MSK 4a: skeletal muscle structure and function

Question 1

Describe circular muscles

Answer 1

Sphincters to adjustopening
Concentric fibres
Attach to skin, ligaments and fascia (not bone)

E.g. orbicularis oculi, orbicularis oris

Question 2

Describe parallel muscles (most common type)}

Answer 2

Fibres run parallel to the force-generating axis. 3 subtypes:

1. Strap: belt-shaped, fibres run longitudinally to contraction direction. E.g. sartorius
2. Fusiform: wider and cylindrical in centre, taper off at ends. E.g. biceps brachii
3. Fan-shaped: fibres converge at one end and spread over a broad area at another end. E.g. pectoralis major

Question 3

Describe pennate muscles

Answer 3

One or more aponeuroses run through the muscle body from the tendon. Feather shape. Fascicles attach to aponeuroses at an angle (pennation angle)
3 types:
1. Unipennate: all fascicles on the same side as tendon. E.g. extensor digitorum longus
2. Bipennate: fascicles on both sides of central tendon. E.g. rectus femoris
3. Multipennate: central tendon branches, e.g. deltoid

Question 4

What is the significance of limbs being divided into compartments by fascia?

Answer 4

Compartment syndrome: trauma in one compartment can cause internal bleeding and increased pressure on blood vessels and nerves
Deep constant pain, aggravated by stretch, paraesthesia, tense and firm swollen shiny skin, prolonged capillary refill
Treated by fasciotomy: cut skin and fascia to relieve pressure, closed by skin graft when the pressure has gone down

Question 5

What is the classification of muscles by movement?

Answer 5

Agonists: main movers
Antagonists: oppose agonists
Synergists: assist agonists,alone cannot perform movement but their angle of pull assists
Neutralisers: prevent unwanted actions that an agonist can perform
Fixators: act to hold a body part immobile whilst another body part is moving (e.g. to stabilise a joint)

Question 6

What is isotonic contraction?

Answer 6

Constant tension, variable muscle length as it changes to move load. Can be:

• concentric: muscle shortens, e.g. lifting a load with arm
• eccentric: muscle exerts a force while being extended e.g. walking downhill

Question 7

What is isometric contraction?

Answer 7

Constant length, variable tension. E.g. pushing against a wall

Question 8

What are 1st, 2nd and 3rd class muscle levers?

Answer 8

1st: effort at one end, load at other; “see-saw”. Usually mechanical disadvantage. E.g. flexion and extension of head
2nd: “wheelbarrow”. Effort at one end, fulcrum at other. E.g. plantar flexion of foot
3rd: “fishing rod”. Effort between load and fulcrum. Mechanical disadvantage, most common type in body. E.g. biceps brachii in elbow flexion

Question 9

What is a motor unit?

Answer 9

An alpha motor neurone + the muscle fibre it innervates

Each fibre innervated by 1 neurone, but a single neurone can innervate many fibres. Muscle fibres of motor unit are all of the same contractile type

Fine control=few fibres, powerful many fibres per motor unit

Question 10

How can different muscle fibres be distinguished?

Answer 10

Stains, as different isoforms have different pH sensitivity

Question 11

State the 3 main fibre types and their characteristics

Answer 11

Slow type I: slow oxidative, high Mb, red, many mitochondria, rich capillary supply, fatigue resistant, first type recruited-for standing/walking

Fast type IIA: fast oxidative, high Mb, pink, many mitochondria, rich cap supply, moderate fatigue resistance, second type recruited-for walking/running

Fast type IIX: fast glycolytic (anaerobic), low Mb, pale, few mitochondria, poorer capillary supply, rapidly fatigue, last type recruited-for running/sprinting/jumping

Question 12

How do muscle fibre types differ molecularly?

Answer 12

Based on myosin heavy chain expression
About 8 types of MHC
It determines function

Question 13

What are muscle spindles?

Answer 13

Intrafusal muscle fibres located in the muscle belly, that sense muscle stretch to facilitate proprioception.

Question 14

How are muscle spindles innervated?

Answer 14

One gamma motor neurone to keep the fibres taught
Type Ia sensory to relay rate of change in muscle length to the CNS
Type II sensory to provide sense of position

Question 15

If a patient can perform accurate movements whilst watching their affected limb but without vision these movements are very inaccurate, what has happened to them?

Answer 15

They have large-fibre sensory neuropathy

Question 16

How is muscle force controlled?

Answer 16

Small and slow motor neurones recruited first
More action potentials=more force
Subsequent AP produce summation, up to a limat–>unfused tetanus–>fused tetanus

Question 17

When are healthy muscles relaxed?

Answer 17

Never fully relaxed except in REM sleep

Question 18

Describe the symptom hypotonia and how it may be caused

Answer 18

Lack of skeletal muscle tone. Commonly in newborn babies: floppy baby syndrome
Causes:
-cerebral/spinal neural shock
-lesions of cerebellum
-lesions of sensory afferents from muscle spindles
-lesion of lower motor neurones e.g. polyneuritis
-primary degeneration of muscle (myopathies)
Disease examples:
-muscular dystrophies
-spinal muscular atrophy
-Charcot-Marie-Tooth disease

Question 19

Myotonia congenita?

Answer 19

Symptoms: muscle stiffness, hypertrophy that is enhanced by cold and inactivity
Dominant (Thomson type) or recessive (Becker type)
CLCNI channel forms a homodimer
Myotonic discharges as buffering capacity lost