L14: Neural Control Of CVS

Question 1

Central processing areas for CVS

Answer 1

1. Higher centres
   - respond to emotional/Psychological responses (limbic system)
   - influence hypothalamus, medulla
2. Motor cortex
   - origin of Sympathetic Cholinergic nerve
   - pass through hypothalamus but bypass medullary centres (CV centre, dorsal motor vagal nucleus)
   - very few blood vessel received Sympathetic Cholinergic nerve
3. Hypothalamus
   - no tonic activity
   - assumes control over CV centre during Physiological stress (exercise, temperature)
4. Cardiovascular centre (medulla)
   - sympathetic tone on heart and vessel
   - receives and integrates inputs from all CVS receptors
   - Presser region: Tonically active —> Sympathetic Noradrenergic nerve supplying heart and vessel
   - Depressor region: Inhibition of tonic activity —> inhibit sympathetic tone
5. Dorsal Motor Vagal nucleus (medulla)
   - origin of Vagus nerve
   - Parasympathetic nerve supplying heart only, blood vessels very few

Question 2

***Efferent nerves of CVS

Answer 2

Sympathetic noradrenergic nerve:
(L: ventricle —> control Force, R: atria + nodal tissue —> control HR)
- Heart: ↑ rate, ↑ force (atria and ventricles)
- Blood vessel (α1 receptor): Vasoconstriction (all vascular smooth muscle)

Parasympathetic cholinergic nerve:

• Heart: ↓ rate only (atria and nodal tissue)
• Blood vessel: Vasodilation (facial skin, external genitalia)

Sympathetic cholinergic nerve:
- Blood vessel: Vasodilation (muscle and skin only)

Question 3

Sympathetic noradrenergic nerve

Answer 3

• MOST important neural control of blood vessels
• Noradrenaline as neurotransmitter
• Heart: ↑ rate, ↑ force (atria and ventricles)
• Blood vessel (α1 receptor): Vasoconstriction (all vascular smooth muscle)
• Sympathetic tone ALWAYS present, complete removal: BP ↓ to very low
• Arterioles: reflex changes in vascular resistance by changing tone
• Veins: noradrenergic tone ↓ compliance —> ↑ venous pressure

Question 4

Parasympathetic nerves

Answer 4

• ACh as neurotransmitter
• Heart: ↓ rate
• Blood vessels: very few vessels (facial skin, external genitalia) —> Vasodilation —> local response only —> do NOT change vascular resistance

Question 5

Sympathetic Cholinergic nerve

Answer 5

• ACh as neurotransmitter
• Blood vessel: Vasodilation
• not involved in day-to-day reflex
• CAN change vascular resistance (because skin and muscle receive large blood supply)
• fight/fright/flight situation: Vasodilation to increase muscle blood flow

Question 6

Control of heart rate

Answer 6

Control of HR: Nerves changing myogenic activity of SA node (intrinsic rate: 100 beats/min)

Mechanisms:

1. Change in pacemaker potential slope (change rate of decline in K permeability)
   - steeper slope: K channel close faster —> faster to depolarise —> faster HR
2. Change K permeability during repolarisation
   - lower level of “resting potential” —> take longer to depolarise —> slower HR
3. Change in threshold
   - higher threshold by changing L type Ca channel permeability —> slower HR

Question 7

Action of parasympathetic nerve on HR

Answer 7

Vagal tone always present to control HR below intrinsic rate of myogenic activity

ACh bind to Muscarinic receptor
—> open ACh-activated K channel / G-protein-gated K channel (IKach)
—> ↑ K permeability during repolarisation and pacemaker potential
1. Shorten action potential (steeper repolarisation)
2. Hyperpolarisation (lower “resting potential”)
3. Shallower slope of pacemaker potential
—> ↓ HR

ACh also ↓ AV conduction velocity
—> ↑ delay between atrial and ventricular systole
—> longer systole

Question 8

Action of sympathetic nerve on HR

Answer 8

Left sympathetic nerve: ventricle —> control Force
Right sympathetic nerve: nodal tissue + atria —> control HR

NE bind to β1 receptor
—> cAMP ↑
—> funny Na channel (If channels) open earlier
—> ↑ slope of pacemaker potential
—> earlier closing of inward-rectifier K channel (Ik1) / faster ↓ in K conductance
—> ↑ HR

Question 9

Reflexes affecting HR

Answer 9

1. Baroreflex (Baroreceptor at Carotid sinus + Aortic arch)
   - detect BP and send to CV centre
   - ↑ BP —> ↓ HR and force
2. Bainbridge reflex (Atrial volume receptor)
   - ↑ blood volume —> ↑ stretch of atria —> ↑ HR (↓SV —> avoid overstretching of atrium, ↑urine formation)
3. Peripheral chemoreceptors (Carotid bodies + Aortic bodies)
   - ↓O2, ↑CO2, ↓pH —> ↑ ventilation —> ↑ HR (VQ matching)
4. Lung stretch receptors (lung interstitium)
   - lung inflation —> ↑ HR