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General Organization of Motor Systems Zoran Đogaš Motor Systems • Functions – movement – posture & balance – communication • Guided by sensory systems – internal representation of world & self – detect changes in environment • external & internal ~ Three Classes of Movement • Voluntary – complex actions reading, writing, playing piano, etc. – purposeful, goal-oriented – Learned • may improve with practice Three Classes of Movement • Reflexes – involuntary, rapid, stereotyped eye-blink, coughing, knee jerk – graded control by eliciting stimulus • Rhythmic motor patterns – combines voluntary & reflexive acts chewing, walking, running – initiation & termination voluntary – once initiated, repetitive & reflexive Movement & Muscles • Movement occurs at joints • Contraction & relaxation of opposing muscles – agonists • prime movers – antagonists • counterbalance agonists • decelerate movement Movement & Muscles • Movement control more than contraction & relaxation – Accurately time control of many muscles – Make postural adjustment during movement – Adjust for mechanical properties of joints & muscles • inertia, changing positions Sensorimotor Integration • Sensory inputs guide movement – visual, auditory, tactile • location of objects in space – Proprioceptive & vestibular • position of our body • Critical for planning & refining movements Error Correction: Feedback • During or after movement • Compare actual position with intended position – if different ----> make correction • muscle contractions • Limited to slow movements Error Correction: Feed-forward • Sensory events control movements in advance – ballistic movements • Prediction • internal model of events • e.g. catching ball – representation of ball trajectory – properties of musculoskeletal system • Reevaluation after response completed SENSORY INPUTS GUIDE VOLUNTARY MOVEMENT THROUGH FEED-BACK AND FEED-FORWARD MECHANISMS EXAMPLE OF FEEDBACK AND FEEDFORWARD MOVEMENT CONTROL: CATCHING A FALLING BALL Visual input provides feed-forward control of the task enabling us to: 1) Position hand under where ball is anticipated to fall 2) Partially stiffen joints in anticipation of ball’s impact on hand Somatosensory and proprioceptive inputs provide feed-back control used to grasp ball. Some aspects of feedback control involve task-specified programming of spinal reflexes Sensorimotor Impairments • Feed-forward control – eyes open: ballistic movements OK – eyes closed: • ballistic movements highly inaccurate hand drifts at end of movement • Eyes open only prior to movement – errors greatly reduced – lack of info about starting position Motor unit α-motor neuron + all the muscle fibers to which it connects Motor units Three types of motor units • SF motor units (type I) (slow, fatigable) • FF motor units (type IIA) (fast, fatigue resistant) • FFR motor units (type IIB) (fast, fatigable) Distribution of Fiber Types • All muscle composed of ST & FT fibers • Distribution varies from muscle to muscle within an individual • Most individuals possess between 45 and 55% ST Distribution of Fiber Types • Vastus lateralis on average 52% SO, 33% FOG, 14% FG. Same for deltoid, biceps brachii. • Soleus may have as much as 85% SO. • Triceps brachii may have as few as 30% SO. Distribution of Fiber Types • Great variation between individuals • Vastus lateralis of elite distance runners had 79% ST, untrained had 58% • Available evidence indicates that the distribution of slow and fast twitch fibers is genetically determined and not altered by training Large and small α-motoneurons Strength of muscle contraction • Recruitment (Henneman’s Size Principle) • Frequency code More precise movements Less precise movements Motor Units and Muscle Force Production • The All-or-None Law (Bowditch’s Law) for motor units – – Applies to individual motor units, but not the entire muscle. The all-or-none law is based upon the difference between graded potentials and action potentials • • – – – Stimulation threshold A motor unit is either activated completely or is not activated at all If there is enough graded potential to create an action potential that travels down the α-motor neuron of a motor unit, then all of the fibers in that motor unit will contract. The level of force production of a single motor unit is independent of the intensity of the stimulus, but it is dependent on the frequency of the stimulus This law implies a stimulation threshold important for the Henneman’s Size Principle Gradation of Muscle Force • Two neural mechanisms responsible for force gradations: 1. Recruitment Spacial summation 2. Rate coding Temporal summation Recruitment • Varying the number of motor units activated. Rate Coding Rate coding 100 Larger muscles Smaller muscles % Maximal Voluntary 50 Motor Unit Recruitment Motor unit firing frequency 0 0 50 % Maximal Voluntary Force Production 100 Important! • Smaller muscles (ex: first dorsal interosseous) rely more on rate coding • Larger muscles of mixed fiber types (ex: deltiod) rely more on recruitment Strength of muscle contraction • PRINCIPLE OF ORDERLY RECRUITMENT ACCORDING TO SIZE: INTRACELLULAR, physiological stimulation of nerves, the smallest axons are stimulated first (have the lowest threshold), and largest axons are stimulated last. • This is another way to increase the force with which a muscle contracts: "Recruit" more alpha-neurons to fire on the muscle. In this case, again, the smallest neurons will fire first (small twitch tension), and larger neurons will fire later (larger twitch tension). • V = IR: Small neurons have a higher resistance, which means they will show a stronger depolarization (V) for the same current (I). • That's why they fire first. This is the opposite of external stimulation, where smaller neurons are stimulated last. • In physiological stimulation, smaller neurons have a smaller threshold potential than larger neurons. Strength of muscle contraction FORCE TRANSDUCTION: • Twitch Tension in a muscle is increased by increasing the frequency with which alpha-motor neurons are fired. • With high frequency firing, the muscle doesn't get a chance to completely relax before the next action potential. • Tetanus is maximal twitch tension. Organization of Motor Control • Hierarchical & Parallel • Parallel – pathways active simultaneously – e.g. moving arm 1. muscles producing movement 2. postural adjustments during movement • Recovery of function after lesion – overlapping functions Hierarchical Control of Movement • 3 levels of control • Spinal cord (SC) • Brainstem • Cortex • Division of responsibility – higher levels: general commands – spinal cord: complex & specific • Each receives sensory input – relevant to levels function Hierarchical Control: Spinal Cord • Automatic & stereotyped responses – reflexes – rhythmic motor patterns • Can function without brain • Spinal interneurons – same circuits as voluntary movement • Pathways converge on α motor neurons – final common path Hierarchical Control: Spinal Cord • Motor neurons in ventral horn • Topographical organization of motor nuclei • motor neuron pools – longitudinal columns across 1-4 spinal segments – according to 2 rules Topographical organization of motor nuclei • F P D E Flexor-Extensor rule ventral: extensors dorsal: flexors • Proximal-distal rule – medial: proximal muscles – lateral: distal muscles • Parallel control systems – proximal: postural – distal: manipulative Somatotopic distribution Hierarchical Control: Brain Stem • Modulates neurons in spinal cord • interneuerons & motor neurons – 2 main parallel pathways • Medial – to ventromedial spinal cord – postural / proximal muscles • Lateral – to dorsolateral spinal cord – manipulative / distal muscles Hierarchical Control: Cortex • 2 tracts • Corticobulbar --->cranial nerves – facial muscles • Corticospinal ---> spinal nerves – Origin of axons 1/3 from primary motor cortex (M1) 1/3 from premotor areas 1/3 from somatosensory cortex Descending motor pathways Descending motor pathways • Medial system pathways (axial and proximal muscles) • Lateral system pathways (distal muscles) • Monoamine descending pathway (general level of excitability) Descending motor pathways Descending motor pathways Positive Signs: Babinski Sign • Lesion of corticospinal tract • Plantar reflex – Stroke firmly stroke sole of foot • heel ---> toe • Normal: flexion – toe curl down • Lesion: Extension – toes curl up and fan Babinski sign Muscle tone Unconscious nerve impulses maintain the muscles in a partially contracted state. If a sudden pull or stretch occurs, the body responds by automatically increasing the muscle's tension, a reflex which helps guard against danger as well as helping to maintain balance. Such near-continuous innervation can be thought of as a "default” or "steady state" condition for muscles. There is, for the most part, no actual "rest state" insofar as activation is concerned. Both the extensor and flexor muscles are involved in the maintenance of a constant tone while "at rest". In skeletal muscles, this helps maintain a normal posture. Muscle tone Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Hypotonia can present clinically as muscle flaccidity, where the limbs appear floppy, stretch reflex responses are decreased, and the limbʼs resistance to passive movement is also decreased. Hypertonia can present clinically as either spasticity or rigidity. While spasticity is velocity-dependent resistance to passive stretch (i.e. passively moving an elbow quickly will elicit increased muscle tone, but passively moving elbow slowly may not elicit increased muscle tone), rigidity is velocityindependent resistance to passive stretch (i.e. there is uniform increased tone whether the elbow is passively moved quickly or slowly). Muscle tone testing Muscle tone testing Muscle Weakness • Lesions produce different syndromes • Lower motor neuron syndrome • spinal motor neurons – lesion: soma or axon – Symptoms • weakness • fasciculations • atrophy Muscle Weakness • Upper motor neuron syndrome • descending motor pathways – imbalance of excitatory/inhibitory interneurons – symptoms spasticity occurs ↑ tonicity & deep tendon reflexes atrophy is rare no fasciculations Parallel Control & Recovery • Fractionation of movement – independent control of single muscles – via direct input from corticospinal tract • Lesion in medullary pyramids – can no longer grasp objects – locomotion, posture unaffected • Parallel pathways assume control Parallel Control & Recovery • Monkeys: If premotor outflow spared • indirect control via brainstem – strength returns – but movement slow • M1 ---> lateral brainstem intact – cortico-rubrospinal & cortico-reticulospinal tracts assume control – humans: fewer fibers ---> less recovery