UNIT 11

Chapter 47: Animal DevelopmentChapter 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 LayersEctoderm skin epidermis, epithelium of mouth

and anus, nervous system, and tooth enamelEndoderm 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 DevelopmentStep 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

• 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

Membrane Potential

• Maintained by ions, proteins, amino acids, etc.

• 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

Changes in 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

• 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

Nerve Impulses Propagate Along an Axon

• Saltatory conduction• In myelinated neurons only unmyelinated

regions of the axon depolarize• Impulse moves faster than in unmyelinated neurons

• Electrical Synapses• Action potentials travel directly from the

presynaptic to the postsynaptic cells via gap junctions

Synapses

• 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 DefinitionsSensations – 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 & TransductionSensory 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. Sensory transduction2. Amplification3. Transmission4. 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 ReceptorsSensory 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 EyeThe 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 & LocomotionMovement and locomotion is

made possible by the nervous system, skeleton, and muscle.

Structure & Function of Vertebrate MuscleThe 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 FunctionsMuscle 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