Microstructure and function of Human Muscle Systems

Question 1

5 tissue types

Answer 1

Epithelium  (sheets of cells)
Connective tissue (support and strength)
Blood
Muscle tissues
Neural tissues (CNS & nerves)

Question 2

Muscle

Answer 2

a contractile tissue

Question 3

classes of muscle

3 main?
2 less major?

Answer 3

Striated (striped)
Skeletal
Cardiac

Non-striated
Smooth muscle

Myoepithelium (in glands, iris of eye)
Myofibroblasts (in healing wounds)

Question 4

Differential functions of muscles

skeletal?
cardiac?
smooth?

Answer 4

Skeletal - rapid contraction but subject to fatigue. Voluntary control.

Cardiac (heart only) - quite rapid, resists fatigue. Can contract regularly for 90+ years. Involuntary control (most people).

Smooth - slower contraction but very powerful, energy-efficient, and little fatigue. Usually involuntary control. E.g. uterus, gut.

Question 5

Skeletal muscle fibres
length?
nuclei?

Answer 5

Very long (up to ~4 cm), giant thread-like cells with thousands of nuclei (= multinucleate or syncytial cells)

Question 6

How can you get a multinucleate cell?

Answer 6

Skeletal muscle fibres are formed by thousands of precursor cells in the embryo (myoblasts) fusing together.

Question 7

Thick and thin filaments

what does transverse section show?

Answer 7

The thick and thin filaments are made of many molecules of the proteins myosin and actin - the core of the contractile apparatus.
Transverse sections show the myosin is in a hexagonal array, with more actin filaments than myosin.

Question 8

What gets narrower in sliding filament theory?

Answer 8

I gets narrower BUT not A

Question 9

How does skeletal muscle know when to contract?

what carry signal to all parts of the cell?

Answer 9

Neural stimulation via motor end-plates hence one nerve can supply many muscle fibres
Special intracellular membrane systems carry the signal to all parts of this very large cell. (Diffusion not fast enough)

Question 10

How does the muscle carry the signal to all parts of the cell?

Answer 10

Membrane systems: convey stimulus rapidly inside fibre, via T-tubules to SR, and initiate contraction.

Question 11

Example of skeletal muscle pathology: Duchenne muscular dystrophy

Answer 11

Dystrophic. Smaller fibres, more connective tissue. Damage/ death and repair of fibres occurring.

Question 12

Overview of cardiac muscle fibres (in heart) versus skeletal

similar?
;ength?
key difference?
nucleus?

Answer 12

Similar (striated) but
Much smaller fibres, joined end-to-end by specialized junctions, intercalated disks
Fibres have only one nucleus (or two)

Question 13

Junction types in the intercalated disk

Answer 13

FA: Fascia adherens. Special sheet-like intermediate junction, strong adhesion. Actin attached
GJ: Gap junction. Ionic communication, contraction wave
D: Desmosome. Adhesion
Z: Z-line

Question 14

Functional aspects - cardiac muscle

why branching fibres?
why smaller diameter fibres?
why numerous mitochondria?
why disks?

Answer 14

Branching fibres provide extra strength and resistance to splitting (by high-pressure blood).

Smaller diameter fibres (than skeletal muscle) allow rich blood supply and additional connective tissue (for strength).

Numerous mitochondria allow continuous energy supply - resistance to fatigue.

Intercalated disks give very strong attachments and ionic communication between fibres.

Question 15

Initiation of contraction in cardiac muscle

starts where? how does it start?
spreads where? how?

what carry it to ventricles?

Answer 15

Myogenic stimulus – muscle-generated. Action potential starts from pacemaker region in R. atrium, and carries a wave of contraction across heart, assisted by ion diffusion through the gap junctions in the intercalated disks.

Purkinje fibres (larger, modified cardiac muscle fibres) carry stimulus rapidly to ventricles.

Question 16

Sites & features of smooth muscle

where?
why smooth?

Answer 16

Generally internal organs other than heart. Digestive system, uterus, lungs, bladder, also glands, blood vessels, etc.
Also called “involuntary” muscle. Why?
“Smooth” because no striations.
Initiation of contraction is varied - neural, hormonal etc.

Question 17

Smooth muscle - overview of fibres

shape of fibres?
nucleus?

Answer 17

Spindle-shaped fibres.
One sausage-shaped nucleus.
Long, spindly cells, one nucleus each. No stripes.

Trichrome stain shows collagen (blue-green) around each individual smooth muscle fibre.
The fibres are attached to this collagen, thus to each other

Question 18

Contractile system in smooth muscle

features?
what is visbile/
what is caveolae?

Answer 18

Features:
no myofibrils
no striations
no complex membrane systems

But:
actin and myosin filaments visible
plasma membrane has caveolae, containing many kinds of receptors hence signals to contract
nearby ER may be a version of sarcoplasmic reticulum

Question 19

Dense Bodies
what are they?
what do they contain?

Answer 19

anchors for contraction
Dense bodies in smooth muscle function like Z lines.
Contain α-actinin, and actin attaches to them.

Question 20

Smooth muscle overview

transverse section shows? hexagonal shape? effect?

Answer 20

Transverse section shows actin and myosin filaments, but relatively few myosin (thick). No hexagonal array.
Structure allows force to be exerted at greater degree of stretch than for striated muscle.

Smooth muscle is the simplest and looks the most primitive, least organized type of muscle in mammals. But well adapted to function.
Contraction is slower than in skeletal, but tissue more elastic, energy-efficient, well adapted to functions in organs.