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Science for Medicine 16 > Muscle > Flashcards

Flashcards in Muscle Deck (39):

Describe the structure and ultrastructure of skeletal muscle.

Skeletal muscle cells are multinucleated striated fibres, which are created in utero by myoblasts. Muscles are made of bundles of these fibres wrapped in connective tissue sheaths. The striations are caused by sarcomeres.


Define isometric and isotonic twitches.

Isotonic – Contraction with shortening length
Isometric – Contraction with constant length

Lengthening - contraction with increased length


Explain the process of excitation-coupling contraction in skeletal muscle contraction.

Once an AP has fired in the muscle, Ca2+ ions are released from the sarcoplasmic reticulum. These ions bind to the troponin on the actin filament and remove the blocking protein, tropomyosin, exposing the actin binding sites. The myosin filaments can then bind to the actin and from cross bridges. This is what causes the “contraction” of the muscle fibres. The calcium ions then return to the sarcoplasmic reticulum through the use of ATP. This leaves the tropomyosin free to return to its original position, and so the contraction ends and cross bridges are broken.


What occurs in the sarcomere during muscle contraction?

As the muscle contracts, various changes occur to the sarcomere, i.e. the A (myosin only) band and the H band reduce, as the filaments slide over each other. More overlap means more tension and vice versa. However, too much overlap, and the filaments will interfere with each other.


Describe the generation of controlled force through the recruitment of motor units.

As the load increases, more motor units are required to compensate. This process is known as recruitment, and allows more muscle fibres to be involved in a movement.


Explain tetanus

Tetanus is a state of extended contraction via the summation of action potentials. Tetanic tension is much greater than twitch tension, as there is no respite in which the calcium can return to the sarcoplasmic reticulum. As such, tropomyosin is always held in the non-blocking state and so the cross bridges cannot be broken.


Explain fatigue

Fatigue is a state that is caused by repeated muscle stimulation. It prevents using up vast quantities of ATP in breaking and reforming cross bridges to sustain contractions. How much stimulation is required to trigger fatigue depends entirely on the individual’s fitness, muscle fibre type and the length of the contraction.


Describe the aerobic properties of muscle

There are a few different types of skeletal muscle fibres, and they are classified according to their speed and whether they are glycolytic or oxidative.

o Oxidative Fibres
These fibres have more mitochondria for oxidative phosphorylation, and are more vascular to allow for the delivery of O2 and nutrients. They also contain myoglobin to help with oxygen delivery. The fibres are red and tend to be of low diameter.
• Slow oxidative fibres –Resist fatigue
• Fast oxidative fibres - Intermediate resistance to fatigue


Describe the anaerobic properties of muscle

There are a few different types of skeletal muscle fibres, and they are classified according to their speed and whether they are glycolytic or oxidative.
o Glycolytic Fibres
These have very few mitochondria, but have a higher concentration of glycolytic enzymes and glycogen. They have a lower blood supply, and are white fibres with larger diameters.
• Fast glycolytic fibres – Fatigue quickly


Describe the structure and function of smooth muscle

Smooth muscle exists in hollow organs, such as the GI Tract, the bladder, airways etc, and is supplied by the autonomic nervous system. The cells are mononucleate non-striated spindles and can divide throughout life. Like skeletal muscle, they have actin and myosin filaments, except they are arranged diagonally across cells and are anchored to membranes and cell structures by dense bodies.


Explain contraction and relaxation in smooth muscle.

Smooth muscles also form cross bridges between myosin and actin filaments, however the mechanism is different.
As calcium is released from the sarcoplasmic reticulum, it binds to calmodulin, which binds to myosin light chain kinase. This phosphorylates the myosin with ATP and forms cross bridges to actin filaments, resulting in contraction and tension.
Relaxation is brought about through the action of myosin light chain phosphatase, which dephosphorylates the cross bridges. Contraction can be held for longer in smooth muscle due to the low rate of consumption of ATP. This is useful in blood vessels, which must stay open for long periods of time.


Explain the sliding-filament theory of muscle contraction and its relationship to the length-tension properties of muscle.

Sliding filament theory - explains the mechanism of muscle contraction based on muscle proteins that slide past each other to generate movement.
According to the sliding filament theory, the actin (thin) filaments of muscle fibers slide past the myosin (thick) filaments during muscle contraction, while the two groups of filaments remain at relatively constant length.

Length tension relationship – there is an optimal length of filament required for optimal tension generation.
Muscle length for greatest isometric tension = optimal length (lo)


What are the main functions of muscle?

• Generate force & movement
• Allow us to express & regulate ourselves


Describe the cross-bridge contraction cycle

1. ATP binds and myosin detaches.
2. ATP hydrolysis provides energy for the myosin head to rotate and reattach to actin.
3. The power stroke. The power stroke (cross-bridge tilting) begins after Ca2+ binds to troponin to uncover the rest of the myosin binding site.
4. At the end of the power stroke, myosin releases ADP, allowing the myosin head to be tightly bound to the actin filament in the rigor state. The cycle is ready to begin once more as a new ATP binds to myosin.


Describe briefly a motor unit

Motor neurons + muscle fibres = motor unit


Define tension and load

• Force exerted by muscle = tension
• Force exerted on muscle = load


What factors influence contractile activity?

Dynamic balance of all the following:
– Spontaneous electrical activity in muscle membranes = Pacemaker activity
– Autonomic neurotransmitters from varicosities
– Hormones (e.g. oxytocin)
– Local factors (paracrine agents, pH, O2, osmolarity, ions, NO)
– Stretch


What happens to skeletal muscle when its damaged?

Damaged cells are replaced by satellite cells which differentiate into new muscle fibres. Other fibres undergo hypertrophy to compensate.


What is a sarcomere?

Basic unit of striated muscle, and are composed of long fibrous proteins called filaments which slide past each other when a muscle relaxes or contracts


What are the thick and thin filaments made up of?

Thick - myosin
Thin - actin, bound to Z lines


What is the Z line?

Borders of sarcomere


What is the M line?

Myosin linked with accessory proteins


What is the H line?

Myosin only zone near M line in centre of sarcomere


What is the I band?

Actin only zone in sarcomere


Describe the role of ATP in muscle contraction and relaxation

ATP hydrolysis energises the cross bridges to cause the myosin heads to release actin to bind a new actin molecule

ATP also powers CA2+-ATPase in SR to bring about muscle relaxation by causing tropomyosin to reattach to myosin binding sites.


What does fatigue depend on?

Type of muscle fibres
Length of contraction
Fitness of individual
K+, lactic acid and ADP + Pi all inhibit contraction short term
Long term depends on glucose, glycogen and dehydration


Describe the difference between oxidative and glycolytic muscle fibres

Oxidative- increased mitochondria, vascularizarion and myoglobin. Red fibres with low diameters

Glycolytic - few mitochondria, increased glycolytic enzymes, lower blood supply. White fibres with larger diameters


What are the three different types of muscle fibre?

Slow oxidative (I) - resist fatigue
Fast oxidative (II) - intermediate resistance to fatigue
Fast glycolytic (III) - fatigue quickly


Describe muscle fibre recruitment

Slow oxidative are activated first, then fast oxidative, then fast glycolytic.


What changes in muscle fibre do you see with aerobic and anaerobic exercise

Aerobic - increased mitochondria, vascularisation and fibre diameter

Anaerobic (weightlifting) - increased glycolysis and diameter

Both cause hypertrophy of different muscle fibres


Describe some features of smooth muscle cells

Spindle shaped
No striations
Thick and thin filaments arranged diagonally across cells


How does smooth muscle contraction and relaxation differ from that of striated muscle?

Released Ca binds calmodulin, and this complex binds to myosin light chain kinase which phosphorylates cross bridges with ATP. These phosphorylated cross bridges then bind to actin filaments causing contraction.

Relaxation occurs due to myosin light chain phosphatase which dephosphorylates cross bridges.


Describe some sources of cytosolic calcium

Sarcoplasmic reticulum
Extracellular Ca - voltage gated channels

While AP released enough Ca to saturate all binding sites in skeletal muscle, in smooth muscle only some sites are activated.


Which muscle type shows tone, or a basal level of Ca inside cells to produce a constant level of tension?

Smooth muscle


What factors influence contractile activity?

Electrical activity
Autonomic neurotransmitters
Local factors e.g. NO, pH, paracrine factors


Describe single unit and multiunit smooth muscle types

Single unit e.g. GIT, uterus, small blood vessels - many cells linked by gap junctions to allow synchronised contraction which can be evoked by stretch. Pacemaker cells

Multiunit e.g. airways, arteries, hairs - few or no gap junctions, richly innervated by ANS, don't respond to stretch


Describe the latent period and contraction of isometric twitches

Short latent period but long contraction (compared to isotonic)


What is ATP needed for in muscle contraction?

Hydrolysis of X bridges to reattach to myosin
Powers Ca2+-ATPase of the SR


What is calmodulin involved in?

Smooth muscle contraction
Ca-calmodulin binds myosin light chain kinase
This phosphorylates X bridges with ATP