Spinal Inputs to Reticulospinal Müller Cells During Locomotor Activity

Doris Kim
Marquette University
Mentor: Dr. James Buchanan

Reticulospinal neurons make up the main descending pathway in lamprey, involved in the control and initiation of locomotion in the spinal cord. Via an ascending feedback pathway, these reticulospinal neurons receive rhythmic ascending spinal input during locomotor activity. These ascending inputs to the reticulospinal neurons can be characterized by the amplitude and timing of membrane potential oscillations using intracellular electrodes.  In this experiment, ascending spinal inputs were recorded in 8 different bilateral pairs of uniquely identifiable reticulospinal neurons, called Müller cells, in sea lamprey (Petromyzon marinus) in order to determine if different reticulospinal neurons receive inputs from different ascending spinal neurons. 

The experiments were performed on in vitro lamprey brainstem-spinal cord preparations in which a Vaseline diffusion barrier was constructed just caudal to the brainstem. Locomotor activity was induced in the spinal cord with perfusion of D-glutamate (0.8mM) to the spinal cord bath. Polysynaptic pathways in the brainstem were suppressed by elevating calcium and magnesium in the brainstem bathing solution. Locomotor output was recorded with extracellular electrodes on the ventral on each side of the spinal cord. Reticulospinal Müller cells were impaled with sharp intracellular microelectrodes and their membrane potential oscillations were recorded. For each cell, the ventral root and intracellular recordings were averaged with respect to the ventral root burst onsets and measured for average peak and trough latencies and peak-to-trough amplitudes of potential oscillations. Locomotor spinal inputs were statistically analyzed between and within cells based on these measurements. 

Results of these experiments demonstrated that Müller cells receive direct rhythmic inputs from the spinal locomotor networks with significantly different peak and trough timings of membrane potential oscillations. These timing differences suggest that individual Müller cells receive ascending locomotor input from different subsets of spinal neurons. Results also show a strong inverse correlation between the distance of the Müller cell body from the spinal cord and the oscillation amplitude. 
 
 

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