MUSCLES
and
MOVEMENT
General Function
of Muscle Tissue
• Movement or locomotion
• Heat production – maintain the
body’s homeostasis of temperature
• Posture – using partial contraction
of the “anati-gravity” muscles
Specific Function of Skeletal
Muscle Tissue
•Excitability or irritability
The ability to be stimulated or simply stated
“responds to stimulus” from nerve signals
•Contractility
The ability to contract or shorten
including contraction without movement
isometric contraction is muscle tissue contracting, tension increasing but the
there is not enough strength to “move the fixed object”
•Extensibility
•The ability to extend or stretch
allowing muscles to return to their
resting length after a contraction
They may also extend while still exerting force as when lowering a
heavy object
Muscle cells are different
than other body cells
•Muscle cells have a long,
thin, thread-like shape
•During tissue development
cells fuse together to form a
multi-nuclei cell that
becomes a muscle cell
Adult muscle cells can even have some
of the stem cells hanging out to
produce more muscle cells after an
injury.
Muscle cells have similar parts but
different names
Sacrolemma – plasma membrane
Sarcoplasm – cytoplasm contains many
mitochondria and more than one nuclei
Sarcoplasmic reticulum (SR) – a type of
endoplasmic reticulum however the SR stores and
pumps calcium ions
T tubules – transverse tubules that allow
electrical impulses traveling along the
sarcolemma to move deep into the cell
The SR butts up against both
sides of the T tubule in a
muscle fiber (called a triad)
Impulses travel along the T
tubule to stimulate the
membranes of the SR to
pump Ca++ into the channels
of the SR causing the
contraction to happen
Myofribrils – not found in other cells
cytoskeletal filaments that extend lengthwise
along the muscle fiber and fill the sarcoplasm.
Myofribrils are made up of finer fibers called
Myofilaments of which there are two types
Thick filaments
Thin filaments
Sarcomere – is a contractile unit located between the Z
lines.
Many sarcomeres line up end to end to form a myofibril
Look at page 399 box 11-1 - use overhead for picture
The microscopic bands give the muscle the “striated” name
Myofilaments
Myofilaments are made of 4 types of proteins
•myosin make up the
thick filaments
actin, tropomyosin and
troponin make up the
thin filaments
The myosin heads of the thick filaments are chemically
attracted to the actin molecules on the thin filaments.
They are called “cross bridges” because they bridge the
gap between the think and thin filaments
Muscles are at
rest unless they
are stimulated by
a motor neuron
•The motor neuron endplate forms a junction called a
neuromuscular junction.
•The narrow gap between the nerve and the fiber is called a
synapse or synaptic cleft
•The nerve releases a neurotransmitter acetylcholine into the
gap to begin the electrical impulses in the sarcolemma
-excitation-
1. The impulse is conducted over the
muscles fiber’s sarcolemma
2. When it reaches the t-tubule it
spread deeper into the fiber
where it release Ca++ into the SR
3. The calcium ions combine with
troponin molecules in the thin
filament causing it to shift to
expose the actin molecule
4. The myosin heads are chemically
attracted to the actin so they “grab” it
and pull, then grab the next actin and
pull past it and so on until an 80%
contraction is reached
This is called the sliding-filament
model
Muscle fibers contract to about 80% or their
starting length
If a muscle is pulling against an immovable object
it is still in a state of active contraction and uses
energy to maintain the contraction.
Types of Muscle Fibers
•Slow twitch
Contains “red fibers”
Red fibers have a high concentration of myoglobin, a
redish pigment used by muscle cells to store oxygen
in the muscle cell itself – gives the muscle its red
color
Contains type I myosin in the thick filaments
that reacts or contracts at a slow rate and is able to
produce ATP at a rate that keeps pace with the
muscle’s energy needs, this type of muscle is found
in anti-gravity muscles--because they can contract
for a long time and not fatigue—and in the muscles
used for endurance activies
•Fast Twitch
White fibers
They contain very little myoglobin
Contain type II myosin that contracts more rapidly than slow fibers
and has t-tubules that are more efficient at delivering Ca++ to the
SR
Rapid contraction depletes ATP rapidly even though they tend to
contain a higher concentration of glycogen
they have fewer mitochondria and rely on anaerobic respiration to
regenerate more ATP
Fast twitch muscles have great force but cannot sustain activity for
very long because of lactic acid production from anaerobic activity
Fingers, eyes, muscles used by sprinters
•Intermediate fibers
Have characteristics in between both fast and slow twitch
muscle fibers
They are more fatigue resistant than fast fibers yet can
generate more force more quickly than slow fibers
Example would be:
Muscles that are anti-gravity yet are needed for other
activities such as jumping
muscles of the leg would be a good example
Page 406 in the text
Motor Unit
Consists of the nerve that stimulates the muscles
fibers and the muscle fibers it is stimulating
Some motor units are
only a few muscle fibers
Some motor units have
thousands of muscle
fibers
Fine motor movements have smaller motor units that only stimulate a few
fibers at time , gross motor movements have large motor units that stimulate
many fibers at once
Myography
Graphing muscle contraction
Terms you need to know
Threshold stimulus
The amount of stimulus applied to make a muscle contract
Twitch contraction
A quick jerk of the muscle
Use the information that you learned about the physiology to explain what is
happening on the myograph
Twitch contractions rarely happen the body
•We can’t make our muscles twitch individually
•Our nervous system “smooths out” our muscle
movements
•Smooth movements are more useful – sustained
movements
•Prevents injury to muscle groups
The staircase effect is why we warm up before exercise.
Calcium diffused through the sarcoplasm of muscles
has not had time to relax and pump all of it back into
their SR so the muscle can contract with more force.
The relaxation phase become shorter and finally
disappears – the muscle stays in partial contraction
called a contracture
After a while the muscle becomes fatigued and won’t
respond to even the strongest stimuli
Ever had your muscles shake
after over working them?
That is what you are feeeling!
It is rarely the entire muscle but certain
motor units that are fatigued causing
the shaking
Tetanus
This graphic represents what a
sustained contraction looks like.
In a normal body tetanus results
from two factors working
together at the same time
•Rapid –fire of nerve fibers that
permit this wave summation to
happen in each fiber
•Coordinated contractions of
different motor units in a
muscle organ in an overlapping
sequence or “relay team” effect
This is called muscle Tone
Tonic contraction or “tone”
Muscles organs with less than normal
“muscle tone” are called flacid
Muscles organs with more than normal
muscle tone are called spastic
Stretch sensors in muscles and tendons detect the
degree of stretch and adjust the strength of the tonic
contraction. This mechanism is a negative feedback
mechanism located in the spinal cord.
Graded Strength Principle
Muscle organs can produce varying degrees of
strength
•Influenced by the individual “type” of fibers
Fast twitch, slow twitch or intermediate
•Number of motor units contracting
more units are recruited when more strength is
needed
The maximum strength of a muscle is directly
related to the initial length of it fibers
•If a muscle contractions begins from a shortened
length its sarcomers are already compressed
•If the muscle begins a contraction from an
overstretched length it cannot develop much tension
because the thick and thin myofilaments are too far
apart to effectively pull and shorten the sarcomeres.
Try flexing you biceps: extended, 90 degrees and completely
flexed Page 411 bottom of the 2nd column
Now feel the stretch reflex, page 412
Effects of Exercise on Skeletal Muscles
Page 412
Concentric Contraction
Movement result in
shortening of muscle
Eccentric Contraction
Movement results in
lengthening of muscle
Isometric contraction
Muscle stays the same
length as tension
increases
“Tightening to resist a force”