06: Muscle

Types of muscle

Skeletal, cardiac, smooth

Muscle movements

Contraction and relaxation

Muscle function

Movement and stabilization

Cala-

Insert

Fasci-

Bundle

What causes striations

Different bands of actin and myosin

Voluntary muscle types

Skeletal

Involuntary muscle types

Cardiac and smooth

Muscle cell

Myofibril

Group of muscle cells

Myofibers

Group of myofibers

Muscle

Why are muscles multinucleated

Multiple myofibrils means multiple nuclei

CT that holds muscle together

Muscle fascia

Fascia that surrounds myofibrils

Endomysium

Fascia that surrounds myofibers

Perimysium

Fascia that surrounds a group of myofibers (muscle)

Epimysium

Purpose of fascia

• Coordinated movement

• Resists stretching and tearing to preserve integrity

Muscle cell plasma membrane

Sarcolemma

What does the sarcolemma surround

A myofiber

T tubule

Sarcolemma invagination that helps conduct the AP

Muscle cell cytoplasm

Sarcoplasm

What does the sarcoplasm surround

Myofibrils

Muscle smooth ER

Sarcoplasmic Reticulum

Function of sarcoplasmic reticulum

Stores and releases calcium

Triad

T-tubule plus 2 cisternae

Functional contractile unit of the myofibril

Sarcomere

Thick filament

Myosin

Thin filament

Actin

Z disc/line

Attachment point for actin filaments, within I band

M line

Attachment point for myosin filaments, in the H band

A band

Dark band consisting of overlapping actin and myosin

H band

Lighter band within the A band that only has myosin

I band

Light region that only has actin

Skeletal muscle morphology (what do you see on histo)

• Lots of striations

• Multiple, peripherally located nuclei

Skeletal muscle locations

Bones

Consequence of skeletal muscle cells being electrically and mechanically isolated

They need some other structure to transfer APs and contractile force

What structure mediates the transfer of APs in a skeletal muscle fiber

T-tubules

What happens once the AP crosses the sarcolemma and reaches the myofibril

The sarcoplasmic reticulum releases calcium ions

Function of calcium in muscle contraction

Allows myosin to grab the actin and pull

Cardiac muscle morphology

• Lots of branching

• Intercalated discs

• Round, centrally located nuclei

Mechanism for automatic contraction of cardiomyocytes

They don’t have lots of T-tubules, instead they have a slow leak of calcium from the SR to cause continuous, forceful contractions

Why are fibroblasts found in cardiac muscle

They produce the ECM that supports the cells

Function of intercalated discs in cardiac muscle

• Mechanical attachment that transmits contractile force

• Communication of electrical signal

Junction on intercalated disc that facilitates attachment

Desmosomes (and fascia adherens)

Junction on intercalated discs that facilitates spread of AP

Gap junction

Special cardiac fibers that carry electrical signals through the heart

Purkinje fibers

Smooth muscle nickname

Visceral muscle

Smooth muscle location

Tubular or hollow organs

Smooth muscle morphology

• No striations (no myofibrils!)

• Spindle shaped (fusiform) cells

• Central, long nuclei

Organizational unit of smooth muscle

Fascicles

What factors control the involuntary contraction of smooth muscle

• Autonomic nervous system

• Hormones

• Local metabolites

Peristalsis

Slow, low-force, long lasting contraction characteristic of smooth muscle

Structures on smooth muscle sarcolemma that communicate the AP

• Gap junctions (like cardiac!)

• Caveolae (analogous to T-tubules)

Dense bodies

Actin anchor points on the sarcolemma, analogous to Z-disc in skeletal muscle

External lamina

Extracellular glycoprotein coat