Neural regulation of circulation

1. Nervous Regulation of Circulation
Dr Shamshad
Majamaah University
Lecture 48

2. Objectives:
1. Describe the physiological anatomy of autonomic supply to
the heart & blood vessels.
2. Discuss the nervous control of circulatory functions esp.
heart rate.
3. Describe the role of vasomotor center in regulation of
circulatory functions.
4. Describe the various factors affecting activity of vasomotor
center.

3. Control of Heart & Vessels
ANS Innervation
Cardiac Peripheral
Sympathetic Vasoconstrictors
 Epicardial
Parasympathetic Vasodilators.
 Endocardial
Preganglionic
Postganglionic

4. 1:Sympathetic N cells lie in the
intermediolateral horn of spinal
cord T1-T5 segments.
2:Myelinated preganglionic fibers
pass into the sympathetic trunk to
superior , middle & inferior cardiac
ganglia.
3:Synapse then unmylienated post-
ganglionic fibers pass via the
superior, middle & inferior cardiac
sympathetic fibers.
Supply: Nodal tissues: SAN,
AVN & muscles of atria &
ventricles. T1 –T5 segments
1
2
3

5. Brief
Sympathetic NS
Ionotropic
Chronotropic
Dromotropic
Bathmotropic
Target
NT substances
Vessels with
smooth muscles
Norepinephrine
ATP
Neuropeptide Y
γ 1NPYVC
Receptors
Purnergic
P2 ATP VC
Adrenergic
α1VC
α2VC/VD
β1 & β2VD
Vascular
Heart
+Ve

6. 1. Cell bodies of Vagus nerves
lies in medulla oblongata in
Nucleus ambiguous
2. Myelinated preganglionic fibers
travel in vagi & synapse with
ganglionic cells located near the
SAN , AVN & in atria.
3. Unmylienated Postganglionic
fibers arise & are distributed to
SAN , AVN & atrial muscles.
Vagal tone
At rest : Vagal tone predominates
over sympathetic tone in heart .
1
2
3
Ventricles have NO Vagal Fibers

7. Peripheral vascular innervation
I:Vasoconstriction
By Sympathetic NS : Sympathetic tone
 Cell bodies from intermediate lateral horn cells of T1 – L2
spinal segments. Tonic effect on all vessels.
 Except: Cerebral vessels , true capillaries & coronary vessels
 Dense innervation :Arterioles, metarterioles ,pre-capillary
sphincters & post-capillary venules.

8. Actions :
1:Vasocontriction:Through β2 adrenergic receptors .
*On resistance vessels: Peripheral resistance  Regulate BP.
*Capacitance vessels(veins): Regulate blood Volume.
*Arterioles Vasoconstriction regulated VR & thus control BP.
2:Vasodilation: On skeletal muscles, erectile tissue of
reproductive system & facial blood vessels .

9. II:Vasodilation
1: Reduction of sympathetic constrictors tone.
2:Specific activation of vasodilators through
A:Sympathetic cholinergic Nn :Ex: Skeletal muscles, heart, lungs
liver, kidneys & uterus .Exercise /child birth increases blood flow.
B:Parasympathetic vasodilator nerves: Nervi erigentes
Ex: Nn : erectile tissue of reproductive system, salivary gland.
3:Dorsal root vasodilation : Axon reflex: Local neural
mechanisms: :

10. Brief
Parasympathetic
NS
Ionotropic
Chronotropic
Dromotropic
Bathmotropic
Target
NT substances
Arteries & Veins
Ach
Receptors
Muscuranic
M3VD
Few no.
Vascular
Heart
-Ve
Receptors
M2
Ventricles have NO Vagal Fibers

12. Medullary control the heart
Cardiovascular center are group of neurons in CNS that regulate
HR, SV and vascular tone .
Medullary regulation of CVs is brought by two different centers
located in the medulla:
1: Medullary cv center: Vasomotor center / vasomotor area
• Pressor and Depressor areas
2:Cardiac vagal center
Regulated by signals from baroreceptors.

14. Vasoconstrictor center/vasopressor area
Location: Bilaterally in the antrolateral portion of the upper medulla.
Neurons: Glutaminergic neurons excitatory effect on spinal
sympathetic neurons.
 Under normal conditions, transmits signals continuously to the
sympathetic vasoconstrictor nerve fibers over the entire body, 
slow firing of these fibers at a rate of about ½-2 impulses/ second.
 lead to sympathetic vasoconstrictor tone.
 Significance: Impulses normally maintain a partial state of
contraction in the blood vessels, called vasomotor tone.

15. Pressor effect on CVS
 Increase HR(Heart Rate)
 Increase BP (Blood pressure)
 Vasoconstriction leads to decrease blood stored in venous
reservoir and increase venous return (VR).

16. Depressor area
Location: Near medial & caudal parts of 4th ventricle in medulla.
Action:
Stimulation leads to decrease in sympathetic activity by
inhibiting the tonically discharging impulses of the Pressor area
neurons
Effect:
 Decrease HR,SV
 BP (due to vasodilation)
 Venodilatation increases storage of blood in venous
reservoirs & decrease VR.

17. Cardiovagal Centre
 Inhibitory pathways in the form of vagal fibers from the
medulla to converge on Sympathetic preganglionic neurons of
the spinal cord to decrease sympathetic activity.
On Heart: Decrease the HR & force of contraction.

18. Vagal fibers arise from 3 different nuclei located in the medulla.
A:Dorsal motor nucleus of the Vagus (Cardio-
inhibitory Centre).
B: Nucleus of tractus solitarius (NTS)
 Receives afferents from most of the baroreceptors & its fibers
are projected to the nucleus ambiguous & dorsal motor nucleus
of Vagus.
C: Nucleus ambiguous : lies lateral to the medullary
reticular neurons .
 Receives afferents via NTS and in turn sends impulses to the
heart.
 Also called as cardiac vagal center .
 Neurons are not tonically active.

19. Factors affecting the VMC
1: Neural factors:
A: Higher centers: Cerebral cortex, Hypothalamus ,Respiratory
center..
2:Afferent impulses: Circulatory system
Baroreceptors of Arterial & Atria & Chemoreceptors..
3: Other parts of the body: Cutaneous pain, Visceral pain
Proprioceptors, Loven’s reflex, Cold Pressor test.
2: Chemical factors :CO2.O2,H+ ions.

20. Nervous Regulation of HR
1:Impulses from higher centers:
Cerebral cortex :Motor cortex ,Ant temporal lobe, frontal cortex
,amygdala hippocampus act through hypothalamus.
Action:
Excitatory/ Inhibitory
Direct /indirect action the hypothalamus (emotions)
Conditioned reflex in to response to visual, smell, hearing stimuli
or even thoughts.

21. 3:Respiratory center: HR ↑ with inspiration & ↓ with expiration .
Respiratory sinus arrhythmia".
Mechanism: i: Central mechanism: Active inspiratory center
activation causing stimulation of the CAC.
II: Peripheral mechanism: Lung inflation stimulates the stretch
receptors in the alveoli -> afferent impulses via the pulmonary vagi
--> -- CIC & ++ CAC.
* Harrison's reflex: During expiration, HR ↓ due to decreased
activity of inspiratory center, elastic recoil of the lungs and
decreased venous return.

22. 2: Hypothalamus
Anterior hypothalamic nuclei :Control parasympathetic activities
-> ↓ HR stimulate CIC. During sleep & severe emotion.
Posterior hypothalamic nuclei: control sympathetic activities -> ↑
HR stimulate CAC.
Mild emotion, stress, muscular exercise or anxiety.
Voluntarily: ↑ /↓ HR : meditation/yoga.
Emotions : Mild : fear, anger, sexual excitation etc. -> ↑ HR.
Severe: Sudden shocking news -> ↓ HR.

23. Baroreceptors:
Mechanoreceptors/Stretch receptors
Sensitive to stretch of vessels by dynamic blood flow
Increased baroreceptors discharge inhibit tonic discharge of pressor
area of VMC & excite CVC.
Range of activation is 60 to 200 mmHg
60 mmHg is the threshold of baroreceptor reflex.
1:Arterial baroreceptors
Innervation: Vagus nerve
Except carotid sinus:
By carotid sinus N branch of IX N.

24. Afferents : To NTS secretes NT Glutamate.
Efferents: Excitatory glutaminigergic fibers terminates into VMC pressor
area thus stimulate GABA secreting inhibitory neurons
NA & Dorsal motor nucleus of vagus thus activate the neurons.
Buffer nerves : IX & X N branches  Sino aortic nerves regulate BP
2:Atrial receptors/Nonmylinated atrial afferents
Innervated by nonmylinated C fibers of X N.
Discharge is irregular & slow .Present all over atria & interseputm of atria
Function: atrial pressure impulses vasodilation mainly in renal
vascular circuit.

25. pO2 <50 mmHg ++Chemoreceptors impulses along IX a& X N
chemosenstive area of medullastimulating respiratory center
Ventilation drive &Parasympathetic --HR
Sensitive to chemical changes in blood :Co2 ,O2 and pH.
Carotid bodies: At bifurcation of CC A:N:Carotid sinus N IX N
Aortic bodies : around arch of aorta N: Aortic N of X N
Afferent: to NTS of medulla
Operate best 40 -100 mmHg. When stimulated they stimulate VMC
& respiratory neurons in medulla.
3:chemoreceptors

26. 2:Afferent impulses from the body other than the CV system:
Stimulation of the central end of any nerve containing afferent
fibers e.g. radial, sciatic or splanchnic ↑ / ↓ HR according to
the nature of the stimulus.
a- Impulses from skeletal muscles (Alams Smirk reflex)
Contraction of voluntary muscles produces reflex tachycardia.
Mechanism: Muscle contraction --> + + Proprioceptors in the
muscle & joints
--> Afferent somatic sensory nerves --> + + CAC & - - CIC -->
Increase HR.
Significance: Helps to ↑ blood supply to the active muscle.

27. 1) Cutaneous pain: Mild pain---> ++ VMC ,severe pain--> -- VMC
2) Severe visceral pain afferent fibers from the trigger zones -- VMC
Trigger zone reflexes : Carotid sinus, Eye balls, Larynx, Epigastrium &
Testes are sensitive.
A blow on these areas produces reflex severe bradycardia /fainting /
cardiac arrest/.
3) Afferent impulses from the Proprioceptors of skeletal muscles
during exercise ++ VMC.
Impulses from pain receptors:

28. 4) Loven reflex:
Stimulation of the afferent nerve of an organ produces local
vasodilatation in the active organ.
But generalized vasoconstriction due to stimulation of the VMC.
Leads to HT & localized vasodilation and increased blood flow to
the organ.
5) Cold pressor test:
If a normal person drips his hand in a cold water (4oC), ABP rises by
10-20 mmHg, due to stimulation of the VMC.

29. McDowall reflex:
Decreased venous return, i.e., decreased right atrial pr as in
hemorrhage leads to reflex increase of HR & of vasoconstrictor tone
of arterioles and venules.
Significance:
This helps to maintain the normal ABP to allow sufficient
supply of blood flow to the vital organs like brain, heart, & kidneys.

30. Marey's law
HR is inversely proportional to the ABP provided that other factors
that affect the HR remain constant. i.e., ↑ ABP  ↓ HR Stimulus: ↑
ABP.
Receptors: Arterial baroreceptors. Carotid sinus & aortic arch.
Afferent: sinus nerve of IX & aortic nerve of X. Called "depressor
nerves" as they resist rise in ABP.
Center: + + CIC & - - CAC.
Efferent: X N.
Response: - - SAN  ↓ HR.
Significance:↓ ed HR antagonizes the elevation of ABP & helps to
restore it to its normal level.

31. Carotid sinus syndrome
Condition in which the carotid sinus is abnormally hypersensitive.
Manifestations:
A slight pressure on the carotid sinus (e.g. during shaving or by
tight collar) stimulates the baroreceptors in the carotid sinus 
reflex ↑ vagal tone to the heart & ↓ sympathetic vasoconstrictor
tone to the blood vessels  bradycardia (↓ HR) and hypotension
↓ ABP cerebral ischemia & momentarily loss of consciousness
(fainting).

32. Treatment:
Denervation of the hypersensitive carotid sinus cures the
condition.
Clinical significance:
Carotid sinus reflex could be used to terminate an attack
of paroxysmal atrial tachycardia (PAT) but not of ventricular
origin, as the vagus does not supply the ventricles.