UNIT 11
Chapter 47: Animal Development
Chapter 48: Nervous Systems
Chapter 49: Sensory & Motor Mechanisms
Fertilization

Fertilization is the union of a sperm and egg
nucleus

acrosomal reaction: sperm releases hydrolytic
enzymes (from the acrosome) to break down
the outer layer of the egg
 Once sufficiently weakened, the haploid
nucleus of the sperm can enter the egg
 Na+ channels in the egg’s membrane open,
Na+ flows into the cell

change in charge inside the egg prevent any more
sperm from entering = fast block to polyspermy

A second process, the cortical reaction, will
take place to guarantee no polyspermy

Ca2+ channels open, allowing it to flow into the
egg

cortical granules will fuse with the inside of the
egg’s membrane and form an impenetrable
fertilization envelope = slow block to polyspermy
The Zygote

Once nuclei have fused, the zygote will undergo
cleavage




Approx. 90 minutes for first division
Increases number of cells, but NOT total volume
of cells
Cleavage results in a solid ball of cells called a
morula
Cells in this ball will continue to divide and
rearrange to form a hollow space (called the
blastocoel)

Hollow ball called a blastula

Gastrulation further rearranges the embryo to
create a blastopore and a triploblastic embryo
The Germ Layers
Ectoderm  skin epidermis, epithelium of mouth
and anus, nervous system, and tooth enamel
Endoderm  digestive tract lining, respiratory
system lining, pancreas, portions of the urinary
system, and portions of the reproductive
system
Mesoderm  notochord, skeletal system, muscle,
circulatory/lymphatic system, reproductive
system, and lining of the coelom
Amniote Embryos


Amniotic organisms develop within a fluid filled
sac in either a shell or uterus
Four extraembryonic membranes are associated
with amniotes

Yolk sac, amnion, chorion, allantois
Mammalian Development
Step 1 
Blastocyst reaches endometrium and the inner
cell mass is surrounded by the trophoblast
Step 2 
Trophoblast
secretes enzymes
that make it
possible for the
blastocyst to
implant in the
endometrium
Step 3 
Extraembryonic membranes develop
Step 4 
Gastrulation occurs
Organogenesis begins with the formation of the
notochord
END
Membrane Potential
• A membrane potential is a localized electrical
gradient across a membrane
• Negative ions (anions) are more concentrated in
a cell; positive ions (cations) are more
concentrated outside
• Maintained by ions,
proteins, amino
acids, etc.
Changes in Membrane Potential
• Excitable cells have the ability to generate
changes in their membrane potentials
• Ion channels open or close in response to stimuli
• Diffusion of ions leads to a change in the membrane
potential
• 2 types ions channels
• Chemically-gated ion channels: open or close in
response to a chemical stimulus
• Voltage-gated ion channels: open or close in
response to a change in membrane potential
• Hyperpolarization
• K+ channels open  K+
diffuses out of the cell
 the membrane
potential becomes
more negative
+
+
-
-
+ + +
+
• Depolarization
• Na+ channels open 
Na+ diffuses into the
cell  the membrane
potential becomes less
negative
+
+
-
-
+ + +
+
• The Action Potential
• If graded potentials
sum to -55mV,
threshold potential is
achieved
• Step 1: Resting State
• Step 2: Threshold
• Step 3: Depolarization
• Step 4: Repolarizing
• Step 5: Undershoot
Nerve Impulses Propagate Along an Axon
• The action potential is repeatedly regenerated
along the length of the axon
• “An action potential achieved at one region of the
membrane is sufficient to depolarize a
neighboring region above threshold”
• Triggers a new action potential
• Refractory period assures that impulse conduction is
unidirectional
• Saltatory conduction
• In myelinated neurons only unmyelinated
regions of the axon depolarize
• Impulse moves faster than in unmyelinated neurons
Synapses
• Electrical Synapses
• Action potentials travel directly from the
presynaptic to the postsynaptic cells via gap
junctions
• Chemical Synapses
• More common than electrical synapses
• Postsynaptic chemically-gated channels for ions
such as Na+, K+, and Cl• Can depolarize or hyperpolarize
• Excitatory postsynaptic potentials (EPSP)
• Cause depolarization
• EASIER for an action potential to occur
• Inhibitory postsynaptic potential (IPSP)
• Cause hyperpolarization
• MORE DIFFICULT for an action potential to occur
END
A Couple Definitions
Sensations – action potentials that reach the
brain via sensory neurons …
… Sensations are universal.
Perceptions – the awareness and interpretation
of the sensation …
… Perceptions are personal.
Sensory Receptors & Transduction
Sensory reception begins with the detection of
stimulus by sensory receptors.


Exteroreceptors – outside the body
Interoreceptors – inside the body
There are four steps involved with sensory
reception:
1.
2.
3.
4.
Sensory transduction
Amplification
Transmission
Integration
Sensory Transduction:
The conversion of the stimulus into a
change in membrane potential.
Amplification:
The strengthening of the stimulus so that it
can be detected by the nervous system.
Transmission:
The conduction of sensory impulses (action
potentials) to the CNS.
Integration:
The processing of sensory information by
the CNS.
Types of Sensory Receptors
Sensory receptors
are categorized
by the type of
stimulus they
detect.
Mechanoreceptors:
Detect mechanical (“physical”) energy
Nocioreceptors:
Detect pain (pain receptors)
Thermoreceptors:
Detect relative heat or coolness
Chemoreceptors:
Detect chemicals
Electromagnetic receptors:
Detect EM radiation
Function of the Vertebrate Eye
The structure and function of most vertebrate
eyes are strikingly similar.
Light enters the eye and is focused on the
retina, which is a collection of
photosensitive cells.
Rods: sensitive to light, but not color (~12
million)
Cones: not as sensitive to light, but can
detect colors (~6 million)
The stimulation of photosensitive rod cells is
made possible by a light-absorbing pigment:
rhodopsin (ALWAYS present in rod cells).
In the dark, rhodopsin is inactive, Na+ channels
are open, and a neurotransmitter is
released inhibiting the postsynaptic neuron.
With light, rhodopsin is activated, Na+ channels
are closed, and the inhibitory
neurotransmitter is not released allowing
the postsynaptic neuron to generate an
action potential.
Cone mechanisms are much more complex
with many types of pigments (photopsins).
Movement & Locomotion
Movement and locomotion is
made possible by the
nervous system,
skeleton, and muscle.
Structure & Function of Vertebrate Muscle
The sarcomere is the
functional unit of muscle
contraction. Thin
filaments consist of two
strands of actin and one
tropomyosin coiled
about each other. Thick
filaments consist of
myosin molecules.
The way in which muscle contracts is explained
by the sliding-filament model.
At rest, tropomyosin is blocking the myosin
binding site on the actin molecule.
However, if calcium (Ca2+) binds to
tropomyosin, a conformational shift will
occur which will expose the myosin binding
site.
Normally calcium is
not available for
binding to
tropomyosin,
BUT …
… in response to an
action potential,
a muscle cell
will release its
stored calcium
ions from the
sarcoplasmic
reticulum (SR).
Muscle Functions
Muscle cells are either in a state of contraction
or relaxation.
To achieve varying degrees of “strength” in
contractions, there are a couple ways to
make this happen.
One way is by varying
the frequency of
action potentials.
Another way is by
a process
called
recruitment.
This basically
activates more
of the muscle
fibers by
generating
action
potentials from
more motor
neurons in a
motor unit.
Fatigue of the muscle is avoided by rotating
which motor units are actively generating
action potentials.
Some muscles, such as those involved in
balance and posture are always partially
contracted.
Other Muscle Types
END