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1、1,Central Nervous System Control of Movement,XING,GUOGANG(邢国刚),MD.Ph.D.北京大学 神经科学研究所Neuroscience Research Institute,Peking University,2,Introduction,生命在于运动运动是动物维系个体生存和种族繁衍的基本功能之一,3,How does a coordinated movement achieved?,4,The mental body image seems to be generated by somatosensory,proprioceptive,
2、and visual inputs to the posterior parietal cortex(area 5,area 7),A baseball pitcher planning a pitch,5,The highest level strategyRepresented by the association areas of neocortex and basal ganglia of the forebrainIs concerned with strategy:The goal of the movement and the movement strategy that bes
3、t achieves the goal,The motor control hierarchy have three levels,6,The middle leveltacticsRepresented by the motor cortex and cerebellumIs concerned with tactics The sequences of muscle contractions Arranged in space and timeRequired to smoothly and accurately achieve the strategic goal,The motor c
4、ontrol hierarchy have three levels,7,The lowest level execution Represented by the brain stem and spinal cord Is concerned with execution Action of the motor neuron and interneuron pools that generate the goal-directed movement and make any necessary adjustments of posture,The motor control hierarch
5、y have three levels,8,Overview,脊髓内的“下运动神经元(Lower neurons)”，除了受到脊髓内局部环路的影响外，还受到大脑皮层运动区及脑干中许多“上运动神经元(Upper neurons)”的支配和协调基底神经节和小脑则向那些上运动神经元提供某种感觉、认知或感性的信息，使运动更加精确和协调,9,10,These programs are accessed,executed,and modified by descending commands from the brain,The brains command and control of the moto
6、r programs in the spinal cord,The motor system consists of all our muscles and the neurons that command them,The spinal cord contains certain motor programs for the generation of coordinated movements,The motor control can be divided into two parts:,The spinal cords command and control of coordinate
7、d muscle contraction,11,Part 1 Spinal Control of Movement,12,Introduction,“running around like a chicken with its head cut off”The rhythmic movements could be elicited in the hind legs of cats and dogs long after their spinal cords had been severed from the rest of the central nervous systemCharles
8、Sherington&Graham Brown(English)The importance of circuitry within the spinal cord for the coordinated control of movements,13,Spinal circuitry control of movement,14,Spinal motor neurons,Alpha motor neurons Innervate skeletal muscle(extrafusal muscle)Function:directly command muscle contractGamma m
9、otor neurons Innervate muscle spindle(intrafusal muscle)Function:regulating the muscle spindleInterneurons Allows coordinated motor programs to be generatedFinal common pathway(directly command muscle contract)Compare:Upper motor neurons,Lower motor neurons,15,The Lower Motor Neurons Alpha motor neu
10、rons,Alpha motor neurons are directly responsible for the generation of force by muscleMotor unit:one alpha motor neuron and all of the muscle fibers it innervates collectively make up motor unit,the elementary component of motor controlMotor neuron pool:The collection of alpha motor neurons that in
11、nervates a single muscle,16,The motor unit is an alpha motor neuron and all of the muscle,A motor unit and motor neuron pool,The motor neuron pool is all of the alpha motor neurons that innervate one muscle,17,Muscle innervation by lower motor neurons,30 mixed spinal nervescervical 1-8thoracic 1-12l
12、umbar 1-5sacral 1-5,18,The cervical enlargement of the spinal cord contains the motor neurons that innervate the arm musclesThe lumbar enlargement contains neurons that innervate the muscles of the leg支配上肢的神经元集群位于颈膨大处，支配下肢的则在腰膨大处,The distribution of motor neurons in the spinal cord,The motor neurons
13、 that innervate distal and proximal musculature are found mainly in the cervical and lumbar-sacral segments of the spinal cordWhereas those innervating axial musculature are found at all levels,19,Motor neurons controlling flexors lie dorsal to those controlling extensors Motor neurons controlling a
14、xial muscles lie medial to those controlling distal muscles支配躯干部肌肉的神经元位于脊髓前角灰质最内侧，由此向外排列的神经元则支配肢体由近及远分布的肌肉,The distribution of lower motor neurons in the ventral horn,20,Muscle weakness and paralysis所支配的骨骼肌瘫痪、肌张力下降、腱反射消失(软瘫)肌萎缩、纤维颤动或肌束颤动见于如肌萎缩侧索硬化症(amyotrophic lateral sclerosis,ALS)：Selective damage
15、 to alpha motor neurons(degeneration)脊髓-运动神经元的进行性溃变为主,下运动神经元的损伤表现,21,Lou Gehring,a star baseball player with New York Yankees,who died of ALS(amyotrophic lateral sclerosis)in 1936,22,Inputs to alpha motor neurons,23,SPINAL CONTROL OF MOTOR UNITS,How the activity of the motor neuron is itself control
16、led?,24,Reflex,25,The myotatic reflex(stretch reflex),Two types of myotatic reflex Tendon reflex and muscle tonusA.Tendon reflex 快速牵拉肌腱而发生的牵张反射Clinic application：了解脊髓不同节段的功能状态,单突触反射潜伏期很短，约0.7s 只够一次突触传递时间延搁,26,+,27,B.Muscle tonus,肌肉受到缓慢而持续的牵拉而发生的收缩 使骨骼肌能保持一定的肌肉张力 意义:维持身体的姿势(posture)，而不表现明显的动作 表现：exte
17、nsor(伸肌)和flexor(屈肌)都发生肌紧张直立时，以伸肌紧张为主，因重力作用于关节，使关节趋向弯曲，伸肌受到牵拉，引起肌紧张反射，肌紧张度增加以对抗关节屈曲 因重力持续作用于关节，肌紧张也就持续发生。使直立姿势得以维持,多突触反射潜伏期较长，经过多个突触传递,28,Muscle spindles:Consists of specialized skeletal muscle fibersIn this middle region,group Ia sensory axons wrap around the muscle fibers of the spindleThe spindles
18、 and their associated Ia axons,specialized for the detection of changes in muscle length(stretch):proprioceptors and propriceptionIa axons are the thickest myelinated axons so they conduct action potentials very rapidlyIa axons enter the spinal cord via the dorsal roots,branch repeatedly,and form ex
19、citatory synapses upon both interneurons and alpha motor neurons of the ventral horns,Proprioception from muscle spindles,Muscle spindle 是一种可感受肌肉长度变化或感受牵拉刺激的 本体感受器梭内肌感受部装置位于中间,收缩成分位于两端,梭内肌收缩时或牵拉梭外肌时,感受装置对牵拉敏感性增高,29,+,30,Gamma motor neurons,31,The function of gamma motor neurons,Activation of alpha m
20、otor neurons shortens the extrafusal muscle fibers.If the muscle spindle becomes slack,it goes“off the air”and no longer reports the length of the muscleActivation of gamma motor neurons contracts the poles of the spindle,keeping it“on the air”,32,The Gamma loop,Changing the activity of the gamma mo
21、tor neurons changes the set point of the myotatic feedback loopThe Gamma loopGamma motor neuron intrafusal muscle fiber Ia afferent alpha motor neuron extrafusal muscle fibersAlpha and gamma motor neurons are simultaneously activated by descending commands from the brain,33,Proprioception from Golgi
22、 tendon organs,Golgi tendon organActs like a strain gauge,it monitors muscle tension,or the force of contractionLocated at the junction of the muscle and the tendon and are innervated by group Ib sensory axons that are slightly smaller than the Ia axons innervating the muscle spindles,34,35,The Ib a
23、fferents enter the spinal cord,branch repeatedly,and synapse on interneurons in the ventral hornSome of these interneurons form inhibitory connections with the alpha motor neurons innervating the same muscle.This is the basis for the reverse myotatic reflex,Circuitry of the reverse myotatic reflex,-
24、,Muscle spindle situated in parallel with the muscle fibers,Golgi tendon organs are situated in series,Ia activity from the spindle encodes muscle length informationWhile Ib activity from the Golgi tendon organ encodes muscle tension information,36,Significance of the reverse myotatic reflex,In extr
25、eme circumstances,this reflex arc protects the muscle from overloadThe normal function is to regulate muscle tension within an optimal rangeAs muscle tension increases,the inhibition of the alpha motor neuron slows muscle contractionAs muscle tension falls,the inhibition of the alpha motor neuron is
26、 reduced,and muscle contraction increasesThis type of proprioceptive feedback is thought to be particularly important for the proper execution of fine motor acts,such as the manipulation of fragile objects with the hands,which require a steady but not too powerful grip,37,Spinal interneurons,Most of
27、 the input to the alpha motor neurons comes from interneurons of the spinal cordInterneurons receive synaptic input from primary sensory axons,descending axons from the brain,and collaterals of lower motor neuron axonsThe interneurons are themselves networked together in a way that allows coordinate
28、d motor programs to be generated in response to their many inputs,38,Reciprocal inhibition of flexors and extensors of the same joint,+,39,(A&C fibers),Circuitry of the polysynaptic flexor reflex,40,Circuitry of the crossed-extensor reflex,41,The generation of spinal motor programs for walking,Headl
29、ess chickens;behaviorA complete transectionof a cats spinal cord at the mid-thoracic level leaves the hind limbs capable of generating coordinated walking movements,When you walk,you alternately withdraw and extend your two legs What is the mechanism to coordinate the timing?,This could be descendin
30、g commands from upper motor neurons?,No!,This control is exerted from within the spinal cord?,Yes!,42,43,Central pattern generators within the spinal cord,The circuit for the coordinated control of walking must reside within the spinal cordIn general,circuits that give rise to rhythmic motor activit
31、y are called central pattern generators How do neural circuits generate rhythmic patterns of activity?The simplest pattern generators,are single neurons whose membrane properties endow them with pacemaker properties(pacemaker neurons),44,Rhythmic activity in a spinal interneuron(Pacemaker neuron),So
32、me neurons respond to the activation of NMDA receptors with rhythmic depolarization,45,A possible circuit for rhythmic alternating activity(walking),Walking is initiated when a steady input excites two interneurons that connect to the motor neurons controlling the flexors and extensors,respectively.
33、The interneurons respond to a continuous input by generating bursts of outputs.The activities of the two interneurons alternate because they inhibit each other via other(inhibitory)interneurons.Thus,a burst of activity in one interneuron strongly inhibits the other,(Pacemaker neuron),(Pacemaker neur
34、on),46,Part 2 Brain Control of Movement,47,How does the brain communicate with the motor neurons of the spinal cord?,48,Overview,来自高级中枢的下行投射对脑干和脊髓环路的运动控制主要影响随意运动和朝向目标的运动发出下行投射的中枢位于脑干某些部位以及大脑皮质的运动区域脑干内的两个主要结构前庭核和网状结构对维持姿势很重要皮质主要运动区和次要运动区(运动前区)主要对运动进行计划并使运动高效率执行，皮质的这种作用是通过对脑干内的某些高级中枢以及脑干、脊髓中运动神经元和环路中间
35、神经元的控制来完成的,49,The descending spinal tracts control of movement,50,The descending spinal tracts,51,Control fine movements of the arms and fingers,The effects of lateral pathway lesions(in monkeys)by Donald Lawrence&Hans Kuypers(in the late 1960s),The Lateral Pathways,X,52,The Ventromedial Pathways,Co
36、ntain four descending tracts the vestibulospinal tractthe tectospinal tractthe pontine reticulospinal tractthe medullary reticulospinal tract Oringinate in the brain stem Terminate among the spinal interneurons Controlling proximal and axial musclesUse sensory information about balance,body position
37、,and the visual enviroment to reflexively maintain balance and body posture,53,The vestibulospinal and tectospinal tracts keep the head balance on the shoulders as the body moves through space,and they turn the head in response to new sensory stimuli,(midbrain),Recive direct input from retinaRecive
38、projections from visual cortex,as well as afferents carring somatosensory and auditory informationConstruct a map of the word around us,The motion of the fluid in this labyrinth,which accompanies movements of the head,activates hair cells that signal the vestibular nuclei via cranial nerve VIII,One:
39、Projects bilaterally down the spinal cord and activates the cervical spinal circuits that control neck and back muscles and guid head movement,Another:Projects ipsilaterally as far down as the lumbar spinal cordIt helps us maintain an upright and balanced posture by facilitating extensor motor neuro
40、ns of the legs,Stimulation at one site in this map Lead to an orienting response that directs the head and eyes to move,54,The pontine reticulospinal tract enhance the antigravity reflexes of the spinal cordBy facilitating the extensors of the lower limbsHelps maintain a standing posture by resistin
41、g the effects of gravity,The medullary reticulospinal tract has the opposite effect:it liberates the antigravity muscles from reflex control,Activity in both reticulospinal tracts is controlled by descending signals from the cortexControl posture of the trunk and the antigravity muscles of the limbs
42、,55,A summary of the major descending spinal tracts and their points of origin,Motor cortex directly activates spinal motor neurons and liberates them from reflex control by communicating with the nuclei of the ventromedial pathwaysSo,the cortex is key for voluntary movement and behavior,The ventromedial pathways originate from several regions of the brain stem and participate mainly in the maintenance of posture and certain reflex movements,Initiation of a voluntary movements,requires instructions that descend from the motor cortex along the lateral pathways,56,our attention,
