Skeletal Muscles, Control of Heart Rate, The Kidneys and Homeostasis

What is the SAN an its function

Distinct group of cells in the wall on the right atrium that have spontaneous rhythmic electrical activity (action potential)

Referred to as the pacemaker because it initiates the heart heart

What is the AVN and its function

Distinct set of cells between the atria

Responsible for conveying electrical activity down to the ventricles

What is the bundle of His and its function

Specialised conducting tissues through the septum

Divides into smaller branches called purkyne tissues which spread into the ventricles

How does the heart beat

• SAN is myogenic so initiates the heart beat

• SAN causes a wave of electrical activity to spread across both atria causing them to contract

• A layer of non-conductive tissues prevents electrical activity spreading to the ventricles

• The wave of electrical activity enters the AVN and causes a short delay which allows atria to empty before ventricles contract

• The AVN conveys a wave of electrical activity along the bundle of his to the bas of the ventricle

• Purkyne tissues conduct electrical activity into the ventricles, causing them to contract from the base upwards

How does the nervous system react when blood pressure is higher than normal

• If blood pressure is higher than normal, pressure receptors in the aorta and wall of carotid artery sends more impulses via sensory neurones to the cardiac centre

• Cardiac centre sends more impulses via parasympathetic neurones to the SAN and acetylcholine is released

• Heart Rate decreases

• Blood Pressure decreases

How does the nervous system react when blood pressure is lower than normal

• If blood pressure is lower than normal, pressure receptors in the aorta and wall of carotid artery sends more impulses via sensory neurones to the cardiac centre

• Cardiac centre sends more impulses via sympathetic neurones to the SAN and nor-adrenaline is released

• Heart Rate increases

• Blood Pressure increases

How does the nervous system react when blood pH is lower than normal/ high CO2 concentration

• Chemoreceptors in wall of carotid artery sends more impulses to the cardiac centre Cardiac

• Cardiac centre sends more impulses via sympathetic nervous system to the SAN and noradrenaline is released

• Heart rate increase

• Increased blood flow removes CO2 faster via lungs, CO2 levels hence blood pH returns to normal

How does the nervous system react when blood pH is higher than normal/ low CO2 concentration

• Chemoreceptors in wall of carotid artery sends more impulses to the cardiac centre Cardiac

• Cardiac centre sends more impulses via parasympathetic nervous system to the SAN and acetylcholine is released

• Heart rate decreased

• Decreased blood flow removes CO2 slower via lungs, CO2 levels hence blood pH returns to normal

Structure of a muscle

• A muscle contains ___________ that are arranged parallel along the length of the muscle

• Each muscle fibre is a single muscle cell

• Inside muscle cells/muscle fibres are many _________

• Myofibrils are ________________ that cause contraction

• These split up into sections striped called ____________

• Sarcomeres contain thin filament (____) and thick filament (____)

• A muscle contains bundles of muscle fibres that are arranged parallel along the length of the muscle

• Each muscle fibre is a single muscle cell

• Inside muscle cells/muscle fibres are many myofibrils

• Myofibrils are bundles of protein filaments that cause contraction

• These split up into sections striped called sarcomeres

• Sarcomeres contain thin filament (actin) and thick filament (myosin)

Describe the structure of the sarcomere

• Contains thick (myosin) and thin filament proteins (actin)

• I band (Light band) only actin, no myosin

• A band (Dark band) myosin and actin

• H Zone, at centre of each A band: myosine without actin (Slighlty lighter than A band)

• M Line, middle of sarcomere where myosin fillaments held together

• Z line, centre of each I band, actin fillaments attached

Describe what happen when a muscle contracts

• Muscle fibres contract in response to being stimulated by a nerve

• Myofibrils shorten due to sarcomere shortening

• I bands get shorter

• A bands stay the same length

• H zone get shorter

• Z lines move closer together

• Sarcomere gets shorter because fillaments slide over each other

Describes what happens at the neuromuscular junction after the arrival of an action potential

• An action potential arrives at the end of a motor neurone, at the neuromuscular junction

• Voltage gated calcium ion channels open and calcium ions diffuse into the synaptic knob via facilitated diffusion

• This causes synaptic vesicles to move down and fuse with the pre synaptic membrane, releasing neurotransmitters (acetylcholine) into the synaptic cleft

• Acetylcholine diffuses across synaptic cleft and binds to sodium ion channels on the sarcolemma (muscle cell membrane)

• This causes sodium ion channels to open and sodium ions diffuse into via facilitated diffusion, causing depolarisation of the the sarcolemma

• If threshold value is reached, an action potential is propagated along the sarcolemma and down transverse tubules

• The transverse tubules are in contact with the sarcoplasmic reticulum so the action potential causes calcium ion channels in the sarcoplasmic reticulum to open and causes calcium ions to diffuse out of it into the sarcoplasm via facilitated diffusion

Describe the process of the cross bridge cycle in the neuromuscular junction after an action potential has been received and calcium ions enter sarcoplasm

• Calcium ions cause tropomyosin on action to be moved out of the way of the binding sites for myosin heads on actin. It also activates the ATP hydrolyse on the myosin heads

• Myosin heads attach to the binding sites on actin forming actinomyosin bridges

• ADP (+Pi) is released from myosin head. This causes myosin heads to bend (power stroke), pulling actin along so that actin filaments slides over myosin

• Sarcomere shortens (Contracts)

• ATP binds to each myosin heads and breaks actiomyosin bridges so that myosin heads detach from binding sites on actin

• ATP in hydrolysed by ATP hydrolyse on myosin heads. The energy is used to recock the myosin heads back to their original position

• Each mysoin head attaches to a binding sites further along the actin

Describe how a muscle will relax

• Stimulation of muscle cell via neurone stops - no more neurotransmitter released, no more depolarisation of sarcolemma, no more action potentials propagated down T-tubules, calcium ion channels on sarcoplasmic reticulum close, calcium ions actively transported back into the sarcoplasmic reticulum

• Tropomyosin returns to original position, covering binding sites for myosin heads on actin

• ATP hydrolyse on myosin heads no longer activated

• No actinomyosin bridges, sarcomere returns to original length before contractions

• Muscle relaxes

Describe the properties of slow twitch muscles

• Adapted for aerobic respiration and so can continue contracting for long periods

• Slow contraction speed, limited rate of oxygen supply

• No lactate produced, so not susceptible to muscle fatigue

• Contain many mitochondria and myoglobin. Myoglobin is similar to haemoglobin, and is used as an oxygen store in these muscles, helping to provide oxygen for aerobic respirations. Myoglobin has higher affinity for oxygen so only relates O2 when PO2 is very low (e.g. emergency situations)

• Good blood supply - many capillaries bringing O2 and glucose

Describe the properties of fast twitch muscles

• Adapted for anaerobic respiration and can Therfore only sustain short bursts of activity

• Fast contraction speed that is not limited by blood supply

• Lactate production leads to low pH and muscle fatigue

• Contains lots of glycogen which can be hydrolysed to glucose for more ATP production

• Little myoglobin/blood supply

• Few mitochondria as these are not needed for anaerobic respiration

• Contains phosphocreatine which provides Pi directly to ADP to regenerate ATP

• Allows rapid generation of ATP

• As anaerobic respiration only produces 2x ATP per glucose

What is the role of ATP in muscle contraction [4]

• Breaks actinomyosin bridges, allowing myosin heads to detach from actin

• ATP hydrolysis releases energy to recock myosin heads

• Causes power stroke

• Active transport of calcium ions back into sarcoplasmic reticulum

What is the role of Calcium ions in muscle contraction [2]

• Moves the tropomyosin to expose myosin binding sites on actin, allowing myosin to binds

• Actives ATP hydrolyse on myosin heads

What is the role of tropomyosin in muscle contraction [2]

• Blocks myosin binding sites on actin during muscles relaxation

• Moves out of the way in response to calcium ions, allowing myosin head to bind in muscle contraction

What is the role of myosin in muscle contraction [2]

• Myosin heads bind to actin and pull actin along

• Detach from actin and re-set further along actin the bind to next binding sites using ATP

What is osmoregulation

Daily balance between loss and gain of water

Optimum is maintained to ensure constant water potential of blood plasma and tissue fluid

Describe the process of ultrafiltration in the Bowman’s Capsule of the Kidney

1) Blood enters glomerulus via afferent arteriole

2) The diameter of the afferent arteriole > efferent arteriole resulting in a build up of hydrostatic pressure

3) Water, glucose, mineral ions and urea squeezed out of capillary to form glomerular filtrate

4) Blood cells and proteins can’t pass through as they are too big (unless damage to basement membrane)

5) This is known as ultrafiltration

What resists ultrafiltration in the Bowman’s Capsule (in the kidney)

• Capillary endothelial cells of the glomerulus

• Basement membrane (connective tissue) which acts as filtration barrier

• Hydrostatic pressure of fluid in renal capsule space (glomerular filtrate)

• Lower water potential of blood in glomerulus (created by large proteins and RBCs)

• Epithelial cells of the renal capsule

What are the two main adaptions of the Bowman’s capsules allowing for ultrafiltration

Podocytes

• Specialised cells on inner layer of renal capsule

• Spaces between them create filtration slits

• Glomerular filtrate can pass between cells

Endothelium of Glomerular Capillaries

• Tiny pores allow fluid to pass between cells, therefore hydrostatic pressure in the blood in the glomerulus is overcome

How is glucose reabsorbed in the proximal convoluted tubule of the kidney

• Sodium potassium pump actively transports sodium out of epithelial cells into blood

• This maintains a low concentration gradient of Na+ in the epithelial cells lining the PCT

• Na+ and glucose diffuse into the cells by co-transport from the filtrate

• Glucose goes against its concentration gradient, sodium goes down its concentration gradient

• Once glucose is inside the cell it can move by facilitated diffusion via a carrier protein into the blood capillaries

• 100% of glucose is reabsorbed

• Amino acids also reabsorbed this way

How is water reabsorbed in the proximal convoluted tubule