Autonomic nervous system: The dive response Flashcards
what is the dive response?
*Extreme cardiovascular and respiratory changes during a dive
*Apnoea
*Intense bradycardia
*Peripheral vasoconstriction
*Conserve limited oxygen store for the benefit of the heart and brain, permitting prolonged survival under water.
Dive reflex is partially triggered by the coldness of the water and in the lab this means water needs to be <7 degrees c to trigger the reflex
(see diagram)
Function/purpose of dive response
1.Defence mechanism against asphyxia (suffocation) in that it helps to conserve oxygen and to ensure that the brain preferentially receives a supply of blood when the organism as a whole is deprived of oxygen. Vasoconstriction redistribute CO.
2.Prevents inhalation of harmful substances e.g. smoke Apnoea partly reflexly engendered, acts as a defence mechanism for the lungs in that noxious gases and vapours are prevented access to the lower airways.
3.Allows for longer dives - Functionally useful marine mammals where it is particularly well developed, it allows a longer period of time for foraging for food in water than otherwise be possible.
Integrative mechanisms concerning the respiratory and cardiovascular responses.
*A central feature of physiology of breath-hold diving, is the cessation of external respiration during the period of submersion, with the progressive development of asphyxia (combination of arterial hypoxaemia low O2 and hypercapnia high CO2)
*Inputs to the nervous system are involved in the diving response Trigeminal + Arterial Baroreceptors + Carotid Body Chemoreceptors.
Neurogenic effects of phasic changes in respiration on the CVS
*HR: Respiratory Sinus Arrhythmia (Carl Ludwig, 1847)
*Breathe in increase HR: Breathe out decrease HR-transient and normal
*Central and pulmonary mechanisms with efferent pathway largely vagal
*Cardiac vagal tone – inspiration causes cardioacceleration (inhibiting cardiac vagal motor neurones to change level of cardiac vagal tone) can only be brought about by reducing pre-exisiting vagal activity
Central activity in the medulla, normally cardiac vagal motor neurones keep tone low, breathing in activates pulmonary stretch receptors (lung inflation) which modulate HR via neural integration within the medulla. Respiratory drive neurones also interact. Breathing out heart rate decreases
^this process is modified in the dive reflex
Breath-hold dive
These dives are associated with:
- decrease PaO2 and increase PaCO2
–being more evident the longer the dive
Prolonged dives marine mammals:
– PaO2 less than 20 mmHg (2.67kPa)
–PaCO2 100 mmHg (13.3kPa)
Degrees of hypoxaemia and hypercapnia which cause vigorous CB chemoreceptor activity
Role of the trigeminal receptors
*located on the skin of the cheeks
*Face immersion into cold water –bradycardia and apnoea
*Face immersion + breath-hold -greater bradycardia than immersion or breath-holding alone (facilitated reflex response)
*Remove face from water but still hold breath, HR & forearm blood flow return toward normal
*Stimulation of trigeminal receptors that initiates the bradycardia
(sends signals back to the brainstem)
Immersing the face whilst holding your breath changes the overall response allowing longer underwater
^Making your dive response longer
Role of the chemoreceptors
see diagrams
chemoreceptors are located in the carotid body between the external and internal carotid artery they detect levels of H+ ions and CO2 resulting in changing respiratory rate. They indirectly measure O2 also although a major decrease is required for these receptors to be stimulated in this way to alter breathing pattern.
The signal to breathe does not immediately result in a breath during dive response – why?
*Carotid body (Chemoreceptor stimulation)
*In a spontaneously breathing anaesthetised animal, stimulation of isolated perfused carotid bodies with hypoxic blood results in an increase pulmonary ventilation with variable changes in HR, BP, CO, TPR
*Repeating this experiment in the absence of breathing, this results in bradycardia and peripheral vasoconstriction to redirect blood away from peripherals and to heart,lungs and brain.
Role of the chemoreceptors
*Absence of lung inflation has a central effect on the cardiac vagal motoneurones to increase their excitibility to give bradycardia
*Chemoreceptor effect in the absence of lung inflation contributes to bradycardia and also produces selective vasoconstriction
*Cessation of breathing playing a dominate role in the integration of the cardiovascular effects.
Summary of roles
Trigeminal stimulation (+ breath hold)
*Bradycardia
Chemoreceptors
*Bradycardia
*Vasoconstriction
Baroreceptors
*bradycardia
Ways to stimulate dive response
Breath-Hold Dive
–triggered by immersion face in cold water & stimulate receptors of the face innervated by trigeminal nerves
Other ways elicit diving response:
- Stimulate receptors in upper airway (nasal, mucosa, larynx) which are not involved in the response to breathe-hold diving)
- This is useful to prevent inhalation of smoke
Death due to hypothermia is more likely than death due to drowning – normally people surface but water is too cold to survive in
Medical implications
Medical implications
*Nasal Stimulation, a rare cause of death- water entering nose-cardiac arrest
“The Case of the Brides in the Bath” George Joseph Smith 1915 – George married many women then either abandoned them or killed them. The detective Arthur Neil and Bernard Spilsbury needed to figure out why a couple of George’s wives had died in the bath. Found that pulling a test subject underwater by the ankles forced water up the nose resulting in cardiac arrest.
*Sudden Infant Death Syndrome-SIDS (Cot Death)
–Premature/full term infant – caused by having neural reflex in the medulla not fully developed resulting in period between breaths becoming longer and longer, this bradycardic stimulation leads to the heart stopping
–Episodes periodic breathing + apnoea
–CB stimulated leads to bradycardia and heart stops
–Does not start rebreathing
–Immature/impaired central drive to respiration
Benefits of the dive reflex
–Treat cardiac irregularities & sinus arrest
*Supraventricular paroxysmal tachycardia in adult
(abnormal condition from accessory pathways-alter route + sequence of ventricular depol -arrhythmia)
*Elicit diving response -stop heart to revert to normal rhythm
*Can be used to slow the heart rate :Apply cold, wet towel to face patient + ask hold breath in end expiratory position or adenosine injection can be used
Summary
*Diving response
–Apnoea
–Bradycardia
–Vasoconstriction (selective)
*Trigeminal receptors - apnoea+ bradycardia vasoconstriction
*Chemoreceptors -bradycardia + vasoconstriction
*Baroreceptors -bradycardia
