The muscle cell and the contractile apparatus

Lecture 1

Development of skeletal muscle

Derived from mesoderm germline

mesoderm cells differentiate into progenitor cells called myoblasts

myoblasts fuse into multinucleated myotubes

myotubes differentiate into muscle fibres

some myoblasts remain free and become (myo)satellite cells

Satellite cells function

Help repair damaged skeletal muscle tissue

move to site of injury

proliferate, differentiate (behave as embryonic stem cells) and fuse to muscle fibres

In severe injury/elderly —→ scar tissue develops instead

How does cardiac tissue/smooth muscle repair?

Cardiac/smooth muscle do not have satellite cells

cardiac muscle - limited repair using stem cells in the blood, does not regenerate well and ultimately leads to scar tissue

smooth muscle - not post-mitotic so can regenerate

Epimysium

sheath of connective tissue that covers the whole muscle

connected to tendons

perimesium

connective tissue that covers fascicles

Microtubules

thickest and most rigid cytoskeletal filaments

tracks for intracellular trafficking

form the mitotic spindle that separates chromosomes during cell division

Intermediate filaments

thinner and more flexible

functions in cell mechanics and maintaining cell structural integrity

actin filaments

thinnest and most flexible

key structural polymer

support cell shape and drive cell deformations

contractile structures in muscle and non-muscle cells

display structural polarity with a plus and minus end

myosin

Actin-based motor

cargo transport

attachment of actin to the plasma membrane

generate contractility by cross-linking and pulling actin filaments

skeletal and cardiac have one group; smooth and non muscle have another group.

tropomyosin

rod-shaped molecule

forms alpha helical subunits

troponin

binds tropomyosin

alpha-actinin

rod-shaped homodimer of 35nm in length

actin binding site on each end

actin cross-linker (binds 2 actin filaments to each other) and the Z disk

Nebulin and titin

large structural proteins anchored at the Z disk

Titin stabilises myosin thick filaments in the middle of the sarcomere

Nebulin - regulates thin filament length

Intercalated disks

connects adjacent cardiac cells electrically

results in syncytium (transmits both electrical and mechanical forces)

Phasic contraction

used in smooth muscle to propel contents through gut

Tonic contraction

used by smooth muscle to maintain pressure in blood vessels

Smooth muscle

contains dense bodies (connected to adhesion plaques at membrane and analogous to Z disks)

contain gap junctions to allow propagation of waves of electrical excitation or intracellular messengers throughout the tissue.

Functions of skeletal muscle

produce body movement

maintain posture and body position

support soft tissues

guard body entrance/exits

maintain body temperature

store nutrients

produce body movement

muscle tendons pull and move bones

maintain posture and body position

stabilises joints

support soft tissues

surround, support and shield internal structures, such as tissues and organs

guard body entrance/exits

sphincters encircle openings

provide voluntary control of swallowing, defecation, urination

maintain body temperature

contraction uses energy; energy use generates heat

store nutrients

muscle proteins can break down to release amino acids

can be used to sythesise glucose or provide energy in emergencies.

skeletal muscle appearance

very long cylindrical cells

multinucleated

striated

skeletal muscle function

control voluntary movement

maintain posture

generate heat

skeletal muscle control

voluntary

skeletal muscle power

high

cardiac muscle appearance

shorter, branched cells

one nucleus per cell

striated

cardiac muscle function

drive heart contractions

cardiac muscle control

involuntary

cardiac muscle power

high

smooth muscle appearance

short cells, often spindle shaped

one nucleus per cell

non-striated

smooth muscle function

surround hollow organs and drive movement of food, secretions, blood in arteries

control respiration

smooth muscle control

involuntary

smooth muscle power

low