Fig. 7.2 The structures of two important neurotransmitters, acetylcholine (neuro-transmitter in the parasympathetic nervous system, parts of the sympathetic nervous system and in the somatic nervous system responsible for activating muscle contraction), and noradrenaline (neurotransmitter in the peripheral parts of the sympathetic nervous system). The route of synthesis of noradrenaline was given in Fig. 5.10.
An axon is a prolongation of a cell (see Fig. 7.1). As in all cells, the cytoplasmic K+ concentration (about 150 mmol/l) is considerably greater than that outside (in the interstitial fluid and plasma - about 5 mmol/l). The nerve cell membrane is selectively permeable to K+ ions, which therefore diffuse out (through specific K+ channels) down their concentration gradient. They take with them positive charge - leaving the interior of the cell with a negative charge relative to the outside. This is known as the resting membrane potential. It can be measured with a voltmeter in a large nerve, and is about - 70 mV. (The negative sign is conventional, implying that the inside is negatively charged with respect to the outside.) Na+ ions have the opposite distribution: they are present at higher concentration outside (about 150 mmol/l) than inside (about 15 mmol/ l). However, the membrane is less permeable to Na+ ions than to K+ ions, so the potential difference is maintained. In addition, nerve cell membranes contain the Na+-K+-ATPase which pumps out 3 Na+ ions in exchange for 2 K+ ions from outside (and uses ATP for this). This further maintains the resting energy potential (since there is a net outward movement of positive charge). This is illustrated in the top panel of Fig. 7.1.1.
The above is true for most cells. However, nerve cells and skeletal muscle cells have the characteristic of excitable membranes (Fig. 7.1.1, lower panel). They possess proteins in the membrane which are voltage-gated sodium channels: they have pores which can be opened to allow Na+ ions to pass through,
Extracellular Quiescent cell
Resting membrane f\j potential ^ -70 mV
Intracellular K* channel [K4] 150 mmol/l open [Na*] 15 mmol/l closed
Passage of an action potential
Na* channel open Depolarization depolarization
K* channel opens; membrane potential restored
but these pores are normally closed by the negative membrane potential. An action potential is started by depolarisation of the membrane (the negative membrane potential is lost) in a specific area. This allows the opening of voltage-gated sodium channels in adjacent parts of the membrane, so that Na+ ions can flow in (down their concentration gradient), thus depolarising yet more of the membrane. Thus, this depolarisation spreads like a wave (the nerve impulse) along the length of the axon. It passes any one point very rapidly: as sodium ions flow in and the local membrane potential falls to zero (or becomes positive) the entry of further sodium ions is restricted. In addition, voltage-gated K+ channels open a short time after the voltage-gated Na+ channels, allowing potassium ions to leak out again. The normal resting membrane potential is therefore re-established after about 2 msec.
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