PSIO 202, Lecture 8 - PSIO 202 Human Anatomy and Physiology...

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Unformatted text preview: PSIO 202 Human Anatomy and Physiology Lecture 8 Control of Heart Rate Objectives and Reading Assignment Reading: Tortora, pages 742-744 Objectives Measure HR from the EKG Define systole, diastole and contractility Describe how the autonomic nervous system (ANS) controls heart rate Explain how activation of the ANS changes neurotransmitter release leading to changes in HR Describe how oxygen and hormones influence HR and contractility ANS-review ANScomparison of sympathetic and parasympathet ic divisions ANSinnervation of effector organs The Cardiac Cycle The cardiac cycle refers to all of the events associated with one heartbeat Systole is the period of ventricular contraction and blood ejection Diastole is the period of ventricular relaxation and filling Each cardiac cycle takes about 0.8 seconds at rest The cardiac frequency, or heart rate, can be computed from the cardiac cycle period Cardiac Cycle, Con't. heart rate (beats / minute) = 1 / cycle period (sec) x 60 sec / min example: 1 / 0.8 sec x 60 sec / min = 75 "beats" / min 0.8 sec (period) Control of Heart Rate by the Autonomic Nervous System The heart rate is modified by changes in the ANS The effects of the ANS on heart rate are called "chronotropic effects" ANS activity can be modified by circulating hormones, and by higher brain structures The sympathetic neurons that send nerve fibers to the heart are located in the caudal portions of the medulla oblongata (the "cardiovascular center") Control of Heart Rate by the Sympathetic Nervous System, Con't. Neurons in the CV center synapse upon preganglionic neurons in the intermediolateral cell columns of the thoracic spinal cord The preganglionic neurons then synapse on postganglionic neurons, which are located in the ganglia of the sympathetic chain The postganglionic neurons then send their axons to the SA and AV nodes, and the cardiac muscle fibers Control of Heart Rate by the Sympathetic Nervous System, Con't. preganglionic neurons release acetylcholine (ACh) onto postganglionic neurons the postganglionic neurons then release norepinephrine onto the target cells NE is the postganglionic neurotransmitter for the sympathetic nervous system NE activates beta-1 receptors, which have wide-spread effects on the cardiac nervous and muscle tissue Mechanisms of Heart Rate Control an increased rate of depolarization in the cells of the SA and AV nodes, due to an increase in the inward Na+ current Mechanism: the neurons are brought to threshold more frequently, leading to an increase in the number of action potentials generated in a given unit of time Control of Heart Rate by the Parasympathetic Nervous System Parasympathetic neurons in the CV centers of the brain stem send their axons to the heart via the vagus nerves ("vagal motorneurons") The neurons synapse directly on the SA and AV nodes, and on the atrial muscle fibers The neurons release ACh onto the postsynaptic cells, leading to slowing of the heart rate. Mechanism of HR Control by the PSNS This occurs because the ACh increases potassium permeability of the cells, and the rate of potassium diffusion out of the cells is increased This results in hyperpolarization of the cell membrane As a result, it takes more time for the spontaneous upward drift in membrane potential to reach the threshold for Na+ channel activation Chemical Regulation of the Heart The hormones epinephrine and thyroxine increase heart rate Intra- and extracellular ion concentrations must be maintained for normal heart function Ionic Influences on Heart Rate and Contractility Hypocalcemia reduced ionic calcium depresses contractility Hypercalcemia dramatically increases heart irritability and leads to spastic contractions Hypernatremia blocks heart contraction by inhibiting ionic calcium transport Hyperkalemia leads to heart block and cardiac arrest Regulation of Cardiac Output: Review ...
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This note was uploaded on 09/23/2008 for the course PSIO 202 taught by Professor Staff during the Fall '08 term at University of Arizona- Tucson.

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