A spinal interneuron, found in the spinal cord, relays signals between (afferent) sensory neurons, and (efferent) motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. Most interneurons are found in the grey column, a region of grey matter in the spinal cord.
Structure The grey column of the spinal cord appears to have groups of small neurons, often referred to as spinal interneurons, that are neither primary sensory cells nor motor neurons. The versatile properties of these spinal interneurons cover a wide range of activities. Their functions include the processing of sensory input, the modulation of motor neuron activity, the coordination of activity at different spinal levels, and the relay of sensory or proprioceptive data to the brain. There has been extensive research on the identification and characterization of the spinal cord interneurons based on factors such as location, size, structure, connectivity, and function. Generally, it is difficult to characterize every aspect of the neuronal anatomy of a vertebrate's spinal cord. This difficulty is due not only to its structural complexity but also to the morphology and the connectivity of neurons. For instance, in the spinal cord of a 19-day-old rat embryo, at least 17 different subclasses of interneurons with ipsilateral axon projections were found. In addition, 18 types of commissural interneurons have been identified on the basis of morphology and location.
Location In particular, the cell bodies of the spinal interneurons are found in the grey matter of the spinal cord, which also contains the motor neurons. In 1952, the grey matter of the cat's spinal cord was investigated, and it was shown to have ten distinct zones referred to as Rexed laminae. Eventually, the lamination pattern was also observed in several species including humans. Rexed laminae VII and VIII are locations where most of the interneurons are found.
Development In the mouse's dorsal alar plate, six progenitor domains give rise to dI1-dI6 neurons and two classes of dorsal interneurons. In addition, in the ventral half of the neural tube, four classes of (CPG) interneurons known as V0, V1, V2, and V3 neurons are generated. V0 neurons are commissural neurons that extend their axons rostrally for 2-4 spinal cord regions in the embryonic spinal cord. V3 neurons are excitatory commissural interneurons that extend caudally projecting primary axons. The V1 neurons are inhibitory interneurons with axons that project ipsilaterally and rostrally. V2 neurons, which include a population of glutamatergic V2a neurons and inhibitory V2b neurons, project ipsilaterally and caudally across multiple spinal cord regions. The class V1 neurons give rise to two local circuit inhibitory neurons known as Renshaw cells and Ia inhibitory interneurons.
Function The integration of the sensory feedback signals and central motor commands at several levels of the central nervous system plays a critical role in controlling movement. Research on cat's spinal cord has shown that at the spinal cord level sensory afferents and descending motor pathways converge onto common spinal interneurons. Human studies since the 1970s have documented how this integration of motor commands and sensory feedback signals is used to control muscle activity during movement. During locomotion, the sum of convergent inputs from the central pattern generator (CPG), sensory feedback, descending commands and other intrinsic properties turned on by different neuromodulators give rise to the activity of the interneurons. Further, this interneuronal activity was either recorded directly or inferred from the modulation of response in their postsynaptic targets, most often motoneurons. The most efficient way to gate sensory signals in reflex pathways is to control the firing level of interneurons. For example, during locomotion, the interneuronal activity is modulated via excitation or inhibition depending on the reflex pathways. Thus, different patterns of interneuronal activity will determine which pathways are open, blocked, or modulated.
Neurotransmitter The sensory information that is transmitted to the spinal cord is modulated by a complex network of excitatory and inhibitory interneurons. Different neurotransmitters are released from different interneurons, but the two most common neurotransmitters are GABA, the primary inhibitory neurotransmitter and glutamate, the primary excitatory neurotransmitter. Acetylcholine is a neurotransmitter that often activates interneurons by binding to a receptor on the membrane.
Cell types
Renshaw cells Renshaw cells are among the first identified interneurons. This type of interneuron projects onto α-motoneurons, where it establishes inhibition by expressing its inhibitory neurotransmitter glycine. However, some reports have indicated that Renshaw cells synthesize calcium-binding proteins calbindin-D28k and parvalbumin. Further, during spinal reflex, Renshaw cells control the activity of the spinal motoneurons. They are excited by the axon collaterals of the motor neurons. In addition, Renshaw cells make inhibitory connections to several groups of motor neurons, Ia inhibitory interneurons as well as the same motor neuron that excited them previously. Furthermore, the connection to the motor neurons establishes a negative feedback system that may regulate the firing rate of the motor neurons. Moreover, the connections to the Ia inhibitory interneurons may modulate the strength of the reciprocal inhibition to the antagonist motor neuron.
… excerpt ends here. Continue reading the full article.
