User:NiayeshRahimiCortese/sandbox/Spinal Interneuron

A Spinal interneuron is found in the spinal cord, and relays signals between afferent neurons and efferent neurons. Further, this particular type of interneuron is involved in the process of sensory-motor integration. The region of the spinal cord called the gray matter is the place where many types of neurons synapse.

Anatomy and Morphology
The gray matter of the spinal cord appears to have groups of small neurons, often referred to as spinal interneuron, that are neither primary sensory cells nor motoneurons. The versatile properties of these spinal interneurons cover a wide range of activities. Their functions include processing sensory input, modulating motoneuron activity, coordinating activity at different spinal levels and relaying sensory or proprioceptive data to the brain. There has been extensive research on the identification and characterization of the spinal cord interneurons,while focusing on factors such as location, size, structure, connectivity, and function. In a general sense, it is difficult to characterize every aspect of the neuronal anatomy of the higher vertebrates’ spinal cord such as mammals. This difficulty arises due to the extent of complexity observed. For instance, in the 19 day embryo rat spinal cord, at least 17 different subclasses of interneurons with ipsilateral axon projections and 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 gray matter of the spinal cord, which also contains the motoneurons. in 1952, the gray matter of the cat spinal cord was investigated, and it was shown to have ten distinct zones often referred to as laminae. Eventually, the lamination pattern was also observed in several species including humans and has been known as Rexed’s laminae. Lamina VII and VIII are locations where most of the interneurons are found (Harry G).

Renshaw cells
Renshaw cells are among the very first morphologically and physiologically identified interneurons. This type of interneurons project onto α-motoneruons, where they establish inhibition by expressing their 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 collaterals of the motor neurons axons and make inhibitory connections to several groups of motor neurons, Ia inhibitory interneurons as well as the same motor neuron that excited them previously. In addition, the connection to the motor neurons establishes a negative feedback system at 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

Ia inhibitory interneuron
These interneurons are involved in the spinal neuronal networks that contribute to the integration of the reflex responses. Ia inhibitory interneurons are activated by monosynaptic input from la afferent fibers of agonist muscles and inhibit antagonists motoneurons. In other words, when a muscle stretches, the antagonists relax. In addition, the reciprocal innervation(or inhibition) is important for mechanism underlying voluntary movement. When the antagonist muscle relaxes during movement, this increases efficiency and speed. This prevents moving muscles from working against contraction force of antagonist muscles. Thus, during voluntary movement, the Ia inhibitory interneurons are used to coordinate muscle contraction. Further, the Ia inhibitory interneurons allow the higher centers to coordinate commands sent to the two muscles working opposite of each other at a single joint via a single command. The interneuron receives the input command from the corticospinal descending axons in such a way that the descending signal, which activates the contraction of one muscle causes relaxation of other muscles.

Origins of interneurons during spinal development
In the mouse dorsal alar plate, six projenitor 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 pitutatibe core (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 ipsilaterraly and rostally. V2 neurons, which include a population of glutamatergic V2a neurons and inhibitory V2b neurons, project ispilaterally and causally across multiple spinal cord regions. The class V1 neurons give rise to two local cicuit inhibitory neurons known as Renshaw cells and Ia inhibitory interneurons.