Download lower motor neurons

No. 28
1. Motor Pathways
Ⅱ. The Motor (descending)
Pathways
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The motor pathways are concerned with motor
function, and composed of upper motor neurons
and lower motor neurons. The upper motor
neurons are the efferent neurons from the
cerebral cortex to the motor nuclei of cranial
nerves and anterior horns of spinal cord. The
lower motor neurons are the nerves in the
motor nuclei of cranial nerves and the anterior
horns of spinal cord. The cell bodies and axons of
lower motor neurons serve as the final common
pathway connecting motor impulses.
The motor pathways include pyramidal and
extrapyramidal systems.
Ⅰ) The Pyramidal System
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It is concerned with the voluntary
movement of the skeletal muscles and is
composed of two orders of neurons, i.e.
the upper and lower motor neurons.
The upper motor neurons are composed
of the giant pyramidal cells (Betz cells)
and other pyramidal cells of various sized
which are located in the precentral gyrus
and the anterior part of paracentral lobule
and the pyramidal cells in the some areas
of frontal, parietal lobes.
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Their axons form the descending
pyramidal tract, among which, the fibers
ending in the cranial motor nuclei are
designated as the corticonuclear tract
and those terminating in the anterior horn
of the spinal cord as corticospinal tract.
The lower motor neurons include the
cranial motor cells of the brain stem and
spinal motor cells of the spinal cord.
1. The corticospinal tract
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The upper motor neurons are the
pyramidal pyramidal cells in the superior
and middle parts of the precentral gyrus
and the anterior part of paracentral lobule.
The axons arising from these upper motor
neurons form the corticospinal tract.
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It traverses the posterior limb of internal capsule,
the intermediate 3/5 of the crus cerebri, the
basilar part of the pons, and the ventral part of
the medulla oblongata. In the caudal part of the
medulla oblongata the greater part (75-90%) of
the tract crosses to the opposite side to form the
pyramidal decussation and continues as the
lateral corticospinal tract in the lateral
funiculus of the spinal cord. The fibers of the
lateral corticospinal tract terminate in the
anterior gray horns of all the spinal segments
controlling the movements of muscles of the
limbs.
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The non-crossed fibers continue as the anterior
corticospinal tract directly into the anterior
funiculus of the same side and cross the median
plane in the anterior white commissure and
synapse, as do those of the lateral tract, directly
or indirectly with the motor neurons of the
anterior gray horn. The anterior corticospinal
tract is generally believed to exist above the level
of the mid-thoracic segments, controlling the
muscles of trunk and limbs. A small part in the
anterior corticospinal tract do not crosses to the
opposite side, terminating in the ipilateral
anterior horns of the spinal cord. These
uncrossed fibers innervate the trunk muscles by
way of the motor cells of the anterior gray horn.
So that the trunk muscles are controlled by
bilateral motor cortex of the hemisphere.
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The lower motor neurons are the large
multipolar cells of the anterior gray horn.
They give rise to the motor fibers that
leave the spinal cord through the anterior
roots to be distributed by way of the
spinal nerves to the skeletal muscles.
Because of the inhibitory functions of the
upper motor neurons to the lower ones,
there are different clinical signs in the
cases of damage to the upper or lower
motor neurons
2. The corticonuclear tract
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The upper motor neurons are the giant
pyramidal and certain other smaller
pyramidal neurons in the inferior part of
the precentral gyrus.
The axons arising from these upper motor
neurons form the corticonuclear tract.
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In the course of descending through the
genu of internal capsule and the brain
stem, it give rise to the collaterals of the
bilateral oculomotor, trochlear, trigeminal
motor, ambiguous, accessory nuclei and
superior part of the facial nucleus, and to
the contralateral hypoglossal nucleus and
the inferior part of the facial nucleus. So,
the hypoglossal nucleus and inferior part
of the facial nucleus receive the fibers only
from the contralateral corticonuclear tract.
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The lower neurons are the cells in the above
cranial motor nuclei.
The lower neurons give rise to axons joining the
corresponding cranial nerves that control the
movements of the innervated skeletal muscles.
Because the inferior part of the facial nucleus and
hypoglossal nucleus receive fibers just from the
contralateral corticonuclear tract, the injury of
the unilateral corticonuclear tract can usually
cause paralysis of the contralateral glossal
muscles and facial muscles below the palpebral
fissure. This is designated as supranuclear
paralysis.
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Paralysis of the unilateral facial muscles
caused by an injury of the homolateral
facial nerve is termed as infranuclear
paralysis.
Form 1. The comparison of expressions
after injury to the upper and lower motor
neurons
Upper motor neurons
(suppranuclear paralysis)
Lower motor neurons
(infranuclear paralysis)
Injured sites
①Pyramidal cell body
(Precentral gyrus and the
paracentral lobule)
②Pyramidal tract
(Corticonuclear tract
Corticospinal tract)
①Cranial motor nuclei,
Motor neurons of the
anterior gray horn.
②Cranial nerves,
Spinal nerves
Paralytic
characteristic
Hard paralysis
Spasmoparalysis
Central paralysis
Soft paralysis
Flaccid paralysis
Peripheral paralysis
Muscular
tonicity
Increasing
Decreasing
Deep reflex
Hyperfunction
Decreasing or disappear
Superficial reflex Decreasing or disappear
Decreasing or disappear
Pathological
reflexes
Existence (+)
(Babinski sign)
Nonentity (-)
Muscular
atrophy
Not have (-)
Have (+)
Ⅱ) The Extrapyramidal System
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It is a common name for the descending
pathways regulating and controlling the voluntary
movements except the pyramidal system.
Functions:
The main functions of the extrapyramidal system
in man are to regulate the tonicity of the muscles,
coordinate the muscular activities, maintain the
normal body posture and produce habitual and
rhythmic movements. For example, riding and
running are initiated in the beginning by the
pyramidal system, but are controlled by the
extrapyramidal system when the motions later
become habitual and rhythmic.
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So the skeletal muscular movements are
controlled by the cortex of the hemisphere
by way of the pyramidal and
extrapyramidal systems to produce
coordinated, precise motions. The two
systems have the coordinated and
dependent functions with each other.
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Structures of the extrapyramidal system
are very complicated including cerebral
cortex (somatic motor area and somatic
sensory area), corpus striatum, dorsal
thalamus, subthalamic nucleus, tectum of
midbrain, red nucleus, substantia nigra,
pontine nucleus, vestibular nucleus,
cerebellum, reticular formation of brain
stem, and their fibrous connections.
1. The cortex-neostriatum-thalamus-cortical
circuit
The somatic motor and sensory areas in the
cerebral cortex
↓(corticotriatum fibers)
neorstriatum
↓(striatopallidal fibers)
globus pallidus
↓ (globus pallidothalamus fibers)
ventral anterior nucleus and ventral lateral nucleus
↓
dorsal thalamus
↓ (internal capsule)
the somatic motor area in the cerebral cortex.
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2. The neostriatum-substantia nigra circuit
The caudate nucleus and putamen→substantia
nigra→caudate nucleus and putamen.
Recent researches indicate that the cause of
Parkinsonism is degeneration of the substantia
nigra and reduction of dopamine in the corpus
striatum. Lesions of the substantia nigra and
globus pallidus cause Parkinsonism which
manifests muscular stiffness and a coarse tremor,
especially involving the more distal extremities.
3. The cortex-pons-cerebellum-cortical circuit
The frontal, parietal, occipital, and temporal lobes in the cerebral cortex.
(corticopontine fibers: frontopontine, parietopontine, occippitopontine and
temporopontine tracts)
↓
pontine nuclei (pontocerebellar tract)
↓
cerebellar cortex
↓
dentate nucleus
(crossing)↓
↓(crossing)
(superior cerebellar peduncle)↓
↓(middle cerebellar peduncle)
red nucleus
dorsal thalamus
(corssing)↙
(ventral anterior and lateral nuclei)
(rubrospinal tract)↙
↘
motor cells in the anterior horn somatic motor area in cerebral cortex
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In the extrapyramidal system, cerebellum
plays an important part in the
coordination and regulation of large
movement complexes including posture
and equilibrium adjustments
Injury of the route, at any levels, causes
the loss of the above functions.