement, and consciousness ❑ Motor and sensory areas ▪ Primary motor area - In frontal lobe anterior to central sulcus - Each hemisphere controls voluntary commands to skeletal muscle of the opposite side of the body ▪ Primary somatosensory area - Posterior to central sulcus in parietal lobes - Each hemisphere receives sensory information from the skin 37 and skeletal muscles on the opposite side of the body  Somatotopical organization of the primary motor and sensory cortex -Cortical Homunculus Cortical Homunculus are the physical representation of the body: ▪ The motor homunculus- in the primary motor cortex in precentral gyrus ▪ The somatosensory homunculus – in the primary sensory cortex in postcentral gyrus ▪ They are not proportionate to the body 38 ❑ Primary taste area ▪ Located in insula ▪ Taste sensations ❑ Primary visual area ▪ Located in the occipital lobe ▪ Receives information from our eyes ❑ Primary auditory area ▪ Located in the temporal lobe ▪ Receives information from our ears 39 Association areas ❑ The sensory and motor regions of the cortex are connected to nearby association areas. ❑ Regions that interpret incoming data or coordinate a motor response ❑ Integration and learning ❑ Memory are stored (involve hippocampus) ❑ Premotor area organizes motor functions for skilled motor activities and then the primary motor area sends signals to the cerebellum and the basal nuclei, which integrate them. ▪ Cerebral palsy https://www.youtube.com/watch?v=cOfUGUNxEqU 40 Processing centers ▪ Prefrontal area - Uses information from other association areas & perform reasoning and planning actions ▪ Motor speech area (Broca’s area) - speaking is partially depends on it - damage →inability to control the muscles of the face and neck that allow speech ▪ Wernicke’s area (general interpretive area) - receives information from all other sensory association areas. - damage →hinder a person’s ability to interpret written and spoken messages https://www.youtube.com/watch?v=ZNsOZvEmhQc 41 Central white matter a. Tracts communicate information between different sensory, motor and association areas b. Tracts communicate with other brain areas c. Tracts communicate with the spinal cord 1) Descending tracts come from the primary motor area 2) Ascending tracts go to the primary somatosensory area d. Corpus callosum joins the two cerebral hemispheres Cerebral white matter tracts https://www.youtube.com/watch?v=yh_argH-hFA  Basal nuclei ▪ Masses of gray matter deep in the cerebrum ▪ Integration and selection of voluntary behaviour ▪ Contains caudate nucleus, putamen, globus pallidus ▪ Most neurons here use gamma-aminobutyric acid (GABA) as the inhibitory neurotransmitter ▪ are responsible for voluntary motor control, procedural learning and eye movement, as well as cognitive and emotional (ie. motivation) functions. ▪ Huntington disease and Parkinson’s disease – uncontrollable movements due to loss of cells. 2-Minute Neuroscience: Direct Pathway of the Basal Ganglia – YouTube 2-Minute Neuroscience: Indirect Pathway of the Basal Ganglia - YouTube Central Nervous System The diencephalon ❑ Hypothalamus ▪ Forms the floor of the third ventricle ▪ Integrating center involved in homeostasis ▪ Regulates hunger, sleep, thirst, body temperature and water balance ▪ Link between nervous and endocrine systems ❑ Thalamus ▪ Located in sides and roof of the third ventricle ▪ Functions as a sensory relay center ▪ Involved in arousal of the cerebrum ▪ Participates in memory and emotions ❑ Epithalamus ▪ Pineal gland (secrete melatonin) regulates body’s daily rhythms 44 45 ❑ Limbic system ▪ Group of brain regions including cerebral (ie. cingulate and gyrus, amygdala, hippocampus, septal nuclei, anterior insula) diencephalon (ie. hypothalamus and thalamus) components ▪ Contains neural pathways that connect portions of the cerebral cortex and the temporal lobes with the thalamus and the hypothalamus ▪ involved in behavioural and emotional responses, especially survival response, such as feeding, reproduction and caring for young and fight or flight responses ▪ facilitate memory (storage & retrieval) and learning ▪ Hippocampus involved in processing short-term memory to become long-term memory 46 The Limbic System  The cerebellum ❑ Separated from brain stem by the fourth ventricle ❑ Two hemispheres composed primarily of white matter ❑ Receives sensory input from the eyes, ears, joints, and muscles ❑ Receives motor output from the cerebral cortex ❑ Integrate & send motor impulses via brainstem to the skeletal muscle ❑ Maintains posture and balance and ensures smooth, coordinated voluntary movement 48 ❑ Assists in learning of new motor skills 49  The brain stem ❑ Midbrain ▪ Relay station between cerebrum and the spinal cord or cerebellum ▪ Reflex centers for visual, auditory and tactile responses ❑ Pons ▪ Contains bundles of axons travelling between the cerebellum and the rest of the CNS ▪ Helps regulate breathing rate with the medulla oblongata ▪ Reflex centers for head movements in response to visual and auditory stimuli ❑ Medulla oblongata ▪ Vital reflex centers for regulating heartbeat, breathing and vasoconstriction. ▪ Reflex centers for vomiting, coughing, sneezing, hiccupping, and swallowing 50 51 o Reticular formation Tracts and nuclei connected to many other brain areas Involved in maintaining muscle tone with the cerebellum Assists in regulating respiration, heart rate and blood pressure with the medulla and pons Helps rouse a sleeping person with the thalamus Processes sensory stimuli to keep us mentally alert Damage may result in coma Peripheral Nervous System o Lies outside of CNS o Composed of nerves and ganglia o Subdivisions Afferent (sensory) ❑ Somatic sensory ▪ Serves the skin, skeletal muscles, joints, and tendons ▪ Special senses ❑ Visceral sensory supplies the internal organs Efferent (motor) ❑ Somatic motor system carries commands only from CNS to the skeletal muscles ❑ Autonomic motor system regulates cardiac and smooth muscle and glands 53 54 Peripheral Nervous System: Cranial nerves o Twelve named pairs o Sensory nerves contain only sensory fibers (nerves I, II, and VIII) o Motor nerves contain only motor fibers (nerves III, IV, VI, XI, and XII) o Mixed nerves contain both sensory and motor fibers (nerves V, VII, IX, and X) o Mostly involved with the head, neck, and facial regions of the body o The vagus nerve (cranial nerve X) has sensory and motor branches to the face and most of the internal organs Cranial Nerves  Spinal nerves o 31 pairs o Designated according to their location in relation to the vertebrae o Spinal nerves are mixed nerves a. Sensory fibers enter at the posterior root b. Motor fibers exit at the anterior root c. The cell body of a sensory neuron is in a posterior-root ganglion d. The cell body of a motor neuron is in the anterior gray matter of the cord Spinal Nerves Spinal Nerves Peripheral Nervous System o Somatic Motor Nervous System and Reflexes Voluntary actions that originate in the motor cortex Utilize 1 somatic motor neuron to travel directly to effector e.g. skeletal muscle fiber without synapse at a ganglion Reflexes are automatic involuntary responses ❑ Cranial reflexes involve the brain ❑ Spinal reflex involves only the spinal cord 61 Spinal，somatic reflex arch Peripheral Nervous System: ANS o Autonomic Motor Nervous System (ANS) and Visceral Reflexes sympathetic & parasympathetic Two divisions of ANS ❑ Function automatically and involuntarily ❑ Innervate all internal organs ❑ Utilize two motor neurons and one ganglion for each impulse Visceral reflexes are important to the maintenance of homeostasis 63 Autonomic Motor Nervous System: subdivisions A. Sympathetic division of ANS (“Fight or Flight”) ❑ Preganglionic fibers arise from the thoracic-lumbar portion of the spinal cord ❑ Preganglionic fibers are short and postganglionic fibers are long ❑ Accelerates heartbeat and dilates the bronchi ❑ Inhibits the digestive tract ❑ Neurotransmitter released by the postganglionic neuron is primarily norepineprhine (NE) 64 B. Parasympathetic division of ANS (“Rest and Digest”) ❑ Includes a few cranial nerves and preganglionic fibers that arise from the sacral portion of the spinal cord (craniosacral portion of ANS) ❑ Preganglionic fiber is long and postganglionic fiber is short ❑ Promotes digestion of food, slows heart rate, and decreases the strength of cardiac contraction ❑ Acetylcholine(ACh) is the neurotransmitter of the parasympathetic division 65 Comparison of ANS Divisions Sympathetic division Parasympathetic division 66  Comparing Sympathetic and Parasympathetic nervous System 67 68 Neurological problems Alzheimer Disease (AD) o Genetic defect on chromosome 21 o People with Down syndrome tend to develop AD o Abnormally structured neurons and a reduced amount of Ach o Affect reasoning and memory o Pathological features: Neurofibrillary tangles, accumulation of amyloid plaques o Treatment: such as Rivastigmine / Galantamine (cholinesterase inhibitor), memantine(N-methyl D-aspartate (NMDA) antagonist, aducanumab (helps to reduce amyloid deposits in the brain) 69 https://www.nia.nih.gov/health/how-alzheimers-disease-treated Spinal cord injuries (SCI) https://www.youtube.com/watch?v=dKtBC2Sg_Bg Causes: o compressions or bruising to one side of the spinal cord due to accidents or trauma e.g. thrown from horse Effect: o depends on the nerve pathways that are interrupted: Paraplegia: T1 and L2 Quadriplegia: Cervical vertebrae and T1 Lost of sensation Loss of breathing ability, pain, absent or exaggerated reflexes, skeletal muscle spasms, loss of bowel and bladder control 70 Effects of Ageing o Brain mass decreases o Learning, memory, and reasoning decline o Neurotransmitter production decreases o Although structural changes occur, mental impairment is not an automatic consequence of getting older 71  Homeostasis and nervous system o Detects, interprets, and responds to changes in the internal and external environment o With the endocrine system, it coordinates and regulates the functioning of other systems o The hypothalamus and medulla oblongata control vital functions o Sympathetic division of the ANS works to keep us from danger 72