Figure 3.

Competitive pattern classification between spiny neurons in the neostriatum. (a) Dendrites (above) and projection axons (below) of two spiny neurons. Two synaptic inputs from the cerebral cortex are illustrated. In the middle are two inhibitory collaterals. Note that one collateral inhibits a dendrite to mediate postsynaptic inhibition, whereas the other inhibits a synaptic terminal to mediate presynaptic inhibition. (b) Schematic illustration of why competition mediated by presynaptic inhibition is more effective than competition mediated by postsynaptic inhibition. The two time plots show net excitatory synaptic input (g_s) from cortex and membrane potential (V_m) of a spiny neuron as the cortical input slowly increases (between the two vertical dashed lines). In the absence of synaptic input, V_m is near the potassium equilibrium potential E_K. As synaptic input g_s increases, V_m moves in the positive direction in a sigmoidal fashion (typical of a down- to up-state transition). The upward arrows indicate times of GABA release from inhibitory collaterals. The open arrows illustrate how postsynaptic inhibition actually depolarizes (excites) spiny neurons that are in the down-state and only mediates shunting inhibition when V_m is at the chloride equilibrium potential E_Cl. The downward closed arrows show that presynaptic inhibition always decreases membrane potential (inhibits) and therefore is qualitatively more effective than postsynaptic inhibition. (c) Membrane potential responses of two model spiny neurons in response to stimulus A followed by stimulus B. The AB neuron responds strongly when B is delivered after A. The effect of presynaptic inhibition is shown by the suppression (arrow) of the BA neuron membrane potential. It is produced by collateral inhibition from unit A in figure 4, which fires in response to stimulus A (figure 5).