proprioception

Question 1

unconscious proprioception

Answer 1

• The impulses arising from the proprioceptors mediating this type of sensation (muscle spindles and Golgi tendon organs) are relayed to the cerebellum rather than to the cerebral cortex

Question 2

muscle spindles - structure

Answer 2

• Muscle spindles are present in skeletal muscles
• Each spindle consists of a connective tissue capsule in which there are 8 to 10 specialized muscle fibers called intrafusal fibers.
• The intrafusal fibers and the connective tissue capsule in which they are located are oriented parallel to the surrounding skeletal muscle fibers called extrafusal fibers.
  • The intrafusal fibers are innervated by gamma motor neurons, whereas the extrafusal fibers receive motor innervation from alpha motor

Question 3

muscle spindles - intrafusal fibre types

Answer 3

• There are two types of intrafusal fibers.
  1. The nuclear chain fiber contains a single row of central nuclei and is smaller and shorter than the nuclear bag fiber.
  2. The nuclear bag fiber has a bag-like dilation at the centre where a cluster of nuclei is located.
• Efferent innervation is provided to the polar ends of both types of intrafusal by axons of gamma motor neurons
• Two types of afferents arise from the intrafusal fibers:
  1. Annulospiral endings (primary afferents - 1a), which are located on the central part of the fibres and are activated by brief stretches/vibration and by prolonged stretches/vibration
  2. Flower-spray endings (secondary afferents - 2), are located on each end of the annulospiral endings. –> only prolonged stretches/vibration

Question 4

golgi tendon organs

Answer 4

• These high-threshold receptors are located at the junction of the muscle and tendon.
• Golgi tendon organs are arranged in series with the muscle fibers, in contrast to muscle spindles, which are arranged parallel to the extrafusal muscle fibers.
• A tendon is composed of fascicles of collagenous tissue that are enclosed in a connective tissue capsule.
• A Golgi tendon organ consists of a large myelinated fiber that enters the connective tissue capsule of a tendon and subdivides into many unmyelinated receptor endings that intermingle and encircle the collagenous fascicles.
• Active contraction of the muscle or stretching of the muscle activates the Golgi tendon organs.
  • Thus, Golgi tendon organs are sensitive to increases in muscle tension caused by muscle contraction.

Question 5

inverse myostatic reflex

Answer 5

• Activation of the Golgi tendon organ produces an action potential in the associated afferent 1b fiber
  • This afferent fiber makes an excitatory synapse with an interneuron which then inhibits the alpha motor neuron, which innervates the muscle group.
  • The net effect is that the contraction of the muscle is reduced –> the inverse myotatic reflex

Question 6

balance strategies

Answer 6

• Three mechanisms by which balance can be maintained:
  1. By moving the centre of pressure with respect to the vertical projection of the centre of mass
• Use area of feet as support for the CoM of the whole body to stay balanced
• Applied when standing and walking on any flat surface

2. By counter-rotating segments around the CoM
   - in situations where the base of support available to the CoP’ is insufficiently wide to accommodate sufficient displacement of the CoP. In such cases it is seen that parts of the trunk are rotated with respect to the CoM
   - hip strategy
3. By applying an external force, other than the ground reaction force.

Question 7

COP and COM during walking

Answer 7

• For any balancing task, the CoM position is always above the CoP and this identifies a potentially unstable Inverted Pendulum (IP), with an (unstable) equilibrium state characterized by the fact that CoM and CoP are aligned on the same vertical
  , in the case of upright standing, the natural tendency to fall is counteracted mostly by the feedback activation of ankle muscles  proprioception
• When walking the CoP and CoM constantly change  balancing strategies of previous LG aim to counteract this and keep the CoM above the CoP by correct foot placement, arm, leg and trunk rotation and possibly external force application