Musculoskeletal 1b – Skeletal Muscle Function in Humans
Anil Chopra
1. Outline the sequence of steps in the “chain of command” leading to skeletal
muscle contraction and be able to explain its significance in terms of normal and
disease states.
2. Motor units:
 Define, sketch and label and explain how the motor unit functions.
 Describe motor unit size, distribution within a muscle, and fibre type.
 Describe changes in motor units that result from of denervation and reinnervation.
3. Fibre types: Describe the properties (physiological, structural, and metabolic) of
the 3 main types in human skeletal muscle. How are they distinguished
histologically?
4. Muscle plasticity, changes in performance
 Describe briefly the changes in strength and endurance that results from training
and from disuse. Be aware of changes that occur in the muscle and other factors
that contribute.
 Describe satellite cells and how they are involved in skeletal muscle repair.
 Describe changes in muscle that occur during aging.
5. Know the definitions of the terms in the glossary.
Motor Unit: the motor neuron and all the fibres it innervates (including the
neuromuscular junctions between them).
- cell bodies for motor neurons are almost always in the anterior ventral horn of
the spinal cord
Brain
- the terminals branch out as they reach the muscle.
Spinal cord
Peripheral nerve
Neuromuscular Junction
Muscle fibre membrane
Transverse tubular system
Muscle Fibre Types
There are 3 different types of muscle:
Ca release
Fast,
Fatigue
Resistant
Fast, Oxidative, Glycolytic
Actin - myosin activation
Cross-bridge formation
FORCE
Motor unit location
The number of muscle fibers within each unit can vary:
thigh muscles can have a thousand fibers in each unit, eye
muscles might have ten. In general, the number of muscle
fibers innervated by a motor unit is a function of a muscle's
need for refined motion. The smaller the motor unit, the
more precise the action of the muscle. Muscles requiring
more refined motion are innervated by motor units that
synapse with fewer muscle fibers. The distribution of
muscle fibres within a unit can vary.
Slow, Oxidative
Motor Unit Type
Slow
I
(S, SO)
IIA
(FR,FOG)
IIB
(FF, FG)
Property
Contractile profile
Metabolic profile
Histological
Identification
Protein
Twitch speed
Max shortening speed
Fatigue resistance
Red colour
Myoglobin content
Capillary supply
Mitochondria
Oxidative enzymes
Glycogen
Alkaline ATPase
Acid ATPase
Myosin isoform
slow
slow
high
dark
high
rich
many
high
low
low
high
I
fast
fast
high
fast
fast
low
dark
high
rich
many
med/high
high
pale
low
poor
few
low
low
high
low
high
moderate
IIA
IIB
Fast, Fatigable
Fast, Glycolytic
The duration of force response longer lasting in S fibres, and is
about the same in FR and FF fibres. When a repeated isometric
tetanus is applied to a muscle, S fibres are able to sustain without
fatigue, FR fibres fatigue but are resistant for quite some time,
and FF fibres are easily fatigued and their force declines rapidly.
All the muscle fibres of a particular motor unit are of the same
type. The CNS activates specific motor units with specific
contractile properties that match the task to be performed.
Muscle Plasticity
Type of training:
- Strength Training: high force & few repetition
o growth partially due to muscle hypertrophy
o number of muscle fibres is determined by genetics and
not affected by strength training.
o individual muscle cells get bigger
o more muscle filaments therefore more cross-bridges
producing force
o neural mechanisms are also involved in strength  skill i.e. learning to keep
the opposing muscles relaxed.
- Endurance Training: Low force & many repetitions
o Endurance training
o Improves fatigue resistance
o Increase duration of performance
o Increase in capillary density
o Increase in mitochondrial content and oxidative enxyme activity
o The force declines quickly after training stops
Disuse:
- Disuse results in weakness and fatigue
- Absence of load bearing – bed rest
- Anti gravity muscles are most affected.
- Atrophy occurs. (muscle fibres get smaller)
- Reduced recruitment of the motor units.
- Opposite of training effects
Injury and Repair
- Common causes of injury
o Disease
o External trauma
o Internal trauma
o Eccentric exercise
- Denervation and reinnervation
- Muscle fibre repair
Damage and Repair of muscle
fibres – Role of Satellite Cells
Membrane damaged
Ca2+ enters
Macrophages enter
Macrophages remove debris
Infarct spreads
Satellite cell activation
Myoblasts
DEATH OF FIBRE
Myotube
REPAIR COMPLETE
Ageing
Performance declines with increasing age after around 20-25 years. This is due to a
decrease in muscle mass and motor neuron loss after the age of around 60.
Mean number of
motor units for
under 60’s
Lower limit of the
range of number of
motor units for under
60’s