N1 motor neurons
backfilled with cobalt.
(Mulloney & Hall,
J Comp Neurol. 2000) Click to enlarge.
Cross
section of N1 showing bundles of motor and sensory
axons.
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View the videos Crayfish Swimming and Swimmeret
Movements to see the rhythmic movements of the swimmerets.
Listen to a
recording from the first roots (N1)
of the third (A3, top trace) and fourth (A4, bottom
trace) abdominal ganglia after activation of the
swimmeret motor pattern by 50 µM carbachol.
You will hear bursts of action potentials in the
axons of swimmeret motor neurons.
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ANATOMY: The crayfish
swimmeret central pattern generator is an example of a
network of neurons that creates a behavioral pattern, the
rhythmic movement of the swimmerets. (In an isolated nerve
cord, the pattern is sometimes referred to as "fictive
swimming" since the muscles and appendages are no longer
attached.) Four abdominal ganglia (A2-A5) contain left and
right clusters of about 60 motor neurons each. Each
cluster is located near the base of the first root,
N1, in a region called the lateral neuropil (LN), as
shown in the micrograph at the left and the diagram below.
The motor neuron axons leave the ganglion in the first root,
which splits into an anterior branch that goes to the
swimmeret's return-stroke muscles (RS), and a
posterior branch to the power-stroke muscles (PS).
Recording from the two branches of N1 reveals alternating
bursts of spikes in the axons of the return-stroke and
power-stroke motor neurons. These bursts activate muscle
contractions, producing alternating forward and backward
movements of the swimmerets.
An abdominal ganglion,
showing the lateral neuropil (LN) where the
swimmeret motor neurons are located, the first
roots (N1) carrying axons from the motor neurons, and
examples of extracellular recordings from the
return-stroke (RS) and power-stroke (PS) branches of the
first root. Source: Mulloney B, Skinner FK, Namba H, Hall
WM (1998) Intersegmental coordination of swimmeret
movements: mathematical models and neural circuits.
Ann N Y Acad Sci 860:266-80.
The excitatory motor neurons release glutamate at
the muscle cells, causing them to depolarize and contract.
There are about 30 excitatory motor neurons in each RS and
each PS group. Like other crustacean muscles, the swimmeret
muscles also receive axons from two or three large
inhibitory motor neurons. These neurons
release GABA at the muscle cells, opposing depolarization
and reducing contraction. When the excitors to one class of
muscles are active, often the inhibitors to the antagonist
muscle are also active.
CENTRAL PATTERN
GENERATOR: The RS and PS motor neurons in each LN
cluster are driven by four non-spiking local
interneurons to form a pattern-generating module.
The modules on each side of a ganglion and in adjacent
ganglia are linked to each other by coordinating neurons. In
addition, five command interneurons run the length of the
nerve cord and send branches to activate the swimmeret
modules.
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A pattern-generating
module. This example is
from the left side of the third ganglion (A3).
RS3 and PS3: Clusters of motor neurons
innervating return-stroke and power-stroke
muscles.
1, 2: Local interneurons
(two of each type) that inhibit each other and one
cluster of motor neurons each, establishing the
central pattern. They also excite coordinating
interneurons.
ASC, DSC: Ascending and descending
coordinating interneurons that carry timing
information to adjacent ganglia.
C1: Commissural interneuron
receiving excitation from adjacent ganglia and
inhibiting CPG neurons to convey timing.
Command neurons: five
neurons that synapse with each module to activate
swimming.
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Four pattern-generating
modules on the left side of the
abdominal nerve cord, showing the coordinating
neurons that establish a phase relationship between
adjacent swimmerets, and the command neurons that
activate swimming.
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Modified from: Smarandache, C,
Hall, WM, and Mulloney, B (2009) Coordination of
rhythmic motor activity by gradients of synaptic
strength in a neural circuit that couples modular
neural oscillators. J. Neuroscience 29:
9351-9360, Fig 9.
Enlarge
this diagram.
See
another diagram of the pattern generating
module. (Adapted from
Mulloney, B (2003) During fictive locomotion,
graded synaptic currents drive bursts of impulses
in swimmeret motor neurons. J Neuroscience
23(13): 5953-5962, Figure 10.)
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In each pattern generating module, the two pairs of
non-spiking local interneurons (1 and 2) inhibit
their respective clusters of motor neurons and also each
other. The circuit is an example of reciprocal
inhibition, which guarantees that only one pair of local
interneurons (1 or 2) and thus one set of motor
neurons (RS or PS) will be active at one time. This
establishes the pattern of alternating bursts of RS and PS
spikes, leading to alternating forward and backward
swimmeret movements. Interestingly, it is not yet known
which neurons excite the motor neurons, since only
inhibitory connections to motor neurons have been discovered
from the local interneurons.
The system lends itself to pharmacological intervention.
Acetylcholine (and its analog, carbachol) turn on the
swimming CPG, as does the peptide proctolin.
Proctolin, a five-amino-acid peptide, is known to be
contained in three of the five pairs of command
neurons that activate the swimmeret system, so it is
plausible that applying proctolin to a ganglion mimics the
release of this transmitter from command neurons to activate
the CPG system. It is also known that acetylcholine directly
depolarizes RS and PS motor neurons by acting on muscarinic
receptors. However, it is not yet clear what the source of
that acetylcholine might be. At the bottom left of the
figure, sensory feedback from stretch receptors and
other sensory neurons is shown connecting (in an undefined
way) to the motor neurons and local interneurons. It is
likely that at least some of these sensory neurons release
ACh as their transmitter, but this is unlikely to be the
sole source of cholinergic excitation of the CPG
circuits..
COORDINATION: The
local interneurons also excite coordinating
interneurons that fire bursts of spikes simultaneously
with the return stroke (DSC, descending to the next
posterior ganglion) or the power stroke (ASC,
ascending to the next anterior ganglion). In the target
ganglia, the ASC and DSC axons synapse on non-spiking
commisural interneurons (C1) that convey timing
information to the CPG module.
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Coordinating interneurons.
A Diagram
emphasizing that four abdominal ganglia (A2-A5)
each contain independent left and right modules for
generating the swimmeret pattern.
B Each module
sends ascending and descending axons to adjacent
ganglia. Only modules from the left side of the
nerve cord are shown.
C A filled example
of a descending (DSC) interneuron, viewed from the
dorsal surface of the ganglion. The neuron's cell
body is at the top (anterior); its neurite branches
extensively in the lateral neuropil (center left),
and its axon leaves in the connective to the next
ganglion (posterior, bottom).
D The CPG module
from the left side of ganglion 4, showing
coordinating neurons (ASC4, DSC4) sending axons to
adjacent ganglia, and a commissural interneuron
(C1) receiving excitatory synapses from adjacent
ganglia.
Modified from: Smarandache, C,
Hall, WM, and Mulloney, B (2009) Coordination of
rhythmic motor activity by gradients of synaptic
strength in a neural circuit that couples modular
neural oscillators. J. Neuroscience 29:
9351-9360, Fig 2.
See
the original version of this figure and its
caption.
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In experiments, the motor pattern can be recorded from
the first roots of abdominal ganglia A2-A5 using suction or
pin electrodes. Adding the acetylcholine mimic
carbachol or the peptide proctolin to the bath
can activate the CPG pattern. It is not yet certain where
those drugs act on the swimmeret system, although (as
mentioned) some command neurons contain proctolin, and motor
neurons (and probably some interneurons) respond to
acetylcholine.
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