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See the following figure (source):

enter image description here

The pathway on the right (SN $\rightarrow$ interneuron $\rightarrow$ F Neuron $ \rightarrow $ flexor muscle), is explained as follows:

The action potential in the sensory neuron invades the synaptic terminal of the sensory neuron causing the release of transmitter, and subsequent excitation of the postsynaptic interneuron colored black. This neuron is called an interneuron because it is interposed between one neuron (here the SN) and another neuron (here the MN). The excitation of the interneuron leads to the initiation of an action potential and the subsequent release of transmitter from the presynaptic terminal of the interneuron (black triangle), but for this branch of the circuit, the transmitter leads to an IPSP in the postsynaptic flexor (F) motor neuron (colored red). The functional consequences of this feedforward inhibition it is to decrease the probability of the flexor motor neuron becoming active and producing an inappropriate flexion of the leg.

Specifically, it states that but for this branch of the circuit, the transmitter leads to an IPSP in the postsynaptic flexor (F) motor neuron. But it doesn't give an explanation of how this works.

How can a neuron generate an inhibitory postsynaptic potential when it is given an action potential? Can this specific neuron inverse the signal? How does that work?

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The University of Texas site where you obtained this picture states in section 6.7:

What about the transmitter substance that is released by the inhibitory interneuron in the spinal cord? The transmitter substance is glycine, an amino acid which is used frequently in the central nervous system as a transmitter that produces inhibitory actions. It is not the most common, however. The most common transmitter with inhibitory actions is gamma amino butyric acid (GABA).

Glycine receptors activate a chloride-selective transmembrane channel, which is predominantly expressed in the spinal cord and brain stem (Rajendra et al, 1997). Chloride hyperpolarizes the neuron, thereby inhibiting the motorneuron controlling the flexor muscle, and thereby facilitating the extensor muscle to contract. If the opposing flexor muscle would not be inhibited, the extensor muscle would be counteracted and the knee jerk reflex would be less efficient, if present at all.

Typically, an inhibitory interneuron will only release inhibitory neurotransmitter. In the example here, the opposite circuit to bend the knee would activate the flexor muscle. This would be accompanied by opposite events. However, the inhibitory F neuron above in your figure would not reverse its action though, it will simply be silent, thereby allowing the acetylcholine from the E neuron do its job. In more general terms, neurons cannot reverse their action from inhibitory to excitatory. Their outputs are typically simply the release of a pre-defined vesicle pool, which is a yes/no event without further qualities. In other words, an inhibitory neuron will be inhibitory and it cannot change the nature of the neurotransmitters released (excitatory versus inhibitory) based on different inputs.

Reference
- Rajendra et al, Pharmacology & Therapeutics (1997), 73(2): 121–46

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    $\begingroup$ Thank you! That explains a lot. Just one more thing so I fully grasp the concept: some neurons will only release inhibitory neurotransmitters when receiving an action potential. These neurons cannot release excitatory neurotransmitters. Am I correct? $\endgroup$ – Jean-Paul Nov 1 '15 at 15:16
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    $\begingroup$ @Jean-Paul Yes, a neuron will release the same set of transmitters at all of its synapses--often it will release just one neurotransmitter, but sometimes two or more at the same time. An inhibitory interneuron will not, for instance, release an inhibitory transmitter onto one cell and an excitatory transmitter onto a different cell. This is known as Dale's principle (en.wikipedia.org/wiki/Dale%27s_principle). $\endgroup$ – yamad Nov 1 '15 at 18:35
  • $\begingroup$ @yamad To be very precise, the effect upon the postsynaptic neurone is determined by the type of receptor that is activated. With regard to the figure, is it true that the sensory neurons in the knee transmit neurotransmitters that generate an excitatory effect in combination with the receptors of the motor neuron that flexes the extensor muscle, and an excitatory effect within the inhibitory interneuron too? Then, does the inhibitory interneuron release neurotransmitters that in combination with the receptors of the motor neuron that flexes the flexor muscle yield an inhibitory effect? $\endgroup$ – Jean-Paul Nov 1 '15 at 19:04
  • $\begingroup$ @Jean-Paul Close, but with one major misunderstanding. In this reflex, the flexor muscle is not flexed (contracted). It is being relaxed by the reflex. How does this work? In the diagram, activating the E neuron contracts the extensor muscle, activating the F neuron contracts the flexor muscle. No activity in these neurons means the corresponding muscle is relaxed. By inhibiting activity in the F neuron, the interneuron indirectly causes the flexor to relax. All the connections in the diagram are excitatory, except for the interneuron to F neuron connection which is inhibitory. $\endgroup$ – yamad Nov 1 '15 at 20:25
  • $\begingroup$ @yamad That is exactly what I meant: I referred to the motor neurons as "flexing the ... muscle" to indicate which motor neuron I was referring to (I should have used their names, E and F, to avoid confusion). But your comment nevertheless completely answered my final question. I fully understand the concept now. Thank you :) $\endgroup$ – Jean-Paul Nov 1 '15 at 20:38

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