Baroreceptors

[Ref: WG21:chp31]

Stretch receptors in the walls of the heart and blood vessels.

Stimulated by distention of the structures

 

Neural pathways

Step 1

Afferent fibres from baroreceptors project to medulla.

Most end in the nucleus of the tractus solitarius (NTS)

Neurotransmitter: probably glutamate

Step 2

From NTS to caudal and intermediate ventrolateral medulla (CVLM, IVLM)

--> Excitatory (?glutaminergic) projections

Step 3

From CVLM, IVLM to RVLM

--> Inibitory (GABA) projections

Step 4

From RVLM

--> IML

 

Thus,

Increase in BP

--> Increase in baroreceptor activity

--> Increase in inhibition of RVLM (where stimulatory sympathetic neurons are)

Resulting in:

• Inhibition of tonic discharge of vasoconstrictors
• Excitation of vagal innervation of heart

End-result:

• Vasodilation and venodilation
• Drop in BP, HR, and CO

 

High pressure baroreceptors

i.e.

• Carotid sinus baroreceptor AND
• Aortic arch baroreceptor

Note:

• Baroreceptors are in the adventitia of the vessels
• Increase in BP
  --> Increase nerve discharge
• Chronic hypertension resets the reflex mechanism
  --> Less discharge at the same BP

Carotid sinus baroreceptors

... small dilation of the internal carotid cartery just above the bifurcation of the common carotid artery

• Responds to both sustained BP and pulsatile BP
• Drop in pulse pressure (MABP kept same)
  --> Decreased discharge
• Maximum firing at about 200mmHg
• Minimal firing at about 50mmHg [BL8:p189]

Afferent neural pathway from carotid sinus

--> Sinus nerve

--> Glossopharyngeal (IX) nerve

--> NTS

• Same sinus nerve innervates carotid body [WG21:p608]

Aortic arch baroreceptors

... in the wall of the arch of the aorta

• Innervated by vagus (X) nerve
• No distinct branch (unlike sinus nerve of CN IX)
• Responds to changes in pulse pressure, but not as sensitive to MABP
  [BL8:p188]

Note:

• Buffer nerves - Sinus nerve and the non-distinct vagal fibres innervating aortic arch baroreceptors

Cardiopulmonary baroreceptors

aka Low pressure baroreceptors

aka Volume receptors

Located in the walls of right and left atria, at the entrance of SVC and IVC

Atrial stretch receptors

Two types:

• Type A: Discharges primarily in atrial systole
• Type B: Discharges primarily in atrial filling

Discharge of type B is increased by increased distention of atrial walls

e.g. increase venous return,

Note:
Dischage is decreased by PEEP

Increased discharge of atrial stretch receptors

... causes:

• Vasodilation
• BP decreased
• HR increase (rather than decrease)
  --> Basis of Bainbridge reflex
• Inhibition of angiotensin, aldosterone, and vasopressin release
  --> Plays a role in long term BP control

Other notes

[KB2:p23]

• Threshold for volume receptor = 7-10% change in volume
• Threshold for osmoreceptor = 1-2% change in osmolality
• Volume receptor is less sensitive but more potent than osmotic stimuli