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Motor Neurons: • Alpha Motor Neurons Motor Unit • Gamma Motor Neurons Muscle Spindle • Interneurons , Renshaw Cells

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Motor Neurons: • Alpha Motor Neurons Motor Unit • Gamma Motor Neurons Muscle Spindle • Interneurons , Renshaw Cells Motor System Elements of Motor System Effectors MOTOR SYSTEM Skeletal Muscle SOMATIC MOTOR SYSTEM Smooth Muscle AUTONOMIC MOTOR SYSTEM Glands (sympathetic and Parasympathetic) Somatic Motor System Autonomic Motor System sympathetic response Central nervous Spinal Brain system cord (CNS) (Input to CNS (Output from CNS from periphery) to periphery) Peripheral Afferent nervous Efferent division system division (PNS) Sensory Visceral Somatic Autonomic stimuli stimuli nervous system nervous system Motor Sympathetic Parasympathetic neurons nervous system nervous system Skeletal Smooth muscle Cardiac muscle muscle Glands Effector organs (made up of muscle and gland tissue) The Central Nervous System • The Central Nervous System Has Main Parts: • The spinal cord • The brain stem : The medulla oblongata, The pons ,The midbrain • The cerebellum • The diencephalon • The cerebrum CNS &Movement •Spinal cord • Brain stem • Cortex •Cerebellum • Basal ganglia Motor systems • The highest :the prefrontal cortex, deals with the purpose of a movement. • The next level, formation of a motor plan, involves interactions between the posterior parietal and premotor areas of the cerebral cortex. The premotor cortex specifies the spatial characteristics of a movement based on sensory information from the posterior parietal cortex about the environment and about position of the body in space. • The lowest level :coordinates the spatiotemporal details of the muscle contractions needed to execute the planned movement. • This coordination is executed by the primary motor cortex, brain stem, and spinal cord. This serial view has heuristic value, but evidence suggests that many of these processes can occur in parallel. Somatic Motor System Upper Motor Neuron Descending Brain Stem UMN Pathways pyramidal Final Common Pathway tract Lower Motor Neuron AUTOMATIC MOTOR VOLUNTARY CONTROL LMN CONTROL motor nerve reflex arc Skeletal Muscle UPPER MOTOR NEURONE SIGNS • The lesion is above anterior horn cell (spinal cord, brain stem, motor cortex). • Increased muscle tone (spasticity), • Weakness :flexors weaker than extensors in the legs and the reverse in the arms - pyramidal pattern • increased reflexes, an up-going plantar response and sustained clonus LOWER MOTOR NEURONE SIGNS • the lesion is in the anterior horn cell or distal to the anterior horn cell ( anterior horn cell, root, plexus, peripheral nerve). • Decreased muscle tone, • Atrophy: weakness and wasting in muscles supplied by that motor nerve • Arreflexia • Muscle fasciculations. – Back pain and sciatica suggests a root problem – Weakness of the biceps with absence of the biceps reflex, with upper motor neurone signs in the legs suggests cord disease (eg a disc) at C5/6 Predominantly Motor Syndromes • Poliomyelitis (Infantile Paralysis) - viral infection of lower motor neuron - LMN syndrome at the level of lesion • Amyotrophic Lateral Sclerosis (ALS) - combined LMN and UMN lesion - LMN syndrome at the level of lesion - UMN syndrome below the level of lesion Amyotrophic Lateral Sclerosis (ALS) Stephen Hawking (1946- ) British Physicist, A Brief History of Time Type of movement • Voluntary movements: are those that are under conscious control by the brain. • Rhythmic movements :can also be controlled Voluntarily, but many such movements differ from voluntary movements in that their timing and spatial organization is to a large extent controlled autonomously by spinal or brain stem circuitry. • Reflexes are stereotyped responses to specific stimuli that are generated by simple neural circuits in the spinal cord or brain stem. Voluntary movements Rhythmic movements Reflexes The spinal cord The most caudal part of the central nervous system, receives and processes sensory information from the skin, joints, and muscles of the limbs and trunk and controls movement of the limbs and the trunk. It is subdivided into cervical, thoracic, lumbar, and sacral regions Spinal Cord for Motor Functions • The cord gray matter is integrative area for reflexes. • Anterior motor neurons: • Alpha motor neurons motor unit • Gamma motor neurons muscle spindle • Interneurons , Renshaw cells Anterior motor neurons Reflexes Reflexes • Reflexes are stereotyped responses to specific stimuli that are generated by simple neural circuits in the spinal cord or brain stem. • Several different circuits exist to connect sensory and motor neurons, they cannot be directly controlled voluntarily. The Stretch Reflex Muscle Sensory Receptors And Their Roles in Muscle Control • (1) muscle spindles send information to the nervous system about muscle length or rate of change of length • (2) Golgi tendon organs transmit information about tendon tension or rate of change of tension. • Control of muscle function requires excitation of the muscle by spinal anterior motor neurons continuous feedback of sensory information from each muscle to the spinal cord • functional status of each muscle at each instant. • The signals from these two receptors are entirely for the purpose of intrinsic muscle control. • They operate almost completely at a subconscious level. • They transmit amounts of information to the spinal cord ,to the cerebellum and to the cerebral cortex. • These portions of the nervous system function to control muscle contraction. Muscle Spindles • Muscle spindles are small encapsulated sensory receptors that have a spindle-like or fusiform shape and are located within the fleshy part of a muscle. • Their main function is to signal changes in the length of the muscle within which they reside. • Changes in length of muscles are closely associated with changes in the angles of the joints that the muscles cross. • Thus muscle spindles are used by the central nervous system to sense relative positions of body segments. • Each spindle has three main components: • (1) a group of specialized intrafusal muscle fibers with central regions that are non contractile • (2) sensory fibers that terminate in the non contractile central regions of the intrafusal fibers • (3) motor axons that terminate in the polar contractile regions of the intrafusal fibers • When the intrafusal fibers are stretched, the sensory nerve endings are also stretched and increase their firing rate. • Because muscle spindles are arranged in parallel with the extrafusal muscle fibers that make up the main body of muscle, the intrafusal fibers change in length as the whole muscle changes. • Thus, when a muscle is stretched, activity in the sensory endings of muscle spindles increases. • When a muscle shortens, the spindle activity decreases. gamma motor neurons • The intrafusal muscle fibers are innervated by gamma motor neurons, whereas the extrafusal muscle fibers are innervated by alpha motor neurons. • Activation of gamma motor neurons causes shortening of the polar regions of the intrafusal fibers. This is turn stretches the central region from both ends, leading to an increase in firing rate of the sensory endings • Thus the gamma motor neurons adjust the sensitivity of the muscle spindles. • When a muscle is stretched the change in length has two phases: a dynamic phase, the period during which length is changing, and a static or steady-state phase, when the muscle has stabilized at a new length. • Structural specializations within each component of the muscle spindles allow spindle afferents to signal aspects of each phase separately. intrafusal muscle fibers • Nuclear bag fibers and nuclear chain fibers. • The nuclear bag fibers can be divided into two groups, dynamic and static. • A typical spindle has two or three bag fibers and a variable number of chain fibers, usually about five. • • The intrafusal fibers receive two types of sensory endings. • A single Ia axon spirals around the central region of all intrafusal muscle fibers and serves as the primary sensory ending • A variable number of type II axons, located adjacent to the central regions of the static bag and chain fibers, serve as secondary sensory endings. • The gamma motor neurons can also be divided into two classes: • Dynamic gamma motor neurons innervate the dynamic bag fibers, whereas the static gamma motor neurons innervate the static bag fibers and the chain fibers. • Increases in firing rate of dynamic gamma motor neurons increase the dynamic sensitivity of primary sensory endings. • Increases in firing rate of static gamma motor neurons increase tonic level of activity in both primary and secondary sensory endings • The tonic discharge of both primary and secondary sensory endings signals the steady- state length of the muscle. • The primary sensory endings are, highly sensitive to the velocity of stretch, allowing them to provide information about the speed of movements. • they are highly sensitive to small changes, the primary endings rapidly provide information about sudden unexpected changes in length, which can be used to generate quick corrective reactions. "Damping" Function of Dynamic and Static Stretch Reflexes Role of the Muscle Spindle in Voluntary Motor Activity • 31 per cent of all the motor nerve fibers to muscle are the gamma efferent fibers • Whenever signals are transmitted from motor cortex or from any other area of brain to alpha motor neurons, in most instances gamma motor neurons are stimulated simultaneously ,an effect called coactivation of the alpha and gamma motor neurons. • Coactivation keeps muscle spindle reflex from opposing muscle contraction and maintains damping function of muscle spindle Brain &Control of the Gamma Motor System • The gamma efferent system is excited
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