INTEGRATION OF RESPIRATORY CENTERS
Role of Medullary
Centers
Rhythmic discharge of
inspiratory impulses
Dorsal respiratory group of neurons are responsible for the normal rhythm of respiration. These neurons maintain the normal rhythm of respiration by discharging impulses (action potentials) rhythmically. These impulses are transmitted to respiratory muscles by phrenic and intercostal nerves.
Inspiratory ramp
Inspiratory ramp is the pattern of
impulse discharge from dorsal respiratory group of neurons. These impulses are
characterized by steady increase in amplitude of the action potential. Impulse
discharge from these neurons is not sudden and it is also not uniform.
Inspiratory ramp
signals
To start with, the amplitude of action
potential is low. It is due to the activation of only few neurons. Later, more and
more neurons are activated, leading to gradual increase in the amplitude of
action potential in a ramp fashion. Impulses of this type discharged from
dorsal group of neurons are called inspiratory ramp signals. Ramp signals are
not produced continuously but only for a period of 2 seconds, during which
inspiration occurs. After 2 seconds, ramp signals stop abruptly and do not
appear for another 3 seconds. Switching off the ramp signals causes expiration.
At the end of 3 seconds, inspriatory ramp signals reappear in the same pattern
and the cycle is repeated. Normally, during inspiration, dorsal respiratory
group neurons inhibit expiratory
neurons of ventral group. During expiration, the expiratory neurons
inhibit the dorsal group neurons.
Thus, the medullary respiratory centers control each other.
Significance
of inspiratory ramp signals
Significance of
inspiratory ramp signals is that there is a slow and steady inspiration, so
that the filling of lungs with air is also steady.
Role of Pontine
Centers
Pontine respiratory centers regulate
the medullary centers. Apneustic center accelerates the activity of
dorsal group of neurons and the
stimulation of this center causes prolonged inspiration.
Pneumotaxic center inhibits the
apneustic center and restricts the duration of inspiration.
Pre-Bötzinger Complex
Pre-Bötzinger complex (pre-BötC) is an additional respiratory
center found
in animals. It is formed by a
group of neurons called pacemaker neurons, located in the
ventrolateral part of medulla. Pacemaker neurons generate the rhythmic respiratory
impulses. Medullary centers send nerve fibers into this complex.
Exact functioning
mechanism of this complex is not known.
PERIPHERAL
CHEMORECEPTORS
Peripheral chemoreceptors are the chemoreceptors
present in carotid and aortic region.
Mechanism of Action
Hypoxia is the most potent stimulant
for peripheral chemoreceptors. It is because of the presence of oxygen
sensitive potassium channels in the glomus cells of peripheral chemoreceptors. Hypoxia
causes closure of oxygen sensitive potassium channels and
prevents potassium efflux. This leads to depolarization of glomus cells (receptor potential)
and generation of action potentials in nerve
ending. These impulses pass through
aortic and Hering nerves and excite the dorsal group of neurons. Dorsal group
of neurons in turn, send excitatory impulses to respiratory muscles, resulting
in increased ventilation. This provides enough oxygen and rectifies
the lack of
oxygen.
In addition to hypoxia, peripheral
chemoreceptors are also stimulated by hypercapnea and increased
hydrogen ion concentration. However,
the sensitivity of peripheral chemoreceptors to hypercapnea and
increased hydrogen ion concentration
is mild.
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