Healthy Adult Aging Lecture Notes (KNES 365) - University of Calgary

FACULTY OF KINESIOLOGY – KNES 365 Sensorimotor Neuroscience Lecture 19 Healthy Adult Aging Date: Thursday November 21, 2024  Outline ▪ Aging of the Motor System ▪ Aging of the Sensory Systems ▪ Implications: Balance Control and Falling Motor Performa...

FACULTY OF KINESIOLOGY – KNES 365 Sensorimotor Neuroscience Lecture 19 Healthy Adult Aging Date: Thursday November 21, 2024  Outline ▪ Aging of the Motor System ▪ Aging of the Sensory Systems ▪ Implications: Balance Control and Falling Motor Performance Declines with Age Average World Record Speeds (5 km) Open circles = Men Filled circles = Women Enoka, Figure 9.34  Aging of the Motor System Number of Motor Neurons in the Number of Motor Neurons in the Lumbosacral Spinal Cord (human cadavers) Tibialis Anterior Muscle (human) *Paradoxically, a reduction in motor neuron number is accompanied by larger amplitude EMG; this is due to a re-modelling process that results in a greater proportion of LARGE MUs. Enoka, Figure 9.35  Aging of the Motor System ▪ This re-modelling of the motor pool ends up in a general reduction in muscle mass (a process called sarcopenia) because some muscle fibres become deprived of innervation and are subsequently lost. As motorneuron number declines, the muscle fibres they innervated either die (atrophy) or get innervated by a branch of another motorneuron. This causes a shift toward LARGER MOTOR UNITS. Tresilian, Figure 2.42  Aging of the Motor System ▪ There is a larger atrophy effect in the lower-limbs compared to the upper-limbs (note the steeper slope for the filled circles in the graphs below). ▪ Below are MRI measurements of total muscle mass vs. age, starting the 5th decade of life. 268 Men 200 Women Lower-Limb = Filled Circles Upper-Limb = Open Circles Enoka, Figure 9.37  Aging of the Motor System ▪ The loss of muscle fibres is smallest for the Type I (slow, oxidative) muscle fibres – ~20% loss compared to about ~40% for other muscle fibre types.  Aging of the Motor System ▪ The loss of muscle fibres is smallest for the Type I (slow, oxidative) muscle fibres – ~20% loss compared to about ~40% for other muscle fibre types. CSA = Cross-Sectional Area b Black Bars = Young slow, fast, fast, Grey Bars = Old oxidative oxidative glycolytic Enoka, Figure 9.38  Aging of the Motor System ▪ In addition, older motor units fire at lower rates and older muscles have slower twitch contractions; therefore, older adults develop torque in their muscles at a slower rate. Enoka, Figure 6.16  Aging of the Motor System ▪ In addition, older motor units fire at lower rates and older muscles have slower twitch contractions; therefore, older adults develop torque in their muscles at a slower rate. Klass et al (2008) found that the maximal rate of contraction in older adults dropped by ~50%, and was associated with a ~30% increase in the intervals between first four APs. *dorsiflexion torque Open Circles = Young Filled Circles = Old Enoka, Figure 9.39  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging.  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 1: Vestibulo-ocular reflex gain is slightly reduced (due to central gain enhancement). *HVOR gain measured for head rotations. Note: despite losing ~40% of all vestibular hair cells and afferents by age 60-70, there is very little drop in the VOR gain! This is likely because the CNS (e.g., vestibular nucleus) has “turned up the volume” on the remaining receptors (this is the idea of CENTRAL GAIN ENHANCEMENT). Peterka et al. 1990  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 1: Vestibulo-ocular reflex gain is ONLY slightly reduced (due to central gain enhancement). Note: Ink dots on the white of the eye (sclera) are used to measure eye torsion. *A torsional VOR is observed with the traditional EVS setup; importantly, the EVS-evoked VOR by-passes mechanotransduction, and therefore, it has been used as a measure of CENTRAL GAIN. Jahn et al. 2003  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 1: Vestibulo-ocular reflex gain is ONLY slightly reduced (due to central gain enhancement). VOR GAIN TO EVS!! Note: loss of hair cell is continuous, but the afferent pathway starts degenerating later. Jahn et al. 2003  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 2: Gain enhancement is also observed in the vestibulo-spinal reflex (as assessed using SVS). Stochastic Vestibular Stimulation (SVS) is a form of electrical vestibular stimulation where small amplitude, noisy currents are applied and balance responses are measured in the legs with EMG or force plates. Dalton et al. 2014  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 2: Gain enhancement is also observed in the vestibulo-spinal reflex (as assessed using SVS). Grey = Older Adults Black = Young Adults Soleus Medial Gastrocnemius Tibialis Anterior Gain Frequency (Hz) *Note that older adults again had greater gain to EVS in the 0.5 to 5 Hz range compared to young adults (these are VERY important frequencies for the vestibular system, as these are the frequencies of head movement that occur naturally: the PHYSIOLOGICAL RANGE of head movements). Dalton et al. 2014  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 3: Reduced perceptual sensitivity to head rotations and translations (again, counteracted by central gain enhancement). Rotation Stimuli Performance Plot Psychometric Curve Angular Velocity (deg/s) Angular Velocity (deg/s) ”Which direction were you rotated, LEFT or RIGHT?” Peters et al. 2016  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 3: Reduced perceptual sensitivity to head rotations and translations (again, counteracted by central gain enhancement). REAL ROTATIONS Note: 0.1 Hz is really outside of the Note: The difference typical range of vestibular stimulation between age groups experienced naturally, however, 1 Hz is smaller at 1 Hz, is right in this physiological range. which we went on to Threshold for young adults show is likely due were 3 deg/s at 0.1 Hz, and to CENTRAL GAIN 1.3 deg/s at 1 Hz. ENHANCEMENT for Threshold for older adults frequencies that are were 7.2 deg/s at 0.1 Hz, and most important and 2.4 deg/s at 1 Hz. common in daily life *The effect at 1 Hz failed to (~0.5 to 5 Hz). reach significance (p = 0.08). Peters et al. 2016  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 3: Reduced perceptual sensitivity to head rotations and translations (again, counteracted by central gain enhancement). Note: Positioning the head downwards on the platform allowed the EVS-rotation vector (which roughly goes through the middle of the face) with the axis of the rotary chair. The result is that, in the dark, real and virtual rotation are nearly indistinguishable. Peters et al. 2016  Aging of the Sensory Systems ▪ There is a ~40% loss of hair cell receptors and ~40% loss of vestibular nerve fibres with aging. ▪ Effect 3: Reduced perceptual sensitivity to head rotations and translations (again, counteracted by central gain enhancement). VIRTUAL ROTATIONS (EVS) Young Older Note: Older adults were more sensitive to virtual Threshold for young adults rotations evoked with EVS, were 0.5 mA at 0.1 Hz, and and as expected, it was the 1.1 mA at 1 Hz. 1 Hz stimuli that showed the biggest effect (right in Threshold for older adults the important frequency were 1.2 mA at 0.1 Hz, and range for the vestibular 0.6 mA at 1 Hz. system – ~0.5 to 5 Hz). Peters et al. 2016  Aging of the Sensory Systems ▪ During aging, Macular Degeneration occurs, leading to a gradual reduction in visual acuity (letter chart). Young Healthy Retina Macular Degeneration The fovea is a tiny pit in the retina aligned with the central axis of the lens, whereas the macula is a larger area including and Yellow spots indicate build- ups of lipids and proteins surrounding the fovea. called Drusen, which damage photoreceptors in the macula.  TOP HAT TIME! Please login to your Top Hat accounts now!  Aging of the Sensory Systems ▪ The eyes also become misshapen, resulting in the focus point of light entering the eye to land before or after the retina causing blurriness of vision. Eye-ball too short or cornea not curved enough. Eye-ball too long or cornea too curved. Hyperopia (farsightedness) Corrective Lenses  Aging of the Sensory Systems ▪ With age, there is also a reduction in the Useful Field of Vision, which is the leading predictor of accidents in older adult drivers…. To probe the Useful Field of Vision, participants are required to detect a peripheral signal while fixating on a central signal. Peripheral signal size and location is varied to change the difficulty level. Roge ́ et al 2004  Driving a Car ▪ Useful Field of Vision refers to the area of the visual scene from which you are able to process information.  Aging of the Sensory Systems ▪ Cutaneous input from the foot soles is critical for human balance control. Peters et al. 2017  Aging of the Sensory Systems Summary ▪ Measured vibration detection thresholds, cutaneous reflexes, and standing balance stability (Posturography) in young (mean age = 26) and older (mean age = 68) adults. ▪ Older adults had higher detection thresholds (needed LARGER vibrations in order to feel them), and WEAKER cutaneous reflexes. ▪ Importantly, detection thresholds correlated with postural sway in the Anterior- Posterior plane, making sensitivity of the foot sole an important predictor for standing balance stability. Peters et al. 2017  Balance Control and Falling 1:58 minute mark – Paul Zehr (UVic) 9 minute mark – Bill McIroy (Waterloo) https://www.youtube.com/watch?v=X3meWWwaLss  Balance Control and Falling ▪ 1-in-3 adults aged 65 and older falls each year, making falling the number one killer of older adults. ▪ The CDC estimates that falls will account for 54.9 billion dollars a year in costs to the U.S. Government by 2020. ▪ 2.8 million injuries, 800,000 hospitalizations, 30,000 deaths per year!