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MOTOR SYSTEM PHYSIOLOGY April 6 and 7, 1998 D. Michele Basso, Asst. Prof. Office Hrs. by Appointment; 2-0754; [email protected] Reading Assignment: Kandel, Schwartz, Jessell "Principles of Neural Science" 3rd edition. Chapters 36, 37, 38. Book is on closed reserve at the library. **figure taken from Kandel, chwartz, Jessell 3rd edition Objectives: Students will be able to: to 1. Identify ultrastructural components of muscle and describe their relationship muscle contraction, force development and movement. 2. Classify each type of motor unit and discuss how motor unit recruitment is modulated to meet the demands of the movement and/or task. 3. Identify all components of the muscle spindle and golgi tendon organ and discuss: the range of sensitivity for both structures; the relationship between sensitivity and muscle tone; and, the CNS mechanisms that modulate muscle tone. 4. Describe the role of the muscle spindle in producing simple and complex movements. 5. Classify the different types of interneurons and describe their roles in modulating movement. 6. Describe the structure, organization and interaction of spinal cord systems which produce simple, complex and locomotor movements. SAMPLE QUESTIONS Students: I have prepared a few tips and questions to help you prepare for the exam. Listed below are several questions. Once you have answered them, you can check your answers to some I thought up. Note that many of these questions do not have ONE, Single correct answer. If you have difficulty understanding the logic behind my answers, please drop me an email with your question and I'll respond within 24 hrs. [email protected] Tips on preparing for motor system physiology portion of the exam: 1. Integrate the material, be prepared to describe movement in terms of its underlying details such as changes in the muscle spindle, gamma biasing, muscle mechanics. Remember we started off the lecture series with the question "How do we move?" 2. Some things to think about: a. What movements utilize greater dynamic than static b. What movements use greater static than dynamic c. What conditions rely primarily on primary spindle d. What conditions rely primarily on secondary spindle e. What would the firing rate of the spindle afferents, both primary during the following activities: - gamma biasing? gamma biasing? afferent firing? afferent firing? and secondary, be Sleeping on a moving subway train: Standing to deliver a speech: riding a horse: f. Does spasticity affect the amount of actin-myosin overlap at rest? How is a movement successfully executed? Muscle Ultrastructure 1. The basic contractile element of muscle is the sarcomere a. The sarcomere is made up of overlapping thick and thin filaments b. Thick filaments are made up of ~200 myosin molecules c. Each molecule has a double helix tail of heavy chain polypeptides and two globular protein heads d. Thin Filament is comprised of an actin backbone wound in a helix with tropomyosin proteins. Troponin molecules are located on ach tropomyosin protein. Muscle Contraction 1. Each muscle fiber contains hundreds to thousands of sarcomeres. Within the sarcomere, the myosin filament lies between the thin actin filaments. 2. Muscles contract by a process called excitation-contraction coupling which causes the actin filaments to slide over the myosin filaments. First: An action potential from the motor neuron causes a release of Ca++ into the intracellular space of the muscle fiber. Second: The Ca++ binds to troponin and, through mechanisms not fully understood, produces a conformational change in the actin filament therby exposing active binding sites. Third: The myosin heads bindto these active sites forming a cross-bridge between actin and myosin filaments. Fourth: Binding of myosin heads to actin continues down the length of the filament which rotates the head of the myosin and slides the actin across the myosin. Fifth: This process will continue as long as ATP, Ca++ and active binding sites on actin are available. Movement is the result of force development and muscle contraction. affecting contraction force include: Factors a. Initial length of the muscle b. Velocity of length change c. External loads opposing the muscle MNs are topographically organized 1. In the horizontal plane the motor neurons controlling proximal muscles are located medially and those controlling muscles of the digits are found laterally 2. MNs are also organized longitudinally in muscle specific groups call motor neuron pools. 3. Mediolateral position in the horizontal plane indicative of distal musculature 4. Muscle Contraction is the direct result of firing of the alpha motor neurons in the ventral horn of the spinal cord. 1. A single ? motor neuron supplies many muscle fibers and is called a motor unit. 2. Each muscle fiber is innervated by only 1 motor neuron 3. The number of fibers innervated by a single motor neuron varies according to the size and precision of the muscle. This is the innervation ratio. a. Precise Motor Control: Extraocular muscles have an innervation ratio of 1:10 b. Intermediate Motor Control: Intrinsic muscles of the hand have an innervation ratio of 1:200 c. Gross Motor Control: Leg muscles (gastrocnemius) have an innervation ratios of at least 1:2000 Classification of Motor Units 1. There are at least 3 types of motor units based on physiological and anatomical parameters: Slow Oxidative Fast Fatigue Resistant Fast Fatiguable Metabolism Aerobic, oxidative Anaerobic, oxidative and Anaerobic, glycolytic enzymes glycolytic enzymes enzymes Contraction Force Low Intermediate High Contraction Velocity Slow Fast Fast Motor Function Postural Control, Intermediate Endurance Endurance Rapid, Powerful Movements Fiber Type Type I Type IIB Type IIA Motor Unit Recruitment 1. Motor Units are recruited in a fixed order according to the size of the motor neuron and the force generating capacity of the motor unit. 2. Orderly recruitment was first described based on the size of the motor neuron. The surface area of the cell is inversely related to its input resistance. Thus, smaller neurons have higher input resistance and are more likely to fire given that: V (action potential) = I (current) x R (resistance) Therefore, if a small and large MN receive the same synaptic drive or current, the small MN will reach threshold and fire before the large MN because of its greater input resistance. 3. Recruitment is also related to motor unit force. Motor neurons demonstrate intrinsic differences in excitability which are related to contraction force of the muscle fibers. Cells with high levels of excitability produce low twitch tension values during muscle contraction. 4. Thus, the nervous system uses stereotypic recruitment of motor units to meet the demands of the task. The weakest motor units are recruited first and as synaptic input increases, progressively stronger motor units are recruited. In this way, a continuing rise in muscular force is ensured and smooth gradations in force result in smooth, yet powerful movements. Rate Modulation 1. Muscle force can be increased by increasing the firing frequency of the motor neuron. 2. A single action potential causes a twitch of the muscle fiber which dissipates rapidly. When the firing rate increases, multiple action potentials bombard the muscle fiber before the force has completely dissipated. 3. The series of muscle twitches will summate, and the force will ramp up so that a greater level of force is attained. This is referred to as Temporal Summation. 4. Once the firing rate reaches a high enough level ~30 hz the muscle fiber force production rapidly increases to high levels and maintains this level of force. The individual action potentials result in small fluctuations in force. This is known as unfused tetanus. 5. Temporal summation and unfused tetanus are used under normal conditions to produce force. In experimental settings, the muscle nerve can be stimulated much faster than the physiological firing rate of the MN. In this condition, maximal force is produced by the muscle and there is no opportunity for relaxation in the fibers. This is called fused tetanus. Monitoring the State of the Muscle 1. The CNS receives information about the state of the muscle from two receptors within the muscle itself. 2. The muscle spindle provides information about the length of the muscle. 3. The Golgi Tendon Organ signals changes in muscle tension. ** The Muscle Spindle 1. Muscle spindles are distributed throughout the fleshy part of the muscle and run parallel to the individual muscle fibers. 2. Each encapsulated spindle contains: a. A group of small specialized muscle fibers called intrafusal fibers b. Sensory or Afferent axons c. Motor or Efferent axons 3. The intrafusal fibers do not contribute to force production which distinguishes them from skeletal muscle fibers called extrafusal fibers. a. Intrafusal fibers include Static and Dynamic Nuclear Bag fibers as well as Nuclear chain fibers. 4. There are two sensory axons that wrap around the intrafusal fibers. a. The primary ending is a group Ia axon wrapped around the nuclear bag and chain fibers of the spindle. b. The secondary ending is a group II axon wrapped around the static nuclear bag fiber and the nuclear chain fibers but NOT the dynamic nuclear bag fiber. c. When the muscle is stretched, the intrafusal fibers are elongated which causes the primary and secondary endings to depolarize. Stretch of intrafusal fibers causes increased firing rate of the afferent axons. d. The primary endings are sensitive to the rate of change in muscle length which is referred to as velocity sensitivity. Higher firing rates of the primary endings occur during faster stretches. 3. We can see the differences in the 1? and 2? endings by recording their firing rates during various types of stretches. A linear stretch increased the firing rate of both 1? and 2? endings. A brisk tendon tap only increases the firing rate of the primary ending. This indicates that the primary endings are not only sensitive to the length of the muscle but also to the rate of change of the length. Primary nerve endings are especially sensitive to very small stretches. 4. If an intermittent stretch in the form of vibration is applied to the muscle, only the primary afferents have an increase in firing rate. The intermittent stretch is occurring too fast to affect the steady state firing of the secondary endings. 5. The motor endings regulate the sensitivity of the muscle spindle. Principles of Gamma Activation a. Axons from gamma motor neurons in the ventral horn of the spinal cord terminate near the ends of the nuclear bag and chain fibers where the contractile elements are located. b. Input from the gamma motor neuron stimulates a contraction at the ends of the intrafusal muscle fibers which causes them to become more taut. c. The more tight or stretched the intrafusal fibers, the higher the firing rate of the sensory axons thus increasing spindle sensitivity. ** The muscle spindle is also sensitive during muscle contraction 1. It is clear that the muscle spindle is capable of conveying information to the CNS about the muscle when it is stretched. However, during states of muscle contraction the muscle spindle would be rendered incapable of assessing the status of the muscle because the intrafusal fibers would be put on slack. 2. The CNS compensates for this through a process called alpha-gamma coactivation. The CNS stimulates alpha and gamma motor neurons simultaneously. 3. The extrafusal fibers contract due to firing of the alpha motor neuron. The intrafusal fibers are prevented from going slack because the gamma motor neurons cause the intrafusal fibers to contract and remain tight. 4. Alpha-gamma coactivation is an important component of normal movement because it enables the muscle spindle to convey information about the rate of change of the muscle length. The CNS can then adjust or correct the movement trajectory. Principles of Gamma Activation a. stimulation of gamma MNs results in contraction of the intrafusal fibers thereby tightening the muscle spindle and ensuring its sensitivity b. gamma MNs are most responsive to descending input from the brain and show little or no response to peripheral input. This means that higher centers can control the sensitivity of the muscle spindle by activating or biasing the gamma MNs. c. alpha gamma coactivation occurs under most conditions but level of activity may be higher in gamma MNs. This is called gamma biasing - gamma biasing occurs in one of three ways based on type of gamma - MNs there are two types of gamma - MNs: static and dynamic dynamic supply the dynamic nuclear bag fiber static supply the static nuclear bag fiber and the nuclear chain fibers static and dynamic gamma biasing means that both types of MNs are activated before a movement which results in increased sensitivity of both primary and secondary endings - dynamic gamma bias increases the sensitivity of only the dynamic nuclear bag fiber, therefore only the primary ending will be sensitive - as we have already seen the primary ending detects rapid changes in length and are especially sensitive to small length changes. - Thus, dynamic gamma bias is best suited for maintaining spindle sensitivity during small postural adjustments. - Static gamma bias: primarily the static gamma MNs are activated prior to and during a movement. - because static gamma MNs supply most of the intrafusal fibers in the spindle, all but the dynamic bag fiber, they will affect the sensitivity of both primary and secondary endings. - static gamma bias keeps shortened spindles active during small length changes as well as prevents their slackening during large changes. The Golgi Tendon Organ 1. The Golgi Tendon Organs (GTO) are located within the collagen at the myotendinous junction. 2. Each GTO is innervated by a group Ib axon which intertwines with the collagen fascicles. 3. Action potentials are elicited from the group Ib axons when the collagen is deformed by tension developed during muscle contraction. The muscle spindle is sensitive to stretch, muscle length, and rate of stretch while the GTO is sensitive to tension and force of muscle contraction. Why do we need GTO? 1. To prevent muscle damage due to excessive force generation. 2. To prevent fatigue of a motor unit. Clinical Importance of Muscle Spindles Muscle spindles are a key factor in determining muscle tone. 1. Muscle tone is the resistance of a muscle to passive stretch. 2. Muscle tone is assessed by passively flexing and extending the limb and feeling the degree of resistance encountered. 3. Under normal conditions, the muscle spindle and stretch reflex are making continual adjustments to maintain the optimal length of the muscle. The optimal length of the muscle is the sum of the excitatory and inhibitory inputs reaching the motor neuron from the brain. 4. Normal muscle tone allows us to stand erect and overcome the pull of gravity. It also provides a spring like quality to the muscle which means that muscles can store energy and release it during movements such as running. 5. Normally muscle tone is relatively low; however, lesions of the CNS produce dramatic increases in muscle tone known as hypertonus or spasticity; or decreases in tone referred to as hypotonus or flaccidity. 6. There are at least three levels within the spinal cord at which alterations in neural input will result in tone changes. a. The CNS can change the amount of fusimotor activity; i.e. the firing rate of the gamma motor neurons, thereby changing the sensitivity of the muscle spindle. b. Changes in direct descending inputs to the alpha motor neurons will reset the optimal length of the muscle, inducing changes in the activity of the stretch reflex. c. Presynaptic changes in the effectiveness of inputs to the motor neurons will drive the firing rate of the motor neuron. What are the supraspinal structures involved in the control of muscle tone? 1. Damage to supraspinal structures results in hypotonia or hypertonia depending on the structures involved. 2. If only the pyramidal tract is lesioned, hypotonia results indicating that the tract plays a facilitory role in the control of tone. 3. however, if the primary motor cortex or extrapyramidal structures are lesioned hypertonia or spasticity is seen. Indicating that they play an inhibitory role in the control of tone. 4. extrapyramidal structures include: a. Lateral Vestibular Nucleus: direct excitation of alpha MNs to the extensor muscles of the limbs, normally inhibited by cortex and cerebellum b. Reticular formation: loss of inhibitory control over the reticular formation by the cortex, cerebellum, and striatum, leads to excitation of flexor and extensor MNs of the limbs. c. Red Nucleus: provides excitatory input to flexor MNs in the cervical cord Decorticate vs Decerebrate Posturing or Rigidity 1. two clinically relevant conditions that you may see following CNS trauma in the comatose patient. 2. By applying a startling painful or auditory stimulus to the comatose patient it is possible to localize a lesion in the brainstem. 3. Damage between the red nucleus and the vestibular nuclei results in what is known as decerebrate posturing. 4. Decerebrate Posture: the comatose patient will extend the upper and lower limbs. a. Due to the loss of inhibitory influence from the cortex on the lateral vestibular and reticular nuclei. The LVN and reticular system facilitates activity in the alpha MNs to the limb extensors and rigidity ensues. 5. If the damage is located rostral to the red nucleus, in the area of the thalamus and hypothalamus, decorticate rigidity occurs. 6. Decorticate Posture: The lower limbs will extend and the upper limbs will flex. Because the RN has not been damaged, it is thought that the rubrospinal axons facilitate upper extremity flexion. Muscle Spindle Afferents Influence Alpha Motor Neurons and Produce Movement 1. The simplest motor output that results directly from activation of the muscle spindle is the stretch reflex. 2. The Ia afferent input enters the spinal cord through the dorsal root and branches. Some of the branches terminate directly on motor neurons to the same muscle, some synapse on motor neurons to synergists or antagonists and some end on interneurons. 3. The simple stretch reflex First: a brisk, passive stretch of a muscle which increases the firing rate of the group Ia afferents in the muscle spindle. Second: The action potentials enter the spinal cord and make excitatory synapses with three classes of neurons Homonymous Motor Neurons: Cells to the same muscle. Motor neurons of Synergist Muscles: Muscles that produce the same action at the same joint. Interneurons: Neurons whose cell bodies and axons are located in the grey matter of the spinal cord. Interneurons synpase on motor neurons to the antagonist muscle, the opposing muscle. Third: The motor neurons stimulate contraction of the stretched muscle and its synergists. The interneurons inhibit the motor neuron to the antagonist muscle thereby minimizing resistance. This dual action is called reciprocal innervation. Fourth: The result is contraction of the stretched muscle. Complex movements are the result of convergent input to interneurons and motor neurons 1. Excitatory and inhibitory input from the brain, spinal cord and muscle converges on the interneurons. Thus, the output of the interneuron is the sum of these inputs. 2. The interneurons serve as gates and determine whether peripheral inputs reach the motor neuron. 3. Specific reflex responses may be inhibited because descending brain systems override the peripheral input at the level of the interneuron. There are Three types of interneurons 1.Ia inhibitory interneuron: Allows higher centers to control the activity of opposing muscles at a joint through a single command. A descending signal that stimulates muscle contraction at a joint will also simultaneously relax the opposing muscles. 2. Renshaw Cells: These cells are stimulated by the motor neuron and then inhibit the same motor neuron which is called recurrent inhibition. The renshaw cell also inhibits Ia interneurons. Thus, stimulation of the renshaw cell controls the excitability of the motor neurons around the joint. 3. Group Ib Inhibitory Interneurons: The afferent fibers from the Golgi Tendon Organ synapse on Ib inhibitory interneurons which decrease the firing rate of the homonymous motor neuron. The Ib fibers also synapse on an excitatory interneuron which stimulates the motor neuron of the antagonist muscle. In this way, muscle contraction is reduced and opposing resistance is increased around the joint. A convergence of input from joint afferents, cutaneous afferents and descending systems also occurs on the Ib inhibitory interneuron thereby coordinating movements that rely on an integration of multiple inputs. Organization of complex movements is done at the level of the spinal cord by several processes some of which are used to elicit simple movements such as the stretch reflex. 1. Divergence A. A single stimulus synapses on many neurons including MNs and interneurons. B. It enables a single focal stimulus to produce coordinated contraction of several muscle groups and inhibition of others through divergent synapses on many cells. 2. Convergence A. Most neurons in the spinal cord receive input from higher centers, interneurons and peripheral receptors. The output of the MN is the sum total of all the excitatory and inhibitory input it receives. 3. Gating by interneurons A. the interneurons play a very big role in mediating coordinated muscle contractions and joint movements. B. Input from the periphery and the CNS converge on the interneuron. The output of the interneuron will reflect the majority of its inputs. C. In this way, the interneuron serves a gating function. Signals are passed on or kept from the MN depending on the activation level of the interneuron. D. gating allows higher centers in the CNS to preselect a particular response without having to receive and process afferent input directly. - For example, the CNS may increase the acitivity level of a particular set of interneurons so that only small excitatory input will be needed to fire the cell. 4. Presynaptic inhibition A. descending systems synapse on axon terminals of the presynaptic cell rather than directly on the interneuron. The action potential of the presynaptic cell is dissipated before neurotransmitter is ever released thus inhibiting input to the postsynaptic cell. 5. Reverberating circuit A. interneurons are arranged in such a way as to be self stimulating. B. excitation of the interneuron causes excitation of other interneurons which converge back on the original interneuron. C. these circuits control the temporal aspects of the response so that the response can outlast the stimulus. Central Pattern Generators are examples of reverberating circuits. Coordination of Whole Limb Movements via Spinal Mechanisms 1. The spinal cord is capable of producing complex, bilateral movements of the limbs without descending brain input. 2. The flexor withdrawal reflex is an example of the movement complexity mediated by the interplay of afferents, interneurons and motor neurons. First: A painful stimulus elicits action potentials in small A? fibers. These fibers synapse on multiple excitatory interneurons. Second: The some of the interneurons synapse on ipsilateral motor neurons to the flexors or extensors. Some of them synapse on motor neurons to the contralateral limb. Third: The ipsilateral flexors are excited while the extensors are inhibited thereby allowing the limb to be removed from the painful stimulus. At the same time, the extensors to the contralateral limb are stimulated and the flexors inhibited so that the opposite leg can provide support. Fourth: This interneuronal pathway is an efficient means to coordinate limb movements across the pelvic girdle and appears to operate during voluntary movement as well. 3. Locomotion is a sophisticated movement mediated by circuits in the spinal cord. A. An as yet undefined set of neural circuits within the spinal cord are capable of generating locomotion in the absence of any descending brain input. B. Animals with a complete transection of the spinal cord will walk when placed on a moving treadmill. The transection is at midthoracic levels and the hindlimbs take alternating, rhythmic steps. The sequence of muscle activation is complex and similar to that in normal animals. C. Circuits responsible for these movements are termed Central Pattern Generators and appear to be present at birth. They are an integral component of the spinal cord. D. There are central pattern generators for each limb. Each CPG can operate independently but are usually linked to one another. E. While afferent input from the limb moving on the treadmill belt is important to stimulate stepping in the transected animal, it is not necessary. Stepping can be elicited in animals with no afferent input reaching the cord by administration of L-dopa which demonstrates that the CPG can function in the absence of afferent input. F. The CPGs are able to adapt to changing environments. A spinal animal on a treadmill will lift the hindlimb higher as if to clear an obstacle if the top of the foot is stimulated during the swing phase. The same stimulation during stance produces greater hindlimb extension. Thus, the CPG adapts to afferent input and more importantly responds in a context specific manner. ** ANSWERS TO REVIEW QUESTIONS 2. a. Quick postural adjustments like slipping on the ice, b. Sitting, Standing, slow walking c. Quick movements: ie, pitching a baseball; muscle twitches d. Stationary Positions; holding a pose; slow movements like Tai Chi - Sleeping on a moving subway train: Movement is imposed on the body which requires quick adjustments of postural muscles, therefore, dynamic is high. Static is low because the muscles are at rest when the perturbations occur. - Standing to deliver a speech: static-moderately high; dynamic-low (unless your knees are shaking due to apprehension J). - Riding a horse: Static high because trying to maintain a posture with movement imposed on it. Dynamic would also be high especially when unexpected perturbations occur like jumping over a ditch. f. Yes, the resting length of the muscle is usually shorter so that there is greater overlap which makes sense given that one of the determinants of tone is actin-myosin overlap.