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
Dr. SaadKlemanAbd ]MOTOR SYSTEMS, POSTURE, SOMATIC SENSATION, [ MOTOR NEUROPHYSIOLOGY Shortcut to MOTOR SYSTEM.pdf.lnk Shortcut to Medical_Physiology__The_Big_Picture.pdf.lnk http://slidepl ayer.com/slide/4280903/ Motor output is of two types: Reflexive. Involuntary, and voluntary. Some would add as a subdivision of reflex responses rhythmic responses;such as swallowing, chewing, scratching, and walking, which are largely involuntary but subject to voluntary adjustment and control. Much is still unknown about the control of voluntary movement. To move a limb, for example, the brain must plan a movement, arrange appropriate motion at many different joints at the same time, and adjust the motion by comparing plan with performance. The motor system “learns by doing,” and performance improves with repetition. This involves synaptic plasticity. Muscles containing many type I fibers called red muscles (darker than other muscles), respond slowly and have a long latency, are adapted for long, slow, posture-maintaining contractions (The long muscles of the back ). White muscles, which contain mostly type II fibers, have short twitch durations and are specialized for fine, skilled movement. The extraocular muscles and some of the hand muscles. -The axial muscles are concerned with postural adjustments and gross movements. -The distal limb muscles are those that mediate fine, skilled movements. SOMATIC MOTOR SYSTEMS The human skeleton is a system of levers that are moved by contraction of skeletal muscles. Muscle contraction only occurs in response to action potentials in alpha motor neurons, which originate in the ventral gray matter of the spinal cord and brainstem nuclei. A. Functions 1. Posture: maintain appropriate body position against external forces 2. Movement: change of body position to accomplish desired act 3. Coordination: control pattern and sequence of muscle contraction for smooth, effective action B. Initiation of Function 1. Involuntary: motor act, initiated by specific internal or external stimulus, generally stereotyped,don’t need conscious will. 1 Dr. SaadKlemanAbd ]MOTOR SYSTEMS, POSTURE, SOMATIC SENSATION, [ 2. Voluntary: sometimes initiated without any obvious stimulus, often complex and varied, requires conscious will. Note: voluntary and involuntary are not separate systems, since they involve shared effectorsorgans and pathways, and interact with each other C. Effector Structures 1. skeletal muscle: regular skeletal muscle fibers 2.neurons innervating skeletal muscle fibers: spinal cord: alpha motor neuronsin ventral horn, innervating muscles of movement andposture cranial nerves: motor nuclei in the brainstem controlling; 1) Eye movement 2) Mastication (chewing) 3) Muscles of facial expression 4) Muscles of the soft palate and larynx 5) Muscles supporting the head 6) Tongue muscles D. Influences on Motor Neurons (Motoneurons); 1. Reflexes: simple, stereotyped, involuntary movements (e.g. flexion reflex, myotatic reflex) 2. Brain stem nuclei (tone) 3. Brain stem pattern generators: more complex movements, oftenvoluntarily initiated (e.g. chewing, swallowing, walking) 4. Motor cortex; essential for voluntary activity 5.cerebellum(coordination, balance, posture/tone) 6. Basal ganglia (elaboration of movement & suppression of unwanted movements) SPINAL CONTROL OF MOVEMENT There are three inputs to alpha motor neurons 1. Upper motor neurons from the cortex or brainstem are important mediators of voluntary movements and are mainly inhibitory in nature. 2. Spinal interneurons may be excitatory or inhibitory . 3. Sensory neurons from muscle proprioceptors provide feedback about muscle length and tension. PROPRIOCEPTORS: Sensory information about body position and its relationship to the environment is relayed from the vestibular, visual, and auditory systems, and is conveyed via three major tracts: 2 Dr. SaadKlemanAbd ]MOTOR SYSTEMS, POSTURE, SOMATIC SENSATION, [ ■ The tectospinal tract receives input from the retina and visual cortex(direct the head and eyes to move toward a selected object in the visual field). ■The vestibulospinal tract provides one of the links between the sensors for balance and the extensor muscles, which are important for maintaining posture. ■ The reticulospinal tract consists of two antagonistic pathways. A balance between the activities of these pathways facilitates fine control of posture through actions on the extensor muscles of the lower limb. A large amount of somatosensory information comes from receptors(e.g.,Pacinian corpuscles) in the skin and joint capsules. Sensory information and feedback is needed at every stage of motor control to ensure smooth, coordinated, and accurate movements. In addition, muscles contain two types of proprioceptors that provide feedback about muscle length and tension: 1. intrafusal&extrafusal muscle fibersspecialized muscle fibers for stretch (length) and the speed with which muscle length is changing. 2. Golgi tendon organs are sensory nerve terminals that are encapsulated within tendons,provide information about muscle force. THE MYOTACTIC REFLEX (muscle stretch); The myotacticreflex is demonstrated by tapping a tendon with a reflex hammer; for example, when testing the knee jerk reflex. ■ Tapping the patellar tendon causes a small degree of stretch in the quadriceps muscle, which results in the generation ofaction potentials in afferents neuronsfrom muscle spindles. ■ A monosynaptic reflex arc afferents neuronssynapse directly on the motor neurons. The physiologic function of the monosynaptic myotactic reflex is to resist gravity. ■A polysynaptic reflex arc there are inter neurons between afferent &efferent neurons. 3 Dr. SaadKlemanAbd ]MOTOR SYSTEMS, POSTURE, SOMATIC SENSATION, [ RECIPROCAL INHIBITION In many cases, muscles work in antagonistic pairs (e.g., the biceps and triceps muscles of the arm). When a muscle produces movement by shortening, it is referred to as an agonist,muscles that oppose the action of the agonist are antagonists. (e.g., flexion of the elbow), the contraction of biceps is accompanied by relaxation of its antagonist, the triceps muscle; this phenomenon is known as reciprocalinhibition and is mediated by spinal interneurons. THE REVERSE MYOTACTIC REFLEX The reverse myotactic reflex is mediated by Golgi tendon organs It is the reflex inhibition of muscle contraction stimulated by active contraction of the muscle itself. protects a muscle from a potentially damaging overload during extreme contraction. under normalloading conditions it regulates muscle tension during a sustained contraction. For example, when picking up a fragile object such as an egg, a steady force that is not too powerful must be applied. THE FLEXOR WITHDRAWAL REFLEX Spinal interneurons are not always inhibitory, flexor withdrawal response, which protects the affected limb by rapidly removing it from an injurious stimulus. Thesteps of the flexor withdrawal reflex responsecan be illustrated, for example, when a painful stimulus is applied to one leg: ■ Pain receptors are activated at the site of stimulation. ■ Afferent pain fibers enter the dorsal root and send collaterals to several spinal segments. ■ Excitatory interneurons that synapse with alpha motor neurons serving flexors are stimulated. Contraction of flexors removes the limb from the aversive stimulus. ■ Reciprocal inhibition suppresses contraction of the extensors of the affected limb. ■ In this example, where one leg would suddenly be lifted, the person would rarely fall down because the postural support of the other limb would be simultaneously increased. 4 Dr. SaadKlemanAbd ]MOTOR SYSTEMS, POSTURE, SOMATIC SENSATION, [ CORTICAL CONTROL OF MOTOR FUNCTIONS: There are numerous connections between the cortical areas related to motor function. THE BASAL GANGLIA:initiation of voluntary movements. THE CEREBELLUM:necessary adjustments to achieve smooth coordinated movements. Functional classification is often used, based on the evolution of cerebellar functions: ■ The vestibulocerebellumcontributes to thecontrol of balance and eye movements. ■ The spinocerebellumcontrol of posture and walking and modulates spinal reflexactivity. ■ The cerebrocerebellumis essential in the decisions regarding the detailed sequence and timing of muscle contractions needed to effect a coordinated movement. E. Terms and Definitions Tone; muscle tension in the absence of voluntary activity synergist muscles; muscles effecting the same action at a joint antagonist muscles; muscles performing opposing actions at a joint (e.g. flexors and extensors) hypertonia / hypotonia;increased / reduced muscle tone compared to normal dystonia; tonically rigid posture hyper- / hyporeflexia; increased / reduced reflex response tremor; involuntary, small, rapid motions rigidity; hypertonia resisting passive movement asthenia; weakness, loss of strength paresis; partial loss of voluntary control paralysis; complete loss of voluntary control Paraplegia; paralysis of the lower half of the body Quadriplegia; paralysis from the neck down Hemiplegia; paralysis of one side of the body Ataxia; awkward, uncoordinated movement 5