Muscles
and Muscle Tissue
Chapter
12
Organization and General Functions
"Myo" refers to muscles.
Sarco = "flesh"--some
muscle structures such as "sarcomere," "sarcoplasmic reticulum," and "sarcolemma."
Sarcophagus
= "flesh eating." Dates back to ancient Egyptians and the belief that
stone tombs (typically limestone) ate away at flesh of the dead, leaving behind
only bones.
Muscle is a principal tissue type (Chapter 6). Specialized for contraction.
Like neurons, muscle is an excitable tissue, in that it can conduct or transmit electrical impulses (respond to stimuli).
3 muscle types: skeletal,
cardiac and smooth.
(review Chapter 6 for muscle as a tissue).
All muscle tissues have 4 characteristics in common:
1) excitability
2) contractility
3) extensibility -
they can be stretched
4) elasticity - they return to normal length after stretching
Functions
1) Movement and Regulation.
Examples related to:
2) Posture and Support
3) Body Temperature Regulation.
Heat is a byproduct of metabolism. Since skeletal muscles make up 40% of total
mass of body, they play a major role in generating heat. During exercise, a
large amount of heat is generated. In cold, muscles will "shiver" to produce
heat.
General structure of a skeletal muscle: Tendon ---- Body of Muscle ----- Tendon
Tendon Histology: dense regular connective tissue
Aponeurosis: tendon that is broad or sheet-like. For example, the aponeurosis of Galen and the lumbar aponeurosis that is the origin of the latissimus dorsi.
Levels of Organization of Skeletal Muscle:
Associated Connective Tissue Organizes Muscle Tissue
Bundles of muscle fibers are grouped or bundled together by connective tissue.
Superficial fascia: layer adjacent to hypodermis. Deep fascia: binds muscles together.
Web sites covering muscle structure:
"Structure of Meat" (you are what you eat?)
Muscle structure (from "meat science")
Blood and Nerve Supply
Muscles are highly vascular organs -- high rate of metabolic activity.
Nerves control or modify muscle contraction. A "motor" nerve is any nerve that innervates a muscle. Skeletal muscles require input from a nerve in order to contract. Cardiac and smooth muscle can contract on their own (they have an intrinsic spontaneous contraction rate), but the rate is controlled by nerves and hormones.
Sensory nerves are abundant
in muscle also--supply nervous system with information on muscle contraction
and joint position.
Muscle
Spindles: monitor tension/stretch within muscles.
Excitation-Contraction Coupling & Sliding Filament Theory
Mechanism by which excitation of muscle cell membrane stimulates muscle cell contraction.
Neuromuscular Junction--3 components:
1) Terminal of motor axon interfaces with muscle cell.
Motor neuron terminal has synaptic vesicles that contain the neurotransmitter acetylcholine (ACh). ACh is released by nerve stimulation (nerve action potential).
2) Synaptic Cleft: Gap thru which transmitter diffuses.
3) Muscle Endplate: Area specialized for reception of neurotransmitter. Endplate has ACh receptors: ACh binds to receptors -- causes endplate potential (EPP) and then a muscle action potential. ACh-esterase: enzyme on endplate that breaks down ACh -- initiation of muscle impulse ends.
Sequence of Events at Neuromuscular Junction and Excitation-Contraction Coupling.
ATP provides energy for strokes. Myosin head is an ATPase in that it cleaves ATP to form ADP and Pi, yielding energy for movement.
Note: myosin is also found
in non-muscle cells that can move.
For
example, myosin in a kidney
epithelial cell.
ATP is also need to disengage actin from myosin head.
Lack of ATP -- rigor mortis
sets in. Muscles are stiff because actin and myosin filaments are cross-linked.
Short term effect.
The follow sites have great
animations of crossbridge formation, stroke and release.
Actin-myosin
Crossbridge 3D animation
Quicktime
animation of crossbridge -- try this site if the site above doesn't work.
gif
animation of the above
Muscle Relaxation: "What ends muscle contraction?"
Other Elements of Muscle Contraction
Length-Tension
Relationship: Based on arrangement of muscle filaments
Quicktime
Animation of Length-Tension Relationship.
Length-Tension Relationship and origin of the Sliding Filament Theory Theory (1954 experiments)
Isometric Vs Isotonic Muscle contraction
Twitch, Summation, and Tetanus (incomplete and complete) Graphic 1, Graphic 2
Staircase effect or Treppe
Tonus: Due to baseline activity of motor units.
Series Elastic Component: Total effect of muscle connective tissues.
Energy Use
Resting muscle uses aerobic respiration pathway to supply ATP. Prime energy sources are fatty acids and glucose (from glycogen via glycogenolysis).
Glucose converted to pyruvic acid, which is broken down by citric acid cycle (in mitochondria, aerobic) to produce ATP.
Aerobic and Anaerobic metabolism of glucose
Advantage of Aerobic respiration: Energy efficient
Disadvantage of Aerobic respiration: Exercising muscle uses ATP, but use soon outweighs ATP production. Muscle respires anaerobically (glycolysis -- lactic acid produced). ATP is also generated from creatine phosphate:
Creatine Phosphate + ADP -----------> ATP + Creatine
Muscle has large stores of creatine phosphate but it is rapidly used up during sustained contraction.
Articles on Creatine Supplements: What are risks? What are the benefits?
Oxygen Debt*
As a result of heavy exercise, lactic acid accumulates from anaerobic respiration. Lactic acid then travels to the liver, which converts it to glucose. Conversion to glucose requires ATP and oxygen. During exercise, available oxygen is used primarily by muscle, so less is available for use by liver.
The "oxygen debt" created is equal to the amount of oxygen needed by liver to convert accumulated lactic acid into glucose, plus the amount needed by muscle for ATP and CP regeneration. Total process may take hours.
(*Note: Exercise physiologists prefer to call oxygen debt "EPOC" for "Excess post-exercise oxygen consumption.")
Powerpoint lecture covering energy use and EPOC
Muscle Fatigue
Accumulation of lactic acid and low ATP results in fatigue (reduction in ability to contract). Other factors include: change in pH, interruption of blood supply, depletion of acetylcholine.
Tolerance to fatigue from athletic training is due to increased muscle blood supply, increased number of mitochondria in muscle fibers.
Energy use of muscles can be modified as a result of exercise.
Effects of Resistance Training: Muscle Hypertrophy and Hyperplasia
Responses of Muscle to Endurance Training: Numerous factors related to aerobic metabolism
Increased size and number of mitochondria.
Less lactic acid produced
Increased myoglobin
Increased intramuscular triglycerides
Increased lipoprotein lipase
Lower rate of glycogen depletion during exercise
Loss of Muscle Mass
Atrophy: wasting away
of muscle tissue. E.g., disuse atrophy, in which muscle mass is lost because
of inactivity.
Also from loss of motor neurons.
Dystrophy: literally,
"defective nutrition." Tissue fails to develop correctly or thrive.
Usually congenital or genetic in cause.
Sarcopenia:
means "vanishing flesh." Age-related loss of muscle with replacement
by fat.
Report
on effect of calorie restriction and sarcopenia.
Effect
of Age on ability of satellite cells to repair muscle
Skeletal Muscle Fiber Types
1) Slow Twitch (Type I)--e.g., soleus. 100 msec for contraction.
2) Intermediate (Type IIA) e.g., gastrocnemius. Fast twitch.
3) Fast Twitch (Fatigable) (Type IIB)--e.g., extraocular muscles. 7.3 msec for contraction.
Fiber type dictated by innervation (experimentally demonstrated) and by genetics.
Effect of training? Twitch type (fast or slow) stays the same. Aerobic capacity/fatigue resistance can be increased in fast-twitch muscles. (Fast-twitch fatigable become intermediate).
Fibers also differ in: Myoglobin content (oxygen supply), Vascularization and Myosin ATPase activity.
Ratio of fast to slow fibers varies among individuals.
Gastrocnemius muscles of runners for example: Sprinters have more fast twitch fibers, long distance runners have more slow twitch fibers.
Muscle Physiology Web Sites of Interest:
Cardiac Muscle
Specialized muscle of the heart.
Structure: Striated, but differs from skeletal muscle in the following ways:
Excitation Contraction Coupling in Cardiac Cells
Smooth Muscle
Typically associated with "viscera" and "involuntary" systems (circulatory, respiratory, digestive, urinary, reproductive and integumentary).
Key characteristics:
Muscular Diseases
Muscular Dystrophy:
MDA-USA
aka "Jerrys Kids"
Major
characteristics of 9 muscular dystrophies.
Master list of 40 neuromuscular diseases
MDA
Canada. In French and English, eh.
MDA
Australia
Duchenne M.D. is well characterized.
It is a genetic disease (sex-linked) in which a protein called "dystrophin"
is either lacking or defective. Dystrophin helps organize the structure of the
muscle fiber.
Duchenne
MD site -- contains histology and diagnostic tools including immunolabeling.
Becker Muscular Dystrophy is a related disease in which dystrophin is present in low amounts.
Myasthenia Gravis:
Myasthenia
Gravis: Diagnostic Assay (Nice
graphic of ACh receptor)
MG,
Ach and Ach receptors
Botulism:
Botulinum
toxin comes from Clostridium botulinum, a soil bacterium. The toxin inhibits
the release of ACh from motor nerve terminals, causing flaccid paralysis.
Tetanus or Lockjaw
Severe muscle spasms from toxin produced by the bacterium Clostridium tetani.Not
a direct muscle effect. The toxin blocks inhibitory neurotransmitters in spinal
cord, so motor neurons are inhibited less. Causes
spastic paralysis.
Muscle Physiology Review Questions