Sensorimotor Integration and Balance

Explain and compare the feed-forward and feed-back mechanisms of postural control

Feed-back mechanisms compensate for a loss of posture. When you lose posture, sensory systems detect this, and a compensatory response occurs. We seem to have a "bottom-up" strategy for this: we stabilise our ankles first, then our knees, then hips, trunk, and so on.

Feed-forward mechanisms are used in voluntary movements, and improve with learning. Feed-forward mechanisms can predict a disturbance, and over time reprogram your body's response so that you become better at handling said disturbance. For example, if you try roller-skating for the first time, there's a good chance that you'll fall on your arse pretty quickly because you're not used to it. Over time, however, your feed-forward mechanisms will predict the changes that occur when you're rolling, and you'll automatically compensate (and therefore stop falling on your arse).

Identify the primary sensory systems that contribute to balance and posture

The main sensory systems that contribute to balance and posture are the proprioceptive system, the visual system and the vestibular system. Let's start with proprioception!

The main proprioceptive receptors are the muscle spindle, the Golgi tendon organ, and joint receptors. Muscle spindles have "polar regions" (ends) made up of actin and myosin. Gamma motor endings, which are located in the middle of the spindle, maintain the sensitivity of muscle spindles. The Golgi tendon organ, located at the musculotendinous junction, is made up of mechanosensitive Ib fibres which are compressed during contraction, causing firing of action potentials. Finally, joint receptors change their firing rate as joint position changes.

The proprioceptive receptors are important in the stretch reflex: when muscle is stretched, the muscle spindles pick this up, causing inhibition of further stretch. The stretch reflex can be modulated by the gamma motor neurons, which regulate the sensitivity of the spindle and prevent it from completely slackening. In cases where there is high gain and/or latency (see here for explanations of these terms), oscillations can occur. An example of this is decerebrate rigidity, caused by lesions of the brain stem in the cerebellum and/or brain stem. Decerebrate rigidity is characterised by increased muscle stiffness, spasticity or clonus (rapid contractions and relaxations in response to stretch).

Describe the different types of eye movement and associated motor control processes

The visual system also contributes to balance and posture, so let's have a look at vision! There are a whole bunch of eye movements that can occur:

• Vestibulo-ocular: Vestibular input stabilises the eyes during rapid head movement
• Optokinetic: Visual input stabilises the eyes during slow head movement
• Saccades: Sharp movements that bring objects into focus
• Smooth pursuit: Eyes follow a moving target
• Vergence: Adjust for viewing at different depths/distances

Describe the vestibular contribution to the control of eye movement and postural reflexes

The main vestibular receptors are the otoliths and semicircular canals, all of which are located in the ear. The otolith organs include the utricle, which detects horizontal acceleration, and the saccule, which detects vertical acceleration. The semicircular canals detect rotational acceleration. As mentioned above, vestibular receptors are important in the vestibulo-ocular reflex, which stabilises the eyes during head movement. Vestibular problems may also generate nystagmus (a rapid, swinging motion of the eyes).

Aside from the vestibulo-ocular reflex, there are several other reflexes that are generated from the vestibular system. The vestibulocollic (neck) and vestibulospinal (limb) reflexes use input from the utricle and saccule. When the head moves back, arms and legs extend, but when the head moves forward, arms and legs flex.