balance Flashcards
what is the vestibular system? when is it activated?
- balance organ
- only activated when it reaches sensory threshold
do you need the vestibular system to stand up?
- no
- can stand without the system if well adapted
- proprioception more important
what is proprioception?
- sense of the relative position of neighbouring parts of the body
can you give an example of proprioception
- relationship of upper arm to trunk
what does proprioception normally involve?
- joint angle
- joint position
what is sometimes included in proprioception?
- vestibular sensation sometimes
what is proprioception highly sensitive for? what is it the most important for?
- highly sensitive for balance
- most important sensory modality
how do I know my elbow is at 90 degrees? what information isn’t useful?
- different sources of information contribute
- joint information found to not be a useful tool
- evidence via animal cases
what was historically thought to be the main source of joint position sense?
- joint capsule receptors
what is contained within the joint capsule?
- ruffini, paciniform, golgi- type and free nerve endings
what do joint capsule receptors provide? what do they detect?
- provide limited information
- detect pressure as they contain different peripheral sense neurons
what are recent developments of joint capsule role based on?
- animal recordings
what do animal recordings involve?
- animal under anaesthesia
- records primary afferents (from joints) & physically rotates to determine how the neuron firing rate encodes the joint position
do the primary afferents recorded give useful information?
- no
- poor information as it is difficult to reconstruct the position of the joints from firing rate
- but do code the limit of the joint accurately
what is the recent understanding of joint capsule afferents?
- provide ambiguous information
- mainly act as limit detectors (injury prevention)
what happens to joint position sense if you remove all sensory input?
- joint position sense doesn’t change
what are the three main sources of evidence showing the joint capsule role?
- normal sense of joint rotation after hip replacement
- knee anaesthesia barely affects sense of joint rotation
- disengaging muscles from joint motion leads to poor motion detection
describe disengaging muscle from joint motion leading to poor motion detection
- decoupled joint movements from its associated tendon & muscle
- take the middle finger and rotate the end of the peripheral joint
- ask people to judge whether its moving or not
in the middle finger experiment, when are we useless at detecting joint motion ?
- useless until you get up to high velocities
what does golgi tendon organ signal? what stimuli?
- signals force and heaviness
- kinetic stimuli
where is the golgi tendon organ situated?
- interface between muscle and tendon
what does golgi tendon organ contribute to? and how?
- indirectly contributes to joint rotation sense
when is golgi tendon organ important?
- important when muscle movement is ambiguous
- concentric vs eccentric
what happens if there is an increase in force through golgi tendon and associated muscle?
- greater firing rate of 1b afferents
what are muscle spindles?
- intrafusal muscle
- embedded throughout muscles are 100s of sensors made by muscle tissue itself but doesn’t produce any useful force
what does muscle spindle provide? what does it contain?
- main signal of joint angle
- contains primary and secondary endings
what do primary endings signal?
- position and velocity
- kinematics stimuli
what are primary endings transmitted by?
- transmitted via 1a afferents
what are primary endings sensitive to?
- vibration
what do secondary endings involve?
- position only
what are secondary endings transmitted by?
- group II afferents
what are gamma fibres?
- contractile elements of a spindle
what do gamma fibres do?
- tighten up the sensors to help maintain their roles
what does alpha- gamma coactivation maintain?
- sensitivity in the face of changing muscle length
what is the alpha motor system?
- contraction of the main body of the muscle to produce force
what is the gamma motor system?
- contraction of the intrafusal fibres
- maintains taut
what evidence is there supporting the role of spindles?
- ways of artificially activating muscle spindles via muscle vibration or associated tendon vibration
- seen by direct recordings
what hertz activates 1a afferents?
- 80 to 100 hertz vibration selectively activates 1a afferents
what does 80 to 100 Hz vibration produce?
- illusions of muscle stretch
- both position and velocity
what is an example of the illusion of muscle stretch?
- pinocchio illusion
- grab your nose and place a muscle vibrator on biceps to evoke the sensation that your biceps is stretching, and nose is growing longer
what does the central nervous system think with the illusion?
- thinks the muscle is elongating
what does calf muscle vibration evoke?
- strong sway response
- in the backwards direction as interpret that you’re going forward so illusion is opposite
what are muscle spindle signals dependent on?
- the nature of mechanical properties of the surrounding muscles
can you bias the direction of the perceived location of the joint?
- yes, you can condition the muscle before a perceptual experiment
what experiment was done to bias the direction of the perceived location of the joint?
- one arm manipulated passively into a position and with the eyes closed match position with other arm
- before intervention people asked to flex or extend
what does flexion or extension before experiment change and what does this lead to?
- changes mechanical properties of the muscle transiently i.e., taut or flexible
- consistent bias in your perception depending upon whether you flexed/ extended
what word describes the factors that affect joint position sense?
- muscle thixotropy
what affects joint position sense?
- prior muscle stretch/ contraction significantly affects joint position sense
why does muscle thixotropy occur?
- due to muscle becoming slack/ taut
- thus affects spindle output
what test was done in standing? what did it test of each system?
- researcher evoked perturbations either by sensory channels including vision, proprioception, vestibular or all
- tests the sensitivity of each system
what was the adaptation for vision?
- person is fixed in space
- has a moving room around them
- room is rotated
- participant asked to judge when they first see it moving
what was the adaptation for vestibular system?
- fixed ankles using blocks
- physically pushed around with eyes closed
what was the adaptation for proprioception condition?
- fixed in space with a balancing inverted pendulum
- fall backwards unless push down on their feet
what did the experiment work out from the different sensory conditions? what determines the usefulness of a sensor?
- worked out the threshold for when you first detect motion to guide how sensitive they are
- better at detecting motion when contracting so state determines how good it is as a sensor
list the systems in order of sensitivity (1= highest, 3= lowest)
1= proprioception
2= vestibular
3= vision
what is standing possible with?
- possible with proprioception alone
what is equivalent body used for?
- used to isolate ankle joint signals
what is used to test balance? what shows a better balance system?
- imperceptible perturbations
- larger response to perturbation shows a better balance system
what information alone can keep the body upright?
- ankle information alone is sufficient
- need muscle spindle and golgi tendon information from calf
what does GVS stand for? where is the GVS?
- Galvanic Vestibular Stimulation
- locked inside skull
how is vestibular information interpreted?
- must be interpreted in context of neck orientations
what does illusions of neck orientation cause?
- vestibular information to be reinterpreted
how does GVS cause problems with balance?
- because head not always fixed in respect to the body
- swaying can alter input
what is a efference copy? what does it do?
- copy of motor command sent to sensory system
- makes us aware of where our limbs are
how do you artificially activate the vestibular system? what happens if you turn the head and why?
- use electrodes to supply small currents across the back of the head
- activates vestibular nerve
- swaying sensation towards cathode electrode
- if you turn the head, you sway in that way
- vestibular info interpreted in context of proprioception
what can proprioception also interact with?
- vision sense
when does evoked sway habituate?
- upon repeated exposure to moving room
which way do you sway in?
- sway in the same way as the room as stationary object
what does foam do?
- reduces ankle input
- leads to larger visually- evoked responses
when does the body no longer react to trial?
- when the body adapts
- due to other sensory inputs conflicting with visual info e.g., proprioceptive
why are calf muscles not useful signals of body position?
- because calf muscle shortens as body sways forward
- it is an ambiguous signal
what is paradoxical motion? what does it tell us?
- as the person sways forwards, the muscle gets shorter
- tells us stretch reflexes cannot be useful for normal control of quiet standing as achilles tendon is elastic under low forces
how does age affect balance?
- ankle proprioception declines with age
is the link of balance with falls strong?
- proprioception falls link not strong
what is associated with falls?
- peripheral neuropathy
what is the link between balance and falls explained by?
- interaction of muscle sensation and strength
does weakness or strength impact ability to sway?
- doesn’t matter when eyes open; however, when closed there was a disproportional greater ^ in sway of weaker people
what is a strong predictor of fall risk?
- reduced strength
is exercise aimed solely at strength training successful?
- no, its not successful in reducing fall risk
what must be challenged to reduce fall risk?
- balance itself
what increases fall risk if included?
- walking exercises
