Skeletal Muscle Morphology, Structure and Mechanics

Question 1

Which types of muscle are described as ‘striated’ and why?

Answer 1

Skeletal and cardiac are classed as striated muscle due to the ordered arrangement of the myofibrillar apparatus producing a characteristic banding pattern when viewed under the microscope.

Question 2

What is the function of skeletal muscle?

Answer 2

• Permit movement and are usually contracted voluntarily and consciously.
• Maintain body posture by stabilising joints even when there is no obvious movement. A
• Body heat is also produced by skeletal muscle as this tissue is inefficient at converting chemical energy into mechanical work.

Question 3

How does the body utilise the inefficiency of skeletal muscle?

Answer 3

Lots of body heat is released, this is utilised by the physiological process of shivering to raise core body temperature by the involuntary activation of skeletal muscle.

Question 4

How are muscle fibres arranged within muscles?

Answer 4

Muscle fibres within muscles can be arranged in several different ways. The most common arrangement is for the fibres to run parallel to the force-generating axis.

Question 5

What are strap muscles?

Answer 5

Strap muscles (e.g. sartorius) are, as the name suggests, shaped like strap or belt with the fibres running longitudinally, parallel to the direction of contraction.

Question 6

What are fusiform muscles?

Answer 6

Fusiform muscles (e.g. biceps brachii) are cylindrical and are wider in the centre with tapering off at the ends.

Question 7

What are fan shaped muscles?

Answer 7

Fan shaped muscles (e.g. pectoralis major) have fibres that converge at one end.

Question 8

What are the the three main categories of parallel muscles?

Answer 8

Strap, fusiform and fan shaped.

Question 9

What are pennate muscles?

Answer 9

Pennate muscles have one or more aponeuroses running through the muscle body from the tendon and the fascicles of muscle fibres attach to these aponeuroses at an angle (called the pennation angle) to the direction of movement.

Question 10

What is the difference between unipennate and bipinnate muscles?

Answer 10

These can be unipennate where all the fascicles are on the same side as the tendon, bipennate with fascicles on both sides of a central tendon or multipennate where a central tendon branches.

Question 11

What are circular muscles?

Answer 11

Circular muscles are where the fibres form concentric rings around a sphincter or opening. Such muscles attach to skin, ligaments and fascia of other muscles rather than to bone. Examples include orbicularis oculi around the eye and orbicularis oris around the mouth.

Question 12

What are synergistic muscles?

Answer 12

Muscles that act to assist the prime mover are called synergists (e.g. brachioradialis and pronator teres in elbow flexion). Acting alone they cannot perform the movement of the agonist but their angle of pull assists the agonist.

Question 13

What are fixators?

Answer 13

Fixators (stabilisers) act to hold a body part immobile whilst another body part is moving. In most activities, proximal joints are stabilised whilst distal joints move. An example is stabilising the shoulder whilst flexing the elbow. The fixators active in elbow flexion are the muscles that stabilise the position of the scapula and those that stabilise the shoulder joint.

Question 14

What is the difference between fixators and neutralisers?

Answer 14

Neutralisers prevent the unwanted actions of a muscle; fixators stabilise a joint.

Question 15

What is a neutraliser?

Answer 15

Neutralisers prevent the unwanted actions that an agonist can perform. For example, the rotator cuff muscles stabilise the glenohumeral joint whilst biceps, (whose long head acts to cause shoulder flexion) acts to cause flexion at the elbow joint.

Question 16

What is isotonic contraction?

Answer 16

Isotonic contraction (lit. ‘equal tension’) is where the tension within the muscle remains constant and the length changes.

Question 17

What are the types of isotonic contraction?

Answer 17

There are two types of isotonic contractions: concentric and eccentric.

Question 18

What is concentric contraction?

Answer 18

Concentric contraction is where the muscle shortens, for example when flexing the elbow to lift a load, the biceps shortens.

Question 19

What is eccentric contraction?

Answer 19

Eccentric contraction refers to active contraction of muscles whilst they are lengthening. Classic examples of this are walking, when the quadriceps (knee extensors) are active just after heel strike whilst the knee flexes, or setting an object down gently (the arm flexors must be active to control the fall of the object).

Question 20

When does isometric contraction occur?

Answer 20

When the load against the muscle equals the contractile force being generated, e.g. holding a weight in a fixed position, or trying to push a brick wall. In both these cases the muscle is generating tension but is not shortening.

Question 21

What is passive stretch contraction?

Answer 21

Muscle is being lengthened while in a passive state (i.e. not being stimulated to contract). An example of this would be the pull felt in the hamstrings whilst touching the toes.

Question 22

What is titin?

Answer 22

Titin connects the Z line to the M line in the sarcomere. It contributes to force transmission at the Z line and resting tension in the I band region. Titin limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle.

Question 23

Which structures are responsible for passive tension?

Answer 23

The structures responsible for passive tension are outside of the cross-bridge itself since muscle activation is not required. The protein titin is now known to be important in the contraction of striated muscle tissues.

Question 24

What stimulates skeletal muscle to contract?

Answer 24

Each skeletal muscle is supplied by a number of motor neurons which stimulate the muscle fibres to contract.

Question 25

Which type of motor neurone is responsible for innervating skeletal muscle?

Answer 25

The type of motor neurons innervating skeletal muscle fibres are called α-motor neurons

Question 26

Where are the cell bodies of alpha motor neurones located?

Answer 26

The cell bodies of these neurons are either located in the ventral horn of the spinal cord, for muscles of the limbs and trunk, or in the motor nuclei of the brainstem for the muscles of the head and face.

Question 27

How do the axons of alpha motor neurone supply skeletal muscle?

Answer 27

The axons of α-motor neurons leave the central nervous system and form part of a peripheral nerve, to supply the muscle fibres of a skeletal muscle.

Question 28

How do muscle fibres and the alpha motor neurone connect?

Answer 28

Via the neuromuscular junction. The neurotransmitter acetylcholine is released from vesicles into the synapse and activates nicotinic acetylcholine receptors on the muscle surface.

Question 29

How would you define a ‘motor unit’?

Answer 29

A motor unit is defined as an α-motor neuron and the group of individual muscle fibres that it innervates. Any single muscle fibre is innervated by only one α-motor neuron, but each α-motor neuron can innervate a number of different muscle fibres.

Question 30

How does the number of muscle fibres in a motor unit vary between different muscles?

Answer 30

Depends on the function of the muscle. Muscles that perform precise fine movements, such as inferior rectus which moves the eyeball have around ten muscle fibres in each unit motor unit whereas powerful muscles, where fine control is less important, such as gastrocnemius may have several thousand muscle fibres in each motor unit.

Question 31

How do skeletal muscle fibres differ?

Answer 31

• Speed at which they contract
• Amount of force that they generate
• Energy requirements/susceptibility to fatigue.
• Different types of muscle fibre differ in the isoforms of contractile proteins they express, most notably the myosin heavy chain.

Question 32

Describe the key feature of slow muscle fibres

Answer 32

Slow muscle fibres (Type I) express the type I myosin heavy chain, they contract relatively slowly and produce low amounts of force. Type I fibres are however extremely resistant to fatigue due to their high mitochondria content and use of oxidative metabolism to produce the ATP they consume.

Question 33

Describe the key features of fast muscle fibres

Answer 33

Fast muscle fibres (Type II) contract relatively quickly and produce large amounts of force.

Question 34

How are the two types of fast muscle fibres in human skeletal muscle classified?

Answer 34

The fast IIX muscle fibres are classified as Fast Glycolytic, while IIA fibres are Fast Oxidative/Glycolytic.

Question 35

Why do type IIA fast fibres represent an intermediate form of muscle fibre between Type I and fast IIX fibres?

Answer 35

They are Fast Oxidative/Glycolytic so represent an intermediate form of muscle fibre between oxidative Type I fibres which produce low force but are fatigue resistant and the fast IIX fibres which rely on glycolytic metabolism, produce greater amounts of force and fatigue rapidly.

Question 36

Small rodents such as rats and mice also have third type of fast fibre called fast IIB. Why is this not present in humans?

Answer 36

The gene for the fast IIB myosin heavy chain is not expressed in humans, presumably because this isoform contracts too quickly for large muscles with many sarcomeres in series.

Question 37

What are the two types of fast muscle fibres in human skeletal muscle?

Answer 37

There are two types of fast muscle fibres in human skeletal muscle termed fast IIA and fast IIX.

Question 38

How can you classify motor units according to muscle fibres?

Answer 38

The muscle fibres making up each motor unit tend to be all of the same contractile type, so each motor unit is either fast, intermediate or slow contracting.

Question 39

Describe the key features of slow motor units

Answer 39

• Very well-vascularised
• High myoglobin content
• Maintain contractions for long periods of time
• Typically found in postural muscles and those used for low intensity long duration activities such as walking.
• e.g. The soleus muscle is composed almost exclusively of slow type I muscle fibres.

Question 40

Describe the key features of fast motor units?

Answer 40

Fast type II muscle fibres have a glycolytic metabolism, and produce ATP quickly to enable a fast, powerful contraction but they also fatigue quickly. Most muscles are composed of a mixture of different fibre types and motor unit types and the proportion of fast to slow fibres depends on the typical function of the muscle.

Question 41

What does the contractile force produced by a muscle depends on?

Answer 41

The size principle and the rate code.

Question 42

Describe what is meant by ‘the size principle’

Answer 42

The size principle simply means that small motor neurons are recruited before large ones. In general this means that motor units with slow type I fibres are recruited first followed by those containing mostly fast IIa fibres and then those containing fast IIX fibres.

Question 43

What is ‘the rate code’?

Answer 43

The Rate code refers to the frequency at which the muscle fibres are stimulated by their α-motor neuron.

Question 44

What is tetany?

Answer 44

Consecutive action potentials in a repetitive train result in summation giving a slightly larger force with each contraction. Eventually a limit is reached where no further force can be produced termed tetany.

Question 45

What is baseline tone?

Answer 45

All muscle has some degree of baseline tone (degree of tension) due to the elasticity of the muscle tissue and low levels of motor neuron activity.

Question 46

How is skeletal muscle tone controlled?

Answer 46

Skeletal muscle tone is controlled by motor control centres in the brainstem. The locus coeruleus, which contains noradrenergic cells, projects ascending axons to spinal motor neurons, where it facilitates muscle tone.

Question 47

Why is muscle tone lost during sleep?

Answer 47

Muscle tone is lost in REM sleep when the locus coeruleus cells shut off.

Question 48

What condition is the result of a lack of skeletal muscle tone? What is the cause?

Answer 48

Hypotonia. This could result from damage to the motor cortex or cerebellum or spinal cord. Alternatively there could be degeneration of the muscle itself (myopathy).

Question 49

How does an action potential arriving at the synapse lead to the generation of action potentials in the muscle fibre?

Answer 49

• Triggers opening of voltage-gated calcium ion channels.
• Increase in intracellular Ca2+ causes vesicles containing acetylcholine (ACh) to release their contents into the synaptic cleft.
• Acetylcholine activates nicotinic ACh receptors in the muscle fibre plasma membrane
• Influx of sodium ions and depolarisation of the muscle fibre membrane potential.
• Local depolarisation activates voltage sensitive sodium channels resulting in the generation of an action potential in the muscle fibre.
• Acetylcholine is rapidly broken down in the synaptic cleft by the enzyme acetylcholinesterase.

Question 50

What is the effect of the action potential generated in the muscle fibre from acetylcholine release?

Answer 50

• Triggers opening of voltage-gated L-type Ca2+ channels which are concentrated in the T-tubules where they come into contact with SR (triads).
• Leads to activation of the ryanodine receptors which allow calcium to flow into the cytoplasm from the sarcoplasmic reticulum (an intracellular store of calcium).
• The influx of intracellular calcium binds to troponin, causing tropomyosin to reveals the actin binding sites for myosin heavy chain heads and therefore initiates muscle contraction.

Question 51

How do muscle fibres relax?

Answer 51

Relaxation of the muscle fibre is facilitated by calcium being pumped back into the sarcoplasmic reticulum by the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA).

Question 52

What is malignant hyperthermia?

Answer 52

Malignant hyperthermia is a rare life-threatening condition triggered by some volatile anaesthetic agents and succinylcholine, a neuromuscular blocking agent. In genetically susceptible individuals such drugs can lead to an uncontrolled increase in oxidative metabolism and increase in body temperature, which can be fatal if not immediately treated.

Question 53

What is the most common genetic cause of malignant hyperthermia?

Answer 53

Polymorphism in the ryanodine receptor. This receptor is activated by some volatile anaesthetic agents leading to a massive increase in intracellular calcium from intracellular stores. The muscle cell SERCA pump (which consumes ATP in its action) is therefore working at a dramatically increased rate ultimately leading to excessive heat production.

Question 54

Describe the short term stores of energy in muscle fibres

Answer 54

Very little ATP is stored in muscle fibres and this is sufficient for only a few seconds of contraction. Additional short term stores in the form of creatine phosphate can be used to rapidly replenish ATP, but again this is only sufficient to provide immediate energy for an initial burst of activity lasting a few seconds.

Question 55

Once ATP and creatine phosphate stores are depleted, where can muscle fibres get their energy?

Answer 55

Further energy to supply the contractile apparatus with the ATP it requires must come from glycolysis (anaerobic or aerobic) and oxidative metabolism (aerobic).

Question 56

How much ATP is provided by glycolysis?

Answer 56

Glycolysis can typically provide ATP sufficient for maximum muscle contraction for around 30-40 seconds.

Question 57

Where is anaerobic glycolysis typically used?

Answer 57

Anaerobic glycolysis is typically used in fast type II muscle fibres to produce short duration maximum force movements such as jumping or sprinting over a short distance.

Question 58

How much ATP is produced by anaerobic glycolysis when the pathway terminates at pyruvate?

Answer 58

Only 2 molecules of ATP are produced when the pathway terminates at pyruvate.

Question 59

How do muscle fibres get energy for prolonged activity?

Answer 59

For prolonged activity however such as running for several miles, the muscle needs to switch to beta oxidation of fatty acids released from triacylglycerol stored in adipose tissue

Question 60

How can patients with malignant hyperthermia be treated?

Answer 60

Patients can be treated with dantrolene, a muscle relaxant which antagonises the ryanodine receptor.

Question 61

What is the advantage of using glycolysis for immediate rapid activity?

Answer 61

This pathway can function under anaerobic conditions and so is not reliant on the blood supplying oxygen at the rate required for maximal contraction.

Question 62

What is the disadvantage of using glycolysis under anaerobic conditions?

Answer 62

The disadvantage of using glycolysis under anaerobic conditions is that lactate is produced from pyruvate by the action of lactate dehydrogenase. The accumulation of lactate is associated (although not necessarily the direct cause) with acidification of the muscle cell environment leading to “cramps” and underlies one of the reasons why maximal activity such as sprinting rapidly fatigues muscle.