Module 15 - Lecture 11 - Vestibular system posture and balance

Question 1

Define Balance.

Answer 1

Balance: ability to control the body mass relative to BOS
Balance disturbances include:

• Gravitational forces
• Forces arising from muscle contraction
• Interaction between limb segments during movement
• External pertubations

Question 2

Define Postural control

Answer 2

Controlling body position in space for dual purpose of orientation and stability

Question 3

Define base of support.

Answer 3

BOS: area of body in contact with support surface

Question 4

Define Center of mass

Define Center of gravity

Define Center of pressure

Answer 4

COM: point that is center of our total body mass. Determined by finding weighted average of the COM of each body segment

COG: vertical projection of the center of mass

COP: center of distribution of total force applied to supporting surface (point where resultant of all ground reaction forces act). It is the response of the nervous system to changes in position of the COM. Moves continuously around the COM to keep the COM within the BOS.

Question 5

What is the difference between postural orientation and stability (balance)?

Answer 5

Postural Orientation: Ability to maintain an appropriate position of the whole body with respect to the environment and task.
Postural stability Stability: Ability to control COM relative to gravity & BOS during perturbations or motor tasks

Question 6

What are the 3 motor control factors that are needed for observed posture?

Answer 6

Control of task, individual, and environment.

Question 7

What 3 factors of the “individual” might be considered relevant for postural control?

Answer 7

Sensory, cognitive, and motor factors.

Question 8

How would you best test an individual’s “limit of stability”?

Answer 8

To test limits of stability define BOS and do not let that change !!!
You would need to know the size of the individuals BOS but then you would want to see how far you can move the person within that BOS
Measuring the postural sway → the “quiet stance” by either kinematic (infrared, video) or COP (ground reaction forces)

Question 9

What is the definitions for proprioception and exteroception and what sensory system might fit in each?

Answer 9

Proprioception: awareness of sensation arising from stimuli within our own body

• Somatosensation (cutaneous, spindles, GTO, joint receptors)
• Vestibular system

Exteroception: awareness of sensation that comes from environment

• Auditory
• Cutaneous sensation
• Vision.

***Sensor basic → for each sense (somatic and beyond)
Stimulus - something in our internal/external environment
Receptor - detects energy for “transduction” - into an AP (how the NS sends information)
Afferent - carries into CNS
Reflex circuits - rapid control of movement
Central pathway: higher level processing

Question 10

For the auditory system what is the stimulus?

Answer 10

Sound waves

Question 11

How is pitch coded in the auditory system?

Answer 11

Frequency of waves as they beat the cilia, subsequent transduction, and afferent pathways to auditory nerves. (deformation of tectorial membrane)

Question 12

How is loudness coded in the auditory system?

Answer 12

Amplitude of waves as they beat the cilia, subsequent transduction, and afferent pathways to auditory nerves.

Question 13

What is the sensory receptor in the auditory system and how does it work?

Answer 13

• Hair cells > mechanoreceptors
• Shear force from sound wave on cilia (hair cell) either causes opening or closing of K+ channels (high outside, low inside > follows gradient)
• Depolarization of receptor causes calcium channels to open, promoting vesicular movement to presynaptic membrane
• Followed by normal neurotransmission to afferent nerve (classic synapse)

*** If the shearing opens K+ ion channels → K+ high outside and low other side = K just passes through and depolarization which will then open the voltage-gated calcium channels

Question 14

What is the basic mechanism for social location (just stimulus basics not the olive)?

Answer 14

1. Time of arrival of AP’s tells us location, via temporal shift in path of soundwaves through the cochlea.
2. By contrasting the loudness of the sound
   
   1. It is going to be slightly louder on the ear that is closest to the sound
   2. They cancel out the noise properties, therefore the ear that is hearing the lower sound will actually be the neuron that travels to the brain.

The pathways to the auditory system to determine sound localization, the louder sound is processed in the contralateral side of our brain and then goes to the cerebral cortex through the thalamus. The information will then go to the primary auditory cortex in the temporal cortex.
The primary motor cortex will then relay the information on the secondary cortex to process more complex “features” contained within primary A1 auditory information.
Auditory system is in REVERSE !!! (if you compare this to the motor system) → motor goes primary then secondary

Question 15

For the vestibular system what is unique about the otolith organs vs the semicircular cannel?

Answer 15

• Otolith organs code for linear acceleration VS semicircular canals for angular acceleration
• Otolith organs have crystals VS only have a gel layer

Question 16

How do the otolith organs determine direction of movement?

Answer 16

Within the saccule and utricular which are found on each side of the head, there are these membranes = “macula” and in the membrane there is a gelatinous layer containing otoconia - crystals

• These crystal allow for us to determine our orientation
• If our head is tilted in different postures, gravity will pull the crystals down and that will then bend the underlying hair cells
• Because the crystals have an actual mass, they will shear the hair cells depending on which direction or orientation
• Linear acceleration can also be determined –> if our head is moving up or down because the head will move before the crystals and the gel which will cause a discrepancy and cause bending of hair cells.
• The gel/crystals might catch up with the head and they might move together causing a constant acceleration.

BIG PICTURE
The hair cells orientated in different ways across region
Direction coded by what hair cells bend
Utricule - orientation is horizontal (maximal response to sideways head tilt
Saccule - orientation is vertical (maximal response to up-down and forward-backward)

Question 17

How does the semicircular cannel determine direction of motion?

Answer 17

• There are 3 orthogonal bones with macula and gelatinous layer but with no crystals.
• The same mechanics of the mechanoreceptor apply → the direction of the shear of the hair cell (cupula) can be depolarizing when the gel flows in one direction or hyperpolarizing by closing those potassium channels if we were to rotate in the other direction.
• To code for direction these 3 orthogonal bones all have unique orientation and planes that they will respond too if they are activated. Each bone has a known orientation that they would preferentially bend their hair cells and the nervous system would learn to associate this with their respective direction.

***The semicircular canal can FURTHER discriminate direction:
They have a counterbalanced design !!!
The right semicircular canal compared to the left are in complete opposite directions
The way the hair cells are set up with this → When moving in a direction → One of the side of our ear will open the potassium channels and firing faster AP rate… while the other side are closing the potassium channels and firing at a slower rate
One more sensitive in the forwards direction and one more sensitive to backwards direction = comparison to Left and right

Question 18

What are the 3 general pathways (what part of the nuclei and the tract) for the 3 vestibular reflexes?

Answer 18

• Lateral Vestibulospinal Tracts → vestibular spinal reflex
  
  • It arises from lateral vestibular nuclei and uncrossed it projects the full length of the spinal cord to activate spinal extensor muscles.
  • Controls extensor muscle tone in anti-gravity
• Medial Vestibulospinal → vestibulo cervical reflex
  
  • It arises from medial vestibular nuclei and descends bilaterally into the spinal cord to terminate in medial gray matter of the cervical cord.
  • Regulates head/neck position as required for fixation of gaze.
• Medial longitudinal fasciculus → vestibulo ocular reflex –
  
  • When you are walking the room does not look like it is bouncing around since we adjust our gaze to when we are moving.
  • Also you are looking at the area where someone dropped something before you even thought about it.

Question 19

Does the vestibular nuclei only produce efferences in response to the vestibular apparatus?

Answer 19

It is not just vestibular information that feeds into the vestibular nuclei. There is also sensory afferent information from a variety of different stimulus. Then it also has a variety of outputs.

Question 20

What are the basic principles of compensatory / feedback/ closed loop control?

Answer 20

CPA = negative feedback from a perturbation → Closed loop control is another name for Compensatory
Sensor detect the actual state, a difference is compared to an expected state, an efferent is produced to restore the system
*Can occur in the pyramidal system → can go to the cortex.

APA’s = feedforward, in anticipation of a perturbation, postural adjustments are made.
Activation of postural muscles before execution of “voluntary goal”

Question 21

What are the principles of anticipatory postural adjustments? What pathways are these anticipatory postural adjustments taking (AKA what structures?)

Answer 21

Voluntary activity → pyramidal system
Corticospinal tract
Corticobulbar tract

Anticipatory postural adjustment → extrapyramidal system
Lateral corticospinal tracts (motor response), along with collaterals in reticulospinal tract (adjustments).

Question 22

Compare and contrast anticipatory with compensatory control.

Answer 22

Anticipatory Postural Control:
· Activation of postural muscles before execution of voluntary goal
· Lateral corticospinal tract (motor response) , reticulospinal tract (adjustments, postural support), corticobulbar tract
· programmed, adaptive, and centrally organized based on prior experience and intention (voluntary movement).

Compensatory Postural Control:
· Uses sensory info during/after movement to make corrections to ongoing or future movement - compare expected with actual
· Activated when APA’s insufficient to maintain balance after internal/external perturbation

Big difference is the error!

Question 23

How does anticipatory postural adjustments provide evidence towards what is involved in motor planning and programming?

Answer 23

Lag in response to perturbations in compensatory movements, suggesting time is needed for motor planning and programming, we don’t notice this as much in APA’s as it is done prior to.

Question 24

How do we maintain balance→ what are the 3 main sources of input to the CNS?

Answer 24

Vison
Vestibular system
Sensation

Question 25

WHERE DOES ALL THIS INFORMATION GO? VESTIBULAR PATHWAY CORTEX?

Answer 25

The information of the vestibular and auditory apparatus will go into the vestibular nuclei to deal with these sophisticated reflexes.
BUT, it can also go to our cerebral cortex, but for it to go there it need to go through the ventral posterior nucleus of the THALAMUS and then.. Goes to no fully defined area … but it goes to S1 and S2 (primary and secondary somatosensory cortex) in part of the insular lobe.

Question 26

If someone is standing there and the only goal is to maintain balance? What are other postural stability strategies that would be happening as the center of mass moves through the base of support???

Answer 26

• The muscle spindles of the ankle could be activated and the stretch reflex could help maintain balance
• Vision is also sensory information that is helping maintain balance
• Vestibular input
• Muscle and joint proprioception