Motor control. Proprioception and movement

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1 Motor control Proprioception and movement 2/24 in general we are not aware of information coming from proprioceptors though they belong to somatosensory receptors these receptors detect stretching of the muscles and tension in tendons and induce reflexes as well as provide information for the control of movement receptors in joints detecting the angle of the joints also belong to this category they also provide input for the control of movement these facts justify treatment of these receptors in the framework motor control 1

2 Final common pathway 3/24 somatic and visceral motor systems represent the output of the CNS in the somatic system the final common pathway is the motor neuron in the ventral horn of the spinal cord or in the motor nucleus of cranial nerves in the brainstem in the visceral system the final common pathway means the motor neuron in the lateral horn of the spinal cord or in the vegetative nucleus of cranial nerves in the brainstem final common pathway means (Sherington) that executive organs can be only accessed through these motor neurons, integration occurs at this or at a higher level in the somatic system, fibers innervate targets (skeletal muscle) directly, in the visceral (smooth muscle and gland cells) through an intercalated neuron Hierarchical organization 4/24 areas where stimulation causes movements are classified as motor areas these areas receive somatosensory input as well, thus they are sometimes referred to as somatomotor areas and systems regulation is done on several levels the higher the level the more complicated movements are controlled, though lower levels are influenced by descending effects as well cortical areas are able to control motor neurons in the spinal cord and brainstem directly, but they also exert their effects on the spinal cord indirectly through the brain stem in addition to these pathways, cerebellum and basal ganglia also participate in motor control by influencing brain stem and cortical areas somatotopy is present at every level 2

3 Organization of motor system cortex basal ganglia thalamus brain stem cerebellum motor neuron sensory pathway brain stem and spinal cord 5/24 Muscle spindle 6/24 muscle spindle is 4-10 mm long, it is made up by 6-8 modified muscle fibers, and is located in parallel with regular muscle fibers muscle spindle is encased in a connective tissue capsule within the muscle spindle, the middle part of intrafusal fibers is modified for receptor function, terminal parts are able to contract usually there are 2 nuclear bag (one dynamic, one static) and 5-6 nuclear chain fibers (all static) in a spindle primary terminals form spirals around the middle part of all three types of receptors - annulospiral terminal - Ia (Aα) afferent secondary terminals (flower spray endings) target static nuclear bag and nuclear chain fibers - II (Aβ) afferent 3

4 Operation of the muscle spindle when extrafusal (regular) fibers contract, intrafusal fibers are shortened and relax when extrafusal fibers are stretched, then intrafusal fibers are also stretched elongation of receptor terminals increase the discharge rate in case of continuous stretch polar regions of dynamic nuclear bag receptors lengthen due to their viscoelastic properties, thus excitation of the nerve terminal decreases regular muscle fibers are innervated by Aα axons, intrafusal fibers by Aγ axons excitation is parallel, thus sensitivity of the receptor remains constant previously a servo-mechanism was suggested 7/24 The tendon organ 8/24 tendon organs are in series with muscles they are 1 mm long structures surrounded by a capsule collagen fibers of the tendon penetrate the muscle perpendicularly Ib afferents, branches run between fibers or wrap around them they are deformed with increasing tension excitation tendon organs inform about the contraction and passive extension of muscles, but they are more sensitive to the former thus, they provide information about forces developing in the muscles 4

5 Spinal reflexes I. 9/24 the lowest level in the hierarchy of the motor control is represented by the spinal reflexes the myotatic (stretch) reflex is monosynaptic: afferents terminate on the motoneuron of the same muscle proprius (Latin): one s own stretching of muscle spindle induces contraction of the same muscle patella reflex, Achilles tendon reflex mostly in extensors, though in flexors as well (biceps) collateral of Ia afferent inhibits antagonist motoneuron through an inhibitory interneuron reciprocal innervation is characteristic for the spinal cord simultaneous contraction is organized always at a higher level myotatic reflex can be dynamic or tonic, in the second type secondary terminals also participate Spinal reflexes II. 10/24 diagnostic value: motoneuron excitability direct stimulation: H or Hoffman reflex its function is to keep posture, thus it is stronger in extensors than in flexors, but sloth divergence and convergence in respect of agonist muscles are both present though the reflex remains segmental muscle tone (resistance against passive moving) is due to this reflex: a subset of motor units are always slightly contracted myotatic reflex, thus muscle tone is continuously modified by descending effects through setting the sensitivity of the motoneuron (e.g. REM) collateral of Ia afferent terminates on neurons belonging to the column of Clarke: spinocerebellar tract 5

6 Spinal reflexes III. inverse myotatic reflex starts from tendon organs incoming fibers target agonist motoneuron through inhibitory interneurons, antagonist through excitatory ones main function is to protect muscle and tendon against overstretching 11/24 it also supplements myotatic reflex: when tension in tendon decreases weaker inhibition contraction flexion withdrawal reflex starts from nociceptors (exteroceptive), its function is to remove extremity from the noxious stimulus source it is polysynaptic, in addition to activate antagonist muscles, it also induces crossed extension reflex the reflex is intersegmental, many muscle groups can participate Motor stereotypes 12/24 spinal cord is able to organize simple motor stereotypes these are intersegmental scratching reflex involves rhythmically alternating movements, frequency is independent from the strength of the stimulus, it only increases duration afterdischarge is characteristic (reverberating circuits) stereotypes are generated by a central rhythm generator no feedback from proprioceptors is needed based on mutual inhibition, adaptation and rebound walking has similar central organization, but feedback from proprioceptors is needed and central descending effects influence frequency (walk, trot, canter, gallop) 6

7 Spinal cord organization I. 13/24 location of motor neurons follows somatototopy motor neurons of proximal and distal muscles are located medially and laterally, respectively axial muscles in the midline of the trunk belong to the most medial motor neurons these motor neurons receive input from interneurons on both sides bilateral control posture motor neurons of extensors and flexors are located ventrally and dorsally, respectively motor neurons controlling a given muscle are found in 1-4 neighboring segments motor neuron pool within the pool muscle fibers innervated by 1 motor neuron form the motor unit 10 (eye), 100 (hand), 2000 (foot) fibers all fibers in a unit are the same type Types of muscle fibers 14/24 tonic fibers postural muscles in amphibians, reptiles and birds muscle spindles and extraocular muscles in mammals no AP, motor axon forms repeated synapses slow shortening effective isometric contraction slow-twitch (type I) fibers mammalian postural muscles slow shortening, slow fatigue high myoglobin content, large number of mitochondria, rich blood supply red muscle fast-twitch oxidative (type IIa) fibers specialized for rapid, repetitive movements flight muscles of migratory birds many mitochondria, relatively resistant to fatigue fast-twitch glycolytic (type IIb) fibers very fast contraction, quick fatigue few mitochondria, relies on glycolysis breast muscles of domestic fowl white muscle 7

8 Spinal cord organization II. 15/24 reflexes activate only part of the motor units fractionization principle reflexes and voluntary movements can be graded more and more units get involved recruitment activation follows size principle first small units are activated motor neurons are also small, EPSPs are more effective the largest units contain fast-twitch glycolytic fibers (white muscle) they are only activated when really necessary in addition to recruitment, frequency can be also increased - during voluntary movements 8-25 Hz causing incomplete tetanus motor units contract asynchronously Inhibitory interneurons 16/24 α-motor neurons are innervated by three types of inhibitory interneurons: Ia, Ib and Renshaw Ia receives input from the muscle spindle in the antagonist muscle, but it is also activated by descending fibers targeting the antagonist motor neuron it also receives inhibitory inputs suspension of reciprocal innervation column function Ib receives input from the tendon organ, but it is also influenced by descending pathways, receptors in the skin and joints these control the strength of contraction: touching, stroking Renshaw receives input from collaterals of α-motor neurons feedback inhibition sensitivity is controlled by descending excitatory and inhibitory pathways 8

9 Brain stem reflexes and posture lesions of the neuraxis change the tone of postural muscles Sherington: tone in these muscles is caused by reflexes tone is modified by descending effects: lifting one leg increases the tone of the others transection between the n.ruber and the Deiters nucleus decerebrate rigidity in tetrapods it can be abolished by cutting the reflex arch Deiters nucleus (tr. vestibulospinalis lat.) and pontine RF (tr. reticulospinalis med.) strongly enhances extensor tone inhibitory effects: cerebellum in tetrapods tr. rubrospinalis from n. ruber in primates it has only effect to the level of cervical segments, cortex is more important medullary RF tr. reticulospinalis lateralis 17/24 Voluntary movements I. the background for voluntary movements is provided by muscle tone control appropriate rearrangement of muscle tone is needed to preserve posture during voluntary, reflex and stereotype movements common characteristic of voluntary movements that they become automated through learning and exercise learning to walk in babies sports, etc. Fritsch and Hitzig 1870: cortical stimulation in dogs might lead to movements information about the organization of cortical motor control was collected from five sources: stimulation studies (Penfield human surgeries) analysis of brain injuries unit recording in monkeys imaging, e.g. PET anatomical studies 18/24 9

10 Voluntary movements II. 19/24 based on these data we know what type of damages impair control of voluntary movements, and which areas are activated first organization of motor control is less clear primary motor area: Br.4 gyrus precentralis somatotopy is similar to the somatosensory area: leg medially, representation is proportional with the sophistication of movements secondary motor cortex: Br.6 in front of primary it consists of two parts: supplementary motor area and premotor cortex their role is in the preparation (premotor), and in the planning (supplementary) of movements: electrophysiological and blood flow changes before the movements and during contemplating of movements Voluntary movements III. 20/24 the corticospinal or pyramidal tract is the most important motor pathway most of the fibers originate from layer V pyramidal neurons in Br.4 and 6, but from other areas as well upper motor neurons 90% of the axons cross over and run in the pyramids on the surface of medulla (name), then in the lateral corticospinal tract, 10% cross in the spinal cord (ant. tr.) before ending direct effect on α-motor neurons (lower motor neurons), indirect effect through interneurons motor cortices receive input from the VL (thalamus) and from the somatosensory cortex VL transmits information from cerebellum and putamen, no direct projection interaction goes both ways (see before) movements can be elicited from other cortical areas as well, but with strong stimuli only 10

11 Cerebellum I. 21/24 cerebellum coordinates motor activities, its injuries impair coordination and execution of voluntary movements motor learning is also lost cerebellum has more neurons than the other parts of the CNS modular structure, János Szentágothai contributed heavily to its description principal neurons are inhibitory Purkinje cells, projecting to deep cerebellar nuclei that in turn project to VL various excitatory (e.g. granule) and inhibitory (e.g. Golgi) interneurons input: climbing fiber (contralateral oliva inferior) and mossy fiber (cortex, spinal cord, brainstem) multiple somatotopical representation in the cortex and deep cerebellar nuclei Cerebellum II. 22/24 three parts (connections, evolution): vestibulocerebellum (archeocerebellum) oldest, caudal part (flocculus, nodulus) direct input from semicircular canals, utriculus, sacculus direct output to Deiters nucleus (it can be considered as a deep cerebellar nucleus) balance and gait, coordination of eye movements, reflex movements of the head spinocerebellum (paleocerebellum) middle part of cerebellum (vermis, central, intermediary parts of the hemispheres) input through the dorsal spinocerebellar tract from sensory afferents information about the position of extremities and about changes that occurred (external feedback) input through the ventral spinocerebellar tract about the activity of interneurons: it reflects descending commands (internal feedback) the cerebellum monitors the execution of motor commands cerebrocerebellum (neocerebellum) lateral part of hemispheres input from the cortex and n. ruber through the pons output to n. dentatus, thalamus, cortex planning, starting and stopping as well as learning of movements 11

12 Basal ganglia I. 23/24 basal ganglia consist of: neostriatum (n. caudatus + putamen) pallidum or globus pallidus (neostriatum+pallidum= corpus striatum, putamen+pallidum= n.lentiformis) substantia nigra (pars compacta and pars reticularis) n. subthalamicus main functions are in the control of movements and muscle tone deducted from impairments caused by lost of different cell groups Parkinson s disease: muscle rigidity, tremor, slowing or loss of physical movements (see Awakenings) caused by loss of dopaminergic cell in substantia nigra pars compacta MPTP (methyl-phenyl-tetrahydropyridine) Huntington s chorea: abnormal, jerky, random movements (chorea: Greek word for dance) cholinergic and GABAergic neurons in the neostriatum die genetic background is known prenatal diagnosis ethical issues Basal ganglia II. 24/24 outdated conception: pyramidal extrapyramidal pathways extrapyramidal was supposed to originate from the neostriatum incorrect, the term is rarely used now neurons of the basal ganglia are not activated before cortical neurons no role in initiating movements they modify through multiple circuits involving thalamic VA (ventralis anterior), VL (ventralis lateralis) and CM (centre median) the functioning of the motor cortex in Parkinson s disease stimulation of the direct and inhibition of the indirect pathway decrease less excitation on VA, VL in Huntigton s chorea the indirect pathway is affected more - VA, VL inhibition decreases 12

13 Muscle spindle Berne and Levy, Mosby Year Book Inc, 1993, Fig Afferents in the muscle spindle Berne and Levy, Mosby Year Book Inc, 1993, Fig

14 Myotatic reflex Berne and Levy, Mosby Year Book Inc, 1993, Fig Inverse myotatic reflex Berne and Levy, Mosby Year Book Inc, 1993, Fig

15 Flexion withdrawal reflex Berne and Levy, Mosby Year Book Inc, 1993, Fig Somatomotor cortex Blumenfeld, Sineauer Assoc. Inc., 2002, Fig

16 Cerebellum Berne and Levy, Mosby Year Book Inc, 1993, Fig Divisions of the cerebellum Berne and Levy, Mosby Year Book Inc, 1993, Fig

17 Somatotopy in the cerebellum Berne and Levy, Mosby Year Book Inc, 1993, Fig Direct and indirect pathways Berne and Levy, Mosby Year Book Inc, 1993, Fig

Voluntary Movement. Ch. 14: Supplemental Images

Voluntary Movement. Ch. 14: Supplemental Images Voluntary Movement Ch. 14: Supplemental Images Skeletal Motor Unit: The basics Upper motor neuron: Neurons that supply input to lower motor neurons. Lower motor neuron: neuron that innervates muscles,