Cell Sytems- Muscle - Lecture notes 15,16,17 PDF

Preview

Summary

Skeletal Muscle Three different types of muscle  Skeletal (aka. voluntary)- muscle we control to move around  Cardiac- keeps our heart beating  SmoothOverview of muscle types Muscles are highly specialised for contraction => electrically excitable, contractile, extensible, elastic (return ...

Description

Skeletal Muscle Three different types of muscle  Skeletal (aka. voluntary)- muscle we control to move around  Cardiac- keeps our heart beating  Smooth Overview of muscle types Muscles are highly specialised for contraction => electrically excitable, contractile, extensible, elastic (return to initial shape and length) 

Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. The presence of sarcomeres manifests as a series of bands visible along the muscle fibers, which is responsible for the striated appearance observed in microscopic images of this tissue

Skeletal muscle The human body contains >600 skeletal muscles- 40-50% of total body mass Functions     

Force production--> for locomotion, postural support and breathing (forcing lungs to contract and expand) Support for soft tissues Control of entrances/ exits (some are smooth)- opening and closing of mouth Heat production- shivering generates heat Nutrient store

Muscle pairs Most skeletal muscles work in opposing pairs E.g. bicep, tricep Flexion of arm muscle  Biceps contracts  Triceps relaxes

Extension of arm muscle  Triceps contracts  Biceps relaxes Tendons attach muscles to bones, which act as levers Tendons and connective tissue     

Epimysium: a big sheet of collagen, collagen coats the muscle to keep it in shape and also transmit forces of contraction of the muscle Fascicles: bundles of muscle fibres Perimysium: surrounds fascicles- predominantly made of collagen Also in this layer- blood vessels and nerves to give a blood and oxygen supply, nerves stimulate muscles to allow contraction Endomysium surrounds each muscle cell and contains collagen- capillaries and mysosatellite cells (Can repair damage to muscle tissue) also contained in that

Muscle fibres (Cells)  Smallest units of muscle that give a normal physiological response – an individual muscle cell, if stimulated will behave exactly the same as the muscle would  Enormous (up to 300mm long, diameter= 100um)  Multinucleate, but enclosed by single plasma membrane (formed by fusion of myoblasts)- this is so that over large distances, you don’t have to wait for proteins to travel to get to the end of a muscle cell- the DNA is always close to where you need it

    

Very long cells Within the muscle fibre, there are myofibril Myofibrils break up into sarcomeres Sarcoplasm- cytoplasm of a muscle cell Sarcolemma- membrane of a muscle cell

Sarcomeres Within the myofibril- 1000s of sarcomeres (~2.4 um long) are arranged in series

Each sarcomere consists of an orderly arrangement of thick (~1.6um)- red and thin filaments (1um)blue: Thin filament (5-6nm): made up of actin – blue Thick filament (10-12 nm): myosin- red Z-line- borders the sarcomeres This regular appearance of sarcomeres give muscle striated appearance. Sarcomere organisation I

Z line

 

Boundary between adjacent sarcomeres Contains proteins that interconnect and anchor thin filaments

 

Middle of sarcomere Contains proteins that stabilise thick filaments

M line

I band: light region

A band: dark

Sarcomere proteins Stabilizing proteins α-actinin 

Forms Z-line

M-line made up of myomesin protein and C-protein Nebulin 

Wraps around actin filaments and keep their particular structure

Titin (elastic protein)  Largest known protein (3.8 million Da)  Restores sarcomere length after contraction  Holds myosin in place and attaches it to Z line Proteins responsible for muscle contraction Actin 

20% of myofibril protein content



60% of myofibril protein content

 

Large family of motor proteins that move along actin filaments Isoform found in skeletal muscle: myosin II (~500kDa)

      

Family of globular multi-functional proteins (~42 kDa) G-actin can polymerise to become F-actin Each G-actin molecule has an active site that can bind a myosin head Within a thin filament, there are about 300-400 molecules of G-actin 2 strands wrap around each other to form filamentous actin, like DNA G-actin is a monomer while F actin is the actin filament + end- attached to Z line

Myosin

Myosin

Actin

Sliding filament model As the muscle contracts, actin filaments slide relative to myosin filaments  Increasing amount of overlap  Shorter distance between Z lines  Length of thin and thick filaments are unchanged Remember- Compression of titin during contraction = compressed spring

Myosin= motor molecule  Converts energy (ATP) to mechanical energy (power stroke)  Actin= track along which myosin moves  Actin-myosin interactions are responsible also for other cell movements (e.g. cell division) Experimental length-tension relationship    

Move transducer up in steps, stimulating each step Force before stimulus= passive (elastic) force Force during stimulus= total force Total- passive = active force

B * Looking at B in image above  There is an initial force (called passive force)  Stimulus- muscle contracts  Increasing length of muscle increases passive force C Overall, no matter what length the muscle was, the total force stayed about the same Increasing length of muscle corresponds to less actual force produced Active force: taking force you see during stimulation subtracted by passive force Longer the muscle gets, less active force used   

Active force profile results from sarcomere structure  Thick myosin overlap thin actin filaments and bind to them and pulling Z-lines in, contracting the sarcomere if muscle is really short- a lot of overlap between the thin and thick filaments=> fewer cross-bridge interactions because too many actin filaments are getting in the way Optimal overlap between thick and thin filaments- Resting state= optimal overlap between thick and thin filaments => maximum cross-bridge interaction if muscle is pulled out really long --> Thin filaments pulled out far from thick filaments => no crossbridge interactions => low tension

Control of muscle contraction As described so far, a muscle would contract continuously until it ran out of ATP- this is NOT how a muscle is controlled  At a molecular level, control is exerted by allowing or preventing myosin from binding to actin Control of actin-myosin binding For muscle contraction to occur, myosin and actin need to bind BUT: in a relaxed muscle, a troponin-tropomyosin complex blocks myosin binding sites on actin filaments and prevents actin-myosin interaction Ca2+ binding by troponin C leads to conformational change => troponin-tropomyosin complex slides away, exposing active sites on actin

 

Tropomyosin mimics actin, coiled-coil formation wraps all the way around the actin filament and completely blocks the actin binding site for myosin Troponin complex: see above

Calcium is important – difference in relaxed and contracted muscle- some change has to occur to expose binding sites  

If calcium is present, it binds troponin C causing a conformational change This pulls troponin complex troponin T pulls tropomyosin away from actin bnding site, allowing myosin to bind

What controls calcium In order to contract, a skeletal muscle must:  be stimulated by a motorneuron  Propagate the electrical signal along its sarcolemma Signal must be distributed quickly throughout the large muscle cell, so all regions contract at the same time  Increase intracellular Ca2+ concentration, the final trigger for contraction  Whole process called excitation-contraction (EC) coupling- linking the electrical signal to the contraction Neuromuscular junction (NMJ)  Site where the motorneuron meets the muscle fibre 

Ach= acetylcholine (a neurotransmitter) NAChR= nicotinic acetylcholine receptor EJP= excitatory junctional potential (equivalent to EPSP in a neuron)  

The nerve terminals are embedded in the sarcolemma If a nerve impulse comes down the motor neuron, this triggers the opening of the presynaptic Ca2+ channels so calcium rushes into the axon, this induces the release of Ach,

Ach that is released will bind to nAChRs on the sarcolemma and this triggers the excitatory-junctional potential (equivalent to EPSP in a neuron) in the muscle.

Muscle membrane action potentials  The muscle membrane (sarcolemma) has a voltage-dependent Na+ and K+ channelssimilar to those in neurons  EJPs are usually supra-threshold, so the muscle membrane spikes when the motorneuron spikes  Muscle spikes enable signal to be conducted rapidly across a large surface  Muscle spikes spread down T-tubules into interior of fibre (T=transverse)  T-tubules are like invaginations in the sarcolemma that allow spike to be transmitted in cell Muscle spikes  Each muscle spike causes a twitch contraction  Giving an electrical signal gives a corresponding contraction Summation and tetanus  Individual twitch contractions can summate  At high frequency, twitches fuse into a tetanic contraction- this is most force a muscle can develop- muscle is permanently contracted until signal is removed or fatigue sets in (e.g. muscle cells run out of ATP or calcium)

How do we go from a muscle action potential to contraction? Membrane systems 2 membrane systems play a key role in EC coupling in skeletal muscles  External: T (transverse) tubules  Internal: sarcoplasmic reticulum (SR) Myofibrils are wrapped in SR and contact T tubules

Functions of T tubules and SR T (transverse) tubules:  Narrow tubes that are continuous with the sarcolemma  Conduct muscle spikes deep into the sarcoplasm

SR (sarcoplasmic reticulum)  Elaborate smooth ER that surrounds each myofibril  SR tubules enlarge, fuse and form chambers (terminal cisternae) on either side of the T tubule- triad  Muscle impulses trigger Ca2+ release from SR into sarcoplasm Dihydropyridine and ryanodine receptors

Ca2+ is stores in SR, pumped in by ATP-dependent calcium pump T tubules and SR are connected by 2 linked proteins:  in T tubules: dihydropyridine receptor (DHPR)- aka L-type calcium channel  In SR: Ryanodine receptor (RyR1: an intracellular calcium channel)

     

DHPR is voltage sensitive DHPR is directly couples to RyR RyR is a calcium channel Spike activates DPHR which opens RyR Allows huge efflux of Ca2+ out of SR and into sarcoplasm (exposing myosin binding sites) Contraction happens

Control of actin-myosin binding

Ca2+ binding by troponin C leads to conformational change by exposing active sites on actin SERCA= sarco-ER calcium ATPase Muscle relaxation Duration of muscle contraction depends on:  Period of stimulation at NMJ  Availability of free Ca2+ in sarcoplasm  ATP availability As APs cease to arrive at NMJ  ACh is broken down by AChE ( a cleft enzyme, acetylcholine esterase)  SR actively reabsorbs Ca2+ (calcium pump)- also some Ca2+ transport into extracellular fluid  Ca2+ concentration declines  Troponin-tropomyosin complex returns to normal position, blocking active sites  Contraction ends, muscle returns passively to resting length

Cardiac Muscle Cardiac muscle Found only in the heart wall and at the base of large veins  Very small proportion of body muscle mass- weight of adult heart- 250-350g  Crucial function: pump blood round the body Our most heavily worked muscle, contracting ~100,000 times and pumping ~8000 litres of blood a day (~3 billion in a lifetime) Cardiac muscle must:  Contract forcefully and rhythmically in a highly coordinated fashion- spiral arrangement  Modify force according to circulatory needs- when we exercise, we want it to pump blood faster



NEVER tire or tetanise- don’t want the muscle - don’t want to be giving repeated signals for it to contract to get summation building up to a point of tetanus (maximum permanent contraction)

Muscle structure  Similar to skeletal muscle  Cardiac muscle cells (cardiomyocytes) also consist of myofibrils and sarcomeres  same general structure- multiple myofibrils packed inside the cell and myofibrils being repeated units of sarcomere along the cell



Cardiac muscles do not divide => injury repaired by connective tissue, more of like a patch so lose function since connective tissue does not do the same job (does not contract)

Cardiac membrane systems Some basics components as skeletal muscle: T-tubules and SR But some structural differences:  Larger T tubules  SR less well organised- forms diad with T-tubules (rather than triad as in skeletal muscle)  Also difference in mechanism of Ca2+ release from SR Other important structural differences:  Individual muscle cells are much smaller- 50-200um long, diameter- 10-20um  Single nucleus- when heart is formed, cardiomyocytes do not fuse with other cells  Cells are joined together in branching linear array, so looks like long fibres that branch and converge  Cells contain large, densely packed mitochondria  Reliable energy supply What are intercalated disks?  Dense bands that cross muscle, typically in irregular lines  Occur at junction between individual fibres  They are only found in heart muscle Intercalated disks  Specialised structures that join neighboring cells end-to-end



Contain 2 types of membrane junction: desmosomes and gap junctions



Desmosomes: strengthen tissue (mechanically hold cells together through intercalated disks) Gap junctions: communicating junctions that connect cytoplasm of adjacent cells Connexins- proteins in cell membranes that group together to form connexonshydrophilic channels that when cells are in close contact, can join together to form one continuous channel thus allowing material from one cell to pass through to the other cell

 

Gap junctions gap junctions are an electrical synapse  Electrical signal transmitted and synchronized throughout tissue  electrical signal can be transmitted through the gap junction from one cell to the other Direct exchange of ions and small molecules (e.g. calcium)  Common response to local regulators Intercalating disks are essential for cardiac muscle function:  They connect cells mechanically, electrically and chemically  Once cell is contracting, if they aren't held together, the cells can rip apart and cause extensive damage  Allow for branching by joining >2 cells  

Strong and efficient force-conducting network Synchronized contraction due to electrical synapses

Cardiac muscle contraction Many similarities to skeletal muscle in basic contraction mechanism  Begins with action potential across cell membrane  This causes rise in sarcoplasmic Ca2+ level by release from SR  This activates sliding filament mechanism But some key differences  Heart muscle contracts spontaneously- a myogenic rhythm. It is modulated by neural input but it is NOT directly driven by it  A skeletal muscle does not contract unless activated by its motorneuron  Action potential involves calcium inflow as well as sodium  Ca2+ release from SR through RyR is triggered by this inflow of Ca2+, rather than by direct coupling to DHPR Skeletal muscle action potential Nerve impulse--> pre-synaptic Ca2+ channels open --> ACh release --> ACh binds to nAChRs on sarcolemma--> EJP in muscle  Mimics the same as neurons  Stable resting potential exists in muscle cells

  

When there is a trigger from a neuron releasing ACh ligand, channels open, allowing sodium to come into cell to depolarise membrane potential turning it +ve When it hits a threshold, small depolarisation triggers voltage gated sodium channels to open and massive spike occurs causing a massive influx of sodium ions Then sodium channels close, potassium ions opens causing efflux of potassium ions

 https://www.verywellmind.com/what-is-an-action-potential-2794811 Pacemaker activity Muscle generates its own rhythm- pacemaker activity Two specialised types of cardiac muscle cells: 1. Contractile cells (~99%)  These do the work  They are NOT pacemakers

2. Pacemaker cells  Membrane potential shows regular spikes  These generate the rhythm Complex ionic mechanisms

    

Rising phase (depolarisation) is driven by inflow of Ca2+ --> differs from skeletal muscle, where driven by Na+ Mediated through voltage-dependent Ca2+ channels which inactivate- these are similar to but slower than, the standard the Na+ channels of the nerve action potential Falling phase is driven by outflow of K+ Mediated by voltage-dependent K+ channels- very similar to those in neurons But the key to pacemaker activity is the slow depolarization before the spikes

The "funny" current  The slow depolarization is driven by the funny current (If)  It is mediated by slow influx of Na+ making it more positive  The funny channel is activated by hyperpolarization and inactivated by depolarizationmost unusual, normally the other way around  Turned on by the extreme hyperpolarization after the spike  It belongs to HCN channel family  HCN- hyperpolarization-activated cyclic-nucleotide gated channel  Turned off at about the level the Ca2+ channels open  Its slope is key to controlling the timing of the heartbeat It belongs to HCN channel family- hyperpolarization-activated cyclic-nucleotide gated channels Spreading of contraction Pacemaker cells lie in four sites: atrioventricular node, sinoatrial node, bundle of his and Purkinje fibres Generates APs (action potential) at different rates:  Fastest: SA (sino-atrial) node -generates the rhythm of heart- others are basically accessory cells  AP spreads through gap junctions in atrium  Delay in AV node so atria contract before ventricles



AP spreads through ventricle

Organised into nodes and bundles to transmit contractile impulse to different parts of heart in precise sequence AP in contractile cardiomyocytes  Contractile (non-pacemaker) muscle fibres get driven by pacemaker fibres are transmitted through gap junctions (electrical synapses) in intercalated disks  They form an electrically-coupled network, and pass on the excitation to other contractile fibres Basic mechanism above:  Rapid depolarization- influx of sodium through voltage dependent ion channels  Cells triggers by AP of pacemaker cells  Transient K+ channels start to cause a repolarisation (decrease in membrane potential) rather than being rapid falling, there is just a small decrease  L-type calcium channels open at this point  L-type channels- slow long lasting channels open to let calcium back into the cell give  Creating a plateau through AP where they are maintaining the membrane potential being positive- slow drop off but very slow, steady decrease  When potential is 0 they close and get a rapid repolarisation- K+ leaves cell so bring membrane potential back down to resting level Key features: contractile fibre APs are very long duration (250ms) due to long-lasting Ca2+ current



Prolonged refractory period- membrane potential is effectively positive for a large part of it, if there was another stimulus, cells would not be able to respond

Calcium release in cardiac muscle     

DHPR is a voltage dependent Ca2+ channel- L-type for long lasting DHPR is not directly linked to RyR2 Instead, RyR2 is itself Ca2+ activated A small amount of Ca2+ enters through DHPR during an action potential- DHPR release calcium slowly and steadily This small amount of calcium triggers the RyR2 to release a large amount of Ca2+ release from the store in the SR

Summary of cardiac EC coupling  

More calcium you have to bind to tropomyosin complex, the more that is uncovering the myosin binding s...