Lecture 12: Posture

What are the three goals of postural control?

1. Postural Stability

2. Postural Orientation

3. Readiness to Move

Describe the goal of postural stability.

• Maintain stability of body relative to gravity during a task-directed motion

• Keep COM within base of support

• Eseentially, we do not want to fall down

Describe the goal of postural orientation?

• Maintain postural and visual orientation of body relative to task and environmental constraints

• Postural alignment and gaze stabilization

• Essentially, am I aligned and is my gaze oriented to the right place

Describe the goal of readiness to move.

• Postural tone

• Derived from both neural and non-neural mechanisms

Why do some postural control mechanisms have different goals?

• Postural control mechanisms have different goals based on the context and goal of the motion

• Examples: running to catch a ball vs hiking

  • Running to Catch Ball: goals involve visual and body orientation on the ball (not really paying attention to ground)

  • Hiking: goals involve visual orientation on the ground to detect obstacles
    

What three mechanisms are utilized to fulfill goal #1 (COM within base of support)?

• Proactive Control

• Reactive Control

• Steady State Control

What is proactive control (with goal #1)?

• Anticipatory postural and alignment adjustments based on internal predictions of movement's affect on postural stability

• "We have the luxury of anticipating how changes to our alignment will have changes to our stability"

What is reactive control (of goal #1)?

• Postural reactions to unexpected, unplanned events resulting in instability

• Example: someone runs into shoulder → didn't expect it but need to fix it to keep COM within BOS (so we do not fall)

  • It also could be just that our motor plan was bad and we need to fix it
    

What are the two different types of reactive control?

• Automatic Postural Reactions

  • Mechanisms used to recover balance from an unexpected perturbation

  • E.g. -- someone running into shoulder

• Volitional Postural Reactions 

  • Used to modify or override an automatic postural correction

What is steady state control (for goal #1)?

• Maintenance of postural stability under predictable and non-changing conditons

• Example: standing in a quiet position with no external demands (conditions are predictable and non-changing)

What strategies are utilized in automatic postural reactions?

• Ankle Strategy

  • Utilized when perturbation is relatively small

  • Example: forward perturbation → plantarflexors used to "come back"

• Hip Strategy

  • Utilized when larger perturbation occurs or there is a narrow base of support

• Stepping Strategy

  • Utilized for larger perturbation where you need to take a step to prevent falling, etc.

• Suspensory Strategy

  •  More of a protective strategy

  • Example: walking over an icy surface, you already have knees bent; it is a "protective" and automatic strategy

Patient's with Parkinson's Disease have issues with _______

Automatic postural reaction strategies

What are volitional postural corrections?

• Reaction Time

  • How fast they can supplement automatic postural reactions

  • E.g.: reaching for counter when you have to step to prevent falling
    

• Conscious Override

  • Override it from cortex

  • E.g.: Gymnasts perform vaults and maybe they land wrong; they know that taking a step during landing is a point deduction so they completely override that mechanism and do everything else to regain balance and not take a step

What are the sensory contributions of goal #1?

• Vision

• Vestibular

• Somatosensation

What does vision contribute to goal #1?

• Body orientation and move via optic flow 

• As you move through the environment, you are seeing things/objects move past you (or as you fall you see things moving past you) -- this is optic flow

What does vestibular contribute to goal #1?

• Orientation of the body relative to gravity

• It can also help with understanding if you are accelerating or decelerating

• More active if there discrepancy b/w vision and somatosensation (to determine who is right/wrong)

What does somatosensation contribute to goal #1?

• Center of mass displacement and position

• Example:

  • I know my body is more anterior if I can feel more pressure or weight on my toes

  • Also can give a sense of the surface we are on (e.g., grass, concrete, etc.)

• We rely a lot on somatosensation because it is the quickest

What structures contribute to the neural control of goal #1?

• Spinal Cord

• Brainstem

• Cerebellum

• Basal Ganglia

• Supplementary Motor Area

• Temporparietal Cortex

• Limbic Regions

What role does the spinal cord play in the neural control of goal #1?

• Maintenance of anti-gravity support

• It has Short Latency Responses (Nonfunctional) 

  • "Nonfunctional" because it cannot correct in reactive scenario (basically, cannot work alone)

• Example: feel stretch in gastroc --> you then turn on gastroc --> but if this is all you do you will still fall down

What role does the brainstem play in the neural control of goal #1?

• Automatic postural responses (from the reticular formation)

• Has medium latency but is functional 

"What role does the cerebellum play in neural control of goal #1?"

• Response magnitude adaptation 

• Matching the response based on the magnitude

• (Sets sensitivity of muscle spindles and APRs)

What is the role of the basal ganglia in neural control of goal #1?

• Response context 

• Basal ganglia helps us pick the correct (APR) strategy for the context (e.g., ankle, hip, step, etc.)

• ** people with Parkinson's Disease tend to pick the wrong strategy

What is the role of the supplementary motor cortex in neural control of goal #1?

• Proactive / Anticipatory control 

• Has long latency 

• Does a better job planning for perturbation compared to reacting to it

• SMA is internally driven (more of a "planner" versus a "do-er")

What is the role of the temporoparietal cortex in neural control of goal #1?

• Perception of body verticality 

• Has long latency 

• Example: Individuals who have experienced a stroke have an issue with this; patient will see something in the right orientation (like a door frame) but still feel like they are falling down

What is the role of the limbic regions in neural control of goal #1?

• Emotional influence over postural correction

• Has long-latency 

• Neurons might change the magnitude of a response

• Example: if you are nervous, you over-recruit for a motor response

________ are always part of the motor plan

Autonomic Postural Alignment Reflexes
FOR GOAL #2"

What are automatic postural alignment reflexes?

• Maintains postural orientation of eyes, head, and body to vertical

• Reflexes bias by feedforward planning 

• Example: catching a ball vs hiking --> goal #2 is still reflexive but how we utilize it is biased by volitional goals

• Can also be triggered by feedback from sensory input

What automatic postural alignment reflexes are used for gaze stabilization (for goal #2)?

• Vestibulo-Ocular Reflex 

  • This reflex serves to move your eyes accordingly in response to head movement

  • Cerebellar Role in VOR:

    • VOR Cancellation

    • Sets Reflex Gain

    • Learns via Retinal Slip

What automatic postural alignment reflexes are used for postural alignment (for goal #2)?

• Vestibulo-spinal Reflexes

• Vestibulo-collic Reflexes

• Asymmetric Tonic Neck Reflex (ATNR)

• Symmetrical Tonic Neck Reflex (STNR)

What is the vestibulo-spinal reflex?

Alignment of the body based on head movement

What is the vestibulo-collic reflex?

Activating neck movement to keep head upright

What is the asymmetric tonic neck reflex (ATNR)?

• If you turn your head to one side → arm on same side will extend and opposite arm will flex

• Example:

  • Turn head to right

  • Right arm extends

  • Left arm flexes

What is the symmetrical tonic neck reflex (STNR)?

• The arms follow the movement of the head, while the legs are opposite 

• Example:

  • Head flexes

  • Arms flex

  • Legs extend

The ATNR and STNR are obligatory in ________ and become integrated into the motor plan as you ________

Children, Age
Cortex usually overrides reflex in older children/adults

What structures are responsible for postural alignment and gait stabilization?

• Spinal Effectors

• Brainstem Centers

  • Lateral Vestibular Nuclei

  • Medial Vestibular Nuclei

  • Medial Longitudinal Fasciculus

  • Reticular Formation

• Cerebellum

  • Vestibulocerebellum

  • Spinocerebellum

• Temporoparietal Cortex

What is the function of spinal effects for goal #2 (postural alignment / gaze stabilization)?

Tonic facilitation of anti-gravity muscle groups (in most contexts, this is the extensors)

What is the function of lateral vestibular nuclei for goal #2 (postural alignment / gaze stabilization)?

Compensates for body movement/tilt via activation of anti-gravity muscles (facilitates ext. m tone to keep alignment)

What is the function of medial vestibular nuclei for goal #2 (postural alignment / gaze stabilization)?

Adjusts head position to upright, especially during walking

What is the function of the medial longitudinal fasciculus for goal #2 (postural alignment / gaze stabilization)?

• Facilitates gaze and postural alignment reflexes via conjugate eye movement

• Want both our eyes on the same thing at the same time

What is the function of reticular formation for goal #2 (postural alignment / gaze stabilization)?

• Internal represenation of postural orientation

• "uprightness" / "verticality"

What is the function of the vestibulocerebellum for goal #2 (postural alignment / gaze stabilization)?

• Modulates VOR, saccades, vergence

• Generates smooth pursuits (cerebellum is required for this!)

• Balance reactions for head/body

• (Triggers balance reactions by acting on vestibular nuclei to influence vestibulospinal tracts)

What is the function of the spinocerebellum for goal #2 (postural alignment / gaze stabilization)?

• Internal representation of postural orientation

• (Internal model for posture / postural alignment)

What is the function of the temporoparietal cortex for goal #2 (postural alignment / gaze stabilization)?

Sensory integration and perception of body verticality

What three structures deal with verticality/uprightness?

• Reticular Formation

• Temporoparietal Cortex

• Spinocerebellum

What is the role of the reticular formation in "upright" sensation?

Informs vestibular nuclei

What is the role of the spinocerebellum in "upright" sensation?

Internal model of "upright"

What is the role of the temporoparietal cortex in "upright" sensation?

Conscious perceptionof body verticality

"Accurate perception of verticality is essential for goal(s) ________ and ________

#1 (COM within BOS), #1 (COM within BOS) and #2 (postural alignment / gaze stabilization)

What three structures are involved in goal #3 (postural tone)?

• Reticular Formation

  • Ceruleo/Raphe Nuclei

  • Pedunculopontine Nucleus

• Cerebellum

• Basal Ganglia

What is the function of the ceruleo and raphe nuclei in goal #3 (postural tone)?

• Faciitation of muscle tone (inc. baseline activation; inc. activity of LMN)

• (part of the emotional motor system)

What is the function of the pedunculopontine nucleus in goal #3 (postural tone)?

Inhibition of the reticulospinal tract to inhibit muscle tone (at rest)

What is the function of the cerebellum in goal #3 (postural tone)?

Cerebellar inputs regulate the gain on muscle spindle settings

What is the function of the basal ganglia in goal #3 (postural tone)?

Modulates postural tone via connections with pedunculopontine nucleus (to influence the reticulospinal tracts)

Pontine Reticulospinal Tract acts on ________ muscles

Extensor

Medullary Reticulospinal Tract acts on ________ muscles

Flexor