Download PG1006 Lecture 4 Skeletal Muscle

PG1006
Lecture 4
Skeletal Muscle
Dr. Neil Docherty
My Teaching Objec/ves 1.  To describe the hierarchical structure of skeletal muscle fibres 2.  To relate the structure of myofibrils to force genera:on thorugh the descrip:on of sarcomeres and the sliding filament theory 3.  To introduce the concept of how skeletal muscle excitability translates into contrac:lity The Role of Skeletal muscle  
Skeletal movement and support  
Support of the body cavi:es  
Reflex control of body temperature Mechanical Proper/es of Skeletal Muscle  
Contrac:lity  
Tensile strength effect on tendons  
Series elas:city Skeletal Muscle from Outside-­‐In Note packeting of functional units and compare with peripheral nerve packaging
Skeletal Muscle fibres -­‐ H&E stain F = fasiculi
P = perimysium
En = endomysium
E = epimysium
Longitudinal and transverse sec/on of muscle fibres – H&E stain Transverse sec/on through skeletal muscle fibres -­‐ iron haematoxylin Very high magnifica:on (x1200) Only 1 nucleus visible Individual packets of filaments within muscle fibre These are the myofibrils Muscle Fibre Organisa/on Muscle Fibre (>100µM diameter, >0.75m length)
Composed of
Myofibrils(1µm diameter whole length of muscle)
Repeating units of
Sarcomeres
Regular arrays of
Thick filaments
(myosin polymer)
18nm diameter
1.6µm length
Thin filaments
(actin polymer)
5-8nm diameter
1µm length
Muscle Fibre Structure Transmission Electron Microscopy Regular striated patterns
of filaments and organelles
hint at functionality
The Sarcomere Visualised A band-stacked thick filaments and
overlapping regions of thin filaments
H-zone-Central thick filament only
Contains M line
I band-Peripheral thin filament only
Terminates at Z line
Area between Z-lines
=SARCOMERE
2.5µm length (resting)
Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L
The Sarcomere Order Hints at Func/on -T.E.M. patterning reflects
molecular composition
Adapted from Human Physiology- Cells to Systems (7th Ed.) Sherwood L
and Wheater’s Functional Histology (5th Ed.) Burkitt H.G. Young B. & Heath J.W.
Sarcomere Geometry Depending how you look at it!
-Thin filaments form a hexagon around a thick filament
-Thick filaments triangulate a central thin filament
A single thick filament can cross bridge to six thin filaments
Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L.
Myosin-­‐The Thick Filament Protein Several hundred myosin
molecules/thick filament
Mirror Image arrangement
Cross bridging to actin by
globular head
Crucial!
-ATPase site
-Actin binding site
Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L
Ac/n-­‐The Primary Thin Filament Protein Spherical polymer of actin monomers
Two polymeric actins intertwine
Each actin monomer contains a myosin binding site
Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L.
Sliding filament theory of contrac/on Sarcomere – thick and thin filaments
- Thick filaments are composed of myosin
- Thin filaments are composed of actin
During contraction the thick and thin filaments
slide over each other, shortening the
sarcomere
N.B. the filaments do not shorten, by sliding
over each other the space between them is
Shortened and the fibril contracts/shortens
Organisa/on of Neuromuscular Communica/on The Motor Unit Motor unit
Adapted from Human Physiology Cells to Systems (7th Ed.) Sherwood L.
Conduc/ng system for contrac/le s/muli T tubule system
- allows synchronous contraction of
all sarcomeres in a muscle fibre
-  Extends transversely through into
muscle cell to surround each
myofibril
- continuous with extracellular space
Conduc/ng system – longitudinal sec/on, EM x33,000 A and I bands
Td = tubular triads, tubule (T) of T
system and terminal cisternae
(TC) of sarcoplasmic reticulum
SR = tubular elements connecting TC
M = mitochondria
Aerobic and Anaerobic Fibres succinate dehydrogenase (x200)
A = aerobic (type I) muscle fibres
slow-twitch fibres
An = anaerobic (type II) muscle fibres
fast-twitch
Myosin ATP-­‐ase (x600) Your Learning from Today Should focus on being able to;
1.  Outline structural organisation of skeletal muscle how
peripheral nerves are packaged
2. Describe skeletal muscel force generation via the sliding
filament theory
3.Provide a basic description of the motor unit structure of
neuromuscular communication