| Electrophysiological
Study of Spinobulbar Neuron Distribution in Lampreys
Wai
Hong Leong
Marquette
University
Milwaukee,
WI
Faculty
Mentor: Dr. James Buchanan
Locomotor
behaviors, such as swimming, flying, and walking, have been studied in
many animals to understand the neurobiological basis of rhythmic motor
activity. The basic motor pattern for locomotion is generated by centrally
located neuronal circuits, known as a central pattern generators (CPG).
However, the basic pattern produced by the CPG is usually modified by sensory
information from peripheral receptors and signals from other regions of
the central nervous system, such as reticulospinal cells. Ascending spinal
neurons that terminate in the brain stem (ie, spinobulbar cells) transmit
sensory and motor information about the environment and the internal state
of the spinal cord to the brain in order to modulate the activity of several
groups of descending brain neurons, which in turn regulate the locomotor
CPG.
Previous
anatomical studies of the spinobulbar cells in lampreys have shown that
the majority of these neurons are located in the rostral spinal cord. The
present study was aimed at determining whether spinobulbar neurons activated
during locomotor activity have a similar distribution in the spinal cord.
Our hypothesis is that the functional distribution of spinobulbar cells
is similar to the anatomical distribution.
Lamprey,
the most primitive living vertebrate, is used as the experimental model
in this study due the relative simplicity of its nervous system compared
to higher vertebrates and the capability of the isolated spinal cord to
remain viable for several days in cold physiological solution. Spinal cord
activity was examined in an in vitro preparation and the spinal locomotor
networks were activated by applying 0.5 mM D-glutamate to three spinal
pools separately: rostral, middle and caudal, each consisting of about
16 spinal segments. The rhythmic activity induced in the spinal cord by
D-glutamate is referred to as “fictive swimming”. The spiking frequency
and spiking activity amplitude of spinobulbar cell axons recorded at the
rostral end of the cord were measured and analyzed.
In
all five preparations, the spiking frequency and spiking amplitude of the
cord were found to be highest when fictive swimming was induced in the
rostral spinal pool, followed by the middle pool and lastly the caudal
pool. In addition, we found that the spiking activity is often rhythmic
indicating that the ascending signal is composed in part of locomotor signals
from the CPG. However, the rhythmic activity in the cord was relatively
weak compared to ventral roots. This could possibly due to the smearing
effect of rhythmic bursting activity coming from a number of segments and
the overlap of ipsilateral and contralateral signals, which are active
in an alternating pattern.
Our
functional distribution matches closely a previous anatomical study in
which the spinolbulbar cells were found to be mostly distributed in the
rostral spinal cord. This leads us to suggest that the rostral compartment
of the spinal cord is more important in conveying information to the brain
about the internal state of the network activity during locomotion. |
