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The nervous system links sensory receptors & motor effectors in all vertebrates (and most invertebrates). Central Nervous System (CNS). Peripheral Nervous System (CNS). 23.1 Evolution of the Animal Nervous System. - PowerPoint PPT Presentation
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GEORGE B. JOHNSON
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PowerPoint® Lectures prepared by Johnny El-Rady
23 The Nervous System
Essentials ofThe Living
WorldFirst Edition
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23.1 Evolution of theAnimal 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
Motor (or efferent) neurons carry impulses away from CNSSensory (or afferent) neurons carry impulses to CNS
Central Nervous System (CNS)
Peripheral Nervous System (CNS)
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Fig. 23.1 Organization of the vertebrate nervous system
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Fig. 23.2 Three types of neurons
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Sponges are the only major phylum of multicellular animals that lack nerves
Invertebrate Nervous Systems
Cnidarians have simplest nervous systemNeurons are linked to one another through a nerve net
First associative activity is seen in free-living flatworms
Two nerve cords run down bodies
Fig. 23.3No associative activityJust reflexes
Permit complex control of muscles
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Evolutionary path to vertebrates
3. Differentiation of sensory and motor nerves
4. Increased complexity of association
5. Elaboration of the brain
Fig. 23.3
1. More sophisticated sensory mechanisms
2. Differentiation into central and peripheral nervous systems
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23.2 Neurons GenerateNerve 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
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Most neurons require nutritional support provided by companion neuroglial cellsSchwann 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 RanvierNerve impulses jump from node to node
Multiple sclerosis and Tay-Sachs disease result from degeneration of the myelin sheath
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Fig. 23.4 Structure of a neuron and formation of the myelin sheath
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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 insideCell membrane is said to be “polarized”
Resting potential is the charge separation between cell’s interior and exterior
– 70 millivolts
The Nerve Impulse
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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+ channelsThus, an action potential is produced
After a slight delay, K+ voltage-gated channels openK+ flows out of the cell
The negative charge in the cell is restoredNa+ channels snap close again
The resting potential is restored by the action of the sodium-potassium pump
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Fig. 23.5 How an action potential works
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Fig. 23.5 How an action potential works
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23.3 The SynapseA synapse is the junction of an axon and another cell
Presynaptic membrane Located on the near (axon) side of the synapse
Postsynaptic membraneLocated on the far (receiving) side of the synapse Fig. 23.6
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Neurotransmitters are chemical messengers that carry nerve impulses across synapses
Bind to receptors in the postsynaptic cellCause chemically-gated channels to open
Fig. 23.7
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Excitatory synapse Receptor protein is a chemically-gated sodium channel
On binding the neurotransmitter, the channel opensNa+ floods inwards
Action potential begins
Inhibitory synapseReceptor protein is a chemically-gated potassium or chloride channel
On binding the neurotransmitter, the channel opensK+ floods outwards or Cl– floods inwards
Action potential is inhibited
Kinds of Synapses
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An individual nerve cell can possess both kinds of synapses
Kinds of Synapses
Integration Various excitatory and inhibitory electrical effects cancel or reinforce one anotherOccurs at the axon hillock
Fig. 23.8a
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AcetylcholineReleased at the neuromuscular junctionHave an excitatory effect on skeletal muscle and inhibitory effect on cardiac muscle
Glycine and GABAInhibitory neurotransmittersImportant for neural control of brain function
Biogenic aminesDopamine – Control of body movementsSerotonin – Sleep regulation and mood
Neurotransmitters and Their Functions
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Fig. 23.9
23.4 Addictive Drugs Acton 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 serotoninProzac, inhibits its reabsorption
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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
Drug Addiction
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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
Drug Addiction
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Fig. 23.10 How drug addiction works
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“Nicotine receptors” normally served to bind acetylcholine
Brain adjusts to prolonged exposure to nicotine by1. Making fewer nicotine receptors2. 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 outThe only way to quit is to quit!
Addiction to Smoking
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23.5 Evolution of the Vertebrate BrainBrains of primitive fish, while small, already had the 3 divisions found in contemporary vertebrate brains
1. HindbrainRhombencephlon
2. MidbrainMesencephlon
3. ForebrainProsencephlon
Fig. 23.12
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Hindbrain Major component of early fishes, as it is todayAn extension of the spinal cord devoted primarily to coordinating muscle reflexes
Most coordination is done by the cerebellum
MidbrainComposed primarily of optic lobes
Receive and process visual information
ForebrainDevoted for processing olfactory (smell) information
Note:Brains of fishes continue growing throughout their lives!
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Starting with the amphibians, sensory information is increasingly centered in the forebrain
DiencephalonThalamus – Relay center between incoming sensory information and the cerebrumHypothalamus – Coordinates nervous and hormonal responses to many internal stimuli and emotions
TelencephalonDevoted largely to associative activityCerebrum (mammals)
Dominant part of the brainReceives sensory data and issues motor commands
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Fig. 23.13 The evolution of the vertebrate brain
Cerebrum dominance is greatest
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23.6 How the Brain WorksCerebrum 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 layerFig. 23.14
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Fig. 23.15 The major functional regions of the human brain
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The cerebrum is divided by a groove into right and left halves called cerebral hemispheres
Linked by bundles of neurons called tractsServe 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
StrokeA disorder caused by blood clots blocking blood vessels in the brain
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Fig. 23.16
ThalamusMajor site of sensory processing in the brainControls balance
HypothalamusIntegrates internal activities
Body temperature, blood pressure, etc.
Controls pituitary gland secretionsLinked to areas of cerebral cortex via limbic system Memory center
Center for pain, anger, sex, hunger, etc.
Responsible for deep-seated drives and emotions
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CerebellumExtends back from the base of the brainCoordinates muscle movementEven better developed in birds
Brain StemMade up of midbrain, pons, and medulla oblongataConnects rest of brain to spinal cordControls breathing, swallowing, digestion
As well as heart beat and blood vessel diameter
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Language and other higher functionsLeft hemisphere is “dominant” hemisphere for language
It is adept at sequential reasoningThe “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
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Alzheimer DiseaseMemory and thought processes become dysfunctionalTwo hypotheses have been proposed for the cause
1. Brain nerve cells are killed from the outside inAccumulation of plaques of abnormal external proteins called -amyloid peptides
2. Brain nerve cells are killed from the inside outAccumulation of tangles of abnormal internal proteins called tau ()
Researchers continue to study whether tangles and plaques are causes or effects of Alzheimer disease
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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 centerGray matter
Axons and dendrites on the outsideWhite matter
It is surrounded and protected by the vertebraeThrough them spinal nerves pass out to the body
Motor nerves from spine control most of the muscles below the head
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Fig. 23.19 The vertebrate nervous system
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23.8 Voluntary and AutonomicNervous Systems
Are two subdivisions of vertebrate motor pathways
Fig. 23.20
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The voluntary nervous system relays commands to skeletal muscles
Reflexes are sudden involuntary movements
Fig. 23.21 The knee-jerk reflex
Can be controlled by conscious thought
Are rapid because sensory neuron passes information directly to a motor neuronMost involve single connecting interneuron between sensory and motor neurons
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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 systemDominates in time of stress Controls the “fight-or-flight” reaction
Increases blood pressure, heart rate, breathing
Parasympathetic system Conserves energy by slowing down processes
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Fig. 23.22 How the sympathetic and parasympathetic nervous systems interact
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23.9 Sensory Perception The sensory nervous system tells the central nervous system what’s happenin’! Sensory receptors
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
Adapted to nocturnal life
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The path of sensory information is a simple one1. Stimulation
Physical stimulus impinges on a sensory receptor2. Transduction
Stimulus-gated ion channels in sensory neuron are opened or closed
An action potential is generated3. Transmission
Nerve impulse is conducted to the CNS
Two main types of sensory receptorsExtroreceptors sense stimuli in external environmentIntroreceptors sense stimuli in internal environment
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Vertebrates use many different sensory receptors to respond to changes in internal environment
Temperature ChangeTwo nerve endings in the skin
One stimulated by cold, the other by warmth
Blood chemistryReceptors in arteries sense blood CO2 levels
PainSpecial nerve endings within tissues near the surface
Sensing the Internal Environment
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Muscle contractionSensory receptors embedded within muscle
Fig. 23.24
TouchPressure receptors buried below skin
Blood pressureNeurons called baroreceptors in major arteries
Fig. 23.25
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23.10 Sensing Gravity and Motion Receptors in the ear inform the brain where the body is in three dimensionsBalance
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
MotionMotion 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
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Fig. 23.26 How the inner ear senses gravity and motion
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Fig. 23.27
23.11 Sensing Chemicals:Taste and Smell
TasteTaste buds are located in raised areas called papillae
Food chemicals dissolve in saliva and contact the taste cells
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23.11 Sensing Chemicals:Taste and Smell
SmellOlfactory receptor cells are embedded in the epithelium of the nasal passage
These are far more sensitive in dogs than in humans
Fig. 23.28
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23.12 Sensing Sounds: Hearing
When a sound is heard, air vibration is detectedEardrum 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
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Cochlea sound receptors are hair cells that rest on a membrane running up and down the chamber
They are covered by another membraneSound 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
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Fig. 23.29 Structure and function of the human ear
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Supplements the fish’s sense of hearingFish 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 cupulaHair cells bend and depolarize associated sensory neurons
The Lateral Line System
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Fig. 23.30 The lateral line system
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Some mammals perceive distance by sonarBats, shrews, whales
Sonar
Fig. 23.31 Using ultrasound to locate a moth
They emit sounds and then determine the time it takes for the sound to return
This process is called echolocation
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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
Fig. 23.32 Simple eyespots in the flatworm
Perceive light direction but not a visual image
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23.13 Sensing Light: Vision
Members of four phyla have evolved well-developed, image-forming eyes
AnnelidsMollusksArthropodsVertebrates
The eyes are strikingly similar in structureBut are believed to have evolved independently
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Fig. 23.33 Eyes in three phyla of animals
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The vertebrate eye works like a lens-focused camera
Structure of the Vertebrate Eye
Fig. 23.34
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Cornea – Transparent covering that focuses lightLens – Completes the focusingCiliary muscles – Change the shape of the lens Iris – Shutter that controls amount of lightPupil – Transparent zoneRetina – The back surface of the eye
Contains two types of photoreceptorsRods and cones
Fovea – Center of retinaProduces the sharpest image
Structure of the Vertebrate Eye
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Rods are extremely sensitive to dim light
How Rods and Cones Work
Fig. 23.35
Cannot distinguish colors Do not detect edges
Produce poorly defined images
Cones can detect colorDetect edges well
Produce sharp images
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When light is absorbed by cis-retinal, it changes shape to trans-retinal
Pigment in rods and cones are made from carotenoidscis-retinal is attached to a protein called opsin
This light-gathering complex is called rhodopsin
This induces a change in the shape of the opsin protein
A signal-transduction pathway is initiated leading to generation of a nerve impulse
Fig. 23.36
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Three kinds of cone cells exist, each with its own opsin typeDifferences in opsin shape, affect the flexibility of the attached cis-retinal
Color Vision
420 nm – Blue530 nm – Green560 nm – Red
This shifts the wavelength at which it absorbs light
Fig. 23.37
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Colorblindness is a condition in which a person cannot see all three colors
Color Vision
Fig. 23.38
It is inherited as a sex-linked trait
More likely to affect males
Caused by a lack of one or more types of cones
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Rods and cones are at the rear of the retina, not front!
Conveying the Light Information to the Brain
Fig. 23.39
Light passes through four types of cells before it reaches themPhotoreceptor activation stimulates bipolar cells, and then ganglion cells
Nerve impulse travels through the optic nerve to the cerebral cortex
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Primates and most predators have eyes on front of the head
The two fields of vision overlapAllows the perception of 3-D images and depth
Binocular Vision
Prey animals generally have eyes located on sides of the head
This prevents binocular visionHowever, it enlarges the perceptive field
Fig. 23.40
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23.14 Other Types ofSensory Reception
Heat Fig. 23.41
Pit vipers can locate warm prey, using infrared radiation
Heat-detecting pit organs
ElectricityUsed by aquatic vertebrates to locate prey and mates Magnetism
Eels, sharks and many birds orient themselves w.r.t the Earth’s magnetic field