Action Potential and Germ Layers

action potential and diagram

Action potential=nerve impulse

• All-or-none event
• Size of action potential not affected by strength of triggering stimulus

Action Potential in a Neuron

1. Environmental changes alter cell’s membrane potential
2. Neuron hyperpolarized by stimuli that open potassium channels, or
3. Neuron hyperpolarized by stimuli that open sodium channels
4. Depolarizing stimulus of sufficient strength will change the membrane potential to critical level (threshold potential)
5. Triggers action potential

The role of voltage-gated ion channels in the action potential

1. Resting state
1. Both sodium and potassium channels are closed
2. Resting potential of membrane is maintained
2. Threshold
1. Stimulus opens some sodium channels
2. Sodium influx achieves threshold potential
3. Additional sodium gates open
4. Triggering action potential
3. Depolarization phase of the action potential
1. Activation gates of sodium channels are open
2. Potassium channels remain closed
3. Sodium ions rush into cell
4. Interior of cell becomes more positive
4. Repolarization phase of the action potential
1. Inactivation gates close sodium channels
2. Potassium channels open
3. Potassium ions leave the cell
4. Loss of positive charge causes inside of cell to become negative relative to outside
5. Undershoot
1. Both gates of sodium channels are closed
2. Potassium channels remain open

i. Relatively slow gates have not had time to respond to the repolarization of the membrane

3. Resting state restored within another 1-2 milliseconds
4. Ready to respond to another stimulus

(Diagram p. 1030)

Chemically-gated ion channels open or close in response to a chemical stimulus

• Neurotransmitter released from synaptic terminal

Voltage-gated ion channels respond to a change in membrane potential

An action potential is an example of positive feedback

Potassium channel gates are the main cause of undershoot, or hyperpolarization

The refractory period sets the limit on the maximum frequency with which action potentials caan be generated

Strong stimuli result in a greater frequency of action potentials than weaker stimuli

The number of action potentials per second, not their amplitude, code for stimulus intensity in the nervous system

germ layers

Embryo becomes layered through the process of gastrulation

• Rearrangement of the embryo
• One end of the embryo folds inward, expands, fills the blastocoel
• Producing layers of embryonic tissues
• Ectoderm: outer layer
• Endoderm: inner layer

Germ layers=concentric layers

• Form various tissues and organs of the body

1. Ectoderm
1. Covers surface of embryo
2. Gives rise to outer coverings of the embryo
3. Origin of nervous system
2. Endoderm
1. Innermost germ layer
2. Lines developing digestive tube (archenteron)
3. Gives rise to lining of digestive tract and organs derived from it

i. Liver

ii. Lungs

3. Mesoderm
1. Third germ layer
2. Between endoderm and ectoderm
3. Forms muscles
4. Forms most organs between digestive tube and outer covering of the animal

Quiz:

1. The outer-to-inner sequence of tissue layers in a post-gastrulation vertebrate embryo is

a. ectoderm->endoderm->mesoderm

b. endoderm->ectoderm->mesoderm

c. endoderm->mesoderm->ectoderm

d. mesoderm->endoderm->ectoderm

e. ectoderm->mesoderm->endoderm

2. The "threshold" potential of a membrane

a. is the minimum hyperpolarization needed to prevent the occurrence of action potentials

b. is the peak amount of depolarization seen in an action potential

c. is the lowest frequency of action potentials a neuron can produce

d. is the point of separation from a living to a dead neuron

e. is the minimum depolarization needed to operate the voltage-gated sodium and potassium channels

Answers: 1e, 2e