Topic 6: Organisms response: Control of heart rate

Autonomic Nervous System

Heart rate is controlled as part of the autonomic nervous system. The autonomic nervous system controls involuntary (subconscious) activities of internal muscles and glands

Autonomic Nervous system is made up of two parts:

1. Sympathetic nervous system - causes any response that prepares body for action - fight or flight - generally stimulates effectors and speeds up activity

2. Parasympathetic nervous system - causes any response that restores normal function - rest or digest- generally inhibits effectors and slows down activity

Heart contraction step 1

• myogenic - contraction is initiated from within the muscle itself, rather than by nervous impulses from outside

• the sinoatrial node (SAN- the heart’s pacemaker) generating the electrical impulse

• located within the walls of the right atrium

Heart contraction step 2

SAN sends a wave of electrical activity (depolarisation) across both atria causing atrial contraction

Heart contraction step 3

• non-conducting tissue (atrioventricular septum) prevents the wave crossing to the ventricles and contracting at the same time as atria

• the atrioventricular node (AVN) delays electrical activity allowing atria to contract and fully empty before ventricles contract

Heart contraction step 4

AVN sends wave of electrical activity (depolarisation) between the ventricles along a series of specialised muscle fibres called the purkyne tissue, which collectively make up a structure called the Bundle of His

Heart contraction step 5

• the bundle of His conducts the wave through the ventricular septum to the base of the ventricles, where the bundle branches into smaller fibres of the Purkyne tissue

• the wave of activity from the Purkyne tissue causes the ventricles to contract from the bottom, upwards

What changes happen to heart rate during exercise? Why?

Increases because:

• increased blood flow

• more oxygen (and glucose) to respiring tissue

• during exercise more carbon dioxide is produced from aerobic respiration and lactic acid from anaerobic respiration

Heart rate is controlled by the part of the brain called the medulla oblongata. Has two centres:

1. One centre increases heart rate linked to the sinoatrial node by the sympathetic nervous system

2. One centre decreases heart rate linked to the sinoatrial node by the parasympathetic nervous system

The centre of the medulla oblongata that is stimulated depends on:

Chemical changes or pressure changes in the blood

Control by chemoreceptors

• found in the walls of the carotid arteries (arteries that serve the brain) and walls of aorta

• Sensitive to changes in the pH of the blood that result from changes in carbon dioxide concentration (In solution carbon dioxide forms an acid, lowering pH)

Pathway of control by chemoreceptors

1. Stimulus - decreased pH

2. Receptor - chemoreceptors in walls of aorta and carotid artery

3. Coordination - more electrical impulses sent to medulla oblongata (acceleratory centre stimulated) and then impulses sent via sympathetic nervous system to SAN. SAN stimulated by noradrenaline (neurotransmitter) and frequency of electrical impulses increases

4. Effector - SAN tissues - cardiac muscle

5. Response - increase heart rate to deliver blood to lungs rapidly to remove carbon dioxide

(Important as excess acid must be removed from blood rapidly to prevent enzymes from denaturing)

Heart rate control involves the parasympathetic and sympathetic nervous system. Suggest and explain how heart rate may change at rest. Explain why.

• less respiration in tissues

• less carbon dioxide produced, so pH increases

• activity in parasympathetic nervous system is greater than sympathetic nervous system

• SAN lowers heart rate/does not increase

Pathway of control by pressure receptors (high pressure)

1. Stimulus - increase ion pressure

2. Receptor - pressure receptor (baroreceptor) in walls of aorta and carotid artery are stretched if high blood pressure

3. Co-ordination - more electrical impulses sent to medulla oblongata and then impulses sent via parasympathetic nervous system to SAN. SAN stimulated by acetylcholine and frequency of electrical impulses decreases

4. Effector - SAN tissue - cardiac muscle

5. Response - reduced heart rate

(important as if blood pressure too high can cause damage to the walls of the arteries)

Pressure receptors are found in the walls of the carotid arteries and walls of the aorta

Sensitive to changes in blood pressure (force of blood pushing against artery walls)

Factors affecting force:

• cardiac output = heart rate x stroke volume

• resistance - artery diameter and flexibility, blood viscosity

Pathway of control by pressure receptors (low pressure)

1. Stimulus - decreased pressure

2. Receptor - pressure receptors in walls of aorta and carotid artery are not stretched if low pressure

3. Coordination - more electrical impulses sent to medulla oblongata and then impulses sent via sympathetic nervous system to SAN.

4. Effector - SAN tissue - cardiac muscle

5. Response - increased heart rate

(Important as if blood pressure too low, there may be insufficient supply of oxygenated blood to respiring cells and removal of waste)

Cardiac muscle is myogenic, which means that the contraction of the cardiac muscle initiates within the heart itself

The basic rhythm of the heart is maintained by a wave of electrical excitation

The wave of excitation travels through the following structures:

1. Sinoatrial node (SAN)

2. A layer of collagen fibres

3. Atrioventricular node (AVN)

4. Bundle of His

5. Purkyne fibers

Sinoatrial node (SAN)

initiates the heartbeat by stimulating the atria to contract

A layer of collagen fibres

prevenst direct electrical flow from atria to contract

Atrioventricular node (AVN)

picks up the electrical activity from the SAN and imposes a slight delay

Bundle of His

receives electrical activity from the AVN and conducts the wave of excitation to the apex (base) of the heart

Purkyne fibers

these branch off of the bundle of His, causing the right and left ventricles to contract from the bottom upwards

Nervous control of the heart rate

The SAN sets the heart’s basic rythm. The brain and autonomic nervous system can adjust the heart rate based on the body’s demand

The medulla oblongata in the brain stem acts as the control centre. It receives information from receptors in the carotid arteries and aorta

the medulla oblongata then regulates the SAN’s firing rate to adjust the heart rate

Information from receptors in the carotid arteries and aorta:

1. Baroreceptors - blood pressure receptors

2. Chemoreceptors - chemical receptors

How the autonomic nervous system responds during exercise:

1. Blood has a higher concentration of carbon dioxide, and so a lower pH

2. Chemoreceptors in the carotid arteries and aorta detect this and increase the frequency of nervous impulses sent to the medulla oblongata

3. The medulla oblongata increase the rate of impulses to the sinoatrial node (SAN) via the sympathetic nervous system

4. Heart rate increases

5. This provides the extra oxygen required for increased respiration in active muscles

6. The increased blood flow also helps remove excess carbon dioxide via the lungs

7. Carbon dioxide concentration reduces

Hormones, such as adrenaline and noradrenaline, can also influence heart rate.

In times of stress, these hormones are released and cause the SAN in the heart to increase the heart rate as part of the ‘fight or flight’ response

High Blood Pressure: Baroreceptors → Parasympathetic

Neurotransmitter: Acetylcholine

Effect on SAN: decreased electrical activity

Change in heart rate: Slows down

Low Blood Pressure: Baroreceptors → Sympathetic

Neurotransmitter: Noradrenaline

Effect on SAN: increased electrical activity

Change in heart rate: Speeds up

High oxygen, low carbon dioxide or high pH:Chemoreceptors → Parasympathetic

Neurotransmitter: Acetylcholine

Effect on SAN: decreased electrical activity

Change in heart rate: Slows down

Low oxygen, high carbon dioxide or low pH:Chemoreceptors → Sympathetic

Neurotransmitter: Noradrenaline

Effect on SAN: increased electrical activity

Change in heart rate: Speeds up