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Essentials of The Living World First Edition GEORGE B. JOHNSON 23 The Nervous System PowerPoint® Lectures prepared by Johnny El-Rady Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.1 Evolution of the Animal Nervous System The nervous system links sensory receptors & motor effectors in all vertebrates (and most invertebrates) Association neurons (or interneurons) are located in the brain and spinal cord Central Nervous System (CNS) Motor (or efferent) neurons carry impulses away from CNS Peripheral Nervous System Sensory (or afferent) neurons (CNS) carry impulses to CNS Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.1 Organization of the vertebrate nervous system Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.2 Three types of neurons Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Invertebrate Nervous Systems Sponges are the only major phylum of multicellular animals that lack nerves Cnidarians have simplest nervous system Neurons are linked to one another through a nerve net Fig. 23.3 No associative activity Just reflexes First associative activity is seen in free-living flatworms Two nerve cords run down bodies Permit complex control of muscles Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Evolutionary path to vertebrates 1. More sophisticated sensory mechanisms 2. Differentiation into central and peripheral nervous systems Fig. 23.3 3. Differentiation of sensory and motor nerves 4. Increased complexity of association 5. Elaboration of the brain Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.2 Neurons Generate Nerve Impulses All neurons have 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 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Most neurons require nutritional support provided by companion neuroglial cells Schwann cells (PNS) and oligodendrocytes (CNS) envelop the axon with fatty material called myelin Myelin acts as a electrical insulator During development cells wrap themselves around each axon several times to form a myelin sheath Uninsulated gaps are called nodes of Ranvier Nerve impulses jump from node to node Multiple sclerosis and Tay-Sachs disease result from degeneration of the myelin sheath Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.4 Structure of a neuron and formation of the myelin sheath Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The Nerve Impulse When a neuron is “at rest”, active transport channels in cell membranes move Na+ out and K+ into cells Concentration of Na+ builds up outside the cell K+ may diffuse out through open channels Thus, neuron’s outside is more positive than inside Cell membrane is said to be “polarized” Resting potential is the charge separation between cell’s interior and exterior – 70 millivolts Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A nerve impulse results from ion movements of in and out of voltage-gated channels A sensory input causes Na+ channels to open Sudden influx of Na+ into cell causes “depolarization” Local voltage change opens adjacent Na+ channels Thus, an action potential is produced After a slight delay, K+ voltage-gated channels open K+ flows out of the cell The negative charge in the cell is restored Na+ channels snap close again The resting potential is restored by the action of the sodium-potassium pump Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.5 How an action potential works Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.5 How an action potential works Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.3 The Synapse A synapse is the junction of an axon and another cell Presynaptic membrane Located on the near (axon) side of the synapse Postsynaptic membrane Located on the far (receiving) side of the synapse Fig. 23.6 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Neurotransmitters are chemical messengers that carry nerve impulses across synapses Bind to receptors in the postsynaptic cell Cause chemically-gated channels to open Fig. 23.7 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 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 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Kinds of Synapses An individual nerve cell can possess both kinds of synapses Integration Various excitatory and inhibitory electrical effects cancel or reinforce one another Occurs at the axon hillock Fig. 23.8a Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Neurotransmitters and Their Functions Acetylcholine Released at the neuromuscular junction Have an excitatory effect on skeletal muscle and inhibitory effect on cardiac muscle Glycine and GABA Inhibitory neurotransmitters Important for neural control of brain function Biogenic amines Dopamine – Control of body movements Serotonin – Sleep regulation and mood Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.4 Addictive Drugs Act on Chemical Synapses Neuromodulators are chemicals that prolong the effect of neurotransmitters Aid their release Prevent their reabsorption Example: Depression may be caused by a shortage of serotonin Prozac, inhibits its reabsorption Fig. 23.9 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Drug Addiction A cell that is exposed to a chemical signal for a prolonged time, loses its “sensitivity” It tends to lose its ability to respond to the stimulus with its original intensity Nerve cells are particularly prone to this loss of sensitivity They respond to high neurotransmitter exposure by inserting fewer receptor proteins Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Drug Addiction 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 Addiction occurs when chronic exposure to a drug induces the nervous system to act physiologically Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.10 How drug addiction works Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Addiction to Smoking “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! Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.5 Evolution of the Vertebrate Brain Brains of primitive fish, while small, already had the 3 divisions found in contemporary vertebrate brains 1. Hindbrain Rhombencephlon 2. Midbrain Mesencephlon 3. Forebrain Prosencephlon Fig. 23.12 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Hindbrain 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 Composed primarily of optic lobes Receive and process visual information Forebrain Devoted for processing olfactory (smell) information Note: Brains of fishes continue growing throughout their lives! Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Starting with the amphibians, sensory information is increasingly centered in the forebrain 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 (mammals) Dominant part of the brain Receives sensory data and issues motor commands Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.13 The evolution of the vertebrate brain Cerebrum dominance is greatest Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.6 How the Brain Works Cerebrum is ~ 85% of the weight of the human brain Functions in language, thought, personality and other “thinking and feeling” activities Much of activity occurs in the cerebral cortex Gray outer layer Fig. 23.14 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.15 The major functional regions of the human brain Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The cerebrum is divided by a groove into right and left halves called cerebral hemispheres Linked by bundles of neurons called tracts 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 Stroke A disorder caused by blood clots blocking blood vessels in the brain Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Thalamus Major site of sensory processing in the brain Controls balance Fig. 23.16 Hypothalamus Integrates internal activities Body temperature, blood pressure, etc. Controls pituitary gland secretions Linked to areas of cerebral cortex via limbic system Center for pain, anger, sex, hunger, etc. Memory center Responsible for deep-seated drives and emotions Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 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 As well as heart beat and blood vessel diameter Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Language and other higher functions Left hemisphere is “dominant” hemisphere for language It is adept at sequential reasoning The “nondominant” hemisphere (the right hemisphere in most people) is involved in Spatial reasoning (assembling puzzles) Musical ability 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 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Alzheimer Disease Memory and thought processes become dysfunctional Two hypotheses have been proposed for the cause 1. Brain nerve cells are killed from the outside in Accumulation of plaques of abnormal external proteins called b-amyloid peptides 2. Brain nerve cells are killed from the inside out Accumulation of tangles of abnormal internal proteins called tau (t) Researchers continue to study whether tangles and plaques are causes or effects of Alzheimer disease Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.7 The Spinal Cord The spinal cord is 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 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.19 The vertebrate nervous system Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.8 Voluntary and Autonomic Nervous Systems Are two subdivisions of vertebrate motor pathways Fig. 23.20 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The voluntary nervous system relays commands to skeletal muscles Can be controlled by conscious thought Reflexes are sudden involuntary movements Are rapid because sensory neuron passes information directly to a motor neuron Most involve single connecting interneuron between sensory and motor neurons Fig. 23.21 The knee-jerk reflex Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 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 Sympathetic nervous system Dominates in time of stress Controls the “fight-or-flight” reaction Increases blood pressure, heart rate, breathing Parasympathetic system Conserves energy by slowing down processes Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.22 How the sympathetic and parasympathetic nervous systems interact Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.9 Sensory Perception The sensory nervous system tells the central nervous system what’s happenin’! Sensory receptors Adapted to nocturnal life Specialized sensory cells that detect changes inside and outside the body Sensory organs Complex sensory receptors Eyes, ears, taste buds Fig. 23.23 Kangaroo rats have specialized ears Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The path of sensory information is a simple one 1. Stimulation Physical stimulus impinges on a sensory receptor 2. Transduction Stimulus-gated ion channels in sensory neuron are opened or closed An action potential is generated 3. Transmission Nerve impulse is conducted to the CNS Two main types of sensory receptors Extroreceptors sense stimuli in external environment Introreceptors sense stimuli in internal environment Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sensing the Internal Environment Vertebrates use many different sensory receptors to respond to changes in internal environment Temperature Change Two nerve endings in the skin One stimulated by cold, the other by warmth Blood chemistry Receptors in arteries sense blood CO2 levels Pain Special nerve endings within tissues near the surface Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Muscle contraction Sensory receptors embedded within muscle Fig. 23.24 Touch Pressure receptors buried below skin Fig. 23.25 Blood pressure Neurons called baroreceptors in major arteries Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.10 Sensing Gravity and Motion Receptors in the ear inform the brain where the body is in three dimensions Balance Gravity is detected by shifting of otolith sensory receptors These are located in a gelatin-like matrix in the utricle and saccule chambers of the inner ear Motion Motion is detected by the deflection of hair cells by fluid in a direction opposite to that of motion These hair cells are found in the cupula Tent-like assemblies in the three semicircular canals Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.26 How the inner ear senses gravity and motion Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.11 Sensing Chemicals: Taste and Smell Taste Taste buds are located in raised areas called papillae Fig. 23.27 Food chemicals dissolve in saliva and contact the taste cells Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.11 Sensing Chemicals: Taste and Smell Smell Olfactory receptor cells are embedded in the epithelium of the nasal passage Fig. 23.28 These are far more sensitive in dogs than in humans Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.12 Sensing Sounds: Hearing When a sound is heard, air vibration is detected Eardrum membrane is pushed in and out by waves of air pressure Three small bones (ossicles) located on other side of eardrum increase the vibration force Amplified vibration is transferred to fluid within the inner ear Inner ear chamber is shaped like a tightly coiled snail shell and is called cochlea Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Cochlea sound receptors are hair cells that rest on a membrane running up and down the chamber They are covered by another membrane Sound waves entering the cochlea cause this membrane “sandwich” to vibrate Bent hair cells send nerve impulses to brain Sounds of different frequencies cause different parts of the membrane to vibrate Different sensory neurons are fired Sound intensity is determined by how often the neurons fire Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.29 Structure and function of the human ear Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The Lateral Line System Supplements the fish’s sense of hearing Fish are able to sense objects that reflect pressure waves and low-frequency vibrations The system consists of canals running the length of the fish’s body under the skin Canals have sensory structures containing hair cells projecting into a gelatinous cupula Vibrations produce movements of the cupula Hair cells bend and depolarize associated sensory neurons Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.30 The lateral line system Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sonar Some mammals perceive distance by sonar Bats, shrews, whales They emit sounds and then determine the time it takes for the sound to return This process is called echolocation Fig. 23.31 Using ultrasound to locate a moth Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.13 Sensing Light: Vision Vision begins with the capture of light energy by photoreceptors Many invertebrates have simple visual systems Photoreceptors are clustered in eyespot Perceive light direction but not a visual image Fig. 23.32 Simple eyespots in the flatworm Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.13 Sensing Light: Vision Members of four phyla have evolved well-developed, image-forming eyes Annelids Mollusks Arthropods Vertebrates The eyes are strikingly similar in structure But are believed to have evolved independently Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.33 Eyes in three phyla of animals Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Structure of the Vertebrate Eye The vertebrate eye works like a lens-focused camera Fig. 23.34 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Structure of the Vertebrate Eye Cornea – Transparent covering that focuses light Lens – Completes the focusing Ciliary muscles – Change the shape of the lens Iris – Shutter that controls amount of light Pupil – Transparent zone Retina – The back surface of the eye Contains two types of photoreceptors Rods and cones Fovea – Center of retina Produces the sharpest image Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display How Rods and Cones Work Rods are extremely sensitive to dim light Cannot distinguish colors Do not detect edges Produce poorly defined images Cones can detect color Detect edges well Produce sharp images Fig. 23.35 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Pigment in rods and cones are made from carotenoids cis-retinal is attached to a protein called opsin This light-gathering complex is called rhodopsin When light is absorbed by cis-retinal, it changes shape to trans-retinal Fig. 23.36 This induces a change in the shape of the opsin protein A signal-transduction pathway is initiated leading to generation of a nerve impulse Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Color Vision Three kinds of cone cells exist, each with its own opsin type Differences in opsin shape, affect the flexibility of the attached cis-retinal This shifts the wavelength at which it absorbs light 420 nm – Blue 530 nm – Green 560 nm – Red Fig. 23.37 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Color Vision Colorblindness is a condition in which a person cannot see all three colors Caused by a lack of one or more types of cones It is inherited as a sex-linked trait More likely to affect males Fig. 23.38 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Conveying the Light Information to the Brain Rods and cones are at the rear of the retina, not front! Light passes through four types of cells before it reaches them Photoreceptor activation stimulates bipolar cells, and then ganglion cells Nerve impulse travels through the optic nerve to the cerebral cortex Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 23.39 Binocular Vision Primates and most predators have eyes on front of the head The two fields of vision overlap Allows the perception of 3-D images and depth Fig. 23.40 Prey animals generally have eyes located on sides of the head This prevents binocular vision However, it enlarges the perceptive field Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 23.14 Other Types of Sensory Reception Fig. 23.41 Heat Pit vipers can locate warm prey, using infrared radiation Heat-detecting pit organs Electricity Used by aquatic vertebrates to locate prey and mates Magnetism Eels, sharks and many birds orient themselves w.r.t the Earth’s magnetic field Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display