What is proprioception?

Proprioception refers to the detection of stimuli relating to body position in space and postural equilibrium. Mechanoreceptors within muscle, joints and tendons relay sensory information about joint position, movement, vibration and pressure to the central nervous system. The major sensory receptors involved include:

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  Muscle spindles, which detect changes in muscle length;

  
  
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  Golgi tendon organs, which detect muscle tension.

What is a muscle spindle?

The muscle spindle is an encapsulated structure containing between 3 and 12 specialised intracapsular muscle fibres, known as intrafusal fibres. Intrafusal fibres are arranged in parallel with contractile extrafusal fibres; stretch of the extrafusal fibres therefore alters the intrafusal fibre length. An intrafusal fibre consists of a central non-contractile elastic portion with outer contractile ends.

There are two morphologically distinct types of intracapsular muscle fibre (Figure 55.1):

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  Nuclear bag fibres, in which the nuclei are collected in a central dilated portion of the fibre;

  
  
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  Nuclear chain fibres, in which the nuclei are distributed along the fibre without a dilatation.

Figure 55.1 Basic layout of the muscle spindle.

How are muscle spindles innervated?

Muscle spindles have their own dedicated afferent and efferent nerve supplies (Figure 55.1):

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  Afferent nerve supply: sensory afferent neurons wind around the central portion of the intrafusal fibres. There are two types:

  
  
  
  
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     – Type la afferent neurons receive inputs from both nuclear bag and nuclear chain fibres. Electrical activity in type Ia fibres reflects changes in both static intrafusal fibre length and rate of such change.

     
     
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     – Type II afferent neurons receive inputs from nuclear chain fibres only. Type II neurons relay information only about the static intrafusal fibre length.

  
  
  
  
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  Efferent nerve supply: γ-motor neurons innervate the contractile outer portions of the intrafusal fibres. Each γ-motor neuron innervates a number of muscle spindles.

Voluntary contraction involves the simultaneous activation of both α- and γ-motor neurons. Contraction of the outer portions of the intrafusal fibres prevents slackening of the spindle, despite the shortening of the extrafusal fibres. The sensitivity of the muscle spindles is therefore maintained despite skeletal muscle shortening and variations in its load.