0. In Brief

• Proprioceptors provide information of current states to motor plant of joint angles and muscle tension
  • info used to optimise and control movements
• Simple Reflexes help maintain balance and avoid unpleasant situations that can result in injuries
• Most reflexes are executed without consciousness, but may be modified by higher supraspinal brain areas
• Reflexes are often associated with a fixed motor action, but can depend on current state of system
• Spinal Reflexes rely on local proprioceptive signals indicating limb orientation and progress of movements
• Spinal Reflexes reduce computational workload of higher brain centres

1. Proprioception - Muscle Spindle

Muscle Spindle

• Muscle Spindle are small encapsulated sensory receptors
  • Oriented parallel to contractile elements of skeletal muscles
  • Enclosed by a capsule
  • Signal changes in length of the muscle, which is associated with changes in angles of joints
• Three main Components that terminates at central non-contractile regions:
  • Intrafusal Muscle Fibres
    • Nuclear Bag Fibres
      • Dynamic Fibre Bags
      • Static Nuclear Bags
    • Nuclear Chain Fibres
  • Afferent Sensory Fibres
  • Efferent γ-motor Neuron
• Extrafusal Muscle Fibres are connected to tendon and generate movement, and are innervated by α-motor Neuron

Innervation

• Extrafusal Muscle Fibres are innervated by α-motor Neurons
• Intrafusal Muscle Fibres:
  • Group Ia Afferent Axon innervates all Intrafusal Fibres, which serves as Primary Sensory Ending
  • Group II Afferent Axon innervates Nuclear Bag Fibres and Nuclear Chain Fibre, which serves as Secondary Sensory Ending
  • Dynamic Efferent γ-motor Neuron innervates Dynamic Bag Fibres
  • Static Efferent γ-motor Neuron innervates Static Bag Fibres

Transduction Mechanism of Afferent Fibres

• Muscle Spindle detects the rate of change in length and the length of muscle
  • Receptor’s initial response amplitude encodes the rate of change in length
  • Steady state of response shows final length
• Rate of change and final length are different for different stimuli

Group Ia Afferent Fibres are responsive to rate of change in length

Group II Afferent Fibres are mainly responsive to the static component of stretch and is involved in muscle length sensing

• At the terminal of Afferent Fibre, neighbouring ion channels are connected by Cytoskeletal Strands
  • channels are mechanically opened by stiff Cytoskeletal Strands when stretched
  • Stretching causes the Muscle Spindle sensory ending activity to increase and vice-versa
• The graph shows the Patch-clamp recording of an individual ion channel
  • Both opening probability and opening duration of channels depend on stretch of muscle

The Fusimotor System - α and γ-motor Neurons

• Activity of Muscle Spindles are modulated by changing γ-motor Neurons activity

A

• Passive stretch of muscle by a pull resulted in a change in length that is picked up by Muscle Spindle and Group Ia Fibres fire AP

B

• When only α-motor Neurons are stimulated, Group Ia Fibres firing from Muscle Spindle pauses as during contraction, the muscle is unloading the Muscle Spindle

C

• If α and γ-motor Neurons are stimulated at the same time, the pause is eliminated as the contraction of intrafusal fibres keeps the Muscle Spindle under tension
• α-γ Coactivation stabilises sensitivity of Muscle Spindles and used in voluntary movements

Servomechanism System - α-γ Coactivation

• Stretch-reflex pathways contribute to regulation of Motor Neurons during Voluntary Movements and maintenance of posture because they form feedback loops

  • Stretching a muscle increase activity in Muscle Spindle Afferents
  • Leading to muscle contraction and consequent shortening of the muscle
  • Muscle shortening in turn leads to decreased activity in Muscle Spindle Afferents
  • Reduction in muscle contraction and lengthening of the muscle

• The reference value is set by descending signals that act on α and γ-motor Neurons

  • If the shortening of the whole muscle is less than required by a task (when the load is greater than anticipated), the Muscle Spindle Afferents increase AP rate because contracting intrafusal fibres are loaded by the relatively greater length of the whole muscle
  • If the shortening of the whole muscle is greater than required, the Muscle Spindle Afferents decrease AP rate because intrafusal fibres are relatively slackened