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

1. Motor Unit

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

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

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

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

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

organization of motor subsystems

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

Final common path - -motor neuron (-) muscle fibers (+) (-) axon hillock motor nerve fiber NM junction Schwann cells Receptors? acetylcholine esterase Transmitter?

Final Common Pathway,   a motor pathway consisting of the motor neurons by which nerve impulses from many central sources pass to a muscle in the periphery.

II Motor Functions of the Spinal Cord – Spinal Reflex

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

1. Stretch Reflex

(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 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 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 Dynamic intrafusal fiber Static intrafusal fibers Static intrafusal fibers Primary ending } Ia Afferent axons II Secondary ending }

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

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

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

Muscle stretch reflex

(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 Muscle spindle Dorsal root Muscle fiber Tendon Ventral root -mn

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

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

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

The types of the Stretch Flex 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 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 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

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

Activation of the g-loop WHAT IS THE g-LOOP? Descending influence (UMN) Muscle spindle 1a a g Activation of the g-loop results in increased muscle tone MUSCLE

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.

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

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 Less frequent than muscle spindle.

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 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 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 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

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 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 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

5. Spinal cord transection and spinal shock (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)

(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）,

(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.

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

Facilitated and inhibitory area 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.

4. Reticular inhibitory area; 5. Reticular facilitated area; 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.

(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.

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

Properties of the Facilitated Area  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.

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.

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

(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.

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.

Decerebrate Rigidity 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

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).

Activation of the g-loop 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? Descending influence (UMN) Muscle spindle 1a a g Activation of the g-loop results in increased muscle tone MUSCLE

IV. The cerebellum and its motor functions

Cerebellar Input/Output Circuit

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

Without normal cerebellar function, movements appear jerky and uncontrolled

Functional Divisions-cerebellum Vestibulocerebellum (flocculonodular lobe)

The vestibulocerebellum input-vestibular nuclei output-vestibular nuclei

The vestibulocerebellum 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.

Spinocerebellum (vermis & intermediate)

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

Spinocerebellum (vermis & intermediate) 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.

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

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)

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

V The motor functions of basal ganglia

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

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

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

somatosensory cortices cortex to basal ganglia to thalamus to cortex somatosensory cortices Thalamus Putamen GPe GPi STN SNc motor cortices excitation inhibition D1 D2 direct indirect D1 & D2 Dopamine receptors GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia Nigra)

somatosensory cortices Direct Pathway: Disinhibition of the thalamus facilitates cortically mediated behaviors somatosensory cortices Thalamus Putamen GPe GPi STN SNc motor cortices excitation inhibition D1 & D2 Dopamine receptors D1 D2 direct indirect GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia nigra))

somatosensory cortices Indirect pathway: Inhibition of the thalamus inhibits cortically mediated behaviors somatosensory cortices Thalamus Putamen GPe GPi STN SNc motor cortices excitation inhibition D1 D2 direct indirect D1 & D2 Dopamine receptors GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia nigra)

Medical Remarks

somatosensory cortices 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). somatosensory cortices Thalamus Putamen GPe GPi STN SNc motor cortices excitation inhibition D1 D2 D1 & D2 Dopamine receptors direct indirect GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia nigra)

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

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

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

somatosensory cortices 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. somatosensory cortices Thalamus Putamen GPe GPi STN SNc motor cortices excitation inhibition D1 D2 direct indirect D1 & D2 Dopamine receptors GPe/i: Globus pallidus internal/external STN: Subthalamus Nucleus SNc: Pars Compacta (part of substantia nigra)

SYDENHAM’S CHOREA Clinical Feature - 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 Principal Pathologic Lesion: Corpus Striatum

Principal Pathologic Lesion: HUNTINGTON’S CHOREA Clinical Feature - 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. Principal Pathologic Lesion: Corpus Striatum (esp. caudate nucleus) and Cerebral Cortex

HEMIBALLISM Clinical Feature Lesion: Subthalamic Nucleus - 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 Lesion: Subthalamic Nucleus

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

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

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.

Projects directly Projects indirectly 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 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 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

Premotor area composed of supplementary motor area and lateral Premotor area

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