Initiation of locomotion: A review


So, what starts escape and also other forms of movement? Let’s start by looking at a primitive animals: the leech and lamprey. Also included are higher animals: zebrafish and rat.

Escape for the leech is swimming, as long as it is in relatively deep water. A tap on its tail will result in fast swimming!

How does a leech swim? It creates an approximately sinusoidal, undulatory traveling wave. Contraction and relaxation of dorsal and ventral longitudinal muscles are primarily responsible for swimming undulations (Lamb & Calabrese, 2011).

NOTE: for scientists, there is neural activity ‘sign’ that an isolated or nearly isolated leech preparations would be swimming if it was in an intact leech. The ‘sign’ is a bursting pattern of a dorsal excitatory motoneuron in these preparations, and the behavior is called “fictive” swimming. Researchers frequently refer to “fictive swimming” as a “swim”, “swim episode”, “swimming”, and “swim activity” (Mullins et al. 2011a).

What are the pathways activated in these muscles? At the base is a central pattern generator (CPG) circuit that is composed of complex segmental oscillators which rely heavily on intersegmental connectivity (Lamb & Calabrese, 2011).

Midbody ganglia contain a bilateral, triphasic oscillator CPG circuit composed predominately of bilaterally paired interneurons. And the output from these oscillator interneurons controls the activity of the excitatory and inhibitory motoneurons. It is these interneurons that provide the final common pathway to the longitudinal muscles used for swimming. Otherwise, a leech is simply resting.


Okay, then what depolarizes the oscillators initially in the swim burst? Unpaired cells, number 204,  located also in midbody ganglia, appear to be briefly depolarized, opening a gate on its axon along which spikes travel to evoke the oscillatory burst that drive oscillation until some other process terminates swimming. These command gating neurons are in turn activated by trigger neurons in the rostral (head) brain or by a ‘novel’ cell, E21, in the rearmost midbody ganglion next to the caudal (tail) brain. Cell E21 has only a rostrally (headward) projecting axon that connects to all the other ganglia; it has been labeled as “trigger command neuron” (Mullins et al. 2011b).

The ability of leech cell E21 to elicit swimming is shown in Figure 1. The drawing shows the head region is intact and the location of the 21st ganglion, next to the tail (T) brain. Stimulation of cell E21 can initiate swimming (A) and prolong it (B).

The Lamprey is a ‘primitive’ vertebrate. And its means of swimming is remarkably similar to the leech undulating motion (Mullins et al. , 2011a). Due to its rigid notochord, it swims by using left-right myotomal muscles activity that are out-of-phase with each other. More is known about what initiates lamprey’s swimming: water waves, vestibular stimulation, illumination of its eyes, and illumination of caudal dermal photo receptors.

What do these receptors stimulate? Sensory information travels to the brainstem, for example, to the mesencephalic locomotor region (MLC). Stimulation of cells in the MLC will bilaterally excite reticulospinal (RS) command neurons. Command cells to oscillatory cells and to interneurons. Interneurons to motoneurons. Motoneurons produce swimming (for details, see (Mullins et al. , 2011a).


Zebrafish and rat




Eklöf Ljunggren E. (2012) “Organization of the spinal locomotor network in zebrafish: Pattern of recruitment and origin of excitatory drive.” (Doctoral dissertation). Retrieved from

Grillner S. (2003) The motor infrastructure: from ion channels to neural networks. Nat Rev Neurosci 4: 573-586.

Lamb DG, Calabrese RL. (2011) Neural circuits controlling behavior and autonomic functions in medicinal leeches. Neural Systems & Circuits 1: 13-21.

Mullins OJ, Hackett JT, Buchanan J, Friesen WO. (2011a) Neural control of swimming behavior: comparison of vertebrate and invertebrate model systems. Prog Neurobiol 93: 244-69.

Mullins OJ, Hackett JT, Friesen WO. (2011b) Local-distributed integration by a novel neuron ensures rapid initiation of animal locomotion. J. Neurophysiology 105: 130-144.

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