Muscle 3 Flashcards
Describe the control of the diaphragm
What happens when the diaphragm is paralysed?
- The diaphragm is a sheet of skeletal muscle. It can be controlled voluntarily via the phrenic nerve.
- Breathing is usually controlled automatically, but ‘manual override’ is possible.
- When a patient is given muscle relaxants during surgery under general anaesthesia the diaphragm will also be paralysed and the patient has to be ventilated.
What are the four steps in the excitation, contraction, and relaxation processes that require ATP
–Splitting of ATP by myosin ATPase provides the energy for power stroke of cross bridge
–Binding of ATP to myosin breaks cross bridge
–Active transport of Ca2+ back into lateral sacs of the SR during relaxation (SERCA pumps Ca2+ back into sarcoplasmic reticulum)
–Activity of Na+/K+ pump to maintain gradient across membrane
Do muscle fibres have alternate pathways for forming new ATP?
–Creatine phosphate
–Oxidative phosphorylation (requires mitochondria)
–Glycolysis (net 2 molecules produced from glucose)
–Lactate production
Muscle fibres have alternate pathways for forming new based on differences in ATP hydrolysis and synthesis?
What different types are there?
–Slow-oxidative (type I) fibers
–Fast-oxidative (type IIa) fibers
–Fast-glycolytic (type IIx) fibers
Describe the gross anatomy of skeletal muscle and what happens if structures dont function properly
- Muscle fibres are arranged in bundles called fascicles.
- You will need to know the arrangement of the epimysium, perimysium and endomysium, as shown in the diagram.
- Fasciculation is involuntary twitching of muscle.
- This occurs normally, but an increased incidence can be associated with conditions such as motor neurone disease.
- Motor neurone disease is a condition in which motor neurones gradually die .
- It is progressive and of unknown aetiology.
What are the 3 types of muscle fibres?
What enzyme is used for the stain?
Type I- slow twitch
Type IIA- fast twitch
Type IIX- fast twitch
From the diagram:
Stained based on the density of mitochondria, (Succinate dehydrogenase is the enzyme contained within the mitochondria)
More densely stained (darker)=rich in mitocondria= slow twitch
Lighltly stained=not many mitochondria= fast twitch
Descirbe the different skeletal muscle fibers
- Each skeletal muscle consists of a variety of fibre types.
- The image below shows a cross-section of skeletal muscle in which mitochondria have been stained.
- Some of the fibres appear dark due to a high density of mitochondria ; these are the Type I or slow twitch fibres. Some fibres appear very light due to low numbers of mitochondria; these are the Type IIx or fast twitch fibres.
- Some of the fibres are of intermediate staining and are classed as Type IIa fast twitch fibres.
- The proportion of each fibre type varies in each muscle. For example, the oculomotor muscles have a predominance of Type IIx fast twitch fibres, whereas postural muscles have a high proportion of Type I slow twitch fibres.
- The proportion of muscle fibre types appears to be determined genetically, however it has been claimed that it can be altered by training, although this is controversial.
- Fibre types can be analysed in a sample of muscle tissue obtained via a needle biopsy.
Draw a table to compare the properties of each of the 3 muscle fibre types
Comparisons between the muscle fibre types are shown in the Table:
- The Type 1 fibres have a high oxidative capacity and are resistant to fatigue. They can maintain contractions over long periods of time and are very important in postural muscles. The Type IIb fibres mostly obtain energy via glycolysis and rapidly fatigue.
Myoglobinn is an oxygen storage molecule in muscle
Comparing Skeletal Muscle Fibres: Fast versus slow fibers
–Fast fibers have higher myosin ATPase (ATP-splitting) activity than slow fibers
Comparing Skeletal Muscle Fibres: Oxidative versus glycolytic fibers
–Fiber types differ in ATP-synthesizing ability
Comparing Skeletal Muscle Fibres: Genetic endowment of muscle fiber types
Largely determined by the type of activity for which the muscle is specialized
How do muscle fibres adapt considerably in response to the demands placed on them?
What happens is muscles arent used enough?
–Improvement in oxidative capacity (more capillaries produced)
–Muscle hypertrophy (resistance training makes the muscle fibers larger)
–Influence of testosterone (favours production of actin and myosin and the buildup of muscle bulk)
–Interconversion between fast muscle types (not between fast and slow fibers)
Muscle atrophy (if you dont use the muscles then they start to break down- they get smaller)
Limited repair of muscle (muscle doesnt have many stem cells, there are satellite cells on the outside of muscle fibers but they are few)
Give a comparison of the characteristics of the three types of muscle
Figure 8-1 Characteristics of three types of muscle. The photos in (a), (b), and (c) are light micrographs of longitudinal sections of skeletal, cardiac, and smooth muscle, respectively.
Where is smooth muscle found in the body?
- Mostly in the walls of tracts which lead to the outside of the body
- Digestive tract – peristalsis
- Respiratory tract
- Reproductive tract – uterus
- Urinary tract - bladder
- Exception: walls of blood vessels – control of blood pressure
Smooth muscle:
Descibre their characteristics
What is contraction activated by?
How does it contract?
What do they form?
–Smooth muscle cells are small and unstriated
–Smooth muscle cell contraction is activated by Ca2+- dependent phosphorylation of myosin (Ca2+ binds to calmodulin which activates myosin light chain kinase which phosphorylates myosin)
–Phasic smooth muscle contracts in bursts; tonic smooth muscle maintains tone
–Multiunit smooth muscle is neurogenic (orgin of contraction is from nerves)
–Single-unit smooth muscle cells form functional syncytia e.g cells in uterus during childbirth (greek for ‘cells together’)
Describe autonomic innervaation of smooth muscle
From the diagram:
A single neuron from the autonomic NS
The axon runs out over smooth muscle creating a synapse with individual smooth muscle cells (no NMJ)
Not just acetylcholine- neurotransmtter could be histamine or 5-HT (5-hydroxytryptamine receptors)
Describe the contraction of smooth muscle
From the diagram:
Arrangement of thick and thin filaments in a smooth muscle cell in relaxed and contracted states
No troponin in smooth muscle
Dense body acts as an anchorpoint for the filaments
Describe the Calcium activation of myosin cross bridge in smooth muscle
- Increase in intracellular Ca2+ ions
- Ca2+ binds to Calmodulins to activate it and then that complex goes on to activate myoson light chain kinase
- Active myosin light chain kinase forms and adds a phosphate group on the myosin head
- Phosphorylated myosin cross bridge can bind with actin
Explain smooth muscle potentials:
- Pacemaker potentials
- Slow wave potentials
Pacemaker potentials:
- Self-generated electrical activity in smooth muscle.
- (a) With pacemaker potentials, the membrane gradually depolarizes to threshold on a regular periodic basis without any nervous stimulation.
- These regular depolarizations cyclically trigger self-induced action potentials.
- Approx 1 contraction every 10 mins
Slow wavepoteintials:
- (b) In slow-wave potentials, the membrane gradually undergoes self-induced hyperpolarizing and depolarizing swings in potential.
- A burst of action potentials occurs if a depolarizing swing brings the membrane to threshold.
- e.g. in the gut- peristalsis
Explain the organization of cardiac muscle fibers
Bundles of cardiac muscle are arranged spirally around the ventricle. When they contract they ‘wring’ blood from the apex to the base where the major arteries exit
Explain Excitation–contraction coupling in cardiac contractile cells
- Action poteintial in cardiac contractile cell travels down T tubule
- Entry of small amount of Ca2+ from the ECF through L-type (long acting) Ca2+ channels
- Release of a large amount of Ca2+ from sarcoplasmic reticulum through ryanodine Ca2+ release channels
- This calcium induced calcium release is very specific to cardiac muscle
- Cytosolic Ca2+ concentraion increases
- Troponin-tropomyosin complex in thin flimaents pulled aside
- Cross bridge cycling between thick and thin filaments
- Thin filaments slide inward between thick filaments
- Contraction
Describe the relationship of an action potential and the refractory period to the duration of the contractile response in cardiac muscle
Very negative membrane poteintial (approx -90mv)
Upon stimulation there is an increase in the permeability to sodium ions and AP occurs
There is an influx of Ca2+ (opening of L-type calcium channels) which lengthens the AP
Opening of potassium channel brings the membrane potentials back down
The is a very long refractory period this causes a substantial contraction
Describe the difference between slow and fast twitch fibres upon stimulation
From the diagram:
- The fast twitch fiber will contract and relax very quickly (in under 40 milliseconds)
- The slow twitch fiber is much slower to get to peak force and then it relaxes (in about 100 milliseconds)
