Musculoskeletal: Anatomy - Overview of the muscular system

Question 1

What characteristics are used to classify skeletal muscle?

Answer 1

Whether willfully controlled (voluntary vs involuntary)
Whether appearing striped under a microscope (striated vs smooth)
Whether located in the body wall and limbs or the hollow organs (somatic vs visceral/autonomic)

Question 2

Compare and contrast the three different types of skeletal muscle in terms of their location, cell appearance, activity type, and stimulation

Answer 2

Question 3

List the 7 different shape classifications of skeletal muscle and give an example of each

Answer 3

Flat: parallel fibres, often with an aponeurosis (flat sheet of tendons anchoring muscle to skeleton)
E.g. external oblique, sartorius

Pennate: feather-like; may be unipennate, bipennate, or multipennate
E.g. extensor digitorum longus (unipennate), rectus femoris (bipennate), and deltoid (multipennate)

Fusiform: spindle-shaped
E.g. biceps brachii

Convergent: arise from a broad area and converge to form a single tendon
E.g. pectoralis major

Quadrate: four equal sides
E.g. rectus abdominis

Circular or sphincteral: surround a body opening or orifice, constricting it when contracted
E.g. orbicularis oculi

Multi-headed or multi-bellied: more than one head of attachment or more than one contractile belly
E.g. biceps brachii (two heads), triceps brachii (three heads), digastric and gastrocnemius (two bellies)

Question 4

Define origin

Answer 4

Usually the proximal end of muscle, which remains fixed during muscular contraction

Question 5

Define insertion

Answer 5

Usually the distal end of muscle, which is mobile

Question 6

What are the three functions of skeletal muscle?

Answer 6

1. Automatic (reflexive) movements
   (e. g. respiratory movements of the diaphragm, myotatic/stretch reflex)
2. Maintaining muscle tone (tonic contraction, even when “relaxed”)
3. Phasic (active) contraction

Question 7

What are the two types of phasic contraction? Define each

Answer 7

Isotonic: involves a change in muscle length
Isometric: muscle length remains the same, but muscle tension is increased above tonic levels to resist gravity or other antagonistic force

Question 8

What are the two types of isotonic contraction?

Answer 8

Concentric: produces movement by muscle shortening
Eccentric: produces movement by controlled relaxation

Question 9

Which type of isotonic contraction uses less metabolic load? Which is capable of generating higher tension?

Answer 9

Eccentric contraction requires less metabolic energy at the same load, but with maximal contraction can generate much higher tension levels than concentric contraction

Question 10

What is a motor unit?

Answer 10

Functional unit of a muscle, consisting of a motor neuron and the muscle fibre it controls

Question 11

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

Answer 11

According to muscle size and function
E.g. large motor units in trunk and thighs, small motor units in eyes and hands where precision is needed

Question 12

Define prime mover

Answer 12

Muscle primarily responsible for particular movement (can also be gravity)

Question 13

Define fixator

Answer 13

Steadies part of a limb through isometric contraction while another part is moving

Question 14

Define synergist in terms of muscle activity

Answer 14

Complements the action of prime movers

Question 15

Define antagonist in terms of muscle activity. What is the difference between a primary and secondary antagonist?

Answer 15

Oppose the actions of another muscle (eccentrically contract as the active movers concentrically contract - to produce a smooth movement)
Primary antagonist: directly opposes the prime mover
Secondary antagonist: opposes synergists

Question 16

Define shunt muscle

Answer 16

A muscle that’s pull is exerted along a line that parallels the axis of the bones to which it is attached - at a disadvantage for producing movement, but instead acts to maintain contact between the articular surfaces of the joint it crosses (i.e. resists dislocating forces; e.g. brachioradialis, brachialis)
Proximal attachment is nearer the joint it acts over and distal attachment is further away

Question 17

Define spurt muscle

Answer 17

A muscle that’s pull is oriented obliquely relative to the bone it moves - capable of rapid and effective movement (e.g. biceps)
Proximal attachment further away from the joint and distal attachment closer

Question 18

What causes DOMS?

Answer 18

Can be caused by eccentric contractions that are either excessive or associated with a novel task
Muscle lengthening is more likely to produce microtears and/or periosteal irritation (usually muscles cannot elongate beyond one third of their resting length without sustaining damage)

Question 19

How can skeletal muscle fibres be replaced? How does the composition of new skeletal muscle compare with the original?

Answer 19

Skeletal muscle fibres cannot divide but can be replaced individually by new muscle fibres derived from satellite cells
Satellite cells are a potential source of myoblasts (muscle cell precursors), which are capable of fusing to form new skeletal muscle fibres if required
Number of new fibres that can be produced is insufficient to compensate for major muscle degeneration or trauma, and the new skeletal muscle is composed of disorganised mixture of muscle fibres and fibrous scar tissue

Question 20

How do muscles increase in size with frequent strenuous activity?

Answer 20

Via hypertrophy of existing fibres

Question 21

Can cardiac muscle be replaced?

Answer 21

No: cannot divide and there is no equivalent to the satellite cells of skeletal muscle to produce new fibres

Question 22

What are pharyngeal arches?

Answer 22

Branchial arches in mammals
Primordia for a multitude of structures including face, jaw and pharynx (structurally same as skeletal muscle but usually covered by mucosa or near a mucosocutaneous junction; involved in superficial reflexes and glandular secretion)

Question 23

What forms the vasculature of the pharyngeal arches?

Answer 23

Aortic arches

Question 24

Describe the development of the branchial arches

Answer 24

Develop during fourth week as a series of mesodermal outpouchings on sides of developing pharynx
Pouches form on endodermal side, grooves or clefts form on ectodermal surface
Nerve supply is from cranial nerves and splanchnic mesoderm

Question 25

Describe Hilton’s law

Answer 25

The nerves supplying a joint also supply the muscles moving the joint and the skin covering their distal attachments