B. Medulla (Respiratory Center) ("pacemaker"
for ventilation)
1. anatomy: two groups of cells
a. dorsal group: mainly inspiratory (I)
neurons
b. ventral group: both inspiratory (I) and
expiratory (E) neurons
2. necessary for respiration: destruction leads to immediate
and permanent cessation of automatic respiration
3. sufficient for respiration: even when isolated from other
neural inputs (by section of higher structures and vagus nerve), can support
a relatively normal pattern of ventilation
4. most cells in region show a discharge pattern synchronous
with either inspiration ("I" neurons) or expiration ("E"
neurons), although some cells discharge in during both inspiration and expiration
("I-E" neurons)
5. inspiratory "I" neurons control the descending
spinal cord pathways to the motor neurons innervating the muscles of inspiration,
and expiratory neurons "E" control the descending spinal cord pathways
to the motor neurons innervating the muscles of expiration (note:
these are somatic motor nerves, not autonomic nerves)
(Note: expiratory muscles are active during forced expiration,
not during quiet breathing)
7) the inspiratory and expiratory neurons exhibit reciprocal
innervation (mutually inhibitory), so not generally
active at the same time
8) deficits of the isolated respiratory center in controlling
ventilation include
- ataxic (irregular) breathing
- poor adjustment to varying body need
