23.1 evolution of the animal nervous system

GEORGE B. JOHNSON

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

PowerPoint® Lectures prepared by Johnny El-Rady

23 The Nervous System

Essentials ofThe Living

WorldFirst Edition


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)


Fig. 23.1 Organization of the vertebrate nervous system


Fig. 23.2 Three types of neurons


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


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


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


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


Fig. 23.4 Structure of a neuron and formation of the myelin sheath


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


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


Fig. 23.5 How an action potential works


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


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


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


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


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


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


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


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


Fig. 23.10 How drug addiction works


“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


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


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!


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


Fig. 23.13 The evolution of the vertebrate brain

Cerebrum dominance is greatest


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


Fig. 23.15 The major functional regions of the human brain


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


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


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


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


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


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


Fig. 23.19 The vertebrate nervous system


23.8 Voluntary and AutonomicNervous Systems

Are two subdivisions of vertebrate motor pathways

Fig. 23.20


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


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


Fig. 23.22 How the sympathetic and parasympathetic nervous systems interact


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


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


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


Muscle contractionSensory receptors embedded within muscle

Fig. 23.24

TouchPressure receptors buried below skin

Blood pressureNeurons called baroreceptors in major arteries

Fig. 23.25


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


Fig. 23.26 How the inner ear senses gravity and motion


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


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


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


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


Fig. 23.29 Structure and function of the human ear


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


Fig. 23.30 The lateral line system


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


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


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


Fig. 23.33 Eyes in three phyla of animals


The vertebrate eye works like a lens-focused camera

Structure of the Vertebrate Eye

Fig. 23.34


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


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


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


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


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


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


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


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

23.1 evolution of the animal nervous system

Documents