Skeletal Muscle Tissue Contraction

Chapter 10

Anatomy and Physiology

Mr. Knowles

Liberty Senior High School

A Brief Review of Skeletal Muscle Tissue

Connective Tissue• Epimysium separates

the…

• Perimysium divides the muscle into…

• Endomysium surrounds the…

Structures Entire muscle from other

tissue.

Bundles of muscle fibers- fascicles.

Individual skeletal muscle fibers.

http://harveyproject.science.wayne.edu/development/muscle/grsphysw.html

How does a skeletal muscle cell differ from most other eukaryotic

cells?

A ComparisonMost Cells

• Small; < 100 μm long• One Nucleus• Normal metabolism-needs

normal enzymes; one copy of genes.

• 100’s of mitochondria• Endoplasmic Reticulum• No myofibrils

Skeletal Muscle • Large; 12 inches long• Multiple Nuclei• High metabolism-needs

more enzymes; more genes.

• 1,000’s of mitochond.• Sarcoplasmic Reticulum• Myofibrils

Nucleus

Sarcolemma

Sarcoplasm

Special Terms for Muscle Fibers• Sarcoplasm- cytoplasm of a muscle fiber.• Sarcolemma- unique cell membrane of a

muscle fiber.• The sarcolemma forms tubes that travel into

sarcoplasm at right angles- Transverse Tubules (T tubules).

• Action Potentials (unequal charges) travel down these T tubules.

Inside a Muscle Fiber…• Each T tubule encircles cylindrical structures-

myofibrils.• Myofibrils- 1-2 μm in diameter and as long as

entire cell. 100’s – 1,000’s of myofibrils/cell.• Myofibrils – are bundles of myofilaments – 2

kinds of protein filaments called actin (thin filaments) and myosin (thick filaments).

http://www.blackwellscience.com/matthews/myosin.html

Inside the muscle fiber…• Myofibrils can shorten and contract the

muscle fiber.• Myofibrils are attached to the sarcolemma on

its inner surface.• Collagen fibers are attached to the

sarcolemma on its outer surface. These fibers extend into the tendon.

• Myofibrils pull on sarcolemma pulls on tendon muscle contraction.

Muscle Fiber

Tendon

Intracellular Extracellular

Collagen

Myofibrils

Sarcolemma

Terminal Cisternae

T Tubule

Triad

Sarcoplasmic Reticulum• Pumps Ca+2 out of the sarcoplasm and stores

it.• The resting cell has very little Ca+2 in the

sarcoplasm. 1000X more Ca+2 in the SR than in the sarcoplasm.

• The SR is made up of terminal cisternae that lie on above the junction of thin and thick filaments in the sarcomeres.

The Organization of the Myofibril

• Myofilaments are organized into repeating functional units- Sarcomere- smallest functional unit of the fiber.

• Sarcomere = myosin + actin + other stabilizing proteins.

Fig. 10-3. P. 282

Inside the Sarcomere…• Have two regions or Bands:

1. A BandsA Bands- are dArk bands and have three parts. a. M line- a protein that connects

neighboring thick filaments, keeps their position.b. H zone- a region with only thick filaments.c. Zone of Overlap- thin and thick filaments

overlap.

Inside the Sarcomere…• Have two Bands:

2. I BandsI Bands- are lIght bands; only find thin filaments.a. Z lines- mark the boundaries between the adjacent sarcomeres on the myofibril; have connectin – a connecting protein that interconnects thin filaments.b. Titin- a protein that aligns thick and thin filaments; resist extreme stretching.

A Band

Z Line

I Band

Sarcomere

Z lines

M line

Myofibril

Thin Filaments (Actin)Contains three proteins:

• F actin- a twisted strand of 300-400 globular (G) actin molecules.– Each G actin has an active site- a region

where thick filaments can bind.• Tropomyosin are strands of protein that wrap

around F actin. They cover the active site. This prevents actin-myosin interaction.

Thin Filaments (Actin)• Troponin- three globular subunits (parts).

– One subunit binds to tropomyosin and locks the two together.

– Another subunit binds to a G actin molecule, holding troponin and tropomyosin to the actin.

– The third subunit binds to a Ca+2 ion. Resting cells have low Ca+2 and so this subunit is empty in the resting cell.

Thick Filaments (Myosin)• Made of 500 myosin molecules.• Each myosin – two subunits twisted

around each other.• Myosin has a long, attached tail

bound to other myosin molecules in the thick filament.

Thick Filaments (Myosin)• The free head projects outward, toward the

nearest thin filament. Head can bind to the active site on the G actin.

• Between the head and the tail there is a flexible hinge that lets the head swing back and forth.

• All myosin molecules are arranged with their tails pointing toward the M line.

Supporting the Myofibrils• Desmin- is a complex network of

protein that twists around each Z line and connects adjacent myofibrils.

• Vertical bands of desmin can be seen with a light microscope and give the fiber a banded appearance-striated muscle.

The Sliding Filament

TheoryAn explanation of how

skeletal muscle cells contract.