Week 13 - Ageing Neuromechanics BW PDF

The neuromechanics of ageing MEDI258: HUMAN NEUROMECHANICS Learning outcomes u Understand that changes in physical capabilities, both during childhood and older adulthood, are linked to changes in the CNS and musculoskeletal systems u Understand that a ‘critical period’ of learning...

The neuromechanics of ageing MEDI258: HUMAN NEUROMECHANICS Learning outcomes u Understand that changes in physical capabilities, both during childhood and older adulthood, are linked to changes in the CNS and musculoskeletal systems u Understand that a ‘critical period’ of learning exists and is related to large-scale structural changes within the CNS u Describe some of the common changes in motor function that occur in older adulthood and ways in which negative changes can be minimised Musculoskeletal development Musculoskeletal development u Our size and proportionality change tremendously over the first few years of life u Body dimensions (e.g. head size) constrain movement abilities u Changes in body dimensions, bone size, muscle capacity and nervous system organisation occur in parallel u These parallel changes allow a rapid expansion in behavioural repertoire 5 Bone ossification Osteoclasts destroy bone tissue in the core of the bone, creating the bone marrow cavity The number of osteoclasts must be tightly regulated to avoid excess bone removal – this results in osteoporosis 6 Epiphyseal growth plates u Allow bone growth to continue after birth u Can be damaged by large compressive loads u Growth plate closure is caused by high levels of estrogen or testosterone u Estrogen plays a critical role in growth plate closure even in males 7 Muscle fibre development Myoblast fusion occurs as membrane proteins allow increased calcium flow between cells Some myoblasts remain unfused and become critically involved in regenerating muscle fibres after injury Muscle capacity u Children produce less force than adults, even after size- normalisation u Children less able to recruit Type II muscle fibres u Why? u Children’s muscles contain a smaller proportion of Type IIX muscle fibres u Smaller forces with lower rate of force production, but less fatiguable Main Points u Children’s bones and muscles develop in parallel due to continued differentiation of developmental cells (chondrocytes and myoblasts) u Children have a lower capacity for muscular force production, due to having fewer type II muscle fibres and potentially reduced frequency of firing of inputs to motoneurons Nervous system development Post-natal brain development u At birth, the brain is ~25% of adult brain size u Contains as many neurons as an adult brain u By 3 years old, the brain is ~80% of adult brain size u Why the volume increase? u Neural development proceeds from the brain to the periphery (cephalocaudally) Changes in neuronal communication u During the first year of life, the number of synapses formed between neurons in the cortex more than doubles u Indicates increased and refined patterns of communication u Within areas of similar function u Between functionally distinct areas u Changes in visual cortex allow improved vision u Changes in sensorimotor cortex allow improved proprioception and movement The importance of synapse elimination u The maturation of the corticospinal tract and cerebellum is critically involved in the development of motor skills u Elimination of terminations in the spinal cord are critical u Also, growth of new terminations u The cerebellum triples in volume in first year, cerebrum increases by 1/3 u These processes are guided by activity (basic movements and sensory feedback) u Neural pruning Cerebellar development Main Points u Infant brain development is associated with rapid synapse formation, synapse elimination and myelination – not with increases in the number of neurons. u Synapse formation and elimination are guided by sensory feedback. u The cerebellum undergoes more change in the first year of life than the rest of the brain. This involves growth of axons and dendrites, as well as synapse formation. Behavioural development Midbrain reflexes u Righting reflexes u Labyrinthine righting reflex u Gravity u otolith organs u head kept level against gravity u Neck righting reflex u Stretching of neck muscles u righting of thorax, shoulders and pelvis u body remains tilted, the neck muscles are stretched and initiate a wave of stretch reflexes to right the thorax, abdomen and hindquarters Cerebral cortex reflexes u Optical righting reflex u visual cues –> righting of the head u Placing reaction u visual, & proprioceptive cues u foot placed on supporting surface u Stepping reflex u Sole of foot touches hard surface u Extension of standing leg and flexion of opposite leg u Reproduces a primitive walking pattern Development of postural control u Also proceeds cephalocaudally, from control of head to trunk to legs 1-3.25 yrs u Average time ranges for postural milestones can be used to detect deviations associated with movement disorders 8-30 mths Development of locomotor control u Unlike many animals, humans are not able to walk at or near birth u Walking alone usually successful between 9-18 months 1-4 yrs u Why is this? Discuss! Most running – yikes! Development of locomotor control Development of arm movements u First grasping movements are reflexive u First arm movements are relatively random u Opportunity to learn arm dynamics u Voluntary movement gradually increases u Fingers initially move together, then independently u Thumb opposition takes time to develop Development of arm movements What is the benefit of unstructured limb movements? Internal model development Improvements in information processing u We can tell that there are improvements in information processing by looking at reaction time u RTs are longest in young children, and their RTs become much longer in more complex situations u Older children are no more sensitive to complexity than adults, but still have longer RTs u Older children (8+) are capable of dealing with complex environments, but have a general delay in selecting and executing movements Critical period for learning? u Is early childhood the best time to learn movement (and other) skills? u It depends. u Children have all of the opportunities we do for synaptic elimination/growth. u But, they have greater opportunities for elimination due to the large number of unrefined corticospinal connections u However, learning complex movements cannot happen until simpler tasks are learned Teenage motor control u We experience a rapid period of physical growth in our early teenage years u Our controller (internal model) no longer makes accurate predictions about the effects of each motor command u It takes time for our brain to learn the new relationships between motor commands and movement Main points (Childhood motor development) u The development of mature movement patterns during childhood is linked to the development of the CNS u More efficient/effective movements are associated with the development of an internal model of our body and limb dynamics u Growth spurts cause temporary inaccuracy of our internal models, resulting in clumsiness u Childhood represents a critical period for motor learning due to the unique opportunity for neural pruning Common motor changes in older adulthood Does neuron density decrease with during healthy ageing? u Not greatly. u Some decreases occur in specific areas of the brain u Some differences between men and women u The idea that age-related decreases in function are related to large-scale decreases in neuron density in the brain appear to be false. Changes in reaction time u Increases in RT and MT in later life reflect: u Changes in sensory receptors u Reduction of motor units (especially fast twitch) u These changes effectively reduce signal-to-noise ratios in sensory and motor systems Changes in balance u Reductions in signal-to-noise ratio in the vestibular system delays postural responses u Decrease the ability to respond appropriately to balance disruptions u Increases risk of falls u 25% of older adults (65+) falls once each year u 30% require hospital treatment u 40% of have multiple falls Effects of exercise for older adults u Resistance training can increase the rate of force development u Faster application of force in balance (and other) situations u Sensory training can induce neural plasticity u Vestibular system u Kinaesthetic system u Aerobic exercise can reduce the likelihood of cardiovascular(and other) disease Main points u Deterioration of movement functions in older adulthood are associated with changes in the CNS and musculoskeletal systems u Neuronal loss and sensory receptor changes can make perception, decision making and movement execution more difficult and slower u Exercise – particularly resistance exercise – can help to limit movement deterioration