Functional properties of the descending medial longitudinal fasciculus

Abstract

The functional importance of the medial longitudinal fasciculus (MLF) in carrying vestibular impulses into the spinal cord has been studied in cats. The amplitude, duration, and latency of the gross motor responses evoked by vestibular stimulation and recorded from cervical and lumbar levels do not show any significant or persistent changes after a selective bilateral disconnection of the descending MLF. Thus, in evaluating the hierarchical importance of the three descending vestibulofugal pathways it becomes obvious that the MLF offers a much weaker link than the connections represented by the vestibulospinal and reticulospinal tracts. By sectioning all pathways other than the MLF in the brain stem at the rerebellopontine angle this tract could be investigated in anatomical isolation from adjacent vestibular connections. Vestibular stimulation applied to this “MLF animal” preparation evoked motor responses which could be recorded as far down as mid-thoracic levels. No sign of activity was ever recorded from lumbosacral levels. Judged by the response, the MLF seems to have a homogenous fiber spectrum, and the mean speed of conduction of the neurons is about 63 m/sec. Single MLF axon recordings demonstrated that the discharge frequency in response to vestibular stimulation may reach values that are more than twice the value of ventral root alpha-fiber discharge in response to identical stimulation, and that the synaptic transmission across the vestibular nuclei occurs with a considerable safety factor. Excitatory and inhibitory postsynaptic potentials evoked by vestibular activity conducted in the MLF have been recorded intracellularly from flexor and extensor motoneurons at cervical levels, but the relative amounts of excitatory or inhibitory action on a test motoneuron are variable. Pure EPSP or IPSP can be recorded but the majority of motoneurons showed EPSP; the long latencies of each were approximately the same and indicate that spinal interneurons are involved in the transmission. These depolarizing and hyperpolarizing effects could be recorded in a random fashion from flexor and extensor neurons, as identified by antidromic stimulation, but no consistent sign of reciprocity was obtained when recording from neurons with antagonistic function.