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Module 632
Lecture 7 JCS
Muscle types, structure,
activation and energy use
MODULE - 632
Lecture 7
Lecture outcomes:
At the end of this lecture a student will be aware of:
1)
the different types of muscle
2)
that basic function of muscle is to produce force and movement
•
3)
that a variety of muscles exist where the outputs – force and
movement occur to different degrees
•
4)
that most muscles work by being attached to a skeleton but that,
•
5)
some work between or within non-skeletal tissues– e.g. heart,
vascular
•
6)
most muscles work in pairs,
•
7)
that muscles are attached to skeletons through tendons or tendonlike structures
•
8)
that muscles move skeletons
•
9)
that pairs of muscles move joints,
•
10) the ultrastructure of striated muscle
•
11) the energy sources for muscle contraction and
•
12) How muscle contraction of is activated
Striated muscle
Three main types of vertebrate muscle
• Smooth (smooth muscle myosin II – 1 isoform)
– Smooth appearance (no cross-striations)
– Involuntary
– Blood vessels, gut, sphincters
• Skeletal (striated muscle myosin II – 8 isoforms,
including a cardiac isoform in ‘slow’ muscle)
– Striated appearance
– Voluntary control
– Biceps, triceps, quadriceps etc.
• Cardiac (cardiac myosin II – 2 isoforms a & b)
– Sarcomeric structure (striated) - not as ordered as skeletal.
– Rhythmic contractions
– Highly specialised function
Striated skeletal muscle is very diverse:
Within an organism (e.g. human muscles):
• Structural architectures (pennate, styloid, long/short
sarc.)
• Fibre types – many muscles contain a mix of the two
Type 1 – slow – postural/slow to fatigue
Type II –fast
• Myosin isoforms (fast, intermediate and slow ATPase
activities)
Striated Skeletal Muscle Architecture:
triangular
Unipennate
penna - Lat. wing or
fusiform
feather
fusus - Lat. spindle
feather pen!
bipennate
styloid
stulos - Gk. pillar
Muscles usually work in antagonistic pairs
Frog leg muscles
Vert. upper limb muscles human
Flexion, extension, adduction, abduction
Muscles are often named by the effects of their action and the
bones they attach to. In general:
Flexors: bends a joint; move limbs away from ‘corpse’ position
Extensors: straightens a joint; returns them to corpse position
Adductors: ‘add’ the limb towards the rest of the body (pulls
the body to wards the midline)
Abductors: moves them away from the midline
2006
Note: Not same as handout
tibialis cranialis
gastrocnemius
muscle
tarsal joint
Muscles work in opposing pairs
often called the flexor and
extensor muscles
Muscle has a hierarchical structure:
Each muscle is a contractile organ: it contains:
muscle fibres
blood vessels
peripheral ends of nerves/muscle endplates
fibrous connective tissue/tendons
and is covered with a connective layer.
Each muscle fibre is a multinucleated single cell (a syncitium)
It contains approx. 1000 myofibrils; specialised contractile
organelles, which run the length of the fibre.
Each myofibril consists of serial contractile units known as
sarcomeres.
Muscle has a hierarchical structure
UK
‘fibre’
Muscle – Fibre – myofibril –
myofilaments
Acto-myosin in muscle :
0.5mm
Myosin containing, thick filament
1 mm
0.5 mm
Actin containing, thin filaments
0.1 mm
Sarcomere
acto-myosin “cross-bridges”
Filament sliding causes muscle to shorten
Light micrograph
myofibril
Electron micrograph
sarcomere
Myosin molecules (purple bars) move over the F-actin (turquoise).
This movement is powered by ATP.
Highly specialised striated muscles (1):
Many specialised muscles exist in different animals e.g.
• Asynchronous insect flight muscle
- drives insect wingbeats at >200Hz (Drosophila 220Hz).
• Tympal muscles that allow crickets to sing
• Molluscan “Catch” muscle
- keeps shell closed for long periods
• Molluscan adductor muscle
- fast closing of shell for swimming.
Highly specialised striated muscles (2):
Asynchronous insect flight muscle
- isometric; requires Ca++ + applied strain to activate
- contracts in an oscillatory fashion at frequencies
>200Hz myosin.
Highly specialised striated muscles (3):
Molluscan catch + adductor muscles:
Pecten maximus
Catch muscle – keeps
shell closed with
minumum energy
requirement (slow)
Adductor muscle –
for swimming.
Catch muscle
Adductor muscle
Highly specialised striated muscles (4):
Forces produced by the muscle are
easily measured
How clams etc. can close their shells –
a single muscle working against a stiff elastic hinge
Crossbridge Cycle
All muscle contraction is powered by the cyclical interactions of myosin and
actin – the so-called crossbridge cycle.
Myosin is an ATPase. By coupling its ATPase to a conformational change,
dependent on binding actin we can get:
Mg.ATP + H2O  Mg.ADP + Pi + H+ + mechanical work
The crossbridge cycle (more in the next lecture) consists of:
- a biochemical cycle (changes in nucleotide state - ATP, ADP etc. - and
in protein binding actin + myosin) and,
- a biomechanical cycle (conformation of the motor molecule – myosin)
that are completely functionally inter-dependent.
.
Energy Sources for Muscle Contraction (1) :
For the crossbridge cycle outputs:
Mg.ATP + H2O  Mg.ADP + Pi + H+ + mechanical work
the primary energy source is clearly ATP (produced by the
mitochondria)
For very short bursts of activity you will use up your ATP pool. The
ATP needs to be replaced.
How?
- immediate stores of energy – creatine phosphate
- new ATP production:
from glycolysis (glucose, glycogen)
– but product is lactic acid
from oxidative phosphorylation (ATP production
by the mitochondria)
Energy Sources for Muscle Contraction (2) :
The energy sources used are reflected by the physiological properties of skeletal
muscles:
Vetebrate skeletal muscle contains two major types of fibres that differ in:
- speed of contraction – ‘Fast’ (type 2) or ‘slow’ (type 1), and
- their major energy supply
- their neural activation – ‘twitch’ and ‘phasic’ (see later).
Fast fibres (for sprinting) are ‘glycolytic’
Slow fibres (for slower movements, maintained peformance – e.g. over long
distance or time outputs and posture etc) are ‘oxidative’
Most skeletal muscles are a combination of ‘fast’ and ‘slow’ fibres.
There is also natural variation in the proportion of these fibre types between
individuals in particular muscles – sprinters vs marathon runners.
Fibre-typing is now a routine part of assessing ‘olympians’
‘Red’ and ‘white’ meat reflect these differences.
Red – mostly oxidative; white is mostly glycolytic.
Energy Sources for Muscle Contraction (3) :
(Fast fibres predominantly)
For fairly short bursts of activity you will use up an energy reserve
(effectively an ATP storage pool) of creatine phosphate,
Cr.P + Mg.ADP  Mg.ATP + Cr
Cr = creatine
This reaction is readily reversible; the energy from CrP is
released very quickly (a few seconds) allowing sprints.
In insect muscles:
Arginine.P replaces Creatine.P
Energy Sources for Muscle Contraction (4) :
ATP ADP
Pi PCr Cr Mg2+ Ca2+
Total (mmol/kg)
5
0.8
3
25
13
10
Free (mM)
4
0.02
2
25
13
3
1
0.0001
Note: More Cr.P than ATP
Once Cr.P is used up the high requirement for more ATP is met by
glycolysis (end-product lactic acid)
So these muscle fibres can work anaerobically for brief periods, but
accumulate lactic acid.
Typically this metabolism predominates in muscles used for sprints fast glycolytic fibres predominate.
After exercise ceases the ATP and PCr must be regenerated and lactic
acid metabolised.
Energy Sources for Muscle Contraction (5) :
Type 1 fibres: For long periods of sustained work – type
1/oxidative/ tonic or slow twitch fibres i.e.
These muscle fibres require a supply of oxygen to enable oxidative
phosphorylation to go on in the mitochondria to produce a
continuous supply of ATP.
Energy sources here are primarily glucose (glycolysis and the TCA
cycle) and over longer periods the glycogen stores.
Requires the mitochondria.
Mitochondria occupy about 30% of volume of the heart and these
muscle fibres.
It is the high(er) concentration of cytochromes/myoglobin etc.
which give these fibres their red colour
Muscle activation by nerves (1)
Neuromuscular junction – muscle endplate
Transmitter is usually acetylcholine
Muscle activation by nerves (2)
Events Leading to Muscle Contraction
e.g. acetylcholine
Muscle activation by nerves (3)
3-D section of a skeletal muscle cell
Muscle activation by nerves (4)
• Control by nerves – action potentials in motor neurons
– Neuromuscular junction
• Muscle plasma membrane depolarises
• Propagates down ‘T’ tubules into centre of fibre
• ‘T’ tubule close to sarcoplasmic reticulum (SR)
• Di-hyropyridine receptor  ryanodine receptor
• Calcium release from SR - induces further calcium
release
• Ca++ binds to troponin complex – troponin C (part
of the sarcomeric thin filament)