Explore Introductory Mammalian Physiology with these specialized notes tailored for Year 1 students at University College London. Dive into the intricacies of the Cardiovascular and Nervous System, exploring topics such as cardiac and smooth muscle dynamics, cardiac output, electrical and contracti...
Cardiac and Smooth Muscle
Cardiac muscle
• Objectives
o Structure of cardiac muscle
o Origin of the heart beat
o Action potential of cardiac muscle
o Regulation of the force of the heart beat
• Muscle
o 3 types of muscle
▪ Skeletal
• Striated – due to regular packing of actin and myosin within the muscle
▪ Smooth
• Irregular
▪ Cardiac
• Striated – due to regular packing of actin and myosin within the muscle
o Muscle fibre
▪ In striated muscle – cardiac and skeletal muscle have a similar structure
▪
o Filament
▪
▪
• Actin-myosin structures = same in skeletal and cardiac muscle
o Binding sites on actin molecules – regulated by calcium binding protein
(troponin)
• Cardiac muscle structure
,Cardiac and Smooth Muscle
o
▪ Intercalated discs
• Structural formations between myocardial cells of the heart
• Play roles in bonding cardiac muscles together + transmitting signals between cells
o Attachment points between cardiac muscle cells
• Not present in skeletal muscle
o Skeletal muscle cells fuse together during development = multinucleated
o Cardiac cells are individually nucleated cells → strongly bonded to one
another using intercalated discs
▪ Gap junctions
• Allows cardiac muscle cells to be electrically connected to one another – connect
cytoplasm of cardiac muscle cells
o Cells need to be connected to each other = allowing the heart to beat as a
syncytium – all of the cells beat together
• Ions flow through gap junctions between cells
▪ Attachment proteins
• Desmosomes
o Specialised adhesive proteins that anchor the ends of cardiac muscle fibres
together – so the cells don’t pull apart during contraction
• Contractile activity of a cardiac muscle
o Regular beating of the heart (myogenic activity) – due to an intrinsic pacemaker activity of the heart
o Cardiac muscle
▪ Supplied by nerve fibres that have their origin in the autonomic ganglia
• Autonomic nerves – modulate the rate and force of contraction of cardiac muscle
• Cardiac action potential
o
▪ AP in ventricles
• Larger magnitude than skeletal muscle
,Cardiac and Smooth Muscle
• Rapid activation → followed by slow repolarisation phase
▪ AP in atria
• Shorter than AP in ventricles
▪ AP in sinoatrial node
• Sinoatrial node
o Specialised myocardial structure – initiates the electrical impulses to
stimulate contraction
▪ Determines the heart rate
o Found in the superior vena cava and the right atrium
• Slow area of response – slow to reach threshold = to initiate depolarisation (defines
rate at which next action potential will be fired)
o Followed by rapid depolarisation → repolarisation
o AP in ventricles
▪
• Action potential – rapid depolarisation → followed by slow repolarisation
• Contraction – action potential – followed by muscle contractions
o Delay between action potential and contraction
• Action potential overlaps the muscle contraction
o Because the action potential is taking a longer period of time – there is
overlap with the contraction = co-incident
▪ No wave summation → no unfused tetani → no tetanus
• During action potential – muscle cannot be reactivated
• Repolarisation time gives a protective time where the
cardiac muscle cannot be activated again
• Shape of response
o Rapid opening of voltage gated Na+ channels – causing depolarisation
o At peak – voltage gated Na+ channels close + voltage-gated Ca2+ channels
open + voltage-gated K+ channels open
▪ Ca2+ + K+ enters cell = repolarisation
• Changing the contraction of cardiac muscle
o Force of contraction of cardiac muscle depends on the degree of stretch of the muscle fibres
▪ More blood entering the heart = the more blood pumped around the body = greater
contraction of cardiac muscle
• Greater stretch of cardiac muscle = greater force of contraction generated by muscle
= more force blood can be pumped around the body
o Regulated by hormonal signals – inotropic response
, Cardiac and Smooth Muscle
▪ Skeletal muscle does not respond to hormonal changes
▪ Cardiac muscle can be regulated by hormones
• Inotropic response
o Responses that will increase the intrinsic contractile component of the heart
o Strength of an individual contraction
• Length-tension relationship of cardiac muscle
o Stretch muscles = results in greater force of contraction
▪ Active tension – muscle is activated electrically
▪ Resting tension – muscle is stretched
o Muscles have a normal operating range = optimal length of muscles
▪ In cardiac muscle
• Work in normal operating range – below maximal level
o Never under so much stress that they become damaged
▪ In skeletal muscle
• Works in normal operating range – at maximal level
o Can be damaged, but heals over time
• Chronotropic regulation
o Regulation that will alter the rate of the response of a contraction
▪
o Pacemaker activity of the SA node- can be modulated by autonomic nerves
▪ Vagal stimulation
• Vagus = 10th cranial nerve
o Major input from gastrointestinal tract back to
the brain
• After we eat – vagus is stimulated
o Slows heart rate
▪ By reducing the rate at which the SA node reaches threshold
• By opening voltage-gated Na+ channels slower
▪ Sympathetic stimulation
• Sympathetic nervous system – fight or flight response
o Increases heart rate
▪ By increasing the rate at which the SA
node reaches threshold
• By open ing voltage gated Na+ channels quicker
• Excitation-contraction coupling within cardiac muscle – same as skeletal muscle
o Nerve action potential → ACh secretion by nerve ending → end
plate potential → muscle action potential → depolarise T-tubules
and open Ca2+ channels of SR → increase sarcoplasmic Ca2+ →
contraction → pump Ca2+ into SR → relaxation
▪ T-tubules – mostly in ventricular muscle
• Contraction cycle (crossbridge recycling) – same as skeletal muscle
o Myosin heads hydrolyse ATP and become reorientated and
energized
o Myosin heads bind to actin → forming crossbridges
o Myosin crossbridges rotate towards centre of the sarcomere –
power stroke
o As myosin heads bind ATP – crossbridges detach from actin
• Cardiac muscle summary
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