Today's video is on central pattern
generators
or CPG. After watching this video
you should be able to do the following:
first,
you should be able to describe the
characteristics
central pattern generator or CPG; and
second, you should be able to design an
experiment to determine which of the two
main types of
CPGs an experimental animal has. In
the last lecture we talked about how a
person with a spinal cord injury
can no longer have voluntary movement. It
turns out that if you take an
animal's spinal cord injury like a
cat or rat,
in some certain circumstances it can
still walk,
although it does not have voluntary management. If you take your spinally transected
animal
and partially support it like with a
harness
on the belly and you put it on a treadmill. You can start the treadmill
and the animal will start walking. You
may be wondering how this spinalized
animal can walk. It does not have any
descending drive from those upper motor
neurons to initiate behavior.
But it turns out that the spinal
circuits left
have all the neurons necessary to start
a rhythmic
walking behavior. This group cells is
term the locomotor central pattern
generator
or CPG. [However] you do need something to get the central pattern generators started,
so it turns out sensory input from the
moving treadmill can get it
started in some animals, like cats or rats. There are many different types a central
pattern generators or CPG
that mediate a variety of rhythmic motor
movements,
for instance, chewing, breathing
and swimming, if your fish. You can also
record
what's called fictive locomotion in an
isolated spinal cord.
You first have neuromodulators like
NMDA
or 5-HT to get the fictive locomotion
started
just like you need to move the treadmill
to get the locomotion started in this
spinalized animal.
So if you take out your spinal cord
and then place electrodes on the L2
lumbar 2 lumbar 5 right and left
ventral roots, so those contain the
motor neuron axons.
You can report alternating behavior,
shown here.
So the burst are activities or
activation of the motor neurons in that
root, In the L2 route contain
the flexor motor neurons, and the L5
you have the extensors. So if you look
just on the right side, the flexors
and extensors alternate back and forth.
Similarly, that alternation occurs
between the left L2 flexor and extensors
and you also have alternation
between the right L2
and left L2. So that back and forth
alternation is
similar to what you would see if you
recorded from muscles,
the flexors and extensors on both sides of the
body when animal was walking.
These
isolated spinal cord experiments provide
evidence that there's a spinal cord
locomotor CPG
surprisingly though, we do not know exactly
which cells make up this locomotor CPG.
We know that that lumbar and thoracic
spinal cord are necessary,
but no one has been able to pinpoint the
exact cells
that make the locomotor CPG. Figuring out
how the locomotor CPG works
is a very active area research due to
the many clinical implications
of treating patients with spinal cord
injuries. A few features a CpG control
locomotion are well-known it's known
that supraspinal or brain
structures are not necessary for the
basic motor pattern.
The rhythm is produced by neuronal
circuits that are contained entirely
within the spinal cord.
Like I said it's in the lumbar and thoracic portion of spinal cord.
These spinal circuits can be activated by
tactonic
descending signals from the brain, i.e., the
start of
locomotion can be signaled by the brain,
but those decending signals
don't contain the rhythmic pattern that
they evoking the spinal cord.
And finally, spinal CPG networks don't
require sensory input
but are strongly regulated by that input,
so sensory information from your propriocepters
can shape the locomotor behavior.
While sensory feedback is not necessary
for the locomotor pattern to be
generated as we saw that isolated spinal
cord preparation.
It does play a very important role in
normal locomotion
sensory feedback controls the timing of
the different phases in the step cycle.
it shapes the pattern of muscle activities
within a step cycle
through reflex pathways to motor neurons.
Sensory feedback contributes to excitatory
drive of the motor neurons.
And sensory feedback also contributes to
long-term adaptation
of locomotion, especially in development.
since you get better and better
with your locomotor pattern as you
get older. Compare your walking
pattern to that
have a 2-year-old.There are two major forms
of rhythm generation, the first involves
pacemaker neurons.
Pacemaker neurons like the red neuron
shown here,
have in endogenous bursting activity, they
fire
and then stopped firing.This endogenous
bursting activity is due to the membrane
properties of the neuron.
If you couple an endogenous burster neuron
like this red neuron with a green tonically active neuron,
you can get rhythm generation. As you
when the red neuron is on the green neuron
is off.
When the gree neuron is on,
the red neuron is off. An example of this
type
of rhythm generator can be found in the
vertebrate respiratory
central pattern generator. One of the
options for your swimmey CPG
is an endogenous burster. The circuit is
shown in your SWIMMY lab manual
and reproduced here. In the endogenous burster CPG,
you have an endogenously bursting cell here,
X, so even if you only record from
X you'll see this bursting on and off 
activity. If you hook up
X, an excitatory cell with A, just
normal neuron.
You can get A to fire at the same time that
X does.
If you then excite an excitatory
interneuron
on tonically active Cell B, so if you
had no
inhibition and B was just on its own,
you'd always have firing
in B. Now you have inhibition
when X is on, so there's no firing in B
when X turns off
Then you get your firing. If A and B
are your motor neurons now you can get
your alternating
rhythmic locomotor pattern. The other form
of rhythm generation involves emergent, synaptic interaction
based rhythms. So if you have two
neurons here
red and green that fire non-rhythmically
in isolation.
If you hook them up with reciprocal inhibition, they each have an inhibitory
Synaptic connection on each other, you
can start to get this
alternating on and off behavior between
the two neurons.
This type of rhythmic generation is used
in a CPG that controls the heart beat.
This form of rhythm generation is used
in one in your SWIMMY CPG options.
The mutually depressing inhibition oscillator. This type of ocillator is shown
below. If you have 2 tonically active
cells
X & Y, you can hook them up
with an excitatory synapse onto motor
neurons A and B
and you can also have an excitatory
synapse from
X & Y onto inhibitory interneurons which
synapse onto the opposite cell.
So X excites an inhibitory interneuron,
which inhibits Y, and vice versa. You
can get
oscillating behavior if you have synaptic depression
at the synapse between Y and its
inhibitory interneuron
and X and its inhibitory interneuron.
So,
if X is firing, Y is going to be inhibited
until there's enough synaptic depression 
at the interneuron
between X & Y, so once it
the syntactic depression occurs then the
inhibition on Y will be removed
and then Y can start firing and then Y can fire
until the synaptic depression between it
and its interneuron is enough
that it removes the inhibition on X and
then X starts to fire again.
Since X is hooked up to motor neuron A,
A will fire when X fires, Y is hooked up to
motor neuron B, so B will fire
when y is firing and you can get this
alternating
locomotor behavior.
That concludes our video on central
pattern generators.
Hopefully by now your able to describe the
characteristics of a
central pattern generator
and you're also able to design an
experiment to determine which of the two
main types of CPG's
an experimental animal has, in fact
you'll be doing this with the SWIMMY lab.
