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:
Ø Define, sketch and label and explain how the motor unit functions.
Ø Describe motor unit size, distribution within a muscle, and fibre type.
Ø Describe changes in motor units that result from of denervation and re-innervation.
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
Ø 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.
Ø Describe satellite cells and how they are involved in skeletal muscle repair.
Ø 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
- the terminals branch out as they reach the muscle.
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.
Muscle Fibre Types
There are 3 different types of muscle:
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 à 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
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.