Muscle Contraction, Tissue and Plasticity etc

Question 1

What are the structures and functions of smooth muscle?

Answer 1

• Involuntary.
• It’s found in the walls of your hollow internal organs such as your gut, blood vessels and uterus.
• Mononucleated and spindle shaped.
• Contract slowly and don’t fatigue.
• Regulated by the autonomic nervous system.

Question 2

What are the structures and functions of cardiac muscle?

Answer 2

• Not a true syncytium
• Myogenic
• Made of muscle fibres connected by intercalated disks.
• Mononucleated and cross striated.
• Contract rhythmically and don’t fatigue.
• Central nuclei.

Question 3

What are the structures and function of skeletal muscle?

Answer 3

• Voluntary.
• Regular cross-striations.
• Force production for locomotion and breathing.
• Force production for postural support.
• Heat production during cold stress.
• Largest protein store in the body.

Question 4

How are skeletal muscle fibres arranged? ( connective tissue covering)

Answer 4

Each fibre is arranged in bundles called fascicles:

• Epimysium surrounds entire muscle
• Perimysium surrounds fascicles
• Endomysium surrounds individual muscle fibres

Question 5

What are 3 functions of connective tissue covering?

Answer 5

• Protective role
• Fibrosis
• Collagenisation

Question 6

What is the microstructure of the skeletal muscle?

Answer 6

• Sarcolemma ( muscle cell membrane)
• Myofibrils ( tubular structures that pack the fibres)
• Myofilaments ( threadlike strands within myofibrils)
• Actin (thin filament) troponin and tropomyosin
• Myosin (thick filament)

Question 7

What occurs during muscle contraction?

Answer 7

1) Depolarisation from the sarcolemma -> T tables -> Sarcoplasmic reticulum causes release of Ca2+ ions into the sarcoplasm.
2) Calcium ions bind to troponin causing a conformational change and the actin-myosin binding site is exposed.
3) Calcium ions activate ATPase which breaks ATP down which is used to move the myosin head.
4) ATP also provides energy to break the actin myosin cross bridge.

Question 8

What 3 complexes is troponin compromised of?

Answer 8

Troponin T: attaches to tropomyosin.
Troponin I: binds to actin head
Troponin C: captures Ca2+ and undergoes a conformational change that lifts tropomysin away from the actin filament.

Question 9

Define excitation contraction coupling

Answer 9

It is a way to link muscle excitation (the depolarisation of the action potential) to Ca2+ release from the sarcoplasmic reticulum.

Question 10

Explain excitation contraction coupling

Answer 10

1) Acetylcholine is released by the axonal ending, diffuses to the muscle cell, and attaches to the ACh receptors on the sarcolemma.
2) Depolarisation occurs, and the action potential is generated along the sarcolemma.
3) The action potential, carried deep into the cell via T tubules, causes the sarcoplasmic reticulum to release calcium ions.
4) The calcium concentration at the myofilaments increas; the myofilaments slide past one another, and the cell shortens.
5) After the action potential ends, calcium concentration at the myofilaments decreases.
6) The muscle cell relaxes and lengthens.

Question 11

What are the 3 types of muscle fibre types?

Answer 11

Type I fibres- slow twitch and slow oxidative.
Type IIa fibres- intermediate and fast-oxidative glycolytic fibres.
Type IIb fibres- fast twitch and fast glycolytic fibres.

Question 12

Define motor unit

Answer 12

A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron’s axonal terminals.

Question 13

What is meant by motor unit recruitment and what order does it occur in?

Answer 13

Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle, and therefore is a measure of how many muscle fibers of that muscle are activated.
The higher the recruitment the stronger the muscle contraction will be. Motor units are generally recruited in order of smallest to largest ( slow to fast twitch) as contraction increases.

Question 14

Define:

1) Hypertrophy
2) Hyperplasia
3) Atrophy

Answer 14

1) Cells getting bigger( increased number of myofibrils)
2) Increasing cell number
3) Cells getting smaller

Question 15

How can mechanical stimuli produce cellular hypertrophy?

Answer 15

1) A mechanical stimulus comes in.
2) IGF1 and other compounds( ANG II) are produced by muscle fibres.
3) These compounds are fed back to the muscle fibre.
4) Satellite cells are then activated which then enter a cell and add a new nucleus.

Question 16

What is a ligament?

Answer 16

They are bands of connective tissue which cross the joint space and therefore support and modify joint movement.
They join bone to bone.

Question 17

Describe the contents of ligaments

Answer 17

• Classed as dense regular connective tissue.
• Collagen fibres arranged in multiple directions, but not irregular like fascia.
• Allows strength in multiple directions.
• Very similar makeup to tendons, but:
  Usually have about 10% elastin content.
  More type III collagen.
• 85% type I collagen plus other types to make up 75% of the dry weight.
• 25% of dry weight is proteoglycans, glycoproteins, elastin and other proteins.
• Like tendons, water content very important to functional properties.

Question 18

What are bursae?

Answer 18

• Tough connective tissue sacs.
• Contain synovial fluid.
• Placed under areas of pressure.
• Mainly under tendons near their insertion.

Question 19

What are tendon sheaths?

Answer 19

• These are present where tendons cross joints.
• Synovial fluid is secreted to lubricate the tendons over joints.
• Blood supply in sheathed tendons is carried by the mesotenon where the sheath folds inwards.

Question 20

How can a muscle be injured externally and what are the consequences?

Answer 20

Can be caused by trauma or by injection which leads to severe necrosis of muscle fibres:

• Torn plasmalemma leads to influx of Ca2+ leading to a necrotic zone.
• Distal to the necrotic region the fibre is functionally denervated.

Question 21

Describe the major causes and consequences of myopathies in skeletal muscle.

Answer 21

Myopathy can occur in horses after exercise: cell membrane damaged, myoglobin is then released into the blood which can lead to renal failure.

Question 22

Describe the major causes and consequences of dystrophies in skeletal muscle.

Answer 22

Duchenne muscular dystrophy is a mutation in gene coding for the membrane protein dystrophin that occurs in humans, dogs and mice.
This leads to Degeneration/regeneration resulting eventually in failure of the respiratory muscles.

Question 23

Explain the role of satellite cells in the repair and regeneration of muscle fibres.

Answer 23

First, satellite cells( quiescent adult stem cells) are activated.
Then they migrate to a necrotic zone and form myotubes in a process similar to normal embryological development.

Question 24

Outline the effects of endocrine and nutritional factors in myopathies.

Answer 24

• Nutritional:Primarily a condition found in herbivores due to selenium or vitamin E deficiency. Lambs and calves of deficent mothers are weak and have impaired movement.
• Endocrine: Cushing’s disease, hypothyroidism and diabetes.
• Both can lead to heart failure and renal disease.

Question 25

What are the two most common inflammatory conditions in dogs?

Answer 25

• Polymyositis: Generalised inflammatory myopathy. Muscle damage is thought to the result of cell-mediated immunity.
• Masticatory muscle myositis (MMM): focal inflammatory myopathy that selectively affects the muscles of mastication.

Question 26

Describe what is meant by sarcopenia.

Answer 26

Atrophy associated with raging is known as sarcopenia.

Question 27

What does HVPP stand for?

Answer 27

Equine Hyperkalemic Periodic Paralysis Disease.
This hereditary disease (autosomal dominant) affects horses under stress( training or racing.)
The horse undergoes a paralytic attack affecting most of the skeletal muscles.
HYPP is a sodium channelopathy- there is a single nucleotide change, which leads to a single amino acid change in the transmembrane ion channel.