Fig. 7

Schematic diagram showing the sympathetic excitation and increase in muscle force afforded by muscle contraction. When the alpha motor neurons innervating the triceps surae muscles where cell bodies are localized in the L4–5 spinal cord and their axons included in the tibial nerve are excited and the triceps surae muscles contract (route 1), the group III and IV afferents of the muscles are excited (route 2). The afferent information is transmitted to the central nervous system via the L3–6 dorsal roots, and then reflexively excites the sympathetic preganglionic neurons in the spinal cord (T11–L2), which are connected through LST to the postganglionic neurons in the L2–4 sympathetic ganglia. Subsequently, the sympathetic postganglionic efferent nerve activity reaches the triceps surae muscles (route 3) and increases their contractile force