Neurophysiology of Nerve Impulses
O B J E C T I V E S 1. To define the following terms: irritability, conductivity, resting membrane potential, polarized, sodium-potassium pump, threshold stimulus, depolarization, action potential, repolarization, hyperpolarization, absolute refractory period, relative refractory period, nerve impulse, compound nerve action potential, and conduction velocity. 2. To list at least four different stimuli capable of generating an action potential. 3. To list at least two agents capable of inhibiting an action potential. 4. To describe the relationship between nerve size and conduction velocity. 5. To describe the relationship between nerve myelination and conduction velocity.
eurons have two major physiological properties: excitability, or the ability to respond to stimuli and convert them into nerve impulses, and conductivity, the ability to transmit an impulse (in this case, to take the neural impulse and pass it along the cell membrane). In the resting neuron (that is, a neuron that does not have any neural impulses), the exterior of the cell membrane is positively charged and the interior is negatively charged relative to the outside. This difference in electrical charge across the plasma membrane is referred to as the resting membrane potential, and the membrane is said to be polarized. The sodium-potassium pump in the membrane maintains the difference in electrical charge established by diffusion of ions. This active transport mechanism moves 3 sodium ions (Na ) out of the cell while moving in 2 potassium ions (K ). Therefore, the major cation (positively charged ion) outside the cell in the extracellular fluid is Na , and the major cation inside the cell is K . The inner surface of the cell membrane is more negative than the outer surface, mainly due to intracellular proteins, which, at body pH, tend to be negatively charged. The resting membrane potential can be measured with a voltmeter by putting a recording electrode just inside the cell membrane with a reference, or ground, Voltmeter
Plasma membrane Microelectrode inside cell
Microelectrode outside cell
F I G U R E 3 . 1 Resting membrane potential is measured with a voltmeter.
[K+] + + + +
–– – – –
+++++++++++++++++++++++++++++++++ ––––––––––––––––––––––––––––––––– [Na ] +
Since in the resting cell Na+ ions tend to diffuse into the cell and K+ ions tend to diffuse out of the cell, the resting potential is maintained by the active sodium-potassium pump.
Diff usio n
––––––––––––––––––––––––––––––––– +++++++++++++++++++++++++++++++++ (a)
u s i on
Na + D
––– ++++++++++++++++++++++++++++++ +++ –––––––––––––––––––––––––––––
+ + + + (e)
–– – – –
K p Pum
+++ –––––––––––––––––––––––––––––– ––– ++++++++++++++++++++++++++++++ Na+ Stimulus (b)
+ + + +
–– – – –
+++++–––––––––––––––––––––––––––– –––––++++++++++++++++++++++++++++ (c)
+++++–––––––––––––––––––––––––––– –––––++++++++++++++++++++++++++++ + + + K+ + –––––++++++++++++++++++++++++++++ + +++++–––––––––––––––––––––––––––– (d)
F I G U R E 3 . 2 The nerve impulse. (a) Resting membrane potential ( 70 mV). There is an excess of positive ions outside the cell, with Na the predominant ion in extracellular fluid and K the predominant intracellular ion. The plasma membrane has a low permeability to Na . (b) Depolarization—reversal of the resting potential. Application of a stimulus changes the membrane permeability, and Na ions are allowed to diffuse rapidly into the cell. (c) Generation of the action...