Motor systems... the only thing mankind can do is to move things... whether whispering or felling a forest. C. Sherrington 1
Descending pathways: CS corticospinal; TS tectospinal; RS reticulospinal; VS vestibulospinal; RbS - rubrospinal Conclusions. 1. Motor behavior involves a coordinated contraction of many muscles and is controlled by motor control systems. CNS contains neuronal networks mediating different types of motor coordination. A typical network consists of a group of interneurons that activate a specific group of motoneurons in a certain sequence and inhibit other motoneurons that may counteract the intended movement. There are preformed networks that allow performance of a basic movement repertoire (e.g. locomotion, posture, breathing, eye movements, etc.), as well as basic networks that underlie reaching hand and finger movements, and sound production as in speech. This constitutes the motor infrastructure that is available to a given individual, after maturation of the nervous system has occurred. In addition, we are able to learn new motor coordinations through plastic changes taken place in genetically expressed motor infrastructure. 2. In CNS one can distinguish three levels of motor control: spinal cord, brainstem and cortical motor areas. They are organized hierarchically and in parallel. Spinal cord contains networks that mediate spinal reflexes (e.g. flexion reflex, crossed extension reflex) and some rhythmical motor patterns (e.g. locomotion, scratching). Brainstem contains networks that mediate bulbar reflexes (e.g. swallowing, coughing) and such rhythmical motor patterns as breathing, chewing, etc. It also contains neuronal systems whose axons project to the spinal networks. Through these descending systems brainstem controls movements generated by the spinal cord. Cortical motor areas are responsible for voluntary movements, as well as for corrections of motor patterns generated by the spinal cord and brainstem. Cerebellum, basal ganglia and thalamus do not participate directly in generation of movements, but are involved in motor coordination. Cerebellum improves a "quality" of movements by coordinating activity of motor cortex, brainstem and spinal cord on the basis of peripheral and central feedback signals. Basal ganglia are responsible for selection and initiation of proper type of motor behavior. Thalamus is a "relay station" transmitting different type of signals to the cortex. 3. Each level of motor control affects its subordinate level via descending pathways. The spinal cord receives commands via five descending tracts. The vestibulo-, reticulo-, rubro- and tectospinal tracts originate from vestibular, reticular, and red nuclei of the brainstem and from tectum, respectively. The corticospinal tracts originate from the primary motor cortex, and to a lesser extent from the premotor and supplementary motor areas. Descending tracts have different basic functions. The reticulospinal tract is responsible for initiation of locomotion and control of upright body posture. The vestibulospinal tract is involved in generation of tonic activity in the antigravity muscles during standing and walking, as well as in different postural reflexes. Tectospinal tract is important in coordinating head and eye movements. The rubro- and corticospinal tracts play two major roles: (1) Transmission of commands for non-stereotypic (voluntary) movements, and (2) Correction of the motor patterns generated by the spinal cord. Parallel descending pathways, with partly overlapping functions, offer the advantage that, if one pathway is lesioned, the remaining ones can to some extent take over its functions. 2
Skeletal muscles are composed of elongated cells called muscle fibers 3
Alpha-motor neurons Alpha-motoneurons (motor nuclei of cranial nerves in brainstem) 4
muscle twitch Unfused tetanus Fused tetanus 5
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Motor neurons Muscle fibers 7
Motoneurons influence muscle fiber type Normal muscle Postpolio muscle 8
(Size low law or Size principle or Henneman s principle) Law: 9
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Conclusions 1. Muscle contraction is produced by an orderly sequence of electrical and chemical events, beginning with an action potential originating at the neuromuscular junction. 2. Individual muscle fibers are classified into three types (1, 2a and 2b) according to their contractile and metabolic properties. The properties of a whole muscle depend on the proportion of muscle fiber types it contains and on the architectural arrangement of muscles fascicles with respect to connective tissues, bones and joints. 3. The motor unit is defined as a single motor neuron and the group of muscle fibers it innervates. All muscle fibers in a single motor unit consist of the same muscle fiber type. The amount of the force produced by the muscle fibers of a motor unit is governed by the pattern and frequency of action potentials produced by the motor neuron. Three types of motor units slow, fast fatigue-resistant, and fast fatigable can be categorized on the basis of their twitch speed and fatigability. 4. Each muscle is innervated by a pool of motor neurons, which typically contains a mixture of motor unit types, although in different proportions depending on the typical use of that muscle. An orderly sequence of motor neuron activation within a pool leads to activation of units producing the smallest amount of force before those producing larger amounts of force. This sequence, known as a size principle, results from passive electrical properties of motor neurons and their synaptic inputs. Alternative recruitment sequences can occur when synaptic inputs have a specialized distribution among motor neurons that overrides the contribution of the passive electrical properties of motor neurons. 5. Precise control of movement is complicated because the mechanical response of muscles to neural activity is slow, changes in muscle tension do not represent a simple one-to-one correspondence to the firing patterns of motor neurons. Rather, the temporal pattern of the incoming train of action potentials is modified by the muscles themselves. Because of this filtering action, muscles faithfully reproduce only those signals that vary slowly. To produce rapid changes in tension, the motor systems must alternate contraction in opposing muscles. MUSCLE RECEPTORS 11
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Golgi tendon organs are sensitive to change in tension Figure 16.12 The function of muscle spindles and Golgi tendon organs (Part 1) 14
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Activity of alpha and gamma motor neurons during locomotion Alpha motor neuron (m.gastrocnemius) Gamma motor neuron (m.gastrocnemius) 16
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Reciprocal coordination of muscles The stretch reflex participates in stabilization of the muscle tone through stabilization of the muscle length 18
The stretch reflex participates in stabilization of the muscle tone through stabilization of the muscle length Gamma motoneurons CNS can regulate the effectiveness of the stretch reflex through: (i) Gamma motoneurons; The stretch reflex participates in stabilization of the muscle tone through stabilization of the muscle length Presynaptic Inhibition of 1a afferents Gamma motoneurons CNS can regulate the effectiveness of the stretch reflex through: (i) Gamma motoneurons; (ii) Presynaptic inhibition of 1a afferents; 19
The stretch reflex participates in stabilization of the muscle tone through stabilization of the muscle length Presynaptic Inhibition of 1a afferents Gamma motoneurons CNS can regulate the effectiveness of the stretch reflex through: (i) Gamma motoneurons; (ii) Presynaptic inhibition of 1a afferents; (iii) Inputs affecting alpha motoneurons Modulation of effectiveness of the stretch reflex during locomotor cycle 20
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Reciprocal coordination of muscles Conclusions 1. Specialized sensory receptors (spindles and tendon organs) in the muscle provide feedback to the CNS regarding the amount the muscle stretch and tension. 2. Muscle spindles are located within the muscle and provide signals on the muscle length. The length of the muscle and changes in length are coded by the pattern and frequency of action potentials in the primary or Ia afferents and secondary or Group II afferents. Gamma motor neurons innervate the spindle fibers and can adjust the sensitivity of the spindle. 3. Golgi tendon organs lie within the musculotendinous junctions and are activated when tension is produced by nearly active motor units. 4. Muscle spindles and tendon organs provide the CNS with continuous information about the mechanical state of the muscle. Sensory signals from muscle receptors are transmitted to the spinal cord as well as to the higher levels of the central nervous system. 5. Discharge of muscle spindle afferents evokes stretch reflex. It is a monosynaptic spinal reflex, which allows muscle tone to be regulated quickly and efficiently without direct intervention by higher centers. Descending control signals adjust the gain of the reflex loops, adapting them to the requirements of specific motor acts. 6. Discharge of Golgi tendon organ Ib afferents disynaptically inhibits the discharge of homonymous alpha motor neurons, thus providing a negative feedback mechanism for regulation of muscle tension. 23
Spinal reflexes 24
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Reciprocal coordination of muscles 26
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Scratching CPG is located in lumbosacral enlargement of the spinal cord Signals from receptive field activate the scratch CPG through propriospinal neurons 28
Conclusions 1. Spinal reflexes are initiated by sensory stimuli that activate the receptors in muscles, joints, and skin. These stimuli activate the neuronal network in the spinal cord. This network affects a specific group of muscles. 2. Spinal reflex networks perform three main functions: (1) control of individual muscles, (2) coordination of muscle action around a joint, (3) coordination of muscles at different joints. 3. Interneurons are important elements of spinal networks. They (i) mediate influences of sensory input upon motoneurons and (ii) constitute the networks generating complicated patterns (reverberating network, rhythm generator). Three groups of inhibitory interneurons (1a, 1b and Renshaw cells) coordinate muscle action around a joint. 4. Spinal reflex circuits provide the higher centers with a set of elementary patterns of coordination, from relatively simple combinations, like reciprocal innervation at a single joint, to more complex spatial patterns of movement, such as flexion reflex, and temporal patterns, as in the scratch reflex. 5. Spinal reflexes are not entirely stereotyped but rather are adapted to the initial position of the body segments and the external loads acting to oppose movement. Because this information reaches the lower levels directly, higher centers can activate these reflex circuits to produce voluntary movements and need not be concentrated with the details of shaping the movement patterns to current circumstances. 29