Lecture 8: Control of Posture & Balance PDF

Lecture 8: Control of posture & balance ● Center of gravity (COG): ○ place in the middle of the object, which has to be within a limit of stability (LOS) ○ in humans is in lower abdomen ○ if we draw our LOS, it forms a circle (almost). ○ in order to stay upright, COG cannot fall beyond the LOS ○ if...

Lecture 8: Control of posture & balance ● Center of gravity (COG): ○ place in the middle of the object, which has to be within a limit of stability (LOS) ○ in humans is in lower abdomen ○ if we draw our LOS, it forms a circle (almost). ○ in order to stay upright, COG cannot fall beyond the LOS ○ if COG falls beyond the limit, you will fall, unless you move your limbs to make your “base” bigger. ○ there are devices that measure the pressure our feet exert on the surface. ○ it shows that our body does not oscillate much, even when compared to cats/dogs who have a shorter COG height. ○ what does this mean? it means that we have a powerful homeostatic mechanism to prevent our COG from moving out of the LOS ● Principal components of the posture stabilization system ○ Environmental interaction → will disturb our stability , which will be detect by: 1) Visual system (most imp. for balance) 2) Vestibular system (least imp) 3) Somatosensation (mainly in our feet) ○ These 3 systems give feedback to the brain to compare select & combine these senses to determine the best body position for the body ○ Brain chooses which body movement to do → select & adjust muscle contractile patterns (via cortex & cerebellum) → ankle, thigh, trunk muscles → generation of body movement → again ○ Ex: you want to throw a ball ■ cortex & cerebellum determine the movement of your body ■ move your ankle, thigh, & trunk muscles ■ this will require adjustment of body posture, which is determined by the 3 systems. ■ these 3 systems give feedback to the brain → then the brain will determine the ideal body position to do the movement ● Computerized dynamic posturography: To test the 3 systems to see how imp. they are: ○ put the subject on a surface that can move, and it detects the pressure while in front there is a curved screen which virtually project something (ex. street) ○ We can control: ■ Open or close the subject’s eye (visual system) ■ sway referencing: project a situation on the screen where everything is moving forwards or backwards. E.g. shows a street moving forward (so visual system tells you that you are moving forward, but you are actually not) ■ make surface uneven/angled → give information to somatosensory system ○ 100% = the pressure that my feet exert on the surface did not change at all (subject was stable) ○ if you move or are wobbly, you exert less pressure, % drops ○ if you fall = pressure is 0 ○ Results: check pics ■ conditions (1, 2, 3, 4) are very successful in maintain balance ■ 5 & 6 are a bit less successful (where only vestibular sys. is functioning). ■ vestibular system is least important out of the 3 in maintaining balance ○ in case of bilateral damage to vestibular system: ■ as long as vision or somatosensory system from feet is preserved ■ (1, 2, 3, 4), balance is good (they are more important) ■ when they’re not preserved → FALL ■ if you compare condition 5 & 6, you can see that the vestibular system still plays an important role ○ graph 1: only vestibular system intact → maintained posture ○ graph 2: no vestibular system → fall ● Surface-electrode recordings of muscle tone during various maneuvers: ○ muscle activity is triggered very fast. how is that possible? bc we already have 2 main patterns on how to maintain balance (ankle & hip strategy) ○ Ankle strategy: ■ Only when we have enough surface for support ■ ankle strategy invokes muscles so that the upper body becomes rigid & the only muscles that move the body are muscles of lower feet (gast.) ● Ex: when the surface is moving forward or backwards you sway the opposite way so you won't fall ■ (does not involve moving hips/changing the angle of hips) ○ Hip strategy: ■ don’t have enough surface for support ■ = make an angle with hip ■ when surface moves, you bend your body to oppose the movement ○ It takes <100 ms to get a reaction/movement → involuntary ○ Muscles engaged are opp. to the direction of the movement, so the brain prepared for when there is disturbances of balance when surface is moving ○ The cerebellum has no primary role in posture & balance (not involved in this reflex) but has role in adjusting the ongoing movement ● The Vestibular System, Animationwatch this <3 The vestibular system: ○ a system of 5 different sensors: utricle, saccule & 3 semicircular canals ○ utricle & saccule (otolith organs) → detect linear movements/acceleration ■ have many epithelial cells with their cilia orientated in ALL different directions (bc cilia can only send signal to brain if it moves in the 2 directions [drawing], so there are many cilia orientated in all directions so you can sense all the movements) ● vs in semicircular canal: all in one direction bc they only want to detect if the endolymph/head is moving left to right or right to left ■ midline = striola → cilia on one side are mirror image of cilia of other side ● In utricle → cilia in either side of striola are polarized toward the striola ● In saccule → cilia in either side of striola are polarized away from the striola ■ Bcs the striola in each otolith organ is curved, there are hair cells with all orientations in the plane of the organ. When the head is tilted to a new position, the orientation of the otolithic membranes in relation to the gravitational vector changes & so the cilia of the hair cells changes the pattern of input from the otolith organs to the CNS & creates the sensation of the movement, as well as possibly triggering various reflexes. Similarly, linear acceleration caused by other forces, such as might occur in a fall or the angular acceleration when a car turns around a curve (angular accelerations have linear centripetal & instantaneous tangential components) also affects output from the otolith organs. ○ 3 semicircular canals → detect angular acceleration ■ canals are perpendicular to each other ■ imp bc angular acceleration is detected by: difference in the movement in the left vs right side ■ each semicircular canal has an enlargement called an ampulla ■ inside ampulla is layer of epithelial cells (SENSORS) = hair cells = long stereocilia & kinocilium (the longest) going through thick jelly substance called cupula ■ the canal is filled with endolymph (very rich in K+, poor in Na; which is opposite to ECF/other fluids of body) ■ cilia are orientated in same direction that canal is going (parallel not perpendicular) ■ angular acceleration (move head) → fluid moves back & forth → bends cupula → bends cilia (only in 2 directions) → endolymph pressure to the opp side & deflection of the cupula to that side → send signal to brain ● Vestibular transduction: ○ if you move your head one way, fluid (& bending of cilia) will move in opposite direction → ending of the cilia stimulates neurons just beneath them: ■ no bending: resting rate of firing ■ bend to one side: ↑ rate of firing ■ bend to opp. side: ↓ rate of firing ○ Why? ■ cilia are rich in K channels that are linked to the cilium via proteins (threads) ■ normally (cilia in upright position), these channels are half open, so little K is leaking inside → depolarization → resting rate of firing ■ when cilia bend towards longest cilium (kinocilium): the cilia PULL on the protein thread & open the channels → depolarization → ↑ rate of firing ■ cilia bend to opp side: less pulling → channels close → ↓ rate of firing ■ ↑ concentration K go inside in the endolymph & since they’re +ve they cause depolarization ○ so if you move your head in direction: ■ one side will send signal to brain with↑ rate of firing ■ opposite side will have proportionally ↓ rate of firing ■ by comparing signals from these 3 pairs (left & right) (of semicircular canals), our brain can determine exactly how our head is moving ○ the neurons (bipolar & myelinated) that innervate those epithelial cells come from vestibular aka Scarpa’s ganglion → axons go to vestibular nuclei → to many pathways (in order from most to least imp): ■ vestibulo-ocular pathway: automatically adjusts the gaze (if your head moves, eyes move to become fixed to the object you are looking at) ● innervates the CNs related to eye muscles: CN3, 4 & 6 ■ Vestibulospinal ● medial (for muscles of neck) & lateral (for antigravity muscles ie extensors) ■ cerebellum: MODULATION of ongoing movements (NOT for immediate maintains of balance) ■ reticular formation: causes brain to “wake up” eg. you sleep on your hand & it slips → you immediately wake up ■ cerebral cortex through the thalamus ■ vestibular nuclei of OPPOSITE side: to get integration of what happens on both sides ○ In conclusion: 1) Cilia bends 2) K+ ↑ or ↓ 3) Membrane de/hyper polarizes 4) Ca+ enter 5) Vesicle with glutamic acid fuse & release 6) Neurons with cell bodies in Scarpa’s ganglion get excited from there they send signals to different areas of the brain ● Connections of the vestibular system to the cerebellum: ○ vestibular ganglion has 2 sets of ganglions: ■ one set: synapse in vestibular nuclei then go to cerebellum ■ other set: goes from the vestibular nuclei DIRECTLY to the cerebellum. this shows how important it is to adjust the ongoing movements immediately ○ red nucleus function: ■ what fine tuning by cerebellum does is affecting the red nucleus so the distal muscles of our limbs can get fine tuned (inhibiting extensors & stimulating flexors) ■ this signal may shut down red nucleus to allow antigravity muscles to contract ● Postural reflexes (VSR, Vestibulospinal reflexes) ○ medial vestibulospinal tract causes contractions of neck muscles that oppose the induced movement ○ lateral vestibulospinal tract activates extensor muscles that support posture ○ Semilunar canals → vestibular afferents → vestibular nuclei → thalamus/cerebellum & MRST/VST → neck motor neurons → neck muscles ● Main tracts involved in the control of posture & balance: ○ corticospinal tract (least imp in posture & balance, bc its involved in VOLUNTARY movement) ○ Rubruspinal & tectospinal tract (very imp) ~ visual ○ Vestibulospinal tract ● Gaze stabilization system (VOR, vestibuloocular reflex) (explained in later lectures) ○ rotation of the head → one signal on (eg. right side & opposite signal on left side) ○ eg. excitation of left vestibular nucleus & inhibition of right vestibular nucleus → go to CNs of eye: ■ left side: excitation of CN3 & inhibition of CN6 ■ right side: inhibition of CN3 & excitation of CN6 → eyes move in OPPOSITE DIRECTION of head movement ○ vestibular system tells the vestibular nuclei that the head is moving to the right, so then the vestibular nuclei sends info to CNs of the eye to make them move the eye in opposite direction (left) in order to keep the gaze focused on one object ● Asymmetric tonic neck reflexes: ○ if you turn/rotate the baby’s head to one side, there will be: ■ extension of limbs on that side ■ flexion of the limbs on other side ● Decerebrate and decorticate rigidity (see pic) ○ A circuit drawing representing lesions produced in experimental animals to replicate decerebrate & decorticate deficits seen in humans ○ cerebral cortex is SUPPRESSING tone of the muscles (that’s why in UMN → hypertonia/reflexia) so in both A & D you are cutting off the motor cortex so you get ↑ tone ■ D: damage to upper pons ABOVE red nucleus (ie still active) → decorticate posture ● bc of vestibulospinal tract is active → anti-gravity muscles enhanced ● no damping from cortex ● red nucleus disinhibited → flexion of upper limbs ■ A: damage to lower pons BELOW red nucleus → decerebrate posture ● no red nucleus: no more inhibition to extensors in upper limbs ■ C: cut at cerebellum: ● what is cerebellum doing? activates red nucleus to inhibit extensors ● so no cerebellum → no inhibition of extensors → the rigidity will be WORSE ■ B: cut gamma motor neurons: ● Righting reflex: ○ maintains head upright relative to how the trunk is moving ○ Normal: move trunk → head always stays upright ● ● ● func: determine tone of muscles so cutting it → NO rigidity What's the mechanism? the vestibulospinal tract acts on 𝛄 MN to ↑ tone of the muscles