Principles of Muscular System (Anatomy)

Question 1

Describe the three types of muscle

Answer 1

Skeletal

Cardiac

Smooth

Skeletal Muscle

Parallel bundles with stripes = striated

Majority of muscle in body in terms of volume

Capable of powerful contractions - force proportional to cross-sectional area

Moves bones, provide support to body for posture

Innervated by somatic and branchial motor nerves

(muscles in the head and neck are branchial, rest of the muscles in the body are somatic)

Each skeletal muscle cell forms from fusion of many hundreds of precursor cells called myblasts

Mature cells are multinucleated

Nuclei are peripheral

Cardiac Muscle

Striped muscle similar but not identical to skeletal

Found in the heart wall (myocardium) and great vessel walls

Branching network of individual cells linked electrically and mechanically work as a unit - linked by intercalated disc

• Ensures the heart/myocardium contracts as one unit

Contractions less powerful than skeletal but resistant to fatigue

Innervated by visceral motor nerves

• Parasympathetic - vagus, slower, less forceful
• Sympathetic trunk, powerful contraction

Central nucleus, shorter cells than skeletal

Fibers arranged in whorls and spirals

Purkinje fibers - specialised conduction system

Smooth Muscle

No stripes

Elongated spindle-shaped fibers

Capable of slow sustained contractions

Found in:

• Blood vessel walls (tunica media)
• Eyeball
• Walls of structures associated with viscera

Innervated by autonomic secretomotor nerves

Question 2

Describe the components of muscle tissue

Answer 2

Muscle Cells

Contractile cells

Adapted to generate motile forces

Interactions of the proteins: Actin and Myosin

Muscle Structure

Epimysium - surround entire muscle

Perimysium - surrounds muscle bundle

Endomysium - surrounds muscle fibres

Muscle Fascile - bundle of muscle fibres (muscle cells)

Muscle cell structure

Cell membrane - sarcolemma

Cell cytoplasm - sarcoplasm

Endoplasmic reticulum - sarcoplasmic reticulum

Functional contractile unit - sarcomere

Question 3

Describe the structure and function of skeletal muscle (including innervation)

Answer 3

Origin is the most proximal part to the trunk, stationary end of muscle

Insertion is the most distal and largely responsible for movement, mobile end of muscle

Tendons

Cylindrical

Tightly packed longitudinally running collagen fibers

• Elongated flattened muscle nuclei of inactive fibrocytes
• Cannot repair well due to bad blood supply

Tendon Sheath

Protects tendon from repeated friction

Tendons traverse over bony surfaces

Innervation

Somatic motor nerves

Muscles that have same action on a joint tend to be innervated by the same nerve

Motor never acts at motor end plate known as neuromuscular junction

Cardiac cells have their own electrical cells - Purkinje cells

Some enteric smmoth cells have a specialist plexus in their walls

Neuromuscular Junction

Release of Ach

Opens sodium channels

Elicits an action potential = contraction

Question 4

Classify skeletal muscle based on the arrangement of muscle fibres and functional groups

Answer 4

Classified by Morphology (shape, attachment, fiber orientation), Function and action

Morphology

Muscles whose fibers run obliquely are pennate (like a feather)
Unipennate – tendon along one side – e.g. EDL
Bipennate – tendon in centre with fibers both sides – e.g. rectus femoris
Multipennate – may be a series of bipennate e.g. acromial part of deltoid or muscle fibers converging from all sides e.g. tibialis anterior

Pennate muscles are stronger than parallel fiber muscles as bigger cross sectional area, but have less excursion

Muscles may be named by shape:
- Rhomboid – rhomboids!
- Quadrilateral – Quadratus lumborum, quadratus femoris, quadratus plantae
- Strap – Thyrohyoid, omohyoid, sternohyoid, sternothyroid
- Fusiform – coracobrachialis, biceps brachii, gastrocnemius
- Two bellies – biceps femoris
- Two headed – rectus femoris
- Triangular – Trapezius

Functional

Agonists - Prime movers

Antagonist - opposes agonist, relaxes synergistically

Synergist - prevents unwanted movements

Fixator - Contracts isometrically = increases tone but does not produce movement

Classification of Skeletal Muscle by Action

Flexors

Extensors

Abductor

Adductor

Rotator

Question 5

Understand repair and regeneration of different muscle types

Answer 5

Cardiac

no resident stem cell population in cardiac muscle

Damage to cardiac muscle leads to replacement of dead cells by collagenous scar

Skeletal

In adult muscle, there is a resident population of muscle precursor cells (statellite cells), which can divide to form new muscle cells after tissue damage

Satellite cells = myogenic cells located between the sarcolemma and basement membrane of muscle fibres.

Satellite cells divide and then fuse with existing muscle fibers to lay down new proteins and hypertrophy
Cardiac muscles incapable so not replaced if die

Hypertrophy

• Increase in cell size that lead to increase in overall muscular size – usually through exercise
• Increase in mass and cross-sectional area

Hyperplasia

Increase in number of muscle cells – probably does not occur in normal conditions in humans, only in tumours

Atrophy

• Wasting of muscles

Causes:

• Lack of exercise
• Poor nutrition
• Poor circulation
• Neurological
• Genetic