An ultrastructural study of serially sectioned Renshaw cells. II. Synaptic types

Area of Science:

• Neuroscience
• Cell Biology
• Spinal Cord Anatomy

Background:

• Renshaw cells are interneurons in the spinal cord that play a crucial role in motor control.
• Understanding the synaptic connections of Renshaw cells is essential for deciphering motor circuitry.
• Previous studies have provided limited ultrastructural detail on Renshaw cell inputs.

Purpose of the Study:

• To ultrastructurally characterize the synaptic boutons and contacts on presumed Renshaw cells.
• To classify these synaptic inputs based on vesicle morphology.
• To determine the specific type of synapse formed by alpha-motoneuron axon collaterals on Renshaw cells.

Main Methods:

• Ultrastructural analysis of 17 presumed Renshaw cells.
• Intracellular horseradish peroxidase (HRP) staining of triceps and surae alpha-motoneurons.
• Classification of synaptic boutons based on vesicle shape (spherical vs. flattened).

Main Results:

• Two main categories of synaptic boutons were identified: S-type (spherical vesicles) and F-type (flattened vesicles).
• All studied Renshaw cells received input from axon collateral boutons of alpha-motoneurons.
• These alpha-motoneuron collateral boutons were exclusively classified as S-type.
• Some S-type boutons contained neurofilaments, and others were contacted by small presynaptic boutons.

Conclusions:

• Renshaw cells receive distinct types of synaptic input, differentiated by vesicle morphology.
• Alpha-motoneuron axon collaterals specifically utilize S-type synapses to contact Renshaw cells.
• These findings contribute to a detailed understanding of spinal alpha-motoneuron and Renshaw cell synaptology.