WEEK 1: Posture and balance

Question 1

What is posture?

Answer 1

A position of the body (Taber’s Cyclopedic Medical Dictionary).

Question 2

Describe upright posture.

Answer 2

Upright posture: body’s standing position such that:

*COG or mass situated approx. in front of S2
*Even in quiet standing many small corrective muscle actions needed to prevent toppling
*Complex CNS network working with automaticity & imbued with
Precision, speed, flexibility, and adaptability

Question 3

Control of posture - What is a stable posture?

Answer 3

*Centre of gravity must fall between the body supports.

*If you stand on one leg reflexive adjustment moves COG to opposite side

*If you move too far over, hopping or stepping reflexes “kick in” to prevent falling

Question 4

This is a highly fine-tuned machine…really, precision beyond human design. The control of posture and balance is a highly complex phenomenon.

What organs or systems participate in the control of posture and balance?

Answer 4

Eye, ears, skin, skeletal muscles, brain stem.

Question 5

Discuss the role of the brainstem in posture & balance.

Answer 5

1. Inputs come from the :
   *EYE: Visual input
   *EARS: Vestibular input
   *RECEPTORS IN THE SKIN: Cutaneous input
   *RECEPTORS IN JOINTS AND MUSCLES: Proprioceptive inputs
2. The inputs are sent to the vestibular nuclei in the brainstem.
3. Motor inputs from the brainstem are then sent to the
   *Limb and torso muscles to maintain balance and a desired posture
   *External eye muscles to control the movement of the eyes
   *Output to the CNS for perception of motion and orientation
4. The vestibular inputs can also be sent to the cerebellum which are then coordinated and processed before sending to the vestibular nuclei in the brainstem.

Question 6

Discuss Integrated control of posture.

Answer 6

1. RETINA: Eye position & velocity in visual space
2. Oculomotor System: Eye position & velocity in head
3. Semicircular Canals: Head movements
4. Otolith Organs: Head position relative
   to gravity
5. Neck Proprioceptors: Head position relative to body
6. Skin Pressure Receptors: Integrated sense of body position in space

Question 7

Discuss Several contributors to posture.

Answer 7

1.Brainstem:
The brainstem plays a crucial role in posture control.
*It receives inputs from the vestibular apparatus, visual system, and proprioceptive system, and processes and integrates this information to generate appropriate motor commands for postural adjustments.
*The brainstem also houses the vestibular nuclei, which receive vestibular inputs and send out efferent signals to control head and body position, eye movements, and posture

2. Cerebellum:
   *It receives inputs from the vestibular nuclei and other brain regions, and it plays a role in coordinating postural adjustments.

*The cerebellum monitors vestibular performance and can make adjustments to central vestibular processing if necessary.

*It is also involved in vestibular reflexes, such as the vestibulocochlear reflex (VOR), which helps stabilize gaze during head movements

3. Vestibular Apparatus:
   *The vestibular apparatus, located in the inner ear, is responsible for detecting head movements and providing information about head position and motion.

*It consists of the semicircular canals, which detect rotational head movements, and the otolith organs (utricle and saccule), which sense linear acceleration and head position relative to gravity.

*The vestibular apparatus sends signals to the brainstem and cerebellum, contributing to the control of posture and balance.

4. Specific Reflexes:

*Several reflexes contribute to posture control.
These include:

*The vestibulo-ocular reflex (VOR), which stabilizes the eyes during head movements.

*The vestibulospinal reflex (VSR), which helps maintain postural stability by adjusting muscle tone and activity in response to vestibular inputs.

*These reflexes involve connections between the vestibular nuclei, brainstem, cerebellum, and motor output pathways

5. Several Contributors to Posture:

In addition to the brainstem, cerebellum, vestibular apparatus, and specific reflexes, there are several other contributors to posture control.

These include specific motor and sensory components, such as:

*The oculomotor system (eye movements in the head)
*Neck proprioceptors (head position relative to the body)
*Skin pressure receptors (sense of body position in space).
*These components provide additional sensory information that is integrated with vestibular inputs to maintain postural stability

Question 8

Visual System & Posture

1. Get a partner
2. Stand on one leg with eyes open - partner observes.
3. Repeat with eyes closed
4. What’s the difference?
5. What can we conclude?

Answer 8

When standing on one leg with eyes open, the visual system provides important information about body position and helps maintain postural stability.

The eyes can fixate on a reference point, allowing the brain to assess the relative position of the body in space and make adjustments to maintain balance and posture.

However, when standing on one leg with eyes closed, the visual input is removed, and the reliance on other sensory systems, such as the vestibular and proprioceptive systems, increases.

Without visual cues, the brain must rely more on the information from these other systems to maintain postural stability.

Question 9

Visual world.

Movement of surrounding environment interpreted relative to our position as being caused by us moving and the world standing still

True, but can occasionally lead to problems when we are standing still and the whole visual world is moving. How would you feel if you…

Stood on a tall building or hill, looked up at a sky full of moving clouds?

Stopped (car) at traffic light next to a big truck or bus and the bus starts moving?

Answer 9

1. When standing on a tall building or hill and looking up at a sky full of moving clouds, you might experience a sensation of movement or vertigo.

This occurs because your visual system interprets the movement of the clouds as if you were moving and the world around you was standing still.

This can lead to a feeling of instability or dizziness, as your brain tries to reconcile the conflicting sensory information from your visual system and other sensory systems.

2. Similarly, when you are stopped in a car at a traffic light next to a big truck or bus, and the bus suddenly starts moving, you might feel a moment of disorientation or imbalance.

This is because your visual system perceives the movement of the bus as if it were your own movement, leading to a temporary mismatch between the visual input and the lack of corresponding movement sensed by your vestibular system and proprioceptive system.

These situations can occasionally lead to problems or discomfort, known as visually induced motion sickness or vection-induced motion sickness.

It occurs when there is a conflict between the information perceived by the visual system and the other sensory systems involved in maintaining balance and spatial orientation.

Question 10

Describe Vestibulo-ocular system.

Answer 10

The vestibulo-ocular system, also known as the vestibulo-ocular reflex (VOR), is a reflex that helps stabilize gaze during head movements. It ensures that the eyes move in the opposite direction to the head movement, allowing for a steady visual field and clear vision.

The VOR is driven by signals arising from the vestibular system, which is located in the inner ear.

The semicircular canals detect head rotation, while the otolith organs detect head translation.

These signals are processed and integrated in the brainstem, specifically the vestibular nuclei, which then send out efferent signals to the oculomotor nuclei.

The oculomotor nuclei innervate the eye muscles, causing them to move in a compensatory manner to counteract the head movement.

The VOR is essential for maintaining stable gaze and clear vision during everyday activities such as walking, running, or turning the head. It allows the eyes to remain fixated on a target, even when the head is in motion. The VOR has impressive adaptation capabilities, allowing it to adjust to different behavioral requirements.

CLINICAL REVELANCE
Disruptions or impairments in the vestibulo-ocular system can lead to problems such as:

1.Oscillopsia (blurred vision during head movement)

2. Abnormal nystagmus (involuntary eye movements).

Testing methods such as caloric testing, head impulse testing, and rotational chair testing can be used to assess VOR function.

Question 11

Vestibulo-ocular system.

What does it rely on?

When is it most effective?

How does it compensate for head rotation?

Answer 11

Relies on semicircular canals to determine rate at which head is rotated in any direction

Most effective at higher speeds of rotation

Compensates for rotation by activating the extraocular muscles to produce a perfectly matched counter-rotation of eyes.

Question 12

Vestibulo-ocular reflex (VOR)

-System in constant use, such as during walking

-Precise, neural system through which head rotations detected by SC canals & eyes counter-rotated (i.e. in opposite direction) by equal amts to stabilize line of sight

What happens if head rotation continues?
Why?

What makes the 2 phases of the Vestibulo-ocular reflex?

Answer 12

-System in constant use, such as during walking

-Precise, neural system through which head rotations detected by SC canals & eyes counter-rotated (i.e. in opposite direction) by equal amts to stabilize line of sight

-If head rotation continues, counter-rotation stops to avoid pointing eyes backwards.
Eyes reset to new central position in the orbits.
Compensatory counter-rotation then resumes.

Comparatively slow compensatory movements & quick resetting movements make the 2 phases of the VOR.

Question 13

Comparatively slow compensatory movements & quick resetting movements make the 2 phases of the VOR.

What do slow phase compensate for?

What do quick phase compensate for?

What is characteristic pattern of alternating quick & slow phases during sustained rotations called?

Answer 13

*SLOW phase compensates for head rotation
*QUICK phase returns the eye to the center of the socket//orbit

Characteristic pattern of alternating quick & slow phases during sustained rotations called nystagmus.

Question 14

Ablation & transection studies

Define ablation.

Answer 14

Ablation = “removal of a component of an input or an upper level of control from the circuit”

1. Ablation studies involve the removal or destruction of specific areas of the CNS to observe the effects on postural control.
2. Transection studies involve cutting or severing specific pathways or connections within the CNS to study their role in postural control. By interrupting the communication between different parts of the CNS, researchers can observe the resulting effects on postural control.

These studies involve the removal or transection of specific parts of the CNS to understand the functions and contributions of those parts to postural control.

Ablation studies involve the removal or destruction of specific areas of the CNS to observe the effects on postural control.

By removing a particular region, researchers can assess the impact on postural stability and determine the role of that region in the overall postural control system.

Question 15

State the significance of ablation.

Answer 15

*Helps isolate what the system can/cannot do w/out the part removed

*Useful in localization of CNS function
One early application was the study of postural control by the CNS.

Question 16

Ablation & transection studies

Charles Sherrington, a renowned neurophysiologist, conducted systematic and progressive separation studies of the neuraxis (at different levels) to investigate the effects on postural correction, also known as “righting reactions.” These studies involved removing or transecting specific parts of the central nervous system (CNS) to observe the resulting postural changes.

State the differing degrees of postural correction (“righting reactions”) observed in each of the following.

1. Below brainstem (SC level)
2. Above Medulla (thru pons)
3. Pinna stimulated
4. 4. Paw stimulated.

c) Cat placed on the side – upper limb assumed flexed and lower limb extended positions on the side in contact with the floor

5. Above mesencephalon (midbrain)
6. Decorticate animal (w/out cerebral cortical tissue)

Answer 16

Charles Sherrington: systematic, progressive separation of the neuraxis (at different levels) resulted in differing degrees of postural correction (“righting reactions”):

1. Below brainstem (SC level) – limp posture, no correction
2. Above Medulla (thru pons) – rigid posture (tonic contraction of antigravity limb extensors, trunk, neck)  exaggerated standing posture. However,
3. Pinna stimulated – turns head to opposite side & corrective limb movements.
4. Paw stimulated – turns head to same side and corrective limb movements.
5. Above mesencephalon (midbrain) – effective righting in chronically surviving animals
6. Decorticate animal (w/out cerebral cortical tissue) – observed many normal postural reactions, and lacked only some placing and stepping reactions

Question 17

State the 2 conclusions which can be derived from
Charles Sherrington: systematic, progressive separation of the neuroaxis (at different levels) resulted in differing degrees of postural correction (“righting reactions”).

Answer 17

1) As the level of transaction is raised, posture and balance progressively approach normal.

-Sherrington observed that as the separation of the neuraxis was performed at higher levels, the resulting postural correction and balance became more similar to normal.

-This suggests that different levels of the CNS contribute to postural control and that higher levels play a more significant role in achieving a stable and balanced posture.

2) lower levels of mammalian brain capable of autonomous simple actions

-Sherrington’s studies revealed that even at lower levels of the mammalian brain, autonomous simple actions, such as muscular coordination, equilibrium, and posture control, can occur.

-This suggests that basic motor functions and postural control can be mediated by lower levels of the CNS, including the spinal cord, medulla, pons, and cerebellum.

Question 18

Lateral & ventromedial systems

Discuss Lateral system.

Answer 18

Originates in red nucleus.
For corticospinal control of limbs
For fine movements and dexterity

1. The lateral corticospinal tract is a descending motor pathway that originates in the primary motor cortex, specifically the precentral gyrus, and controls voluntary movement of the contralateral limbs.
2. It is responsible for the fine movement of ipsilateral limbs, although it lies contralateral to the corresponding motor cortex.
3. The lateral corticospinal tract is the largest part of the corticospinal tract and extends throughout the entire length of the spinal cord.
4. It influences spinal motor neurons, especially those controlling fine movements of the distal musculature.

Lesions of the lateral corticospinal fibers on one side of the cervical cord result in ipsilateral paralysis of the upper and lower extremities (hemiplegia)

Question 19

Describe the ventromedial system.

Answer 19

Descending pathways from the brainstem motor centers grouped based on placement in Sp. cord white/gray matter:

Ventromedial system

-Originate in vestibular nuclei, reticular formation, & other brainstem nuclei

-Transection in medulla results in difficulty maintaining posture, balance, walking, & climbing, but NOT manipulating objects

Question 20

State the components of the vestibular apparatus.

Answer 20

Semicircular canals (3 in each ear)

Otolith organs (1 utricle and 1 saccule in each ear)

Question 21

What is the vestibular apparatus needed for?

Answer 21

Provides information needed for:
-Sense of equilibrium (balance)
-Integrating movement of the head with those of the eye & posture

Question 22

Semicircular canals

How are they oriented?

Cover 3 degrees of freedom of head movement.
State the 3 movements they are able to make.

What type of head movements do they detect?

Answer 22

Oriented orthogonally (90 degrees) to each other.

Cover 3 degrees of freedom of head movement (nodding, shaking, moving ear towards shoulder)

*Detect head movement (if accelerating or decelerating)

*Rotational (angular; not linear) *acceleration/deceleration

Do not signal steady position of head or steady velocity.

Question 23

State semicircular canal specifically separately for the following.

1. Shaking
2. Nodding
3. Moving the ear to the shoulder

Answer 23

1. Shaking [“no”] of the head (horizontal = lateral)
2. Nodding (posterior)
3. Moving the ear to the shoulder (superior = anterior)

Question 24

What is a Sensory end-organ?

Answer 24

A sensory end-organ refers to a specialized cluster of cells that encapsulates the receptor ends of certain sensory axons.

These end-organs play a crucial role in affecting the response of the axons.

Some examples of sensory end-organs include Meissner corpuscles, Pacinian corpuscles, Ruffini corpuscles, and Golgi tendon organs.

These sensory end-organs are involved in various sensory functions, such as touch, pressure, vibration, and proprioception.

They are responsible for detecting and transmitting sensory information to the central nervous system, allowing us to perceive and interpret the world around us.

Question 25

Each Semicircular canal contains a sensory end-organ ridge.

Answer 25

*Rests in ampulla (swelling at base) near the end of each canal

*Hair cells extend from ridge into the cupula (gelatinous)

*Cupula protrudes into the endolymph

*Hair cells with 1 kinocilium and many stereocilia

*Endolymph of same density as cupula

Question 26

Sensory End-Organ and the Cupula in the Vestibular System

The sensory end-organ in the vestibular system includes the cupula, which is a gelatinous structure found in the semicircular canals of the inner ear.

The cupula is involved in detecting rotational movements of the head and plays a crucial role in our sense of balance and spatial orientation.

State some key functions of the CUPULA and describe its function.

Answer 26

1. Cupula Weightlessness:

The cupula is not affected by gravity and remains stationary when the head is not in motion.

2. Inertial Flow of Endolymph:

When the head rotates, the endolymph (fluid) within the semicircular canals lags behind due to inertia. This lag causes the endolymph to push or deflect the cupula in the opposite direction of the head movement.

3. Deflection of Stereocilia:

The physical movement of the cupula pushes the stereocilia of the hair cells within the cupula. Deflection of the stereocilia towards the kinocilium (a specialized cilium) depolarizes the hair cells, leading to the generation of electrical signals.

4. Inhibition of Hair Cells: Deflection of the stereocilia away from the kinocilium hyperpolarizes the hair cells, inhibiting their activity.
5. Directional Sensitivity:

The deflection of the cupula in response to head movement allows for the detection of the direction and speed of rotational movements.

Question 27

The moving head I

Signals from the semicircular canals fed to the extraocular eye muscles & help fix gaze on a target when the head is moving.

1. Observe partner’s eyes as they gaze at you.
2. While still looking at you, ask them to turn their head. Watch their eye movement.

What will happen?

Answer 27

When observing someone’s eyes while they gaze at you and then turn their head, you may notice vestibular nystagmus.

This is a reflex rotation of the eyeball in response to head rotation.

The eyes make a comparatively slow compensatory rotation (vestibulo-ocular reflex) to maintain gaze stability, followed by a quick resetting of the eyes to a new central position.

Question 28

The moving head II

1. Hold object at arm’s length
2. Focus on the object
3. Move the head (up & down, side to side) vigorously while trying to maintain focus on the object.
4. Note the apparent movement of the object
5. Hold the head still and move the object instead

What is the difference and how are the semicircular canals and extraocular eye muscles involved?

Answer 28

Semicircular Canals:

The semicircular canals are part of the vestibular system and are responsible for detecting rotational movements of the head.

They contain fluid-filled canals that are oriented in different planes. When you move your head, the fluid inside the canals lags behind due to inertia, causing the cupula (a gelatinous structure within the canals) to be deflected in the opposite direction of the head movement.

Vestibulo-ocular Reflex (VOR):

The vestibulo-ocular reflex is a reflex that helps stabilize gaze during head movements. It involves the coordination between the semicircular canals, vestibular nuclei, and extraocular eye muscles.

When the semicircular canals detect head movement, they send signals to the brainstem, which then activates the appropriate extraocular eye muscles to generate compensatory eye movements in the opposite direction of the head movement.

This helps to maintain gaze stability and keep the object in focus.

When you hold your head still and move the object instead, the semicircular canals detect the movement of the object and send signals to the brainstem. The brainstem then activates the appropriate extraocular eye muscles to generate compensatory eye movements that keep the object in focus.

In summary, the semicircular canals detect head movements, and the vestibulo-ocular reflex coordinates the movement of the extraocular eye muscles to maintain gaze stability and focus on a target. This interaction between the semicircular canals and extraocular eye muscles allows us to perceive the apparent movement of objects when we move our heads.

Question 29

Describe Otolith Organs.

Answer 29

1 utricle and 1 saccule in each ear

Small regions (swellings) at the base of semicirc canals.

Each contains a macula – small region containing hair cells & receptors innervated by vestibular afferents

Macula hair cells with celia embedded in gel mass

Gel contains otoconia or statoconia - dense Ca carbonate crystals => entire mass above cells more dense than endolymph

Mass thus affected by gravity or linear acceleration

The otolith organs, specifically the utricle and saccule, are part of the vestibular system in the inner ear. They play a crucial role in sensing linear movements, head tilt, and gravity. Here’s what we know about the otolith organs based on the search results:

Utricle and Saccule: The utricle and saccule are the two otolith organs in the human ear. They respond to gravitational forces and are also known as gravity receptors. Each sac has a patch of sensory cells called a macula, which monitors the position of the head relative to the vertical.

Sensory Epithelium: Both the utricle and saccule contain a sensory epithelium called the macula. The macula consists of hair cells and associated supporting cells. Overlying the hair cells and their hair bundles is a gelatinous layer, and above this is a fibrous structure called the otolithic membrane, which contains calcium carbonate crystals called otoconia or otoliths.

Role in Sensing Linear Movements: The otolith organs, particularly the utricle and saccule, sense linear head acceleration and head tilt. The utricle detects horizontal movements, while the saccule senses vertical motion.

Otoconia: The otolithic membrane contains otoconia, which are calcium carbonate crystals. These crystals make the otolithic membrane heavier than the surrounding structures and fluids. When the head tilts, gravity causes the otolithic membrane to shift relative to the sensory epithelium, leading to the stimulation of hair cells.

Vestibulospinal Reflex and Otoliths: The otolith organs provide primary input for the vestibulospinal reflex, which helps control posture and balance. They also contribute supplementary information, along with the semicircular canals, for the vestibulo-ocular reflex (VOR).

Role in Spatial Orientation and Memory: Emerging research suggests that the otoliths have a role in spatial orientation and memory.

Question 30

Describe Otolith Organs function.

Answer 30

1. Respond to linear and gravitational acceleration.
2. Detect angle (position) of head with respect to effective direction of gravity.
3. Effective direction of gravity includes other accelerations or decelerations
   gravity gravity
   Effective gravity
   *No stimulation - at rest
   *Reactive force of bus when bus moves  (linear acceleration)
4. Changes in linear motion irrespective of direction
5. Hair cells depolarized when stereocilia deflected towards kinocilium

Question 31

Describe the utricle.

Answer 31

Largest cavity of the vestibular labyrinth

Utricular macula lies on floor of utricle

At rest, in upright or upside position, no stimulation

At any other orientation with respect to gravity, hair cells stimulated

Hair cells also stimulated by change in horizontal linear motion

Microscopic exam: kinocilia oriented in different directions
*Suggests different head tilts stimulate different populations of afferents
*Some hair cells increase and others decrease firing frequency

Question 32

Discuss the saccule.

Answer 32

-Maculae lie on the sides of the sacculae & oriented approx in parasagittal plane

-Thus, strong stimulation to hair cells when head erect than on the sides/sleeping (little or no stim)

Also stimulated during vertical acceleration/deceleration

-Microscopic exam: kinocilia oriented in different directions

*Suggests different head tilts stimulate different populations of afferents
*Some hair cells increase and others decrease firing frequency

Question 33

Discuss Conditions associated with vestibular damage.

Answer 33

1. Can be due to vestibular neuritis, peripheral or central tumors or infarction, surgery, etc.
2. In chronic B/L loss of vestibular input, one may maintain balance when visual & somatosensory input are present but fail when support surface & visual surrounding sways.
3. However, in acute B/L vestibular loss or in pts who have not compensated for the loss, balance poor if either vision or somatic sense is sway-referenced.

Question 34

Discuss conditions associated with vestibular damage.

Answer 34

(a) Motion sickness

Motion sickness is a common condition characterized by symptoms such as nausea, dizziness, and vomiting that occur when there is a conflict between visual input and the body’s sense of motion.

(b) Meniere’s disease
*Increase in endolymph associated with endolymphatic distention

*Damage to the membranous labyrinth commonly occurs

*Possible viral infection?, but cause of syndrome often unknown

-Severe dizziness (vertigo) within minutes, subsiding within hours

-Unsteadiness may persist

-Pronounced ear ringing (tinnitus)

-Episodes occur at irregular intervals over years

-Loss of hearing eventually permanent

-Inability to stand upright

-Self spinning

-Other objects spinning

-Drugs, surgery may stop vertigo, but no treatment for hearing loss & tinnitus

Question 35

Discuss the Tonic Neck Reflex:

Answer 35

Tonic Neck Reflex:

*When the infant’s head is turned to a particular side, the leg and arm on that side will extend, while the leg and arm on the opposite side will flex.

For example, if the infant’s head is gently turned to the right side the right leg and right arm will slightly extend, while the left leg and left arm will slight flex.

It disappears around 4 months of age.

Question 36

Discuss the vestibulospinal reflex.

Answer 36

Definition: The vestibulospinal reflex is a reflex that uses the organs of the vestibular system (located in the inner ear) and the skeletal muscles to maintain balance and posture. It involves the transmission of signals from the vestibular nuclei in the brainstem to the interneurons in the anterior gray column of the spinal cord.

Types: The vestibulospinal reflex can be categorized into different types based on timing and sensory input. These include dynamic reflexes, static reflexes, and tonic reflexes. The reflex can also be categorized based on sensory input as either canal-based, otolith-based, or a combination of both.

Role in Posture Control: The vestibulospinal reflex is essential for maintaining postural stability and adjusting muscle activity in response to changes in head position and movement.

It helps to counteract the effects of gravity and maintain an upright posture.

The reflex contributes to the development of muscle tone, proprioceptive and vestibular senses, and opportunities to practice balance.

Clinical Implications: The vestibulospinal reflex is involved in various clinical assessments and conditions.

It is examined as part of the investigation of balance apparatus activity.

Tests such as the Romberg test, Hautant’s test, Fukuda’s stepping test, and writing test can be used to assess the vestibulospinal reflex and its impact on coordinated movement. Impairments in the vestibulospinal reflex can be observed in conditions such as Parkinson’s disease.

Question 37

Discuss Anticipatory postural control (reflexes).

Answer 37

-Occurs in anticipation of destabilization of the body

-Thus preparatory e.g. soleus, trunk muscles…

-Lacking in Parkinson’s disease