Mechanics Of Skeletal Muscle

Question 1

What are the four main functions of skeletal muscle?

Answer 1

• Heat Generation
• Posture
• Movement
• Joint Stability

Question 2

What are Fasciculations?

Name one condition where they are more frequent than usual

Answer 2

Small involuntary skeletal muscle contractions used to maintain muscle tone.

In motor neurone disease the number of fasciculations is much greater than normal

Question 3

Give a brief description of compartment syndrome, and give three direct causes

Answer 3

A condition whereby a fascial compartment becomes filled with blood, leading to neuro and vascular compression and potential Volkmann’s contracture.

It can be caused by:

• Direct Trauma and subsequent haemorrhage
• Heavily Bruised Muscles
• Crush fractures

Question 4

By what mechanism do circular muscles contract?

Give two examples of circular muscles

Answer 4

Concentric Contraction

• Orbicularis Occuli
• Orbicularis Oris

Question 5

What are parallel muscles?

Answer 5

Muscles which contract parallel to the force generating axis

Question 6

Give three types of parallel muscle and give at least one example of each type of parallel muscle

Answer 6

• Strap muscle eg Sartorius
• Fusiform Muscle eg Biceps Brachii
• Fan Muscle eg Pectoralis Major

Question 7

What are pennate muscles and give one example of a multi-pennate muscle

Answer 7

Pennate muscles can be either unipennate, bipennate or multipennate. A good example of a multi-pennate muscle is the deltoid. In pennate muscles one or more aponeuroses run through the body of the muscle and the muscle fibres come off of it

Question 8

What would be described as the Pennation angle of a muscle?

Answer 8

The angle the muscle fibres originate off of a aponeurosis in a pennate muscle

Question 9

If a muscle is described as an agonist what is it?

Answer 9

The muscle which is the prime mover in the movement of a joint

Question 10

If a muscle was described as an antagonist for a movement what would it be?

Answer 10

The direct opposition muscle to an agonist, which is the agonist for the opposite joint movement

Question 11

What is the difference between a fixator muscle and a neutraliser muscle?

Answer 11

Fixator muscles during a movement prevent the joint doing any other movements than the one being performed by immobilising the joint.

Neutralisers prevent the unwanted movements during a certain muscle contraction, eg Pronator Teres preventing supination of the forearm during flexion

Question 12

Finally what is the role of a synergist?

Answer 12

Aid the action of the agonist in the movement. Cannot perform the entire movement by themselves but assise

Question 13

Describe Isotonic Muscle Contraction, and therefore explain concentric and eccentric skeletal muscle contraction

Answer 13

A variant of muscle contraction where the tension in the muscle remains constant but the size of the muscle changes. Concentric contraction is when the muscle is shortening and exerting a force.
Eccentric contraction is when a force is being exerted during muscle extension, such as walking down hill

Question 14

What are the three main types of biomechanics in terms of muscle contraction? Give one example of each class

Answer 14

First Class (See-saw) - example is in neck extension
Second Class (Wheelbarrow) - example gastrocnemius in Plantatflexion
Third Class (Fishing Rod) - example is in forearm flexion

Question 15

In what order are muscle fibres recruited to increase the force of muscle contraction?

Answer 15

• Small fibres before larger fibres (larger fibres lead to a larger increase in tension)
• Slow type 1 fibres, then Fast type IIa and finally fast type IIB

Question 16

How does the Rate code affect muscle contraction?

Answer 16

Rate Codes are the rate of stimulus a muscle receives from the nervous system. A larger rate code leads to an increase in muscle tension up to the point of tetany by the process of SUMMATION.

Question 17

When a muscle is in tetany what does this describe?

Answer 17

Tetany is a state of constant tension in the muscle, where the tension cannot get any higher and the muscle is described as fused

Question 18

Describe the stages of muscle tension with an increasing rate code

Answer 18

• Twitch
• Wave Summation
• Unfused Tetany
• Fused Tetany

Question 19

What is a motor unit? What would the effect on muscle control be if the motor unit has fewer innervated muscle fibres?

Answer 19

All the muscle fibres innervated by a single motor neurone. By having fewer muscle fibres per motor neurone there is greater control over the muscle contraction

Question 20

What are the three main fibre types in skeletal muscle? List some of the properties for each….

Answer 20

-Slow Type 1
Aerobic Respiration
Lots of mitochondria
High myoglobin
Highly vascularised
- Fast Type IIa
Same as the slow bar the fact it is only moderately resistant to fatigue
- Fast Type IIX(B)
Anaerobic
Few mitochondria
Grey/white in appearance
Poorly vascularised
Low myoglobin

Question 21

What factor leads to the type of muscle fibre is present?

Answer 21

The expression of the heavy myosin light chain

Question 22

What is an alternative name for intrafusal fibres and their main function?

Answer 22

Alternatively Intrafusal fibres are called Muscle spindles and they are used for proprioception within muscle

Question 23

Where are intrafusal fibres located and how are they innervated?

Answer 23

Inside a collagen sheath within the muscle and are innervated by two sensory neurones and one gamma motor neurone. The motor neurone is used to keep the fibres taught.

Type 1 sensory neurone relay rate of change back to the CNS. Type 2 provides the position sense

Question 24

Explain the injury to intrafusal fibres in large fibre sensory neuropathy

Answer 24

The type 2 sensory neurones in the intrafusal muscle fibre cannot relay sensory system about position to the CNS.

As a result the patient will make inaccurate movements.

Question 25

What are the two main influences on muscle tone?

Answer 25

The motor neurone activity.

The muscle elasticity

Question 26

What is the function of maintaining a base muscle tone?

Answer 26

Protects the muscles against sudden forces to prevent muscular damage.

Also helps maintain posture by maintaining tension in the muscle

Question 27

Define Hypotonia

Answer 27

A lack of skeletal muscle tone

Question 28

List a few examples of pathological causes of hypotonia

Answer 28

Myopathies
Cerebral or spinal nerve shock
Sensory afferent lesions in muscle spindles
Motor neurone lesions

Question 29

Alternatively What are some conditions which could lead to hypotonia?

Answer 29

Charcot- Marie Tooth Disease
Landry- Guillain barre syndrome
Muscular Dystrophies

Question 30

Describe Floppy Baby Syndrome

Answer 30

A condition where a baby has hypotonia on birth

Question 31

Define Myotonia

Answer 31

An inability to relax skeletal muscle at will

Question 32

How do Cl- leak channels in the plasma membrane of Skeletal muscle affect the resting potential of the skeletal muscle

Answer 32

Drop the resting membrane potential down by about -20mV leading to a resting potential ranging from -80 to -90 mV

Question 33

What is the error which occurs in myotonia congenita?

Answer 33

Mutation in the ClCN1 receptor preventing the opening of Cl- channels in the plasma membrane

Question 34

Explain the sustained muscle contraction pathology in Myotonia Congenita

Answer 34

The mutation in the ClCN1 receptor leads to hyper-excitability of the cell, as the muscle cells cannot repolarise. This leads to “myotonic Discharges” under exertion because the cells cannot repolarise quickly.

As a result at rest myotonia congenita isn’t as much of an issue

Question 35

Describe the inheritance patterns of myotonia congenita

Answer 35

Autosomal Recessive - Becker’s Type

Autosomal Dominant - Thomson’s Type

Question 36

What are some of the aggravating and relieving factors of myotonia Congenita?

Answer 36

Aggravated by the cold and immobility, relieved by exercise. Symptoms of the condition are muscle stiffness and hypertrophy