control of heart rate

heart

• cardiac muscle

• myogenic

• contractions can be altered by the SNS and PNS

myogenic

the heart can beat without an external nerve impulse - regular heart beat and contraction is initiated from within the muscle itself (SAN)

SAN

sinoatrial node

SAN (sinoatrial node)

has a basic rhythm of stimulation, determining the beat of the heart - “pacemaker”

AVN

atrioventricular node

autonomic nervous system

- responsible for controlling heart rate
- controls the involuntary activities of internal muscles and glands
- splits off into SNS and PNS - these 2 systems work together and are described as antagonistic

sympathetic nervous system

stimulates effectors, speeds up activity

parasympathetic nervous system

inhibits effectors, slows down activity

the heart beat

1. the SAN initiates the heart beat. it produces an impulse in the atria, causing them to contract. non-conducting tissue between the atria and ventricles (atrioventricular septum) prevents immediate contraction of ventricles.
2. the impulse enters the atrioventricular node (AV node), which after a delay sends the impulse down the Purkyne fibres in the Bundle of His.
3. the delay means the atria contracts first, allowing the atria to empty before the ventricles contract.
4. the ventricles contract from the bottom up.
5. this results in the blood being forced out the ventricles and in to the artery.

altering the heart beat

• heart rate can be increased or decreased by nerve impulses from the medulla oblongata in the brain

• parasympathetic nerve impulses connected to the SAN slows the heart rate

  • this uses acetyl-choline

• sympathetic nerve impulses connected to the SAN increase the heart rate

  • this uses nor-adrenaline

chemoreceptors - sympathetic nervous system

- usually linked with exercise
- higher respiration rate increases CO2 conc → lowers pH
- this reduction is detected by the chemoreceptors in the carotid artery
- impulses sent to medulla in brain
- medulla sends more impulses along the sympathetic NS to the SAN
- this stimulates SAN to increase rate at which it sends impulses, stimulating AVN, increasing impulses sent from AVN and increasing heart rate, removing CO2 from blood

pressure receptors - parasympathetic nervous system

- pressure receptors in the carotid artery
- if the pressure gets too high then impulses are sent to the medulla in the brain
- here, impulses are sent along the parasympathetic pathway to the SAN, which inhibits the SAN
- this decreases the impulses from the SAN, which reduces rate AVN is stimulated
- this reduces the heart rate, bringing the pressure down