This raises the question of how and when this organization arises during development. These findings suggest that the spatial segregation of functionally distinct cell types at stereotypic positions underlies the specificity of circuit formation in the spinal cord. Similarly, the position of distinct subtypes of V1 neurons – inhibitory neurons that provide negative feedback to MNs and that are crucial for the alternating motor input to flexor and extensor muscles – seems to correlate with distinct proprioceptive input onto these neurons ( Bikoff et al., 2016).
Although neuronal settling position does not seem to be important for projecting to the right target muscle, it is essential for establishing the correct neuronal connections with sensory afferents ( Balaskas et al., 2019 Demireva et al., 2011 Sürmeli et al., 2011).
The position of the neuronal cell body in the spinal cord matches the projection pattern of the MN, with proximally projecting neurons occupying more ventral positions, and distally projecting pools located in progressively more dorsal positions. This is most obvious at limb levels, where MNs are organized into ∼50 motor pools that innervate distinct limb muscles. In contrast to the laminar structure of the dorsal horns, MNs within the ventral spinal cord are organized into distinct clusters that occupy invariant positions.
The outer laminae (laminae I and II) are innervated by afferents transmitting noxious stimuli, including pain, itch and thermosensation, whereas touch afferents project towards the deeper laminae (laminae II-V) and proprioceptive afferents project towards more ventral laminae or directly onto motor neurons (MNs). For example, neurons in the dorsal horns are arranged in distinct laminae and each lamina receives sensory input from neurons involved in the transmission of distinct signals (reviewed by Lai et al., 2016).
Functional characterization of the adult spinal cord, based on morphology, neuronal connectivity, axonal projections, neurotransmitter types and molecular markers, has revealed a modular organization with neurons performing specific tasks occupying stereotypical positions.
0 kommentar(er)
