Transcript Muscle contraction

Yesterday we finished off talking about the different kinds of muscle
contractions. Today we are going to look at how a muscle produces these
contractions.
Think of jumping, maybe going to block a shot in
basketball or spiking a volleyball. I want you to write
down everything that goes into you being able to jump.
So how do muscles know how and when to contract?
Neuromuscular System
“nerve”
“muscle”
• complex linkage between the nervous
system and the muscular system
NEURON
• transmits demands of the
brain, via ELECTRICAL
IMPULSES
• the movement of an
impulse from the cell
body to terminal
branches is called an
ACTION POTENTIAL
-only one direction
• 2 types of neurons
1) Sensory –detects a
stimulus; message to brain
2) Motor –produces
movement; message from brain
When the terminal branches of a motor neuron attach to a muscle, we get a. . .
• A single action potential to the muscle will cause a twitch, but nerves
transmit impulses continuously to ensure smooth movement
• 2 types:
Doesn’t
denote
size
•Small: stimulates few fibres  fine motor movements (eye)
•Large: stimulates many fibres  gross motor movements (quads)
• In order for a muscle force to be produced, ALL MOTOR UNITS in the group
MUST BE RECRUITED
All the motor units need to have a sufficient impulse reaching them
ALL-or-NONE PRINCIPLE
“ when a motor unit is stimulated to contract, it will do
so to its fullest potential, or not at all”
WHERE ARE WE?
We have an electrical impulse (action potential) sent from the brain
(motor neuron) and it has reached the muscle
But how does the muscle receive the impulse? How is the
connection made from the nervous to the muscular system?
Neuromuscular Junction
-video
• simply put, the neuromuscular junction is a space between the terminal
branches of the neuron (axon) and the sarcolemma of the muscle fibre
• the junction is called a “synapse”
• this is the point where electrical energy transforms into
chemical energy
The crossing of the NMJ boils down to 3 main steps:
1.
Action potential (AP) moves along the axon till it reaches the presynaptic cleft;
causes an influx of Ca 2+ ions into the cleft
2.
The Ca 2+ ions cause a release of a neurotransmitter acetylcholine (Ach) that
diffuses across the synapse
3.
ACh is detected by receptors on the postsynaptic cleft; causes influx of Na + ions
into the sarcolemma causing a new AP to propagate into the muscle fibre
WHERE ARE WE?
We have an electrical impulse (action potential) sent from the brain
(motor neuron) and it has reached the muscle.
The action potential crossed a synapse at the neuromuscular
junction via a neurotransmitter (Acetylcholine) and entered the
muscle
-we have seen a change from electrical energy (AP) to chemical
energy (Ca and Ach) and back to electrical energy (new AP)
-Video
But how does this secondary AP cause a muscle to contract and how does
the muscle itself contract?
Excitation-Contraction Coupling
This process is one of changing chemical energy into mechanical energy
The excitement of a muscle leading to its contraction
1.
The AP is rapidly conducted along the muscle fibre till it reaches the
transverse tubules (This is occurring all over the muscle fibres)
-video
2.
The AP causes a change in the tubules, which in turn forces the terminal
cisternae to release Ca 2+ ions into the sarcoplasm
So calcium is not just important for keeping our bones strong
Calcium ions are also the reason that our muscles contract.
But how do calcium ions cause a contraction?
Sliding Filament Theory
We have now reached the smallest parts that make up a muscle: the filaments.
Before we look at how the filaments work, we need to look at what they
look like
MYOSIN
• protein
• made up of a “head” and “tail” portion
• contains an attachment site for actin
• simpler of the two filaments
Structure of a myosin filament
ACTIN
• protein
• also contains 2 other proteins
1) Troponin –binding site for calcium!
2) Tropomyosin –cord like structure covering actin binding sites
Working together, these two accessory proteins will NOT allow the myosin to
interact with actin unless calcium is present
So, how does the interaction of actin and myosin result in a
contraction?
** Muscles ALWAYS PULL, never push **
** This PULL is a result of the OVERLAPPING of actin and myosin **
Part 1
1.
Released Ca2+ binds to the troponin.
2.
Excited troponin causes a shift in the tropomyosin, which exposes actin
(binding site for myosin)
Part 2
3.
Myosin will attach to the exposed actin, (forming a myosin crossbridge)
flex rapidly, causing the actin to slide over the myosin
4.
The presence of ATP (Adenosine triphosphate) or energy causes the bridge
to “break”
5.
Since calcium is still present, the actin will remain exposed and therefore the
myosin will continue to attach, pull, detach, etc
The result. . .
CONTRACTION!
The Sarcomere
Z-line: edge of a sarcomere
M-line: midpoint of myosin
I-band: distance between successive myosin
H-zone: distance between successive actins
A-band: length of a myosin filament
What is happening to all those areas of the sarcomere during contraction and relaxation?
H-zone decreases
I-band decreases
Video
WHERE ARE WE?
That’s your job to finish
In groups of 2 or 3 you are going to write out the entire
process of muscle contraction from impulse to myosin
crossbridges
You’re not done yet!
Now you are going to look over your process and
eliminate everything unnecessary. Your job is to
make the process as straightforward and simple as
possible without losing the meaning.