Excessive short-latency stretch reflexes in the calf muscles do not cause postural instability in patients with hereditary spastic paraplegia.

OBJECTIVE: To identify the role of hyperexcitable short-latency stretch reflexes (SLRs) on balance control in people with hereditary spastic paraplegia (PwHSP). METHODS: Sixteen PwHSP with triceps surae spasticity and 9 healthy control subjects were subjected to toes-up support-surface perturbations. EMG data were recorded from gastrocnemius, soleus and tibialis anterior. Furthermore, center-of-mass trajectories were recorded. RESULTS: PwHSP were less able to withstand the perturbations. Triceps surae SLRs (40-80 ms post perturbation) in PwHSP were increased compared to healthy subjects. Furthermore, a sustained triceps surae EMG activity at 220-320 ms post perturbation was observed in PwHSP, whereas control subjects demonstrated suppression of triceps surae activity. Center of mass trajectories started to diverge between PwHSP and controls only after ∼500 ms, with greater excursions being observed in the PwHSP. CONCLUSIONS: The present results confirm that balance control is impaired in PwHSP. However, the late instant of center of mass divergence argues against a direct, causative role of hyperexcitable SLRs in the triceps surae. SIGNIFICANCE: We postulate that enhanced short-latency stretch reflexes of the triceps surae do not underlie poor balance control in PwHSP. Instead, we suggest the lack of suppression of later triceps surae activity to be the main cause.

Characterization of the Salmonella typhimurium phoE gene and development of Salmonella-specific DNA probes.

In Escherichia coli K-12, the phoE gene, encoding a phosphate-limitation-inducible outer membrane pore protein (PhoE), is closely linked to the genes proA and proB. When the corresponding fragment of the Salmonella typhimurium chromosome was transferred to E. coli K-12 using an RP4::miniMu plasmid, pULB113, no production of S. typhimurium PhoE could be detected. Nevertheless, DNA hybridization studies revealed that the corresponding plasmid did contain S. typhimurium phoE. Production of S. typhimurium PhoE in E. coli was detected only after subcloning the gene in a multicopy vector. Nucleotide (nt) sequence analysis showed extensive homology of S. typhimurium phoE to the E. coli gene and suggested possible explanations for the low expression of S. typhimurium phoE in E. coli. In addition, the sequence information was used to develop Salmonella-specific DNA probes. Two oligodeoxyribonucleotides were synthesized based on nt sequences encoding the fifth and eighth cell-surface-exposed regions of PhoE. When used in polymerase chain reactions, these probes turned out to be specific, i.e., no crossreactions occurred with the non-Salmonella strains, whereas 132 out of 133 tested Salmonella strains were recognized.