Histology of Skeletal Muscle

Question 1

Types of muscle tissue

Answer 1

There are three types of muscle: skeletal, cardiac and smooth. All have parallel, elongated cells called myofibres or muscle fibres.

Question 2

How does muscle tissue function?

Answer 2

In all muscles energy from hydrolysis of ATP (chemical energy) is transferred into mechanical energy that results in movement (contraction). This occurs via myofilaments (actin & myosin) which fill most of the muscle cell. Muscle cells also need an exciteable cell membrane which responds to a stimulus to initiate cellular contraction. There are other types of contractile cells found in the body: 1. Myoepithelial cells 2. Myofibroblasts 3. Pericytes

Question 3

What are the different microscopic appearances classifications of muscle?

Answer 3

Smooth, striated (skeletal and cardiac).

Question 4

Innvervation

Answer 4

(1) Voluntary (skeletal) (2) involuntary (smooth and cardiac)

Question 5

Skeletal muscle

Answer 5

Skeletal muscle fibres are long tubular cells. The average length of skeletal muscle cells in humans is about 3cm (range 1mm – 100cm). Diameters vary from 10um to 100um. Skeletal muscle fibres contain up to several hundred small peripherally located nuclei.

Question 6

Development of skeletal muscle

Answer 6

Origin is from mesoderm –> mesenchyme -> myoblasts. A muscle fibre is formed by the fusion of multiple small muscle cells (myoblasts) making a large multinucleated cell (syncytium). Initially the nuclei are centrally located, but as the cells produce their actin and myosin filaments the nuclei are pushed peripherally.

Question 7

Skeletal muscle organisation

Answer 7

A muscle is made up of many fascicles. A fascicle has many myofibres (myofibre = muscle cell). Myofibres are polygon shaped in cross-section and has many myofibrils. A myofibril is composed of bundles of myofilaments (thick and thin).

Question 8

Epimysium

Answer 8

surrounds the whole muscle (dense connective tissue), contains larger vessels and nerves.

Question 9

Perimysium

Answer 9

dense CT surrounds the fascicles, contains blood vessels and nerves.

Question 10

Endomysium

Answer 10

loose CT = reticular fibres, surrounds individual muscle cells contain very small blood vessels and nerves.

Question 11

How is a tendon form and attached to a bone?

Answer 11

The coverings of the muscle (epimysium, perimysium and endomysium) continue beyond the muscle tissue and fuse to become a tendon which then blends with the fibrous layer of the periosteum and the bone via Sharpey’s fibres. Muscle-tendon (musculotendinous) junction is a weak point.

Question 12

What are myofibrils composed of and what are their action?

Answer 12

Myofibrils contain actin and myosin (myofilaments). Myofilaments (actin and myosin) are proteins, hence the highly acidophilic staining pattern of muscle. Muscles contract when actin and myosin filaments slide over each other (sliding filament mechanism). Actin and myosin are proteins that form the filamentous structures called myofilaments.

Question 13

Can mitochondria be identified under EM in skeletal muscle?

Answer 13

At low power EM the mitochondria can be identified as small electron dense bodies between myofibrils.

Question 14

Actin

Answer 14

Actin are thin filaments, with a diameter of 7nm and a length of 1 micron. Thin filaments also contain troponin, tropomyosin (both wrap around actin filaments) and nebulin (anchors from Z line to the end of the thin filament, thought to help anchor the thin filaments and regulate where they lie).

Question 15

Myosin

Answer 15

Myosin are thick filaments. They are 15nm in diameter and 1.5 microns in length. Thick filaments also contain titin (helps regulate where the thick filaments thick in between the thin filaments).

Question 16

What is the structure of a sarcomere?

Answer 16

• M line tethers myosin • H-band = myosin only (no overlapping actin) • A-band = all myosin (plus actin between A and H bands) • I-band = actin only (no overlapping myosin) • Z line = disc in centre of I band from which actin extends.

Question 17

Innervation of skeletal muscle

Answer 17

Innervation is by the somatic nervous system – this is conscious control of muscle contraction. The junction between a nerve and a skeletal muscle cell is called a neuromuscular junction. One axon (nerve cell) may innervate many muscle cells – motor unit.

Question 18

Motor unit

Answer 18

A motor unit is composed of a neuron and all of the muscle cells it innervates.
The terminal branches of the neuron end on individual muscle fibres. A single neuron may innervate a few to many muscle fibres (e.g. eye one neuron to three muscle fibres; erector spinae one neuron to several hundred fibres).

Question 19

Neuromouscular junction

Answer 19

The neuromuscular junction is composed of a synapse (axon terminal) and infoldings of sarcolemma (junctional folds). These would contain receptors for acetylcholine.
If muscle contraction starts at the plasma membrane (sarcolemma), but the cell is tightly packed with myofibrils how does the action potential travel to inside the cell?

Question 20

T-tubule system

Answer 20

The t-tubule system is an invagination of the sarcolemma of the muscle cell into and between the myofibrils of the muscle cell.

Question 21

teh sarcoplasmic reticulum

Answer 21

The sarcoplasmic reticulum: the endoplasmic reticulum of these cells is greatly expanded to form a network of cisternae that surrounds the myofibrils.

Question 22

What is a triad?

Answer 22

In the histology of skeletal muscle, a triad is the structure formed by a T tubule with a sarcoplasmic reticulum (SR) known as the terminal cisterna on either side.

Question 23

At death

Answer 23

At death there is initially still some ATP in the muscle. Ca no longer actively pumped into sarcoplasmic reticulum – leaks into cytosol and causes muscles to contract.
However, need ATP to release Ca and relax muscle = rigor mortis (starts about 3 hours after death).
Eventually muscle starts to break down and the body ‘relaxes’ (12 – 72 hours).

Question 24

Type I skeletal muscle

Answer 24

(red) slow twitch-fatigue resistant but generated less tension. Many mitochondria, wider Z bands. Depends on cellular respiration for ATP production. Rich in myoglobin = red colour. Activated by slow-conduction motor neurons which have a small diameter.

Question 25

Type IIa skeletal muscle

Answer 25

(white) fast twitch, fatigue resistant. Many mitochondria and high glycogen, narrower Z bands.

Question 26

Type IIb skeletal muscle

Answer 26

(white) fast twitch, fatigue prone. Fewer mitochondria and high glycogen stores, narrower Z bands. High peak muscle tension. These depend on glycolysis for ATP production. Low in myoglobin hence whitish in colour. Activated by large-diameter motor neurons (fast).

Question 27

Location of different skeletal mucle types.

Answer 27

• Type I: dominate in postural muscle (erector spinae).
• Type IIb: dominate in power muscles or those used in fine precision movemnts.
Athletes have more of one type of fibre:
• Type I: more common in endurance athletes
• Type IIa: middle distance athletes
• Type IIb: sprinters

Question 28

Myoglobin

Answer 28

Muscle contains myoglobin, a protein containing iron that binds to oxygen. Iron makes meat red.

Question 29

Lymph location

Answer 29

Lymph located in perimysium and epimysium.

Question 30

Where are neurovascular bundles located?

Answer 30

Epimysium, perimysium and endomysium.

Question 31

Muscle repair

Answer 31

Muscle repair is by satellite cells (limited supply), after injury satellite cells become active, generate new myoblasts which form muscle fibres.
More severe injury has repair by fibroblasts (forming scar tissue)
Muscular dystrophy constant damage to muscle fibres eventually exhausts supply of satellite cells.

Question 32

Sensory receptors

Answer 32

Muscles also contain sensory receptors (muscle spindles) that respond to muscle stretch. Similar receptors are also found in tendons.