Decreased Axosomatic Input to Motoneurons and Astrogliosis in the Spinal Cord of Aged Rats

Abstract

An increasing body of evidence indicates that aging-related impairments of nervous functions are caused by damage to neuron integrity rather than by loss of neurons. By using electron microscopy, we have examined axosomatic boutons on spinal cord motoneurons derived from aged and young adult Sprague-Dawley rats. The main finding was that about half of the examined motoneuron somata from aged rats had a reduced (50%) bouton coverage, which seemed to be caused by a smaller number of axosomatic bouton profiles. Long stretches of the cell body plasma membrane were apposed by pale processes, and immunolabeling for glial fibrillary acidic protein (GFAP) disclosed that a number of the aged motoneurons appeared embedded in GFAP immunopositive processes. Lumbar motoneurons seemed to be more severely affected than cervical motoneurons. At the ultrastructural level, affected motoneurons disclosed plasma membrane irregularities with appendages/sprout-like extensions that in some cases were sites for axosomatic contacts.

Evidence of self-sustained motoneuron firing in young and older adults

“…A second possibility is that only small concentrations of these neuromodulators are required to modify human motoneurons and produce plateau potentials. The density of axosomatic synapses is reduced in aged rats [28], leading to the speculation that descending influences on motoneuron dendrites may be reduced as well. A related possibility is that human motoneurons are adaptive, and that the L-type Ca ++ channels are more sensitive to these neuromodulators in aging humans.…”

Decreases in motor unit firing rate during sustained maximal-effort contractions in young and older adults

“…Synaptic transmission from spindles is altered in aged cats [12], resulting in diminished la drive to the motoneuron pool. The density of synaptic input to the motoneuron pool is also diminished in aged rats [36]. Any or all of these factors could be important in determining maximal firing rate, however, the exact mechanism through which motor unit firing rates decline with age remains to be determined.…”

Voluntary running attenuates age-related deficits following SCI

“…These conditions greatly affect the integrity and functionality of the aged neural systems, which often manifests in age-related dementia and cognitive decline (Laurin et al, 2001), the latter being associated with age-related demyelination of axons (Kovari et al, 2004). The loss of myelin integrity with age has been documented both in the brain and in the spinal cord (Kullberg et al, 1998;Peters and Sethares, 2003;Raz and Rodrigue, 2006).…”