No. 28No. 28

1. Motor Pathways1. Motor Pathways

ⅡⅡ. The . The Motor (descending) Motor (descending) PathwaysPathways

The motor pathways are concerned with motor functiThe motor pathways are concerned with motor function, and composed of upper motor neurons and lower on, and composed of upper motor neurons and lower motor neurons. The motor neurons. The upper motor neuronsupper motor neurons are the eff are the efferent neurons from the cerebral cortex to the motor nerent neurons from the cerebral cortex to the motor nuclei of cranial nerves and anterior horns of spinal coruclei of cranial nerves and anterior horns of spinal cord. The d. The lower motor neuronslower motor neurons are the nerves in the mo are the nerves in the motor nuclei of cranial nerves and the anterior horns of stor nuclei of cranial nerves and the anterior horns of spinal cord. The cell bodies and axons of lower motor npinal cord. The cell bodies and axons of lower motor neurons serve as the eurons serve as the final common pathwayfinal common pathway connecti connecting motor impulses.ng motor impulses.

The motor pathways include pyramidal and extrapyraThe motor pathways include pyramidal and extrapyramidal systems.midal systems.

ⅠⅠ) The ) The Pyramidal SystemPyramidal System

It is concerned with the voluntary movement oIt is concerned with the voluntary movement of the skeletal muscles and is composed of two f the skeletal muscles and is composed of two orders of neurons, i.e. the upper and lower moorders of neurons, i.e. the upper and lower motor neurons.tor neurons.

The The upper motor neuronsupper motor neurons are composed of t are composed of the giant pyramidal cells (Betz cells) and other he giant pyramidal cells (Betz cells) and other pyramidal cells of various sized which are locapyramidal cells of various sized which are located in the precentral gyrus and the anterior pated in the precentral gyrus and the anterior part of paracentral lobule and the pyramidal cellrt of paracentral lobule and the pyramidal cells in the some areas of frontal, parietal lobes. s in the some areas of frontal, parietal lobes.

Their axons form the descending Their axons form the descending pyramidal trpyramidal tractact, among which, the fibers ending in the cra, among which, the fibers ending in the cranial motor nuclei are designated as the nial motor nuclei are designated as the corticcorticonuclear tractonuclear tract and those terminating in the a and those terminating in the anterior horn of the spinal cord as nterior horn of the spinal cord as corticospinacorticospinal tractl tract..

The The lower motor neuronslower motor neurons include the cranial include the cranial motor cells of the brain stem and spinal motor motor cells of the brain stem and spinal motor cells of the spinal cord.cells of the spinal cord.

1. The 1. The corticospinal tractcorticospinal tract

The The upper motor neuronsupper motor neurons are the pyramidal are the pyramidal pyramidal cells in the superior and middle parpyramidal cells in the superior and middle parts of the precentral gyrus and the anterior part ts of the precentral gyrus and the anterior part of paracentral lobule.of paracentral lobule.

The axons arising from these upper motor neuThe axons arising from these upper motor neurons form the corticospinal tract.rons form the corticospinal tract.

It traverses the posterior limb of internal capsule, the iIt traverses the posterior limb of internal capsule, the intermediate 3/5 of the crus cerebri, the basilar part of ntermediate 3/5 of the crus cerebri, the basilar part of the pons, and the ventral part of the medulla oblongatthe pons, and the ventral part of the medulla oblongata. In the caudal part of the medulla oblongata the grea. In the caudal part of the medulla oblongata the greater part (75-90%) of the tract crosses to the opposite ater part (75-90%) of the tract crosses to the opposite side to form the side to form the pyramidal decussationpyramidal decussation and continue and continues as the s as the lateral corticospinal tractlateral corticospinal tract in the lateral funic in the lateral funiculus of the spinal cord. The fibers of the lateral corticoulus of the spinal cord. The fibers of the lateral corticospinal tract terminate in the anterior gray horns of all tspinal tract terminate in the anterior gray horns of all the spinal segments controlling the movements of mushe spinal segments controlling the movements of muscles of the limbs.cles of the limbs.

The non-crossed fibers continue as the The non-crossed fibers continue as the anterior cortianterior corticospinal tractcospinal tract directly into the anterior funiculus of th directly into the anterior funiculus of the same side and cross the median plane in the anterioe same side and cross the median plane in the anterior white commissure and synapse, as do those of the lar white commissure and synapse, as do those of the lateral tract, directly or indirectly with the motor neuronteral tract, directly or indirectly with the motor neurons of the anterior gray horn. The anterior corticospinal ts of the anterior gray horn. The anterior corticospinal tract is generally believed to exist above the level of thract is generally believed to exist above the level of the mid-thoracic segments, controlling the muscles of tre mid-thoracic segments, controlling the muscles of trunk and limbs. A unk and limbs. A small partsmall part in the anterior corticospin in the anterior corticospinal tract do not crosses to the opposite side, terminatinal tract do not crosses to the opposite side, terminating in the ipilateral anterior horns of the spinal cord. Thg in the ipilateral anterior horns of the spinal cord. These uncrossed fibers innervate the trunk muscles by wese uncrossed fibers innervate the trunk muscles by way of the motor cells of the anterior gray horn. So that ay of the motor cells of the anterior gray horn. So that the trunk muscles are controlled by bilateral motor cothe trunk muscles are controlled by bilateral motor cortex of the hemisphere.rtex of the hemisphere.

The The lower motor neuronslower motor neurons are the large multi are the large multipolar cells of the anterior gray horn.polar cells of the anterior gray horn.

They give rise to the motor fibers that leave thThey give rise to the motor fibers that leave the spinal cord through the anterior roots to be e spinal cord through the anterior roots to be distributed by way of the spinal nerves to the sdistributed by way of the spinal nerves to the skeletal muscles.keletal muscles.

Because of the inhibitory functions of the uppBecause of the inhibitory functions of the upper motor neurons to the lower ones, there are er motor neurons to the lower ones, there are different clinical signs in the cases of damage tdifferent clinical signs in the cases of damage to the upper or lower motor neuronso the upper or lower motor neurons

2. The 2. The corticonuclear tractcorticonuclear tract

The The upper motor neuronsupper motor neurons are the giant pyra are the giant pyramidal and certain other smaller pyramidal neumidal and certain other smaller pyramidal neurons in the inferior part of the precentral gyrus.rons in the inferior part of the precentral gyrus.

The axons arising from these upper motor neuThe axons arising from these upper motor neurons form the corticonuclear tract.rons form the corticonuclear tract.

In the course of descending through the genu In the course of descending through the genu of internal capsule and the brain stem, it give rof internal capsule and the brain stem, it give rise to the collaterals of the bilateral oculomotise to the collaterals of the bilateral oculomotor, trochlear, trigeminal motor, ambiguous, acor, trochlear, trigeminal motor, ambiguous, accessory nuclei and superior part of the facial ncessory nuclei and superior part of the facial nucleus, and to the contralateral hypoglossal nucleus, and to the contralateral hypoglossal nucleus and the inferior part of the facial nucleuucleus and the inferior part of the facial nucleus. So, the hypoglossal nucleus and inferior part s. So, the hypoglossal nucleus and inferior part of the facial nucleus receive the fibers only froof the facial nucleus receive the fibers only from the contralateral corticonuclear tract.m the contralateral corticonuclear tract.

The The lower neuronslower neurons are the cells in the above cranial are the cells in the above cranial motor nuclei.motor nuclei.

The lower neurons give rise to axons joining the corresThe lower neurons give rise to axons joining the corresponding cranial nerves that control the movements of ponding cranial nerves that control the movements of the innervated skeletal muscles.the innervated skeletal muscles.

Because the inferior part of the facial nucleus and hypBecause the inferior part of the facial nucleus and hypoglossal nucleus receive fibers just from the contralatoglossal nucleus receive fibers just from the contralateral corticonuclear tract, the injury of the unilateral coeral corticonuclear tract, the injury of the unilateral corticonuclear tract can usually cause paralysis of the corticonuclear tract can usually cause paralysis of the contralateral glossal muscles and facial muscles below tntralateral glossal muscles and facial muscles below the palpebral fissure. This is designated as he palpebral fissure. This is designated as supranuclesupranuclear paralysisar paralysis..

Paralysis of the unilateral facial muscles causeParalysis of the unilateral facial muscles caused by an injury of the homolateral facial nerve id by an injury of the homolateral facial nerve is termed as s termed as infranuclear paralysisinfranuclear paralysis..

Form 1Form 1. The comparison of expressions after i. The comparison of expressions after injury to the upper and lower motor neuronsnjury to the upper and lower motor neurons

Upper motor neuronsUpper motor neurons(suppranuclear paralysis) (suppranuclear paralysis)

Lower motor neuronsLower motor neurons(infranuclear paralysis) (infranuclear paralysis)

Injured sites Injured sites ①①Pyramidal cell bodyPyramidal cell body(Precentral gyrus and the par(Precentral gyrus and the paracentral lobule)acentral lobule)②②Pyramidal tractPyramidal tract(Corticonuclear tract(Corticonuclear tractCorticospinal tract)Corticospinal tract)

①①Cranial motor nuclei, Cranial motor nuclei, Motor neurons of the Motor neurons of the anterior gray horn.anterior gray horn.

②②Cranial nerves,Cranial nerves,

Spinal nerves Spinal nerves

Paralytic Paralytic characteristic characteristic

Hard paralysisHard paralysisSpasmoparalysisSpasmoparalysisCentral paralysis Central paralysis

Soft paralysisSoft paralysis

Flaccid paralysisFlaccid paralysis

Peripheral paralysis Peripheral paralysis

Muscular Muscular tonicity tonicity

Increasing Increasing Decreasing Decreasing

Deep reflex Deep reflex Hyperfunction Hyperfunction Decreasing or disappear Decreasing or disappear

Superficial reflex Superficial reflex Decreasing or disappear Decreasing or disappear Decreasing or disappear Decreasing or disappear

Pathological Pathological reflexes reflexes

Existence (+)Existence (+)(Babinski sign) (Babinski sign)

Nonentity (-) Nonentity (-)

Muscular Muscular atrophy atrophy

Not have (-) Not have (-) Have (+) Have (+)

ⅡⅡ) The ) The Extrapyramidal SystemExtrapyramidal System

It is a common name for the descending pathways regIt is a common name for the descending pathways regulating and controlling the voluntary movements exceulating and controlling the voluntary movements except the pyramidal system.pt the pyramidal system.

FunctionsFunctions:: The main functions of the extrapyramidal system in mThe main functions of the extrapyramidal system in m

an are to regulate the tonicity of the muscles, coordinan are to regulate the tonicity of the muscles, coordinate the muscular activities, maintain the normal body ate the muscular activities, maintain the normal body posture and produce habitual and rhythmic movemenposture and produce habitual and rhythmic movements. For example, riding and running are initiated in the ts. For example, riding and running are initiated in the beginning by the pyramidal system, but are controlled beginning by the pyramidal system, but are controlled by the extrapyramidal system when the motions later by the extrapyramidal system when the motions later become habitual and rhythmic.become habitual and rhythmic.

So the skeletal muscular movements are contrSo the skeletal muscular movements are controlled by the cortex of the hemisphere by way oolled by the cortex of the hemisphere by way of the pyramidal and extrapyramidal systems to f the pyramidal and extrapyramidal systems to produce coordinated, precise motions. The twproduce coordinated, precise motions. The two systems have the coordinated and dependeo systems have the coordinated and dependent functions with each other.nt functions with each other.

Structures of the extrapyramidal system are veStructures of the extrapyramidal system are very complicated including cerebral cortex (somry complicated including cerebral cortex (somatic motor area and somatic sensory area), coratic motor area and somatic sensory area), corpus striatum, dorsal thalamus, subthalamic nupus striatum, dorsal thalamus, subthalamic nucleus, tectum of midbrain, red nucleus, substacleus, tectum of midbrain, red nucleus, substantia nigra, pontine nucleus, vestibular nucleus,ntia nigra, pontine nucleus, vestibular nucleus, cerebellum, reticular formation of brain stem, cerebellum, reticular formation of brain stem, and their fibrous connections.and their fibrous connections.

1. The 1. The cortex-neostriatum-thalamus-cortical circuitcortex-neostriatum-thalamus-cortical circuit The somatic motor and sensory areas in the cerebral cThe somatic motor and sensory areas in the cerebral c

ortexortex ↓ ↓(corticotriatum fibers)(corticotriatum fibers) neorstriatumneorstriatum ↓ ↓(striatopallidal fibers)(striatopallidal fibers) globus pallidusglobus pallidus ↓ ↓ (globus pallidothalamus fibers)(globus pallidothalamus fibers) ventral anterior nucleus and ventral lateral nucleusventral anterior nucleus and ventral lateral nucleus ↓ ↓ dorsal thalamusdorsal thalamus ↓ ↓ (internal capsule)(internal capsule) the somatic motor area in the cerebral cortex.the somatic motor area in the cerebral cortex.

2. The 2. The neostriatum-substantia nigra circuitneostriatum-substantia nigra circuit The caudate nucleus and putamen→substantia nigraThe caudate nucleus and putamen→substantia nigra

→caudate nucleus and putamen. →caudate nucleus and putamen. Recent researches indicate that the cause of ParkinsoRecent researches indicate that the cause of Parkinso

nism is degeneration of the substantia nigra and redunism is degeneration of the substantia nigra and reduction of dopamine in the corpus striatum. Lesions of tction of dopamine in the corpus striatum. Lesions of the substantia nigra and globus pallidus cause Parkinshe substantia nigra and globus pallidus cause Parkinsonism which manifests muscular stiffness and a coarsonism which manifests muscular stiffness and a coarse tremor, especially involving the more distal extremite tremor, especially involving the more distal extremities.ies.

3. The 3. The cortex-pons-cerebellum-cortical circuitcortex-pons-cerebellum-cortical circuitThe frontal, parietal, occipital, and temporal lobes in the cerebral cortex.The frontal, parietal, occipital, and temporal lobes in the cerebral cortex.(corticopontine fibers: frontopontine, parietopontine, occippitopontine and tempor(corticopontine fibers: frontopontine, parietopontine, occippitopontine and tempor

opontine tracts) opontine tracts) ↓ ↓ pontine nuclei (pontocerebellar tract) pontine nuclei (pontocerebellar tract) ↓ ↓ cerebellar cortex cerebellar cortex ↓ ↓ dentate nucleusdentate nucleus (crossing)↓ ↓(crossing)(crossing)↓ ↓(crossing) (superior cerebellar peduncle)↓ ↓(middle cerebellar peduncle)(superior cerebellar peduncle)↓ ↓(middle cerebellar peduncle) red nucleus dorsal thalamusred nucleus dorsal thalamus (corssing)↙ (ventral anterior and lateral nuclei)(corssing)↙ (ventral anterior and lateral nuclei) (rubrospinal tract)↙ ↘(rubrospinal tract)↙ ↘ motor cells in the anterior horn somatic motor area in cerebral cortexmotor cells in the anterior horn somatic motor area in cerebral cortex

In the extrapyramidal system, cerebellum playIn the extrapyramidal system, cerebellum plays an important part in the coordination and res an important part in the coordination and regulation of large movement complexes includigulation of large movement complexes including posture and equilibrium adjustmentsng posture and equilibrium adjustments

Injury of the route, at any levels, causes the losInjury of the route, at any levels, causes the loss of the above functions.s of the above functions.