ANS 331: Muscle
I. General Functions
1. Movement
A. Skeleton
B. Blood
C. Ingesta
D. Gametes/Conceptus
2. Support
3. Make up 45-50% of Body Mass
II. Classifications
1. Smooth
A. Not Striated, spindle-shaped cells
B. Uninuclear, centrally located nuclei
C. Regulated by Autonomic Nervous System
D. Found in ducts, blood vessels, digestive
and reproductive tracts
2. Cardiac
A. Branching, striated
B. Multinucleated, Nuclei centrally located
C. Divided by intercalated disks
--Facilitates Nerve impulses
C. Regulated by Autonomic Nervous System
D. Form Heart Muscle
3. Skeletal
A. Long Bundles of muscle cells, striated by myofibrils
B. Multinucleated, form at periphery of cells
C. Innervated by Spinal or Cranial Nerves
a. Each Fiber Needs to be Stimulated
D. Form Skeletal Muscle
E. Three Types
a. Red or Dark
--Slow Contraction and Fatiguability
--"Dark Meat"
b. White or Pale
--Faster Contraction and
Fatiguability
--"White Meat"
c. Intermediate
F. Comprises most of Muscle Mass
G. Described by type of movement
a. Flexors
--Decrease Joint Angle
b. Adductors
--Pull Towards median plane
c. Abductors
--Pull Away from median plane
d. Sphincters
--Constrict body openings
II. Arrangement
1. Function
A. Contract or Shorten
a. Move Body Part
b. Move Body Contents
c. Provide Resistance for Movement
2. Structure unique to these functions
A. Sheets
B. Sheets rolled into tubes
C. Bundles
D. Rings
E. Cones
F. Discrete cells or clusters of cells
3. Cardiac and Smooth Muscle
A. Inside the visceral organ
B. Directly associated with what is being moved
4. Skeletal Muscle
A. Attached to structure or organ at some
distense
a. Directly
b. Tendon
B. Connections
a. Origin
--Least moveable part
b. Insertion
--Most moveable part
c. Contraction bring Origin and
Insertion closer together
III. Skeletal Muscle Harnessing
1. Connective tissue makes up the harness
for skeletal muscle fibers
2. Three Layers (Inner to Outer)
A. Endomysium
--Surround muscle fiber bundle
B. Perimysium
--Surround a collection of muscle bundles
C. Epimysium
--Surround whole muscle
--Continuous with tendon or aponeurosis
IV. Microstructure of Skeletal Muscles
1. Muscle Fiber (Cell)
A. Nuclei (Multi)
B. Mitochondria
--Especially in Dark Fibers
B. Sarcolemma
--Plasma Membrane
C. Myofibrils
--Bundles of Myofilaments
D. Sarcotubular System
--Modified Endoplasmic Reticulum
2. Myofibrils
A. 100-1000 per muscle fiber
B. Divided into Sarcomeres
(Repetitive Units)
C. Contain Protein Myofilaments
a. Myosin
--Thick Filaments
b. Actin
--Thin Filaments
c. Found in 2:1 (Actin:Myosin) ratio
D. Striated or Banded
a. Z-line
--located at each end of
sarcomere
--Actin backbone that
projects filaments
through myofibril
b. I-band
--Light band
--Contains Actin only
c. A-band
--Dark band
--Contain Actin and Myosin
overlapped
d. H-zone
--Zone within A-band
containing only
myosin
--Slightly lighter zone
e. M-line
--Darker line through the
H-zone
--Backbone for Myosin
2. Sarcotubular System
A. Network of tubules in skeletal muscle
B. Located outside of the myofibrils
C. Two separate tubule sets
a. Sarcoplasmic Reticulum
b. T-tubules
D. T-tubules
a. Arranged transversely (right angles)
to myofibrils
b. Open to the outside of fibers
--contain extracellular fluid
c. Found at boundary of A- and I-bands
--Each sarcomere has two
E. Sarcoplasmic Reticulum
a. Arranged mostly parallel to
myofibrils
b. Contain intracellular Fluid
c. Bulbous ends (Lateral Sacs)
--Lie in close proximity to T-tubules
--2 Ends and a T-tubule=Triad
d. Function
--Conduction of impulses from
surface of muscle fiber
to myofibrils
3. Neuromuscular Junction
A. Each skeletal muscle fiber is innervated
by a motor neuron
B. Axon terminal end bulb forms synapse
with muscle fiber
a. Release of Acetylcholine (ACh)
causes Depolarization
C. One motor neuron innervates many
muscle fibers
a. Motor Unit
b. Lower the Ratio,
Higher the Precision
V. Skeletal Muscle Contraction
1. Depolarization
A. Acetylcholine released into synapse
a. Ca++ facilitates release
b. Parturient Paresis occurs if not
enough Ca++
B. Opens Ligand-gated Na+ channels
C. Causes a Propagated Action Potential
a. Sarcolemma
b. T-tubule
c. Sarcoplasmic Reticulum
D. Cholinesterase destroys ACh
D. Depolarization of Sarcoplasmic
Reticulum causes Ca++ release
E. Ca++ diffuses to myofibrils
F. Ca++ initiates contraction
G. Ca++ pumped back into sarcoplasmic
reticulum
a. Active Transport
b. Ca++ ATPase Pump (uniport)
2. Contraction Process
A. Interaction between Actin and Myosin
B. Actin Filament Components
a. Actin
b. Tropomyosin
--Combines with actin to form helix
c. Troponin
i. Binds Actin and Tropomyosin
together
ii. Binds Ca++
C. Ca++ binding with Troponin causes Actin
"active sites" to be exposed
a. Covered by Tropomyosin
in relaxed state
b. Active sites form cross bridges
with Myosin
D. Myosin filaments
a. Heads
i. Have high affinity to actin
active site
ii. Cleave ATP (ATPase)
b. Upon cleaving ATP heads cock
(perpendicular)
c. Binding with active site causes
uncocking
i. Heads tilt
--towards center of sarcomere
ii. Pull actin with it
d. Tilting causes release of ADP +Pi
e. Bind new ATP
--Relaxation
f. Cleaves and Cocks
g. Awaits next Depolarization
3. Contraction vs. Contracture
A. Muscle shortening can happen in absence
of Action Potentials
a. Rigor or Physiologic Contracture
B. Muscles remain in contracted state
because ATP is not available
for Relaxation
C. Rigor Mortis (Best Example)
a. Contracture after death
b. Relaxation only occurs after autolysis
of the crossbridges
c. Muscles most active before death
develop rigor fastest
4. Contraction Strength
A. Motor Unit Summation
a. The more motor units that fire the
stronger the contraction
b. All gradations of contraction strength
possible
B. Wave Summation
a. Frequency of contraction is increased
b. Stimulation before relaxation
increase strength
c. Tetany
--When high frequency muscle
twitches become fused into a
single prolonged contraction
C. Tetanus
A. Tetany caused by bacterial
neurotoxin
(Clostridium tetani)
B. Prevents release of inhibitory
neurotransmitter glycine by CNS
C. Increase synaptic sensitivity to
excitatory impulses
D. Tetany
--Lockjaw
D. Treppe
A. Also called staircase phenomena
B. Warming-Up to a maximal
contraction
C. Causes increase Ca++ in sarcoplasmic
reticulum
5. Comparisons Among Three Muscle Types
A. All three have similar contraction process
--Use myosin and actin
B. Cardiac Muscle
a. Striated like Skeletal
b. Myofibrils join together and vary in
diameter
--Cause Circular Contraction
c. Join to each other rather than C.T.
harness
d. Receive impulse from pacemaker cells
--Innervated by A.N.S.
e. Conduction
--cell to cell from intercalated disks
--Purkinje fibers
d. Sarcoplasmic reticulum not well
developed
C. Smooth Muscle
a. Myofilaments not aligned into
myofibrils
b. 15:1 Actin to myosin ratio
c. Third filament present
--Intermediate filament
i. Harnesses Myosin and Actin
ii. Attached to Dense Bodies and
Cell Membrane
iii. Causes Shortening along cells
longitudinal axis
d. Each cell harnessed to external C.T.
e. Innervated by A.N.S.
--Each cell has to be innervated
f. Poorly developed Sarcoplasmic
reticulum
g. T-tubules are vesicles
VI. Changes in Muscle Size
1. Hypertrophy and Hyperplasia
A. Hypertrophy
--Increase in muscle fiber size
--Caused by increased work load
over time
B. Hyperplasia
--Increase in number of muscle fibers
C. Skeletal and Smooth Muscle
--Can regenerate
--Undergo both Hypertrophy
and Hyperplasia
D. Cardiac Muscle
--can only undergo hypertrophy
--Cells don't regenerate
2. Atrophy
A. A decrease in size of muscle
B. Causes
a. Immobilization
b. Loss of nerve supply
--Draft Horse Example
~~~~~Revised 9/19/96~~~~~ TAW