Lecture 20 The Nervous System

1. Lecture 20The Nervous System

2. The Nervous System • The master controlling and communicating system of the body • Functions • Sensory input – monitoring stimuli occurring inside and outside the body • Integration – interpretation of sensory input • Motor output – response to stimuli by activating effector organs

3. Evolutionary Path to Vertebrate Nervous Systems • Cnidarians have simplest nervous system • Neurons are linked to one another through a nerve net • No associative activity, justreflexes • First associative activity is seen in free-living flatworms • Two nerve cords run down bodies • Permit complex control of muscles • More complex animals developed: • More sophisticated sensory mechanisms • Differentiation into central and peripheral nervous systems • Differentiation of sensory and motor nerves • Increased complexity of association • Elaboration of the brain

4. Organization of the vertebrate nervous system • The nervous system links sensory receptors & motor effectors in all vertebrates (and most invertebrates) Central Nervous System (CNS) Association neurons (or interneurons) are located in the brain and spinal cord Peripheral Nervous System (PNS) Motor(or efferent)neuronscarry impulses away from CNS Sensory (or afferent) neurons carry impulses to CNS

5. Neurons Generate Nerve Impulses • All neuronshave the same basic structure • Cell body – Enlarged part containing the nucleus • Dendrites – Short, slender input channels extending from end of cell body • Axon – A single, long output channel extending from other end of cell body • Most neurons require nutritional support provided by companion neuroglial cells • Schwann cells (PNS) and oligodendrocytes(CNS) envelop the axon with fatty material called myelin which act as a electrical insulator • During development cells wrap themselves around each axon several times to form a myelin sheath • Uninsulated gaps are callednodes of Ranvier • Nerve impulses jump from node to node • Multiple sclerosis and Tay-Sachs disease result from degeneration of the myelin sheath

6. Three types of neurons

7. The Nerve Impulse • Ionic differences are the consequence of: • Differential permeability of the cell membrane to Na+ and K+ • Operation of the sodium-potassium pump • Graded potentials are short-lived, local changes in membrane potential • Decrease in intensity with distance • Their magnitude varies directly with the strength of the stimulus • Sufficiently strong graded potentials can initiate nerve impulses called action potentials • The potential difference (–70 mV) across the membrane of a resting neuron is generated by different concentrations of Na+, K+, and Cl PLAY Action Potential

8. How an Action Potential Works • An action potential forms when the membrane potential reaches -55 to -50 mV • The action potential results from ion movements in and out of voltage-gated channels • The change in membrane potential causes Na+ activation channels to open • Sudden influx of Na+ into cell causes “depolarization” • Local voltage change opens adjacent Na+ channels and an action potential is produced • When the membrane potential reaches +100 mV, K+ voltage-gated channels open • K+ flows out of the cell • Na+ inactivation channels snap close • The negative charge in the cell is restored • The Na+ channels remain closed until the membrane potential normalizes (-70 mV), keeping the action potential from moving backward • The ion balance across the membrane is restored by the action of the sodium-potassium pump

9. Synapses • A junction that mediates information transfer from one neuron: • To another neuron • To an effector cell • Presynaptic neuron – conducts impulses toward the synapse • Postsynaptic neuron – transmits impulses away from the synapse PLAY Transmission Across A Synapse

10. Kinds of Synapses • Excitatory synapse • Receptor protein is a chemically-gated sodium channel • On binding the neurotransmitter, the channel opens • Na+ floods inwards • Action potential begins • Inhibitory synapse • Receptor protein is a chemically-gated potassium or chloride channel • On binding the neurotransmitter, the channel opens • K+ floods outwards or Cl– floods inwards • Action potential is inhibited • An individual nerve cell can possess both kinds of synapses • Integration (Summation) • Various excitatory and inhibitory electrical effects cancel or reinforce one another • Occurs at the axon hillock

11. Are chemical messengers that carry nerve impulses across synapses Bind to receptors in the postsynaptic cell causing chemically-gated channels to open Neurotransmitters • Acetylcholine • Released at the neuromuscular junction • Have an excitatory effect on skeletal muscle and inhibitory effect on cardiac muscle • GlycineandGABA • Inhibitory neurotransmitters • Important for neural control of brain function • Biogenic amines • Dopamine– Control of body movements • Serotonin– Sleep regulation and mood • Neuromodulatorsare chemicals that prolong the effect of neurotransmitters by aiding their release or preventing their reabsorption • Example: Depression may be caused by a shortage of serotonin • Prozac, inhibits its reabsorption

12. Drug Addiction • Cells that are exposed to a chemical signal for a prolonged time, lose their “sensitivity” • They lose their ability to respond to the stimulus with their original intensity • Nerve cells are particularly prone to this loss of sensitivity • They respond to high neurotransmitter exposure by inserting fewer receptor proteins

13. Drug Addiction • Addiction occurs when chronic exposure to a drug induces the nervous system to act physiologically • Cocaine is a neuromodulator • It causes large amounts of neurotransmitter to remain in synapses for long periods of time • Dopamine transmits pleasure messages in the body’s limbic system • High levels for long periods of time, cause nerve cells to lower the number of receptors • Tobacco • “Nicotine receptors” normally served to bind acetylcholine • Brain adjusts to prolonged exposure to nicotine by • 1. Making fewer nicotine receptors • 2. Altering the pattern of activation of nicotine receptors • Addiction occurs because the brain compensates for the nicotine-induced changes by making others • There is no easy way out • The only way to quit is to quit!

14. Evolution of the Vertebrate Brain • Brains of primitive fish, while small, already had the 3 divisions found in contemporary vertebrate brains • Hindbrain (Rhombencephlon) • Major component of early fishes, as it is today • An extension of the spinal cord devoted primarily to coordinating muscle reflexes • Most coordination is done by the cerebellum • Midbrain (Mesencephlon) • Composed primarily of optic lobes thatreceive and process visual information • Forebrain (Proencephlon) • Devoted for processing olfactory (smell) information • Note: • Brains of fishes continue growing throughout their lives!

15. How the Human Brain Works • Diencephalon • Thalamus– Relay center between incoming sensory information and the cerebrum • Hypothalamus– Coordinates nervous and hormonal responses to many internal stimuli and emotions • Telencephalon • Devoted largely to associative activity • Cerebrum (~ 85% of the weight of the human brain) • Dominant part of the brain, receives sensory data and issues motor commands • Cerebral cortex (Gray outer layer)Functions in language, thought, personality and other “thinking and feeling” activities

16. The cerebrum is divided by a groove into right and left halves called cerebral hemispheres Linked by bundles of neurons calledtracts that serve as information highways In general: The left brain is associated with language, speech and mathematical abilities The right brain is associated with intuitive, musical, and artistic abilities The Central Sulcus divides the front and back of the cerebrum The front is associated with motor functions The back with sensory Higher association functions are in the prefrontal area Basic Geography of the Human Brain • Stroke • A disorder caused by blood clots blocking blood vessels in the brain

17. Thalamus Major site of sensory processing in the brain Controls balance Hypothalamus Integrates internal activities: body temperature, blood pressure, etc. Controls pituitary gland secretions Linked to areas of cerebral cortex via limbic system The Diencephalon

18. Cerebellum Extends back from the base of the brain Coordinates muscle movement Even better developed in birds Brain Stem Made up of midbrain, pons, and medulla oblongata Connects rest of brain to spinal cord Controls breathing, swallowing, digestion, heart beat, and blood vessel diameter The Brain Stem & Cerebellum

19. Memory Processing • Memory is the storage and retrieval of information • The three principles of memory are: • Storage – occurs in stages and is continually changing • Processing – accomplished by the hippocampus and surrounding structures • Memory traces – chemical or structural changes that encode memory • Short-term memory –appears to be stored electrically in the form of a transient neural excitation • Long-term memory –appears to involve structural changes in certain neural connections

20. Types of Sleep • There are two major types of sleep: • Non-rapid eye movement (NREM) • Rapid eye movement (REM) • One passes through four stages of NREM during the first 30-45 minutes of sleep • REM sleep occurs after the fourth NREM stage has been achieved

21. Importance of Sleep • Slow-wave sleep is presumed to be the restorative stage • Those deprived of REM sleep become moody and depressed • REM sleep may be a reverse learning process where superfluous information is purged from the brain • Daily sleep requirements decline with age • Sleep Disorders • Narcolepsy – lapsing abruptly into sleep from the awake state • Insomnia – chronic inability to obtain the amount or quality of sleep needed • Sleep apnea – temporary cessation of breathing during sleep

22. Degenerative Brain Disorders • Alzheimer’s disease – a progressive degenerative disease of the brain that results in dementia • Parkinson’s disease – degeneration of the dopamine-releasing neurons of the substantia nigra • Huntington’s disease – a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei

23. The Spinal Cord • Thespinal cordis a cable of neurons extending from the brain down through the backbone • Neuron cell bodies in the center • Gray matter • Axons and dendrites on the outside • White matter • It is surrounded and protected by the vertebrae • Through them spinal nerves pass out to the body • Motor nerves from spine control most of the muscles below the head

24. Major Nerves of Humans

25. Voluntary and Autonomic Nervous Systems Are two subdivisions of vertebrate motor pathways

26. The Voluntary Nervous System • Relays commands to skeletal muscles • Can be controlled by conscious thought • Reflexes are rapid involuntary movements • Are rapid because sensory neuron passes information directly to a motor neuron • Most involve single connecting interneuron between sensory and motor neurons

27. The Autonomic Nervous System • Stimulates glands and relays commands to smooth muscles • Cannot be controlled by conscious thought • Composed of elements that act in opposition to each other • Parasympathetic nervous System • Controls normal functions • Conserves energy by slowing down processes • Sympathetic nervous system • Dominates in time of stress • Controls the “fight-or-flight” reaction • Increases blood pressure, heart rate, breathing