S1B4 - Ionic Basis of Excitation Flashcards
What are the two main effects of hyperkalemia on cardiac myocytes?
Hyperkalemia has two main effects on cardiac myocytes:
- hyperkalemia increases myocyte excitability by making the membrane potential less negative and thus closer to threshold potential. This is because the increased blood potassium causes decreased potassium efflux (decreased concentration gradient) out of the cell during phase 4.
- as the levels of hyperkalemia continue to increase, the slope of phase 0 (the upstroke) decreases with this increasing hyperkalemia. This is because as the resting membrane potential becomes less negative, some sodium channels become inactivated, and thus less sodium channels are available when there is an action potential → decreased slope of phase 0.
What channels are responsible for the upstroke of neuronal action potential?
Upstroke of the AP: Local depolarization → activation of nearby fast-acting voltage gated Na channels → influx of Na → depolarization of the next section of membrane.
What is the effect of hypocalcemia on skeletal muscle cells?
Hypocalcemia → muscle spasms and tetany. (Hypocalcemia affects the voltage at which sodium channels are activated, causing increased excitability.)
What is responsible for the repolarization of a neuronal action potential?
Down stroke (repolarization) of the AP: Shortly after initial depolarization Na channel inactivation gates close and slow-acting voltage gated K channels open.
Which phase of the action potential does the absolute refractory period occupy? The relative refractory period? Can action potentials be initiated during these refractory periods?
The absolute refractory period refers to the period at which the cell is depolarized. During the absolute refractory period, another action potential cannot be initiated.
The relative refractory period refers to the period at which the cell is hyperpolarized. During the relative refractory period, a larger than normal stimulus is required to initiate another action potential.
What are normal concentrations (mmol/L) for extracellular and intracellular…
…Na+?
…K+?
…Ca2+?
The main idea is there is more NaCl and bicarbonate outside, more K inside.
What is the mechanism by which lidocaine functions as an anesthetic?
Certain anesthetics (e.g. lidocaine) function by inhibiting these Na+ channels and thereby blocking signal (pain) transmission.
What are the typical resting membrane potentials in nerve cells and muscle cells?
Typical values of resting membrane potential are -70 mV in nerve cells and -90 mV in muscle cells.
Which has faster action potentials, thick axons or thin axons?
Fiber diameter - conduction velocity is proportional to the square root of the diameter. Larger neurons have faster conduction velocities.
What’s the difference between type A nerve fibers and type B nerve fibers?
Motor neurons.
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Type A fibers are myelinated. The myelinated efferent Type A motor neurons are often divided into two groups.
- The type A-alpha motor neurons are large, myelinated fibers that innervate extrafusal skeletal muscle.
- The type A-gamma motor neurons are smaller myelinated fibers that innervate intrafusal muscle. The latter have slower conduction velocities and are involved in reflex tone adjustments.
- Type C fibers are small and unmyelinated. This designation usually refers to sensory afferent fibers rather than motor fibers. The type C designation is another way of referring to type III and type IV fibers (see 3 below).
What do you need to know about sensory nerve classification?
Sensory nerve classification.
- Somatosensory neurons are often referred to by roman numerals related to size and myelination: I, II, III, IV.
- Type I afferents are large myelinated nerves carrying information like tactile sensation to the brain.
- Types III and IV sensory afferents are unmyelinated and carry information about sensations like proprioception, pain and temperature.
