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
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 by signals from bulboreticular facilitatory region of brain stem and, secondarily, by impulses transmitted into bulboreticular area from the cerebellum, the basal ganglia, and the cerebral cortex. • The bulboreticular facilitatory area is concerned with antigravity contractions, and antigravity muscles have high density of muscle spindles, emphasis is given to the importance of the gamma efferent mechanism for damping movements of different body parts during walking and running. decerebrate animal
• A decerebrate animal has stereotyped and usually heightened stretch reflexes • Without control by higher brain centers, descending pathways from the brain stem powerfully facilitate the neuronal circuits involved in the stretch reflexes of extensor muscles. • This results in a dramatic increase in extensor muscle tone that sometimes suffices to support the animal in a standing position. • In normal animals, owing to the balance between facilitation and inhibition , reparation. The effect of this procedure is stretch reflexes are weaker and considerably more variable in strength than those in decerebrate animals.
Clinical Applications of the Stretch Reflex • The purpose is to determine how much background excitation, or "tone," the brain is sending to spinal cord. • The muscle jerks are used to assess degree of facilitation of spinal cord centers. When large numbers of facilitatory impulses are being transmitted from upper regions of central nervous system into the cord, the muscle jerks are greatly exaggerated. • if facilitatory impulses are depressed muscle jerks are weakened or absent. These reflexes are used in determining presence or absence of muscle spasticity caused by lesions in motor areas of brain or diseases that excite bulboreticular facilitatory area of the brain stem. • large lesions in motor areas of the cerebral cortex but not in the lower motor control areas (lesions caused by strokes or brain tumors) cause greatly exaggerated muscle jerks in muscles on the opposite side of the body. Clonus-Oscillation of Muscle Jerks. Golgi tendon organs
• Golgi tendon organs are slender encapsulated structures approximately 1 mm long and 0.1 mm in diameter • located at the junction between skeletal muscle fibers and tendon. • Each capsule encloses several braided collagen fibers connected in series to a group of muscle fibers. • Each tendon organ is innervated by a single Ib axon that branches into many fine endings inside the capsule; these endings become intertwined with the collagen fascicles
Golgi tendon organs • Stretching of the tendon organ straightens the collagen fibers, thus compressing the Ib nerve endings and causing them to fire. • Because the nerve endings are so closely associated with the collagen fibers, even very small stretches of the tendons can compress the nerve endings. • Whereas muscle spindles are most sensitive to changes in length of a muscle, tendon organs are most sensitive to changes in muscle tension. • Contraction of the muscle fibers connected to the collagen fiber bundle containing the receptor is a particularly potent stimulus to a tendon organ. • The tendon organs are thus readily activated during normal movements. Transmission of Impulses from Tendon Organ into CNS
• Ib nerve fibers transmit signals both into local areas of the cord and, after synapsing in a dorsal horn of the cord, through long fiber pathways such as the spinocerebellar tracts into the cerebellum and through still other tracts to the cerebral cortex. • The local cord signal excites a single inhibitory interneuron that inhibits the anterior motor neuron. • This local circuit directly inhibits the individual muscle without affecting adjacent muscles. Tendon Reflex and Its Importance • This reflex is inhibitory,provides a negative feedback mechanism that prevents development of too much tension on muscle. • lengthening reaction; it is probably a protective mechanism to prevent tearing of the muscle or avulsion of the tendon from its attachments to the bone. Convergence onto lb interneurons Golgi tendon organs
• Golgi tendon organs were first thought to have a protective function, preventing damage to muscle, for it was assumed that they always inhibited homonymous motor neurons and that they fired only when tension in the muscle was high. • But we now know that these receptors signal minute changes in muscle tension, providing nervous system with precise information about the state of a muscle's contraction. withdrawal reflex Flexor Reflex Withdrawal Reflexes • cutaneous sensory stimulus from a limb is likely to cause the flexor muscles of the limb to contract, thereby withdrawing the limb from the stimulating object. • the flexor reflex is elicited most powerfully by stimulation of pain endings nociceptive reflex, or simply a pain reflex Flexion-withdrawal reflex
• The sensory signal activates divergent polysynaptic reflex pathways. • One excites motor neurons that innervate flexor muscles of the stimulated limb • Whereas another inhibits motor neurons that innervate the limb's extensor muscles . • Excitation of one group of muscles and inhibition of their antagonists-those that act in the opposite direction-is called reciprocal innervation, a key principle of motor organization basic types of circuits
• Diverging circuits to spread the reflex to the necessary muscles for withdrawal • circuits to inhibit the antagonist muscles, called reciprocal inhibition circuits • circuits to cause after discharge lasting many fractions of a second after the stimulus is over
Flexion-withdrawal reflex • The reflex can produce an opposite effect in the contralateral limb, that is, excitation of extensor motor neurons and inhibition of flexor motor neurons. • This crossed-extension reflex serves to enhance postural support during withdrawal of a foot from a painful stimulus. • Activation of the extensor muscles in opposite leg counteracts the increased load caused by lifting the stimulated limb. • Thus, flexion-withdrawal is a complete, albeit simple, motor act.
principle of "local sign
• the integrative centers of the cord cause those muscles to contract that can most effectively remove the pained part of the body away from the object causing the pain. • Although this principle, called the principle of "local sign," applies to any part of the body, it is especially applicable to the limbs because of their highly developed flexor reflexes. Reflexes of Posture and Locomotion • Positive Supportive Reaction:Pressure on the footpad of a decerebrate animal causes the limb to extend against the pressure applied to the foot. • Cord "Righting“ Reflexes • Stepping and Walking Movement • Scratch Reflex
. Spinal Cord Reflexes That Cause Muscle Spasm
• Muscle Spasm Resulting from a Broken Bone
• Abdominal Muscle Spasm in Peritonitis
• Muscle Cramps Autonomic Reflexes in the Spinal Cord • Many types of segmental autonomic reflexes are integrated • in the spinal cord 1. changes in vascular tone resulting from changes in local skin heat 2. sweating, which results from localized heat on the surface of the body 3. intestinointestinal reflexes that control some motor functions of the gut 4. peritoneointestinal reflexes that inhibit gastrointestinal motility in response to peritoneal irritation 5. evacuation reflexes for emptying the full bladder or the colon Mass Reflex
• In a spinal animal or human being, sometimes the spinal cord suddenly becomes excessively active, causing massive discharge in large portions of the cord. • The usual stimulus that causes this is a strong pain stimulus to the skin or excessive filling of a viscus, such as overdistention of the bladder or the gut • (1) a major portion of the body's skeletal muscles goes into strong flexor spasm; • (2) the colon and bladder are likely to evacuate; • (3) the arterial pressure often rises to maximal values, sometimes to a systolic pressure well over 200 mm Hg; • (4) large areas of the body break out into profuse sweating.
• Because the mass reflex can last for minutes, it presumably results from activation of great numbers of reverberating circuits that excite large areas of the cord at once. This is similar to the mechanism of epileptic seizures Spinal Cord Transection and Spinal Shock • When the spinal cord is suddenly transected in the upper neck, at first, essentially all cord functions, including cord reflexes, immediately become depressed to the point of total silence • The reason for this is that nomal activity of the cord neurons depends to a great extent on continual tonic excitation by the discharge of nerve fibers entering the cord from higher centers, particularly discharge transmitted through the reticulospinal tracts, vestibulospinal tracts, and corticospinal tracts. • After a few hours to a few weeks, the spinal neurons gradually regain their excitability. This seems to be a natural characteristic of neurons everywhere in the nervous system that is, after they lose their source of facilitatory impulses, they increase their own natural degree of excitability to make up at least partially for the loss. • In most nonprimates, excitability of the cord centers returns essentially to normal within a few hours to a day or so, but in human beings, the return is often delayed for several weeks and occasionally is never complete; conversely, sometimes recovery is excessive, with resultant hyperexcitability of some or all cord functions. spinal shock
• 1. At onset of spinal shock, the arterial blood pressure falls instantly and drastically-sometimes to as low as 40 mm Hg-thus demonstrating that sympathetic nervous system activity becomes blocked almost to extinction. The pressure ordinarily returns to normal within a few days, even in human beings. • 2. All skeletal muscle reflexes integrated in the spinal cord are blocked during the initial stages of shock. In lower animals, a few hours to a few days are required for these reflexes to return to normal; in human beings, 2 weeks to several months are sometimes required. In both animals and humans, some reflexes may eventually become hyperexcitable, particularly if a few facilitatory pathways remain intact between the brain and the cord while the remainder of the spinal cord is transected. • The first reflexes to return are the stretch reflexes, followed in order by the progressively more complex reflexes: flexor reflexes, postural antigravity reflexes, and remnants of stepping reflexes. • 3. The sacral reflexes for control of bladder and colon evacuation are suppressed in human beings for the first few weeks after cord transection, but in most cases they eventually return.