YAWNING
I. Yawning Behavior
A. a paroxysm of the respiratory cycle
1. sudden attack/expression of inhalation
2. lasts 5-10 s
a. duration is not fixed
i. appears to be fixed
ii. can be modulated voluntarily
B. Standard cascade of movements: 3 distinct phases
1. Initiation: opening the mouth widely
a. allows for slow, deep inspiration
i. almost entirely through the mouth
b. accompanied by expansion of the pharynx / larynx & thorax
c. & vocal cord abduction
2. Acme Phase
a. thorax reaches maximum capacity
i. followed by a brief interruption of inspiration
b. eyes close
i. limb stretching
a) yawning + stretching = Pandiculation
c. generalized stretching of respiratory tract muscles
i. diaphragm, intercostals, face and neck
3. Conclusion: short expiration and relaxation
II. Respiratory Cycle
A. Regulated by brain stem chemosensitive area
1. Medulla oblongata and pons
2. Sensitive to D in blood PCO2 or [H+]
B. Chemosensory neurons excite respiratory premotor centers
1. Inhalation: Nucleus of solitary tract (NST = Dorsal respiratory group)
a. signal begins weakly, increases for 2-3 s
b. NST stimulates motor neurons
i. Üphrenic (C3-C5) & intercostal (T1-T11) nerves
ii. Üdiaphragm + intercostal muscles
iii. Ramp signal Ü steady ñ thoracic volume
a) sensory feedback inhibits diaphragm excitation
c. elastic recoil Ü expiration
2. Forced Exhalation
a. Nucleus ambiguus (Ventral respiratory group)
3. Pontine pneumotaxic & apneustic centers
a. rate + coordination of inhalation and exhalation
i. stimulate NST
C. Yawning caused by a transient ñ sympathetic arousal
1. not deep inhalation
III. Evolution of/Purpose for Yawning