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

Motor Units

• 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

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

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

Reflexes

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.

• 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

References

1. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
2. ANZCA March/April 2000