2.2 Excitable tissue: Muscle Flashcards
What are the three types of muscle?
Skeletal - cross-striated, generally voluntary
Cardiac - cross-striated, functionally syncytial, involuntary
Smooth - not cross-striated
- visceral (unitary) - functionally syncytial
- and multiunit - eye, not spontaneously active
Discuss Skeletal muscle structure
What are characteristics of a muscle fiber?
made up of individual muscle fibres, most of which begin + end in tendons, arranged in parallel between tendinous ends so contractile force is additive.
NO syncytial bridges between cells in SkM
Each muscle fiber = single cell, multinucleated, long, surrounded by “sarcolemma” (cell membrane)
Muscle fiber is made up of “myofibrils” which are each made up of “myofilaments” with contractile proteins.
What are the key contractile proteins in skeletal muscle?
myosin-II
actin
tropomyosin
troponin (2 subunits, TnI, TnT, TnC)
What are striations?
See diagram in notes page 21 / Ganong 99
Cross-striations of skeletal muscle as seen under microscope, occur due to orderly arrangement of contractile proteins.
Z-lines allow for anchoring of thin filaments
Sarcomere = space between 2 adjacent Zs
What are the thick and thin filaments?
Thick filaments - Myosin-II
Has 2 heads containing an actin-binding site + ATP hydrolysis catalytic site.
Thin filaments
Actin - 2 chains of actin, forms a double helix
Tropomyosin - in the length of groove between the actin chains
Troponin - TnT binds tropomyosin, TnI inhibits interaction between myosin + actin, TnC has Ca2+ binding site to initiate contraction.
What are three additional structural proteins important to skeletal muscle function?
Actinin - binds actin to Z-lines
Titin - connects Z-lines to M-lines to scaffold the sarcomere, elastic domains
Desmin - binds Z-lines to cell membrane
Explain the sarcotubular system
The sarcotubular system is a system of membranous structures which surround the myofibrils
- Allows action potential to reach all myofibrils (even those located deeper) in the muscle fiber for coordinated contraction.
- Consists of T-system of transverse-tubules, which are continuous with the sarcolemma of muscle fiber, + the SR, which forms an irregular curtain around each myofibril.
- SR has enlarged “terminal cisterns” in close contact with T system at junctions between A+I bands.
“triads” - arrangement of central T-tubule with cistern of SR on either side.
What is the RMP of skeletal muscle?
- 90mV
What is a muscle twitch?
A muscle twitch is the brief contraction + relaxation caused by a single action potential.
It begins ~ 2ms after start of membrane depolarisation.
Duration of twitch varies depending on type of muscle fibers - “fast-twitch” as short as 7.5ms, “slow-twitch” up to 100ms.
Describe/illustrate excitation-contraction coupling in skeletal muscle.
See notes page 21 / Ganong 103.
What is the SERCA pump?
Sarcoplasmic + endoplasmic reticulum Calcium ATPase - hydrolyses ATP to actively transport Ca2+ back into SR to facilitate muscle relaxation.
What is the mechanism of skeletal muscle contraction?
Sliding of thin filaments over thick filaments -
AP causes rise in cytosolic Ca2+
Ca2+ binds to TnC, releasing TnI to expose actin binding site for myosin head
Myosin-actin cross-bridge formation
Myosin releases ADP, undergoes conformational change of myosin head that moves thin filament relative to thick filament = “power-stroke”.
ATP binds to free site on myosin, causes detachment of myosin from actin.
ATP is hydrolysed, causes re-cocking of myosin head
Cycle ready to be repeated.
What is isometric vs isotonic contraction?
isometric - contraction without change in muscle length (possible because of elastic + viscous elements also contained in addition to contractile proteins)
isotonic - contraction with change in muscle length against a constant load, may be concentric (shortening) or eccentric (lengthening) - does work (positive + negative work)
Explain summation of contractions
Electrical response of muscle fibre is similar to a neuron, in that there is a refractory period.
However the contractile mechanism does not have a refractory period, so repeated electrical stimulation results in an additive response = “summation of contractions”
Fusion into one continuous contraction = “tetanic contraction”.
“Complete tetanus” - no relaxation between stimuli - the tension developed is 4x that of single twitch contractions.
“Incomplete tetanus” - period of incomplete relaxation between summated stimuli.
Required stimulation frequency for tetanus depends on twitch duration of particular muscle
What are the types of skeletal muscle fibres and their properties?
Type - I - slow-oxidative IIA - fast-oxidative-glyclolytic IIB - fast-glycolytic See notes 22 / Ganong 106 for properties
What is phosphorylcreatine?
- Phosphorylcreatine - hydrolysed to creatine + phosphate groups with release of considerable energy.
At rest, mitochondrial ATP transfers its phosphate to creatine to build up a phosphorylcreatine store - during exercise, this can then be used to form ATP and permit contraction to continue.
Discuss metabolism of carbohydrates and lipids in muscle
Rest and during light exercise - muscles use lipids in form of FFAs as energy source
Increased intensity, lipids cannot supply energy fast enough -> carbohydrate is predominant energy source
Glucose -> CO2 + H2O
How? Glucose (and glycogen) are broken down into pyruvate. Pyruvate enters TCA cycle and is metabolised to CO2 and H2O via AEROBIC GLYCOLYSIS
If insufficient O2 (cannot use TCA cycle), pyruvate is reduced to lactate - ANAEROBIC GLYCOLYSIS (lower energy yield) - short term, self-limiting as lactate accumulation eventually exceeds buffering capacity, causing enzyme-inhibiting decline in pH.
What is the oxygen debt mechanism?
Increased blood flow to exercising muscles delivers increased O2 supply.
To a point, increase in O2 consumption is proportional to energy expended + energy needs are met by aerobic metabolism.
When exertion is very great (utilisation of energy stores > aerobic resynthesis) -
1) Phosphorylcreatine is still used to resynthesise ATP, and
2) Anaerobic metabolism of pyruvate to lactate
E.g. 100m sprint - 85% anaerobic, vs long-distance/steady-state 5% anaerobic
After exertion, extra O2 is consumed to remove excess lactate, replenish ATP + phosphorylcreatine stores + replace myoglobin O2
= the OXYGEN DEBT, which is the proportional to extent to which energy demands exceeded capacity of aerobic metabolism.
What are the types of heat production in skeletal muscle?
Resting heat - given off at rest, external manifestation of basal metabolic processes
Initial heat - heat produced in excess of resting heat, during contraction (made up of activation heat - due to contraction, and shortening heat, proportional to distance muscle shortens)
Relaxation heat - due to lengthening of muscle after isotonic contraction
Recovery heat - in excess of resting heat after contraction - heat liberated by restorative metabolic processes
What is the motor unit? Do sizes vary? What about muscle fibre types? What is the association between size + type?
What is the size principle?
- Motor unit = a single motor neuron + the muscle fibres it innervates
- Size varies - e.g. hand, eye - fine, graded, precise - 3-6 muscle fibres / neutron
vs leg - 600 fibres / neutron - All muscle fibres in a unit are of ONE type (i.e. duration of twitch contraction) - this defines types of motor units as S, FR, FF.
- S fibres tend to be small units (low innervation ratio), FF are larger units.
- smaller, S units are recruited first, then FR, then FF for most demanding tasks
Discuss cardiac muscle morphology
- Striations with Z-lines, similar to skeletal muscle. T-system is at Z-lines though, not A-I junction
- Muscle fibres branch + interdigitate, each is complete unit surrounded by cell membrane.
- One muscle fibre joins the next at “intercalated disks”, both membranes parallel each other in a series of folds - at Z lines - maintains cell to cell cohesion for uniform contractility
- Next to intercalated disks are “gap junctions” for spread of excitation - allow function as a syncytium
What is the RMP of cardiomyocytes?
-80 mV
Draw typical ventricular cardiomyocyte action potential
See notes page 23 / Ganong 112
How are myosin isoforms distributed in the heart?
alpha and beta myosin isoforms
alpha predominating in atria
beta in ventricles
spatial expression contributes to well-coordinated contraction of heart
