The heart is lined with a endothelial layer called the endocardium.
Surrounding the endocardium is a thick layer of cardiac muscle refered to as myocardium.
The heart is covered by a thin tissue layer refered to as epicardium.
The heart muscles are supplied with blood via the coronary vessels.
Primitively, the vertebrate heart consists of two large main chambers and two smaller accessory chambers.
The two main chambers are the Atrium and the Ventricle.
The two accessory chambers are the Conus Arteriosus and the Sinus Venous.
Unoxygenated blood flows thru the sinus venous to the atrium and thence to the ventricle, and finally passes through the conus arteriosus to the ventral aorta (truncus arteriosus) of the pharyngeal-arch system where the blood becomes oxygenated.
The primitive heart is S-shaped with sinus venous and the atrium are dorsal the ventricle and conus arteriosus, offering gravity assistance to blood flowing from the atrium to the ventrical.
In more advanced vertebrates the sinus venous and conus arteriosus have become vestigial or have been lost as the heart and pharyngeal-arch system have become remodeled.
Myogenic electrical impulses begin at the SA node and are conducted to the AV node.
The cardiac rhythm originating in the SA node is the hearts primary "pace-maker" and conducts impulses at about 1 to 1.2 m/s.
The cardiac rhythm generating at the AV node is the hearts secondary "pace-maker".
At the AV node the impulses travel, via the bundle of His, to the left and right bundle branch and onward to the Purkinje fibers that spread through out both the left and right ventricles where they bring about cardiac muscle contractions.
The Purkinje fibers making up the bundle of His are electrically-coupled and conduct impulses at about six times the rate of the pacemaker regions (2-4 m/s).
Purkinje fibers are large specialized cardiac muscle fibers that have exploited the "excitablity" of muscle tissue to form a rapid conduction system for myogenic electrical transmission.
Purkinje fibers are not contractile due to the fact that they contain only a few disorganized actin and myosin fibrils instead of the regular arrays of sarcomeres that characterize contractile cells.
In agnatha the atrial myocardium is innervated by unmyelinated parasympathetic vagal efferents only.
These vagal efferents exert a cardioexcititory influence on the heart via
In primitive gnathostomes, the newly emergent myelinated sympathetic efferents also come to innervate the atrial myocardium as well as the ventricular myocardium and the sino-atrial node.
| Neural Regulation of the Heart as a Function of Vertebrate Phylogeny | |||||
|---|---|---|---|---|---|
| Group | CHM | DVC | SNS | AD/m | VVC |
| Jawless Fish | X+ | X+ | none | none | none |
| Cartilaginous Fish | X+ | X- | none | none | none |
| Bony Fish | X+ | X- | X+ | none | none |
| Amphibians | X+ | X- | X+ | none | none |
| Reptiles | X+ | X- | X+ | X+ | none |
| Mammals | X+ | X- | X+ | X+ | X- |
|
Abbreviations: CHM, chromaffin tissue; DVC, dorsal vagal complex with vagal efferent pathways originating in the dorsal motor nucleus of the vagus and vagal efferents terminating in the nucleus of the solitary tract (NTS); SNS, spinal sympathetic nervous system; AD/m, adrenal medulla; and VVC, ventral vagal complex with efferent pathways originating in the nucleus ambiguus that regulate visceral structures (heart, bronchi, thymus) and striated muscles via special visceral efferents and afferents via the solitary tract, trigeminal and facial nerve.
X+ indicates a cardioexcitatory influence (e.g. Increases heart rate). X- indicates a cardioinhibitory influence (e.g. Decreases in heart rate). Source: Stephen W. Porges in: The Polyvagal Theory: Phylogenetic Substrates of a Social Nervous System – International Journal of Psychophysiology, Vol. 42, pp. 123-146, 2001. This is Table 2 on page 128. |
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In mammals the ventricular myocardium is innervated by sympathetic efferents and very sparsely innervated by vagal efferents.
The sino-atrial node (SA node) is mainly innervated by the right vagus nerve and sympathetic efferents.
The atrio-ventricular node (AV node) is mainly innervated by the left vagus nerve.
There is significant overlap in the overall anatomical distribution of nerve endings for both left and right vagal nerves.
Baroreceptors are specially adapted collections of stretch receptor nerve fibres within the walls of the carotid sinus, internal carotid artery, aortic arch, blood vessels, and heart that are responsible for monitoring blood pressure.
Baroreceptor input to the CNS is modulated by the mean blood pressure and the rate of change of blood pressure within the respective vessel.
Baroreceptors respond blood pressures within the range of 60 to 170 torr.
An increase in blood pressure is detected by baroreceptors in the carotid sinus and the baroreceptros activate afferent pathways which send collaterals to two different sets of neurons withing the solitary nucleus.
One set of neurons corresponds to a parasympathetic branch of the solitary nucleus, the other to a sympathetic branch.
The parasympathetic branch activates the dorsal motor nucleus of the vagus via medullary interneurons.
The unmyelinated vagus nerve travels from the dorsal motor nucleus to the intramural ganglia, which projects parasympathetic neurons with a cardio-inhibitory effect to the heart.
The sympathetic branch activates the interneurons that travel to the rostral ventrolateral medulla to activate the reticulospinal tract.
The reticulospinal tract synapses with preganglionic neurons in the intermediolateral cell column (IML).
The neuronal output of the IML influences the sympathetic ganglia that exert a cardio-excititory effect on the heart, as well as, decreasing in the peripheral resistance of the arterioles so as to lower blood pressure.
A network of interneurons located in NTS and NA generate the respiritory rhythm and communicate with the motor neurons that control respiratory, laryngeal, and cardiac function.
Respiration results in rhythmic fluctuations of heart rate that are refered to as respiratory sinus arrhythmia (RSA).
RSA is due to the actions of NA projections to the sino-atrial node of the heart
RSA reflects the influence of respiration on the flow of sympathetic and DVC/DMNX-vagal impulses to the sinoatrial node of the heart.
VVC/NA-mediated vagal activity is inhibited by inspiration causing heart rate to accelerate in response to the removal of inhibition on the sympathetic input to the sino-atrial node of the heart.
VVC/NA-mediated activity resumes upon expiration and heart rate slows as inhibition of sympathetic input is reinstated.
Mammals are endotherms capable of maintaining a relatively narrow range of variance in internal body temperature in the face of wide ranging environmental temperature changes.
With mammals, the closing of the cardiopulmunary loop and establishment of the four-chambered heart brings about a significant increase in blood oxygen content thereby endowing mammals with a burst of metabolic resources relative to their non-mammalian ancestors.
The acquistion of endothermy facilitated a shift to nocturnal lifestyle in early mammalian evolution, possibly as a means to avoid diurnal predators (Geiser et al. 2002).
The stabilization of internal body temperature has important implications from a biochemical and physiological perspective with respect to metabolic rate and the neural mechanisms of temporal synchronization of activity across widely distributed cell populations in the body.
It is well known that changes in temperature affect metabolic rates - a 10 degree celsius change in temperature results in a approximate doubling of the metabolic rate.
With the stabilization of temperature, selection pressures on metabolic temperature compensation mechanisms are released thereby lifting a set of previously conserved structural and kinetic constraints on protein evolution. .
The embryonic heart tube develops sequentially in an anterior to posterior direction.
Ventricle --> Atrium --> Sinus Venous
Initially heart contractions appear in the presumptive region that will form the ventricular myocardium, which is the first region of the heart to develop.
As each region of the heart tube becomes established it begins to beat.
The atrium is the second area to develop and it initiates contractions that are quicker than those of the ventricle.
The contractions of the ventricle quicken to keep pace with the atrial contractions.
The third region of the heart tube to develop is the sinus venous.
The contractions of the sinus venous are more rapid than those of the atrium and ventricle, and the two chambers increase their contractile rates to match those of the sinus venous.