Local-Distributed Integration by a Novel Neuron Ensures Rapid Initiation of
Olivia J. Mullins,1,2John T. Hackett,2,3and W. Otto Friesen1,2
1Department of Biology,2Neuroscience Graduate Program, and3Department of Molecular Physiology and Biological Physics, University
of Virginia, Charlottesville, Virginia
Submitted 4 June 2010; accepted in final form 25 October 2010
Mullins OJ, Hackett JT, Friesen WO. Local-distributed integration
by a novel neuron ensures rapid initiation of animal locomotion. J
Neurophysiol 105: 130–144, 2011. First published October 27, 2010;
doi:10.1152/jn.00507.2010. Animals are adapted to respond quickly
to threats in their environment. In many invertebrate and some
vertebrate species, the evolutionary pressures have resulted in rapidly
conducting giant axons, which allow short response times. Although
neural circuits mediating escape behavior are identified in several
species, little attention has been paid to this behavior in the medicinal
leech, a model organism whose neuronal circuits are well known. We
present data that suggest an alternative to giant axons for the rapid
initiation of locomotion. A novel individual neuron, cell E21, appears
to be one mediator of this short-latency action in the leech. In isolated
nerve cord and semi-intact preparations, cell E21 excitation initiates
and extends swimming and reduces the cycle period. The soma of this
cell is located caudally, but its axon extends nearly the entire length
of the nerve cord. We found that cell E21 fires impulses following
local sensory inputs anywhere along the body and makes excitatory
synapses onto the gating cells that drive swimming behavior. These
distributed input–output sites minimize the distance information trav-
els to initiate swimming behavior, thus minimizing the latency be-
tween sensory input and motor output. We propose that this single cell
E21 functions to rapidly initiate or modulate locomotion through its
distributed synaptic connections.
I N T R O D U C T I O N
To effectively escape predators and catch prey, animals must
respond rapidly to sensory inputs that indicate threats. The
speed of the response is crucial; a delay lasting a fraction of a
second can be the difference between death and survival.
Neuronal circuits mediating these responses must therefore
relay information quickly and efficiently to initiate locomotion.
In leeches, a common locomotion used in escape and predation
is swimming; however, despite extensive research into swim
circuits, there has been little focus on how this behavior might
be rapidly initiated. Among other sensory modalities, swim
episodes are initiated by touch (Kristan Jr et al. 1982) and
surface water waves (Friesen 1981). Water waves might indi-
cate the nearby presence of a suitable host to feed on (Sawyer
1986) and touch may indicate a threat from a predator. Rostral
touch often initiates a shortening response, whereas caudal
touch usually initiates swimming. If such input is noxious,
swimming may be preceded by shortening, curling, or writhing
(Kristan Jr et al. 1982). These behaviors remove the leech from
the offending stimulus and parallel escape responses observed
in other animals, where initial retreats or turns from noxious
stimuli are followed by rhythmic locomotion (Domenici et al.
2008; Sillar 2009). Escape circuits in species such as crayfish
(Edwards et al. 1999), teleost fish (Eaton and Hackett 1984;
Korn and Faber 1996), cockroaches (Ritzmann and Eaton
1998), rats (Depoortere et al. 1990; Mitchell et al. 1988), and
cats (Mori et al. 1989) have been identified and elucidated to
various degrees. In many invertebrates, the rapid escape re-
sponses are mediated by large interneurons with high impulse
conduction velocities, such as the medial and lateral giant
fibers in the crayfish (Wiersma 1947), the Mauthner neurons in
teleost fish (Sillar 2009), and the giant interneurons in the
cockroach (Kolton and Camhi 1995). In the leech, the large
axon of the S-cell was once thought to mediate a shortening
escape response, however this intersegmental interneuron is
neither sufficient nor necessary for this behavior (Sahley et al.
1994). Since it lacks high conduction velocity axons, the leech
must use alternative strategies to minimize its response latency
to sensory signals.
The latency of a motor response will be reduced if the
distance between the input site and motor system is minimized.
For segmental animals, local sensory input might activate
segmental sensory interneurons that, in turn, activate local
elements of the motor system. Alternatively, multisegmental
interneurons with distributed input–output sites could perform
a similar function. Neurons with multiple spike initiation zones
have been identified in many species. The dorsal gastric (DG)
motoneuron in the stomatogastric system of Cancer borealis,
for example, possesses a secondary spike initiation zone im-
portant for the persistent firing evoked by some stimuli (Le et
al. 2006). Multisegmental tactile interneurons in the crayfish,
stomatogastric and cardiac neurons in the lobster, and the heart
interneurons in the leech can also initiate spikes at multiple
locations, either inter- or intrasegmentally (Calabrese and
Kennedy 1974; Hartline and Cooke 1969; Moulins et al. 1979;
Thompson and Stent 1976). Several interneurons in the cray-
fish especially have been shown to have distributed input sites
(Wiersma and Bush 1963; Wiersma and Hughes 1961;
Wiersma and Ikeda 1964; Wiersma and Mill 1965). A similar
organization is also seen in the lateral giant neurons in the
crayfish and in the S-cell of the leech; these make up networks
of tightly electrically coupled segmental cells that have multi-
ple segmental inputs and outputs (Edwards et al. 1999; Kristan
Jr et al. 2005).
Here, we introduce an excitatory neuron, cell “E21,” that
broadly integrates sensory input from most, and perhaps all,
body segments to trigger swim initiation in the leech nerve
cord. During ongoing fictive swimming, depolarization of cell
E21 decreases the cycle period and prolongs swim episodes.
Address for reprint requests and other correspondence: W. O. Friesen,
Department of Biology, University of Virginia, P.O. Box 400328, Charlottes-
ville, VA 22904-4328 (E-mail: email@example.com).
J Neurophysiol 105: 130–144, 2011.
First published October 27, 2010; doi:10.1152/jn.00507.2010.
1300022-3077/11 Copyright © 2011 The American Physiological Societywww.jn.org
The soma of cell E21 resides in the caudal-most midbody
ganglion but projects to the rostral nerve cord. Through elec-
trophysiological studies on semi-intact and isolated prepara-
tions of Hirudo verbana, we show that cell E21 interactions
link mechanosensory cells with swim-gating neurons. Further-
more, we found that cell E21 receives sensory input, excites
swim-gating cells, and has spike-initiation zones in many
segmental ganglia. We conclude that sensory input anywhere
along the leech body could rapidly initiate or modulate swim-
ming locomotion via the local-distributed structure of cell E21.
This synaptic pathway could prove to be critical in mediating
rapid swim initiation in leeches.
M E T H O D S
Experiments were performed on adult medicinal leeches, Hirudo
verbana, supplied by Niagara Leeches (Cheyenne, WY) or Leeches
USA (Westbury, NY). Leeches were maintained in aquaria in a
temperature-controlled room on a 12-h light/12-h dark cycle at 18–
21°C. Prior to dissection, leeches were anesthetized with 4°C leech
saline, containing (in mmol/l) 115 NaCl, 4 KCl, 1.8 CaCl2, 2 MgCl2,
and 10 HEPES buffer (pH 7.4) (Friesen 1981). During experiments,
preparations were superfused with this normal saline, with saline
containing 50 ?M serotonin to enhance swim initiation (Willard
1981) or with a high divalent cation solution containing (in mmol/l)
91 NaCl, 4 KCl, 10 CaCl2, 10 MgCl2, and 10 HEPES buffer (pH 7.4).
Experiments were performed on isolated nerve cords, on nearly
isolated nerve cord preparations or on semi-intact preparations
(Kristan Jr et al. 2005). Isolated nerve cord preparations comprised a
chain of ganglia extending from M2 (the second segmental ganglion)
through the caudal (tail) brain (T). The rostral (head) brain (H) and
M1 inhibit swim initiation (Brodfuehrer and Friesen 1986a) and thus
they were excluded from most preparations. Preparations, with one or
more segmental ganglia desheathed, were pinned onto a glass-bottom
dish that was covered by a thin layer of resin (?1 mm). To permit
quasi-natural stimulation, we used nearly isolated preparations that
included a flap of tissue from the caudal sucker innervated by the
otherwise isolated nerve cord (M2–T). In semi-intact preparations, the
middle and caudal sectors of the body wall were removed (posterior
to M10 or M11), leaving the posterior nerve cord exposed for
extracellular and intracellular recording. The rostral ganglia, H and
M1, were either disconnected or the anterior body wall was dener-
vated via a small window in the body wall. The preparation was then
placed either in the flat glass-bottom dish or in a dish incorporating a
well to immerse the intact portion of the leech. For both dishes,
threads were attached to the rostral sucker and to the denervated
midbody, permitting undulatory swimming movements by the ante-
rior half of the preparation, but limiting movement in the caudal end
to allow stable recordings. The threads attached to denervated portions
of the leech were taped to the sides of the dish, so the leech was
suspended in the minitrough. In all semi-intact preparations, a petro-
leum jelly dam placed between intact and isolated sections allowed
independent manipulation of saline in anterior and posterior compart-
ments. In semi-intact preparations, swimming was monitored both by
visual inspection of the intact portion of the leech and by electrophys-
iological recording from nerves in the isolated portion of the prepa-
To monitor swimming activity, extracellular suction electrodes
recorded axonal impulses from several dorsal-posterior (DP) nerves.
DP nerves contain the axon of a dorsal excitatory motoneuron, cell
DE-3, whose rhythmic bursting reports swimming (Kristan Jr and
Calabrese 1976). The bursting pattern of cell DE-3 is often called
“fictive” swimming, although here, for convenience, the terms
“swim,” “swim episode,” “swimming,” and “swim activity” are used
to describe the neuronal activity occurring in isolated or nearly
isolated preparations that underlies swimming in intact animals. Suc-
tion electrodes on DP nerves were also used to provide electrical
shocks to initiate swimming. Impulses traveling in the intersegmental
connectives were detected through en passant suction electrode re-
Sharp microelectrodes for intracellular recording were pulled using
a P-87 Flaming Brown Micropipette Puller (Sutter Instrument, No-
vato, CA) and when filled with 2.7 M KAc and 20 mM KCl had a
measured resistance of 30–60 M?. Intracellular recording and cur-
rent injection were accomplished by Axoclamp2A amplifiers (Axon
Instruments, Sunnyvale, CA) in bridge mode. Although the bridge
was balanced immediately before penetration, it often became unbal-
anced over the course of the experiment. Extracellular signals were
amplified by a preamplifier and then, along with intracellular signals,
were further amplified and digitized with PowerLab, then displayed
and stored with Chart software (AD Instruments, Colorado Springs,
CO). Intracellular recordings were obtained from the somata of
neurons identified by location, size, and electrical and functional
properties. We used a microscope reticule to measure cell E21 soma
To initiate swimming, we either shocked a caudal DP nerve with a
train of 2- to 4-V, 5-ms pulses at 25 Hz (referred to as nerve shock)
or injected depolarizing current into cell E21. For experiments to
compare stimulus duration with swim duration, the level of current
injection was kept constant, although this sometimes led to a variable
impulse frequency. To examine swim duration in terms of impulse
frequency, we varied the current injection amplitude and used a
constant stimulus duration within each experiment.
We evaluated the effects of sensory stimulation in nearly isolated
preparations by applying electrical shocks to body wall flaps. Pulses
of 2, 5, or 7 V, lasting about 1.5 ms, were delivered at about 6 Hz via
silver wires attached to a stimulator. In semi-intact preparations, we
stroked, poked, or pinched innervated regions of the body wall with a
small probe or a forceps.
To visualize the neurite and axon of cell E21, we injected the soma
with Alexa Fluor hydrazide 568 (Molecular Probes, Eugene, OR).
Glass micropipettes were backfilled with the dye dissolved in 100 mM
lithium chloride (Fan et al. 2005). Dye injection was accomplished by
applying hyperpolarizing current pulses (?0.8 nA) superimposed on
a constant depolarizing current (?0.1 nA), with a duty cycle of about
1 s for 30–70 min. Ganglion M21 was then placed in saline on a glass
slide under a coverslip and visualized under green fluorescent light
with a Zeiss fluorescence microscope. Pictures were obtained with a
MagnaFire SP digital camera (Olympus America, Melville, NY).
Chart software (AD Instruments, Mountain View, CA) was used to
store and analyze physiological data. Swim duration was measured by
counting the number of spike bursts from cell DE-3 that occurred
during a swim episode. Cycle periods were obtained by exporting data
from Chart for analysis with Matlab (The MathWorks, Natick, MA)
and the Rhythm Analysis System (RAS) (Hocker et al. 2000), which
uses the median spike in each burst as a reference point.
There is large variability in swim duration among preparations;
consequently, we normalized swim durations by expressing values as
a fraction of the average swim duration in a given experiment. This
131 LOCAL-DISTRIBUTED INTEGRATION
J Neurophysiol • VOL 105 • JANUARY 2011 • www.jn.org
normalized value was then plotted against either stimulus duration or
cell E21’s impulse frequency. To determine the effect of cell E21
impulse frequency on cycle period, we injected single 400-ms depo-
larizing current pulses after the fifth or sixth burst in a series of swim
episodes. Results are reported as relative differences in the cycle
periods following and those immediately preceding the current injec-
tion. To detect cell E21 spikes in the suction electrode recordings from
the intersegmental connectives, averaged records were triggered on
soma spikes. We also used spike-triggered averaging to analyze
synaptic transmission from sensory neurons to cell E21. We estimated
excitatory postsynaptic potential (EPSP) duration from the exponen-
tial decay of these averaged data.
Statistical analysis was performed using Prism5 (GraphPad, La
Jolla, CA). Prism5 was also used to generate all graphs. Results are
reported as means with SE. Unless otherwise noted, the “n” value
refers to the number of leech preparations from which the data were
R E S U L T S
Here, we introduce an excitatory, command-type neuron,
cell E21, whose soma is located in the caudal midbody gan-
glion 21 (M21) of the leech nerve cord. Excitation of this cell
powerfully influences swimming activity, suggesting that this
cell may be important for the rapid onset of locomotion.
Cell E21 triggers swimming
To investigate cell E21’s swim-initiating properties, we
recorded from its soma in isolated nerve cord preparations
while monitoring swimming activity via motoneuron output
with suction electrode recordings from DP nerves. Cell E21
displays high-frequency firing during and just after a swim-
initiating DP nerve shock (Fig. 1A1). This property is shared by
previously identified trigger neurons Tr1 and Tr2 in the rostral
brain (Brodfuehrer and Friesen 1986b). Trigger neurons are
so-called because swim duration is independent of their initi-
ating intensity. We found that like these cephalic trigger
neurons, brief excitation of cell E21 by depolarizing current
injections effectively initiated swim episodes similar to those
elicited by DP nerve shock (Fig. 1, A2 and A3). Pulse durations
of 350–400 ms initiated swimming if the impulse frequency of
cell E21 was relatively high (?50–60 Hz), whereas lower
impulse frequencies (?30 Hz) were effective when injected
current pulses were longer (?1 s). Swimming was more
reliably initiated as we increased the frequency or number of
impulses in cell E21. However, unlike the cephalic trigger
neurons, cell E21 exhibits small membrane potential oscilla-
tions that are phase-locked with DP bursts (Fig. 1A2, inset).
These often give rise to impulses, indicating that cell E21
receives some feedback from the swim oscillator circuit.
We next investigated whether the swim duration was inde-
pendent of stimulus intensity, as it is for the previously iden-
tified trigger neurons. We injected current pulses into cell E21
with either fixed duration and varying amplitude or fixed
amplitude and varying duration, and observed the length of
swim episodes elicited by these pulses. Figure 1, A2 and A3
illustrates that brief (0.36-s) and longer (1.48-s) injected cur-
rent pulses elicit swim episodes with similar durations. Overall,
swim length was not correlated with either the duration (Fig.
1B1) or the impulse frequency (Fig. 1B2) of the initiating
stimulus in the parameter range tested. These data show that
cell E21 is a trigger neuron by the criteria used to classify
trigger neurons Tr1 and Tr2. The small depolarization and
membrane potential oscillations observed in cell E21 suggest,
however, that there are interesting differences in circuit inter-
actions and potentially in functional properties between cell
E21 and the cephalic trigger neurons.
In isolated nerve cord preparations swimming is often initi-
ated by peripheral nerve stimulation, which activates the tactile
A1: swimming activity initiated by electrical stimulation (gray
bar) of dorsal-posterior (DP) nerve (R,17). Cell E21 fires in
response to the stimulus and, at a reduced rate, during the swim
episode. A2: brief current injection into cell E21 (1.2 nA, 0.36
s, gray bar) elicits a swim episode like episodes initiated by DP
nerve shock, but with shorter duration. Inset: enlargement of
data within the gray rectangle; dotted lines indicate timing of
DP bursts. A3: prolonged current injection (1.2 nA, 1.70 s, gray
bar) into cell E21 elicits a swim episode with a duration similar
to that of a brief current pulse (A2). B: cell E21 is a swim trigger
neuron. B1: swim duration is not altered by the duration of
depolarizing current pulses injected into cell E21; linear regres-
sion; slope is not different from zero; P ? 0.91. B2: there is no
correlation between cell E21 firing frequency and swim dura-
tion; dashed line is the linear regression; slope is not different
from zero; P ? 0.96.
Cell E21 triggers swimming. A: swim initiation.
132O. J. MULLINS, J. T. HACKETT, AND W. O. FRIESEN
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Crisp KM. Multiple spike initiation zones in a neuron implicated in learning
in the leech: a computational model. Invert Neurosci 9: 1–10, 2009.
Debski EA, Friesen WO. Habituation of swimming activity in the medicinal
leech. J Exp Biol 116: 169–188, 1985.
Debski EA, Friesen WO. Intracellular stimulation of sensory cells elicits
swimming activity in the medicinal leech. J Comp Physiol A Sens Neural
Behav Physiol 160: 447–457, 1987.
Depoortere R, Di Scala G, Sandner G. Treadmill locomotion and aversive
effects induced by electrical stimulation of the mesencephalic locomotor
region in the rat. Brain Res Bull 25: 723–727, 1990.
Domenici P, Booth D, Blagburn JM, Bacon JP. Cockroaches keep predators
guessing by using preferred escape trajectories. Curr Biol 18: 1792–1796,
Eaton RC, Hackett JT. The role of the Mauthner cell in fast-starts involving
escape in teleost fishes. In: Neural Mechanisms of Startle Behavior, edited
by Eaton RC. New York: Plenum, 1984, p. 213–266.
Edwards DH, Heitler WJ, Krasne FB. Fifty years of a command neuron: the
neurobiology of escape behavior in the crayfish. Trends Neurosci 22:
Esch T, Mesce KA, Kristan WB Jr. Evidence for sequential decision making
in the medicinal leech. J Neurosci 22: 11045–11054, 2002.
Fan RJ, Marin-Burgin A, French KA, Friesen WO. A dye mixture (Neu-
robiotin and Alexa 488) reveals extensive dye-coupling among neurons in
leeches; physiology confirms the connections. J Comp Physiol A Sens
Neural Behav Physiol 191: 1157–1171, 2005.
Friesen WO. Physiology of water motion detection in the medicinal leech. J
Exp Biol 92: 255–275, 1981.
Friesen WO, Poon M, Stent GS. An oscillatory neuronal circuit generating a
locomotory rhythm. Proc Natl Acad Sci USA 73: 3734–3738, 1976.
Friesen WO, Poon M, Stent GS. Neuronal control of swimming in the
medicinal leech. IV. Identification of a network of oscillatory interneurones.
J Exp Biol 75: 25–43, 1978.
Funch PG, Faber DS. Impulse propagation along a myelinated vertebrate
axon lacking nodes of Ranvier. Brain Res 190: 261–267, 1980.
Gamkrelidze GN, Laurienti PJ, Blankenship JE. Identification and charac-
terization of cerebral ganglion neurons that induce swimming and modulate
swim-related pedal ganglion neurons in Aplysia brasiliana. J Neurophysiol
74: 1444–1462, 1995.
Gray J, Lissmann HW, Pumphrey RJ. The mechanism of locomotion in the
leech (Hirudo medicinalis Ray). J Exp Biol 15: 408–430, 1938.
Hartline DK, Cooke IM. Postsynaptic membrane response predicted from
presynaptic input pattern in lobster cardiac ganglion. Science 164: 1080–
Hocker CG, Yu X, Friesen WO. Functionally heterogeneous segmental
oscillators generate swimming in the medical leech. J Comp Physiol A Sens
Neural Behav Physiol 186: 871–883, 2000.
Hughes GM, Wiersma CAG. Neuronal pathways and synaptic connexions in
the abdominal cord of the crayfish. J Exp Biol 37: 291–307, 1960.
Jing J, Gillette R. Central pattern generator for escape swimming in the
notaspid sea slug Pleurobranchaea californica. J Neurophysiol 81: 654–
Kennedy D, Mellon D Jr. Synaptic activation and receptive fields in crayfish
interneurons. Comp Biochem Physiol 13: 275–300, 1964.
Kolton L, Camhi JM. Cartesian representation of stimulus direction: parallel
processing by two sets of giant interneurons in the cockroach. J Comp
Physiol A Sens Neural Behav Physiol 176: 691–702, 1995.
Korn H, Faber DS. Escape behavior: brainstem and spinal cord circuitry and
function. Curr Opin Neurobiol 6: 826–832, 1996.
Kramer AP, Krasne FB. Crayfish escape behavior: production of tailflips
without giant fiber activity. J Neurophysiol 52: 189–211, 1984.
Kristan WB Jr, Calabrese RL. Rhythmic swimming activity in neurones of
the isolated nerve cord of the leech. J Exp Biol 65: 643–668, 1976.
Kristan WB Jr, Calabrese RL, Friesen WO. Neuronal control of leech
behavior. Prog Neurobiol 76: 279–327, 2005.
Kristan WB Jr, McGirr SJ, Simpson GV. Behavioural and mechanosensory
neurone responses to skin stimulation in leeches. J Exp Biol 96: 143–160,
Kristan WB Jr, Stent GS, Ort CA. Neuronal control of swimming in the
medicinal leech. I. Dynamics of the swimming rhythm. J Comp Physiol A
Sens Neural Behav Physiol 94: 97–119, 1974.
Kupfermann I, Weiss KR. The command neuron concept. Behav Brain Sci 1:
Le T, Verley DR, Goaillard JM, Messinger DI, Christie AE, Birmingham
JT. Bistable behavior originating in the axon of a crustacean motor neuron.
J Neurophysiol 95: 1356–1368, 2006.
Masino MA, Calabrese RL. Phase relationships between segmentally orga-
nized oscillators in the leech heartbeat pattern generating network. J Neu-
rophysiol 87: 1572–1585, 2002.
Mathy A, Ho SS, Davie JT, Duguid IC, Clark BA, Hausser M. Encoding of
oscillations by axonal bursts in inferior olive neurons. Neuron 62: 388–399,
McClellan AD, Grillner S. Activation of “fictive swimming” by electrical
microstimulation of brainstem locomotor regions in an in vitro preparation
of the lamprey central nervous system. Brain Res 300: 357–361, 1984.
Meyrand P, Weimann JM, Marder E. Multiple axonal spike initiation zones
in a motor neuron: serotonin activation. J Neurosci 12: 2803–2812, 1992.
Mitchell IJ, Redgrave P, Dean P. Plasticity of behavioural response to
repeated injection of glutamate in cuneiform area of rat. Brain Res 460:
Mori S, Sakamoto T, Ohta Y, Takakusaki K, Matsuyama K. Site-specific
postural and locomotor changes evoked in awake, freely moving intact cats
by stimulating the brainstem. Brain Res 505: 66–74, 1989.
Moulins M, Vedel JP, Nagy F. Complex motor neurone in crustacea: three
axonal spike initiating zones in three different ganglia. Neurosci Lett 13:
Mullins OJ, Hackett JT, Buchanan JT, Friesen WO. Neuronal control of
swimming behavior. Comparison of vertebrate and invertebrate model
systems. Prog Neurobiol In press.
Nicholls JG, Baylor DA. Specific modalities and receptive fields of sensory
neurons in CNS of the leech. J Neurophysiol 31: 740–756, 1968.
Nicholls JG, Purves D. Monosynaptic chemical and electrical connexions
between sensory and motor cells in the central nervous system of the leech.
J Physiol 209: 647–667, 1970.
Nusbaum MP, Friesen WO, Kristan WB Jr, Pearce RA. Neural mecha-
nisms generating the leech swimming rhythm: swim-initiator neurons excite
the network of swim oscillator neurons. J Comp Physiol A Sens Neural
Behav Physiol 161: 355–366, 1987.
Olson GC, Krasne FB. The crayfish lateral giants as command neurons for
escape behavior. Brain Res 214: 89–100, 1981.
Palani D, Baginskas A, Raastad M. Bursts and hyperexcitability in non-
myelinated axons of the rat hippocampus. Neuroscience 167: 1004–1013,
Pearce RA, Friesen WO. Intersegmental coordination of leech swimming:
comparison of in situ and isolated nerve cord activity with body wall
movement. Brain Res 299: 363–366, 1984.
Puhl JG, Mesce KA. Keeping it together: mechanisms of intersegmental
coordination for a flexible locomotor behavior. J Neurosci 30: 2373–2383,
Ritzmann RE, Eaton RC. Neural substrates for initiation of startle responses.
In: Neurons, Networks, and Motor Behavior, edited by Stein PS, Grillner S,
Selverston AI, Stuart DG. Cambridge, MA: MIT Press, 1998, p. 33–44.
Sahley CL, Modney BK, Boulis NM, Muller KJ. The S cell: an interneuron
essential for sensitization and full dishabituation of leech shortening. J
Neurosci 14: 6715–6721, 1994.
Sawyer RT. Ecology of freshwater leeches. In: Leech Biology and Behaviour.
Oxford, UK: Clarendon Press, 1986, p. 524–590.
Shaw BK, Kristan WB Jr. The whole-body shortening reflex of the medicinal
leech: motor pattern, sensory basis, and interneuronal pathways. J Comp
Physiol A Sens Neural Behav Physiol 177: 667–681, 1995.
Sillar KT. Mauthner cells. Curr Biol 19: R353–R355, 2009.
Thompson WJ, Stent GS. Neuronal control of heartbeat in the medicinal
leech. I. Generation of the vascular constriction rhythm by heart motor
neurons. J Comp Physiol 111: 261–279, 1976.
Thorogood MS, Brodfuehrer PD. The role of glutamate in swim initiation in
the medicinal leech. Invert Neurosci 1: 223–233, 1995.
Viana Di Prisco G, Pearlstein E, Le Ray D, Robitaille R, Dubuc R. A
cellular mechanism for the transformation of a sensory input into a motor
command. J Neurosci 20: 8169–8176, 2000.
Viana Di Prisco G, Pearlstein E, Robitaille R, Dubuc R. Role of sensory-
evoked NMDA plateau potentials in the initiation of locomotion. Science
278: 1122–1125, 1997.
Watanabe A, Grundfest H. Impulse propagation at the septal and commis-
sural junctions of crayfish lateral giant axons. J Gen Physiol 45: 267–308,
Weeks JC. Segmental specialization of a leech swim-initiating interneuron,
cell 205. J Neurosci 2: 972–985, 1982a.
J Neurophysiol • VOL 105 • JANUARY 2011 • www.jn.org
Weeks JC. Synaptic basis of swim-initiation in the leech. II. A pattern-generating
neuron (cell 208) which mediates motor effects of swim-initiating neurons. J
Comp Physiol A Sens Neural Behav Physiol 148: 265–279, 1982b.
Weeks JC, Kristan WB Jr. Initiation, maintenance and modulation of
swimming in the medicinal leech by the activity of a single neurone. J Exp
Biol 77: 71–88, 1978.
Wiersma CA. Giant nerve fiber system of the crayfish: a contribution to
comparative physiology of synapse. J Neurophysiol 10: 23–38, 1947.
Wiersma CA, Bush BMH. Functional neuronal connections between the
thoracic and abdominal cords of the crayfish, Procambarus clarkii (Girard).
J Comp Neurol 121: 207–235, 1963.
Wiersma CA, Ikeda K. Interneurons commanding swimmeret movements in
the crayfish, Procambarus clarkii (Girard). Comp Biochem Physiol 12:
Wiersma CA, Mill PJ. “Descending” neuronal units in the commissure of the
crayfish central nervous system, and their integration of visual, tactile and
proprioceptive stimuli. J Comp Neurol 125: 67–94, 1965.
Wiersma CAG, Hughes GM. On the functional anatomy of neuronal units in
the abdominal cord of the crayfish, Procambarus clarkii (Girard). J Comp
Neurol 116: 209–228, 1961.
Wilkinson JM, Coggeshall RE. Axonal numbers and sizes in the connectives
and peripheral nerves of the leech. J Comp Neurol 162: 387–396, 1975.
Willard AL. Effects of serotonin on the generation of the motor program for
swimming by the medicinal leech. J Neurosci 1: 936–944, 1981.
Yu X, Nguyen B, Friesen WO. Sensory feedback can coordinate the swim-
ming activity of the leech. J Neurosci 19: 4634–4643, 1999.
Zottoli SJ. Correlation of the startle reflex and Mauthner cell auditory
responses in unrestrained goldfish. J Exp Biol 66: 243–254, 1977.
144 O. J. MULLINS, J. T. HACKETT, AND W. O. FRIESEN
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