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Chapter 43 Lecture Outline Nervous System Organization • All animals must be able to respond to environmental stimuli • Except for sponges, all animals use a network of nerves to process and integrate information. • Sensory receptors – detect stimulus • Motor effectors – respond to it • Nervous system links the two – Consists of neurons and supporting cells 2 Nervous System Organization • Receptors respond to stimuli • Sensory receptors detect the stimulus • Motor effectors respond to stimulus • Nervous system divisions • Central nervous system – Command center • Peripheral nervous system – Collects and responds 3 Nervous System Organization • Vertebrates have three types of neurons 1. Sensory neurons (afferent neurons) carry impulses toward central nervous system (CNS) 2. Motor neurons (efferent neurons) carry impulses away from CNS to effectors (muscles and glands) 3. Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory) 4 Nervous System Organization • Central nervous system (CNS) • Brain and spinal cord • Peripheral nervous system (PNS) • Sensory • Motor – Somatic - stimulates skeletal muscles – Autonomic - stimulates smooth and cardiac muscles, as well as glands » Sympathetic and parasympathetic 5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PNS CNS Sensory Neurons Interneuron Cell body Touch Direction of conduction Cell body Axon Dendrites Dendrites Taste Direction of conduction Axon Motor Neurons Smooth muscle Skeletal muscle Ganglion Axon terminals Cell bodies Direction of conduction 6 CNS Brain and Spinal Cord Motor Pathways PNS Sensory Pathways Sensory neurons registering external stimuli Sensory neurons registering external stimuli Somatic nervous system (voluntary) Sympathetic nervous system "fight or flight" Autonomic nervous system (involuntary) Parasympathetic nervous system "rest and repose" central nervous system (CNS) peripheral nervous system (PNS) 7 Nervous System Organization • Neurons have the same basic structure – Cell body • Enlarged part containing nucleus – Dendrites • Short, cytoplasmic extensions that receive stimuli – Axon • Single, long extension that conducts impulses away from cell body 8 Nervous System Organization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cell body Dendrites Nucleus Axon Schwann cell Axon Node of Ranvier Myelin sheath Myelin sheath 9 © Enrico Mugnaini/Visuals Unlimited Nervous System Organization • Neuroglia – Support neurons both structurally and functionally – Schwann cells and oligodendrocytes produce myelin sheaths surrounding axons • In the CNS, myelinated axons form white matter – Dendrites/cell bodies form gray matter • In the PNS, myelinated axons are bundled to form nerves 10 Nervous System Organization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon Schwann cell Nucleus Myelin sheath The formation of the myelin sheath around a peripheral axon. 11 Nerve Impulse Transmission • A potential difference exists across every cell’s plasma membrane – Negative pole – cytoplasmic side – Positive pole – extracellular fluid side • When a neuron is not being stimulated, it maintains a resting potential – Ranges from –40 to –90 millivolts (mV) – Average about –70 mV 12 Nerve Impulse Transmission Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Extracellular + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + Electrode outside axon --- +40 mV --- –0 mV --–70 mV Electrode inside axon Oscilloscope screen – – – – – Intracellular – – – proteins and nucleic acids K+ Na+ 13 • The inside of the cell is more negatively charged than the outside (membrane potential) • Cell membrane is impermeable to negative ions (such as Cl-) • Sodium-potassium pump will transport positive ions • Ion channels for K+ are more numerous (allowing more K+ to transport out of cell) • Leads to resting potential of ~ -70mV 14 What is the sodium-potassium pump? • • • Helps to maintain ion imbalance needed to have resting potential Uses ATP Leads to build up of positive outside cell and negative inside cell – More + ions going out, - ions not able to cross 15 • Sudden temporary disruptions to resting membrane potential occur in response to stimuli • 2 types of changes: • Graded potentials – small continuous changes • Ligand-gated channels • Respond to hormones and neurotransmitters • Action potentials – transient disruptions, signals that propagate down the neuron • Voltage-gated channels (Na+ channel and K+ channel) • Action potential “jumps” from node of Ranvier to next node 16 • Disruptions in membrane potential result in • Depolarization – membrane becomes less negative • Hyperpolarization – membrane becomes more negative • With graded potentials change in membrane potential is much less • With action potentials it is greater Nerve Impulse Transmission • Uniqueness of neurons compared with other cells is the production and maintenance of the resting membrane potential 18 • Ligand-gated channels • Ligands are hormones or neurotransmitters • Induce opening and cause changes in cell membrane permeability Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Synaptic cleft Na+ Na+ Na+ Acetylcholine Cell membrane Receptor protein Ion channel Na+ Na+ Cytoplasm in postsynaptic cell 19 • The action potential has three phases – Rising, falling, and undershoot • Action potentials are always separate, allor-none events with the same amplitude 20 Nerve Impulse Transmission • Two ways to increase velocity of conduction – Axon has a large diameter • Less resistance to current flow • Found primarily in invertebrates – Axon is myelinated • Action potential is only produced at the nodes of Ranvier • Impulse jumps from node to node (see next slide) • Saltatory conduction 23 Nerve Impulse Transmission Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Saltatory conduction Action potential Action potential Na+ Na+ Myelin Axon + + Na+ + + + + – 24 Synapses • Intercellular junctions with the dendrites of other neurons, with muscle cells, or with gland cells • Presynaptic cell transmits action potential • Postsynaptic cell receives it • Two basic types: electrical and chemical 25 • Electrical synapses – Involve direct cytoplasmic connections between the two cells formed by gap junctions – Relatively rare in vertebrates • Chemical synapses – Have a synaptic cleft between the two cells – End of presynaptic cell contains synaptic vesicles packed with neurotransmitters 26 Synapses • Chemical synapses – Action potential triggers influx of Ca2+ • Causes synaptic vesicles fuse with cell membrane • Vesicles contain neurotransmitter, released by exocytosis • Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins • Produces graded potentials in the postsynaptic membrane • Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake of neurotransmitter 27 Synapses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neurotransmitter (ACh) Action potential Inward diffusion of Ca2+ Terminal branch of axon Ca2+ Synaptic cleft Synaptic vesicle Na+ Receptor protein 28 Synaptic Integration Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon a. b. 50 µm b: © Science VU/Lewis-Everhart-Zeevi/Visuals Unlimited 29 Drug Addiction • Habituation – Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it – Cell may decrease the number of receptors because there is an abundance of neurotransmitters • In long-term drug use, means that more of the drug is needed to obtain the same effect 30 Drug Addiction • Cocaine – Affects neurons in the brain’s “pleasure pathways” (limbic system) – Binds dopamine transporters and prevents the reuptake of dopamine – Dopamine survives longer in the synapse and fires pleasure pathways more and more 31 Drug Addiction • Nicotine – Binds directly to a specific receptor on postsynaptic neurons of the brain • Actually binds to a receptor for acetylcholine – Explains “excitation” by initial nicotine use – Brain adjusts to prolonged exposure by “turning down the volume” by • Making fewer receptors to which nicotine binds • Altering the pattern of activation of the nicotine receptors 32 The Central Nervous System • Sponges are only major phylum without nerves • Cnidarians have the simplest nervous system – Neurons linked to each other in a nerve net – No associative activity • Free-living flatworms (phylum Platyhelminthes) are simplest animals with associative activity – Two nerve cords run down the body – Permit complex muscle control • All of the subsequent evolutionary changes in nervous systems can be viewed as a series of elaborations on the characteristics already present in flatworms 33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cnidarian Earthworm Human Central nervous system Peripheral nerves Cerebrum Cerebellum Spinal cord Nerve net Cervical nerves Thoracic nerves Arthropod Echinoderm Lumbar nerves Sacral nerves Femoral nerve Radial nerve Brain Nerve ribs Ventral nerve cords Mollusk Sciatic nerve Flatworm Nerve cords Associative neurons Tibial nerve Giant axon Brain 34 Vertebrate Brains • All vertebrate brains have three basic divisions: – Hindbrain or rhombencephalon – Midbrain or mesencephalon – Forebrain or prosencephalon • In fishes, – Hindbrain – largest portion – Midbrain – processes visual information – Forebrain – processes olfactory information 35 Vertebrate Brains • Relative sizes of different brain regions have changed as vertebrates evolved • Forebrain became the dominant feature Mammals Birds Reptiles Amphibians Bony Fish Cartilaginous Fish Jawless Fish Lancelets Tunicates Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chordate Ancestor 37 Vertebrate Brains • Forebrain is composed of two elements – Diencephalon • Thalamus – integration and relay center • Hypothalamus – participates in basic drives and emotions, controls pituitary gland – Telencephalon (“end brain”) • Devoted largely to associative activity • Called the cerebrum in mammals 38 Cerebrum • The increase in brain size in mammals reflects the great enlargement of the cerebrum • Split into right and left cerebral hemispheres, which are connected by a tract called the corpus callosum • Each hemisphere receives sensory input from the opposite side • Hemispheres are divided into: frontal, parietal, temporal, and occipital lobes 39 Cerebrum – don’t have to know each separate part Thalamus Pineal gland Corpus callosum Parietal lobe of cerebral cortex Frontal lobe of cerebral cortex Occipital lobe of cerebral cortex Lateral ventricle Optic recess Optic chiasm Pons Temporal lobe of cerebral cortex Pituitary gland Hypothalamus Cerebellum Medulla oblongata 40 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Motor speech area (Broca's area) Central sulcus Frontal lobe Motor areas involved with the control of voluntary muscles Sensory areas involved with cutaneous and other senses Parietal lobe General interpretative area (Wernicke's area) Occipital lobe Auditory area Lateral sulcus Temporal lobe Interpretation of sensory experiences, memory of visual and auditory patterns Combining visual images, visual recognition of objects Cerebellum 41 Hand Fingers Forefinger Eye Nose Face Gums Teeth Lips Jaw Tongue Wrist Leg Genitals HIp Trunk Arm Elbow Forearm Hand Fingers Toes Knee Hip Trunk Shoulder Arm Elbow Each of these regions of the cerebral cortex is associated with a different region of the body – don’t have to know all of these, just a reference Thumb Neck Brow Eye Face Lips Jaw Tongue Sensor Motor Pharynx 42 Other Brain Structures • Thalamus – Integrates visual, auditory, and somatosensory information • Hypothalamus – Integrates visceral activities – Controls pituitary gland • Limbic system – Hypothalamus, hippocampus, and amygdala – Responsible for emotional responses 43 Complex Functions of the Brain • Language – Left hemisphere is “dominant” hemisphere • Different regions control various language activities • Adept at sequential reasoning – Right hemisphere is adept at spatial reasoning • Primarily involved in musical ability 44 Complex Functions of the Brain • Memory – Appears dispersed across the brain – Short-term memory is stored in the form of transient neural excitations – Long-term memory appears to involve structural changes in neural connections – Two parts of the temporal lobes, the hippocampus and the amygdala, are involved in both short-term memory and its consolidation into long-term memory 45 Complex Functions of the Brain • Alzheimer disease – Condition where memory and thought become dysfunctional – Two causes have been proposed 1. Nerve cells are killed from the outside in – External protein: b-amyloid 2. Nerve cells are killed from the inside out – Internal proteins: tau () 46 Spinal Cord • Cable of neurons extending from the brain down through the backbone • Enclosed and protected by the vertebral column and the meninges 47 Spinal Cord • 2 zones – Inner zone is gray matter (opposite of brain) • Primarily consists of the cell bodies of interneurons, motor neurons, and neuroglia – Outer zone is white matter • Contains cables of sensory axons in the dorsal columns and motor axons in the ventral columns 48 Spinal Cord • It serves as the body’s “information highway” – Relays messages between the body and the brain • It also functions in reflexes – The knee-jerk reflex is monosynaptic – However, most reflexes in vertebrates involve a single interneuron 49 The Peripheral Nervous System • Consists of nerves and ganglia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. – Nerves are bundles of axons bound by connective tissue – Ganglia are aggregates of neuron cell bodies • Function is to receive info from the environment, convey it to the CNS, and to carry responses to effectors such as muscle cells 5 µm © E.R. Lewis/Biological Photo Service 50 The Autonomic Nervous System • Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata 51 Don’t have to know, just for reference Parasympathetic Sympathetic Dilate Constrict Stop secretion Secrete saliva Dilate bronchioles Constrict bronchioles Speed up heartbeat Slow down heartbeat Spinal cord Sympathetic ganglion chain Adrenal gland Stomach Secrete adrenaline Increase secretion Decrease secretion Large intestine Decrease motility Increase motility Small intestine Empty colon Retain colon contents Empty bladder Delay emptying Bladder 52