Skeletal Muscle Flashcards
Muscle types
Skeletal Muscle-Skeletal muscles are attached to bones by tendons, and they produce all the movements of body parts in relation to each other. Skeletal muscles are under voluntary control
Cardiac Muscle-only exists in the heart keeping blood pumping around the body. involuntary striated muscle.
Smooth Muscle-contracts without any voluntary control. It is made up of spindle-shaped, unstriated cells with only one nucleus. contracts slowly
Skeletal muscle structure
Muscles are composed of many cell types
- Muscle fibres (cells)
- Vascular cells
- Fibroblasts
- Satellite cells
skeletal muscle energy metabolism
The nonoverlapped
areas represent
specificity of
metabolic function
among the body’s
three energy
systems; the three
overlapped portions
represent
generality.
Skeletal muscle energy metabolism
1.CreatinePhosphate (CP)
* Provides energy very fast to
form ATP from ADP but lasts
only 1-2 seconds
2.Glycolysis
* Energy from glucose in the
absence of oxygen (anaerobic
metabolism)
3.Oxidative phosphorylation
* Energy from glucose or fat in the
presence of oxygen (aerobic
metabolism)
Replenishing muscle stores of glycogen and CP, and removing lactic acid requires energy.
To achieve this the muscle uses more oxygen to produce the energy needed after the exercise has finished. This is
referred to as the OXYGEN DEBT
Skeletal muscle energy metabolism and fibre
type
Two main types of fibres that differ in three characteristics:
* Primary mechanisms used to produce ATP
* Type of motor neuron innervation
* Type of myosin heavy chain expressed
Type:
* IIx: Fast-twitch, fast-glycolytic fibres
* IIa: Intermediate fibres, fast-oxidative glycolytic fibres
* I: Slow-twitch, slow-oxidative fibres
Neuromuscular junction
- Motor neuron’s action potential arrives at
the axon terminal
↓
Depolarizes plasma membrane - Opening Ca2+ channels
↓
Ca2+ions diffuse into axon terminal
↓
Ca2+binds to proteins - Synaptic vesicles release Ach
- Ach diffuses from axon terminal to motor
end plate, binding to nicotinic receptors
5.Binding of Ach opens an ion channel
↓
Na+ and K+ can pass through these
channels (electrochemical gradient across
plasma membrane means more Na+ moves
in than K+ out) - local depolarization of the motor
end plate - Muscle fibre action potential
initiated - Propagation (end plate potential)
Events at neuromuscular junction: excitation-
contraction coupling
Note: Every action potential in a motor neuron normally
produces an action potential in each muscle fibre in its
motor unit.
This is different from synaptic junctions between neurons, where multiple excitatory
postsynaptic potentials must occur for threshold to be reached and an action potential
elicited in the postsynaptic membrane
Excitation-contraction Coupling
The sequence of events by which an action potential in the plasma membrane
activate the force-generating mechanisms
- An action potential in a skeletal
muscle fibre lasts 1 to 2 ms and
is over before signs of
mechanical activity begin - Mechanical activity following an
action potential may last 100 ms
or more (depending on
availability of intracellular Ca2+)
Excitation-contraction Coupling
- Relaxed Muscle
Low Ca2+
Cross-bridge cannot bind with Actin
because Tropomyosin is covering the
binding site (Troponin holds tropomyosin
over binding site) - Active Muscle
High Ca2+
Ca2+ binds to troponin → tropomyosin
moves away from cross-bridge binding
site → Actin binds to cross-bridge
Calcium and Skeletal muscle contraction
Two proteins are responsible
for linking the membrane
action potential with calcium
release in the cell
– Dihydropyridine(DHP) receptor
(Membrane)
– Ryanodine receptor
(sarcoplasmic reticulum)
* Removal of Ca2+ from the
cytosol requires energy
Sliding filament mechanism
Shortening of the muscle is the result of certain parts of the actin
and myosin filament interacting with each other.
Sliding filament mechanism
Note: Typically, muscle
shortening involves one end
of the muscle remaining at a
fixed position while the other
end shortens toward it.
Cross-bridge cycle: 4 stages
- Energized myosin cross bridges on the thick
filaments bind to actin - Cross bridge binding triggers release of ATP
hydrolysis products from myosin, producing
angular movement - ATP bound to myosin, breaking link between
actin and myosin → cross bridge dissociate - ATP bound to myosin, is split, energizing the
myosin cross bridge
ATPase: an enzyme which
determines the speed of ATP
hydrolysis and resulting
sarcomere shortening velocity
Tension vs load
- Tension: The force that a muscle exerts
on the joint when it is contracting is
called the tension of the muscle. - Load: The force that is exerted on a
muscle by an object is called the load of
the muscle.
Muscle tension must exceed the load in order for the muscle fibres to shorten, and therefore move the object that is responsible for the load.
If muscle tension does not exceed the load then the muscle will either remain at the same length, or it will lengthen.
Types of muscular contractions
- SHORTENING CONTRACTION (concentric contraction)
➢Constant load, muscle shortens
➢Tension > load - ISOMETRIC CONTRACTION
➢Constant muscle length - Free object: load = tension
- Fixed object: load => tension
- LENGTHENING CONTRACTION (eccentric contraction)
➢Muscle length increases
➢load > tension
single fibre contractions
- The mechanical response of a
muscle fibre to a single action
potential is known as a twitch. - After the action potential, there is a
latent period(few milliseconds)
before the tension in the muscle
fibre begins to increase. - The time interval from the beginning
of tension development (at the end
of the latent period) to the peak
tension is the contraction time.
shortening
In a shortening contraction, an increasing load causes:
* The latent period to increase
* The velocity of shortening to slow down
* The total duration of the twitch to become shorter
* The distance shortened to become less
