Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Control of body movement The somato-motor division Somatic Motor Pathways SNS, or the somatic motor system, controls contractions of skeletal muscles Conscious and Subconscious Motor Commands Skeletal muscle contraction results in Posture Reflexes Rhythmic activity (locomotion, breathing) Voluntary movement Somatic Motor Pathways – from brain to effector Always involve at least two motor neurons – upper and lower motor neurons Upper motor neuron Cell body lies in a CNS processing center activity in upper motor neuron may facilitate or inhibit lower motor neuron Lower motor neuron Cell body lies in a nucleus of the brain stem or spinal cord Triggers a contraction in innervated muscle only the axon of lower motor neuron extends outside CNS Parts of CNS that are involved in the SNS Motor areas of the cerebral cortex Basal nuclei Cerebellum Medulla oblongata Descending pathways in spinal cord Ventral horn of spinal cord Primary Motor Cortex Located in the precentral gyrus in the frontal lobe in each hemisphere Pyramidal cells that have long axon project to the spinal cord and form a voluntary motor tracts called pyramidal tracts/corticospinal tracts A pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar neuron found in the cerebral cortex. These cells have a triangularly shaped soma Pyramidal neurons compose approximately 80% of the neurons of the cortex Release glutamate as their neurotransmitters, making them the major excitatory component of the cortex Allows conscious control of precise, skilled, voluntary movements Primary Motor Cortex Homunculus Somatotopy mapping Body is represented upside down Although simplified in the figure, one should remember that: A given muscle is controlled by multiple spots on the cortex Individual cortical neurons send impulses to more than one muscle Neurons that control related movements will overlap Neurons that control unrelated movements do not cooperate Posterior Motor Motor map in precentral gyrus Anterior Toes Jaw Tongue Swallowing Primary motor cortex (precentral gyrus) Figure 12.9.1 Basal nuclei of cerebrum Are masses of gray matter within each hemisphere deep to lateral ventricle floor Provide subconscious control of skeletal muscle tone and help coordinate learned movement patterns Normally do not initiate movement, but provide general pattern and rhythm Cerebellar Function • Adjust ongoing movements on the basis of comparison between arriving sensation to one previously experienced – Posture: • Balance • Equilibrium – Fine Tune Movements • Timing • Rate • Range • Force The Cerebellum Somatic Motor Pathways Three motor pathways Corticospinal pathway Medial pathway Lateral pathway Somatic motor system Corticospinal Medial pathway Vestibulospinal Corticobulbar Corticospinal Tectospinal Lateral corticospinal Lateral pathways Reticulospinal Anterior corticospinal Rubrospinal Body area Voluntary control Skeletal muscles of body Skeletal muscles of head Subconscious Reflex control activity Equilibrium Ascending pathway Corticospinal Upper neuron location Precentral gyrus Lower neuron location Anterior gray horn Corticobulbar Precentral gyrus Nuclei of cranial nerves Reticulospinal Brainstem nuclei (reticular formation) Nucleus of cranial nerve VIII Superior and inferior colliculi Anterior gray horn Vestibulospinal Auditory and Tectospinal visual reflexes Distal Rubrospinal muscles of upper limbs Red nucleus of midbrain Anterior gray horn Anterior gray horn of cervical area Anterior gray horn in cervical area The Corticospinal /pyramidal/direct Pathway Upper motor neurons begin at the primary motor cortex Synapses with lower motor neurons occur in two tracts Corticobulbar (bulbar, brain stem) tracts move the eye, jaw, face, and some muscles of neck and pharynx Synapses in motor nuclei of cranial nerves Corticospinal tracts Provide conscious control over skeletal muscles that move various body areas Synapse in the anterior gray horn in the spinal cord The corticobulbar tracts Cranial nerve Muscles/area The occulomotor III Extrinsic muscles of the eye The trochlear IV Extrinsic muscles of the eye The abducens VI Extrinsic muscles of the eye The facial VII muscles of facial expression The glossopharyngeal IX muscles involved in swallowing The accessory XI muscles of neck and upper back The hypoglossal XII tongue movements Somatic Motor Pathways – the corticospinal pathways – voluntary pathways Begins at pyramidal cells in primary motor cortex Primary motor cortex corresponds point by point with specific regions of the body Upper axons descend into brain stem and spinal cord Synapse with lower motor neurons that control muscles directly Somatic motor system Corticospinal Corticobulbar Corticospinal Lateral corticospinal Anterior corticospinal Somatic Motor Pathways – the corticospinal pathway The lateral corticospinal tracts (85 %) cross over at the level of the medulla (pyramids) Exits at all levels of the spinal cord responsible for the control of the distal musculature fine control of the digits of the hand The anterior/ventral corticospinal (15%) tract crosses over in anterior gray horns before synapsing Exits at C1-L3 responsible for the control of the proximal musculature Somatic Motor Pathways Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Initiation of Skilled Movement Frontal Association and Primary Motor Cortex Frontal cortex can plan and initiate movement but cannot calculate the complex, timed sequence of muscle contraction Send information about intended movements to the cerebellum Basal Ganglia Somatosensory System Information on current position Lateral Zone of Cerebellum When cerebellum receives information about initiated movement it computes the contribution that various muscles will have to make Sends results Dentate Nucleus In cerebellum Via basal ganglia Allows the cerebellum to modify the ongoing movement that was initiated by the frontal cortex Thalamus The cerebellum can also control a skilled movement by timing the movement and by turning on the antagonist muscle. This happens when the movement is rapid and cannot relay on feedback. Somatic Motor Pathways – medial and lateral pathways Involuntary pathways A cross section of the spinal cord showing the locations of the medial and lateral pathways Lateral corticospinal tract Medial Pathway Involved primarily with the control of muscle tone and gross movements of the neck, trunk, and proximal limb muscles Lateral Pathway Involved primarily with the control of muscle tone and the more precise movements of the distal parts of the limbs Rubrospinal tract Reticulospinal tract Vestibulospinal tract Tectospinal tract Anterior corticospinal tract Figure 13.17 3 Somatic motor system Medial pathway Vestibulospinal Tectospinal Reticulospinal The locations of centers in the cerebrum, diencephalon, and brain stem that may issue somatic motor commands as a result of processing performed at a subconscious level Motor cortex Thalamus Basal nuclei Red nucleus Cerebellar nuclei Nuclei of the Medial Pathway Superior and inferior colliculi Reticular formation Vestibular nucleus Medulla oblongata Figure 13.17 2 Body area Voluntary control Skeletal muscles of body Skeletal muscles of head Subconscious Reflex control activity Ascending pathway Corticospinal Upper neuron location Precentral gyrus Lower neuron location Anterior gray horn Corticobulbar Precentral gyrus Nuclei of cranial nerves Brainstem nuclei (reticular formation) Nucleus of cranial nerve VIII Anterior gray horn Superior and inferior colliculi Anterior gray horn of cervical area Anterior gray horn in cervical Reticulospinal (medial pathways) Equilibrium Vestibulospinal (medial pathways) Auditory and Tectospinal visual (medial reflexes pathways) Distal Rubrospinal muscles of (lateral pathway) Red nucleus of midbrain Anterior gray horn Somatic motor system Lateral pathways Rubrospinal Somatic motor pathways - subconscious motor commands Lateral pathway control of muscle tone and the more precise movements of the distal parts of the upper limbs Upper motor neuron in the Red nucleus of the midbrain Found only in the cervical area Voluntary control Skeletal muscles of body Skeletal muscles of head Subconscious Reflex control activity pathway Corticospinal location Precentral gyrus location Anterior gray horn Corticobulbar Precentral gyrus Nuclei of cranial nerves Brainstem nuclei (reticular formation) Nucleus of cranial nerve VIII Anterior gray horn Superior and inferior colliculi Anterior gray horn of cervical area Anterior gray horn in cervical area Reticulospinal (medial pathways) Equilibrium Vestibulospinal (medial pathways) Auditory and Tectospinal visual (medial reflexes pathways) Distal Rubrospinal muscles of (lateral pathway) upper limbs Red nucleus of midbrain Anterior gray horn Three Muscle Types All muscle tissue exhibit: Responsiveness - The ability to receive and respond to a stimulus Conductivity – the ability of the impulse to travel along the plasma membrane of the muscle cell. Contractility - The ability to shorten Elasticity - The ability to recoil and resume original length Skeletal Muscle Functions Movement of bones or fluids (e.g., blood) Maintaining posture and body position Stabilizing joints Heat generation Each muscle is served by one artery, one nerve, and one or more veins Muscle terminology Muscle fiber – muscle cell Sarcolema – cell membrane Sarcoplasm – cytoplasm Sarcoplasic reticulum – endoplasmic reticulum Skeletal Muscle Connective tissue sheaths of skeletal muscle: Epimysium: dense regular connective tissue surrounding entire muscle Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) Endomysium: fine areolar surrounding each muscle fiber connective tissue Microscopic Anatomy of a Skeletal Muscle Fiber Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma Fibers are 10 to 100 m in diameter, and up to hundreds of centimeters long Sarcoplasm has numerous glycosomes (granules that store glycogen) and a unique oxygen-binding protein called myoglobin (similar to hemoglobin) Skeletal Muscle organization In order of decreasing size… Myofiber - entire cell. Myofibrils - bundles of myofilaments inside myofiber. Myofilaments - actin and myosin proteins. Myofibrils Myofibrils are densely packed contractile elements They make up most of the muscle volume Sarcolemma Mitochondrion Myofibril Dark A band Light I band Nucleus (b) Diagram of part of a muscle fiber showing the myofibrils. One myofibril is extended afrom the cut end of the fiber. Sarcomeres (within myofibril) The smallest contractile unit of a muscle The region of a myofibril between two successive Z discs Composed of myofilaments made up of contractile proteins – actin and myosin Sliding Filament Model of Contraction Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree In the relaxed state, thin and thick filaments overlap only slightly Upon stimulation, myosin heads bind to actin and sliding begins When muscle contracts the actin filaments slide into the A/H band overlapping the myosin http://www.brooklyn.cuny.edu/bc/ahp/LAD/C4b/C4b_muscle.html Events at the Neuromuscular Junction Skeletal muscles are stimulated by somatic motor neurons Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles Each axon forms several branches as it enters a muscle Each axon ending forms a neuromuscular junction with a single muscle fiber Events in Generation of muscle contraction 1. Local depolarization (end plate potential): ACh binding opens chemically (ligand) gated ion channels Simultaneous diffusion of Na+ (inward) and K+ (outward) More Na+ diffuses, so the interior of the sarcolemma becomes less negative Local depolarization – end plate potential Excitation-Contraction (E-C) Coupling Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments Latent period: Time when E-C coupling events occur Time between AP initiation and the beginning of contraction Voltage-sensitive proteins stimulate Ca2+ release from SR Ca2+ is necessary for contraction AP FLIX - Excitation-Contraction (E-C) Coupling Muscle Twitch Response of a muscle to a single, brief threshold stimulus Three phases of a twitch: Latent period: events of excitation-contraction coupling Period of contraction: cross bridge formation; tension increases Period of relaxation: Ca2+ reentry into the SR; tension declines to zero Tension Maximum tension development Resting phase Stimulus Contraction phase Relaxation phase Time (msec) The latent period begins at stimulation and typically lasts about 2 msec. During this period, an action potential sweeps across the sarcolemma, and the sarcoplasmic reticulum releases calcium ions. The muscle fiber does not produce tension during the latent period, because the contraction cycle has yet to begin. In the contraction phase, tension rises to a peak. As the tension rises, calcium ions are binding to troponin, active sites on thin filaments are being exposed, and cross-bridge interactions are occurring. The relaxation phase lasts about 25 msec. During this period, calcium levels are falling, active sites are being covered by tropomyosin, and the number of active cross-bridges is declining as they detach. As a result, tension returns to resting levels. Figure 9.6 3 Motor Unit: The Nerve-Muscle Functional Unit Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies Small motor units in muscles that control fine movements (fingers, eyes) Large motor units in large weight-bearing muscles (thighs, hips) Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle Force of Muscle Contraction The force of contraction is affected by: Length-tension relationship — muscles contract most strongly when muscle fibers at relaxation are at 80– 120% of their normal resting length Frequency of stimulation — frequency allows time for more effective transfer of tension to noncontractile components Number of muscle fibers stimulated (recruitment) Relative size of the fibers — hypertrophy of cells increases strength Length-tension relationship The force of muscle contraction depends on the length of the sarcomeres before the contraction begins On the molecular level, the length reflects the overlapping between thin and thick filaments The tension a muscle fiber can generate is directly proportional to the number of crossbridges formed between the filament Response to Change in Stimulus Frequency A single stimulus results in a single contractile response — a muscle twitch Single stimulus single twitch Contraction Relaxation Stimulus A single stimulus is delivered. The muscle contracts and relaxes Response to Change in Stimulus Frequency Increase frequency of stimulus (muscle does not have time to completely relax between stimuli) Ca2+ release stimulates further contraction temporal (wave) summation Further increase in stimulus frequency unfused (incomplete) tetanus If stimuli are given quickly enough, fused (complete) tetany results Rarely happens in the body – mostly in lab conditions Response to Change in Stimulus Strength Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs Muscle contracts stronger as stimulus strength is increased above threshold Contraction force is controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action Response to Change in Stimulus Strength Size principle: motor units with larger and larger fibers (cells) are recruited as stimulus intensity increases Motor unit 1 Recruited (small fibers) Motor unit 2 recruited (medium fibers) Motor unit 3 recruited (large fibers) Contraction does not always shorten a muscle Isotonic contraction: muscle shortens because muscle tension exceeds the load Isometric contraction: no shortening; muscle tension increases but does not exceed the load Type of muscle contraction - Isotonic Contractions Muscle changes in length and moves the load Isotonic contractions are either concentric or eccentric: Concentric contractions — the muscle shortens and does work For example, concentric contraction is used to lift a glass from a table Eccentric contractions — the muscle contracts as it lengthens Example – someone pulls your arm straight while at the same time you try to keep the arm locked in one position Type of muscle contraction - Isometric Contractions • The load is greater than the tension the muscle is able to develop • Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens Muscles need energy to contract ATP is the only source used directly for contractile activities Muscles store few high-energy molecules ATP - Available stores of ATP are used in 4–6 seconds Creatine phosphate (CP) Most energy stored as glycogen May account for 1.5% of total muscle weight Enables extended periods of muscle contractions Figure 9.9 2 ATP production in muscles – 3 sources Glycolysis (anaerobic: does not require oxygen) Occurs in sarcoplasm Produces 2 ATP and 2 pyruvate molecules for each glucose Aerobic metabolism Provides 95% of ATP demands of resting muscle cell Occurs in mitochondria Primarily through electron transport chain activity Creatine phosphate (CP) Creatine assembled from amino acids Facilitates regeneration of ATP ADP + CP ATP + C Muscle Metabolism: Energy for Contraction ATP demand and production at different activity levels At rest Demand for ATP is low Surplus ATP produced by mitochondria (aerobic) Used to build up CP and glycogen reserves At moderate activity levels Demand for ATP increases ATP production by mitochondria (aerobic metabolism) meets demand ATP demand and production at different activity levels At peak activity levels Mitochondria can provide only ~1/3 ATP demand Glycolysis provides most ATP (anaerobic pathways due to low oxygen) Excess pyruvate converts to lactic acid Diffuses into the bloodstream Used as fuel by the liver, kidneys, and heart Converted back into pyruvic acid by the liver Decreases intracellular pH Can affect enzymatic activities and cause fatigue Muscle Fatigue Physiological inability to contract or sustain the expected power output It is a reversible condition Fatigue can potentially occur at any of the points involve in muscle contraction – from the brain to the muscle fibers or in any of the systems that are responsible to supply oxygen to the muscles Total lack of ATP occurs rarely during states of continuous contraction Muscle Fatigue Central fatigue mechanisms – arise from the CNS Includes subjective feeling of tiredness and desire to cease activity Suggested reasons include low pH, failure to produce enough ACh Peripheral fatigue mechanisms – anywhere between the neuromuscular junction and the muscle Lack of glycogen Ionic imbalances (K+, Ca2+, Pi) interfere with E-C coupling Muscle Fatigue An endurance exercise training can delay onset of fatigue by increasing oxidative capacity: Increased number of mitochondria Increased level of oxidative enzymes Increased number of capillary beds to muscle Muscle Performance Power The maximum amount of tension produced Endurance The amount of time an activity can be sustained Power and endurance depend on The types of muscle fibers Physical conditioning Muscle Fiber Types Muscle fiber type is defined by 2 criteria Speed of contraction – determined by speed in which ATPases split ATP The two types of fibers are slow and fast ATP-forming pathways Oxidative fibers – use aerobic pathways Glycolytic fibers – use anaerobic glycolysis Muscle Fiber Type: Functional Characteristics These two criteria define three categories Slow oxidative fibers contract slowly, have slow acting ATPases, and are fatigue resistant (contain myoglobin) Fast oxidative fibers contract quickly, have fast ATPases, and have moderate resistance to fatigue Fast glycolytic fibers contract quickly, have fast ATPases, and are easily fatigued (large glycogen reserve, few mitochondria) Terms of muscle diameter changes Hypertrophy – increased total mass of muscle Result of increased number of the filaments in each fiber The enzyme systems also increase (mainly enzymes for glycolysis) If the muscles are not used for many weeks, the rate of decay of contractile units is more rapid results in atrophy (decrease in mass) Adjustment of muscle length It is a type of hypertrophy in which muscles are stretched to greater than normal length That causes the addition of new sarcomeres at the end of the fibers Anaerobic endurance Refers to the length of time muscle contraction can continued to be supported by glycolysis and existing energy reserve of ATP and CP It is limited by: Amount of ATP and CP Amount of glycogen Ability of the muscle to tolerate lactic acid The use of resistance to muscular contraction to build the strength and size of skeletal muscles. Muscle fatigue occurs within 2 minutes of start of maximal activity Effects of Resistance Exercise (anaerobic) • Resistance exercise results in: • Muscle hypertrophy (due to increase in fiber size) • Increased mitochondria, myofilaments, glycogen stores, and connective tissue • Examples – weight lifting, Machines that offer resistance Aerobic exercise Aerobic exercise is physical exercise that intends to improve the oxygen system Leads to increased: Muscle capillaries Number of mitochondria Myoglobin synthesis Results in greater endurance, strength, and resistance to fatigue May convert fast glycolytic fibers into fast oxidative fibers Examples: running, swimming, cycling Effects of Exercise - Aerobic exercise The length of time a muscle can continue to contract while being supported by mitochondrial activities Among the recognized benefits of doing regular aerobic exercise are: Strengthening the muscles involved in respiration, to facilitate the flow of air in and out of the lungs Strengthening and enlarging the heart muscle, to improve its pumping efficiency and reduce the resting heart rate, known as aerobic conditioning Strengthening muscles throughout the body Improving circulation efficiency and reducing blood pressure Energy systems used in various sports Energy system Sport Creatine phosphate, almost 100 meter sprints, Weight lifting, diving entirely (anaerobic) CP and glycogen-lactic acid 200 meter sprints, basketball (anaerobic) glycogen-lactic acid, mainly 400 meters sprints, 100 meter swim, (anaerobic) tennis glycogen-lactic acid and aerobic 800 meter sprints, boxing, 1 mile run Aerobic system 10,000 meter skating, marathon