Marilee J. Stephens

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1. Intracellular recording from medial gastrocnemius (MG) motoneurones was used to examine postsynaptic potentials produced by electrical stimulation of the plantaris nerve at group I strength at rest and during fictive locomotion. Fictive locomotion was evoked by stimulation of the midbrain locomotor region (MLR) in decerebrate cats or in decerebrate,(More)
Clinical assessment of rigidity in parkinsonian patients is largely qualitative. The reliability and validity of the assessments are sometimes in doubt. Several "engineering" methods of quantifying rigidity have been described, but none has been adopted into general clinical practice. A possible reason is that these methods differ in crucial aspects from(More)
Transient disturbances were applied to the lower limbs of infants (3-10 mo of age) while they were supported to stepped on a treadmill. The aim was to determine how stepping infants respond to novel disturbances that would disrupt equilibrium during independent walking. Their responses were also compared with those from lower mammals and adult humans. In(More)
1. Stepping responses were studied in infants between the ages of 10 days and 10 months while they were supported to step on a slowly moving treadmill belt. Surface electromyography (EMG) from muscles in the lower limb, force exerted by the feet on the treadmill belt, and the motion of the lower limbs were recorded. 2. Two groups of infants were studied,(More)
The brain stem provides most of the noradrenaline (NA) present in the spinal cord, which functions to both increase spinal motoneuron excitability and inhibit sensory afferent transmission to motoneurons (excitatory postsynaptic potentials; EPSPs). NA increases motoneuron excitability by facilitating calcium-mediated persistent inward currents (Ca PICs)(More)
Immediately after spinal cord injury (SCI), a devastating paralysis results from the loss of brain stem and cortical innervation of spinal neurons that control movement, including a loss of serotonergic (5-HT) innervation of motoneurons. Over time, motoneurons recover from denervation and function autonomously, exhibiting large persistent calcium currents(More)
Spinal cord transection leads to elimination of brain stem-derived monoamine fibers that normally synthesize most of the monoamines in the spinal cord, including serotonin (5-hydroxytryptamine, 5-HT) synthesized from tryptophan by enzymes tryptophan hydroxylase (TPH, synthesizing 5-hydroxytryptophan, 5-HTP) and aromatic l-amino acid decarboxylase (AADC,(More)
Activation of group Ib afferents from extensor muscles produces an inhibition in the parent muscle and its synergists in a resting cat. This reflex switches to excitation of the parent muscle and its synergists when the cat walks. This study determined if a similar reflex undergoes the same type of reversal in the intact human. A putative Ib reflex was(More)
Sensory afferent transmission and associated spinal reflexes are normally inhibited by serotonin (5-HT) derived from the brain stem. Spinal cord injury (SCI) that eliminates this 5-HT innervation leads to a disinhibition of sensory transmission and a consequent emergence of unusually long polysynaptic excitatory postsynaptic potentials (EPSPs) in(More)
The contribution of the ryanodine-sensitive fraction of canine cardiac sarcoplasmic reticulum to the total issue calcium uptake was estimated by the oxalate-supported calcium uptake rate in canine whole heart homogenates. Ryanodine stimulated this uptake rate nearly three-fold. Ryanodine stimulated this same activity in isolated SR vesicles only two-fold.(More)