Download Muscle Structure and Function - Queen`s ECE

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ELEC 811:
NERVE & MUSCLE STRUCTURE
AND FUNCTION
Dr. E. Morin, Dept of Electrical & Computer
Engineering, Queen’s University
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Nerve cells: categorization
• Nerve cells are also called neurons and are
categorized by structure and function
• Structural categories: multipolar, unipolar and bipolar
• Functional categories: motor and sensory
Mulitpolar (right), unipolar (middle) and
bipolar (left) neurons
from:http://people.eku.edu/ritchisong/301notes2.htm
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Multipolar neuron
• Dendrites – communicate
information to the cell body
• Axon – transmits signals - action
potentials - from the cell body to
the terminal endings
• Terminal endings – transmit
signals to other nerve or muscle
cells
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Action potential propagation
• Under normal
circumstances, action
potentials propagate –
or transmit information
– in one direction in
nerve cells.
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Muscle: categorization
• Muscle is categorized by function and structure
• Function – voluntary, involuntary
• Structure – striated, smooth
• Types of muscle:
• Smooth, involuntary – smooth muscle
• Striated, voluntary – skeletal muscle
• Striated, involuntary – cardiac muscle
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Muscle Types
Smooth muscle: no striations
or stripes visible in the muscle
cells. Each cell has its own
nucleus.
Image from:
www.uoguelph.ca/zoology/devobio/2
10labs/muscle1.html#skeletal
Skeletal muscle: cells have
striations or stripes; each
cell has many nuclei
Image from:
http://kentsimmons.uwinnipeg.ca
/cm1504/15lab42006/lb4pg7.htm
Cardiac muscle: cells
have striations; each
cell has a single nucleus.
Image from:
http://www.siumed.edu/~dkin
g2/ssb/muscle.htm#1a
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Skeletal muscle structure and function
• Primary function of skeletal muscle – generate forces by
contracting (shortening)
Muscles are connected
to bones via tendons.
Skeletal muscles act on
the bones to produce
movement of a limb.
Image from:
http://biophysics.homestead.c
om/muscular.html
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Skeletal muscle organization
• Muscles generate force by
contracting to pull the tendon
of insertion closer to the
tendon of origin
• In order to move a limb in
opposing directions, muscles
are often organized in pairs,
called agonist-antagonist
pairs.
Biceps brachii contracts to
flex the elbow joint; triceps
brachii contracts to extend
the elbow joint.
Image from:
http://biophysics.homestead.com/
muscular.html
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Skeletal muscle structure
• A muscle is
comprised of
individual muscle
cells or fibres
• Each fibre is
surrounded by
connective tissue endomysium
Image from:
http://faculty.etsu.edu/forsman/hist
ologyofmuscleforweb.htm
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Skeletal muscle structure
• Fibres are organized
into bundles called
fascicles (or
compartments)
• Each fascicle is
surrounded by
connective tissue –
perimysium.
Image from:
http://faculty.etsu.edu/fors
man/histologyofmusclefor
web.htm
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Skeletal muscle structure
• A number of fascicles
are bundled to form a
skeletal muscle
• The muscle is
surrounded by
connective tissue –
epimysium.
Image from:
http://faculty.etsu.edu/fors
man/histologyofmusclefor
web.htm
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Skeletal muscle structure – gross
anatomy
There are different shapes of muscles in the human body – e.g.,
circular, convergent, parallel, pennate and fusiform.
Figure shows: a) fusiform, b) unipennate, c)bipennate, d) triangular,
e) strap
Image from: Muscle, skeletal, Wiley Encyclopedia of Biomedical Engineering.
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Skeletal muscle structure – gross
anatomy
Limb muscles are generally pennate or fusiform.
tendon
tendon
Unipennate and bipennate arrangement
Fusiform arrangement
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Skeletal muscle - microscopic structure
Image from: A Schematic of Skeletal Muscle,
www.uoguelph.ca/zoology/devobio/210labs/sketchmuscle1.html
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The myofibril and sarcomere
Sarcomere bands are labelled: A- anisotropic; I – isotropic; H – Heller
(bright).
Actin filaments are anchored in the z-disc.
Image from: A Schematic
of Skeletal Muscle,
www.uoguelph.ca/zoolog
y/devobio/210labs/sketch
muscle1.html
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Motor control
• Skeletal muscles are under control of the central nervous
system – brain and spinal cord.
Image from: The brain from top to
bottom: Body movement and the brain;
http://thebrain.mcgill.ca/
Motor cortex is at the rear portion of the
frontal lobe
• area 6 – premotor cortex
• area 4 – primary motor cortex
• cerebellum – precisely regulates the
sequence and duration of movement
• basal ganglia – initiate and regulate
motor commands
Primary motor cortex – sends signals,
via the motor nerves (α-motoneurons),
to tell a muscle to contract.
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Neuromuscular organization
• Each α-motoneuron connects with several muscle fibres
• The motoneuron and all its associated muscle fibres is called
a motor unit
Image from:
http://www.uofaweb.ualberta.ca/comp
neurolab/newshome.cfm
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Neuromuscular organization
• The motor unit is the smallest functional unit in a muscle
• A muscle will have 100’s or 1000’s of motor units
Motor nerves
activate
associated
muscle fibres.
Motor units
generate
individual force
twitches, which
are translated to
the muscle
tendon.
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Excitation-contraction coupling
Process by which excitation from a motor nerve is converted
to force generation in the muscle fibre
The action potential travels
along the cell membrane or
sarcolemma of the muscle
fibre.
Image from: http://faculty.etsu.edu/forsman/histologyofmuscleforweb.htm
The T-tubules (yellow)
transmit the excitation
from the sarcolemma
into the interior of the
cell.
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The sarcomere
The sarcomere is the basic functional unit in muscle.
I-band
I-band
A-band
H-zone
z-disc
actin filament
M-band
myosin filament
z-disc
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The sarcomere
There are other proteins in the sarcomere. One of these is titin which
is probably responsible for the elastic properties of the sarcomere.
titin
z-disc
actin filament
M-band
Re-drawn from: P. Luther, www.sarcomere.org
myosin filament
z-disc
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Contraction of the sarcomere
• The myosin filaments have structures called heads
• Upon excitation, Ca++ is released into the sarcomere
 myosin heads link to actin → cross bridges
 heads change conformation to pull actin towards the M-band
sarcomere shortens
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Muscle fibre contraction
Shortening many sarcomeres in series results in
contraction of the entire fibril; contraction of many fibrils
results in contraction of the muscle fibre
Image from: Muscle fibre
structure and function,
http://muscle.ucsd.edu/m
usintro/fibril.shtml
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Model of muscle force generation
Neural
Excitation
u(t)
ActivationexcitationcontractionDynamics
Ca++ activation
of contractile process
Muscle
activation
Muscle
force
Muscle Contraction
Dynamics
FM(t)
a(t)
Cross-bridge
activation
• The muscle activation level, a(t) is often estimated from the
amplitude of the recorded EMG (electromyogram)
• From: Zajac, 1989.
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The muscle force twitch
• In a single motor unit:
 a motoneuron action potential → motor unit action potential → force
twitch.
muap
t (ms)
tension
t (ms)
Muap duration ~ 5 ms; force twitch duration ~ 100-300 ms
There is an electromechanical delay (EMD) between the muap and the start of
the force twitch.
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The muscle force twitch
• The force twitch looks like a critically damped 2nd order
response, where the muap is an impulse
muap
t (ms)
𝑡 −𝑡
𝐹 𝑡 = 𝐹0 𝑒
𝑇
tension
𝑇
t (ms)
T
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Summation of force twitches
• If a 2nd muap occurs shortly after the first, the force twitches overlap and
the resultant force twitch is longer duration and has higher peak force
than the individual twitches.
muaps
t (ms)
tension
t (ms)
T
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Fusion of force twitches
• For a train of closely spaced muap’s, the force twitches will fuse and
produce a smooth force profile called a tetanic contraction
muap
train
t (ms)
tension
t (ms)
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Motoneuron and muscle cell
characteristics
• Motoneurons vary in size and shape and
electrophysiological characteristics → a.p. conduction
velocity
• Muscle cells vary in size and shape and contractile
characteristics → contraction time and sensitivity to fatigue
• Muscle cells within a motor unit will have uniform characteristics
• Muscle fatigue is a physiological process – muscle output force (or
power) decreases over time during a sustained muscle contraction –
can lead to task failure
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Motor unit types
• Three types of muscle fibres have been identified – type I,
IIa and IIb.
Motor Unit
Type
Mechanical
properties
Electrical
properties
Other
Slow twitch Slow, small force
Slower nerve
or Type I
twitch; smaller fibre conduction
diameter and MU size velocity (CV)
Recruited at low
force levels
Fast, fatigue Fast force twitch
resistant or
Type IIa
Recruited at
intermediate
force levels
Fast,
fatiguing or
Type IIb
Intermediate
nerve CV
Fast, large force
Faster nerve
twitch; large fibre
CV
diameter and MU size
Table from: Merletti and Parker, Electromyography
Recruited at high
force levels
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Distribution of fibre types
• The different
Image from:
http://www.lab.anhb.uwa.edu.au/mb140/corepage
s/muscle/muscle.htm
muscle fibre types –
I, IIa and IIb – a are
distributed
throughout a
muscle
• Different types can
be seen under a
microscope, after
special staining of
the muscle tissue
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References
F. Felici and P. Sbriccoli, Muscle, skeletal, in Wiley Encyclopedia of
Biomedical Engineering, Wiley Interscience, Hoboken, NJ, 2006.
T. Moritani, D. Stegeman and R. Merletti, Basic physiology and
biophysics of EMG signal generation (chapter 1) in R. Merletti and P.A.
Parker, (eds.), Electromyography, IEEE Press, Piscataway, NJ, 2004.
D.A. Winter, Biomechanics, 3rd ed., Wiley, NJ, 2005. Chapter 8: Muscle
mechanics.
F. Zajac, Muscle and tendon: Properties, models, scaling and application
to biomechanics and motor control. CRC Crit. Rev. Biomed. Eng., 17:
359-411, 1989.
The Brain from Top to Bottom: Body movement and the brain,
http://thebrain.mcgill.ca/index.php
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Other Web Resources
1. Southern Illinois University - Muscle tissue
http://www.siumed.edu/~dking2/ssb/muscle.htm
2. University of Guelph - Developmental Biology On-line - Muscle tissue
http://www.uoguelph.ca/zoology/devobio/210labs/muscle1.html#skeletal
3. Eastern Kentucky University – Neurons & the Nervous System
http://people.eku.edu/ritchisong/301notes2.htm
4. Dr. J.A. Illingworth, Muscle Structure and Function
http://www.bmb.leeds.ac.uk/illingworth/muscle/
5. Structure and Function of Skeletal Muscle
http://courses.washington.edu/conj/bess/muscle/structurefunc2.html
6. Histology of muscle
http://faculty.etsu.edu/forsman/histologyofmuscleforweb.htm
7. Blue Histology – Muscle (School of Anatomy and Human Biology, University
of Western Australia
http://www.lab.anhb.uwa.edu.au/mb140/corepages/muscle/muscle.htm