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Nervous System Part 2IB-202-154-24-06
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
• 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
• Major neurotransmitters
Table 48.1
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• 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
• Gases such as nitric oxide and carbon monoxide– Are local regulators in the PNS
• 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
Figure 48.19
Central nervoussystem (CNS)
Peripheral nervoussystem (PNS)
Brain
Spinal cordCranialnerves
GangliaoutsideCNSSpinalnerves
• 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
• 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
Gray matter
Whitematter
Ventricles
Figure 48.20
Grey matter is unmylinated axons, dendrites and nerve bodies.
Mylinated axons interconnecting parts of brain and nerve tracks to spinal cord
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
• The PNS can be divided into two functional components– The somatic nervous system and the
autonomic nervous system
Peripheralnervous system
Somaticnervoussystem
Autonomicnervoussystem
Sympatheticdivision
Parasympatheticdivision
Entericdivision
Figure 48.21
Somatic largely voluntary control of muscle in response to external stimuli
Autonomic regulates the internal environment in an involuntary manner.
• The sympathetic and parasympathetic divisions– Have antagonistic effects on target organs
Parasympathetic division Sympathetic division
Action on target organs: Action on target organs:
Location ofpreganglionic neurons:brainstem and sacralsegments of spinal cord
Neurotransmitterreleased bypreganglionic neurons:acetylcholine
Location ofpostganglionic neurons:in ganglia close to orwithin target organs
Neurotransmitterreleased bypostganglionic neurons:acetylcholine
Constricts pupilof eye
Stimulates salivarygland secretion
Constrictsbronchi in lungs
Slows heart
Stimulates activityof stomach and
intestines
Stimulates activityof pancreas
Stimulatesgallbladder
Promotes emptyingof bladder
Promotes erectionof genitalia
Cervical
Thoracic
Lumbar
Synapse
Sympatheticganglia
Dilates pupilof eye
Inhibits salivary gland secretion
Relaxes bronchiin lungs
Accelerates heart
Inhibits activity of stomach and intestines
Inhibits activityof pancreas
Stimulates glucoserelease from liver;inhibits gallbladder
Stimulatesadrenal medulla
Inhibits emptyingof bladder
Promotes ejaculation and vaginal contractionsSacral
Location ofpreganglionic neurons:thoracic and lumbarsegments of spinal cord
Neurotransmitterreleased bypreganglionic neurons:acetylcholine
Location ofpostganglionic neurons:some in ganglia close totarget organs; others ina chain of ganglia near spinal cord
Neurotransmitterreleased bypostganglionic neurons:norepinephrine
Figure 48.22
• 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• In all vertebrates
– The brain develops from three embryonic regions: the forebrain, the midbrain, and the hindbrain
Figure 48.23a
Forebrain
Midbrain
Hindbrain
Midbrain Hindbrain
Forebrain
(a) Embryo at one month
Embryonic brain regions
• By the fifth week of human embryonic development– Five brain regions have formed from the three
embryonic regions
Figure 48.23b
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
(b) Embryo at five weeks
MesencephalonMetencephalon
Myelencephalon
Spinal cord
Diencephalon
Telencephalon
Embryonic brain regions
• As a human brain develops further– The most profound change occurs in the
forebrain, which gives rise to the cerebrum
Figure 48.23c
Brain structures present in adult
Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)
Diencephalon (thalamus, hypothalamus, epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem), cerebellum
Medulla oblongata (part of brainstem)
(c) Adult
Cerebral hemisphereDiencephalon:
Hypothalamus
ThalamusPineal gland(part of epithalamus)
Brainstem:
Midbrain
Pons
Medullaoblongata
Cerebellum
Central canal
Spinal cord
Pituitarygland
• 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
• 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
Frontal lobe
Temporal lobe Occipital lobe
Parietal lobe
Frontalassociationarea
Speech
Smell
Hearing
Auditoryassociationarea
Vision
Visualassociationarea
Somatosensoryassociationarea
Reading
Speech
TasteS
omat
osen
sory
cor
tex
Mot
or c
orte
x
Figure 48.27
The Diencephalon• The embryonic diencephalon develops into
three adult brain regions– The epithalamus, thalamus, and hypothalamus
• The hypothalamus regulates– Homeostasis– Basic survival behaviors such as feeding,
fighting, fleeing, and reproducing
Memory and Learning
• The frontal lobes– Are a site of short-term memory– Interact with the hippocampus and amygdala
to consolidate long-term memory
• 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• Experiments on invertebrates
– Have revealed the cellular basis of some types of learning
Figure 48.31a, b
(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.
(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.
Siphon
Mantle
Gill
Tail
Head
Gill withdrawal pathway
Touchingthe siphon
Shockingthe tail Tail sensory
neuron
Interneuron
Sensitization pathway
Siphon sensoryneuron
Gill motorneuron
Gill
• In the vertebrate brain, a form of learning called long-term potentiation (LTP)– Involves an increase in the strength of
synaptic transmission
Figure 48.32
PRESYNAPTIC NEURON
NO
Glutamate
NMDAreceptor
Signal transduction pathways
NO
Ca2+
AMPA receptor
POSTSYNAPTIC NEURON
Ca2+ initiates the phos-phorylation of AMPA receptors,making them more responsive.Ca2+ also causes more AMPAreceptors to appear in thepostsynaptic membrane.
5
Ca2+ stimulates thepostsynaptic neuron toproduce nitric oxide (NO).
6
The presynapticneuron releases glutamate.1
Glutamate binds to AMPAreceptors, opening the AMPA-receptor channel and depolarizingthe postsynaptic membrane.
2
Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneouslydepolarized, the NMDA-receptorchannel opens.
3
Ca2+ diffuses into thepostsynaptic neuron.
4
NO diffuses into thepresynaptic neuron, causing it to release more glutamate.
7
P
Alzheimer’s Disease
• Alzheimer’s disease (AD)– Is a mental deterioration characterized by
confusion, memory loss, and other symptoms
• AD is caused by the formation of– Neurofibrillary tangles and senile plaques of
protein in the brain
Figure 48.35
Senile plaque Neurofibrillary tangle20 m
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
• Chapt 49 (pp 1063-1074)
• 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
• 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• 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
• 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
• The human skeleton
Figure 49.26
1 Ball-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.
2 Hinge joints, such as between the humerus andthe head of the ulna, restrict movement to a singleplane.
3 Pivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.
keyAxial skeletonAppendicularskeleton
Skull
Shouldergirdle
Clavicle
Scapula
Sternum
RibHumerus
Vertebra
RadiusUlnaPelvicgirdle
Carpals
Phalanges
Metacarpals
Femur
Patella
Tibia
Fibula
TarsalsMetatarsalsPhalanges
1
Examplesof joints
2
3
Head ofhumerus
Scapula
Humerus
Ulna
UlnaRadius
• 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
Human Grasshopper
Bicepscontracts
Tricepsrelaxes
Forearmflexes
Bicepsrelaxes
Tricepscontracts
Forearmextends
Extensormusclerelaxes
Flexormusclecontracts
Tibiaflexes
Extensormusclecontracts
Flexormusclerelaxes
Tibiaextends
Vertebrate Skeletal Muscle• Vertebrate skeletal muscle
– Is characterized by a hierarchy of smaller and smaller units
Figure 49.28
Muscle
Bundle ofmuscle fibers
Single muscle fiber(cell)
Plasma membrane
Myofibril
Lightband Dark band
Z line
Sarcomere
TEM 0.5 mI band A band I band
M line
Thickfilaments(myosin)
Thinfilaments(actin)
H zoneSarcomere
Z lineZ line
Nuclei
Sarcomere
Muscle fiber composed of many individual embryonic muscle cells fused end to end. Note many nuclei.
• 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
• 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
• 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
• As a result of this sliding– The I band and the H zone shrink
Figure 49.29a–c
(a) Relaxed muscle fiber. In a relaxed muscle fiber, the I bandsand H zone are relatively wide.
(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.
0.5 m
Z HA
Sarcomere
Correlation of structure as seen with the electron microscope and function.
• 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
• Myosin-actin interactions underlying muscle fiber contraction
Figure 49.30
Thick filament
Thin filaments
Thin filament
ATPATP
ADPADP
ADP
P i P i
P i
Cross-bridge
Myosin head (low-energy configuration)
Myosin head (high-energy configuration)
+
Myosin head (low-energy configuration)
Thin filament moves toward center of sarcomere.
Thick filament
ActinCross-bridge binding site
1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.
2 The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration.
P
1 The myosin head binds toactin, forming a cross-bridge.
3
4 Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament.
P
5 Binding of a new mole-cule of ATP releases the myosin head from actin,and a new cycle begins.
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
ActinTropomyosin Ca2+-binding sites
Troponin complex
(a) Myosin-binding sites blocked
• 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
Ca2+
Myosin-binding site
(b) Myosin-binding sites exposed
• 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
Motorneuron axon
Mitochondrion
Synapticterminal
T tubule
Sarcoplasmicreticulum
Myofibril
Plasma membraneof muscle fiber
Sarcomere
Ca2+ releasedfrom sarcoplasmicreticulum
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
• 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
ACh
Synapticterminalof motorneuron
Synaptic cleft T TUBULEPLASMA MEMBRANE
SR
ADP
CYTOSOL
Ca2
Ca2
P2
Cytosolic Ca2+ is removed by active transport into SR after action potential ends.
6
Calcium as a regulator of muscle contraction!
Figure 49.33
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.
1
Action potential is propa-gated along plasmamembrane and downT tubules.
2
Action potentialtriggers Ca2+
release from sarco-plasmic reticulum(SR).
3
Myosin cross-bridges alternately attachto actin and detach, pulling actinfilaments toward center of sarcomere;ATP powers sliding of filaments.
5
Calcium ions bind to troponin;troponin changes shape,removing blocking actionof tropomyosin; myosin-bindingsites exposed.
4
Tropomyosin blockage of myosin-binding sites is restored; contractionends, and muscle fiber relaxes.
7
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
• 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
Spinal cord
Nerve
Motor neuroncell body
Motorunit 1
Motorunit 2
Motor neuronaxon
Muscle
Tendon
Synaptic terminals
Muscle fibers
• 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
• 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
Actionpotential Pair of
actionpotentials
Series of action potentials at
high frequency
Time
Ten
sion
Singletwitch
Summation of two twitches
Tetanus
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
• Types of skeletal muscles
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
• 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
• 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
• Walking, running, hopping, or crawling on land– Requires an animal to support itself and move
against gravity
• Diverse adaptations for traveling on land– Have evolved in various vertebrates
Figure 49.36
CONCLUSIONFor 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.
FlyingRunning
Swimming
10–3 103 1061
10–1
10
102
1
Body mass(g)
En
erg
y co
st (
J/K
g/m
)CONCLUSION
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.
RESULTS
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.
EXPERIMENT
•The energy cost of locomotion
–Depends on the mode of locomotion and the environment
Figure 49.37
Comparing Costs of Locomotion
• Animals that are specialized for swimming– Expend less energy per meter traveled than
equivalently sized animals specialized for flying or running
Chapter 47
Animal DevelopmentRead 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
• A human embryo at approximately 6–8 weeks after conception– Shows the development of distinctive features
Figure 47.1 1 mm
Head, with eye plaque, internal organs and tail.
• 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
• Preformation is the idea that the egg or sperm contains an embryo– A preformed miniature infant, or
“homunculus,” that simply becomes larger during development
Figure 47.2
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.
• 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
• 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
• 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!
• The acrosomal reaction
Spermnucleus
Sperm plasmamembrane
Hydrolytic enzymes
Corticalgranule
Cortical granulemembrane
EGG CYTOPLASM
Basal body(centriole)
Spermhead
Acrosomalprocess
Actin
Acrosome
Jelly coatEgg plasmamembrane
Vitelline layer
Fused plasmamembranes
Perivitellinespace
Fertilizationenvelope
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 thevitelline layer, and clipping off sperm-binding receptors. The resulting fertilization envelope is the slow block to polyspermy.
5 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 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, bindingto receptors in the egg cell membrane that extend through the vitelline layer.
2 Contact. The sperm cell contacts the egg’s jelly coat, triggering exocytosis from the sperm’s acrosome.
1
Sperm-bindingreceptors
Entry of sperm nucleus.4
Figure 47.3
You will be able to see the fertilization envelope in lab.
• 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• 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
Figure 47.4
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.
EXPERIMENT
RESULTS
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.
CONCLUSION
30 sec20 sec10 sec afterfertilization
1 sec beforefertilization
Point ofspermentry
Spreading waveof calcium ions
500 m
• These changes cause the formation of a fertilization envelope– That functions as a slow block to polyspermy
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
• In a fertilized egg of a sea urchin, a model organism– Many events occur in the activated egg
Figure 47.5
Binding of sperm to egg
Acrosomal reaction: plasma membranedepolarization (fast block to polyspermy)
Increased intracellular calcium level
Cortical reaction begins (slow block to polyspermy)
Formation of fertilization envelope complete
Increased intracellular pH
Increased protein synthesis
Fusion of egg and sperm nuclei complete
Onset of DNA synthesis
First cell division
1
2
34
6
8
10
20
30
4050
1
2
345
10
20
30
40
60
Sec
onds
Mi n
utes
90
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
• Cleavage partitions the cytoplasm of one large cell– Into many smaller cells called blastomeres
Figure 47.7a–d
Fertilized egg. Shown here is thezygote shortly before the first cleavage division, surrounded by the fertilization envelope. The nucleus is visible in the center.
(a) Four-cell stage. Remnants of the mitotic spindle can be seen between the two cells that have just completed the second cleavage division.
(b) Morula. After further cleavage divisions, the embryo is a multicellular ball that is stillsurrounded by the fertilization envelope. The blastocoel cavityhas begun to form.
(c) 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.
(d)
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.
Figure 47.11
Digestive tube (endoderm)
Key
Future ectodermFuture mesodermFuture endoderm
BlastocoelMesenchymecells
Vegetalplate
Animalpole
Vegetalpole
Filopodiapullingarchenterontip
Archenteron
Blastocoel
Blastopore
50 µm
Blastopore
Archenteron
Blastocoel
Mouth
Ectoderm
Mesenchyme:(mesodermforms future skeleton) Anus (from blastopore)
Mesenchymecells
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.
1
2 The vegetal plate invaginates (buckles inward). Mesenchyme cells migrate throughout the blastocoel.2
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).
3
Contraction of these filopodia then drags the archenteron across the blastocoel.4
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.
5
Sea urchin is a deuterostome so blastopore forms the anus. New opening for mouth. Mesoderm buds off from endoderm.
• 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
• 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
• 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
• Cleavage planes usually follow a specific pattern (Radial cleavage)– That is relative to the animal and vegetal poles
of the zygote
Figure 47.9
Zygote
2-cellstageforming
4-cellstageforming
8-cellstage
Eight-cell stage (viewed from the animal pole). The largeamount 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).
0.25 mm0.25 mm
Vegetal pole
Blastula(crosssection)
Animal poleBlasto-coel
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. Vegetal pole
Blastula(crosssection)
Animal poleBlasto-coel
0.25 mm
0.25 mm
Because of large amount of yolk the animal pole cells smaller!
• 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
Figure 47.10 Epiblast Hypoblast
BLASTODERMBlastocoel
YOLK MASS
Fertilized eggDisk ofcytoplasm
Zygote. Most of the cell’s volume is yolk, with a small disk of cytoplasm located at the animal pole.
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.
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.
3
1
2
In birds embryo forms on top of huge yolk.
• Gastrulation in the chick– Is affected by the large amounts of yolk in the egg
Figure 47.13
Epiblast
Futureectoderm
Migratingcells(mesoderm)
Endoderm
Hypoblast
YOLK
Primitivestreak