Skeletal Muscle Innervation
Explain the concept of motor units
Describe the relationship between muscle length and tension
Describe the monosynaptic stretch reflex, single twitch, tetanus and the Treppe effect
- A motor unit consists of an α-motor neuron and the group of muscle cells that it innervates
- An action potential in this neuron will cause contraction of all the myocytes in the unit
- Large muscles have many myocytes per unit
- Small, precise muscles (e.g. extraocular) have few myocytes per unit
Force of Contraction
Muscle tension is dependent on three factors:
- Initial myocyte fibre length
Optimal stretch maximises the number of overlapping actin and myosin filaments.
- Number of contracting myocytes
Recruitment of additional motor units increases the force of contraction.
- Frequency of Action Potentials
High frequency action potentials cause accumulation of calcium in the cytoplasm (the Bowditch or Treppe effect), increasing force of contraction.
- As the absolute refractory period of skeletal muscle is shorter than cardiac muscle, tetany, or sustained muscle contraction, can occur
Proprioception is the ability of the body to determine it's position in space. There are two key proprioceptive sensors:
- Muscle spindles
- Golgi tendon organs
Muscle spindles sense changes in muscle length. They:
- Are a specialised muscle fibre, known as intrafusal fibres
- Run parallel to myocytes (also known as extrafusal fibres)
- Consist of two elements:
- Central, non-contractile portion which senses tension
- Contractile ends
This allows the muscle spindle to adjust its length with its muscle, so that a constant tension in the non-contractile portion can be maintained over a range of muscle lengths.
Muscle spindles have both afferent and efferent innervation:
- Afferent type Ia fibres adjust their electrical output to signal both current fibre length and rate of change
- Afferent type II fibres only signal fibre length
- Efferent γ neurons innervate the contractile elements
Voluntary muscle contraction results in contraction of both motor units (α1 neurons) and intrafusal fibres (γ-motor neurons).
Tonic innervation of γ-motor neurons increases muscle tone by stretching the non-contractile portions, increasing Ia firing and subsequent α-motor unit firing.
Golgi Tendon Organs
Golgi tendon organs are stretch receptors located between muscle and tendon. They:
- Run in series to myocytes
- Sense stretch
- Cause reflexive muscle relaxation, intended to prevent muscle damage
A reflex is an involuntary, predictable movement in response to a stimulus. There are two types:
- Monosynaptic: Motor neuron synapses directly with the sensory neuron
Monosynaptic reflexes are rapid, but only generate simple responses. There are five components to a monosynaptic reflex:
- Sensory receptor
Typically muscle spindles.
- Afferent neuron
Type Ia afferents relay signal from muscle spindle to ventral horn via the dorsal root.
- Synapse between afferent and efferent neuron
In the ventral horn
- Efferent neuron
α-motor neuron travels from the ventral horn and innervates the motor unit.
- Effector muscle Innervated motor unit contracts in response.
- Sensory receptor
- Polysynaptic: Motor neuron is separated from the sensory neuron by one or more interneurons in the dorsal horn
This allows modulation of signal. Responses are slower but more complex, e.g. withdrawal of a limb from a hot object.
Twitch and Tetany
- A twitch is the response of a muscle to a single stimulus (action potential)
- A tetanic contraction describes the sustained contraction produced by repetitive stimulation before relaxation can occur
- This stimulation must be causing above a critical frequency, which is dependent on the action potential duration for a cell
- Repetitive stimulation causes repeated SR depolarisation, leading to sustained high intracellular Ca2+ levels as Ca2+ entry exceeds Ca2+ exit
- Force from tetanic contraction is up to 4x greater than that of a twitch
- Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
- ANZCA March/April 2000