Lecture 8: Muscle Diversity Flashcards
How do muscles vary?
- force of muscle contraction
- speed of muscle contraction
(Paper 1) What do superfast contracting sonic muscles involve to achieve its speed of contraction?
involves modifications to EC-coupling via:
- large and rapid Ca2+ transients
- low Ca2+ sensitivity
- high rates of dissociation
What is the amount of force that a whole skeletal muscle can generate during a contraction?
depends on how many muscle fibres in that muscle are recruited, and which type(s) of muscle fibres are recruited
- skeletal muscle is composed of a large number of muscle fibres
- it is possible to selectively recruit only small parts of an entire muscle, or the entire muscle at one time, depending on the requirements of each situation – not all fibres always need to be stimulated
How are vertebrate twitch skeletal muscles innervated?
each muscle fibre (skeletal muscle cell) is innervated by a single branch of the axon of a motor neuron
- in muscles requiring very precise control, one neuron innervates only a few muscle fibres
- in some muscles, one neuron innervates several thousand muscle fibres
What is a motor unit?
group of muscle fibres under the control of one motor neuron
- recruitment of motor units increases strength of contraction (allows for diversity in muscle strength)
- more motor neurons → more muscle fibres recruited → more force generation
Fish Muscle Fibres
How are muscle fibres arranged in fish?
fish have spatially separated muscles fibre types
Fish Muscle Fibres
What are the 2 main types of fish muscle fibres? What is the intermediate type of fish muscle fibres?
- main: red muscle fibre and white muscle fibre
- intermediate: pink muscle fibre
Fish Muscle Fibres
What are red muscle fibres specialized for?
specialized for sustained activity
- produce ATP by oxidative phosphorylation
- have high myoglobin content
- have many mitochondria
- are fatigue resistant
- have relatively slow rate of contraction
Fish Muscle Fibres
What are white muscle fibres specialized for?
specialized for very short but rapid bursts of activity
- produce ATP by substrate-level phosphorylation
- have low myoglobin content
- have relatively few mitochondria
- are less resistant to fatigue
- have relatively fast rate of contraction
Vertebrate Muscle Fibres
What are the 3 different types of skeletal muscle fibres?
- slow-oxidative fibres (type I)
- fast-oxidative fibres (type IIa)
- fast-glycolytic fibres (type IIb, IId, or IIx)
Vertebrate Muscle Fibres
What are the characteristics of slow-oxidative fibres (type I)?
more found in elite endurance athletes (ie. long-distance runners, cyclists)
- smaller diameter
- darker colour due to myoglobin
- 60-100 ms to peak tension
- lower myosin-ATPase activity
- high resistance to fatigue
Vertebrate Muscle Fibres
What are the characteristics of fast-oxidative fibres (type IIa)?
more found in elite power athletes (ie. weightlifters, sprinters), people with SCI
- larger diameter
- pale colour
- 20-40 ms to peak tension
- higher myosin-ATPase activity
- intermediate resistance to fatigue
Vertebrate Muscle Fibres
What are the characteristics of fast-glycolytic fibres (type IIb, IId, or IIx)?
more found in elite power athletes (ie. weightlifters, sprinters), people with SCI
- similar to fast-oxidative fibres in speed and myosin-ATPase activity
- low resistance to fatigue
Vertebrate Muscle Fibres
What is myoglobin? What does it do?
iron and oxygen-binding protein found in vertebrate tissue
- related to hemoglobin
- causes red colour in muscles (iron is in ferrous (2+) state) – cooked meat turns brown because iron atom is in ferric (3+) oxidation state (lost an electron)
- helps provide oxygen during times of high oxygen demand – ie. diving, running, etc.
Invertebrate Muscle
Describe the characteristics of obliquely striated muscle.
- found in many invertebrate taxa – ie. nematoda, platyhelminthes, annelids, mollusc
- sarcomeres are not organized into myofibrils
- thin filaments anchor to dense bodies
- dense bodies attach to cell membrane and (via other proteins) extracellular matrix proteins and basal lamina – when sarcomeres contract, they pull on dense bodies to bend basal lamina
Invertebrate Muscle
Describe the diversity in muscle excitation.
in some invertebrate tonic muscle cells:
- single cell is innervated by a single motor neuron at multiple synapses
- graded contraction: summation of excitatory postsynaptic potentials (EPSPs)
in other invertebrate tonic muscle cells:
- single cell is innervated by multiple motor neurons – each neuron may have multiple synapses with this single muscle cell
- some neurons will be excitatory, while others may be inhibitory
- graded contraction
Invertebrate Muscle
How is a weak contraction induced?
single stimulus causes a small excitatory postsynaptic potential (EPSP), leading to a relatively small increase of cytoplasmic [Ca2+], inducing a weak contraction
Invertebrate Muscle
How is a strong contraction induced?
summation of excitatory postsynaptic potentials (EPSPs) – multiple stimuli within a given time period add together, causing a larger depolarization, greatly increasing cytoplasmic [Ca2+], inducing a strong contraction
Invertebrate Muscle
How is an inhibitory postsynaptic potential (IPSP) induced?
release of neurotransmitter from an inhibitory neuron causes hyperpolarization of the cell membrane
Insect Flight Muscle
How do contraction frequencies differ between vertebrates and some insects?
- in vertebrates, maximum contraction frequency is ~100 Hz (sonic muscle)
- some insects can achieve contraction frequencies of up to 1000 Hz
Insect Flight Muscle
What is the downside of insects being able to achieve such high contraction frequencies?
at very high contraction frequencies, many aspects of EC-coupling may be limiting
Insect Flight Muscle
How do insect muscles achieve such high rate of contraction to support high-performance?
by separating excitation from contraction – synchronous muscle vs. asynchronous muscle
Insect Flight Muscle – Asynchronous
Where are asynchronous muscles found?
found in most insects with wing beats > 100 Hz
- enables fast contraction
Insect Flight Muscle – Asynchronous
How does contraction and relaxation of asynchronous muscle occur?
- single Ca2+ pulse maintains muscle in an activated state for successive cycles
- contraction is triggered by stretch, and deactivated by shortening in the presence of elevated myoplasmic Ca2+
- reduction of Ca2+ cycling reduces ATP demand
Insect Flight Muscle – Asynchronous
What are direct flight muscles?
muscles attached directly to wings
- elevator muscles: pull wings up
- depressor muscles: pull wings down – thicker when activated
Insect Flight Muscle – Asynchronous
What are indirect flight muscles?
muscles attached to the walls of the thorax
- vertical muscles: pull on roof of thorax, causing wings to rise – thorax widens and lengthens, and stretches longitudinal muscles
- longitudinal muscles: pull on anterior and posterior ends of thorax, causing wings to lower – thorax narrows and shortens, and stretches vertical muscles
Mollusc Catch Muscle
What are catch muscles?
specialized smooth muscles in bivalves
- often adductor muscles
- capable of maintaining force for long periods of time (up to several hours) with very low ATP turnover
- serotonin (monoamine neurotransmitter) ends the catch state and allows for relaxation
Mollusc Catch Muscle
What is paramyosin?
forms the core of the thick filament around which unique myosin isoforms are attached
- myosin isoforms in catch muscle can be regulated by Ca2+ directly
- mechanisms of Ca2+ regulation is unknown
Mollusc Catch Muscle
Describe EC-coupling, muscle contraction, and relaxation of catch muscles.
- ACh triggers Ca2+ release from SR
- Ca2+ binds to myosin, initiates cross-bridge cycling and contraction
- prolonged cholinergic stimulation keeps results in sustained elevation of Ca2+
- ACh and Ca2+ decline, but muscle remains in contracted state with very little ATP turnover (catch state) – protein twitchin phosphorylation or dephosphorylation may play a role
- stimulation of serotonergic nerves and release of serotonin (5-HT) results in relaxation (end of catch state)
Mollusc Catch Muscle
What is twitchin?
protein that only binds to actin when dephosphorylated, and when not displaced by myosin forming cross-bridges with actin
Mollusc Catch Muscle
What does twitchin do during the catch state, and at the end of the catch state?
- dephosphorylated twitchin tethers the thin filament to the thick filament, maintaining tension after myosin detaches from actin
- when phosphorylated, twitchin releases actin, allowing for relaxation
Mollusc Catch Muscle
How are serotonin receptors and twitchin related?
- during the catch phase, twitchin becomes progressively dephosphorylated via action of calcineurin (calmodulin-sensitive phosphatase)
- 5-HT activates protein kinase A (PKA), which phosphorylates twitchin
Transdifferentiated Muscle – Heater Organs
Describe the heater organ of swordfish.
- swordfish selectively warm the brain, eye, and muscle
- superior rectus muscle is devoid of contractile filaments, but is rich in mitochondria and is highly aerobic
- superior rectus muscle is rich in SR and t-tubules
Transdifferentiated Muscle – Heater Organs
How do heater organs generate heat?
- depolarization causes release of Ca2+ from SR by conformational changes in DHPR and Ryr
- RyR isoform in these cells is extremely slow to close, allowing for prolonged release of Ca2+
- SERCA pumps Ca2+ back into SR
- futile cycling of Ca2+
- oxidative phosphorylation and all energy transforming reactions generate heat
Transdifferentiated Muscle – Electric Organs
In which taxa are electric organs found?
evolved independently in various taxa of teleosts (Actinopytergii) and elasmobranchs (Chondrichthyes)
Transdifferentiated Muscle – Electric Organs
What are electric organs in most taxa of electric fish?
electrolytes are modified skeletal muscle cells – do not have myofibrils
Transdifferentiated Muscle – Electric Organs
How does electroreception work in elephantnose fish?
elephantnose fish use low-voltage electric organ discharge to generate an electric field
- have numerous electroreceptors on the body surface used to sense this electric field
- surrounding objects affect the electric field, allowing them to sense the shape and material of these objects and estimate their distance
Transdifferentiated Muscle – Electric Organs
How do electric eels stun prey?
- use low-voltage electric organ discharge (weak EOD) for electrolocation
- use rapid series of high-voltage (up to 600 V), high frequency (400 Hz) EOD to stun prey – each high-voltage EOD stimulates an AP in the motor neurons of the prey, prey can be put into ‘tetanus’ state
Transdifferentiated Muscle – Electric Organs
How are electrolytes arranged?
- often arranged in series into columns, and multiple columns may be arranged in parallel
- voltage-gated Na+ channels are only located on the innervate side of the electrocyte cells
Transdifferentiated Muscle – Electric Organs
What is the resting membrane potential of electrolytes?
around -85 mV
Transdifferentiated Muscle – Electric Organs
How do electrolytes compare to skeletal muscle cells?
higher density of:
- Na+/K+-ATPase pumps
- nicotinic ACh receptors
- voltage-gated Na+ channels
Transdifferentiated Muscle – Electric Organs
How does excitation occur?
- electromotor neurons release ACh, which binds to nicotinic ACh receptors, generating an endplate potential
- voltage-gated Na+ channels open, depolarizing the cell membrane on the innervated (posterior) side of the cell to +65 mV, while the cell membrane on the non-innervated (anterior) side of the cell remains at -85 mV
- activated electrocyte has a transcellular potential difference of approximately 150 mV
