WEEK 1: Posture and balance Flashcards
What is posture?
A position of the body (Taber’s Cyclopedic Medical Dictionary).
Describe upright posture.
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
Control of posture - What is a stable posture?
*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
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?
Eye, ears, skin, skeletal muscles, brain stem.
Discuss the role of the brainstem in posture & balance.
- Inputs come from the :
*EYE: Visual input
*EARS: Vestibular input
*RECEPTORS IN THE SKIN: Cutaneous input
*RECEPTORS IN JOINTS AND MUSCLES: Proprioceptive inputs - The inputs are sent to the vestibular nuclei in the brainstem.
- 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 - 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.
Discuss Integrated control of posture.
- RETINA: Eye position & velocity in visual space
- Oculomotor System: Eye position & velocity in head
- Semicircular Canals: Head movements
- Otolith Organs: Head position relative
to gravity - Neck Proprioceptors: Head position relative to body
- Skin Pressure Receptors: Integrated sense of body position in space
Discuss Several contributors to posture.
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
- 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
- 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.
- 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
- 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
Visual System & Posture
- Get a partner
- Stand on one leg with eyes open - partner observes.
- Repeat with eyes closed
- What’s the difference?
- What can we conclude?
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.
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?
- 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.
- 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.
Describe Vestibulo-ocular system.
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)
- 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.
Vestibulo-ocular system.
What does it rely on?
When is it most effective?
How does it compensate for head rotation?
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.
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?
-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.
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?
*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.
Ablation & transection studies
Define ablation.
Ablation = “removal of a component of an input or an upper level of control from the circuit”
- Ablation studies involve the removal or destruction of specific areas of the CNS to observe the effects on postural control.
- 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.
State the significance of ablation.
*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.
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.
- Below brainstem (SC level)
- Above Medulla (thru pons)
- Pinna stimulated
- 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
- Above mesencephalon (midbrain)
- Decorticate animal (w/out cerebral cortical tissue)
Charles Sherrington: systematic, progressive separation of the neuraxis (at different levels) resulted in differing degrees of postural correction (“righting reactions”):
- Below brainstem (SC level) – limp posture, no correction
- Above Medulla (thru pons) – rigid posture (tonic contraction of antigravity limb extensors, trunk, neck) exaggerated standing posture. However,
- Pinna stimulated – turns head to opposite side & corrective limb movements.
- Paw stimulated – turns head to same side and corrective limb movements.
- Above mesencephalon (midbrain) – effective righting in chronically surviving animals
- Decorticate animal (w/out cerebral cortical tissue) – observed many normal postural reactions, and lacked only some placing and stepping reactions
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”).
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.
Lateral & ventromedial systems
Discuss Lateral system.
Originates in red nucleus.
For corticospinal control of limbs
For fine movements and dexterity
- 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.
- It is responsible for the fine movement of ipsilateral limbs, although it lies contralateral to the corresponding motor cortex.
- The lateral corticospinal tract is the largest part of the corticospinal tract and extends throughout the entire length of the spinal cord.
- 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)
Describe the ventromedial system.
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
State the components of the vestibular apparatus.
Semicircular canals (3 in each ear)
Otolith organs (1 utricle and 1 saccule in each ear)
What is the vestibular apparatus needed for?
Provides information needed for:
-Sense of equilibrium (balance)
-Integrating movement of the head with those of the eye & posture
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?
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.
State semicircular canal specifically separately for the following.
- Shaking
- Nodding
- Moving the ear to the shoulder
- Shaking [“no”] of the head (horizontal = lateral)
- Nodding (posterior)
- Moving the ear to the shoulder (superior = anterior)
What is a Sensory end-organ?
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.
