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Nervous System 2007-2008 Evolution of the Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. eyespot auricle ventral nerve cord with ganglia cerebral ganglia nerve brain nerve net lateral nerve cords transverse nerves a. Hydra b. Planarian c. Earthworm 2 Evolution of the Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. cerebrum in forebrain hindbrain giant nerve fiber spinal cord brain eye brain thoracic ganglion d. Crab tentacle e. Squid f. Cat 3 Evolution of the Nervous System • Vertebrate Nervous-System Organization – Central nervous system • Develops from an embryonic dorsal neural tube • Cephalization and bilateral symmetry result in several paired sensory receptors – Eyes, ears, olfactory structures – Vertebrate brain is organized into three areas • Hindbrain • Midbrain • Forebrain 4 Evolution of the Nervous System • Division of Nervous System: – The Central nervous system (CNS) • Includes the brain and spinal cord – The peripheral nervous system (PNS) • Consists of all nerves and ganglia that lie outside the CNS • Two divisions – Somatic nervous system » Sensory and motor functions that control skeletal muscle – Autonomic nervous system » Controls smooth muscle, cardiac, muscle, and gland » Divided into sympathetic and parasympathetic divisions 5 Organization of the Human Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. brain cranial nerves cervical nerves thoracic nerves spinal cord radial nerve median nerve ulnar nerve lumbar nerves sacral nerves sciatic nerve tibial nerve common fibular nerve a. 6 Organization of the Human Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central Nervous System brain and spinal cord Peripheral Nervous System somatic sensory fibers (skin, special senses) visceral sensory fibers (internal organs) somatic motor fibers (to skeletal muscles) autonomic motor fibers (to cardiac and smooth muscle, glands) sympathetic division parasympathetic division 7 b. 37.2 Nervous Tissue • Neurons (nerve cells) – Cell body contains nucleus and organelles – Dendrites receive signals from sensory receptors or other neurons – Axon conducts nerve impulses to another neuron or to other cells • Covered by myelin sheath • Any long axon is also called a nerve fiber 8 Nervous Tissue • Types of Neurons • Motor (efferent) neurons – Accept nerve impulses from the CNS – Transmit them to muscles or glands • Sensory (afferent) neurons – Accept impulses from sensory receptors – Transmit them to the CNS • Interneurons – Convey nerve impulses between various parts of the CNS 9 Neuron Anatomy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. cell body dendrite direction of conduction myelin sheath axon terminal node of Ranvier axon a. Motor neuron (multipolar) muscle 10 a: © M.B. Bunge/Biological Photo Service Neuron Anatomy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. axon cell body direction of conduction sensory receptor axon b. Sensory neuron (unipolar) myelin sheath skin 11 Neuron Anatomy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. axon cell body dendrite c. Interneuron (multipolar) c: © Manfred Kage/Peter Arnold, Inc. 12 Nervous Tissue • Transmission of Nerve Impulses – Resting Potential • The membrane potential (voltage) when the axon is not conducting an impulse – The inside of a neuron is more negative than the outside, around -70 mV – Due in part to the activity of the sodiumpotassium pump 13 Nervous Tissue • An action potential is a rapid change in polarity across a portion of an axonal membrane • An action potential is generated only after a stimulus larger than the threshold • Gated channel proteins – One channel protein suddenly allows sodium to enter the cell – Another channel protein allows potassium to leave the cell 14 Cells: surrounded by charged ions • Cells live in a sea of charged ions – anions (negative) • more concentrated within the cell • Cl-, charged amino acids (aa-) – cations (positive) • more concentrated in the extracellular fluid • Na+ Na+ Na+ K+ aa- K+ Na+ aaCl- Na+ ClK+ Na+ aa- Na+ K+ aa- K+ Na+ ClCl- Na+ aa- Na+ Na+ Na+ Claa- Cl- – K+ + channel leaks K+ Cells have voltage! • Opposite charges on opposite sides of cell membrane – membrane is polarized • negative inside; positive outside • charge gradient • stored energy (like a battery) + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + + + + + + Measuring cell voltage unstimulated neuron = resting potential of -70mV How does a nerve impulse travel? • Stimulus: nerve is stimulated – reaches threshold potential • open Na+ channels in cell membrane • Na+ ions diffuse into cell – charges reverse at that point on neuron The 1st domino goes down! • positive inside; negative outside • cell becomes depolarized – + + + + + + + + + + + + + + + – – – – – – – – – – – – – – Na+ + – – – – – – – – – – – – – – – + + + + + + + + + + + + + + How does a nerve impulse travel? • Wave: nerve impulse travels down neuron – change in charge opens + – + next Na gates down the line • “voltage-gated” channels channel – Na+ ions continue to diffuse into cell closed – “wave” moves down neuron = action potential Gate The rest of the dominoes fall! + + channel open – – – + + + + + + + + + + + + + + + – – – – – – – – – – – – Na+ + + + – – – – – – – – – – – – – – – + + + + + + + + + + + + wave How does a nerve impulse travel? • Re-set: 2nd wave travels down neuron – K+ channels open • K+ channels open up more slowly than Na+ channels – K+ ions diffuse out of cell – charges reverse back at that point • negative inside; positive outside Set dominoes back up quickly! K+ + – – – – + + + + + + + + + + – + + + + – – – – – – – – – – Na+ – + + + + – – – – – – – – – – + – – – – + + + + + + + + + + wave How does a nerve impulse travel? • Combined waves travel down neuron – wave of opening ion channels moves down neuron – signal moves in one direction • flow of K+ out of cell stops activation of Na+ channels in wrong direction Ready for next time! K+ + + + – – – – + + + + + + + + – – – + + + + – – – – – – – – Na+ – – – + + + + – – – – – – – – + + + – – – – + + + + + + + + wave How does a nerve impulse travel? • Action potential propagates – wave = nerve impulse, or action potential – brain finger tips in milliseconds! In the blink of an eye! K+ + + + + + + + – – – – + + + + – – – – – – – + + + + – – – – Na+ – – – – – – – + + + + – – – – + + + + + + + – – – – + + + + wave Voltage-gated channels • Ion channels open & close in response to changes in charge across membrane – Na+ channels open quickly in response to depolarization & close slowly – K+ channels open slowly in response to depolarization & close slowly Structure & function! K+ + + + + + + + + + – – – + + + – – – – – – – – – + + + – – – Na+ – – – – – – – – – + + + – – – + + + + + + + + + – – – + + + wave How does the nerve re-set itself? • After firing a neuron has to re-set itself – Na+ needs to move back out – K+ needs to move back in – both are moving against concentration gradients • need a pump!! A lot of work to do here! Na+ + Na+ + K K Na+ + K+ + Na Na+ Na+ K+ K Na+ +Na + Na Na + + + + + + + + + + – – – – + – – +– – – – – – – – + + + + – Na+ Na K+ K+ + + K K++ Na + + + + Na K K Na K Na+ Na+ K+ – – – – – – – – – – + + + + – + + + + + + + + + + – – – – + wave Na+ + How does the nerve re-set itself? • Sodium-Potassium pump – active transport protein in membrane • requires ATP – 3 Na+ pumped out – 2 K+ pumped in – re-sets charge across membrane That’s a lot of ATP ! Feed me some sugar quick! ATP Neuron is ready to fire again Na+ Na+ Na+ K+ aa- aaNa+ Na+ Na+ K+ Na+ Na+ K+ Na+ aa- K+ Na+ Na+ Na+ Na+ K+ aaNa+ Na+ Na+ K+ Na+ Na+ Na+ K+ aa- aa- K+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ resting potential + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + + + + + + Action potential graph 40 mV 4 30 mV Membrane potential 1. Resting potential 2. Stimulus reaches threshold potential 3. Depolarization Na+ channels open; K+ channels closed 4. Na+ channels close; K+ channels open 5. Repolarization reset charge gradient 6. Undershoot K+ channels close slowly 20 mV 10 mV Depolarization Na+ flows in 0 mV –10 mV 3 –20 mV Repolarization K+ flows out 5 –30 mV –40 mV –50 mV Threshold –60 mV 2 –70 mV –80 mV 1 Resting potential Hyperpolarization (undershoot) 6 Resting Resting and Action Potential of the Axonal Membrane Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +60 +40 Na+ moves to inside axon K+ moves to outside axon action potential +20 Voltage (mV) 0 –20 –40 threshold –60 resting potential 0 1 2 3 4 5 6 Time (milliseconds) d. An action potential can be visualized if voltage changes are graphed over time. 28 Myelin sheath Axon coated with Schwann cells signal direction insulates axon speeds signal signal hops from node to node saltatory conduction 150 m/sec vs. 5 m/sec (330 mph vs. 11 mph) myelin sheath action potential saltatory conduction QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Na+ myelin axon + + + + + – – Na+ Multiple Sclerosis immune system (T cells) attack myelin sheath loss of signal Nervous Tissue • Propagation of Action Potentials – In nonmyelinated axons, the action potential travels down an axon one small section at a time – In myelinated fibers, an action potential at one node causes an action potential at the next node • Saltatory (jumping) Conduction – Conduction of a nerve impulse is an all-or-nothing event • Intensity of signal is determined by how many impulses are generated within a given time span 31 Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 32 Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Nervous Tissue • Transmission Across a Synapse – A synapse is a region where neurons nearly touch – Small gap between neurons is the synaptic cleft – Transmission across a synapse is carried out by neurotransmitters • Sudden rise in calcium in the axon terminal of one neuron • Calcium stimulates synaptic vesicles to merge with the presynaptic membrane • Neurotransmitter molecules are released into the synaptic cleft 35 Synapse Structure and Function Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. path of action potential 1. After an action potential arrives at an axon terminal, Ca2+ enters, and synaptic vesicles fuse with the presynaptic membrane. Ca2+ axon terminal synaptic vesicles enclose neurotransmitter cell body of postsynaptic neuron synaptic cleft 2. Neurotransmitter molecules are released and bind to receptors on the postsynaptic membrane. presynaptic membrane neurotransmitter neurotransmitter receptor Na+ postsynaptic neuron postsynaptic membrane 3. When an excitatory neurotransmitter binds to a receptor, Na+ diffuses into the postsynaptic neuron, and an action potential begins. 36 Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. What happens at the end of the axon? Impulse has to jump the synapse! – junction between neurons – has to jump quickly from one cell to next How does the wave jump the gap? Synapse Chemical synapse axon terminal Events at synapse action potential synaptic vesicles synapse Ca++ receptor protein neurotransmitter acetylcholine (ACh) muscle cell (fiber) We switched… from an electrical signal to a chemical signal action potential depolarizes membrane opens Ca++ channels neurotransmitter vesicles fuse with membrane release neurotransmitter to synapse diffusion neurotransmitter binds with protein receptor ion-gated channels open neurotransmitter degraded or reabsorbed Nerve impulse in next neuron K+ • Post-synaptic neuron – triggers nerve impulse in next nerve cell • chemical signal opens ion-gated channels Na+ + binding site • Na diffuses into cell • K+ diffuses out of cell Here we go again! – switch back to voltage-gated channel Na+ ACh ion channel K+ K+ Na+ – + + + + + + + + + + + + + + + – – – – – – – – – – – – – – Na+ + – – – – – – – – – – – – – – – + + + + + + + + + + + + + + Nervous Tissue • Synaptic Integration – A single neuron is on the receiving end of • Many excitatory signals, and • Many inhibitory signals – Integration • The summing of excitatory and inhibitory signals 41 Synaptic Integration Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. cell body of the neuron +20 0 axon terminals excitatory signal integration inhibitory signal –20 –40 threshold –70 resting potential –80 Time (milliseconds) 42 b. (a): Courtesy Dr. E.R. Lewis, University of California Berkeley 37.3 The Central Nervous System • The Central Nervous System – Consists of the brain and spinal cord – Three specific functions: • Receives sensory input • Performs integration • Generates motor output 43 The Central Nervous System • The Central Nervous System – Consists of the brain and spinal cord – Spinal cord and brain are wrapped in three protective membranes called meninges • Spaces between meninges are filled with cerebrospinal fluid • Fluid is continuous with that of central canal of spinal cord and the ventricles of the brain 44 The Central Nervous System • The Spinal Cord – Two main functions • Center for many reflex actions • Means of communication between the brain and spinal nerves – Composed of grey matter and white matter • Cell bodies and short unmyelinated fibers give the gray mater its color • Myelinated long fibers of interneurons running in tracts give white mater its color 45 The Central Nervous System • The Brain – The cerebrum is the largest portion of the brain in humans • Communicates with, and coordinates the activities of, the other parts of the brain • Longitudinal fissure divides the cerebrum into left and right cerebral hemispheres 46 The Human Brain Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cerebrum (telencephalon) opening to lateral ventricle third ventricle skull meninges corpus callosum pituitary gland Diencephalon thalamus (surrounds the third ventricle) hypothalamus pineal gland fourth ventricle Brain stem midbrain Cerebellum pons medulla oblongata a. Parts of brain spinal cord b. Cerebral hemispheres 47 The Lobes of a Cerebral Hemisphere Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. central sulcus Frontal lobe primary somatosensory area primary motor area premotor area motor speech (Broca’s) area prefrontal area Parietal lobe leg trunk arm hand somatosensory association area primary taste area general interpretation area face tongue Occipital lobe primary visual area lateral sulcus Temporal lobe visual association area auditory association area primary auditory area sensory speech (Wernicke’s) area 48 The Central Nervous System • Cerebral Cortex • A thin but highly convoluted outer layer of gray matter • Covers the cerebral hemispheres • Contains motor areas and sensory areas as well as association areas – Primary motor area is in the frontal lobe, just ventral to the central sulcus – Primary somatosensory area is in the parietal lobe, just dorsal to the central sulcus 49 The Central Nervous System • Other Parts of the Brain • Diencephalon • A region encircling the third ventricle • Includes three structures – Hypothalamus • Forms the floor of the third ventricle • Integrating center that maintains homeostasis • Controls the pituitary gland 50 The Central Nervous System • Other Parts of the Brain • Diencephalon (continued) • Thalamus • Consists of two masses of gray matter located in the sides and roof of the third ventricle • Receives all sensory input except smell • Integrates sensory information and sends it to the cerebrum • Pineal gland • Secretes melatonin 51 The Central Nervous System • Other Parts of the Brain • Cerebellum • Separated from the brain stem by the fourth ventricle • Largest portion of the brainstem • Receives sensory input from the eyes, ears, joints, and muscles • Sends motor impulses out the brain stem to the skeletal muscles 52 The Central Nervous System • Other Parts of the Brain • Brain Stem • Contains the midbrain, pons, and the medulla oblongata • Midbrain • Acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum • Pons • Contains axons that form a bridge between the cerebellum and the rest of the central nervous system • Medulla Oblongata • Contains reflex centers for vomiting, coughing, sneezing, hiccupping, and swallowing 53 37.4 The Peripheral Nervous System • Somatic system – Includes cranial nerves and spinal nerves • Gather information from sensors and conduct decisions to effectors • Controls the skeletal muscles – Voluntary • Autonomic system – – – – Controls the smooth muscles, cardiac muscles, and glands Innervates all internal organs Usually involuntary Divided into two divisions • Sympathetic division • Parasympathetic division – Utilizes two neurons and one ganglion for each impulse 54 Cranial and Spinal Nerves Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. spinal cord gray matter vertebra white matter frontal lobe olfactory bulb dorsal root olfactory tract dorsal root ganglion optic nerve optic chiasma spinal nerve ventral root vertebra b. temporal lobe central canal cerebellum gray matter medulla white matter a. c. c: © Karl E. Deckart/Phototake 55 The Peripheral Nervous System • Reflex Arc • Sensory receptors generate a nerve impulse that moves along sensory axons through a dorsal root ganglion toward the spinal cord • Sensory neurons pass signals on to many interneurons in the gray matter of the spinal cord • Nerve pulses travel along motor axons to an effector, which brings about a response to the stimulus 56 A Reflex Arc Showing the Path of a Spinal Reflex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. pin dorsal root ganglion sensory receptor (in skin) axon of sensory neuron central canal white matter dendrites Dorsal gray matter dorsal horn cell body of sensory neuron interneuron dendrites axon of motor neuron cell body of motor neuron effector (muscle) ventral root ventral horn Ventral 57 The Peripheral Nervous System • Sympathetic division – Especially important during fight or flight responses – Accelerates heartbeat and dilates bronchi • Parasympathetic division – Promotes all internal responses associated with a relaxed state – Promotes digestion and retards heartbeat 58 Autonomic System Structure and Function Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. inhibits tears stimulates tears constricts pupils dilates pupils ganglion inhibits salivation Sympathetic Division Parasympathetic Division stimulates salivation cranial nerves slows heart speeds heart dilates air passages cervical nerves constricts bronchioles stimulates liver to release glucose stimulates gallbladder to release bile stimulates adrenal secretion thoracic nerves vagus nerve increases activity of stomach and pancreas inhibits activity of kidneys, stomach, and pancreas increases intestinal activity decreases intestinal activity lumbar nerves ganglion inhibits urination stimulates urination causes orgasmic contractions sympathetic ganglia causes erection of genitals sacral nerves Acetylcholine is neurotransmitter. Norepinephrine is neurotransmitter. 59 Comparison of Somatic Motor and Autonomic Motor Pathways 60