Anatomy of the ANS/Autonomic Reflexes

Question 1

What is the Baroreceptor reflex?

Answer 1

BRR - involved in blood pressure control

Blood pressure = cardiac output / total peripheral resistance

CO = HR X Force Of Contraction

No autonomic control of heart rate, HR = 100-110 BPM
Usually 60-70 BPM.
Parasympathetic (Vagal) influence decreases HR
Detected and controlled by arterial baroreceptors.

Question 2

What do baroreceptors do?

Answer 2

They are sensing pressure in the system and relaying that to the brain.
brain determines level of autonomic activation that is transmitted back down.

If baroreceptor firing rate is normal –> CNS –> parasympathetic nerve to heart to be equally activated + increase inhibitory signal to sympathetic nerve to decrease activation.

If BP increases, baroreceptor firing rate increases to CNS –> increased parasympathetic signal to Heart to increase HR + increased inhibitory signal to sympathetic nerve to inhibit sympathetic activity
–> greater parasympathetic effect to bring BP back down

If BP drops, decrease in BR firing –> decreased parasympathetic nerve signals to heart, and decreased inhibitory signals to sympathetic nerves, increasing sympathetic activity ( and HR and BP)

Question 3

Baroreceptor reflex - change of posture?

Answer 3

This is a severe challenge to human circulation - gravity pushes down on the column of blood as you stand up.

Blood pressure above heart (standing) should be lower than mean (gravity pushing it down)
Blood pressure below should be higher than 120/80mmHg as blood works against gravity

Question 4

What happens with a change of posture?

Answer 4

• Increased hydrostatic pressure in the blood vessels of the legs (VENOUS DISTENSION - more blood in venous system, lower arterial blood pressure)
• Hydrostatic pressure is higher due to gravity increases the force of fluid into tissues from capillaries, CAPILLARY FLUID LOSS.
• this induces a hypotensive effect - less blood returns to the heart, less contractility of the heart, and less blood into the arterial system –> decrease of BP

Question 5

How to normalise Cardiac Output?

Answer 5

Using the sympathetic nervous system
- nerves, preganglionic ACh neruon synpases onto post ganglionic postganglionic sympathetic neuron releases noradrenaline –> heart increasing HR

• preganglionic sympathtetic nerve innervates Adrenal gland - adrenaline produced in the medulla, released into circulation and acts on heart to increase contractility and BP.

Question 6

What is the Pupillary Light Reflex?

Answer 6

a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation to various levels of lightness/darkness.

Question 7

What is the PLR controlled by?

Answer 7

Autonomic control of the IRIS muscle
Parasympathethic preganglionic (ACh) nerve passes to the Ciliary ganglion (close to the eye) synpases to post ganglionic fibre (ACh).
When activated - ACh released and PUPIL CONSTRICTS.
At rest PNS keeps pupil mildly constricted.

Pupil is controlled by IRIS muscle and contracts or dilates as a result.