Craniofacial Muscles

236 M.S. Shall
Fig. 13.2 Contractile responses of the postnatal day 14 tongue retractor musculature in a damreared
( left ) and arti fi cially reared ( right ) rat pup. ( a ) Constant frequency stimulation with 200ms
duration trains at 20, 40, 60, and 80 Hz from bottom to top . The dam-reared rat pup’s fusion frequency
= 80 Hz and maximum tetanic tension = 25.91 g. The arti fi cially reared rat pup’s fusion
frequency = 60 Hz and maximum tetanic tension = 25.91 g. ( b ) Fatigue response to stimulation at
50 Hz for 500 ms, 1 train/s for 2 min. The fi rst response is the top trace (showing greater tension)
and the last response is the bottom trace . The fatigue index (ratio of the last response to the fi rst
trace) of the dam-reared pup = 0.81 and the arti fi cially reared rat pup’s fatigue index = 0.55
of the tongue, which triggers the swallow. If the baby is intubated because of other
health issues, and has no opportunity to “practice” nutritive suckling and swallowing,
then these are activities that require learning later, and the baby may develop
speech at a later developmental age or have a persistent speech disorder (Jennische
and Sedin 1998, 1999 ) .
Newborn rats transition from nutritive suckling to chewing in 30 days (Maeda
et al. 1987 ) , during which time the MyHC isoform composition shifts from developmental
to adult fast MyHC (Brozanski et al. 1993 ) . A rat model mimicking perinatal
infants with disrupted suckling was developed in the Goldberg/McClung lab
to study the neuromuscular development in the absence of suckling experience.
Arti fi cial feeding via a gastric cannula from day 4 to postnatal day 14, eliminating
nutritive suckling behavior during the initial postnatal period, caused at least a
short-term alteration of the contractile characteristics of the SG muscle. Relative to
dam-reared animals, the rats had a decrease in fusion frequency and a decrease in
fatigue resistance. A 1-month resumption of dam-rearing and transition to rat chow
was suf fi cient for recovery of the contractile speed and fatigue characteristics of the
SG (Fig. 13.2 ) .
However, in the long-term, there was an increase in MyHCIIA isoform expression
and a decrease in the MyHCIIB isoform expression (Kinirons et al. 2003 ) . This
subtle change in muscle fi ber MyHC isoform expression may partially explain the
subtle changes in the motor control of the tongue. The rat is not deprived of all sensory
sensation during this period but fails to gain the sensory experience of nutrition
in the mouth to fully develop tongue coordination.
Swallowing is a complex sensorimotor function involving the tongue (see Ertekin
and Aydogdu 2003 ) . The initial “oral phase” is accepted as mainly voluntary, though
the duration of the phase may vary depending on hunger, taste, motivation, etc.
Initially, while still in the oral phase, the food is acquired and chewed into small