Week 3 - Autonomic nervous system + CV control during exercise Flashcards
Two divisions of Autonomic Nervous system
1) Sympathetic NS (fight or flight)
2) Parasympathetic NS (rest and digest)
What are the two neurons that autonomic pathways consist of?
preganglionic neuron and postganglionic neuron - they connect the CNS and the effector via a ganglion
Ganglia
collection of cell bodies that sit outside the spinal cord
Describe the sympathetic neuron pathway and parasympathetic neuron pathway.
Sympathetic
- Neurons travel from the brainstem and exit at certain levels of the spinal cord
- T1-T5 lead to the heart (and upper body blood vessels) and T6 and below lead to lower body blood vessels where they vasoconstrict/vasodilate and maintain bp.
Parasympathetic
- Neurons travel down the vagal nerve from the brain stem to the heart, lungs and gastrointestinal tract.
- Also leave from S2-S4 to bladder and pelvic organs
Describe the cell body location and neurotransmitters released from the preganglionic and postganglionic neurons of the PSN and SNS.
Parasympathetic
- Cell body of preganglionic neurons is located in the brain stem (medulla oblongata) and S2-S4 - cranial and sacral regions.
- Pre and postganglionic neurons release acetylcholine (cholinergic)
Sympathetic
- Cell body of preganglionic neurons located in the thoracic and lumbar regions of the brain.
- Preganglionic neurons release ACh.
- Postganglionic neurons release norepinephrine (adrenergic) which acts on effector organs - smooth muscles, organs, heart.
- One key difference is that postganglionic neurons release ACh for sweat glands.
- Also note T10-L1: preganglionic neuron releases ACh and adrenal medulla then releases 80% E and 20% NE into circulation.
Describe the neuro-anatomical control of the heart.
PNS: parasympathetic fibers come from neurons in the brainstem (medulla oblongata) and they make up the vagus nerve. This nerve leaves the brainstem and the fibers make contact with the SA and AV node. When they stimulate these nerve endings they stimulate ACh which reduces the activity of SA and AV nodes which reduces HR.
SNS: sympathetic fibers arise from neurons located in the spinal cord (T1-T5), the ending of these fibers release NE which acts on beta-1 adrenergic receptors to stimulate SA and AV node to increase HR.
Any neuron/fiber releasing acetylcholine is known as …
cholinergic
Any neuron/fiber secreting norepinephrine is known as …
adrenergic
What is the difference between the preganglionic and postganglionic axon lengths of parasympathetic neurons and sympathetic neurons?
Parasympathetic neurons: axons are long for the preganglionic and they synapse with the postganglionic really close to the effector organ.
Sympathetic neurons: preganglionic is short as the synpase between the ganglia are located close to the spinal cord and the postganglionic neuron is long.
Where is the ganglia located in the PNS and SNS?
PSN: near or in target organ
SNS: in a chain alongside throaco-lumbar spinal region
Role of sympathetic NS during exercise
Regulates blood flow to the working muscles and increase cardiac output.
Explain the autonomic NS control of heart rate at the onset of exercise.
Initial increase in heart rate is due to parasympathetic withdrawal (up to 100bpm)
Further increases are due to increased sympathetic NS outflow.
What is SV the difference between?
End-diastolic volume and end-systolic volume
3 factors that regulate stroke volume
1) End-diastolic volume (EDV)
2) Average aortic blood pressure (during systole)
3) Strength of the ventricular contraction (contractility)
End-diastolic volume
volume of blood in the ventricles at the end of diastole (“preload”) - dependent on venous return
Average aortic blood pressure
Pressure the heart must pump against to eject blood during systole (“afterload”)
What is the strength of the ventricular contraction enhanced by?
- Circulating E and NE
- Direct sympathetic stimulation of the heart
Frank-Starling mechanism
greater EDV (end-diastolic volume) results in a more forceful contraction due to stretch of ventricles (results in greater SV) - dependent on venous return!
What 3 factors increase venous return?
- Venoconstriction (via SNS)
- Skeletal muscle pump: skeletal muscle contractions force blood towards the heart and one-way valves in veins prevent backflow of blood
- Respiratory pump: changes in thoracic pressure pull blood toward the heart.
Cardiac output
the amount of blood pumped by the heart each minute
Equation - heart rate x stroke volume
How does parasympathetic NS and sympathetic NS regulate cardiac output?
PNS reduces cardiac output by reducing HR (cardiac rate)
SNS stimulates cardiac output by increasing HR (cardiac rate) and stroke volume through the increase in contraction strength.
Systolic blood pressure
pressure generated during ventricular contraction
Diastolic pressure
pressure in the arteries during cardiac relaxation
What is Pulse pressure the difference between?
difference between systolic and diastolic blood pressure
Mean arterial pressure (MAP)
- Define
- What are the two determinants?
- Equation
- Average pressure in the arteries
- Cardiac output and total vascular resistance (sum of resistance to blood flow)
- MAP = DBP + 0.33 (SBP - DBP) - Rest
- MAP = cardiac output x total vascular resistance.
Short-term regulation of arterial blood pressure
Regulated by baroreceptors in aorta and carotid arteries.
When BP increases this triggers an increased firing of the baroreceptors that send an impulse to CV control in brain (medulla oblongata) which consequently reduces sympathetic activity and increases parasympathetic activity thus reducing cardiac output (hr), adrenal epinephrine release and vascular resistance (dilation) –> lowers bp.
Low BP leads to a reduction of baroreceptors signaling to the brain. This causes the CV centre to respond by increasing the sympathetic outflow and reducing the parasympathetic activity and thereby either increasing Q (hr), total peripheral resistance (vasoconstriction), or/and adrenal epinephrine release which increase mean arterial pressure/bp.
What two factors does the long-term regulation of blood pressure require?
1) Constant blood volume
2) Constant extracellular Na+ content
What is the role of the kidneys in long-term regulation of blood pressure?
the kidneys maintain a constant amount of extracellular sodium
What two mechanisms increase oxygen delivery and thus blood flow to exercising skeletal muscles?
1) Increased cardiac output (Q)
2) A redistribution of blood flow from inactive organs to the working skeletal muscle (vascular shunt)
Why do trained individuals have a greater Q?
Due to a much greater stroke volume which is caused by:
- the cardiac hypertrophy of the left ventricle walls and a larger left ventricular volume
- a greater venous return which increases filling of the left ventricle (diastole).
What are changes in heart rate and blood pressure dependent on?
- Type, intensity and duration of exercise (prolonged vs intermittent)
- Environment conditions (hot/humid vs cool)
- Emotional influence pre-exercise and during submaximal exercise due to increases in SNS activity.
CV responses to (a) the onset of exercise and (b) during recovery
A
- Rapid increase in HR, SV, cardiac output
- Then get a plateau (after 2-3mins) in CV values during submaximal exercise (below lactate threshold)
B
- Decrease in HR, SV, Q toward resting levels.
- Depends on the duration and intensity of exercise as well as training state of subject.
CV responses to incremental exercise
- HR and cardiac output increase linearly with increasing work rate and reaches a plateau at 100% v02max.
- Mean arterial blood pressure increases linearly.
- Systolic BP increases (due to increase contractility of the heart due to SNS activity)
- Diastolic BP remains fairly constant (dilation of major arteries in working muscles).
Describe the neural control of the adrenal medulla.
Sympathetic preganglionic neurons, cell body located in T10-L1, releases ACh which stimulates the adrenal medulla to release epinephrine (80%) and norepinephrine (20%).
What does the recovery of HR and BP between bouts depend on?
- Fitness level
- Temperature and humidity
- Duration and intensity of exercise
CV responses to prolonged exercise at constant work rate
- Cardiac output maintained (by increase in HR)
- Gradual decrease in stroke volume due to dehydration and reduced plasma volume.
- Gradual increase in HR
Which brain region is responsible for the overarching integration of the ANS?
Hypothalamus
What neurotransmitter does PNS postganglionic neurons release?
acetylcholine - binds to muscarinic receptors on effector cells (cholinergic)
What neurotransmitter does most SNS postganglionic neurons release? What is the expection?
norepinephrine (adrenergic) which binds to adrenergic receptors on effector cells
the exception is the sympathetic nerve to sweat gland acinar cells, where the postganglionic neuron released acetylcholine which binds to muscarinic receptors on the effector cells
What neurotransmitter binds to:
a) muscarinic receptors
b) nicotinic receptors
c) adrenergic receptors
a) acetylcholine released by parasympathetic postganglionic neurons
b) acetylcholine released by parasympathetic and sympathetic preganglionic neurons
c) norepinephrine released by sympathetic postganglionic neurons
What are the effects of sympathetic stimulation on the CV system?
- Increases heart rate and force of contraction.
- Dilates some blood vessels and constricts others to enhance blood flow to active skeletal muscles.
Which branch of the ANS affects both heart rate and stroke volume?
Sympathetic nerve stimulation - parasympathetic nerve stimulation only affects heart rate not stroke volume
What is heart rate normally determined by?
the rate of cycling of the SA node action potentials
Note: changes in cellular cAMP responsible for changes in this^
How does norepinephrine release from the postganglionic sympathetic cardiac fibers lead to an increased HR?
1) stimulates beta-1 adrenergic receptor on SA node cells.
2) cAMP concentration in the cell rises
3) increases frequency of action potentials originating at the SA node
4) increased HR
How does acetylcholine release from the postganglionic parasympathetic neurons lead to a decreased HR?
1) occupies muscarinic receptor linked to G-inhibiting protein.
2) Decreases cAMP concentration in the cell.
3) reduces the action potentials originating at the SA node
4) decreased HR
As end diastolic volume increases to a certain point, stroke volume fails to increase further. How can we explain this?
When the heart chamber becomes overfilled, the cardiac muscle becomes too stretched - this leads to less overlap between myosin and actin which results in fewer crossbridge being formed and explains why we can no longer increase the force of contraction (SV).
How does sympathetic stimulation influence the stroke volume?
It increases the strength of contraction for any end-diastolic volume and thus increases SV.
What interval is used to measure heart rate variability?
RR interval
Which baroreceptor monitors the blood pressure driving most of the systemic circulation?
aortic arch baroreceptors
Which baroreceptor monitors the blood pressure driving the circulation in the brain?
carotid sinus
What are the effects of sympathetic stimulation on:
a) Venous
b) Cardiac
c) Arteriolar
a) decrease venous diameter (vasoconstriction) which increase venous return and preload (EDV)
b) incr. contractility and HR = incr. SV = incr. cardiac output = incr. systolic blood pressure
c) diameter of muscle arterioles incr, diameter of skin and splanchnic bed decr = incr. systemic vascular resistance and afterload (ESV) = incr. systolic blood pressure
What are the two tissues with the greatest change in blood flow during exercise?
1) Skeletal muscle
2) Skin
In order to meet the increased oxygen demand of muscle during exercise, what are the two major adjustments in blood flow that must be made?
1) An increase in cardiac output
2) A redistribution of blood flow from inactive tissues to skeletal muscles
What does Heart rate variability refer to?
the time interval between heart beats - its influenced by the balance between the parasympathetic NS and the sympathetic NS
Does HRV decrease in response to regular aerobic exercise training?
NO
