Download Muscles and Nerves

IE 665Applied Industrial Ergonomics
Suggested external links:
http://people.eku.edu/ritchisong/301notes3.htm
http://www.youtube.com/watch?v=0_ihc26yxN4&NR=1
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Three types of muscle tissues: Skeletal, Smooth and
Cardiac.
Some basic functions and fundamental characteristics of
skeletal muscles.
Function and Structure of skeletal muscle tissue
The nerve tissue and motor unit
Microscopic anatomy of a skeletal muscle tissue
How a muscle tissue contracts
Action potential
Length tension characteristics of a muscle tissue
Force regulation in skeletal muscles
How energy is metabolized for muscle contraction and
cellular respiration
Fatigue in static and dynamic muscular work
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Skeletal muscle attaches to bones, holds the
skeleton against gravitational forces & moves
skeleton to produce motion
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Smooth muscles are present in the walls of blood
vessels, intestine & other 'hollow' organs. Its
rhythmic contraction moves body fluids.
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Cardiac muscle are present in the wall of the
heart. Its rhythmic contraction moves blood.
FOCUS OF THIS COURSE IS THE SKELETAL
MUSCLES
Striations
• It is under voluntary control. The
muscle can be contracted and
relaxed at will.
• It has a striated appearance under
microscope, which is due to the
orderly arrangement of the
contractile proteins within the
tissue.
• The cells are cylindrical and
multinucleated.
Nuclei
• Involuntary muscle, ie.,
not under voluntary
control
• Not striated under
microscope
• Not multinucleated
• Involuntary, ie., not under
voluntary control
• Striated appearance under
microscope
• Auto-rhythmic, ie. contracts
rhythmically without any nervous
impulse (nerve impulse modifies
the rhythm)
• Not multinucleated
• Rectangular in shape
• Produces motion – fundamental
characteristics of all living things
• Produces force (tension)
• Maintains posture – works against
gravitational forces
• Provides joint stability
• Produces heat as a bi-product of
contraction
• Excitability - responds to stimuli (e.g.,
nervous and other impulses)
• Contractility - able to shorten in length
• Extensibility - stretches when pulled
• Elasticity - tends to return to original
shape & length after contraction or
extension
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Each skeletal muscle spans over
one or more skeletal joints and the
muscle contraction produces a
force that tends to turn a bone
about its joint axis.
Skeletal muscles vary in size,
shape, and arrangement of
fibers. They range from extremely
tiny strands such as the stapedium
muscle of the middle ear to large
masses such as the muscles of the
thigh.
A gross muscle contains skeletal
muscle tissues, connective tissues,
nerve tissues, and vascular (blood
circulation) tissues. Out of these,
only the muscle tissue has the
contractile property.
Each muscle is surrounded by a
connective tissue sheath called the
epimysium. Fascia, connective tissue
outside the epimysium, surrounds and
separates the muscles. Portions of the
epimysium project inward to divide the
muscle into compartments. Each
compartment contains a bundle of
muscle fibers. Each bundle is called a
fasciculus and is surrounded by a layer
of connective tissue called the
perimysium. Within the fasciculus, each
individual muscle cell, called a muscle
fiber, is surrounded by connective tissue
called the endomysium. All these
connective tissue fuse together at the
two end and forms tendon, which
connects muscles to bones
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Skeletal muscle cells (fibers),
like other body cells, are soft
and fragile. The connective
tissue covering furnish
support and protection for the
delicate cells and allow them
to withstand the forces of
contraction.
Through these tough tissues
contractile force of the muscle
cells are transmitted to the
bone.
The coverings also provide
pathways for the passage of
blood vessels and nerves.
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Skeletal muscles have an
abundant supply of blood
vessels, approximately 2
capillaries per muscle cell.
Capillaries supply the
essential oxygen and
nutrients to each muscle
fiber.
Since the capillaries spreads
evenly in the muscle body
the smaller muscles cells
have more capillaries.
The connective tissues, the
epimysium, perimysium, and
endomysium extend beyond the fleshy
part of the muscle to form a thick
ropelike tendon or a broad, flat sheetlike aponeurosis.
The tendon form attachments from
muscles to the bones and aponeurosis
forms connection to the connective
tissue of other muscles.
Typically a muscle spans a joint
and is attached to bones by
tendons at both ends. One of the
bones remains relatively fixed or
stable while the other end moves as a
result of muscle contraction.
Ligaments forms joint capsules are
fibrous tissues that connect bone
to bone.
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It is the major controlling, regulatory, and
communicating system in the body. If muscles are
power house, then the nerves are the control
mechanism.
It is the center of all mental activity including thought,
learning, and memory.
Together with the endocrine system (producing
hormones), the nervous system is responsible for
regulating and maintaining homeostasis (regulates
internal environment so as to maintain a stable,
constant condition).
Through its receptors, the nervous system keeps us
in touch with our environment, both external and
internal.
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The nervous system is
composed of central
nervous system (brain
and spinal chord) and
peripheral nervous
system (containing nerve
cells external to the brain
or spinal cord).
These, in turn, consist of
various tissues, including
nerve, blood, and
connective tissue.
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Millions of sensory receptors detect changes, called stimuli, which
occur inside and outside the body. They monitor such things as
temperature, light, and sound from the external environment. Inside
the body, the internal environment, receptors detect variations in
pressure, pH, carbon dioxide concentration, and the levels of
various electrolytes. All of this gathered information is called
sensory input (afferent nervous system).
Sensory input is converted into electrical signals called nerve
impulses that are transmitted to the brain. There the signals are
brought together to create sensations, to produce thoughts, or to
add to memory; Decisions are made each moment based on the
sensory input. This is integration.
Based on the sensory input and integration, the nervous system
responds by sending signals to muscles, causing them to contract,
or to glands, causing them to produce secretions.
The nerve cells that send impulse to muscle cells are called motor
nerve (efferent nervous system).
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Axon terminals of one motor
neuron innervate a number of
muscle cells that are dispersed
randomly in the overall muscle
mass. The muscle cells and
the single motor neuron that
innervates them make one
motor unit.
When the neuron of a motor unit sends a nerve impulse which
exceeds a threshold value, all the muscle cells (fibers) of the motor
unit contract together. All or none principle
Number of muscle cells controlled by a motor neuron varies.
Muscles which require fine controls may have innervations of a few
muscle cells per motor neuron, where as, when gross force
production is the primary objective, motor units innvervates large
(over hundred) number of muscles cells.
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When the nerve impulse (electrical) reaches axon end, the
permeability of the synaptic vesicle membranes at its axon
ends releases chemical neurotransmitter (acetylcholine).
This chemical binds with the muscle cell membrane
molecules at the synaptic cleft (known as motor end
plate), and stimulates the muscle cell.
Microscopic Structure of a Muscle Cell
Neucleus
Sarcolema
Mitochondria
Contractile
proteins
Sarcolemma: Bi-layer lipid membrane, semi-permeable, has
specialized molecules that selectively control inflow and outflow of ions
from the extra-cellular space.
Motochondria: Organelle, where ATP (Adenosine Tri-phosphate) is
synthesized by oxidative process. ATP is only form of energy that
muscle cells can utilize to produce mechanical energy.
Contractile proteins: Responsible for muscle contraction.
T-tubules and Sarcoplasmic reticulum
Arrangement
of Protein filaments
Muscles cells are packed with
myofibrils.
Myofibrils are composed of two main
types of myo-filaments: thick and thin.
They are arranged in a very regular,
precise pattern.
Myosin – thick filaments
Actin – thin filaments
Sliding of the thin filaments over the
thick filaments causes sarcomere to
contract.
Sarcommere: The smallest contractile
unit.
Models of Protein filaments
Review – U-tube video
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http://www.youtube.com/watch?v=EdHzKYD
xrKc&feature=player_embedded
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In a resting muscle, there is a higher concentration of
Na+ ions in the extra-cellular space and a higher
concentration of K+ ions in the intracellular space
(inside the muscle cell membrane).
In resting state the muscle cell membrane remains
electrically polarized (i.e. outside has higher
positive ion concentration than inside). This is due
to the fact that K+ ions are small and can freely defuse
across the cell membrane but larger Na+ ions cannot,
which makes the cell membrane polarized.
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Nerve impulse (electrical) reaches the axon end of the
nerve cell. The Impulse releases a neurotransmitter
chemical (acetylcholine) that binds with specific molecules
at the motor end plate
Due to this chemical reaction, some molecules at the
motor end-plate change their shapes opening gates
(pores) for Na+ ions.
Na+ ions start to diffuse in the muscle cell. The influx of
Na+ ions locally depolarizes the cell membrane.
After the depolarization reaches a threshold level, a local
electric current sets up between the depolarized region at
motor end plate and the neighboring polarized (resting)
regions of the cell membrane.
This electric current opens more voltage sensitive Nagates on the cell membrane and causes Na+ ions influx in
the neighboring region of the cell membrane.
This newly depolarized region, in turn, depolarizes their
neighboring region and the depolarization wave
propagates in the outward direction from the motor end
plate, and travels the entire length of the muscle cell. This
phenomena is called Action Potential.
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The wave also reaches the deep inside of the cell body trough the ttubules.
This whole phenomena starts with a single nerve stimulus that
exceeds a threshold level. Once a single nerve stimulus level
exceeds a threshold value, the action potential starts with the same
intensity (all or none principal). Larger discharge of
neurotransmitter would not produce stronger Action potential.
Right after the depolarization, acetylcholine is broken down by
enzymes and Na+ ions are actively (using energy molecules)
transported back to the outside of cell membrane and the cell
membrane returns to its normal polarized (resting) state.
Action potential reaches deep in the muscle through the Ttubules, which causes release of Ca+2 ions.
 Ca+2 ions binds with tropomyosin protien, and shifts the
troponin molecules to open the binding site of actin and
myosin.
 Myosin molecule attaches to actin molecule and change its
shape, and sliding the actin molecule.
 With the presence of energy molecule (ATP), myosin
combines with ATP, and the mysin-actin bond is broken.
 As long as ATP and Ca+2 ions are present, this process
continues.
 If no new nerve impulse is there, then Ca+2 ions are actively
pushed back in to SR, and binding sites of actin-myosin are
closed and the sliding stops.
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WATCH HOW MUSCLE CELLS CONTRACT
 http://www.youtube.com/watch?v=gJ309LfHQ3M&feature
=player_embedded#!
 http://www.mmi.mcgill.ca/mmimediasampler/
The muscle twitch is a single response to a single stimulus.
Latent period - the period of a few ms for the chemical and physical
events preceding actual contraction.
Contraction period - tension increases (myosin cross-bridges are
swiveling)
Relaxation period – muscle relaxes, relieves tension or comes back
to its original length. Since it occurs due to passive tension from
the connective tissues, takes more time than the contraction phase.
We do not use the muscle twitch as part of our normal muscle
responses. Instead we use graded contractions, contractions of
whole muscles which can vary in terms of their strength and
degree of contraction. In fact, even relaxed muscles are
constantly being stimulated to produce muscle tone, the minimal
graded contraction possible.
Muscles exhibit graded contractions in two ways:
(1) Motor Unit Summation/Recruitment/Quantal Summation:
Increasing numbers of motor units to increase the force of
contraction. (Quantal, because individual muscle cells cannot be
recruited).
(2) Wave Summation/Rate coding & Titanization: This results from
stimulating a muscle cell before it has relaxed from a previous
stimulus by increasing the frequncy of nerve stimulation. This is
possible because the contraction and relaxation phases are much
longer than the refractory period.
Another way in which the tension of a muscle
fiber can vary is due to the length-tension
relationship. Muscle fibers along with its
connective tissues are elastic, i.e., muscle
fibers can be elastically stretched by the
action of external forces. For example, as we
flex our elbow, the length of the biceps muscle
fibers shortens. Opposite happens when the
elbow is extended.
With the change of length of a muscle fiber
from its resting or optimum length, the number
of cross bridges between actin and myosin
filaments decreases. As a result of this, the
force developed for an action potential
decreases as it is stretched or shortened from
its normal resting length.
Force developed due to
sliding action of protein
filaments.
Force due to
stretching of
connective
tissues
Force
The graph shows the force
developed in a muscle fiber for a
single twitch, when it is kept at
various lengths. Lo is the normal
resting length of the muscle fiber.
The black line shows the
contractile force generated by the
action of myosin sliding over
actin filaments. After sufficient
stretch, the elastic contractile
force from the connective tissues
adds a passive tension.
Lo
Length
Lo = Normal resting length of the muscle
Muscle contraction needs energy for myosin-actin sliding, transport
of Na out of plasma membrane, transport of Ca molecules back to
SR etc. all of which are energy intensive.
Muscle cells, like all other cells, use ATP (adinosine tri-phosphate)
as their energy currency.
ATP↔ADP + Energy
Each muscle cell stores some ATP, which can sustain contraction
for 1 to 2 seconds. To continue contraction, other high energy
particles are broken down and the energy liberated from these
reactions is used to re-synthesize ADP back to ATP to sustain
contraction.
Muscle cells store a high energy molecule, Creatine
Phosphate, which can be readily decomposed to
Creatine and phosphate to liberate energy, which then
can be used to re-synthesize ADP to ATP. But this
source of ATP can only supply a cell for 8 to 10 seconds
during the most strenuous exercise.
Creatine phosphate can be stored and is made from
ATP during periods of rest.
The bulk of the energy supply comes from metabolism (destruction) of
glucose molecules, which is stored as glycogen (polymer of glucose) in
muscle cells. Fat ( and protein in extreme cases) molecules, supplied
through blood are also metabolized in some cases. Glucose molecules can
be metabolized in two ways:
Anaerobic: In the absence of oxygen (anaerobic glycolysis) – glucose
molecules are broken down to pyruvic acid and each molecule produces
energy equivalent to 2 ATP molecules. End product of anaerobic glycolysis
is Lactic acid, which builds up in muscle cells causing local fatigue painful
sensation.
Aerobic: In the presence of oxygen (aerobic glycolysis), glucose molecules
break down to simpler molecules (CO2, H2O) and thus produces more
energy, equivalent to 36 ATP molecules. This process of energy production
can continue for long period of time as O2 can be made available through
blood supply.
Glycolysis is the initial way of utilizing glucose in all cells, and is used
exclusively by certain cells to provide ATP when insufficient oxygen is
available for aerobic metabolism. Glycolysis doesn't produce much ATP in
comparison to aerobic metabolism, but it has the advantage that it doesn't
require oxygen. In addition, glycolysis occurs in the cytoplasm, not the
mitochondria. So it is used by cells which are responsible for quick bursts
of speed or strength. Like most chemical reactions, glycolysis slows down
as its product, pyruvic acid, builds up. In order to extend glycolysis the
pyruvic acid is converted to lactic acid. Lactic acid itself eventually builds
up, slowing metabolism and contributing to muscle fatigue.
Ultimately the lactic acid must be reconverted to pyruvic acid and
metabolized aerobically, either in the muscle cell itself, or in the liver. The
oxygen which is "borrowed" by anaerobic glycolysis is called oxygen debt
and must be paid back. But mostly it is the amount of oxygen which will be
required to metabolize the lactic acid produced.
Strength training increases the myofilaments in muscle
cells and therefore the number of crossbridge
attachments which can form. Training does not
increase the number of muscle cells in any real way.
(Sometimes a cell will tear and split resulting in two cells
when healed). Lactic acid removal by the cardiovascular
system improves with training which increases the
anaerobic capacity. Even so, the glycolysis-lactic acid
system can produce ATP for active muscle cells for
only about a minute and a half.
Ultimately, the product of glycolysis, pyruvic acid, must be
metabolized aerobically. Aerobic metabolism is performed
exclusively in the mitochondria. Pyruvic acid is converted to CO2
and H2O and vast majority of ATP. The reactant other than glucose
is O2. Aerobic metabolism is used for endurance activities and has
the distinct advantage that it can go on for hours.
Training Effect:
Aerobic training increases the length of endurance activities by
increasing the number of mitochondria in the muscle cells,
increasing the availability of enzymes, increasing the number of
blood vessels, and increasing the amount of an oxygen-storing
molecule called myoglobin.
Different types of cells perform the differing functions of endurance
activities and speed- strength activities. There are three types, red, white,
and intermediate. The main differences can be exemplified by looking at
red and white fibers and remembering that intermediate fibers have
properties of the other two.
White Fibers are fast twitch, large diameter, used for speed and strength,
fatigable. Depends on the anaerobic energy metabolism, stores glycogen
for conversion to glucose, Fewer blood vessels, Little or no myoglobin.
Red Fibers are slow twitch, small diameter, used for endurance. Depends
on aerobic metabolism. Utilize fats as well as glucose. Little glycogen
storage. Many blood vessels, mitochondria and much myoglobin give this
muscle its reddish appearance.
Intermediate Fibers: sometimes called "fast twitch red", these fibers have
faster action but rely more on aerobic metabolism and have more
endurance.
Most muscles are mixtures of the different types. Muscle fiber types and
their relative abundance cannot be varied by training.
At the onset of muscular work, energy is supplied primarily from
stored high energy particles and from anaerobic glycolysis. This is
because circulatory system takes some time to catch up with the
higher O2 demand at the muscle site.
CO2, and Lactic acid are built up (causing change in Ph level) in
the muscle site triggering the CNS to initiates actions to increase
cellular respiration (CO2 and O2 movement in and out of the cells).
This is achieved in a combinations of ways (1) Redistribution of
blood supply (dilating the arteries near the muscle and constricting
arteries in skin and other organs), and (2) by increasing cardiac
output and ventilation at lungs to maintain the O2 at the working
muscle site. Heart rate, stroke volume, blood pressure and
respiratory rate increase according to the intensity of the muscular
work.
Cellular respiration is affected by constriction of the
nearby arteries and blood capillaries by the mechanical
force developed by the muscle itself.
The blood supply starts to decrease when the muscle
contracts with an intensity of 15% of its maximum
voluntary contraction (MVC) capacity.
The blood supply is completely occluded above 60% of
MVC in most of the muscle cells. Reduction of blood
supply means reduction of cellular respiration (O2 supply and CO2 removal).
An activity which requires muscle to maintain
contraction continuously it is called static muscular
work.
Muscles that are maintaining a static body posture, or
holding a hand tool are example of static muscular
work.
As blood supply is impeded in this kind of muscle work,
depending upon the contraction level, majority of the
energy may be produced through anaerobic pathway.
As a result, metabolite (Lactic acid) accumulates in the
muscle cells and local fatigue of the muscles ensues
quickly.
Typical endurance limits of skeletal muscles in static
muscle contraction
% MVC
Endurance time for static muscle
contractions
100
6 seconds
75
21 seconds
50
1 minute
25
3.4 minute
15
> 4 minute
In dynamic muscular work muscle contraction is followed by a
muscle relaxation. That is static tension interspaced with
relaxation. Work with rhythmic movement, such as walking, is an
example of this kind.
During relaxation phase, the blood supply is restored which washes
away the metabolite (waste byproducts) and supplies nutrients and
oxygen. As a result, this kind of muscle work can be continued for
long time without fatigue. The rhythmic movement also helps
venous return of blood and thus is less taxing on heart
performance.
In dynamic work, maximum intensity of work is determined by the
circulatory systems capacity to supply O2 which is determined by
the Maximum heart rate capacity, or by Maximum O2 (Max VO2 in
L/min) delivering capacity. Fatigue in this kind of work is primarily
from the central fatigue, less blood glucose level, etc.