Excitable tissue, Resting Membrane Potential & Action Potential

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First physio lesson

Define physiology

Study of the functioning of living organisms or their constituent tissues or cells

Many physiological reactions are aimed at preserving a constant physical and chemical internal environment. This is called what?

Homeostasis

At cellular level, communication is based on

Electrical & Chemical signalling

What is irritability?

An ability of all living tissues to respond to stimuli (either external or internal environment)

What is excitability?

An ability of specialized cells to respond to certain stimuli by producing electrical signals known as action potential at its membrane

Give examples of excitable tissues

Nerve cells, muscle (cardiac/skeletal)

What are 2 basic properties of excitable cell membranes?

1. The membranes have an electrical excitability across the membrane. (In response to depolarization of the membrane above a certain threshold voltage) and may transmit an impulse along the membrane
2. The membranes contain a variety of ion channels (pores) that may be opened or closed, allowing specific ions to flow across.

What is Resting Membrane Potential (RMP)?

Membrane potential difference (transmembrane voltage) that exists when cell membranes of excitable tissues are not producing an action potential (at rest)

Why does RMP occur?

Because of build-up of negative charges inside the cells and an equal build-up of positive charges outside the cells.

The greater the difference in charges across the membrane, the ___ the membrane potential (voltage)

Larger

What is the RMP of nerve cells?

-90mV (the potential inside the nerve fiber cells is 90 mV more negative than the potential of the outside)

What is the RMP of skeletal muscle fibers?

-80 to -90mV

What is the RMP of non-excitable tissue?

+20mV out, -20mV in

A cell that exhibits a membrane potential is said to be ___

polarized

2 main factors contribute to RMP. They are?

1. Distribution of ions across the membrane
2. Permeability of the membrane to Na⁺ and K⁺

What is the distribution of ions in extra and intracellular fluid?

Extracellular: rich in Na⁺ and Cl⁻ ions
Intracellular: mainly K⁺ & organic phosphates + proteins

Explain about the permeability of the membrane to Na⁺ and K⁺

Plasma membrane permeability to K⁺ is 50-100 times greater than its permeability to Na⁺

The ion concentrations do not normally change very quickly (with the exception of calcium). However, the membrane permeabilities can change in a fraction of a milisecond, as a result of?

Activation of ligand-gated or voltage-gated ion channels

The change in membrane potential can be large or small, depending on how many ion channels are activated and what type they are. This type of changes of the membrane potential are referred to as ____.

graded potentials

Explain a bit on graded potentials

- affect locally
- Non-propagated potential
- In contrast to action potentials, AP have a constant amplitude and time course, and propagated along the neighbouring cell membranes.

What are action potentials?

Rapid changes of membrane electrical potential on the surface of a cell during excitation and then leads to the transmission of an electrical impulse that travels across the cell membrane.

The action potential comprises of

depolarization phase and repolarization phase

2 types of voltage-gated ion channels are involved in these 2 phases. What are they?

1. Na⁺ channels
2. K⁺ channels

Voltage-gated Na⁺ channels have 2 separate gates. What are they?

1. Activation gates: close in resting membrane, open at threshold
2. Inactivation gates: open in resting membrane, also open at threshold

Following stimulation of excitable cells, a graded potential causes membrane to depolarize to a critical level that is called

threshold

Explain depolarization phase

Rapid opening of Na+voltage-gated channels leads to Na+ inflow. this leads to loss of membrane polarization. (MP = +30mV). An action potential rises to a constant and maximum strength each time

Explain threshold depolarization

Also stimulate slower opening of voltage gated K+ channels. That its opening coincides with voltage-gated Na+ channels closing.

What is the effect of K⁺ outflow?

K+ outflow causes the resting membrane potential to be restored = repolarization phase (MP = -70mV)

While the voltage-gated K⁺ channels are open, a large enough outflow of K⁺ may lead to ????

After-hyperpolarization

What is hyperpolarization?

Polarization more negative than the resting level (about -90mV)

As the voltage-gated K⁺ channels close, what happens?

RMP returns to the resting level (-70mV)

What is the refractory period?

The period when excitable cells cannot generate another action potential

There are two types of refractory periods. What are they?

1. Absolute refractory period
2. Relative refractory period

What is the absolute refractory period?

Refers to the time period during which a 2nd action potential cannot be initiated, even with a very strong stimulus. It coincides with Na⁺ channels activation and inactivation. Inactivated Na⁺ channels cannot reopen, they must first return to resting state.

What is relative refractory period?

Refers to the time period during which a 2nd action potential can be initiated, but only with a larger than threshold stimulus (supra threshold). It coincides with the period when the voltage gated K⁺ channels are still open after inactivated Na⁺ channels have returned to their resting state

Explain propagation (conduction) of action potentials

As Na⁺ flows in, depolarization increases to threshold depolarization. This open voltage-gated Na⁺ channels in adjacent patches of cell membrane

What is the All-or-None principle?

A principle of action potential generation. The depolarization process travels over the entire membrane but if it does not generate sufficient voltage to stimulate the next area of the membrane, the spread of depolarization stops.

Compare between different excitable tissues in terms of resting membrane potential

The initiation and conduction of nerve and muscle action potentials are similar, but they have different RMP's.
Neuron = -70mV
Skeletal and cardiac muscle fibers = -90mV

Compare between different excitable tissues in terms of velocity of conduction

Nerve action potentials are 18x faster than muscle

Compare between different excitable tissues in terms of duration of AP

Neuron = 0.5 - 2.0 msec
Skeletal muscle fibers = 1.0 - 5.0 msec
Cardiac & smooth muscle fibers = 10 - 300 msec

What helps spread the action potential deep with the skeletal muscle fiber?

Transverse tubules. The T tubule action potentials cause release of calcium ions inside the muscle fiber

Cardiac muscle tissues contract without neural stimulation. This property is called

automaticity

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