Motor Unit Lecture 2 PDF

NEUR3101 Motor control Lecture 2 Motor units, motoneuron recruitment and control The size principle A/Prof Ingvars Birznieks  Lower motor neuron circuits Chapter 16 Purves et. al, Neuroscience (6th Ed)  Neural centres responsible for movement con...

NEUR3101 Motor control Lecture 2 Motor units, motoneuron recruitment and control The size principle A/Prof Ingvars Birznieks  Lower motor neuron circuits Chapter 16 Purves et. al, Neuroscience (6th Ed)  Neural centres responsible for movement control Lower motor neurons are neurons which send their axons directly to skeletal muscles. Upper motor neurons control the local circuit neurons and α-motor neurons. Local circuit neurons located in the spinal cord or in the motor nuclei of the brainstem cranial nerves they regulate activity of the lower motor neurons. Cerebellum and basal ganglia regulate activity of the upper motor neurons without direct access to either the local circuit neurons or lower motor neurons.  Lower motor neurons (LMNs) Lower motor neurons are neurons which send their axons directly to skeletal muscles usually meant α-motor neurons however γ-motor neurons controlling muscle spindle sensitivity are also lower motor neurons axons from motor neurons located in the spinal cord travel to muscles via the ventral roots and peripheral nerves lower motor neurons in the brainstem are located in the motor nuclei and axons travel via cranial nerves Note that the upper motor neurons also could be located in the brainstem! all commands for movement (reflexive or voluntary) are ultimately conveyed to muscles only by lower motor neurons Charles Sherrington introduced term “final common path”, because no other cells have direct access to muscles – the path has to involve the lower motor neurons!  Upper motor neurons (UMNs) cell bodies located in the cerebral cortex or brainstem upper motor neurons in the cortex are essential for initiation of voluntary movements essential for complex spatiotemporal sequences of skilled movements axons synapse with the local circuit neurons and in rare cases (mostly for distal muscles) directly with lower motor neurons upper motor neurons in the brainstem are involved in regulation of muscle tone, control of posture and balance in response to vestibular, auditory, visual and somatic sensory inputs  Local circuit neurons are interneurons, which are responsible for activation of α-motor neurons located close to where corresponding α-motor neurons are (in spinal cord or in motor nuclei of brainstem cranial nerves) receive descending projections from higher centres mediate sensory-motor reflexes maintain interconnections for rhythmical and stereotyped behaviour Even without inputs from the brain the local circuit neurons can control involuntary highly coordinated limb movements like walking (has been demonstrated in animals, some success has been seen using electrical stimulation in humans).  Cerebellum and basal ganglia Cerebellum and basal ganglia are called complex circuits and they do not contain any type of motor neurons do not have direct access to either local circuit neurons or lower motor neurons regulate activity of upper motor neurons cerebellum largest subsystem detecting and attenuating the difference between expected and actual movement - “motor error” mediates real-time ongoing error correction (feedback control) responsible for long term reduction of errors (motor learning) basal ganglia supress unwanted movements prepare upper motor neuron circuits for initiation of movement malfunction can lead to Parkinson’s and Huntington’s disease Hierarchical organisation of movement control Figure 16.1  Motor neuron – muscle relationship Each lower motor neuron innervates muscle fibres within a single muscle. Individual motor axons branch within muscles to synapse on many muscle fibres. Each muscle fibre is innervated only by one single α-motor neuron. An action potential generated in the axon bring to the threshold and activate all muscle fibres it innervates.  Motor unit A motor unit is made up of a motor neuron and the skeletal muscle fibres innervated by that axon (Sherrington). Fibres are typically distributed over a relatively wide area within the muscle to ensure that the contractile force is spread evenly, to ensure that local damage to motor neurons or their axons will not have significant effect on muscle contraction. Activation of one motor unit corresponds to the smallest amount of force the muscle can produce. Figure 16.5  Motor neuron – muscle relationship All motor neurons innervating a single muscle are called motor neuron pool for that muscle and are grouped together in one cluster. The motor neuron pools that innervate distal parts of the extremities (fingers and toes) lie farthest from the midline (lateral motor neuron pool). Figure 16.3  Types of motor units Motor units vary in size – both in regard to cell body size of motor neuron and number of fibres it innervates. Small α-motor neurons innervate relatively few muscle fibres to form motor units that generate small forces. Large α-motor neurons innervate larger number of more powerful muscle fibres. Motor units differ in the types of muscle fibres that they innervate. Small α-motor neurons have lowest activation thresholds and thus are first to be recruited.  Types of motor units There are three major motor unit types: Slow (S) motor units – small cell bodies, small number of muscle fibres low activation threshold high endurance - muscle fibres rich in myoglobin, mitochondria, dense capillary network important for activities that require sustained muscular contraction (e.g., posture) connect to Slow Oxidative (SO) type I muscle fibres containing myosin heavy chain MyHC-I isoform Fast fatigue-resistant (FR) motor units – intermediate size generate twice the force of a slow motor unit fatigue resistant connect to Fast Oxidative/Glycolytic (FOG) type IIa muscle fibres with MyHC-IIa Fast fatigable (FF) motor units – largest motor units generate highest level of force in brief contractions such during jumping easily fatigable, sparse mitochondria high activation threshold, nevertheless capable of high firing rates connect to type IIb Fast Glycolytic (FG) muscle fibres with MyHC-IIb and/or MyHC-IIx Note that humans have only type MyHC-IIx isoform  Mapping fibre types to MyHC types Muscle fibre type classification could be done by means of two different methodological approaches ATPase histochemistry and SDS-PAGE electrophoresis. ATPase histochemistry determines fibre type. It is the classical method based on muscle tissue staining and muscle fibre examination under microscope. It has distinguished three fibre types type I, type IIa and type IIb. Electrophoretic SDS-PAGE method determines MyHC protein type. It is based on protein separation by mass. It uses sodium dodecyl sulfate (SDS) molecules to bind to proteins (different MyHC types in this case) to move them in electric field and polyacrylamide gel. There are 4 main MyHC isoforms identified using similar nomenclature: type I, IIa, IIb, but introducing additional type IIx to be placed between IIa and IIb types. The mapping between MyHC isoforms and ATPase histochemistry could be confusing and means that type IIb muscle fibre types may correspond to two different MyHC isoforms IIx or IIb. Force and fatigability of motor units Figure 16.6  Types of motor units Soleus muscle involved in postural control, has predominantly small motor units and average innervation ratio is 180 muscle fibres per motor neuron. Gastrocnemius muscle innervation ratio is 1000-2000 muscle fibres per motor neuron and can generate forces needed for sudden changes in body position. In extraocular muscles average innervation ratio is 3 fibres per unit.  Use dependant motor unit plasticity Pattern of neural activity in a motor nerve provides instructive signal which can influence the expression of muscle fibre phenotype. Following 56 days of chronic electrical nerve stimulation, nearly all fibres acquired the histochemical phenotype of slow oxidative fibres. Implications for FES (functional electrical stimulation) in paralysed patients. Changes in fibre type after chronic electrical stimulation in cats  Star – S (slow) type I muscle fibres  Square – FR (fast fatigue-resistant) type IIa muscle fibres  Circle – FF (fast fatigable) type IIb muscle fibres Box 16A  Neuromuscular matching Experimentally switching a nerve innervating a fast muscle to innervate a slow muscle instead has lead to a slow muscle transition to contractile and histochemical properties of a fast muscle (i.e. a change in muscle phenotype). Alpha motoneurone type: Muscle name: Dominant fibre type: Figure 13.9 p431 Bear, Neuroscience: Exploring the Brain, 2007.  Regulation of muscle force Muscle force could be increased by increasing discharge rate (number of spikes per unit of time) number of active motor units  Effects of firing rate on muscle tension Under normal conditions the maximum firing rate of motor neurons is less than that required for fused tetanus. The asynchronous firing at different lower motor neurons provides a steady level of input to the muscle, which averages out the changes in tension due to contractions and relaxations of individual motor units and achieves apparently smooth overall muscle contraction. Figure 16.8 Henneman’s size principle of motor unit recruitment In 1960s Elwood Henneman from Harvard Medical School observed that gradual increase in muscle tension results from the recruitment of motor units in a fixed order according to their size. During a weak contraction only low threshold small size S motor units are activated. As synaptic activity driving a motor neuron pool increases, the FR units are recruited. To reach the maximum force finally the largest size FF units are recruited last. This systematic relationship between motor neuron size and recruitment order has come to be known as the size principle. S FR FF Regulation of muscle force Figure 16.7 Biophysical principle underlying motor unit recruitment size principle Ohm’s Law: I = V / R; Current = Voltage / Resistance V = I x R; Voltage = Current x Resistance Threshold voltage is the same regardless same I of neuron’s size. Low R Similar synaptic input is capable inducing small V-change the same current I determined by the Higher R small EPSP amount of released transmitter: greater V-change larger EPSP Ismall=Ilarge For larger neurons R is lower as they have large surface area and volume: Rsmall>Rlarge Vsmall = I x Rsmall Vlarge = I x Rlarge  Vsmall > Vlarge Thus with the same synaptic input small motoneuron will reach threshold for action potential generation while it will Recruited first remain subthreshold for a larger neuron. Kandel et al., Fig. 34-5  Strength of muscle contraction is regulated by means of discharge rate and number of active motor units Motor neurons Action potentials transmitted by axon  Violations of the size principle Electrical stimulation electrical stimulation can alter recruitment order, tending to recruit larger motor units first Cat “paw-shake” response? high threshold units preferentially recruited for maximal velocity repetitive, cyclic movement During fast contractions? it has been suggested that larger motor units may be preferentially recruited during fast forceful contractions in humans... During eccentric contractions? It has been suggested that during eccentric contraction larger motor units are preferentially activated  Motor unit changes with aging On average the motor units of old adults have a higher innervation ratio Because muscle fibres that are abandoned when their parent nerve dies are reinnervated by sprouting collateral branches from remaining motoneurons Muscles undergo atrophy as other abandoned muscle fibres die Old adults have fewer motor units than young adults Apoptosis of motoneurones in the spinal cord from age 60 onwards Muscles of old adults also exhibit a shift towards more slow-twitch properties The consequence of these changes is weaker muscles for old adults Thank you for your attention