Download How do muscles contract?

Chapter 47
Lecture 16
How do muscles contract?
Dr. Chris Faulkes
How Do Muscles Contract?
Aims:
•  To examine the structure and function of the
three types of muscle tissue
•  To take a closer look at muscle contraction and
describe the roles of actin and myosin
How Do Muscles Contract?
Aims:
•  To examine the structure and function of the three types of muscle tissue
•  To take a closer look at muscle contraction and describe the roles of actin
and myosin
These lecture aims form part of the knowledge
required for learning outcome 3:
Describe mechanisms for life processes (LOC3)
How Do Muscles Contract?
• 47.1 How Do Muscles Contract?
Essential reading
•  Pages 1004-1013
47.1 How Do Muscles Contract?
Muscles and skeletons: the
musculoskeletal system.
Muscles and skeletons are the effectors
that produce movement.
47.1 How Do Muscles Contract?
Three types of vertebrate muscle:
• Skeletal: voluntary movement,
breathing
• Cardiac: beating of heart
• Smooth: involuntary, movement of
internal organs
47.1 How Do Muscles Contract?
Skeletal muscle (striated):
• Cells are called muscle fibres:
multinucleate
• Form from fusion of embryonic
myoblasts
• One muscle consists of many muscle
fibres bundled together by connective
tissue
Figure 47.1 The Structure of Skeletal Muscle
47.1 How Do Muscles Contract?
Contractile proteins:
Actin: thin filaments
Myosin: thick filaments
Each muscle fibre has several
myofibrils: bundles of actin and
myosin filament.
47.1 How Do Muscles Contract?
Each myofibril consists of repeating
units: sarcomeres.
Sarcomere: overlapping actin and
myosin filaments.
Bundles of myosin filaments are held in
place by the protein titin, the largest
protein in the body.
Figure 47.1 The Structure of Skeletal Muscle (Part 2)
Figure 47.1 The Structure of Skeletal Muscle (Part 3)
47.1 How Do Muscles Contract?
The sliding filament theory of muscle
contraction:
• Depends on structure of actin and
myosin molecules.
• Myosin heads can bind specific sites
on actin molecules to form cross
bridges. Myosin changes conformation,
causes actin filament to slide 5–10 nm.
47.1 How Do Muscles Contract?
Contraction of skeletal muscle
Figure 47.2 Sliding Filaments
47.1 How Do Muscles Contract?
Muscle contraction is initiated by action
potentials from a motor neuron at the
neuromuscular junction.
A motor unit: all the muscle fibres
activated by one motor neuron.
Figure 47.3 Actin and Myosin Filaments Overlap to Form Myofibrils
47.1 How Do Muscles Contract?
One muscle may have many motor
units.
To increase strength of muscle
contraction: increase rate of firing of
motor neuron or recruit more motor
neurons to fire (more motor units
activated).
47.1 How Do Muscles Contract?
Muscle cells are excitable: the plasma membrane
can conduct action potentials.
Acetylcholine is released by the motor neuron at the
neuromuscular junction and opens ion channels in
the motor end plate.
47.1 How Do Muscles Contract?
Action potentials also travel deep within
muscle fibre via T tubules.
T tubules (transverse tubules) descend
into the sarcoplasm (muscle fibre
cytoplasm).
T tubules run close to the sarcoplasmic
reticulum (ER): a closed compartment
that surrounds every myofibril.
Figure 47.5 T Tubules in Action
47.1 How Do Muscles Contract?
Sarcoplasmic reticulum has Ca2+ pumps.
At rest there is high concentration of
Ca2+ in the sarcoplasmic reticulum.
Action potential reaches receptor proteins
and opens the Ca2+ channels, Ca2+ flows
out of sarcoplasmic reticulum and
triggers interaction of actin and myosin.
47.1 How Do Muscles Contract?
Actin filaments also include tropomyosin
and troponin.
Troponin has three subunits: one binds
actin, one binds myosin, and one binds
Ca2+.
At rest, tropomyosin blocks the binding
sites on actin.
47.1 How Do Muscles Contract?
When Ca2+ is released, it binds to
troponin, which changes conformation.
Troponin is bound to tropomyosin—
twisting of tropomyosin exposes
binding sites on actin.
When Ca2+ pumps remove Ca2+ from
sarcoplasm, contraction stops.
Figure 47.6 The Release of Ca2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction
47.1 How Do Muscles Contract?
Cardiac muscle is also striated; cells
are smaller than skeletal and have one
nucleus.
Cardiac muscle cells also branch and
interdigitate: can withstand high
pressures.
Intercalated discs provide mechanical
adhesions between cells.
Figure 47.7 There are Three Kinds of Muscle
47.1 How Do Muscles Contract?
Pacemaker and conducting cells initiate
and coordinate heart contractions.
Heartbeat is myogenic—generated by
the heart muscle itself.
Autonomic nervous system modifies the
rate of pacemaker cells, but is not
necessary for their function.
47.1 How Do Muscles Contract?
Contraction of cardiac muscle:
• DHP proteins in T tubules are Ca
channels; ryanodine receptors are iongated Ca2+ channels, sensitive to Ca2+.
• Action potential causes Ca2+ to flow into
sarcoplasm from T tubules; increase in
Ca2+ opens the Ca2+ channels in
sarcoplasmic reticulum—large increase in
Ca2+ in sarcoplasm—initiates contraction.
Ca2+-induced Ca2+ release
47.1 How Do Muscles Contract?
Contraction of cardiac muscle:
Cardiac muscle cell beating
47.1 How Do Muscles Contract?
Smooth muscle: in most internal organs;
under autonomic nervous system
control.
Smooth muscle cells are arranged in
sheets; have electrical contact via gap
junctions. Action potential in one cell
can spread to all others in the sheet.
47.1 How Do Muscles Contract?
Plasma membrane of smooth muscle
cells sensitive to stretch.
Stretched cells depolarize and fire action
potentials which starts contraction.
47.1 How Do Muscles Contract?
Smooth muscle contraction:
• Ca2+ influx to sarcoplasm stimulated by
stretching, action potentials, or hormones.
• Ca2+ binds with calmodulin: activates
myosin kinase which phosphorylates
myosin heads—can then bind and release
actin.
Figure 47.8 Mechanisms of Smooth Muscle Activation
Figure 47.8 Mechanisms of Smooth Muscle Activation (Part 1)
Figure 47.8 Mechanisms of Smooth Muscle Activation (Part 2)
47.1 How Do Muscles Contract?
Skeletal muscle: minimum unit of
contraction = a twitch.
Twitch measured in terms of tension, or
force it generates.
A single action potential generates a
single twitch. Force generated depends
on how many fibres are in the motor
unit.
47.1 How Do Muscles Contract?
Tension generated by entire muscle
depends on:
• Number of motor units activated
• Frequency at which motor units are
firing
47.1 How Do Muscles Contract?
Single twitch: if action potentials are
close together in time, the twitches are
summed, tension increases.
Twitches sum because Ca2+ pumps can
not clear Ca2+ from sarcoplasm before
the next action potential arrives.
Tetanus: when action potentials are so
frequent there is always Ca2+ in the
sarcoplasm.
Figure 47.9 Twitches and Tetanus
47.1 How Do Muscles Contract?
How long muscle fibre can sustain tetanic
contraction depends on ATP supply.
ATP is needed to break the myosin-actin
bonds, and “re-cock” the myosin heads.
To maintain contraction, actin–myosin
bonds have to keep cycling.
47.1 How Do Muscles Contract?
Muscle tone: a small but changing
number of motor units are contracting.
Muscle tone is constantly being adjusted
by the nervous system.
How Do Muscles Contract?
Check out
•  47.1 Recap, page 1031, questions 2 and 3
•  47.1 CHAPTER SUMMARY, page 1022, See WEB/CD Activity 47.1
Self Quiz
page 1022-1023: Chapter 47, questions 1 and 2
For Discussion
•  page 1023: Chapter 47, question 1
How Do Muscles Contract?
Key terms:
actin, autonomic nervous system, calmodulin, cardiac
muscle, fibres, intercalated discs, musculoskeletal,
myogenic, myoblasts, myosin, myosin kinase, myosin
phosphotase, pacemaker, rigor mortis, sacromere,
sacroplasm, sarcoplasmic reticulum, skeletal (striated)
muscle, smooth muscle, T tubule, titin, tropomyosin,
troponin
Figure 47.15 The Human Endoskeleton
Figure 47.19 Types of Joints
Figure 47.20 Joints, Ligaments, and Tendons