On the intrafusal distribution of dynamic and static fusimotor axons in cat muscle spindles

Locomotor training maintains normal inhibitory influence on both alpha- and gamma-motoneurons after neonatal spinal cord transection

Spinal cord injuries lead to impairments, which are accompanied by extensive reorganization of neuronal circuits caudal to the injury. Locomotor training can aid in the functional recovery after injury, but the neuronal mechanisms associated with such plasticity are only sparsely known. We investigated ultrastructurally the synaptic inputs to tibialis anterior motoneurons (MNs) retrogradely labeled in adult rats that had received a complete midthoracic spinal cord transection at postnatal day 5. A subset of the injured rats received locomotor training. Both γ- and α-MNs were studied. The total number of boutons apposing γ-MNs, but not α-MNs, was reduced after neonatal spinal cord transection. The proportion of inhibitory to excitatory boutons, however, was increased significantly in both α-MNs and γ-MNs in spinally transected rats, but with locomotor training returned to levels observed in intact rats. The specific densities and compositions of synaptic boutons were, however, different between all three groups. Surprisingly, we observed the atypical presence of both C- and M-type boutons apposing the somata of γ-MNs in the spinal rats, regardless of training status. We conclude that a neonatal spinal cord transection induces significant reorganization of synaptic inputs to spinal motoneurons caudal to the site of injury with a net increase in inhibitory influence, which is associated with poor stepping. Spinal cord injury followed by successful locomotor training, however, results in improved bipedal stepping and further synaptic changes with the proportion of inhibitory and excitatory inputs to the motoneurons being similar to that observed in intact rats.

Ultrastructural synaptic features differ between alpha- and gamma-motoneurons innervating the tibialis anterior muscle in the rat

We investigated the synaptology of retrogradely labeled spinal motoneurons after injection of horseradish peroxidase into the tibialis anterior (TA) muscle of adult rat. In total, 32 TA motoneurons were investigated in the electron microscope and demonstrated a bimodal size distribution with cell diameter peaks at 40 microm and 20 microm, likely representing alpha- and gamma-motoneurons, respectively. Both alpha- and gamma-motoneurons were apposed by S- and F-type synaptic boutons, whereas only alpha-motoneurons demonstrated inputs by the large M- and C-type boutons. The proportion of cell body membrane covered by synaptic inputs was surprisingly indistinguishable between alpha-motoneurons (72.2%) and gamma-motoneurons (63.5%). The ratio between the number of F- and S-type boutons in apposition with the motoneuron cell body (F/S ratio) and the ratio between the soma membrane coverage provided by F- and S-type boutons were both significantly higher in alpha- than in gamma-motoneurons. When comparing our data with previous findings in other species, we conclude that rat TA alpha-motoneurons are similar to cat and primate alpha-motoneurons with regard to synaptic terminal morphology, frequency, and distribution. However, rat gamma-motoneurons show a markedly higher total synaptic coverage and frequency than cat gamma-motoneurons, although both species exhibit appositions made by the same synaptic types and similar ratios between inhibitory and excitatory inputs.

Re-afferent effects of individual static and dynamic gamma-stimuli during maintained fusimotor stimulation

The ability of maintained dynamic and static fusimotor stimulation to modulate the primary afferent response of the muscle spindle in the rhythm of gamma-stimulation was investigated using a highly sensitive method for modulation detection. The effect of 41 gamma-fibers (13 dynamic; 28 static) on 38 primary afferents obtained from the tibialis anterior muscle of the cat was studied. It was found that maintained stimulation of 10 out of the 13 dynamic (77%) and of 25 out of the 28 static (89%) gamma-fibers could evoke significant modulations of the primary afferent response in the rhythm of fusimotor stimulation at a minimum of one stimulation rate. Moreover, both static and dynamic gamma-stimulations could evoke significant primary afferent modulations almost over the entire range of stimulation rates studied (30-300 stimuli per second). These results show that both gamma-systems can modulate the primary afferent response in the rhythm of fusimotor stimulation over a wide range of stimulation rates; thus the central nervous system may be provided with re-afferent information about the effect of each individual gamma-motoneuron discharge. Some hypotheses for the internal spindle mechanism responsible for the afferent modulations are discussed.