Download muscle contraction

Motor Unit
A motor unit is described as a single motor
neurone and all of the muscle fibres it
innervates. A motor unit can contain anywhere
between 10 and thousands of muscle fibres.
Muscles which produce large powerful
movements contain motor units with large
numbers of fibres, and those for small intricate
movements contain only a few fibres per
motor unit.
• Where the synaptic knobs of the neurone meet
the muscle fibres is known as the neuromuscular
junction. When an impulse reaches the
neuromuscular junction, a neurotransmitter
called Acetylcholine is released which filters
across the synaptic cleft (microscopic space
between the synaptic knob and motor end
plate). This causes depolarisation of the motor
end plate and puts the sliding filament theory of
muscular contraction into practice.
• The 'all or none' law as mentioned above
also applies to the contraction of fibres
within a motor unit. When a motor unit
activates, all of the fibres within the unit
contract and at full force, there is no strong
or weak contraction. The strength of the
resultant whole muscular contraction
depends upon the number of motor units
recruited
• Another way of increasing the stength of a
muscle contraction is by decreasing the time
between impulses so that the muscle fibres do
not have time to relax, resulting in a continuous
wave of contractions known as wave
summation. To produce a strong contraction all
motor units in the muscle are recruited, but only
for a short time. In order to increase the length of
a contraction a kind of rotation system is
implemented whereby some units contract while
others rest and continuously alternate. This is
known as spatial summation or tetanus
A single contraction of one motor unit
SPATIAL SUMMATION
WAVE SUMMATION
• Skeletal Muscle Structure
Although skeletal muscles come in different shapes and sizes
the main structure of a skeletal muscle remains the same. If
you were to take one whole muscle and cut through it, you
would find the muscle is covered in a layer of connective tissue
known as the Epimysium. The Epimysium protects the muscle
from friction against other muscles and bones.
A large strong muscle, such as thoses forming
your Quadriceps would have a large number of
fibres within each bundle. A smaller muscle used
for precision movement, such as those in the
hand would contain far fewer fibres per
Fasciculi.
Looking at each muscle fibre in
detail, you can see they too are
covered in a fibrous connective
tissue, known as Endomysium which
insulates each muscle fibre. Muscle
fibres can range from 10 to 80
micrometers in diameter and may be
up to 35cm long.
Beneath the Endomysium and surrounding the muscle fibre is the Sarcolemma
which is the fibres cell membrane and beneath this is the Sarcoplasm, which is
the cells cytoplasm, a gelatinous fluid which fills most cells.
This contains Glycogen and Fats for energy and also Mitochondria which are
the cells powerhouses, inside which the cells energy is produced
• Each muscle fibre itself contains cylindrical organelles
known as Myofibrils. Each muscle fibre contains
hundreds to thousands of Myofibrils. These are bundles
of Actin and Myosin proteins which run the length of the
muscle fibre and are important in muscle contraction.
• Surrounding the Myofibril there is a network of tubules
and channels called the Sarcoplasmic Reticulum in
which Calcium is stored which is important in muscle
contraction. Transverse tubules pass inwards from the
Sacrolemma throughout the Myofibril, through which
nerve impulses travel.
• Each Myofibril can then be broken down into functional
repeating segments called Sarcomeres.
SARCOMERE
Sliding filament model of muscle contraction
The sarcomeres are what give skeletal and cardiac muscles their striated
appearance.
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A sarcomere is defined as the segment between two neighbouring Z-lines
(or Z-discs, or Z bodies). In electron micrographs of cross striated muscle
the Z-line (from the German "Zwischenscheibe", the band in between the I
bands) appears as a series of dark lines.
Surrounding the Z-line is the region of the I-band (for isotropic).
Following the I-band is the A-band (for anisotropic). Named for their
properties under a polarizing microscope.
Within the A-band is a paler region called the H-band (from the German
"Heller", bright). Named for their properties under a polarization microscope.
Finally, inside the H-zone is a thin M-line (from the German "Mittel", middle
of the sarcomere). what give skeletal and cardiac muscles their striated appearance.
SARCOMERE - basic repeat unit of striated muscle, delimited by Z-lines
I band - "paler zone" around Z-line
(Isotropic - passes light in all directions)
A band - "dark region" in center of sarcomere (Anisotropic - in different directions)
M line - mid point of the sarcomere
H zone - "paler zone" in the center of sarcomere around M line
•
some definitions related to muscle contraction...
Summation - a 2nd contraction before 1st
subsides fig* (cause is time differential of
nerve/muscle)
Tetany - sustained contractions (requires energy ATP)
Fatigue - under repeat stimulation, contractions get
feebler, lactate accumulates,
pH changes lead to stoppage
of contractions
Shivers - involuntary-summed muscle contractions
which release waste heat, that warms body
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Muscle Cell Proteins [4 types involved in contraction cycle]
1. THICK FILAMENT (A band)
myosin* - 6 polypeptides twisted to form 2 helical fibers with
globular ends,
which have ATPase activity & an affinity to bind to
actin
myosins are... Molecular
Motors & kinesin animations 2. THIN FILAMENT (I band)
G-actin* - globular protein which polymerizes into polymeric
fiber...
each globular actin unit contains a myosin binding
site 3.
Tropomyosin* - fiber-like protein which wraps helically
around thin filament 4.
Troponin - globular protein complex which
binds Ca+2 & initiates contraction cycle
is complex of 3 proteins, Troponins C, I, & T, which bind
Ca; Troponin C (18 kD) binds Ca reversibly.
TnC binds TnI (23 kD) & TnT (37 kD), which change
conformation in response to TC binding Ca.
Muscles can not push, they may only only CONTRACT (shorter via a pull)
A muscle contraction is called a muscle TWITCH
4 parts of a Muscle twitch
[ CONTRACTION CYCLE *]
1) latent period - 5 msec
time between application of AP & initiation of contraction
2) contraction - 40 msec
muscle shortens & does its work
3) relaxation - 50 msec
muscle elongates & returns to original position
4) refractory period
- 2 msec
time of recovery between stimulations of muscle
Myosin is a hexamer with a MW of 520 kD with
2 domains - a head region and a tail region.
The globular head domain binds to actin and use
ATP hydrolysis to change its conformation.
Different isoforms (structural variations) of
myosins are found in different muscle types.
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