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Active Transport of Sodium and PotassiumIons Through the MembraneThe Sodium-

Potassium (Na+-K+) Leakage of Potassium and Sodium Through

the Nerve Membrane

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Contribution of the Potassium Diffusion Potential: Because of the high ratio of potassium ions insideto outside, 35:1, the Nernst potential corresponding

to this ratio is

94 millivolts

Therefore, ifpotassium ionswere the only factor causing theresting potential

Contribution of Sodium Diffusion Through theNerve Membrane : diffusion of sodium ions

through the K+-Na+ leak channels. The ratio ofsodium ions from inside to outside the membrane is0.1

Contribution of the Na+-K+ Pump

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A Positive-Feedback Vicious Cycle Opens the SodiumChannels if any event causes enough initial rise in the membrane potential from90 millivolts toward the zero level, the rising voltage itself causes

many voltage-gated sodium channels to begin opening. This allowsrapid inflow of sodium ions, which causes a further rise in themembrane potential

Threshold for Initiation of the Action Potential A sudden rise in membrane potential of 15 to 30 millivolts usually is

required. Therefore, a sudden increase in the membrane potential in alarge nerve fiber from 90 millivolts up to about 65 millivolts usuallycauses the explosive development of an action potential. This level of65 millivolts is said to be the threshold for stimulation

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TRANSMISI SINAPTIK

ELEKTRIK

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Ner e fiber t pes in mammalian ner e

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Nerve fiber types in mammalian nerve.

Fiber Type Function

Fiber

Diameter

(m)

Conduction

Velocity

(m/s)

Spike

Duration

(ms)

Absolute

Refractory

Period (ms)

A

Proprioception; somatic

motor

12-20 70-120

Touch, pressure 5-12 30-70 0.4-0.5 0.4-1

Motor to muscle

spindles

3-6 15-30

Pain, cold, touch 2-5 12-30

B Preganglionic

autonomic

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Numerical classification sometimes used for sensory

neurons.

Number Origin Fiber Type

Ia Muscle spindle, annulospinal ending. A

Ib Golgi tendon organ. A

II Muscle spindle, flower-spray ending; touch, pressure. A

III Pain and cold receptors; some touch receptors. A

IV Pain, temperature, and other receptors. Dorsal root C