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Male and female rats differ in muscle fibre composition, related motor unit contractile properties, and muscle spindle density but not number. On the other hand, their motoneurons' intrinsic properties, excitability and firing properties are similar. The aim of this study was to investigate whether apparent sex differences in body mass and muscle force influence the proprioceptive input from muscle spindles to motoneurons. Medial gastrocnemius motoneurons were investigated intracellularly in deeply anaesthetised male and female rats. Monosynaptic Ia excitatory postsynaptic potentials (EPSPs) were evoked using electrical stimulation of primary afferents from homonymous muscle. Data were analysed using a mixed linear model. The central latencies of EPSPs were 0.38–0.80 ms, with no differences in means between males and females. The maximum EPSP amplitude varied between 2.03 and 8.09 mV in males and 1.24 and 6.79 mV in females. The mean maximum EPSP amplitude was 26% higher in males than in females. The mean EPSP rise time, half‐decay time and total duration did not differ between the sexes. EPSP amplitudes correlated with the resting membrane potential, input resistance and EPSP rise time in both sexes. The observed sex differences in the Ia proprioceptive input may be related either to mechanical loading differences in males and females associated with their different body mass or hormonal differences influencing the levels of neuromodulation in spinal circuits. The results highlight the importance of taking sex into consideration in the studies on the influence of afferent inputs on MN excitability. [ABSTRACT FROM AUTHOR]

Recent years have seen increased study of dendritic integration, mostly in acute brain slices. However, due to the low background activity in brain slices the integration of synaptic input in slice preparations may not truly reflect conditions in vivo. To investigate dendritic integration, back-propagation of the action potential (AP) and initiation of the dendritic Ca2+ spike we simultaneously recorded membrane potential at the soma and apical dendrite of layer 5 (L5) pyramidal neurons in quiescent and excited acute brain slices. After excitation of the brain slice the somatic input resistance decreased and the apparent passive space constant shortened. However, the back-propagating AP and dendritic Ca2+ spike were robust during increased synaptic activity. The dendritic Ca2+ spike was suppressed by the ionic composition of the bath solution required for slice excitation, suggesting that Ca2+ spikes may be smaller in vivo than in the acute slice preparation. The results presented here suggest that, under the conditions of slice excitation examined in this study, the increased membrane conductance induced by activation of voltage-gated channels during back-propagation of the AP and dendritic Ca2+ spike initiation is sufficiently larger than the membrane conductance at subthreshold potentials to allow these two regenerative dendritic events to remain robust over several levels of synaptic activity in the apical dendrite of L5 pyramidal neurons. [ABSTRACT FROM AUTHOR]

The number and distribution of excitatory and inhibitory inputs affect the integrative properties of neurons. These parameters have been studied recently for several hippocampal neuron populations. Besides parvalbumin- (PV) containing cells that include basket and axo-axonic cells, cholecystokinin (CCK)-containing interneurons also form a basket cell population with several properties distinct from PV cells. Here, at the light microscopic level, we reconstructed the entire dendritic tree of CCK-immunoreactive (IR) basket cells to describe their geometry, the total length and laminar distribution of their dendrites. This was followed by an electron microscopic analysis of serial ultrathin sections immunostained against γ-aminobutyric acid, to estimate the density of excitatory and inhibitory synapses on their somata, axon initial segments and different subclasses of dendrites. The dendritic tree of CCK-IR basket cells has an average length of 6300 µm and penetrates all layers. At the electron microscopic level, CCK basket cells receive dendritic inputs with a density of 80–230 per 100 µm. The ratio of inhibitory inputs is relatively high (35%) and increases towards the soma (83%). The total numbers of excitatory and inhibitory synapses converging onto CCK-IR cells are ∼ 8200. Comparison of the two, neurochemically distinct basket cells reveals that CCK-containing basket cells receive much less synaptic input than PV cells; however, the relative weight of inhibition is higher on CCK cells. Additional differences in their anatomical and physiological properties predict that CCK basket cells are under a more diverse, elaborate control than PV basket cells, and thus the function of the two populations must be different. [ABSTRACT FROM AUTHOR]