Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 310

Like atrial pacemaker myocytes, cardiac Purkinje cells are devoid of transverse
tubules, but have a much larger width. They possess a specialized triplicate layer
of inositol (1,4,5)-trisphosphate-sensitive Ca 2 + channels under the sarcolemma
juxtaposed to a layer (

m) of sarcoplasmic reticulum RyR3 channels, whereas
RyR2 channels lodge in the inner cell region.
In canine Purkinje cells, 3 types of Ca 2 + transients are observed [ 595 ]: (1)
non-propagating Ca 2 + transients that originate directly under the sarcolemma and
lead to (2) small Ca 2 + wavelets in a subsarcolemmal region (thickness 6

∼ 5

m)
that cause (3) large Ca 2 + waves that travel to sarcomeres. Purkinje cells contain
aCa 2 + release structure that translates Ca 2 + transients under the sarcolemma
into rapid Ca 2 + release in the subsarcolemmal region, itself initiating a large-
amplitude long-lasting Ca 2 + liberation. Three layers of Ca 2 + channels actually
exist: (1) channels associated with type-1 IP 3 (IP 3 R1) and type-3 ryanodine
receptors (RyR3) directly under the sarcolemma; (2) RyR3 receptors are dominant
Ca 2 + release channel in a subsarcolemmal layer (between 2 and 4

mbelowthe
sarcolemma); and (3) RyR2 receptors are observed throughout the cell, but not in the
subsarcolemmal layer (between 2 and 4 m below the sarcolemma). The threshold

of RyR3 receptors that cause propagating wavelets transients is lower than that of
RyR2 receptors responsible for large Ca 2 + waves.

6.2.5.8 Innervation of the Nodal Tissue

Both the sinoatrial and the atrioventricular nodes are supplied with sympathetic and
parasympathetic postganglionic fibers from the intrinsic cardiac ganglia. Parasym-
pathetic nerve terminals are more numerous in the atrioventricular node. When
stimulated, the sympathetic and parasympathetic fibers release noradrenaline (NAd)
and acetylcholine (ACh), respectively.
On the other hand, the main bundle and bundle branches lack parasympathetic
nerve endings in some mammalian species such as dogs, but not monkeys [ 596 ].
Purkinje fibers receive a sympathetic input. Terminal axons run in the space between
the Purkinje fibers and the fibrocytic envelope, either naked or accompanied
by Schwann cell processes. The width of the synaptic cleft between an axonal
varicosity and Purkinje cell is about 60 nm.

6.3 Cardiac Regulation of Energy Homeostasis

The liver, adipose tissue, and skeletal muscle are major regulatory organs and tissues
of body's energy expenditure and metabolism, especially of carbohydrates and
lipids. The heart that has the highest rate of oxygen consumption per gram of tissue,
participates to the control of energy homeostasis locally as well as remotely possibly
via cardiac-derived endocrine factors either acting directly or via the central nervous
system.