Neuronal Classification And Internal Connection Patterns In The Fifth Layer Of The Medial Entorhinal Cortex

Neurons have a variety of molecular,morphological,connective and functional properties.The only realistic way to control this complexity is to group neurons and analyze them systematically and repeatedly,thereby paving the way for understanding the structure,function and development of brain circuits.The fifth layer(L5)of the medial entorhinal cortex(MEC)is divided into sublayers a and b(L5a and L5b),in which L5 b receives input from the hippocampus and L5 a produces the main entorhinal output.However,currently,there is no systematic classification of the neuron types of L5,and the connection pattern between L5 a and L5 b is not clear.The purpose of this project is to study the classification and internal connections of neurons in MEC L5 a and L5 b.The connection direction and probability of L5 a and L5 b neurons were clarified to complete the information flow in the hippocampus-MEC-telencephalon circuit.We done signal acquisition first and injected the neurons with biocytin for characterization at the same time.After reconstruction,artificial classification was carried out.Based on morphology,density heat map analysis scheme was used to verify the scientific nature of artificial classification.At the electrophysiological level,we first extracted 28 electrophysiological parameters,extracted the relative importance of corresponding electrophysiological parameters based on morphological classification,and obtained the main electrophysiological parameters which can distinguish different types of neurons.Then the neural connections were recorded with three-electrode patch clamp to explore the internal connection mode in L5.Research findings show: according to morphological characteristics,L5 neurons can be divided into 6 types,and one type of neurons in L5 a has a higher density and longer length of lateral basal dendrites outside.Sag ratio,AP threshold and Input Resistance are the main electrophysiological parameters that can distinguish the six types of neurons.The self-connection probability of L5 a pyramidal neurons was about 10.8%(13/120).The connection probability of L5 b pyramidal neurons to L5 a pyramidal neurons is about 2.3%(1/43).In addition,it was found that the connection rate between L5 a pyramidal neurons and L5 b pyramidal neurons was about 7.0%(3/43).And there are two types of circuit motif,feedback inhibition and emanation in L5 at least.To sum up,large pyramidal neurons with longer length and higher density of lateral basal dendrites were found in this study,and direct evidence that L5 b projected to L5 a was found.Thus,the internal connection mode of L5 neurons was clarified,the connection probability between pyramidal neurons was calculated,and the most important link in the hippocampus-MEC-telencephalon circuit was improved,laying a good research foundation for the in-depth study of MEC and related studies of hippocampus-MEC-telencephalon circuit.