Chemical transmission

We must now consider the question, how does excitation in the presynaptic cell produce a response in the postsynaptic cell? One possibility, first suggested in the nineteenth century, is that the presynaptic cell might release a chemical substance which would then act as a messenger between the two cells.

Fig. 7.2. Electron micrograph showing the structure of the frog neuro-muscular junction. The axon terminal (A) runs diagonally across the middle of the section, covered by a Schwann cell (S) and collagen fibres (Co), and overlying a muscle cell (Mu). Between the axon and the muscle cell is the synaptic cleft (C). The acetylcholine receptors are concentrated at the top of a series of folds (F) in the subsynaptic membrane. The terminal contains mitochondria (Mi) and large numbers of synaptic vesicles (V). Vesicle release probably occurs at presynaptic active zones (Z). Magnification 27000 x. Photograph supplied by Professor J. E. Heuser.

Fig. 7.2. Electron micrograph showing the structure of the frog neuro-muscular junction. The axon terminal (A) runs diagonally across the middle of the section, covered by a Schwann cell (S) and collagen fibres (Co), and overlying a muscle cell (Mu). Between the axon and the muscle cell is the synaptic cleft (C). The acetylcholine receptors are concentrated at the top of a series of folds (F) in the subsynaptic membrane. The terminal contains mitochondria (Mi) and large numbers of synaptic vesicles (V). Vesicle release probably occurs at presynaptic active zones (Z). Magnification 27000 x. Photograph supplied by Professor J. E. Heuser.

An experiment to test this idea for the frog heart was carried out by O. Loewi in 1921. The heart normally beats spontaneously, but it can be inhibited by stimulation of the vagus nerve. Loewi found that the perfusion fluid from a heart which was inhibited by stimulation of the vagus would itself reduce the amplitude of the normal beat in the absence of vagal stimulation. Perfusion fluid from a heart beating normally did not have this effect. This means that stimulation of the vagus results in the release of a chemical substance, presumably from the nerve endings.

It did not take too long to show that the chemical substance was acetyl-choline (Fig. 8.1), a substance whose pharmacological action had previously been demonstrated by H. H. Dale. Dale and his colleagues then went on to show that acetylcholine was released from the motor nerve endings in skeletal muscle when the motor nerves were stimulated electrically.

Muscle fibre

Fig. 7.3. Diagram to show how the end-plate potential is recorded from a frog muscle fibre using an intracellular microelectrode.

Muscle fibre

Fig. 7.3. Diagram to show how the end-plate potential is recorded from a frog muscle fibre using an intracellular microelectrode.

It seems then that synaptic transmission is in most cases a chemically mediated process involving the release of a transmitter substance from the presynap-tic terminals. As we shall see later, acetylcholine is an important transmitter substance, but it is not the only one.

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