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Activity 1 Recall that wave summation occurs when a skeletal muscle is simulated with such frequency that muscle twitches overlap and result in astronger contraction than a single muscle twitch. When the stimulations are frequent enough , the muscle reaches astate of fused tetanus, during wich the individual muscle twitches cannot be distinguished. Tetanus occurs in skeletal muscle because skeletal muscle has a relatively short absolute refractory period (a period during which action potentials cannot be generated no matter how strong the stimulus) Unlike skeletal muscle, cardiac muscle has a relatively long refractory period and is thus incapable of wave summation. In fact, cardiac muscle muscle is incapable of reacting to any stimulus before approximately the middle of phrase 3, and will not respond to a normal cardiac stimulus before phrase 4. The period of time between the beginning of the cardiac action potential and the approximate middle of phrase 3 is the absolute refractory period. The period of time between the absolute refractory period and phrase 4 is the relative refractory period. The total refractory period of cardiac muscle is 200-250 milliseconds-almost as long as contraction of teh cardiac muscle.Activity 2 The autonomic nervous system has two branches : the sympathetic nervous system (figh or flight) and parasympathetic nervous system (resting and digesting) . at rest both the sympathetic and parasympatethic nervous system are working but the parasympathetic branch is more active. The sympathetic nervous system becomes more active when needed , for example, during exercise and when confronting danger . The vagus nerve carries the signal to the heart . if stimulation of the vagus nerve (vagal stimulation)is excessive, the heart will stop beating . after a short time, the ventricles will begin to beat again . teh resumption of the heartbeat is referred to as vagal escape and can be the result of sympathetic reflexes or initation of a rhythm by the Purkinje fibers . The sinoatrial node (SA node) is a cluster of autorhythmic cardiac cells found in the right atrial wall in the human heart. The SA node has the fartest rate of spontaneous depolarization and for that reason it determines the heart rate and is therefore referred to as the heart’s “pacemaker”Activity 3 Humans are homeothermic , which means that the human body maintains an internal body temperature within the 35,8-38,2 C range even though the external temparature is changing . During extreme external temperature conditions , the body might not be able to maintain homeostatis and either hyperthermia (elevated body temperature) or hypothermia (low body temperature ) could result . in contrast, the frog is poikilothermic animal . its internal body temperature changes depending on the temperature of its external environment because it lacks internal homeostatic regulatory mechanismsActivity 4 The sympathetic nervous system is activated in times of “fight or flight “ and sympathetic nerve fiber release norephinephrine (also known as noradrenaline)and ephinephrine (also known as adrenaline ) at their cardiac synapses .Norephinephrine and ephinephrine insrease the frequency of action potentials by binding to B1 adrenergic receptors embedded in the plasma membrane of sinoatrial (SA) node (pacemaker)cellsParasympathetic nerve fibers release acetylcholine at their cardiac synapses .Chemical modifiers that inhibit , mimic, or enhance the action of acetylcholine in the body are labeled cholinergic. Chemical modifiers that inhibit , mimic, or enhance the action of ephineprine in the body are adrenergic . if the modifier works in the same fashion as the neurotransmitter (acethylcoline and norephineprine ) it is an agonist . if the modifier works in oppostion to the neurotransmitter it is an antagonist .Activity 5Modifiers that affect heart rate are chronotropic and modifiers that affect the force of contraction are inotropic .
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