Questions and Answers
What is the effect of inactivation gates on sodium channels?
During the resting membrane potential, what is the state of voltage-gated K+ channels?
What triggers the explosive depolarization of the membrane potential?
What is the primary ion responsible for depolarization?
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During repolarization, what is the primary ion responsible for efflux?
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What is the effect of the simultaneous decrease in sodium influx and increase in potassium efflux?
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What is the purpose of the inactivation gate in sodium channels?
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During the resting membrane potential, what is the membrane potential?
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What is the sequence of events in an action potential?
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What is the ionic basis of hyperpolarization?
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Study Notes
Excitable Tissue
- Nerves and muscles are called excitable tissue because they respond to chemical, mechanical, and electrical stimuli.
- Muscles demonstrate by contraction, while nerves by integration and transmission.
Electrical Signals in Neurons
- Production of signals depend on two basic features of the plasma membrane of excitable cells: resting membrane potential and ion channels.
Ion Channels
- Ion channels open and close due to the presence of gates.
- There are four kinds of ion channels:
- Leakage channels: open and close randomly
- Voltage-gated channels: open in response to a change in membrane potential (voltage)
- Ligand-gated channels: open and close in response to chemical stimulus, such as Ach
- Mechanical gated channels: open or close in response to mechanical stimulus
Resting Membrane Potential (RMP)
- Definition: The difference in voltage across the cell membrane when a neuron or muscle cell is not producing an action potential.
- A typical value is: -70 mV (-50 to -90)
- A cell that exhibits a membrane potential is said to be polarized.
- RMP is negative inside the cell relative to the outside because:
- The resting membrane is 10-100 times more permeable to K+ than to Na+
- Potassium inside the cell is 140meq/L and outside is 4meq/L
- K+ tends to leak out of the cell down its concentration gradient, carrying +ve charge with it
Forces Acting on Cell Membrane at Rest
- Diffusion: movement of molecules from a region of higher concentration to a region of lower concentration
- Electrical gradient: +ve ions move to the –ve area and –ve ions move to +ve area
- Active transport: transport ions against their concentration gradient (e.g., Na+-K+ pump)
The Action Potential (AP)
- Definition: A sudden reversal of membrane polarity by a stimulus
- Importance: AP occurs in living organisms to produce physiological effects such as transmission of impulses along nerve fibers and release of neurosecretions or chemical transmitters in synapses
- Action potential takes place as a result of the triggered opening and subsequent closing of 2 specific types of channels: voltage-gated Na+ channels and voltage-gated K+ channels
Voltage-Gated Na+ Channels
- Most important channels during AP
- Have two gates: activation gate and inactivation gate
- Activation gate opens when membrane potential becomes less negative than during resting state
- Inactivation gate closes when membrane potential becomes less negative, leading to Na+ influx
Voltage-Gated K+ Channels
- Open during RMP, but are closed
- Same stimulus that opens voltage-gated Na+ channels also opens voltage-gated K+ channels
- Due to slow opening of these channels, they open just as Na+ channels are beginning to close
- Simultaneous decrease in Na+ influx and increase in K+ outflux cause repolarization
Phases of Action Potential
- Depolarization
- Repolarization
- Hyperpolarization
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Description
This quiz covers the basics of neurons and excitable tissue, including membrane potentials, electrical signals, and how nerves and muscles respond to stimuli.