nerves and the resting potential

7/27/2019 Nerves and the Resting Potential

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Nerves and the Resting Potential

1. The Neuron

The basic cell unit of the nervous system in the neuron. Neurons come in a variety ofshapes and sizes there are over a billion inthe human body.

Supporting cells are about 5 times moreplentiful than the neurons themselves.

Each neuron is composed of a cell body, or

perikaryon which contains the nucleus,mitochondria and the endoplasmic reticulum.Cell bodies are frequently clustered together.In the brain and spinal chord (CNS) theclusters are called nuclei, in the peripheral areathey are called ganglia.

The dendrites or dendritic branches areextensions that carry an impulse toward thecell body. Usually dendrites are highlybranched to provide connections to up to 100

other neurons.

Axons are longer processes that carry impulsesfrom the cell body. Axons vary in length from1 mm to several meters. Side branches are callcolaterals.

Proteins are transported rapidly down an axonfrom the cell body. Axoplasmic flow (slower)results from rhythmic waves of contraction

that move the cytoplasm. Axonal transport uses microtubules to move selected

substances.

The neurons themselves may be afferent or sensory and conduct impulses to the CNS;motor conducting impulses from the CNS or associative interneurons between the two.

Supporting cells include :Schwann cells (neurolemmocytes) of the peripheral nervous system. These cells formmyelin sheaths as they grow around the peripheral axons


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Satellite cells or ganglionic gliocytes support ganglia of the PNSOligodendrocytes from sheaths around axons in the CNSMicroglia phagocytotic cells of the CNSAstrocytes ion regulators of the CNSAnd Ependymal cells line the ventricles of the brain and spinal chord.

In the PNS axons are surrounded by sheaths of Schwann with The outer surface of thelayer is encased in a glycoprotein basement matrix. ( there is no continuous basementmembrane in the CNS). Nerves in both CNS and PNS are insulated with a myelin sheathformed by successive wrappings of the neurolemmocytes or oligodendrocytes.

The supporting cells themselves remain alive. The oligodendrocytes surround more thanone axon with extensions rather like an octopus to form the white matter of the CNS.

If an axon in the PNS is cut. The severed part degenerates and the myelin sheath forms agrowth tube, secreting factors that allow the original cell to regrow. Injury in the PNSstimulates the growth of the colateral branches but these cells do not regenerate aswell . Further cells of the neighboring area also degenerate. But there are several nerveand myelin growth factors that are present in the fetal brain and can promote CNSregrowth. These factors are being applied to spinal chord injury in an attempt to healdamage.

The Resting PotentialThe Nernst Equation Revisited.

In the section on transport and the movement of ions, we examined the way in which animpermeant ion could cause an uneven distribution of permeant ions across a membranein Donnan equilibrium. We also noted that the charge as well as the concentrationgradients affect this distribution. The Nernst equation allows us to examine theconcentration and charge forces acting on a particular ion to see if that ion is in passiveequilibrium or is actively moving.

The equation states that the equilibrium potential for an ion (E) is equal to the gasconstant (R) times the absolute temperature (T) divided by the charge on the ion (z) timesFaradays constant (F) ; this quantity is multiplied by the natural log (ln) of theconcentration ratio inside and outside the membrane.

E = RT/zF ln [C1]/[C2]

Faradays constant is 96,500 coulombs/ mole. Charge is measured in coulombs. Acoulomb is a charge equivalent to 1/96,500 grams of electrons. The charge on oneelectron is 1.6 x 10-19coulmbs. If this is multiplied by Avagadro;s number, the totalcharge is one faraday or 96,487 coulombs/mole.


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The gas constant is 8.314 joules/ degree Kelvin/ mole.

So E = 8.314 joules/ degree/ mole x degrees x ln [C1]/[C2]96,500 coulombs/mole x charge

E = joules/ coulomb and a joule/ coulomb is equal to a volt.( or a potential difference x a charge = energy)

If we assume a temperature of 18 Centigrade and a monovalent ion and convert thenatural log to log base 10 then

E = .058/z log [C1]/[C2]

In squid nerve the intracellular potassium concentration is 410 mM and the externalconcentration is 22 mM, the measured membrane voltage is 70 mV. Is potassium inequilibrium ?

( by convention the membrane voltage (Vm) is inside with respect to outside and allanimal cells have a membrane potential of 20 to 120 mV.

Note that the equilibrium potential for potassium is negative. This means that the interiorof the cell would have to be 70 to 75 mV negative in order to prevent the outwardmovement of potassium given these concentration gradients.

The Goldman Equation

More than one ion contributes to the overall potential of a membrane and because themembrane is selectively permeable to different ions, when the membrane potential is

calculated, the permeability of the ions must be taken into account. The Goldmanequation expands the Nernst equation with the addition of several ions and their relativepermeabilities.Vm = RT/F ln PK [Kout] + PNa[Naout] + PCl[Clin]

PK[Kin] + PNa[Nain] +PCl[Clout]

Resting Potential

The resting potential of a neuron is approximately 70 mV. Ion concentrations affect the

resting potential, but the nature of the biological membrane itself is also important.

Membrane Capacitance

The bilipid membrane of the nerve is not very permeable to the movement of most ions.The rate at which ions traverse membranes is as little as 10 -8 times the rate at which theycross an equivalent distance of 50-100 A in the cytoplasm. In fact we can think of themembrane as a narrow strip of insulation between two plates where charge accumulates.


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We can say then that the membrane has a capacitance. Capacitance is measured in faradsand a farad is equal to a coulomb per volt. The capacitance of a nerve membrane is aboutone microfarad.

Membrane Conductance

Conductance is the release of the charge difference on either side of a membrane, throughpores, channels or carriers, ions are allowed to move across a membrane and conduct thecurrent from one side to another. Conductance is the reciprocal of resistance G = 1/R andthe units of conductance are siemens. Resistance (R) = E/I so he voltage across themembrane is proportional to the current passed through the membrane and inverselyproportional to the conductance. E = I/G. For each ion the conductance of that ion will beequal to the current carried by that ion divided by the potential difference or emf actingon that ion.

Permeability

If the axon potential becomes more negative than the resting potential, the cell ishyperpolarized, if the potential becomes more positive the cell is depolarized. Ionchannels contribute to changes in permeability of individual ions. The ion channels forNa and K are fairly specific . There are two basic types of K channels; one is not gatedand always open, but the other is gated and closed in the resting cell. Sodium channelsare gated and are closed in the resting cell. So the cell is more "leaky " to potassium thansodium and the membrane potential is just slightly less than the K equilibrium potential .

Over the long term, Na is removed from the cell and K is added by Na/K ATPase and theion gradients are maintained.

nerves and the resting potential

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