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+ BIOL 370 > Lecture 8: Muscle Diversity > Flashcards

Lecture 8: Muscle Diversity Flashcards

1
Q

How do muscles vary?

A
  • force of muscle contraction
  • speed of muscle contraction
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2
Q

(Paper 1) What do superfast contracting sonic muscles involve to achieve its speed of contraction?

A

involves modifications to EC-coupling via:

  • large and rapid Ca2+ transients
  • low Ca2+ sensitivity
  • high rates of dissociation
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3
Q

What is the amount of force that a whole skeletal muscle can generate during a contraction?

A

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
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4
Q

How are vertebrate twitch skeletal muscles innervated?

A

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
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5
Q

What is a motor unit?

A

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
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6
Q

Fish Muscle Fibres

How are muscle fibres arranged in fish?

A

fish have spatially separated muscles fibre types

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7
Q

Fish Muscle Fibres

What are the 2 main types of fish muscle fibres? What is the intermediate type of fish muscle fibres?

A
  • main: red muscle fibre and white muscle fibre
  • intermediate: pink muscle fibre
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8
Q

Fish Muscle Fibres

What are red muscle fibres specialized for?

A

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
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9
Q

Fish Muscle Fibres

What are white muscle fibres specialized for?

A

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
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10
Q

Vertebrate Muscle Fibres

What are the 3 different types of skeletal muscle fibres?

A
  • slow-oxidative fibres (type I)
  • fast-oxidative fibres (type IIa)
  • fast-glycolytic fibres (type IIb, IId, or IIx)
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11
Q

Vertebrate Muscle Fibres

What are the characteristics of slow-oxidative fibres (type I)?

A

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
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12
Q

Vertebrate Muscle Fibres

What are the characteristics of fast-oxidative fibres (type IIa)?

A

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
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13
Q

Vertebrate Muscle Fibres

What are the characteristics of fast-glycolytic fibres (type IIb, IId, or IIx)?

A

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
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14
Q

Vertebrate Muscle Fibres

What is myoglobin? What does it do?

A

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.
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15
Q

Invertebrate Muscle

Describe the characteristics of obliquely striated muscle.

A
  • 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
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16
Q

Invertebrate Muscle

Describe the diversity in muscle excitation.

A

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
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17
Q

Invertebrate Muscle

How is a weak contraction induced?

A

single stimulus causes a small excitatory postsynaptic potential (EPSP), leading to a relatively small increase of cytoplasmic [Ca2+], inducing a weak contraction

18
Q

Invertebrate Muscle

How is a strong contraction induced?

A

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

19
Q

Invertebrate Muscle

How is an inhibitory postsynaptic potential (IPSP) induced?

A

release of neurotransmitter from an inhibitory neuron causes hyperpolarization of the cell membrane

20
Q

Insect Flight Muscle

How do contraction frequencies differ between vertebrates and some insects?

A
  • in vertebrates, maximum contraction frequency is ~100 Hz (sonic muscle)
  • some insects can achieve contraction frequencies of up to 1000 Hz
21
Q

Insect Flight Muscle

What is the downside of insects being able to achieve such high contraction frequencies?

A

at very high contraction frequencies, many aspects of EC-coupling may be limiting

22
Q

Insect Flight Muscle

How do insect muscles achieve such high rate of contraction to support high-performance?

A

by separating excitation from contraction – synchronous muscle vs. asynchronous muscle

23
Q

Insect Flight Muscle – Asynchronous

Where are asynchronous muscles found?

A

found in most insects with wing beats > 100 Hz

  • enables fast contraction
24
Q

Insect Flight Muscle – Asynchronous

How does contraction and relaxation of asynchronous muscle occur?

A
  • 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
25
Q

Insect Flight Muscle – Asynchronous

What are direct flight muscles?

A

muscles attached directly to wings

  • elevator muscles: pull wings up
  • depressor muscles: pull wings down – thicker when activated
26
Q

Insect Flight Muscle – Asynchronous

What are indirect flight muscles?

A

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
27
Q

Mollusc Catch Muscle

What are catch muscles?

A

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
28
Q

Mollusc Catch Muscle

What is paramyosin?

A

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
29
Q

Mollusc Catch Muscle

Describe EC-coupling, muscle contraction, and relaxation of catch muscles.

A
  • 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)
30
Q

Mollusc Catch Muscle

What is twitchin?

A

protein that only binds to actin when dephosphorylated, and when not displaced by myosin forming cross-bridges with actin

31
Q

Mollusc Catch Muscle

What does twitchin do during the catch state, and at the end of the catch state?

A
  • 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
32
Q

Mollusc Catch Muscle

How are serotonin receptors and twitchin related?

A
  • 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
33
Q

Transdifferentiated Muscle – Heater Organs

Describe the heater organ of swordfish.

A
  • 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
34
Q

Transdifferentiated Muscle – Heater Organs

How do heater organs generate heat?

A
  • 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
35
Q

Transdifferentiated Muscle – Electric Organs

In which taxa are electric organs found?

A

evolved independently in various taxa of teleosts (Actinopytergii) and elasmobranchs (Chondrichthyes)

36
Q

Transdifferentiated Muscle – Electric Organs

What are electric organs in most taxa of electric fish?

A

electrolytes are modified skeletal muscle cells – do not have myofibrils

37
Q

Transdifferentiated Muscle – Electric Organs

How does electroreception work in elephantnose fish?

A

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
38
Q

Transdifferentiated Muscle – Electric Organs

How do electric eels stun prey?

A
  • 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
39
Q

Transdifferentiated Muscle – Electric Organs

How are electrolytes arranged?

A
  • 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
40
Q

Transdifferentiated Muscle – Electric Organs

What is the resting membrane potential of electrolytes?

A

around -85 mV

41
Q

Transdifferentiated Muscle – Electric Organs

How do electrolytes compare to skeletal muscle cells?

A

higher density of:

  • Na+/K+-ATPase pumps
  • nicotinic ACh receptors
  • voltage-gated Na+ channels
42
Q

Transdifferentiated Muscle – Electric Organs

How does excitation occur?

A
  • 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

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