Slide1: Part 4 Motor Functions of the Nervous System I Motor Unit and Final Common Pathway


Slide3: 1. Motor Unit


Slide4: Every striated muscle has encapsulated muscle fibers scattered throughout the muscle called muscle spindles. Extrafusal and intrafusal fibers


Slide5: The extrafusal muscle fibers are innervated by Alpha motor neuron The intrafusal muscle fibers are innervated by Gamma motor neurons


Motor units : Motor units A motor unit is a single motor neuron (a motor) and all (extrafusal) muscle fibers it innervates Motor units are the physiological functional unit in muscle (not the cell) All cells in motor unit contract synchronously


Slide7: Motor units and innervation ratio Purves Fig. 16.4 Innervation ratio Fibers per motor neuron Extraocular muscle 3:1 Gastrocnemius 2000:1


Slide8: The muscle cells of a motor unit are not grouped, but are interspersed among cells from other motor units The coordinated movement needs the activation of several motors


Slide9: organization of motor subsystems


Slide11: Overview - organization of motor systems Motor Cortex Brain Stem Skeletal muscle -motor neuron Final common pathway


Slide12: Final common path - -motor neuron (-)


Slide14: II Motor Functions of the Spinal Cord – Spinal Reflex


Spinal Reflexes: Spinal Reflexes Somatic reflexes mediated by the spinal cord are called spinal reflexes These reflexes may occur without the involvement of higher brain centers Additionally, the brain can facilitate or inhibit them


Slide16: 1. Stretch Reflex


(1) Anatomy of Muscle Spindle: (1) Anatomy of Muscle Spindle The muscle spindles detect change in the length of the muscle -- stretch receptors that report the stretching of the muscle to the spine. Each spindle consists of 3-10 intrafusal muscle fibers enclosed in a connective tissue capsule These fibers are less than one quarter of the size of extrafusal muscle fibers (effector fibers)


Anatomy of Muscle Spindle: Anatomy of Muscle Spindle The central region of the intrafusal fibers which lack myofilaments are noncontractile, serving as the receptive surface of the spindle (sensory receptor)


Anatomy of Muscle Spindle: Anatomy of Muscle Spindle Intrafusal fibers are wrapped by two types of afferent endings that send sensory inputs to the CNS Primary sensory endings Type Ia fibers Innervate the center of the spindle Secondary sensory endings Type II fibers Associated with the ends of the spindle


Components of muscle spindle: Components of muscle spindle Static intrafusal fibers Dynamic intrafusal fiber Afferent axons Ia II Static intrafusal fibers Primary ending Secondary ending } }


Anatomy of Muscle Spindle: Anatomy of Muscle Spindle Primary sensory endings Type Ia fibers Stimulated by both the rate and amount of stretch


Anatomy of Muscle Spindle: Anatomy of Muscle Spindle Secondary sensory endings Type II fibers stimulated only by degree of stretch


Anatomy of Muscle Spindle: Anatomy of Muscle Spindle The contractile region of the intrafusal muscle fibers are limited to their ends as only these areas contain actin and myosin filaments These regions are innervated by gamma () efferent fibers


Slide24: Muscle stretch reflex


Slide25: (2) Muscle stretch reflex Definition: Whenever a muscle is stretched, excitation of the spindles causes reflexive contraction of the same muscle from which the signal originated and also of closely allied synergistic muscle. The basic circuit: Spindle Proprioceptor nerve fiber dorsal root of the spinal cord synapses with anterior motor neurons  -motor N. F. the same M. from whence the M. spindle fiber originated.


Circuit of the Strength Reflex: Circuit of the Strength Reflex Dorsal root Ventral root Muscle spindle Tendon Muscle fiber -mn


The Stretch Reflex: The Stretch Reflex Exciting a muscle spindle occurs in two ways Applying a force that lengthens the entire muscle Activating the  motor neurons that stimulate the distal ends of the intrafusal fibers to contact, thus stretching the mid-portion of the spindle (internal stretch)


The Stretch Reflex: The Stretch Reflex Whatever the stimulus, when the spindles are activated their associated sensory neurons transmit impulses at a higher frequency to the spinal cord


The Stretch Reflex: The Stretch Reflex At spinal cord sensory neurons synapse directly (mono- synaptically) with the  motor neurons which rapidly excite the extrafusal muscle fibers of stretched muscle


The Stretch Reflex: The Stretch Reflex The reflexive muscle contraction that follows (an example of serial processing) resists further stretching of the muscle


The Stretch Reflex: The Stretch Reflex Branches of the afferent fibers also synapse with inter- neurons that inhibit motor neurons controlling the antagonistic muscles


Slide33: Inhibition of the antagonistic muscles is called reciprocal inhibition In essence, the stretch stimulus causes the antagonists to relax so that they cannot resist the shortening of the “stretched” muscle caused by the main reflex arc


Slide34: 1)   Tendon reflex (dynamic stretch reflex)  Caused by rapid stretch of the muscle, as knee-jerk reflex;  Transmitted from the IA sensory ending of the M. S.  Causes an instantaneous, strong reflexive contraction of the same muscle;  Opposing sudden changes in length of the M.; A monosynaptic pathway being over within 0.7 ms; The types of the Stretch Flex


Slide35: 2)   Muscle tonus (static stretch reflex):  Caused by a weaker and continues stretch of the muscle,  Transmitted from the IA and II sensory ending of the M. S.  Multiple synaptic pathway, continues for a prolonged period.  Non-synchronized contraction,  M. C. for at least many seconds or minutes, maintaining the posture of the body. The types of the Stretch Flex


The Stretch Reflex: The Stretch Reflex The stretch reflex is most important in large extensor muscles which sustain upright posture Contractions of the postural muscles of the spine are almost continuously regulated by stretch reflexes


(3) Gamma impact on afferent response: (3) Gamma impact on afferent response


Slide39: Muscle spindle: motor innervation Gamma motoneurons: Innervate the poles of the fibers.


Slide40: WHAT IS THE g-LOOP? g MUSCLE Muscle spindle Activation of the g-loop results in increased muscle tone


Functional significance of gamma impact on spindle activity: Functional significance of gamma impact on spindle activity The tension of intrafusal fibers is maintained during active contraction by gamma activity. The system is informed about very small changes in muscle length.


Slide45: 2. The Deep Tendon Reflex (1) Structure and Innervation of Golgi Organ


Golgi tendon organ: structure: Golgi tendon organ: structure Located in the muscle tendon junction. Connective tissue encapsulating collagen fibers and nerve endings. Attached to 10-20 muscle fibers and several MUs. Ib afferent fiber. sensitive to tension


(2) Golgi tendon organ: response properties: (2) Golgi tendon organ: response properties Less frequent than muscle spindle.


Golgi tendon organ: response properties (cont): Golgi tendon organ: response properties (cont) Sensitive to the change of tension caused by the passive stretch or active contraction


(3) The Deep Tendon Reflex: (3) The Deep Tendon Reflex When muscle tension increases moderately during muscle contraction or passive stretching, GTO receptors are activated and afferent impulses are transmitted to the spinal cord


The Deep Tendon Reflex: The Deep Tendon Reflex Upon reaching the spinal cord, informa- tion is sent to the cerebellum, where it is used to adjust muscle tension Simultaneously, motor neurons in the spinal cord supplying the contracting muscle are inhibited and antagonistic muscle are activated (activation)


The Deep Tendon Reflex: The Deep Tendon Reflex Deep tendon reflexes cause muscle relaxation and lengthening in response to the muscle’s contraction This effect is opposite of those elicited by stretch reflexes Golgi tendon organs help ensure smooth onset and termination of muscle contraction Particularly important in activities involving rapid switching between flexion and extension such as in running


Compare spindle and golgi: Compare spindle and golgi


Compare spindle and golgi: Compare spindle and golgi


3. The Crossed Extensor Reflex: 3. The Crossed Extensor Reflex The reflex occur when you step on a sharp object There is a rapid lifting of the affected foot (ipsilateral withdrawal reflex ), while the contralateral response activates the extensor muscles of the opposite leg (contralateral extensor reflex) support the weight shifted to it


4. Superficial Reflexes: 4. Superficial Reflexes Superficial reflexes are elicited by gentle cutaneous stimulation These reflexes are dependent upon functional upper motor pathways and spinal cord reflex arcs Babinski reflex


Babinski reflex - an UMN sign: Babinski reflex - an UMN sign Adult response - plantar flexion of the big toe and adduction of the smaller toes Pathological (Infant) response - dorsoflexion (extension) of the big toe and fanning of the other toes Indicative of upper motor neuron damage


Slide59: (1) Concept: When the spinal cord is suddenly transected in the upper neck, essentially all cord functions, including the cord reflexes, immediately become depressed to the point of total silence. (spinal animal) 5. Spinal cord transection and spinal shock


Slide60: (2) During spinal shock: complete loss of all reflexes, no tone, paralysis, complete anaesthesia, no peristalsis, bladder and rectal reflexes absent (no defecation and micturition ) no sweating arterial blood Pressure decrease（40mmHg）,


Slide61: (3) the reason: The normal activity of the spinal cord neurons depends to a great extent on continual tonic excitation from higher centers (the reticulospinal-, vestibulospinal- corticospinal tracts). (4) The recovery of spinal neurons excitability.


Slide62: III. Role of the brain stem: Support of the Body Against Gravity – Roles of the Reticular and Vestibular nuclei


Slide64: Areas in the cat brain where stimulation produces facilitation (+) or inhibition (-) of stretch reflexes. 1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei. Facilitated and inhibitory area


Slide65: 1. Facilitated area—roles of the reticular and vestibular nuclei.： (1) The pontine reticular nuclei  Located slightly posteriorly and laterally in the pons and extending to the mesencephalon,  Transmit excitatory signals downward into the cord (the pontine reticulospinal tract) motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.


Slide67: (2) The vestibular nuclei  selectively control the excitatory signals to the different antigravity M. to maintain equilibrium in response to signals from the vestibular apparatus. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.


Slide68: MOTOR CORTEX MOTOR TRACTS & LOWER MOTOR NEURON SKELETAL MUSCLE MIDBRAIN & RED NUCLEUS (Rubrospinal Tract) PONS & MEDULLA RETICULAR FORMATION (Reticulospinal Tracts) VESTIBULAR NUCLEI (Vestibulospinal Tract) LOWER (ALPHA) MOTOR NEURON THE FINAL COMMON PATHWAY


Slide69:  Terminate on the motor neurons that exciting antigravity M. of the body (the M. of vertebral column and the extensor M. of the limbs).  Have a high degree of natural (spontaneous) excitability.  Receive especially strong excitatory signals from vestibular nuclei and the deep nuclei of the cerebellum.  Cause powerful excitation of the antigravity M throughout the body (facilitate a standing position), supporting the body against gravity. 1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei. Properties of the Facilitated Area


Slide70: 2. Inhibitory area –medullary reticular system (1) Extend the entire extent to the medulla, lying ventrally and medially near the middle. (2) Transmit inhibitory signals to the same antigravity anterior motor neurons (medullary reticulospinal tract). 1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.


Slide71: MOTOR CORTEX MOTOR TRACTS & LOWER MOTOR NEURON SKELETAL MUSCLE MIDBRAIN & RED NUCLEUS (Rubrospinal Tract) PONS & MEDULLA RETICULAR FORMATION (Reticulospinal Tracts) VESTIBULAR NUCLEI (Vestibulospinal Tract) LOWER (ALPHA) MOTOR NEURON THE FINAL COMMON PATHWAY


Slide72: (3) Receive collaterals from the corticospinal tract; the rubrospinal tracts; and other motor pathways. These collaterals activate the medullary reticular inhibitory system to balance the excitatory signals from the P.R.S., so that under normal conditions, the body M. are normally tense. 1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.


Slide73: Areas in the cat brain where stimulation produces facilitation (+) or inhibition (-) of stretch reflexes. 1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.


Slide74: Decerebrate Rigidity: transection of the brainstem at midbrain level (above vestibular nuclei and below red nucleus) Symptoms include: extensor rigidity or posturing in both upper and lower limbs Decerebrate Rigidity


Slide75: Results from: loss of input from inhibitory medullary RF (activity of this center is dependent on input from higher centers). active facilitation from pontine RF (intrinsically active, and receives afferent input from spinal cord).


Slide76: The extensor rigidity is g-loop dependent section the dorsal roots interrupts the g-loop, and the rigidity is relieved. This is g-rigidity. THE g-LOOP? g MUSCLE Muscle spindle Activation of the g-loop results in increased muscle tone


Slide77: IV. The cerebellum and its motor functions


Slide78: Cerebellar Input/Output Circuit


Slide79: to produce smooth, reproducible movements Based on cerebral intent and external conditions The cerebellum tracks and modifies millisecond-to-millisecond muscle contractions,


Without normal cerebellar function, movements appear jerky and uncontrolled: Without normal cerebellar function, movements appear jerky and uncontrolled


Slide81: Functional Divisions-cerebellum Vestibulocerebellum (flocculonodular lobe)


Slide82: input-vestibular nuclei output-vestibular nuclei The vestibulocerebellum


Slide83: Function: The control of the equilibrium and postural movements. Especially important in controlling the balance between agonist and antagonist M. contractions of the spine, hips, and shoulders during rapid changes in body positions. Method Calculate the rates and direction where the different parts of body will be during the next few ms. The results of these calculations are the key to the brains’s progression to the next sequential movement. The vestibulocerebellum


Slide85: Spinocerebellum (vermis & intermediate)


Slide86: Spinocerebellum (vermis & intermediate) input-periphery & spinal cord: output-cortex


Slide87: Functions: -- Provide the circuitry for coordinating mainly the movements of the distal portions of the limbs, especially the hands and fingers -- Compared the “intentions ” from the motor cortex and red nucleus, with the “performance” from the peripheral parts of the limbs, --Send corrective output signals to the motor neurons in the anterior horn of spinal cord that control the distal parts of the limbs (hands and fingers) --Provides smooth, coordinate movements of the agonist and antagonist M. of the distal limbs for the performance of acute purposeful patterned movements. Spinocerebellum (vermis & intermediate)


Slide88: Cerebrocerebellum (lateral zone) input-pontine N. output-pre & motor cortex


Slide89: Cerebrocerebellum (lateral zone)  Receives all its input from the motor cortex, adjacent pre-motor and somatic sensory cortices of the brain. Transmits its output information back to the brain. Functions in a “feedback” manner with all of the cortical sensory-motor system to plan sequential voluntary body and limb movements, Planning these as much as tenths of a second in advance of the actual movements (mental rehearsal of complex motor actions)


Slide90: Vestibulocerebellum (flocculonodular lobe) Balance and body equilibrium Spinocerebellum (vermis & intermediate) Rectify voluntary movement Cerebrocerebellum (lateral zone) Plan voluntary movement


Slide91: V The motor functions of basal ganglia


Slide93: Putamen Caudate GPi GPe 1. Corpus Striatum Striatum ----- Caudate Nucleus & Putamen Pallidum ----- Globus Pallidus (GP) Components of Basal Ganglia


Slide94: 2. Substantia Nigra Pars Compacta (SNc) Pars Reticulata (SNr) Components of Basal Ganglia 3. Subthalamic Nucleus (STN) STN SN (r & c)


Basal Ganglia Connections: Basal Ganglia Connections Circuit of connections cortex to basal ganglia to thalamus to cortex Helps to program automatic movement sequences (walking and arm swinging or laughing at a joke) Output from basal ganglia to reticular formation reduces muscle tone damage produces rigidity of Parkinson’s disease


Slide96: D1 & D2 Dopamine receptors cortex to basal ganglia to thalamus to cortex GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia Nigra)


Slide97: Direct Pathway: Disinhibition of the thalamus facilitates cortically mediated behaviors D1 & D2 Dopamine receptors GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia nigra))


Slide98: Indirect pathway: Inhibition of the thalamus inhibits cortically mediated behaviors D1 & D2 Dopamine receptors GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia nigra)


Slide99: Medical Remarks


Slide100: Hypokinetic disorders result from overactivity in the indirect pathway. example: Decreased level of dopamine supply in nigrostriatal pathway results in akinesia, bradykinesia, and rigidity in Parkinson’s disease (PD). D1 & D2 Dopamine receptors GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia nigra)


Slide101: Muhammad Ali in Alanta Olympic Parkinson’s Disease Disease of mesostriatal dopaminergic system PD normal


Slide102: Substantia Nigra, Pars Compacta (SNc) DOPAminergic Neuron Slowness of Movement - Difficulty in Initiation and Cessation of Movement Clinical Feature (1) Parkinson’s Disease


Slide103: Clinical Feature (2) Resting Tremor Parkinsonian Posture Rigidity-Cogwheel Rigidity Parkinson’s Disease


Slide104: Hyperkinetic disorders result from underactivity in the indirect pathway. example: Lesions of STN result in Ballism. Damage to the pathway from Putamen to GPe results in Chorea, both of them are involuntary limb movements. D1 & D2 Dopamine receptors GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia nigra)


Slide105: - Fine, disorganized , and random movements of extremities, face and tongue - Accompanied by Muscular Hypotonia - Typical exaggeration of associated movements during voluntary activity - Usually recovers spontaneously in 1 to 4 months Clinical Feature Principal Pathologic Lesion: Corpus Striatum


Slide106: Clinical Feature Principal Pathologic Lesion: Corpus Striatum (esp. caudate nucleus) and Cerebral Cortex - Predominantly autosomal dominantly inherited chronic fatal disease (Gene: chromosome 4) - Insidious onset: Usually 40-50 - Choreic movements in onset - Frequently associated with emotional disturbances - Ultimately, grotesque gait and sever dysarthria, progressive dementia ensues. HUNTINGTON’S CHOREA


Slide107: - Usually results from CVA (Cerebrovascular Accident) involving subthalamic nucleus - sudden onset - Violent, writhing, involuntary movements of wide excursion confined to one half of the body - The movements are continuous and often exhausting but cease during sleep - Sometimes fatal due to exhaustion - Could be controlled by phenothiazines and stereotaxic surgery Clinical Feature Lesion: Subthalamic Nucleus


Slide108: Two principal components Primary Motor Cortex Premotor Areas VI Control of muscle function by the motor cortex


Slide110: The primary motor cortex The topographical representations of the different muscle areas of the body in the primary motor cortex


Slide111: Characteristics of the PMC: 1, It has predominant influence on the opposite side of the body (except some portions of the face) 2. It is organized in a homunculus pattern with inversed order 3. The degree of representation is proportional to the discreteness (number of motor unit) of movement required of the respective part of the body. (Face and fingers have large representative) 4. Stimulation of a certain part of PMC can cause very specific muscle contractions but not coordinate movement.


Slide112: Projects directly to the spinal cord to regulate movement Via the Corticospinal Tract The pyramidal system Projects indirectly Via the Brain stem to regulate movement extrapyramidal system


Descending Spinal Pathways pyramidal system: Descending Spinal Pathways pyramidal system Direct Control muscle tone and conscious skilled movements Direct synapse of upper motor neurons of cerebral cortex with lower motor neurons in brainstem or spinal cord


Descending Spinal Pathwaysextrapyramidal system: Descending Spinal Pathways extrapyramidal system Indirect coordination of head & eye movements, coordinated function of trunk & extremity musculature to maintaining posture and balance Synapse in some intermediate nucleus rather than directly with lower motor neurons


Slide115: Premotor area composed of supplementary motor area and lateral Premotor area


Slide116: Premotor Areas Receive information from parietal and prefrontal areas Project to primary motor cortex and spinal cord For planning and coordination of complex planned movements