CHAPTER 6 • NEUROHUMORAL CONTROL OF THE HEART AND CIRCULATION
125
the brainstem, the hypothalamus, and the
cortical regions work together to regulate
autonomic
function
(Fig.
6.1).
Regions
within the medulla contain the cell bodies for
the parasympathetic (vagal) and sympathetic
efferent nerves that control the heart and vas-
culature. The hypothalamus (in particular,
the paraventricular nucleus and dorsal medial
nucleus) plays an integrative role by modulat-
ing medullary neuronal activity, for example,
during exercise or when the body needs to
adjust blood flow to the skin to regulate body
temperature. Higher centers, including the
cortex and limbic and midbrain structures,
connect with the hypothalamus and medulla.
The higher centers can alter cardiovascular
function during times of emotional stress
(e.g., caused by fear and anxiety).
The central nervous system receives sensory
(afferent) input from peripheral sensors and from
sensors within the brain. Afferent fibers from
peripheral baroreceptors and chemoreceptors,
as well as respiratory stretch receptors, enter the
medulla at the nucleus tractus solitarius (NTS)
(see Fig. 6.2). Inhibitory interneurons from
cells within the NTS project to other medullary
regions containing cell bodies of sympathetic
nerves. In addition, excitatory interneurons from
Cerebral
Cortex
■ FIGURE 6.1 Regions of the central nervous
system involved in cardiovascular regulation. The
prim ary site of cardiovascular regulation resides
in the medulla; the hypothalamus serves as an
integrative region for coordinating cardiovascular
responses. Higher centers such as the cortex influ-
ence cardiovascular function.
the NTS project to medullary regions containing
cell bodies of parasympathetic (vagal) nerves.
Therefore, increased activity of the NTS enhances
vagal efferent nerve activity and inhibits sym-
pathetic nerve efferent activity. The NTS also
sends fibers to the hypothalamus and receives
input from the hypothalamus. Sensors within
the hypothalamus that monitor blood tempera-
ture (thermoreceptors) send fibers to medullary
regions to modulate sympathetic outflow to the
cutaneous circulation.
PARASYMPATHETIC INNERVATION
The parasympathetic vagal fibers innervating
the heart originate from cell bodies located
within the medulla of the brainstem (see
Figs. 6.1 and 6.2). These cell bodies are found
in collections of neurons called the dorsal
vagal nucleus (DVN) and nucleus ambiguus
(NA).
Increased
activity
of
these
nuclei
reduces sinoatrial (SA) nodal firing (negative
chronotropy) and slows AV nodal conduc-
tion (negative dromotropy). It is important
to note that under normal resting conditions,
these neurons are tonically active, thereby
producing what is termed “vagal tone” on
the heart, resulting in resting heart rates sig-
nificantly below the intrinsic firing rate of the
SA nodal pacemaker. Afferent nerves, particu-
larly from peripheral baroreceptors that enter
the medulla through the NTS, modulate the
activity of these vagal neurons. Excitatory
interneurons from the NTS, which normally
are excited by tonic baroreceptor activity,
stimulate vagal activity.
Efferent vagal fibers (also referred to as
preganglionic fibers) exit the medulla as the
tenth cranial nerve (see Fig. 6.3) and travel
to the heart within the left and right vagus
nerves. Branches from these nerves inner-
vate specific regions within the heart such
as the SA and atrioventricular (AV) nodes,
conduction pathways, atrial myocytes, and
the coronary vasculature. The preganglionic
efferent fibers synapse within or near the tar-
get tissue and form small ganglia, from which
short postganglionic fibers innervate specific
tissue sites. The right vagus is usually the
primary vagal branch that innervates the SA
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