Functional Properties of Single Motor Units in Inferior Head of Human Lateral Pterygoid Muscle: Task Relations and Thresholds

• Published in: Journal of neurophysiology Vol. 86; no. 5; pp. 2204 - 2218
• Main Authors: Phanachet, I, Whittle, T, Wanigaratne, K, Murray, G. M
• Format: Journal Article
• Language: English
• Published: United States Am Phys Soc 01-11-2001
• Subjects: Adult; Differential Threshold; Electromyography; Female; Humans; Jaw - physiology; Male; Motor Neurons - physiology; Movement - physiology; Muscle Fibers, Skeletal - diagnostic imaging; Muscle Fibers, Skeletal - physiology; pterygoid muscle; Pterygoid Muscles - diagnostic imaging; Pterygoid Muscles - physiology; Recruitment, Neurophysiological - physiology; Tomography, X-Ray Computed
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.2001.86.5.2204

Jaw Function and Orofacial Pain Research Unit, Faculty of
Dentistry, University of Sydney, Westmead Centre for Oral Health,
Westmead Hospital, Westmead, NSW 2145, Australia
Phanachet, I.,
T. Whittle,
K. Wanigaratne, and
G. M. Murray.
Functional Properties of Single Motor Units in Inferior Head of
Human Lateral Pterygoid Muscle: Task Relations and Thresholds. J. Neurophysiol. 86: 2204-2218, 2001. The
aim of this study was to clarify the normal function of the
inferior head of the human lateral pterygoid muscle (IHLP). The
hypothesis was that an important function of the IHLP is in the fine
control of horizontal jaw movements. The activities of 99 single motor
units (SMUs) were recorded from IHLP (22 recordings from 16 subjects).
Most recording sites were identified by computer tomography (CT). All
99 SMUs were active during contralateral jaw movements with the teeth
apart, and protrusive jaw movements with the teeth apart, and 81% (48 of 59 units studied during all 3 tasks) were active during submaximal
jaw-opening movements. None were active on maximal ipsilateral or
retrusive jaw movements with the teeth apart nor on jaw
closing/clenching in intercuspal position; nor were they spontaneously
active when the jaw was at the clinically determined postural jaw
position. Thresholds of SMUs ranged from <0.2 mm of contralateral or
protrusive horizontal displacements to 61-89% of the maximum
contralateral or protrusive displacement, respectively. For the 35 units continuously active during the contralateral task, 23 (66%) were
recruited within 2 mm of contralateral displacement [25 (63% of 40 units) for protrusion]. Recruitment thresholds (mm) of some of the
units were rate dependent with thresholds significantly decreasing with
increasing rate of horizontal jaw movement in protrusion and
contralateral movements. At eight recording sites where up to six SMUs
were able to be discriminated, the average thresholds of successively
recruited SMUs were within a 1-mm increment of horizontal jaw
displacement. After dividing IHLP into four regions, the SMUs recorded
in the superior-medial zone exhibited significantly lower mean
threshold values than for the SMUs recorded in the other zones (no
units were recorded in the inferior-lateral zone). This provides
suggestive evidence supporting previously proposed notions of
functional heterogeneity within IHLP. Taken together, the data suggest
that specific regions of the IHLP are capable of selective activation in a finely controlled manner to allow the application of the appropriate force vector (magnitude and direction) to effect the required condylar movement needed for the generation and control of
horizontal jaw movements.