1. Mammalian cardiac muscle consists of two kinds of fibers, both derived from embryonic muscle cells (myoblasts) - these are contractile fibers and pacemaker fibers. Pacemaker fibers - conduct electrical potentials through gap junctions between cells; Contractile fibers - short, compact, unlike skeletal muscle cells.

• Contraction of mammalian cardiac muscle depends not only on Ca⁺⁺ release from the sarcoplasmic reticulum but also on Ca⁺⁺ entry from the extracellular fluid, via voltage-gated Ca⁺⁺ channels.

• Mammalian cardiac action potentials are longer (Eckert, Fig. 12-07b), and very strange looking as compared to action potentials on nerves and skeletal muscles (Eckert, Fig. 12-07a). (Eckert, Fig. 10-49a) (Eckert, Fig. 10-49b).

• Pacemaker cells depolarize, producing an action potential. But, soon after the action potential has ended, they produce another action potential. Left to themselves, they will repetitively fire, giving a rhythm to the excitation of the heart. Contractile cells will produce action potentials as the result of voltage-gated Ca⁺⁺ channels opening in response to the pacemaker potentials. The frequency of pacemaker potentials decreases upon parasympathetic stimulation by the vagus (Eckert, Fig. 12-06a) and increases by sympathetic stimulation (Eckert, Fig. 12-06b).

• Important question - what is the adaptive significance of the long cardiac action potential? Answer - The cardiac action potential lasts almost as long as the contractile twitch it produces. After during the period of the action potential, another action potential can't be started. (This period of no ability to produce action potentials is called the refractory period. In non-cardiac muscle, the refractory period, while short, is extended somewhat by a hyperpolarizing afterpotential.) So, cardiac muscle can't go into tetanus - one long, extented contraction. Good thing or else pumping would stop.

2. Biomechanics of human and other types of circulatory systems

• Many invertebrates have an open circulatory system, emptying into a compartment called the hemocoel. (Eckert, Fig. 12-02a) (Eckert, Fig. 12-02b). As animals become larger and more complex, they require higher blood pressure to reach all parts of the body and more efficent delivery of oxygen and removal of carbon dioxide. Notice that the closed circulatory system of the cephalopod (squid, octopus) has two kinds of heart - the brachial or gill hearts deliver blood to the oxygen gathering organ, the gill, and the ventricle or systemic heart delivers the newly oxygenated blood coming from the gill to the rest of the body. (Eckert, Fig. 12-02c)

• We see in the cephalopod that one heart chamber is used to pump blood to the gills and a second to pump blood back to the rest of the body - Two single chamber hearts. This is a very sophisticated system, unlike the heart of a typical fish, in which one heart, even if it is of two chambers, pumps blood to the gills. From the gills, the blood doesn't get a boost in pressure before going to the tissues. This lack of efficiency may have prevented fish from developing homeothermy. (Eckert, Fig. 12-16a) In the African Lungfish, that can survive in either water or air, the swim bladder, normally used to control buoyancy in fish, is modified into a lung-like organ. (Eckert, Fig. 12-16c) Notice how the lung fish almost separates the arterial and venous circulation as a result of the structure of the main, multichambered heart - the oxygenated blood (red) and unoxygenated blood (blue) going to the gills don't mix very much as blood flows through the heart.

• The four-chambered heart of mammals totally separates the the blood flow to the body and to the lungs. (Eckert, Fig. 12-3) (Eckert, Fig. 12-4)

• The four-chambered heart of birds is similar to that of mammals but can have multiple pulmonary veins.

• The three-chambered heart of amphibians and reptiles has two atria but only one ventricle. an intermediate pattern between fish and mammal. Frogs have one large ventricle but manage to keep circulation of oxygenated and unoxygenated blood somewhat separate due to flow patterns. (Eckert, Fig. 12-17) Some lizards have a separation part way up the ventricle. (Eckert, Fig. 12-18) Figures in Eckert's book don't make the separation of flow patterns clear so don't spend too much time worrying about those diagrams.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2006, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.