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Nervous System Part 2. IB-202-15 4-24-06 Chapt 48 pp 1022-1028, 1036 (memory), 1040-1041 (Alzheimer’s and Parkinson’s disease). Direct Synaptic Transmission. The process of direct synaptic transmission Involves the binding of neurotransmitters to ligand-gated ion channels

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Nervous system part 2 l.jpg

Nervous System Part 2

IB-202-15

4-24-06

Chapt 48 pp 1022-1028, 1036 (memory), 1040-1041 (Alzheimer’s and Parkinson’s

disease)


Direct synaptic transmission l.jpg

Direct Synaptic Transmission

  • The process of direct synaptic transmission

    • Involves the binding of neurotransmitters to ligand-gated ion channels

  • Neurotransmitter binding

    • Causes the ion channels to open, generating a postsynaptic potential

  • After its release from channel, the neurotransmitter

    • Diffuses out of the synaptic cleft

    • May be taken up by surrounding cells and degraded by enzymes


Slide3 l.jpg

Table 48.1

  • Major neurotransmitters


Acetylcholine l.jpg

Acetylcholine

  • Acetylcholine

    • Is one of the most common neurotransmitters in both vertebrates and invertebrates. Transmitter for neuromuscular synapses in vertebrates (skeletal muscle).

    • Can be inhibitory or excitatory with other types of muscle.


Biogenic amines l.jpg

Biogenic Amines

  • Biogenic amines

    • Include epinephrine (adrenalin), norepinephrine, dopamine, and serotonin

    • Are active in the CNS and peripheral nervous system (PNS)

  • Various amino acids and peptides

    • Are active in the brain


Gases l.jpg

Gases

  • Gases such as nitric oxide and carbon monoxide

    • Are local regulators in the PNS


Slide7 l.jpg

Central nervous

system (CNS)

Peripheral nervous

system (PNS)

Brain

Cranial

nerves

Spinal cord

Ganglia

outside

CNS

Spinal

nerves

Figure 48.19

  • Concept 48.5: The vertebrate nervous system is regionally specialized

  • In all vertebrates, the nervous system

    • Shows a high degree of cephalization and distinct CNS and PNS components


Slide8 l.jpg

  • The brain provides the integrative power

    • That underlies the complex behavior of vertebrates

  • The spinal cord integrates simple responses to certain kinds of stimuli

    • And conveys information to and from the brain


Slide9 l.jpg

Gray matter

White

matter

Ventricles

Figure 48.20

  • The central canal of the spinal cord and the four ventricles of the brain

    • Are hollow, since they are derived from the dorsal embryonic nerve cord

Mylinated axons interconnecting parts of brain and nerve tracks to spinal cord

Grey matter is unmylinated axons, dendrites and nerve bodies.


The peripheral nervous system l.jpg

The Peripheral Nervous System

  • The PNS transmits information to and from the CNS

    • And plays a large role in regulating a vertebrate’s movement and internal environment

  • The cranial nerves originate in the brain

    • And terminate mostly in organs of the head and upper body

  • The spinal nerves originate in the spinal cord

    • And extend to parts of the body below the head


Slide11 l.jpg

Peripheral

nervous system

Somatic

nervous

system

Autonomic

nervous

system

Sympathetic

division

Parasympathetic

division

Enteric

division

Figure 48.21

  • The PNS can be divided into two functional components

    • The somatic nervous system and the autonomic nervous system

Autonomic regulates the internal environment in an involuntary manner.

Somatic largely voluntary control of muscle in response to external stimuli


Slide12 l.jpg

Parasympathetic division

Sympathetic division

Action on target organs:

Action on target organs:

Dilates pupil

of eye

Constricts pupil

of eye

Location of

preganglionic neurons:

brainstem and sacral

segments of spinal cord

Location of

preganglionic neurons:

thoracic and lumbar

segments of spinal cord

Inhibits salivary

gland secretion

Stimulates salivary

gland secretion

Sympathetic

ganglia

Neurotransmitter

released by

preganglionic neurons:

acetylcholine

Constricts

bronchi in lungs

Relaxes bronchi

in lungs

Neurotransmitter

released by

preganglionic neurons:

acetylcholine

Cervical

Accelerates heart

Slows heart

Inhibits activity of

stomach and intestines

Thoracic

Stimulates activity

of stomach and

intestines

Location of

postganglionic neurons:

in ganglia close to or

within target organs

Location of

postganglionic neurons:

some in ganglia close to

target organs; others in

a chain of ganglia near

spinal cord

Inhibits activity

of pancreas

Stimulates activity

of pancreas

Stimulates glucose

release from liver;

inhibits gallbladder

Stimulates

gallbladder

Lumbar

Neurotransmitter

released by

postganglionic neurons:

acetylcholine

Neurotransmitter

released by

postganglionic neurons:

norepinephrine

Stimulates

adrenal medulla

Promotes emptying

of bladder

Inhibits emptying

of bladder

Promotes erection

of genitalia

Promotes ejaculation and

vaginal contractions

Sacral

Synapse

Figure 48.22

  • The sympathetic and parasympathetic divisions

    • Have antagonistic effects on target organs


Slide13 l.jpg

  • The sympathetic division

    • Correlates with the “fight-or-flight” response

  • The parasympathetic division

    • Promotes a return to self-maintenance functions

  • The enteric division

    • Controls the activity of the digestive tract, pancreas, and gallbladder


Embryonic development of the brain l.jpg

Forebrain

Midbrain

Hindbrain

Midbrain

Hindbrain

Forebrain

(a) Embryo at one month

Embryonic Development of the Brain

  • In all vertebrates

    • The brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrain

Embryonic brain regions

Figure 48.23a


Slide15 l.jpg

Embryonic brain regions

Telencephalon

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

Mesencephalon

Metencephalon

Diencephalon

Myelencephalon

Spinal cord

Telencephalon

Figure 48.23b

(b) Embryo at five weeks

  • By the fifth week of human embryonic development

    • Five brain regions have formed from the three embryonic regions


Slide16 l.jpg

Brain structures present in adult

Cerebrum (cerebral hemispheres; includes cerebral

cortex, white matter, basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

Diencephalon:

Cerebral hemisphere

Hypothalamus

Thalamus

Pineal gland

(part of epithalamus)

Brainstem:

Midbrain

Pons

Pituitary

gland

Medulla

oblongata

Cerebellum

Spinal cord

Central canal

(c) Adult

Figure 48.23c

  • As a human brain develops further

    • The most profound change occurs in the forebrain, which gives rise to the cerebrum


Slide17 l.jpg

  • In humans, the largest and most complex part of the brain

    • Is the cerebral cortex, where sensory information is analyzed, motor commands are issued, and language is generated


Slide18 l.jpg

Frontal lobe

Parietal lobe

Motor cortex

Somatosensory cortex

Somatosensory

association

area

Speech

Frontal

association

area

Taste

Reading

Speech

Hearing

Visual

association

area

Smell

Auditory

association

area

Vision

Temporal lobe

Occipital lobe

Figure 48.27

  • Concept 48.6: The cerebral cortex controls voluntary movement and cognitive functions

  • Each side of the cerebral cortex has four lobes

    • Frontal, parietal, temporal, and occipital


The diencephalon l.jpg

The Diencephalon

  • The embryonic diencephalon develops into three adult brain regions

    • The epithalamus, thalamus, and hypothalamus


Slide20 l.jpg

  • The hypothalamus regulates

    • Homeostasis

    • Basic survival behaviors such as feeding, fighting, fleeing, and reproducing


Memory and learning l.jpg

Memory and Learning

  • The frontal lobes

    • Are a site of short-term memory

    • Interact with the hippocampus and amygdala to consolidate long-term memory


Slide22 l.jpg

  • Many sensory and motor association areas of the cerebral cortex

    • Are involved in storing and retrieving words and images

  • Many sensory and motor association areas of the cerebral cortex

    • Are involved in storing and retrieving words and images


Cellular mechanisms of learning l.jpg

(a) Touching the siphon triggers a reflex thatcauses the gill to withdraw. If the tail isshocked just before the siphon is touched,the withdrawal reflex is stronger. Thisstrengthening of the reflex is a simple formof learning called sensitization.

Siphon

Mantle

Gill

Tail

Head

(b) Sensitization involves interneurons thatmake synapses on the synaptic terminals ofthe siphon sensory neurons. When the tailis shocked, the interneurons releaseserotonin, which activates a signaltransduction pathway that closes K+channels in the synaptic terminals ofthe siphon sensory neurons. As a result,action potentials in the siphon sensoryneurons produce a prolongeddepolarization of the terminals. That allowsmore Ca2+ to diffuse into the terminals, which causes the terminals to release more of their excitatory neurotransmitter onto the gill motor neurons. In response, the motor neuronsgenerate action potentials at a higher frequency,producing a more forceful gill withdrawal.

Gill withdrawal pathway

Touchingthe siphon

Gill motorneuron

Siphon sensoryneuron

Gill

Sensitization pathway

Interneuron

Shockingthe tail

Tail sensoryneuron

Figure 48.31a, b

Cellular Mechanisms of Learning

  • Experiments on invertebrates

    • Have revealed the cellular basis of some types of learning


Slide24 l.jpg

The presynaptic

neuron releases glutamate.

1

Glutamate binds to AMPA

receptors, opening the AMPA-

receptor channel and depolarizing

the postsynaptic membrane.

2

NO diffuses into the

presynaptic neuron, causing

it to release more glutamate.

7

Glutamate also binds to NMDA

receptors. If the postsynaptic

membrane is simultaneously

depolarized, the NMDA-receptor

channel opens.

Ca2+ stimulates the

postsynaptic neuron to

produce nitric oxide (NO).

6

3

Ca2+ diffuses into the

postsynaptic neuron.

4

  • In the vertebrate brain, a form of learning called long-term potentiation (LTP)

    • Involves an increase in the strength of synaptic transmission

PRESYNAPTIC NEURON

NO

NMDA

receptor

Glutamate

AMPA receptor

NO

P

Ca2+ initiates the phos-

phorylation of AMPA receptors,

making them more responsive.

Ca2+ also causes more AMPA

receptors to appear in the

postsynaptic membrane.

5

Ca2+

Signal transduction pathways

Figure 48.32

POSTSYNAPTIC NEURON


Alzheimer s disease l.jpg

Alzheimer’s Disease

  • Alzheimer’s disease (AD)

    • Is a mental deterioration characterized by confusion, memory loss, and other symptoms


Slide26 l.jpg

20 m

Senile plaque

Neurofibrillary tangle

Figure 48.35

  • AD is caused by the formation of

    • Neurofibrillary tangles and senile plaques of protein in the brain


Parkinson s disease l.jpg

Parkinson’s Disease

  • Parkinson’s disease is a motor disorder

    • Caused by the death of dopamine-secreting neurons in the mid-brain. It is characterized by difficulty in initiating movements, slowness of movement, and rigidity

    • Transplantation of stem cells that appear to transform into dopamine-secreting cells alleviate the symptoms but thus far no success in humans


Sensory and motor mechanisms l.jpg

Sensory and Motor Mechanisms

  • Chapt 49 (pp 1063-1074)


Slide29 l.jpg

  • Concept 49.5: Animal skeletons function in support, protection, and movement

  • The various types of animal movements

    • All result from muscles working against some type of skeleton


Types of skeletons l.jpg

Types of Skeletons

  • The three main functions of a skeleton are

    • Support, protection, and movement

  • The three main types of skeletons are

    • Hydrostatic skeletons, exoskeletons, and endoskeletons


Endoskeletons l.jpg

Endoskeletons

  • An endoskeleton consists of hard supporting elements

    • Such as bones, buried within the soft tissue of an animal

  • Endoskeletons

    • Are found in sponges, echinoderms, and chordates


Slide32 l.jpg

  • The mammalian skeleton is built from more than 200 bones

    • Some fused together and others connected at joints by ligaments that allow freedom of movement


Slide33 l.jpg

Head ofhumerus

key

Examplesof joints

Axial skeleton

Skull

Appendicularskeleton

Scapula

1

Clavicle

Shouldergirdle

Scapula

Sternum

1Ball-and-socket joints, where the humerus contactsthe shoulder girdle and where the femur contacts thepelvic girdle, enable us to rotate our arms andlegs and move them in several planes.

Rib

2

Humerus

3

Vertebra

Radius

Ulna

Humerus

Pelvicgirdle

Carpals

Ulna

Phalanges

2Hinge joints, such as between the humerus andthe head of the ulna, restrict movement to a singleplane.

Metacarpals

Femur

Patella

Tibia

Fibula

Ulna

Radius

Tarsals

3Pivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.

Metatarsals

Phalanges

  • The human skeleton

Figure 49.26


Slide34 l.jpg

Human

Grasshopper

Extensormusclerelaxes

Bicepscontracts

Tibiaflexes

Flexormusclecontracts

Tricepsrelaxes

Forearmflexes

Extensormusclecontracts

Tibiaextends

Bicepsrelaxes

Forearmextends

Flexormusclerelaxes

Triceps

contracts

  • The action of a muscle is always to contract

  • Skeletal muscles are attached to the skeleton in antagonistic pairs even with exoskeletons

    • With each member of the pair working against each other

Figure 49.27


Vertebrate skeletal muscle l.jpg

Muscle

Bundle ofmuscle fibers

Nuclei

Single muscle fiber

(cell)

Plasma membrane

Myofibril

Z line

Lightband

Dark band

Sarcomere

TEM

0.5 m

A band

I band

I band

M line

Thickfilaments(myosin)

Thinfilaments(actin)

H zone

Z line

Z line

Sarcomere

Vertebrate Skeletal Muscle

  • Vertebrate skeletal muscle

    • Is characterized by a hierarchy of smaller and smaller units

Muscle fiber composed of many individual embryonic muscle cells fused end to end. Note many nuclei.

Sarcomere

Figure 49.28


Slide36 l.jpg

  • A skeletal muscle consists of a bundle of long fibers

    • Running parallel to the length of the muscle

  • A muscle fiber

    • Is itself a bundle of smaller myofibrils arranged longitudinally

  • The myofibrils are composed to two kinds of myofilaments

    • Thin filaments, consisting of two strands of actin and one strand of regulatory protein

    • Thick filaments, staggered arrays of myosin molecules


Slide37 l.jpg

  • Skeletal muscle is also called striated muscle

    • Because the regular arrangement of the myofilaments creates a pattern of light and dark bands


The sliding filament model of muscle contraction l.jpg

The Sliding-Filament Model of Muscle Contraction

  • According to the sliding-filament model of muscle contraction

    • The filaments slide past each other longitudinally, producing more overlap between the thin and thick filaments


Slide39 l.jpg

0.5 m

(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bandsand H zone are relatively wide.

Z

H

A

Sarcomere

(b) Contracting muscle fiber. During contraction, the thick andthin filaments slide past each other, reducing the width of theI bands and H zone and shortening the sarcomere.

(c) Fully contracted muscle fiber. In a fully contracted musclefiber, the sarcomere is shorter still. The thin filaments overlap,eliminating the H zone. The I bands disappear as the ends ofthe thick filaments contact the Z lines.

Correlation of structure as seen with the electron microscope and function.

  • As a result of this sliding

    • The I band and the H zone shrink

Figure 49.29a–c


Slide40 l.jpg

  • The sliding of filaments is based on

    • The interaction between the actin and myosin molecules of the thick and thin filaments

  • The “head” of a myosin molecule binds to an actin filament

    • Forming a cross-bridge and pulling the thin filament toward the center of the sarcomere


Slide41 l.jpg

Thick filament

Thin filaments

1Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.

5 Binding of a new mole-

cule of ATP releases the

myosin head from actin,

and a new cycle begins.

Thin filament

Myosin head (low-energy configuration)

The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration.

ATP

2

ATP

Cross-bridge binding site

Thick filament

P

Actin

Thin filament moves toward center of sarcomere.

Myosin head (high-energy configuration)

ADP

Myosin head (low-energy configuration)

P i

1The myosin head binds toactin, forming a cross-bridge.

3

ADP

+

Cross-bridge

ADP

P i

P i

Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament.

4

P

  • Myosin-actin interactions underlying muscle fiber contraction

Figure 49.30


The role of calcium and regulatory proteins l.jpg

Tropomyosin

Ca2+-binding sites

Actin

Troponin complex

(a) Myosin-binding sites blocked

The Role of Calcium and Regulatory Proteins

  • A skeletal muscle fiber contracts only when stimulated by a motor neuron

  • When a muscle is at rest the myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin

Figure 49.31a


Slide43 l.jpg

Ca2+

Myosin-binding site

(b) Myosin-binding sites exposed

  • For a muscle fiber to contract the myosin-binding sites must be uncovered

  • This occurs when calcium ions (Ca2+) bind to another set of regulatory proteins, the troponin complex

Figure 49.31b


Slide44 l.jpg

Motorneuron axon

Mitochondrion

Synapticterminal

T tubule

Sarcoplasmicreticulum

Ca2+ releasedfrom sarcoplasmicreticulum

Myofibril

Sarcomere

Plasma membraneof muscle fiber

  • The stimulus leading to the contraction of a skeletal muscle fiber

    • Is an action potential in a motor neuron that makes a synapse with the muscle fiber

Figure 49.32


Skip to figure l.jpg

Skip to figure!

  • The synaptic terminal of the motor neuron

    • Releases the neurotransmitter acetylcholine, depolarizing the muscle and causing it to produce an action potential


Slide46 l.jpg

  • Action potentials travel to the interior of the muscle fiber

    • Along infoldings of the plasma membrane called transverse (T) tubules

  • The action potential along the T tubules

    • Causes the sarcoplasmic reticulum to release Ca2+

  • The Ca2+ binds to the troponin-tropomyosin complex on the thin filaments

    • Exposing the myosin-binding sites and allowing the cross-bridge cycle to proceed


Slide47 l.jpg

Acetylcholine (ACh) released by synaptic terminal diffuses across synapticcleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber.

Synapticterminalof motorneuron

1

PLASMA MEMBRANE

Synaptic cleft

T TUBULE

Action potential is propa-

gated along plasma

membrane and down

T tubules.

2

ACh

SR

4

Action potential

triggers Ca2+

release from sarco-

plasmic reticulum

(SR).

3

Ca2

Calcium ions bind to troponin;

troponin changes shape,

removing blocking action

of tropomyosin; myosin-binding

sites exposed.

Tropomyosin blockage of myosin-

binding sites is restored; contraction

ends, and muscle fiber relaxes.

7

Ca2

CYTOSOL

Cytosolic Ca2+ is

removed by active

transport into

SR after action

potential ends.

6

ADP

P2

Myosin cross-bridges alternately attach

to actin and detach, pulling actin

filaments toward center of sarcomere;

ATP powers sliding of filaments.

5

Calcium as a regulator of muscle contraction!

Figure 49.33


Neural control of muscle tension l.jpg

Neural Control of Muscle Tension

  • Contraction of a whole muscle is graded

    • Which means that we can voluntarily alter the extent and strength of its contraction

  • There are two basic mechanisms by which the nervous system produces graded contractions of whole muscles

    • By varying the number of fibers that contract

    • By varying the rate at which muscle fibers are stimulated


Slide49 l.jpg

Motorunit 1

Motorunit 2

Spinal cord

Synaptic terminals

Nerve

Motor neuroncell body

Motor neuronaxon

Muscle

Muscle fibers

Tendon

  • In a vertebrate skeletal muscle

    • Each branched muscle fiber is innervated by only one motor neuron

  • Each motor neuron

    • May synapse with multiple muscle fibers

Figure 49.34


Slide50 l.jpg

  • A motor unit

    • Consists of a single motor neuron and all the muscle fibers it controls

  • Recruitment of multiple motor neurons

    • Results in stronger contractions


Slide51 l.jpg

Tetanus

Tension

Summation of two twitches

Singletwitch

Time

Actionpotential

Pair ofactionpotentials

Series of action potentials at high frequency

  • A muscle twitch results from a single action potential in a motor neuron

  • More rapidly delivered action potentials produce a graded contraction by summation

  • Tetanus is a state of smooth and sustained contraction produced when motor neurons deliver a volley of action potentials

Figure 49.35


Types of muscle fibers l.jpg

Types of Muscle Fibers

  • Skeletal muscle fibers are classified as slow oxidative, fast oxidative, and fast glycolytic

    • Based on their contraction speed and major pathway for producing ATP


Slide53 l.jpg

  • Types of skeletal muscles


Other types of muscle l.jpg

Other Types of Muscle

  • Cardiac muscle, found only in the heart

    • Consists of striated cells that are electrically connected by intercalated discs

    • Can generate action potentials without neural input


Slide55 l.jpg

  • In smooth muscle, found mainly in the walls of hollow organs

    • The contractions are relatively slow and may be initiated by the muscles themselves

  • In addition, contractions may be caused by

    • Stimulation from neurons in the autonomic nervous system


Slide56 l.jpg

  • Concept 49.7: Locomotion requires energy to overcome friction and gravity

  • Movement is a hallmark of all animals

    • And usually necessary for finding food or evading predator

  • Overcoming friction is a major problem for swimmers

  • Overcoming gravity is less of a problem for swimmers than for animals that move on land or fly


Locomotion on land l.jpg

Locomotion on Land

  • Walking, running, hopping, or crawling on land

    • Requires an animal to support itself and move against gravity


Slide58 l.jpg

  • Diverse adaptations for traveling on land

    • Have evolved in various vertebrates

Figure 49.36


Slide59 l.jpg

EXPERIMENT

Physiologists typically determine an animal’s rate of energy use during locomotion by measuring its oxygen consumption or carbon dioxide production while it swims in a water flume, runs on a treadmill, or flies in a wind tunnel. For example, the trained parakeet shown below is wearing a plastic face mask connected to a tube that collects the air the bird exhales as it flies.

RESULTS

This graph compares the energy cost, in joules per kilogram of body mass per meter traveled, for animals specialized for running, flying, and swimming (1 J = 0.24 cal). Notice that both axes are plotted on logarithmic scales.

Flying

Running

102

10

Energy cost (J/Kg/m)

1

Swimming

10–1

10–3

1

103

106

Body mass(g)

Comparing Costs of Locomotion

  • The energy cost of locomotion

    • Depends on the mode of locomotion and the environment

CONCLUSION

For animals of a given body mass, swimming is the most energy-efficient and running the least energy-efficient mode of locomotion. In any mode, a small animal expends more energy per kilogram of body mass than a large animal.

CONCLUSION

Figure 49.37


Slide60 l.jpg

  • Animals that are specialized for swimming

    • Expend less energy per meter traveled than equivalently sized animals specialized for flying or running


Chapter 47 l.jpg

Chapter 47

Animal Development

Read pages 987-992 and 994-995 for information on sea urchin fertilization and development.


It is difficult to imagine that each of us began life as a single cell a zygote l.jpg

1 mm

Figure 47.1

It is difficult to imagine that each of us began life as a single cell, a zygote

  • A human embryo at approximately 6–8 weeks after conception

    • Shows the development of distinctive features

Head, with eye plaque, internal organs and tail.


Slide63 l.jpg

  • The question of how a zygote becomes an animal has been asked for centuries

  • As recently as the 18th century

    • The prevailing theory was a notion called preformation


Slide64 l.jpg

Figure 47.2

  • Preformation is the idea that the egg or sperm contains an embryo

    • A preformed miniature infant, or “homunculus,” that simply becomes larger during development

We now know that animals emerge gradually from a formless egg in a series of progressive steps as determined by the genome of the zygote.


Slide65 l.jpg

  • An organism’s development is determined by the genome of the zygote and by differences that arise between early embryonic cells. Two terms!

  • Cell differentiation

    • Is the specialization of cells in their structure and function (ectodermal, endodermal and mesodermal cells give rise to specific tissues and organs)

  • Morphogenesis

    • Is the process by which an animal takes shape


Slide66 l.jpg

  • Concept 47.1: After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis

  • Important events regulating development

    • Occur during fertilization and each of the three successive stages that build the animal’s body

    • Next week’s lab we will look at fertilization and early development in the sea urchin.


Fertilization l.jpg

Fertilization

  • The main function of fertilization

    • Is to bring the haploid nuclei of sperm and egg together to form a diploid zygote

  • Contact of the sperm with the egg’s surface

    • Initiates metabolic reactions within the egg that trigger the onset of embryonic development


Rapid events occur when sperm contacts the egg l.jpg

1

Acrosomal reaction. Hydrolytic

enzymes released from the

acrosome make a hole in the

jelly coat, while growing actin

filaments form the acrosomal

process. This structure protrudes

from the sperm head and

penetrates the jelly coat, binding

to receptors in the egg cell

membrane that extend through

the vitelline layer.

2

Contact and fusion of sperm

and egg membranes. A hole

is made in the vitelline layer,

allowing contact and fusion of

the gamete plasma membranes.

The membrane becomes

depolarized, resulting in the

fast block to polyspermy.

3

4

Cortical reaction. Fusion of the

gamete membranes triggers an

increase of Ca2+ in the egg’s

cytosol, causing cortical granules

in the egg to fuse with the plasma

membrane and discharge their

contents. This leads to swelling of the

perivitelline space, hardening of the

vitelline layer, and clipping off

sperm-binding receptors. The resulting

fertilization envelope is the slow block

to polyspermy.

5

Contact. The

sperm cell

contacts the

egg’s jelly coat,

triggering

exocytosis from the

sperm’s acrosome.

Entry of

sperm nucleus.

Sperm plasma

membrane

Sperm

nucleus

Acrosomal

process

Basal body

(centriole)

Fertilization

envelope

Sperm

head

Fused plasma

membranes

Cortical

granule

Actin

Perivitelline

space

Hydrolytic enzymes

Acrosome

Cortical granule

membrane

Vitelline layer

Jelly coat

Egg plasma

membrane

Sperm-binding

receptors

EGG CYTOPLASM

Figure 47.3

Rapid events occur when sperm contacts the egg!

  • The acrosomal reaction

You will be able to see the fertilization envelope in lab.


Slide69 l.jpg

  • Gamete contact and/or fusion

    • Depolarizes the egg cell membrane and sets up a fast block to polyspermy (prevents other sperm from entering egg).


The cortical reaction l.jpg

EXPERIMENT

RESULTS

A fluorescent dye that glows when it binds free Ca2+ was injected into unfertilized sea urchin eggs. After sea urchin

sperm were added, researchers observed the eggs in a fluorescence microscope.

500 m

10 sec after

fertilization

1 sec before

fertilization

30 sec

20 sec

Spreading wave

of calcium ions

Point of

sperm

entry

CONCLUSION

The release of Ca2+ from the endoplasmic reticulum into the cytosol at the site of sperm entry triggers the release

of more and more Ca2+ in a wave that spreads to the other side of the cell. The entire process takes about 30 seconds.

Figure 47.4

The Cortical Reaction

  • Fusion of egg and sperm also initiates the cortical reaction inducing a rise in Ca2+ that stimulates cortical granules to release their contents outside the egg plasma membrane


Slide71 l.jpg

  • These changes cause the formation of a fertilization envelope

    • That functions as a slow block to polyspermy


Activation of the egg l.jpg

Activation of the Egg

  • Another outcome of the sharp rise in Ca2+ in the egg’s cytosol

    • Is a substantial increase in the rates of cellular respiration and protein synthesis by the egg cell

  • With these rapid changes in metabolism

    • The egg is said to be activated


Slide73 l.jpg

Binding of sperm to egg

1

2

Acrosomal reaction: plasma membrane

depolarization (fast block to polyspermy)

3

4

6

Seconds

8

Increased intracellular calcium level

10

20

Cortical reaction begins (slow block to polyspermy)

30

40

50

Formation of fertilization envelope complete

1

2

Increased intracellular pH

3

4

5

Increased protein synthesis

Minutes

10

Fusion of egg and sperm nuclei complete

20

30

Onset of DNA synthesis

40

60

Figure 47.5

First cell division

90

  • In a fertilized egg of a sea urchin, a model organism

    • Many events occur in the activated egg


Cleavage l.jpg

Cleavage

  • Fertilization is followed by cleavage

    • A period of rapid cell division without growth shown in the next slide.


Fertilization is followed by cleavage rapid cell division without growth l.jpg

(d)

Blastula. A single layer of cells

surrounds a large blastocoel

cavity. Although not visible here,

the fertilization envelope is still

present. The blastula will next

undergo gastrulation.

(b)

Four-cell stage. Remnants of the

mitotic spindle can be seen

between the two cells that have

just completed the second

cleavage division.

(c)

Morula. After further cleavage

divisions, the embryo is a

multicellular ball that is still

surrounded by the fertilization

envelope. The blastocoel cavity

has begun to form.

Fertilization is followed by cleavage-- rapid cell division without growth

  • Cleavage partitions the cytoplasm of one large cell

    • Into many smaller cells called blastomeres

(a)

Fertilized egg. Shown here is the

zygote shortly before the first

cleavage division, surrounded

by the fertilization envelope.

The nucleus is visible in the

center.

Figure 47.7a–d


Gastrulation l.jpg

Key

Future ectoderm

Future mesoderm

1

The blastula consists of a single layer of ciliated cells surrounding the

blastocoel. Gastrulation begins with the migration of mesenchyme cells

from the vegetal pole into the blastocoel.

Animalpole

Future endoderm

Blastocoel

Mesenchymecells

Vegetalplate

Vegetalpole

The vegetal plate invaginates (buckles inward). Mesenchyme cells

migrate throughout the blastocoel.

2

2

Blastocoel

Filopodiapullingarchenterontip

3

Endoderm cells form the archenteron (future digestive tube). New

mesenchyme cells at the tip of the tube begin to send out thin

extensions (filopodia) toward the ectoderm cells of the blastocoel

wall (inset, LM).

Archenteron

Mesenchymecells

Blastopore

Blastocoel

Contraction of these filopodia then drags the archenteron across

the blastocoel.

4

50 µm

Archenteron

Ectoderm

Blastopore

Mouth

5

Fusion of the archenteron with the blastocoel wall completes

formation of the digestive tube with a mouth and an anus. The

gastrula has three germ layers and is covered with cilia, which

function in swimming and feeding.

Mesenchyme:(mesodermforms future skeleton)

Digestive tube (endoderm)

Figure 47.11

Anus (from blastopore)

Gastrulation

  • The morphogenetic process called gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, that has a primitive gut. Three germ layers develope.

Sea urchin is a deuterostome so blastopore forms the anus. New opening for mouth. Mesoderm buds off from endoderm.


Slide77 l.jpg

  • The three layers produced by gastrulation

    • Are called embryonic germ layers

  • The ectoderm

    • Forms the outer layer of the gastrula

  • The endoderm

    • Lines the embryonic digestive tract

  • The mesoderm

    • Partly fills the space between the endoderm and ectoderm


Slide78 l.jpg

  • The eggs and zygotes of many animals, except mammals

    • Have a definite polarity

  • The polarity is defined by the distribution of yolk

    • With the vegetal pole having the most yolk and the animal pole having the least


Slide79 l.jpg

  • Holoblastic cleavage, the complete division of the egg

    • Occurs in species whose eggs have little or moderate amounts of yolk, such as sea urchins and frogs


Slide80 l.jpg

Zygote

0.25 mm

0.25 mm

0.25 mm

2-cell

stage

forming

Eight-cell stage (viewed from the animal pole). The large

amount of yolk displaces the third cleavage toward the animal pole,

forming two tiers of cells. The four cells near the animal pole

(closer, in this view) are smaller than the other four cells (SEM).

4-cell

stage

forming

8-cell

stage

0.25 mm

Animal pole

Animal pole

Blastula (at least 128 cells). As cleavage continues, a fluid-filled

cavity, the blastocoel, forms within the embryo. Because of unequal

cell division due to the large amount of yolk in the vegetal

hemisphere, the blastocoel is located in the animal hemisphere, as

shown in the cross section. The SEM shows the outside of a

blastula with about 4,000 cells, looking at the animal pole.

Blasto-

coel

Blasto-

coel

Blastula

(cross

section)

Blastula

(cross

section)

Vegetal pole

Vegetal pole

  • Cleavage planes usually follow a specific pattern (Radial cleavage)

    • That is relative to the animal and vegetal poles of the zygote

Because of large amount of yolk the animal pole cells smaller!

Figure 47.9


Slide81 l.jpg

Fertilized egg

Disk of

cytoplasm

Zygote. Most of the cell’s volume is yolk, with a small disk

of cytoplasm located at the animal pole.

1

2

Four-cell stage. Early cell divisions are meroblastic

(incomplete). The cleavage furrow extends through the

cytoplasm but not through the yolk.

Blastoderm. The many cleavage divisions produce the

blastoderm, a mass of cells that rests on top of the yolk mass.

3

Cutaway view of the blastoderm. The cells of the

blastoderm are arranged in two layers, the epiblastand hypoblast, that enclose a fluid-filled cavity, theblastocoel.

Blastocoel

BLASTODERM

YOLK MASS

Figure 47.10

Epiblast

Hypoblast

  • Meroblastic cleavage, incomplete division of the egg. Occurs on the surface of the yolk!

    • Occurs in species with yolk-rich eggs, such as reptiles and birds


In birds embryo forms on top of huge yolk l.jpg

Epiblast

Future

ectoderm

Primitive

streak

Migrating

cells

(mesoderm)

Endoderm

Hypoblast

YOLK

Figure 47.13

In birds embryo forms on top of huge yolk.

  • Gastrulation in the chick

    • Is affected by the large amounts of yolk in the egg


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