Simulation Of Primary Afferent Synapse In Unmyelinated Nerve Fiber

Neurons in dorsal root ganglion are primary sensory neurons, whose axon has two branches: one projecting to the periphery and one prejecting to the central nervous system. The terminal of the peripheral branch of the axon is the only portion of the dorsal root ganlion neuron that is sensitive to natural stimuli. The properties of the nerve terminal determine the sensory function of each dorsal root ganglion neuron. The remainder of the peripheral branch, together with the central branch, is called the primary afferent fiber; it transmits the encoded stimulus information to the spinal cord or brain stem. Spinal dorsal horn is the region where the primary afferent fibers terminate and form synaptic contacts with the central neurons. In somatosensory system, spinal dorsal horn plays an important role in relaying and processing the sensory information. So it is of great biological significance to comprehensively study the syanptic transmission bentween primary afferent fiber and spinal dorsal horn neuron.Neurons are able to generate action potentials when the stimulation occurred. This is one of the most important functional responses for neurons. In the past study of basic electrophysiological properties of neurons it is found that action potentials have many types of timing pattern, which is also termed as firing pattern. Moreover, it is confirmed that the timing series of action potentials composed can be transmitted to other neurons via synapses. That is, firing pattern in neurons may contain information which needed to be conveyed. However, it is not confirmed whether firing pattern is coding mode of information transmission in nervous system. In addition, it is not known that whether firing pattern have biological significance. Synapse transmission is basic mode of information transmission in central nervous system. Chemical synapse and electrical synapse have two different structure and function. Presynaptic neurons generate action potentials with certain timing series when stimulated. These action potentials form firing pattern, which are delivered to fiber terminal via axon, thus cause transmitter release. In the end, transformation from electrical signal to chemical signal is completed and information transmission is fulfilled. What is the relationship between firing pattern and transmitter release? How do electrical signal transform to chemical signal?The efficiency of synapse transmission is the capability of neural information transmission between synapses. The assessed indexes of synapse transmission are as follows: the magnitude and slope rate of PSPs evoked by signal pulse stimulus; the magnitude and area of PSCs. The another indicator of synaptic transmission efficiency is mutual information.Mutual information,in a word,is the ability of postsynapse in keeping the presynaptic information completely.According to popular views, information can be carried by a series action potential only.As a result,to evaluate the impact factors of transmission efficiency,we should consider the different postsynaptic EPSCs induced by different presynaptic action potential and the mutual information both.So it is in great need of developing the further study how we establish a set of reasonable indexes of evaluation the efficiency of synapse transmission. The present research includes three objectives: (1) We created the simulational model of primary afferent synapses in unmyelinated nerve fiber based on the electrophysiological experimental data.(2)We analyze the dynamical mechanisms who caused different presynaptic firing patterns.(3)Combining the simulational model and the experimental data,we investigated the rules of synaptic transmission and synaptic mechanisms which dominate the bursting firing pattern and the single firing pattern.Main results:1. We finished the simulational model of primary afferent synapses in unmyelinated nerve fibera.We created the presynaptic C-fiber firing model by referred Sriven's workb.Based on Destexhe's model and the experimental data,we created the synaptic transmission model of primary afferent synapses in unmyelinated nerve fiber.c. Based on experimental data,we merge the presynaptic firing model and the synaptic transmission model to our model.d. This model can be modified to short-term plasticity model easily.As a two compartments model,the model can be widely used.For example,it can be modified to the model of primary afferent synapses in myelinated nerve fiber easily.2. We analyzed the dynamical mechanisms of different presynaptic firing patterns.a. Realized repetitive firing and busrting due to different dynamical mechanismb. The different dynamical mechanism can be induced by Na-K pump and inject current jointly.3. Synatpic transmission of different firing pattern on primary afferent synapse-the contrasting between model and experiment. a. In the different presynaptic condition,the low frequency presynaptic firing pattern(0.5~4Hz) can induce the postsynaptic EPSC more easily.And also,the CV is small in the low frequency presynaptic firing pattern condition.b. The CV analyzing show us,in the AMPA ONLY condition,the consecutive stimulus can make the CV stable.At the same , bursing stimulus accommodated the MPA&NMDA condition more.c. By calculating the mutual information between input and output trains, we found that under brief-burst stimulation, the information carried by input trains can be more reliably relayed during synaptic transmission. Conclusion:In the present research, we established our simulation model to study the mechanism of synaptic transmission at primary afferent synapse originally.To analyze the dynamical mechanism,we found the different firing patterns due to different dynamical mechanisms.The low frenquency(0.5~4Hz) repetitive stimulus can indue the EPSC easily and stably. By calculate the mutual information,we found the main temporal structure of presynaptic firing patterns can transfer to the postsynapse,especially in the presynaptic bursting firing patterns.According to distube the presynaptic release function in the model,we found that it has little effect in synaptic transmission process.After cuting off the NMDA component,we found that the reliability of bursting firing pattern transferring slow down sharply.It indicated that the mechanism of reliability of bursting firing pattern transferring is the postsynaptic mechanism,more over,the mechanism is related to NMDA tightly.Experimental data recorded under different conditions were fitted with small errors, using the model of synaptic transmission. This model can also be used to analyze the mechanism of change in transmission synaptic effiency. This method provides a good computational way to study the synaptic transmission in spinal dorsal horn. Our research systematically explored the transmission efficiency at primary afferent synapse and its mechanism. Moreover, the model is a double compartment model,it can be modified to other model easily,just like short-term plasticity model and simulational model of primary afferent synapse in myelinated nerve fiber.