Biomedical Engineering Reference
In-Depth Information
INTRODUCTION
bACkGROUND
Repetitive synaptic activity can induce persistent
modification of synaptic efficacy in many brain
regions in the form of long-term potentiation
(LTP) and long-term depression (LTD). Such
synaptic plasticity provides a cellular mechanism
for experience dependent refinement of develop-
ing neural circuits and for learning and memory
functions of the mature brain. The precise timing
of presynaptic and postsynaptic spiking is often
critical for determining whether an excitatory
synapse undergoes LTP or LTD.
This chapter proposes a new mathematical
analysis to study the temporal structure of neu-
ronal responses to sensory stimulation. We have
applied the coherence on the wavelet based ap-
proach for quantification of the functional stimulus
- neural response coupling and its modulation
when two tactile stimuli appear simultaneously.
This analysis reveals that modulation of sensory
responses may imply an increase/decrease in the
number of spikes elicits by a sensory stimulus and
an increase/decrease in the temporal coherence of
evoked spikes with the stimulus onset, as well.
Recent electrophysiological studies indicate
the existence of an important somatosensory
processing in the trigeminal nucleus which is
modulated by the corticofugal projection from
the somatosensory cortex. The somatosensory
cortex may enhance relevant stimulus. Also, it
may decrease sensory responses in the trigeminal
nuclei when a novel (distracter) stimulus is ap-
plied. We interpret this decrease of the response
as sensory-interference. The objective of the
present chapter is to demonstrate that sensory
interaction may occur in the first relay station of
the trigeminal somatosensory pathway changing
the number of spikes evoked by a tactile stimulus
and temporal coherence with the stimulus onset.
Data suggest the existence of an attentional filter
at this early stage of sensory processing.
Somatosensory information coming from the face
(including the mouth and the cornea) is collected,
processed and finally sent to the thalamus by the
trigeminal complex. For experimental study of
the mechanism of information representation
and processing the vibrissae sensory system of
rodents is one of the most used models since it
is particularly well organized and structured.
Indeed, the large mystacial vibrissae of the rat
are arranged in a characteristic, highly conserved
array of five rows and up to seven arcs (Brecht
et al., 1997; Welker, 1971). Rats use these facial
whiskers to perform a variety of tactile discrimi-
native tasks and behaviors (Carvell & Simons,
1990; Gustafson & Felbain-Keramidas, 1977).
Sensory information from the vibrissae arrives
to the trigeminal complex, which is organized in
three sensory and one motor nuclei. The sensory
trigeminal nuclei include: the principal nucleus
(Pr5), the spinal nucleus (Sp5) and the mesence-
phalic nucleus (Me5). In turn Sp5 is divided into
three subnuclei called oralis (Sp5O), interpolaris
(Sp5I) and caudalis (Sp5C). In the trigeminal
complex primary afferents and neurons form
the “barrelettes”, which replicate the patterned
arrangement of the whisker follicles on the snout
(Ma, 1991).
Three classes of morphologically and physi-
ologically distinguishable neurons reside in the rat
trigeminal nucleus: barrelette cells, interbarrelette
cells, and GABAergic or glycinergic inhibitory
interneurons (Ressot et al., 2001; Viggiano et
al., 2004).
The Pr5 and Sp5 trigeminal nuclei are obliga-
tory synaptic relays for somatic sensory informa-
tion originated in the large mystacial vibrissae or
“whiskers'' on one side of the face to the contra-
lateral ventral posterior medial (VPm) nucleus
of the thalamus (Peschanski, 1984; Smith, 1973).
Pr5 projection neurons are characteristically de-
scribed as having single-whisker receptive fields
(RFs), whereas the rest of the population has RFs
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