Figure 1. A much simplified diagram of the spinal cord (above, shown in cross-section) and lower leg (below) showing the essential elements involved in the stretch reflex and the complementary inhibition of antagonist muscles. An agonist ankle extensor muscle (the stretched muscle) contains a muscle spindle that gives rise to the peripheral process of a group Ia muscle spindle afferent that has its cell body in the dorsal root ganglion. The central process of the afferent enters the grey matter of the spinal cord to make direct excitatory (+) synaptic contacts on a motor neuron (MN) that innervates a collection of muscle fibres (muscle unit) in the agonist muscle. The same group Ia afferent makes excitatory synapses on an interneuron that in turn makes inhibitory (–) synapses on an ankle flexor (antagonist) motor neuron. The individual neurons in the diagram each represent large groups with the same organization.

Figure 2. Diagram of the organization of some of the control systems that make synaptic contact with interneurons in the disynaptic group Ia reciprocal inhibitory reflex pathway. The target interneuron (Ia IN, blue), which inhibits flexor motor neurons (MNs), receives excitatory synapses (+) from extensor group Ia afferents, from several systems descending from the supraspinal brain, and from the extensor half (EXT) of the central pattern generator (CPG) for locomotion in the spinal cord. The interneuron also receives inhibitory synapses (–) from other group Ia inhibitory interneurons (Ia IN, pink), specifically those that are excited by flexor muscle spindle afferents. The cell is also inhibited by synapses from Renshaw interneurons, which are excited by recurrent collateral axons from extensor motor neurons. FLEX, flexor half of the CPG.