Lecture 16

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Cell and Systems Biology
Melanie Woodin

CSB332H1S L16; March. 14, 2012 Motor Control • Muscle contraction begins w small motor units being activated (or recruited) first. • When more contraction is required progressively larger motor units recruited o Because small motoneurons = more easily excited than large motoneurons, so more easily activated • Limb muscles are controlled my motorneurons & interneurons of the spinal motor apparatus • Interneurons w/i spinal cord & brainstem make u central pattern generators that direct motor apparatus • Motor output planned & refined by motor cortex, basal ganglia, & cerebellum • Movement either voluntary or unconscious reflexes • Use reflex to compensate – need sensory fdbk to CNS (spinal or cortex) o Simplest level is reflex • Single muscle spindle fibre (1a) has multiple inputs to • Need something to tell muscle that something is going multiple alpha motoneurons on & something to control output Neural organization of motor control is hierarchical • 1 act pot in a single 1a afferent synaptic pot in 1. Simplest level: sensory neurons synapse w alpha motoneuron – only a motoneurons in spinal cord to mediate simple reflexes fraction of a mV (blue) o Basic reflex – ex. Holding something steady o Very small depolarization 2. Central pattern generators: networks of interneurons in spinal cord & brainstem coordinating interplay of • Presynaptic fiber fires 3 act multiple motor grps (locomotion, respiration) pots rapidly in succession  o Not in this lecture synaptic pots (brown) ride on 3. Motor output that is planned & refined by motor cortex, falling phase of previous one, basal ganglia & cerebellum increasing the depolarization – prob enough to have effect The Motor Unit in motoneuron • Sherrington: spinal motoneuron = final common path because all the neural influences that concern movement or posture converge upon it • α motoneurons – major motoneurons of spinal cord • A muscle (e. soleus in cat) o cause muscles to contract may have 50 1a afferents • γ motoneurons – smaller motoneurons, regulate converging onto 1 motoneuron sensitivity of muscle spindles o Make sure don’t over or under contract muscle, • A strong stretch of muscle fine-tuning • Motor unit – a single α motoneuron & all muscle can activate all 1a afferents o  Indiv EPSPs will fibres it innervates spatially summate to • Motor pool – all motor neurons supplying a depolarize membrane particular muscle o  activation of alpha o Single muscle innervated by multiple grps of motoneurons motoneuron The Motor Unit Agonists & Antagonists • Limb movements are produced by coordinated contraction of grps of muscles that work together. o Agonists – tend to move things toward midline • At the same time opposing muscles are made to relax o Antagonists – tend to move things away from midline • Antagonist muscle opposes action of another muscle o Ex. Relaxing while the other 1 contracts  smooth, coordinated movement. • Extensor muscles – open (or extend) joints • Flexor muscles – close joints & pull limbs toward body • Golgi tendon reflex at ends of muscle • Works w Gamma motoneurons – fine tune, sense stretch in muscle o Fdbk mechanism • … Additional stretch recs – Golgi tendon organs in tendon-muscle junction o Signal thru 1b afferent fibers o  make disynaptic connections onto interneurons o  inhibit α motoneurons in ventral horn supplying agonist muscle of origin o  relaxation • Interneuron afferents also excite antagonist • For motor activity to occur, need to sense within muscle motoneurons • 1a afferents in muscle spindles (part of muscle) • Info they provide about muscle tension helps to shape sense stretch in muscle o So know if stre
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