Neurophysiology of balance

Role of various systems to maintain balance.
Role of sensory systems-vision,proprioceptors,vestibular
Role of Musculoskeletal system
Biomechanics in balance
Contextual factors in balance
Role of nervous system
Strategies-ankle, hip,stepping

1. N europhysiology of Balance By- Dr.Ranjeet Singha,PT(MPT in Neurology) HAAD Licensed Associate Professor, College of Physiotherapy and Medical Sciences, Guwahati,Assam .

2. Topics: Balance Role of sensory systems- vision,proprioceptors,vestibular Role of Musculoskeletal system Biomechanics in balance Contextual factors in balance Role of nervous system Strategies-ankle, hip,stepping

3. Balance  refers to an individuals ability to maintain their line of gravity within their  Base of support  (BOS).  It can also be described as the ability to maintain equilibrium, where equilibrium can be defined as any condition in which all acting forces are cancelled by each other resulting in a stable balanced system.

4. Variation in Terminologies In literature the balance term has been used synonymously with: Postural Control Postural Stability Equilibrium

5. Balance, or postural stability, is a generic term used to describe the dynamic process by which the body’s position is maintained in equilibrium. Equilibrium means that the body is either at rest (static equilibrium) or in steady-state motion (dynamic equilibrium).

6. Balance is greatest when the body’s center of mass (COM) or center of gravity (COG) is maintained over its base of support (BOS).

7. Balance Control Balance is a complex motor control task involving the detection and integration of sensory information to assess the position and motion of the body in space and the execution of appropriate musculoskeletal responses to control body position within the context of the environment and task. Thus, balance control requires the interaction of the nervous and musculoskeletal systems and contextual effects

8. ■ The nervous system provides the (1) sensory processing for perception of body orientation in space provided mainly by the visual, vestibular, and somatosensory systems; (2) sensorimotor integration essential for linking sensation to motor responses and for adaptive and anticipatory (i.e., centrally programmed postural adjustments that precede voluntary movements) aspects of postural control; and (3) motor strategies for planning, programming, and executing balance responses

9. Balance systems The following systems provides input regarding the body's equilibrium and thus maintains balance. Somatosensory /  Proprioceptive  System Vestibular System Visual System

10. The Central Nervous System receives feedback about the body orientation from these three main sensory systems and integrates this sensory feedback and subsequently generates a corrective, stabilizing torque by selectively activating muscles. In normal condition, healthy subjects rely 70% on somatosensory information and 20% Vestibular & 10% on Vision on firm surface but change to 60% vestibular information, 30% Vision & 10% somatosensory on unstable surface.

11. Sensory Systems and Balance Control Perception of one’s body position and movement in space require a combination of information from peripheral receptors in multiple sensory systems, including the visual, somatosensory ( proprioceptive , joint, and cutaneous receptors), and vestibular systems.

12. Visual System The visual system provides information regarding (1) the position of the head relative to the environment; (2) the orientation of the head to maintain level gaze; and (3) the direction and speed of head movements, because as your head moves, surrounding objects move in the opposite direction.

13. Visual stimuli can be used to improve a person’s stability when proprioceptive or vestibular inputs are unreliable by fixating the gaze on an object. Conversely, visual inputs sometimes provide inaccurate information for balance control, such as when a person is stationary and a large object, such as a nearby bus, starts moving, causing the person to have an illusion of movement.

14. For non-impaired individuals, under normal conditions the contribution of visual system to postural control is partially redundant as the visual information has longer time delays as long as 150-200 ms.Friedrich et al.observed that adults with visual disorders were able to adapt peripheral, vestibular, somatosensory perception and cerebellar processing to compensate for their visual information deficit and to provide good postural control.

15. In addition, Peterka found that adults with bilateral vestibular deficits can enhance their visual and proprioceptive information even more than healthy adults in order to reach effective postural stability. The influence of moving visual fields on postural stability depends on the characteristics of the visual environment, and of the support surface, including the size of the base of support, its rigidity or compliance

16. Somatosensory System The somatosensory system provides information about the position and motion of the body and body parts relative to each other and the support surface. Muscle proprioceptors , including muscle spindles and Golgi tendon organs (sensitive to muscle length and tension), joint receptors (sensitive to joint position , movement, and stress), and skin mechanoreceptors (sensitive to vibration, light touch, deep pressure, skin stretch ),are the dominant sensory inputs for maintaining balance when the support surface is firm, flat, and fixed

17. However, when standing on a surface that is moving (e.g., on a boat) or on a surface that is not horizontal (e.g., on a ramp), inputs about body position with respect to the surface are not appropriate for maintaining balance; therefore, a person must rely on other sensory inputs for stability in these conditions.

18. Information from joint receptors does not contribute greatly to conscious joint position sense. It has been demonstrated that local anesthetization of joint tissues and total joint replacement does not impair joint position awareness .

19. Muscle spindle receptors appear to be mostly responsible for providing joint position sense, whereas the primary role of joint receptors is to assist the gamma motor system in regulating muscle tone and stiffness to provide anticipatory postural adjustments and to counteract unexpected postural disturbances.

20. Proprioceptive information from spino-cerebellar pathways, processed unconsciously in the  cerebellum , are required to control postural balance.   Proprioceptive information has the shortest time delays, with monosynaptic pathways that can process information as quickly as 40–50 ms  and hence the major contributor for postural control in normal conditions.

21. Vestibular System The vestibular system provides information about the position and movement of the head with respect to gravity and inertial forces. Receptors in the semicircular canals ( SCCs) detect angular acceleration of the head, whereas the receptors in the otoliths (utricle and saccule ) detect linear acceleration and head position with respect to gravity.

22. The SCCs are particularly sensitive to fast head movements, such as those made during walking or during episodes of imbalance (slips, trips, stumbles), whereas the otoliths respond to slow head movements, such as during postural sway.

23. By itself, the vestibular system can give no information about the position of the body. For example, it cannot distinguish a simple head nod (head movement on a stable trunk) from a forward bend (head movement in conjunction with a moving trunk).

24. Consequently, additional information, particularly from mechanoreceptors in the neck, must be provided for the central nervous system (CNS) to have a true picture of the orientation of the head relative to the body.

25. The vestibular system uses motor pathways originating from the vestibular nuclei for postural control and coordination of eye and head movements.

26. The  vestibular system  generates compensatory responses to head motion via: Postural responses ( Vestibulo -Spinal Reflex) - keep the body upright and prevent falls when the body is unexpectedly knocked off balance. Ocular-motor responses ( Vestibulo -Ocular Reflex) - allows the eyes to remain steadily focused while the head is in motion. ( Vestibulo -Colic Reflex) - help keep the head and neck centred, steady, and upright on the shoulders. To achieve this the vestibular system measures head rotation and head acceleration through semicircular canals and otolith organs (utricle and saccule ).

27. Sensory Organization for Balance Control Vestibular , visual, and somatosensory inputs are normally combined seamlessly to produce our sense of orientation and movement. Incoming sensory information is integrated and processed in the cerebellum, basal ganglia, and supplementary motor area. Somatosensory information has the fastest processing time for rapid responses, followed by visual and vestibular inputs.

28. When sensory inputs from one system are inaccurate owing to environmental conditions or injuries that decrease the information-processing rate, the CNS must suppress the inaccurate input and select and combine the appropriate sensory inputs from the other two systems. This adaptive process is called sensory organization. Most individuals can compensate well if one of the three systems is impaired; therefore, this concept is the basis for many treatment programs.

29. Role of Musculoskeletal system Muscle Skeletal framework integrity Joints integrity

30. ■ Musculoskeletal contributions include postural alignment, musculoskeletal flexibility such as joint range of motion (ROM), joint integrity, muscle performance (i.e., muscle strength, power, and endurance)

31. Biomechanics in balance COG BOS Limit of stability Alignment(LOG position affected) Type of muscle contraction/work(Isometric ,concentric or eccentric)

32. Center of mass The COM is a point that corresponds to the center of the total body mass and is the point at which the body is in perfect equilibrium. It is determined by finding the weighted average of the COM of each body segment

33. Center of gravity The COG refers to the vertical projection of the center of mass to the ground. In the anatomical position,the COG of most adult humans is located slightly anterior to the second sacral vertebra or approximately 55% of a person’s height

34. Base of support The BOS is defined as the perimeter of the contact area between the body and its support surface; foot placement alters the BOS and changes a person’s postural stability. A wide stance, such as is seen with many elderly individuals, increases stability, whereas a narrow BOS, such as tandem stance or walking, reduces it. So long as a person maintains the COG within the limits of the BOS, referred to as the limits of stability, he or she does not fall.

35. Limits of stability “Limits of stability” refers to the sway boundaries in which an individual can maintain equilibrium without changing his or her BOS . These boundaries are constantly changing depending on the task, the individual’s biomechanics, and aspects of the environment.

36. Contextual factors in balance Contextual effects that interact with the two systems are the environment whether it is closed (predictable with no distractions) or open (unpredictable and with distractions), the support surface (i.e., firm versus slippery, stable versus unstable, type of shoes), the amount of lighting, effects of gravity and inertial forces on the body, and task characteristics (i.e., well-learned versus new, predictable versus unpredictable, single versus multiple tasks).

37. Even if all elements of the neurological and musculoskeletal systems are operating effectively, a person may fall if contextual effects force the balance control demands to be so high that the person’s internal mechanisms are overwhelmed.

38. Role of nervous system Cerebellum Higher center

39. Strategies for balance Ankle Hip Stepping

40. Types of Balance Control Functional tasks require different types of balance control, including (1 ) static balance control to maintain a stable antigravity position while at rest, such as when standing and sitting ; (2) dynamic balance control to stabilize the body when the support surface is moving or when the body is moving on a stable surface, such as sit-to-stand transfers or walking; and (3) automatic postural reactions to maintain balance in response to unexpected external perturbations, such as standing on a bus that suddenly accelerates forward .

41. Feedforward (open loop motor control) is utilized for movements that occur too fast to rely on sensory feedback (e.g., reactive responses) or for anticipatory aspects of postural control. ■ Anticipatory control involves activation of postural muscles in advance of performing skilled movements, such as activation of posterior leg and back extensor muscles prior to a person pulling on a handle when standing30 or planning how to navigate to avoid obstacles in the environment. ■ Closed loop control is utilized for precision movements that require sensory feedback (e.g., maintaining balance while sitting on a ball or standing on a balance beam).

42. Motor Strategies for Balance Control To maintain balance, the body must continually adjust its position in space to keep the COM of an individual over the BOS or to bring the COM back to that position after a perturbation. Horak and Nashner described three primary movement strategies used by healthy adults to recover balance in response to sudden perturbations of the supporting surface (i.e ., brief anterior or posterior platform displacements) called ankle, hip, and stepping strategies .

43. Results of research examining the patterns of muscle activity underlying these movement strategies suggest that preprogrammed muscle synergies comprise the fundamental movement unit used to restore balance. A synergy is a functional coupling of groups of muscles, so they must act together as a unit; this organization greatly simplifies the control demands of the CNS. The CNS uses three movement systems to regain balance after the body is perturbed: reflex, automatic, and voluntary systems. ■ “Stretch” reflexes mediated by the spinal cord comprise the first response to external perturbations. They have the shortest latencies (70 ms), are independent of task demands, and produce stereotyped muscle contractions in response to sensory inputs .

44. ■ Voluntary responses have the longest latencies (>150 ms),are dependent on task parameters, and produce highly variable motor outputs (e.g., reach for a nearby stable support surface or walk away from a destabilizing condition). ■ Automatic postural reactions have intermediate latencies (80 to 120 ms) and are the first responses that effectively prevent falls. They produce quick, relatively invarianmovements among individuals (similar to reflexes), but they require coordination of responses among body regions and are modifiable depending on the demands of the task (similar to voluntary responses). The reflex, automatic, and voluntary movement systems interact to ensure that the response matches the postural challenge.

45. References: Kisner Physiopedia Articles from Google site Youtube