Journal of neurophysiology
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Substance P (SP) is an undecapeptide that is co-localized with conventional transmitters in the nucleus accumbens (NAc). Its neurochemical and behavioral effects resemble those of cocaine and amphetamine. How SP accomplishes these effects is not known, partly because its cellular and synaptic effects are not well characterized. ⋯ In addition, the SP-induced synaptic depression was blocked by an adenosine A1 receptor blocker 8-cyclopentyltheophylline (8-CPT) but not the N-methyl-D-aspartate (NMDA) receptor antagonist D-APV. These data show that SP, by activating presynaptic NK1 receptors, depresses excitatory synaptic transmission indirectly by enhancing extracellular dopamine and adenosine levels. Since the cellular and synaptic effects of SP resemble those of cocaine and amphetamine, it may serve as an endogenous psychogenic peptide.
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A total of 20 right-handed subjects were asked to perform a grasp-lift-and-hold task using a precision grip. The grasped object was a one-degree-of-freedom manipuladum consisting of a vertically mounted linear motor capable of generating resistive forces to simulate a range of object weights. In the initial study, seven subjects (6 women, 1 man; ages 24-56 yr) were first asked to lift and hold the object stationary for 4 s. ⋯ Vision of the grasping hand did not significantly correct the finger misalignment after digital anesthesia. Taken together, these results suggest that mechanoreceptors in the fingertips signal the source and direction of pressure applied to the skin. The nervous system uses this information to adjust the fingers and direct the pinch forces optimally for grasping and object manipulation.
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Recently we have shown that acute ethanol (EtOH) exposure suppresses dorsal root-evoked synaptic potentials in spinal motoneurons. To examine the synaptic mechanisms underlying the reduced excitatory activity, EtOH actions on properties of action potential-independent miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) were studied in spinal motoneurons of newborn rats. Properties of mEPSCs generated by activation of N-methyl-D-aspartate receptors (NMDARs) and non-NMDA receptors and of mIPSCs mediated by glycine and gamma-aminobutyric acid-A receptors (GlyR and GABA(A)R) were examined during acute exposure to 70 and 200 mM EtOH. ⋯ Exposure to the higher EtOH concentration had opposite actions on mEPSC and mIPSC amplitudes: it attenuated the amplitude of NMDAR- and non-NMDAR-mediated mEPSCs to ~80% of control and increased GlyR- and GABA(A)R-mediated mIPSC amplitude by ~20%. EtOH-induced changes in the amplitude of postsynaptic currents were not associated with changes in their basic kinetic properties. Our data suggested that in spinal networks of newborn rats, EtOH was more effective in modulating the release of excitatory and inhibitory neurotransmitters than changing the properties of their receptors/channels.
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Although we move our arms rhythmically during walking, running, and swimming, we know little about the neural control of such movements. Our working hypothesis is that neural mechanisms controlling rhythmic movements are similar in the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that seen during leg movement. ⋯ The data support the hypothesis that neural mechanisms regulating reflex pathways in the moving limb are similar in the human upper and lower limbs. However, the inhibition of H-reflex amplitude induced by contralateral leg movement is absent in the arms. This may reflect the greater extent to which the arms can be used independently.
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Transmission in the corticospinal and Ia pathways to soleus motoneurons was investigated in healthy human subjects during bicycling. Soleus H reflexes and motor evoked potentials (MEPs) after transcranial magnetic stimulation (TMS) were modulated similarly during the crank cycle being large during downstroke [concomitant with soleus background electromyographic (EMG) activity] and small during upstroke. Tibialis anterior MEPs were in contrast large during upstroke and small during downstroke. ⋯ These findings suggest that there is a selectively increased transmission in the fast monosynaptic corticospinal pathway to soleus motoneurons in early downstroke during bicycling. It would seem likely that one cause of this is increased excitability of the involved cortical neurons. The increased presynaptic inhibition of Ia afferents in late downstroke may be of importance for depression of stretch reflex activity before and during upstroke.