Distribution of ions in nerve and muscle

With the advent of flame photometry and other microanalytical techniques there is no difficulty in determining the quantities of ions present in a small sample of tissue. In order to arrive at the true intracellular concentrations, it is necessary to make corrections for the contents of the extracellular space, which may be done after measuring its size with the aid of a substance like

Distribution of ions in nerve and muscle 29

Fig. 3.4. Electron micrograph of a freeze fracture preparation of a cell membrane. The proteins appear as globular indentations. Reproduced by courtesy of Professor J D. Robertson.

inulin to which the cell membrane is impermeable. Table 3.1 gives a simplified balance sheet of the ionic concentrations in frog muscle fibres and blood plasma determined in this way.

In the case of the squid giant axon it is possible to extrude the axoplasm just as toothpaste is squeezed from a tube, and so to obtain samples uncont-aminated by extracellular ions. Table 3.2 shows the resulting ionic balance sheet.

The main features of the distribution of ions which all excitable tissues have in common are that the intracellular potassium is 20 to 50 times higher in the cytoplasm than in the blood, and that for sodium and chloride the situation is reversed. The total amount of ions is, of course, about four times greater in a marine invertebrate like the squid whose blood is isotonic with sea water than it is in an amphibian like the frog which lives in fresh water, but the

Table 3.1. Ionic concentrations in frog muscle fibres and plasma

Concentration in

Concentration in

fibre water

plasma water

(mm)

(mm)

K+

124

2.3

Na+

3.6

108.8

Ca2+

4.9

2.1

Mg2+

14.0

1.3

Cl-

1.5

77.9

HCO-

12.4

26.6

Phosphocreatine

35.2

-

Organic anions

c. 45

These figures are calculated from values given by Conway (1957). At pH 7.0, phosphocreatine carries two negative charges; the remaining deficit in intracellular anions is made up by proteins.

Table 3.2. Ionic concentrations in squid axoplasm and blood

Concentration in Concentration in

Table 3.2. Ionic concentrations in squid axoplasm and blood

axoplasm

blood

(mm)

(mm)

K+

400

20

Na+

50

440

Ca2+

0.4

10

Mg2+

10

54

Cl-

123

560

Arginine phosphate

5

-

Isethionate

250

-

Other organic anions

c. 110

c. 30

These values are taken from Hodgkin (1958) and Keynes (1963).

The genesis of the resting potential 31

concentration ratios are not very different. The principal anion in the external medium is chloride, but inside the cells its place is taken by a variety of non-penetrating organic anions. The problem of achieving a balance between intracellular anions and cations is most severe in marine invertebrates, and is met by the presence either of large amounts of aspartate and glutamate or, in squid, of isethionate.

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