control of heart rate

Myogenic

• When a muscle (cardiac muscle) can contract and relax without receiving signals from nerves

Sinoatrial node

• Located in right atrium and is known as the pacemaker

• releases wave of depolarisation across the atria, causing muscles to contract

Atrioventricular node

• Located near the border of the right/left ventricles within the atria

• releases another wave of depolarisation after a short delay when it detects the first wave from the SAN

Bundle of His

• A group of specialised muscle fibres in the heart

• located in the septum between the ventricles

• conducts the wave of depolarisation from the AV node down the septum

• → passes the impulse to the Purkyne fibres, causing ventricles to contract from the base upwards

Purkyne fibres

• Specialised conducting fibres in the walls of the ventricles

• receive the wave of depolarisation from the Bundle of His

• spread the impulse through the ventricular muscle, causing it to contract from the base upwards

• → ensures efficient pumping of blood out of the heart

Role of non-conductive tissue

• Found between the atria and ventricles

• Prevents the wave of depolarisation from passing directly from atria to ventricles

• Forces the impulse to travel through the AV node and Bundle of His

• → Creates a short delay, allowing the atria to fully contract and fill the ventricles before they contract

Importance of short delay between SAN and AVN waves of depolarisation

Ensures enough time for atria to pump all blood into ventricles before the ventricles contract

Role of the medulla oblongata

• Controls heart rate via the autonomic nervous system

• uses sympathetic and parasympathetic nervous system to control SAN rhythm

Chemoreceptors

• Located in carotid artery and aorta

• responds to pH / CO2 conc. changes

Baroreceptors

• Located in carotid artery and aorta

• responds to pressure changes

Response to high blood pressure

• Baroreceptors in the aorta and carotid arteries detect high pressure

• Send more impulses to the medulla oblongata

• Medulla increases impulses along parasympathetic neurones to the SAN

• Acetylcholine is released → slows SAN activity

• → Heart rate decreases, reducing blood pressure

• if blood pressure is too high this can cause damage to the walls of the arteries

Response to low blood pressure

• Baroreceptors in the aorta and carotid arteries detect low pressure

• Send more impulses to the medulla oblongata

• Medulla increases impulses along sympathetic neurones to the SAN

• Noradrenaline is released → increases SAN activity

• → Heart rate increases, raising blood pressure

• if blood pressure is too low there may be insufficient supply of oxygenated blood to respiring cells and removal of waste

Response to low blood pH

• Chemoreceptors in the aorta and carotid arteries detect high CO2 levels (low pH)

• Send more impulses to the medulla oblongata

• Medulla increases impulses along sympathetic neurones to the SAN

• Noradrenaline is released → increases SAN activity

• → Heart rate increases, removing more CO2 via the lungs and returning pH to normal

• During high respiratory rate or exercise, CO₂/lactic acid increases → pH decreases

• excess acid must be removed from blood rapidly to prevent enzymes denaturing

• so increase in heart rate so carbon dioxide can diffuse into the alveoli more rapidly

Response to high blood pH

Chemoreceptors in the aorta and carotid arteries detect low CO2 levels (high pH)

Send more impulses to the medulla oblongata

Medulla increases impulses along parasympathetic neurones to the SAN

Acetylcholine is released → slows SAN activity

→ Heart rate decreases, allowing CO2 to accumulate and pH to return to normal

Structure of myelinated motor neurone

Resting potential

• The difference between the electrical charge inside and outside the axon when a neurone is not conducting an impulse

• more positive ions (Na+ /K+ ) outside axon compared to inside

• inside the axon -70mV

How is resting potential established

• Sodium potassium pump actively transports 3 Na+ out of the axon, 2 K+ into the axon

• membrane more permeable to K+ (more channels and always open)

• K+ diffuses out down conc. gradient - facilitated diffusion

• membrane less permeable to Na+ (closed Na channels)

• higher conc. Na+ outside

Action potential

• Stimulus causes membrane to depolarise past the threshold (–55 mV)

• Voltage-gated Na+ channels open → Na+ diffuses in

• → Inside becomes more positive (depolarisation)

• At +40 mV, Na+ channels close and voltage- gated K+ channels open

• K+ diffuses out, making the inside negative again (repolarisation)

• Hyperpolarisation may occur as K+ channels are slow to close

Action potential: stimulus

• Stimulus causes the membrane to

• depolarise past the threshold (–55 mV)

• Voltage-gated Na+ channels open → Na+ diffuses in

• → Inside becomes more positive (depolarisation)

Action potential: depolarisation

• When a threshold potential is reached, an action potential is generated

• more voltage-gated Na+ channels open

• Na+ move by facilitated diffusion down conc. gradient into the axon

• potential inside becomes more positive

Action potential: repolarisation

• Na+ channels close, membrane becomes less permeable it Na+

• K+ voltage-gated channels open, membrane more permeable to K+

• K+ diffuses out neuron down conc. gradient

• voltage rapidly decreases

Action potential: hyperpolarisation

K+ channels slow to close -> overshoot in voltage

too many K+ diffuse out of neurone

potential difference decreases to -80mV

sodium-potassium pump returns neurone to its resting potential

Action potential graph

All or nothing principle

• If depolarisation does not exceed -55 mV threshold, action potential is not produced

• any stimulus that does trigger depolarisation to -55mVthreshold will always peak at the same maximum voltage

Importance of all or nothing principle

• Ensures that only stimuli above a threshold trigger an action potential

• Prevents response to small, insignificant stimuli — avoids overload

• All action potentials are the same size, so information is transmitted clearly

• → Stimulus intensity is shown by frequency, not size, of impulses

Refractory period

• After an action potential has been generated, the membrane enters a period where it cannot be stimulated

• because Na channels are recovering and cannot be opened

Importance of refractory period

• Ensures discrete impulses produced - action potentials separate and cannot be generated immediately

• unidirectional - cannot generate action potential in refractory region

• limits number of impulse transmissions - prevent overwhelming