Frank-Starling Law of the Heart

Starling's Law of the heart:

• Otto Frank discovered in the late 19th century that ventricular contraction strength in an isolated frog heart increased when the ventricle was stretched before contraction.
• Ernest Starling extended this observation in the early 20th century, finding that increasing venous return leads to increased filling pressure (LVEDP), causing stroke volume (SV) to increase.
• Conversely, decreasing venous return decreases stroke volume.
• The cardiac response to changes in venous return and ventricular filling pressure is intrinsic to the heart, independent of neurohumoral mechanisms.
• The Frack-Starling mechanism explains that the heart's ability to change its contraction force and stroke volume in response to venous return variations.
• The Frank-Starling law states that increased filling pressure results in increased cardiac output.

Frank-Starling Curves

• Frank-Starling curves illustrate how changes in ventricular preload affect stroke volume.
• When venous return increases, the ventricle fills more, increasing end-diastolic volume.
• Increased venous return leads to increased stroke volume.
• This physiological mechanism does not apply to ventricles in failure.

Physiological Significance of the Frank-Starling Law

• The Frank-Starling mechanism explains heart's response to venous return changes.
• Increased venous return causes heart chambers to fill more, leading to more forceful contraction and increased blood pumped to the body.
• Greater preload results in a greater volume of blood in the heart at the end of diastole.
• Starling's Law represents the relationship between contractility and afterload.

Autonomic Nervous System (ANS)

• The ANS is a component of the peripheral nervous system which regulates involuntary physiological processes.
• These processes, including heart rate, blood pressure, respiration, digestion, and glandular functions.
• The ANS consists of the sympathetic and parasympathetic systems.
• Centers in the spinal cord, brain stem, and hypothalamus control the ANS.
• Heartbeats originate from the sinoatrial (SA) node within the heart.
• The SA node's pacing rate is about 100 beats per minute without extrinsic influences.
• Heart rate and cardiac output must adjust to meet the body's needs for oxygen and nutrients.
• The autonomic nervous system, hormones, and other factors regulate heart rate and contractility to meet the body's changing requirements.
• Sympathetic and parasympathetic neurons have opposing effects on the heart.
• The sympathetic system prepares the body for energy expenditure in emergencies or stressful situations.
• The parasympathetic system is most active during restful conditions.
• The sympathetic system is counteracted by the parasympathetic system, restoring the body to a restful state after stress.
• The sympathetic nervous system releases norepinephrine (NE).
• The parasympathetic nervous system releases acetylcholine (ACh).

Effects of Sympathetic Stimulation on the Cardiovascular System

• The sympathetic nervous system activates during exercise, emotional excitement, or pathological conditions like heart failure.
• Stimulation of the sympathetic nervous system results to increased heart rate
• Stimulation of the sympathetic nervous system results to increased myocardial contractility.
• Stimulation of the sympathetic nervous system results to blood vessel constriction.
• Stimulation of the sympathetic nervous system results to increased renin secretion by the kidneys, leading to increased systemic blood pressure.

Effects of Parasympathetic Stimulation on the Cardiovascular System

• The parasympathetic system counteracts the sympathetic system post-stress, returning the body to rest.
• Parasympathetic stimulation decreases heart rate.
• Parasympathetic stimulation decrease cardiac output.
• Parasympathetic stimulation decreases blood pressure.
• Parasympathetic stimulation does not cause vasodilation because most blood vessels lack parasympathetic innervation and vessel diameter is controlled by the tone that allows the reduction in sympathetic stimulation.

Sympathetic Neurotransmitters

• Neurotransmitters are chemical substances released into the synaptic cleft from nerve terminals.
• They transmit signals from a neuron to a target cell across a synapse, such as acetylcholine for neuromuscular junctions.
• Preganglionic neurons of both sympathetic and parasympathetic systems secrete acetylcholine (ACh), hence they known as cholinergic.
• Most postganglionic endings of the sympathetic nervous system release norepinephrine (NE), similar to epinephrine (i.e., adrenalin).
• Sympathetic postganglionic fibers are commonly called adrenergic fibers.

Sympathetic Receptors

• There are two types of adrenergic receptors: β and α.
• In the cardiovascular system, adrenergic receptors include β1, β2, α1, and α2.

Parasympathetic Neurotransmitters

• Acetylcholine is the primary neurotransmitter in the parasympathetic nervous system for both preganglionic and postganglionic neurons.
• Acetylcholine inhibits the contraction of cardiomyocytes by activating muscarinic receptors (M2).
• Parasympathetic postganglionic fibers are called cholinergic fibers because they secrete acetylcholine at nerve endings.
• Parasympathetic receptors: the the parasympathetic postganglionic fibers are cholinergic.
• Acetylcholine binds to nicotinic and muscarinic cholinergic receptor types.