Decrease of synaptic effectiveness
Presynaptic inhibition (PSI) refers to a decrease of transmitter release at central synapses. Five decades ago, it was reported that activation of afferent fibers originating in flexors led to depression of monosynaptic group Ia excitatory postsynaptic potentials (EPSPs) evoked on extensor motoneurones in the cat spinal cord . This depression occurred with no detectable changes in the time course of monosynaptic EPSPs, membrane potential or motoneurone excitability [1,2]. It is now known that PSI occurs broadly within the central nervous system of both vertebrates and invertebrates, and that synaptic efficacy at axon terminals from sensory afferents, descending systems or interneurones  can be subject to an inhibitory control by a number of neurotransmitters and presynaptic receptors .