Segmental Arrangement of Spinal Cord
The spinal cord is arranged as a series of segments, and the motoneurones that control the largest muscle groups
are spread over several segments.
The afferent axons that enter the dorsal roots divide into axon collaterals that synapse on neurones in the same and adjacent segments (see the diagram opposite).
Some of these afferent collaterals end directly on motoneurones
- that innervate the same muscle
- muscles with a synergistic (similar) action
For these, there is only one synapse - between the muscle afferent and the motoneurones - hence the term monosynaptic reflex.
However other axon collaterals of Ia afferents from the same muscles spindles have different destinations:
- Someare involved in initiating relaxation of antagonistic muscles during the reflex contraction.
- Others make contact with neurones that project to higher levels of the nervous system, e.g. the cerebellum
All afferent axons release an excitatory neurotransmitter - glutamate. However the relaxation of antagonistic muscles requires another mechanism, involving axon collaterals that excite an interneurone that in turn releases an inhibitory transmitter.
This mechanism allows antagonistic muscles to be relaxed during the contraction induced by the stretch reflex, and called reciprocal inhibition.
The inhibitory transmitter that hyperpolarises the motoneurones of the antagonistic muscle is glycine.