Unitary Excitatory Postsynaptic Potentials in Clarke's Column Neurones

IT is generally believed that synaptic transmission is effected by a quantal release of transmitter substances, such as has been demonstrated for the neuromuscular junction1. The quantal nature of synaptic transmission in the central nervous system is more difficult to investigate, one reason being that many presynaptic fibres usually contribute to the EPSP (excitatory postsynaptic potential), whereas at the neuromuscular junction the entire effect is evoked from a single presynaptic fibre. Thus at central synapses the total synaptic action should be described both in terms of the number of unitary EPSPs (the EPSP contributed by a single presynaptic fibre) and the quantal composition of each unitary EPSP. In motoneurones, impulses in Ia afferents (large muscle spindle afferents) usually evoke small unitary EPSPs with a low quantal content2, but larger unitary Ia EPSPs with quantal content “possibly as high as ten or fifteen” have also been found3.