L9 Cardiac Cycle
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Transcript of L9 Cardiac Cycle
- The Cardiac Cycle
- How The Heart is Controlled
The brain is able to modulate the heart rate via the Autonomic Nervous System (ANS). This system adapts the strength and timing of the heartbeat during rest and exercise or intense emotion.
- Autonomic Nervous System
Responsible for control of involuntary or visceral bodily functions
Key role in the bodies response to stress
- The autonomic nervous system (ANS)
Although the heartbeat arises in the SA Node it can be altered by the brain via nerve fibres. These nerve fibres are subdivided into two groups
- Automonic Nervous System
Sympathetic nervous system
allow body to function under stress
fight or flight
Parasympathetic nervous system
feed or breed or rest and repose
constant opposition to sympathetic system
- A little video
- The autonomic nervous system (ANS)
- Normal HR
Heart rate is normally determined by the pacemaker activity of the sinoatrial node (SA node) located in the posterior wall of the right atrium. The SA node exhibits automaticity that is determined by spontaneous changes in Ca++, Na+, and K+ conductances. This intrinsic automaticity, if left unmodified by neurohumoral factors, exhibits a spontaneous firing rate of 100-115 beats/min. This intrinsic firing rate decreases with age.
- The medulla, located in the brainstem above the spinal cord, is the primary site in the brain for regulating sympathetic and parasympathetic (vagal) outflow to the heart and blood vessels. The nucleus tractus solitarius (NTS) of the medulla receives sensory input from different systemic and central receptors (e.g., baroeceptors and chemoreceptors).
The medulla also receives information from other brain regions (e.g., hypothalamus). The hypothalamus and higher centers modify the activity of the medullary centers and are particularly important in stimulating cardiovascular responses to emotion and stress (e.g., exercise, thermal stress).
Autonomic outflow from the medulla is divided principally into sympathetic and parasympathetic (vagal) branches. Efferent fibers of these autonomic nerves travel to the heart and blood vessels where they modulate the activity of these target organs.
How do the baroreceptors respond to a sudden decrease in arterial pressure and how is cardiovascular function altered? A decrease in arterial pressure (mean, pulse or both) results in decreased baroreceptor firing. The "cardiovascular center" within the medulla responds by increasing sympathetic outflow and decreasing parasympathetic (vagal) outflow.
Under normal physiological conditions, baroreceptor firing exerts a tonic inhibitory influence on sympathetic outflow from the medulla. Therefore, acute hypotension results in a disinhibition of sympathetic activity within the medulla, so that sympathetic activity increases. These autonomic changes cause vasoconstriction (increased systemic vascular resistance, SVR), tachycardia and positive inotropy. The latter two changes increase cardiac output. The increases in cardiac output and SVR lead to a partial restoration of arterial pressure.
- Arterial blood pressure is normally regulated within a narrow range, with a mean arterial pressure typically ranging from 85 to 100 mmHg in adults. It is important to tightly control this pressure to ensure adequate blood flow to organs throughout the body. This is accomplished by negative feedback systems incorporating pressure sensors (i.e., baroreceptors) that sense the arterial pressure.
The most important arterial baroreceptors are located in the carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch.
These receptors respond to stretching of the arterial wall so that if arterial pressure suddenly rises, the walls of these vessels passively expand, which stimulates the firing these receptors. If arterial blood pressure suddenly falls, decreased stretch of the arterial walls lead to a decrease in receptor firing.
- Parasympathetic Nerves
These DECREASE Heart Rate.
Without any influence from the nervous system the inherent rate of heart beat is 100bpm. This is somewhat higher than the normal resting heart rate which is around 70bpm.
The reason for this is the parasympathetic activity (via vagus nerve) slows down the rate of SA node impulse generation.
At rest the heart is said to be vagal tone. This allows the brain to increase the heart rate by reducing the activity of the vagus nerve.
- Sympathetic Nerves
These INCREASE Heart Rate.
During increased demands on the circulatory system such as occurs in exercise, sympathetic fires release noradrenalin which speeds up the SA impulse generation.
In addition the sympathetic activity increases the speed of the electrical conduction through the AV node, allowing the ventricles to be excited and therefore beat more frequently
As well as exercise, intense emotional states such as fear can increase the HR through the sympathetic system.
The sympathetic nervous system supply to the heart leaves the spinal cord at the first four thoracic vertebra, and supplies most of the muscle of the heart. Stimulation via the cardiac beta-1 receptors causes the heart rate to increase and beat more forcefully
- Circulating catecholamines, epinephrine and norepinephrine, originate from two sources. Epinephrine is released by the adrenal medulla upon activation of preganglionic sympathetic nerves innervating this tissue. This activation occurs during times of stress (e.g., exercise, heart failure, hemorrhage, emotional stress or excitement, pain). Norepinephrine is also released by the adrenal medulla (about 20% of its total catecholamine release is norepinephrine).
The primary source of circulating norepinephrine is spillover from sympathetic nerves innervating blood vessels. Normally, most of the norepinephrine released by sympathetic nerves is taken back up by the nerves (some is also taken up by extra-neuronal tissues) where it is metabolized. A small amount of norepinephrine, however, diffuses into the blood and circulates throughout the body. At times of high sympathetic nerve activation, the amount of norepinephrine entering the blood increases dramatically.
- The fight-or-flight response, also called hyperarousal or the acute stress response, was first described by Walter Cannon in 1915
His theory states that animals react to threats with a general discharge of the sympathetic nervous system, priming the animal for fighting or fleeing. This response was later recognized as the first stage of a general adaptation syndrome that regulates stress responses among vertebrates and other organisms
- The SA node is richly innervated by parasympathetic nervous system fibers (CN X: Vagus Nerve) and by sympathetic nervous system fibers (T1-4, Spinal Nerves). This makes the SA node susceptible to autonomic influences.
Stimulation of the vagus nerve (parasympathetic fibres) causes a decrease in the SA node rate (thereby decreasing the heart rate and force of contraction).
Stimulation via sympathetic fibres causes an increase in the SA node rate (thereby increasing the heart rate and force of contraction).
- Exercise increases blood flow through the heart so that the cardiac cycle accelerates to accommodate the increased demand for oxygen.
The normal cycle is around 0.8 seconds. This accelerates with faster and more powerful atrial and ventricular contraction, which is stimulated by the cardiac centre in the brain.
Heart rate:- is defined as the number of heart contractions in each minute.
There are two distinct periods in the cardiac cycle- one of the heart muscle relaxation (cardiac diastole), the other of contraction (cardiac systole)
- What is the Cardiac Cycle.
The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle:
Diastole Heart Muscles are relaxed.
Systole Heart Muscles contract.
Diastole is the period of time when the heart relaxes after contraction. Ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing.
- During the first phase (diastole), deoxygenated blood enters the right atrium and oxygenated blood enters the left atrium. As these near capacity blood flows into the ventricles.