. In haterolropic control (fig 10A.39) the mechanism 4 as follows:

mechanism ensures, there cannot be excessive secretion of NA. In haterolropic control (fig 10A.39) the mechanism 4 as follows: post ganghonic sympathetic and parasympathetic nerve terminals lie side by side. These two terminals communicate wilh each other by a tunnel. When NA is secreted, part of the NA escapes via this tunnel to enter the cholnergic terminal where it (= the NA) combines with MAgic receptors situated within the cholmergic terminal, this combination inhibits the choiinergic nerve In short, NAgic activity in the penphefy, automatically leads to suppression of cholinergic activity. Note, both autofeed back and heterotropic controls are examples of presynaptic inhibition. SYNAPTIC INHIBITION. The transmission at synaptic level can be, and often is, inhibited by naturally existing mechanisms in our body. Several such mechanisms em si. the notable amongst them being, (i) direct inhibition-(reciprocal inhibition of Sher-ington). (ii) Renshaw cell inhibition, and (iii) presynaptic inhibition. Presynaptic inhibition Consider the fig. IDA 3.5. Axon 'e'of a neuron, which is excitatory is impinging on the soma. 's' of a second neuron Thus, there is a synapse, shown by a red circle between e ands. When is excited s is also excited. Another axon i, is impinging on e and thus there is an­other synapse, shown by a red circle between the Termination of e and i Mark ,this synapse is avoauonic (= between two axons). The presynapthic membrane i has inhibitory effect on the e.