The general principles governing the nervous control of innate motor acts in vertebrates are discussed. Particular consideration is given to the control of locomotion in both mammals and lower vertebrates. One in vitro model of the lamprey central nervous system has been developed. It can be maintained in vitro for several days and the motor pattern underlying locomotion can be elicited in isolated sections of the spinal cord. These findings now allow a detailed analysis of the underlying neural mechanisms. The hypothesis that different parts of the network controlling locomotion can be used in a variety of other motor acts, including learned ones, is reviewed.

https://science.sciencemag.org/content/sci/228/4696/143.full.pdf

Neurobiological Bases of Rhythmic Motor Acts in Vertebrates

We present a theoretical model which is used to explain the intersegmental coordination of the neural networks responsible for generating locomotion in the isolated spinal cord of lamprey.

The nature of the coupling between segmental oscillators of the lamprey spinal generator for locomotion: a mathematical model.

Nerve terminals are unique among cellular secretory systems in that they can sustain vesicular release at a high rate. Although little is known about the mechanisms that account for the distinctive features of neurotransmitter release, it can be assumed that neuron-specific proteins are involved. One such protein family, the synapsins, are believed to regulate neurotransmitter release through phosphorylation-dependent interactions with synaptic vesicles and cytoskeletal elements. Here we show that clusters of vesicles at synaptic release sites are composed of two pools, a distal pool containing synapsin and a proximal pool devoid of synapsin and located adjacent to the presynaptic membrane. Presynaptic injection of synapsin antibodies resulted in the loss of the distal pool, without any apparent effect on the proximal pool. Depletion of this distal pool was associated with a marked depression of neurotransmitter release evoked by high-frequency (18-20 Hz) but not by low-frequency (0.2 Hz) stimulation. Thus the availability of the synapsin-associated pool of vesicles seems to be required to sustain release of neurotransmitter in response to high-frequency bursts of impulses.

Distinct pools of synaptic vesicles in neurotransmitter release

Neural networks that co-ordinate locomotion and body orientation in lamprey

An in vitro preparation of the lamprey spinal cord was developed to enable detailed studies of the neuronal organization of the central spinal network generating fish swimming movements, one basic type of vertebrate locomotion. 1. In the isolated lamprey spinal cord, stable bursting activity recorded in the ventral roots was initiated by adding, e.g., D-glutamate or L-DOPA to the bathing solution. Less stable rhythmic activity could also be induced by tonic electrical stimulation of the spinal cord. 2. The isolated spinal cord is capable of producing rhythmic activity with the same type of intra- and intersegmental coordination as in the swimming fish, i.e., with alternation between the two sides of the segment and an intersegmental phase coupling. Hence, the in vitro preparation of the lamprey spinal cord may be said to represent the neuronal correlate of the undulatory swimming movements of fish. 3. By performing spinal cord transections it was demonstrated that as few as four segments can produce rhythmic activity with maintained coordination. It was concluded that the capacity to produce coordinated burst activity is distributed throughout the lamprey spinal cord. 4. A longitudinal midline lesion as long as four segments did not prevent the ventral roots on each side from bursting with maintained coordination between adjacent hemisegments. Thus, one side of a segment can produce bursting without interaction with its opposite side, at least when connected to its rostral and caudal neighbors. 5. The rate of bursting was found to vary from one cycle to the next with the period length tending to oscillate about a mean value. Burst duration and intersegmental phase lag varied in the same manner.

C. M. Rovainen

Papers

Neurobiology of lampreys.