CHAPTER 6 • NEUROHUMORAL CONTROL OF THE HEART AND CIRCULATION
135
Other Autonomic Reflexes
Affecting the Heart
and Circulation
In addition to the baroreceptor and chemo-
receptor reflexes already described, several
other reflexes affect cardiovascular function.
1. Ischemic brain reflexes. Insufficient blood
flow to the brain (cerebral ischemia), which
occurs during severe hypotension (a mean
arterial pressure <60 mm Hg), or when
there is cerebral vascular occlusion, causes
intense sympathetic activation and con-
striction of the systemic circulation. Mean
arterial pressure
can rise
to well over
200
mm
Hg
during
severe
cerebral
ischemia. This can be thought of as a final
effort by the body to restore perfusion to the
brain. An increase in intracranial pressure,
which can occur following hemorrhagic
stroke or brain trauma, can cause ischemia
within the brainstem. This elicits a strong,
sympathetic-mediated
pressor
response
(Cushing reflex), often accompanied by
baroreceptor-mediated bradycardia.
2. Pain reflexes. Chest pain associated with
myocardial
ischemia
(insufficient
coro-
nary blood flow) or myocardial infarction
can cause generalized sympathetic acti-
vation, leading to elevated arterial pres-
sure, tachycardia, and increased sweating
(diaphoresis). If cardiac output decreases
significantly
because
of
the
ischemic
injury, arterial pressure may fall despite the
enhanced sympathetic activity. Deep pain
produced by trauma or visceral distension
can produce hypotension (i.e., circulatory
shock) caused by enhanced parasympa-
thetic and decreased sympathetic activity.
Another example of a pain reflex is the
cold pressor response. If a person’s hand
or foot is submerged into ice-cold water,
arterial pressure increases as a result of
sympathetic activation. This test is some-
times used clinically to evaluate autonomic
function and vascular reactivity in patients.
3. Bezold-Jarisch reflex. This reflex is triggered
by stimulation of specific types of chemore-
ceptors within the heart and coronary arteries
and produces bradycardia and hypotension
mediated
by
vagus
nerve
afferents
and
efferents. This reflex is sometimes stimu-
lated when dye or other chemical agents are
injected into coronary arteries during coro-
nary
arteriography.
Ventricular
ischemia,
particularly caused by right coronary artery
occlusion, can also trigger this reflex.
4. Pulmonary and muscle stretch receptors.
Lung inflation activates stretch receptors
located in the airways
and respiratory
muscles
that inhibit medullary sympa-
thetic centers and cause arterial pressure to
fall; heart rate increases reflexively. These
receptors contribute to the normal cyclical
changes in heart rate and arterial pressure
associated with respiratory activity. Limb
muscles and tendons also possess recep-
tors that sense tension and length changes.
Passive or active movement of joints can
stimulate sympathetic activity to the heart
and circulation and help to reinforce car-
diovascular responses to exercise.
5. Temperature reflexes.
Changes in
envi-
ronmental temperature sensed by cold and
warm thermoreceptors in the skin can lead
to reflex changes in cutaneous blood flow
and sweating. Similarly, changes in core tem-
perature, sensed by thermoreceptors located
in the hypothalamus, produce changes in
sympathetic activity to the skin circulation.
For example, a decrease in either skin surface
temperature or hypothalamic blood temper-
ature leads to cutaneous vasoconstriction.
HUMORAL CONTROL
In addition to autonomic nerves, many circulat-
ing factors (humoral substances) affect cardiac
and vascular function. Some of these humoral
factors directly influence the heart and blood ves-
sels, whereas others indirectly alter cardiovascu-
lar function through changes in blood volume.
Major humoral factors include circulating cat-
echolamines, the renin-angiotensin-aldosterone
system, atrial natriuretic peptide, and anti-
diuretic hormone (vasopressin). Although not
addressed in this chapter, note that many other
hormones and circulating substances (e.g., thy-
roxin, estrogen, insulin, and growth hormone)
have direct or indirect cardiovascular effects.
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