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.

How does a skeletal muscle cell differ from most other eukaryotic cells?

A Comparison Most 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).

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 P. 282

Inside the Sarcomere… Have two regions or Bands: A Bands 1. A 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: I Bands 2. I 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 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 Theory An explanation of how skeletal muscle cells contract.