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
Chapt 48 pp 1022-1028, 1036 (memory), 1040-1041 (Alzheimer’s and Parkinson’s
Mylinated axons interconnecting parts of brain and nerve tracks to spinal cord
Grey matter is unmylinated axons, dendrites and nerve bodies.
Autonomic regulates the internal environment in an involuntary manner.
Somatic largely voluntary control of muscle in response to external stimuli
Action on target organs:
Action on target organs:
brainstem and sacral
segments of spinal cord
thoracic and lumbar
segments of spinal cord
bronchi in lungs
Inhibits activity of
stomach and intestines
of stomach and
in ganglia close to or
within target organs
some in ganglia close to
target organs; others in
a chain of ganglia near
release from liver;
Promotes ejaculation and
(a) Embryo at one month
Embryonic brain regions
Embryonic brain regions
(b) Embryo at five weeks
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)
(part of epithalamus)
(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.
Gill withdrawal pathway
Figure 48.31a, b
neuron releases glutamate.
Glutamate binds to AMPA
receptors, opening the AMPA-
receptor channel and depolarizing
the postsynaptic membrane.
NO diffuses into the
presynaptic neuron, causing
it to release more glutamate.
Glutamate also binds to NMDA
receptors. If the postsynaptic
membrane is simultaneously
depolarized, the NMDA-receptor
Ca2+ stimulates the
postsynaptic neuron to
produce nitric oxide (NO).
Ca2+ diffuses into the
Ca2+ initiates the phos-
phorylation of AMPA receptors,
making them more responsive.
Ca2+ also causes more AMPA
receptors to appear in the
Signal transduction pathways
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.
2Hinge joints, such as between the humerus andthe head of the ulna, restrict movement to a singleplane.
3Pivot joints allow us to rotate our forearm at theelbow and to move our head from side to side.
Bundle ofmuscle fibers
Single muscle fiber
Muscle fiber composed of many individual embryonic muscle cells fused end to end. Note many nuclei.
(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.
Correlation of structure as seen with the electron microscope and function.
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.
Myosin head (low-energy configuration)
The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration.
Cross-bridge binding site
Thin filament moves toward center of sarcomere.
Myosin head (high-energy configuration)
Myosin head (low-energy configuration)
1The myosin head binds toactin, forming a cross-bridge.
Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament.
(a) Myosin-binding sites blocked
(b) Myosin-binding sites exposed
Ca2+ releasedfrom sarcoplasmicreticulum
Plasma membraneof muscle fiber
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.
Action potential is propa-
gated along plasma
membrane and down
release from sarco-
Calcium ions bind to troponin;
troponin changes shape,
removing blocking action
of tropomyosin; myosin-binding
Tropomyosin blockage of myosin-
binding sites is restored; contraction
ends, and muscle fiber relaxes.
Cytosolic Ca2+ is
removed by active
SR after action
Myosin cross-bridges alternately attach
to actin and detach, pulling actin
filaments toward center of sarcomere;
ATP powers sliding of filaments.
Calcium as a regulator of muscle contraction!
Motor neuroncell body
Summation of two twitches
Series of action potentials at high frequency
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.
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.
Energy cost (J/Kg/m)
Comparing Costs of Locomotion
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.
Read pages 987-992 and 994-995 for information on sea urchin fertilization and development.
Head, with eye plaque, internal organs and tail.
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.
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.
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.
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
egg’s jelly coat,
exocytosis from the
You will be able to see the fertilization envelope in lab.
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.
10 sec after
1 sec before
of calcium ions
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.
Binding of sperm to egg
Acrosomal reaction: plasma membrane
depolarization (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
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
Four-cell stage. Remnants of the
mitotic spindle can be seen
between the two cells that have
just completed the second
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.
Fertilized egg. Shown here is the
zygote shortly before the first
cleavage division, surrounded
by the fertilization envelope.
The nucleus is visible in the
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.
The vegetal plate invaginates (buckles inward). Mesenchyme cells
migrate throughout the blastocoel.
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).
Contraction of these filopodia then drags the archenteron across
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)
Anus (from blastopore)
Sea urchin is a deuterostome so blastopore forms the anus. New opening for mouth. Mesoderm buds off from endoderm.
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).
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.
Because of large amount of yolk the animal pole cells smaller!
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.