Regulation of Cardiovascular System – Part 2

 

Baroreceptor reflex

Baroreceptor reflex inhibits brain stem excitatory signals.

• ·       The baroreceptors are stretch receptors in the walls of the heart and blood vessels, stimulated by distention.

·       The carotid sinus and aortic arch receptors monitor the arterial circulation

1. Carotid sinus is innervated by glossopharyngeal nerve ( carotid sinus nerve )

     2. Arch of the aorta is innervated by vagus nerve ( aortic depressor nerve )

·      The baroreceptors are stretched receptors (stimulated by distention)
 

·      Receptors discharge at an increased rate when the distended pressure in the blood vessels rise.
 

·      The sensory information pass via the glossopharyngeal and vagus nerves to the medulla.
 

·      These impulses coming from baroreceptors when stimulated will
 
-        Inhibit sympathetic activity
-        Stimulate parasympathetic activity to hear and blood vessels

• High blood pressure causes an increase in baroreceptor activity, which signals the brain stem to reduce sympathetic activity in the heart and blood vessels and stimulate parasympathetic activity to the heart 

• Thus, increased, baroreceptor discharge,  inhibits the tonic discharge of sympathetic nerves and excites the parasympathetic activity of the heart.

• So barorecpters when stimulate will lead to vasodilation, venodilation, hypotension, bradycardia, and a decrease in cardiac output.

• when barorecptor discharge is reduce, it will lead to vasoconstriction , elevated blood pressure, increase heart rate and increase cardiac output. 

Baroreceptor reflex pathway

Pathway - 1 

Stretch 🠆 Increased discharge from baroreceptors   🠆      Impulses travel via cranial 

nerves ix and x to the medulla oblangata NTS   🠆      Inhibits the sympathetic discharge 

from caudal ventrolateral medulla ( CVLM) or vaso motor are to heart and blood vessels.

Pathway - 2

NTS   🠆     2nd order neurons   🠆       Stimulate cardiac parasympathetic neurons

Baroreceptor nerve activity

• The baroreceptors reflex constitute a reflex feedback mechanism that operates to stabilise blood pressure and heart rate.

• Any drop in systemic arterial pressure decreases the discharge in the buffer nerves, and there is a compensatory rise in blood pressure and cardiac output.

• Any rise in pressure produces dilation of the arterioles and decreases cardiac output until the blood pressure returns to its previous baseline level.

Relationship between blood pressure and heart rate

• Increase in heart rate, increase blood pressure and decrease in heart rate ,decrease blood pressure

• Exceptions

            -   Cushing's reflex
                             Hypertension and bradycardia

                   Hypertension is elevated blood pressure. Bradycardia is reduced heart rate.
    This cushing's ref;ex happens in head injury patients. In head injury , there will be a less blood flow into the vital organs inside the brains including the brain stem area that controls the heart. That is called vasomotor centre ( VMC). When there is less blood flow to this areas, Oxygen supply is reduced to this area. This situation is called as hypoxia. This hypoxia directly stimulate the sympathetic discharge to the heart and result is increasing blood pressure. This is a hypertension situation.  When blood pressure is increased, the baroreceptors are stimulates and send inhibitory impulses to the brain cardio inhibitory centre to reduce the blood pressure. But the hypoxic effect on the VMA , cannot be override by the baroreceptors. Baroreceptors can stimulate the parasympathetic area. Therefore vagul discharge is increased and heart rate is reduced. Because of that the bradycardia is occurred. This is cushing's reflex. 
     

Head injury observations

- level of consciousness ( level of consciousness is decreased)

- heart rate ( heart rate is decreased)

- blood pressure ( blood pressure is increased)

- reaction of pupil to light ( pupil constricts initially, later dilate)

- respiratory rate

Feedback regulation via the baroreceptors

• Furthermore direct effects on vasomoter discharge, in the baroreceptor pathway causes changes in endocrine function ( chemical regulation) and changes in the renal functions ( reduced urine output in hypovoleamia)

• These include,

                        - Renin - Angiotensin - Aldosteron pathway
                        - ADH ( vasopressin)

Factors that affect the heart rate

- decreased activity of arterial baroreceptors

- increased activity of atrial stretch receptors

- inspiration

- excitement / anger

- most painful stimuli

- hypoxia

- exercise

- thyroid hormones

- fever

• Heart rate slowed by,

            - increased activity of arterial barorecptors

            - expiration

            - fear

            - grief

            - stimulation of pain fibre in trigeminal nerve

            - increased intracranial pressure

Functions of baroreceptors

• stabilise heart rate and blood pressure
• maintenance of arterial blood pressure at rest
• maintenance of blood pressure under different situations 

            eg - blood loss