Neurologic Control of Ventilation
I The following areas located throughout the body each play a specific role in the neurologic control of ventilation. Two general regulatory mechanisms exist: automatic or involuntary control and voluntary or conscious control.
A Located within the medulla oblongata is the respiratory control center, which receives afferent impulses from all other areas in the body (Figure 6-1).
B Afferent impulses are interpreted, and efferent impulses are initiated in the medulla oblongata.
C The medullary respiratory center maintains the normal rhythmic pattern of ventilation.
D The medullary respiratory center is located in the brain stem along with the pons and connects the midbrain and cerebellum with the spinal cord.
E Two fairly distinct areas in the medulla contain respiratory neurons (Figure 6-2).
1. The dorsal respiratory group is located in two elongated bundles of neurons along the lateral walls of the medulla referred to as the nucleus tractus solitarius (NTS).
a. Functions as initial processing centers of afferent impulses.
b. Origin of inspiratory efferent impulses, which travel to ventral respiratory group neurons and spinal cord.
c. The basic rhythm of respiration is generated by the dorsal respiratory group of neurons. Rhythmic ventilatory impulses are generated even when all peripheral nerves entering the medulla have been severed.
d. Inspiration is normally (except during stressed breathing) a ramp signal, increasing steadily in force for approximately 2 seconds.
e. Inspiration then ceases for approximately 3 seconds.
f. The ramp signal is controlled in two ways:
g. If the ramp signal ceases early the length of expiration is also decreased.
h. As a result, the dorsal respiratory group is the primary controller of the depth and rate of inspiration.
2. The ventral respiratory group is located approximately 5 mm anterior and lateral to each dorsal respiratory group. These neurons are located in the nucleus ambiguus anteriorly and the nucleus retroambiguus caudally.
F Areas from which afferent impulses are sent to the medulla oblongata:
A Two distinct centers in the pons contain afferent respiratory neurons.
1. The pneumotaxic center is located dorsally in the nucleus parabrachialis of the upper pons (see Figure 6-2).
a. Afferent impulses from the pneumotaxic center “fine tune” ventilatory rhythmicity by inhibiting length of inspiration.
b. Maximum stimulation in adults can limit inspiration to 0.5 seconds, increasing rate to ≥40 breaths/min.
c. A weak signal reduces respiratory rate to only 3 to 5 breaths/min.
d. If the pneumotaxic center is destroyed, apneustic breathing (long, sustained inspirations) occurs.
2. Apneustic center: Only weak evidence of its existence is available (see Figure 6-2).
b. Afferent impulses from the apneustic center cause a sustained inspiratory pattern with only short expiratory times; apneustic breathing.
c. How and when these neurons are activated is unclear; the vagus nerve must be impaired for this center to be active.
d. If the apneustic and pneumotaxic centers are destroyed, a rapid, irregular, gasping respiratory pattern develops.
A Axons from the higher brain centers descend into the spinal cord.
B These projecting axons influence phrenic intercostal and abdominal motoneuron transmission.
C Thus ventilatory skeletal muscle is stimulated.
D Skeletal muscle is composed of two types of contractile fibers:
1. Extrafusal fibers (main muscle): Contraction of these fibers is responsible for actual muscular contraction.
2. Fusimotor fibers (muscle spindle fibers): These fibers are organs of proprioception that determine the extent of muscle contraction necessary to perform a certain workload.
E Ventilatory reflexes, such as cough and hiccup, are mediated by the spinal cord.
F Ascending spinal pathways transmit sensations of pain, touch, temperature, and proprioception to higher brain centers.
1. Stimulation of nasal mucosa may cause exhalation.
2. Stimulated exhalation is frequently in the form of a sneeze.
3. Apnea and bradycardia also may result from nasal stimulation.
1. Stimulation may cause the sniff or aspiration reflex.
2. A rapid inspiration is initiated to move the irritant from the nasopharynx to the oropharynx.
3. Stimulation may also cause bronchodilation and hypertension.
A Afferent impulses via the vagus nerve originate from two areas:
a. Located in the aortic arch.
b. Stimulated by variation in blood pressure.
c. Afferent impulses from baroreceptors cause alteration of vascular tone to maintain normal blood pressure levels.
d. Ventilatory response is minimal.
a. Pulmonary stretch receptors (Hering-Breuer reflex)
(1) Pulmonary stretch receptors are located in the smooth muscle of conducting airways.
(2) Lung inflation and increased transpulmonary pressures stimulate these receptors.
(3) They are slowly adaptive to changes in inflating pressure but do not appear to be active in humans until tidal volume exceeds 1.5 L.
(4) Stimulation of these receptors may cause:
