Upper Motor Neurons Flashcards
- lowest, and perhaps most simple, level of organization is present within the
- provides the mechanisms for reflex motor functions
spinal cord
final common path for neurons that innervate skeletal muscle, which constitute the___ for motor responses.
lower motor neurons
- lower motor neurons for both the body and head region are controlled by
- innervate lower motor neurons of the spinal cord and brainstem, either directly or through an interneuron
- arise from the brainstem or cerebral cortex
upper motor neurons
pathways of importance
- vestibulospinal tract
- reticulospinal tract
- rubrospinal tract
- tectospinal tract
most significant of all the upper motor neurons are those that arise from the
cerebral cortex
upper motor neurons that project to the spinal cord are called the
corticospinal tract
- project to lower motor neurons of the brainstem are called the ___
- provide the anatomical substrates for voluntary control of movement.
corticobulbar tracts
- participate in the control of movement by receiving significant inputs from the cerebral cortex and feeding back signals to different regions of the frontal cortex involved in the initiation of movement
- serve to modulate the activity of neurons of the motor regions of the cortex
basal ganglia
receives inputs from most parts of the CNS that contribute to motor functions
cerebellum
corticospinal (or pyramidal) tract arises from three different regions of the cortex
- precentral gyrus
- postcentral gyrus
- supplemental motor area (SMA) & premotor cortex (PMC)
- Approximately 30% of the fibers arise from the
- referred to as the primary motor cortex called “MI”
precentral gyrus
- Forty percent of the fibers arise from the
- primary somatosensory cortex [S-1] and includes areas 3, 1, and 2
postcentral gyrus
- 30% of the fibers originate from the region immediately rostral to the precentral gyrus (area 6)
supplemental motor area [SMA] and the premotor cortex [PMC]
Electrical stimulation of the dorsal and medial aspect of the precentral gyrus in humans produces movements associated with the
lower limb
stimulation of more lateral aspects of the motor cortex produces movements of the
upper limb
stimulation of the far lateral aspect of the precentral gyrus produces movements of the
face and tongue
functional representation of the precentral gyrus is referred to as a
motor homunculus
- two layers (external & internal)
- receive information mainly from the thalamus and other regions of the cortex
granule cells
- two layers (external & internal)
- serve as the origins of the efferent pathways of the cortex
pyramidal cells
arise from the internal pyramidal cell layer situated mainly in layer V
corticospinal tracts
located within the region of the genu
corticobulbar fibers
Fibers associated with the ___ are located in a more lateral position than those associated with the arm
leg region
fibers associated with the ___ (i.e., corti- cobulbar fibers) are located medial to the corticospinal fibers
head region
At that level, 90% of the fibers cross over to the opposite side in the pyramidal decussation and descend through the lateral funiculus of the contralateral spinal cord, largely as the
lateral corticospinal tract
remaining 10% of the fibers, which remain uncrossed, descend into the
spinal cord
8% as the
anterior corticospinal tract,
2%
crossed lateral corticospinal tract.
cross over to the contralateral side
anterior corticospinal tract
% of the corticospinal tract fibers project to the contralateral spinal cord
98%
% of the lateral corticospinal tract remains ipsilateral over its entire course
2%
Fibers contained within the corticospinal tract are distributed throughout the entire ___ extent of the spinal cord
rostrocaudal
neurons located medially innervate the ___musculature
axial
neurons located more laterally innervate the ___ musculature
distal
innervate the cervical and lumbar cord serve to control fine movements of the extremities
lateral corticospinal fibers
innervate the medial aspect of the ventral horn serve to regulate postural mechanisms
anterior corticospinal tract
innervate the dorsal horn serve to modulate primary sensory afferent information to the cerebral cortex rather than to produce movement
corticospinal neurons
- receives indirect inputs from several important regions that are known to regulate motor activity
- cerebellum and globus pallidus
primary motor cortex
conscious proprioception, position sense, pain, and tactile information
primary somatosensory cortex
sequences of movements require the transmission of “positive” sensory feedback to the regions of motor cortex associated with those movements
descending fibers
regulation of sensory transmission through the nuclei of the dorsal columns and may respond to one or more of the following aspects of movement
(1) alteration of the position of the limb,
(2) rate of change of the position of the limb,
(3) magnitude of the muscular contraction in relation to the force exerted upon an object, and
(4) a combination of force as well as rate of change
contributes 30% of the fibers to the corticospinal tract, consists of two secondary motor regions: the SMA and the PMC
area 6
- provides integrated somatosensory and visual information to area 6
- necessary for the programming of motor sequences
posterior parietal cortex (PPC)
- coordinating voluntary movements
- governs postural adjustments
- affect both the axial and the distal musculature
SMA
Patients who have lesions of the SMA display
apraxia
inability to initiate specific, purposeful movements, even though the sensory and motor pathways for the execution of the movement remain intact.
apraxia
ability to execute a movement upon request. An example is the failure of a patient to be able to brush his or her hair or tie his or her shoelaces
ideomotor apraxia
- inability to conceptualize the movements, and the patient is unable to identify the sequences of movements that are necessary for carrying out the response in question
- apraxia becomes evident when the patient is asked to perform, simultaneously, different movements of both arms
ideational apraxia
- the patient is unable to coordinate the movement of both arms at the same time.
- loss of strength in the proximal muscles of the contralateral arm or leg and have difficulty in raising or abducting that limb
lesions of the PMC
Somatosensory inputs from the postcentral gyrus as well as vestibular inputs are directed to
area 5
- concerned with visual signals
- receives inputs from area 5
area 7
- patient denies the disease condition or is unaware of it
- patient with a right hemispheric lesion may deny or ignore the fact that he or she cannot move his or her left leg
- patient is asked to draw the numbers on a clock, he or she will draw them all on the right-hand side of the clock, while ignoring the left-hand side
sensory neglect (or anosognosia)
responsible for voluntary control over precise movements that affect primarily (but not exclusively) the distal musculature
primary motor cortex (area 4)
- project to the dorsal column nuclei and dorsal horn of the spinal cord
- serve as a sensory filtering mechanism
- allows specific sensory signals, such as the position of a limb or digit of the hand and the force of contraction of a muscle or group of muscles, to reach the relevant regions of the primary motor cortex while preventing sensory signals irrelevant to the movement in question from reaching the cortical neurons critical for that response sequence
primary somasensory cortex (areas 3,1, and 2)
- supplementary and premotor cortices
- serve to provide the programming mechanism for the sequencing of response patterns that are essential for producing movements, such as lacing up one’s shoes and walking
- project directly to the spinal cord and send signals to the primary motor cortex
area 6
actions of area 6 neurons can only occur if they receive integrated somatosensory and visual signals from the ___
PPC (areas 5 and 7)
arise from the lateral aspect of the primary motor cortex
corticobulbar tracts
- serve as upper motor neurons to the cranial nerve motor nuclei onto which they make synaptic contact
- make direct or indirect synaptic contact with all motor cranial nerves
descending fibers of corticobulbar tracts
serve as the anatomical substrate for voluntary control of the muscles of facial expression, eye movements, jaw opening and closing, and movements of the tongue
upper motor neurons
most corticobulbar fibers that are directed toward cranial nerve motor nuclei innervate these neurons
bilaterally
mild forms of weakness in the affected muscles
central facial palsy
Weakness in muscles that regulate breathing, swallowing, speech, and chewing is referred to as
pseudobulbar palsy
If the lesion involves only the corticobulbar fibers and not corticospinal neurons, then there would be no paralysis of the
upper or lower extremities
serves as a filtering mechanism for sensory inputs associated with the head region
sensory trigeminal nuclei
fibers that arise from the primary and premotor cortices also project ipsilaterally to the
red nucleus
Fibers from the ___ region of the precentral gyrus project to the dorsal part of the red nucleus
upper limb
fibers from the ___ region of the precentral gyrus project to the ventral aspect of the red nucleus
lower limb
of the red nucleus project to the cervical and lumbar regions of the spinal cord.
dorsal and ventral regions
six descending fiber pathways from the brainstem to the spinal cord
- medial and lateral reticulospinal tracts
- medial and lateral vestibulospinal tracts
- rubrospinal tract
- tectospinal tract
six descending fiber pathways from the brainstem to the spinal cord
- medial and lateral reticulospinal tracts
- medial and lateral vestibulospinal tracts
- rubrospinal tract
- tectospinal tract
innervate neurons of the ventral horn that are associated with the axial musculature
reticulospinal and vestibulospinal tracts
supply mainly the neurons that innervate the distal flexor musculature
rubrospinal fibers
two reticulospinal tracts
medulla and pons
the lateral reticulospinal tract
medulla
medial reticulospinal tract
pons
The medial reticulospinal tract arises from large cells located in the medial part of the reticular formation
nucleus reticularis pontis oralis and
nucleus reticularis pontis caudalis
facilitate voluntary or cortically induced movements and to increase muscle tone because of its actions on gamma motor neurons. The lateral reticulospinal tract arises from cells called the
nucleus reticularis gigantocellularis
- descend to all levels of the spinal cord
- inhibits voluntary movements and cortically induced movements and reduces muscle tone by further inhibiting muscle spindle activity through its effects on gamma motor neurons
medial reticulospinal tract
- two vestibulospinal tracts
- both convey impulses to the spinal cord from the labyrinth
medial vestibular nucleus
lateral vestibular nucleus
- arises from the medial vestibular nucleus and projects as the descending component of the medial longitudinal fasciculus to reach cervical levels of the spinal cord bilaterally
- activate lower motor neurons associated with the spinal accessory nerve
- cause rotation and lifting of the head as well as rotation of the shoulder blade around its axis
medial vestibulospinal tract
- arises from the lateral vestibular nucleus and projects to all levels of the spinal cord
- facilitates alpha and gamma motor neurons that innervate extensor motor neurons
lateral vestibulospinal tract
- maintenance of posture by exciting neurons that innervate extensor muscles, mainly of the lower limbs
- modulated by activation of the vestibular apparatus or cerebellum, which provides it with its major afferent supply
lateral vestibulospinal tract
- arises from the red nucleus
- descend to both cervical and lumbar levels
- facilitate motor neurons that innervate flexor muscles
rubrospinal tract
receives major inputs from the primary and premotor cortices, and the entire pathway from the cortex to the spinal cord is somatotopically organized
red nucleus
(1) both systems originate from the same general regions of cortex,
(2) both systems are somatotopically organized, and
(3) both systems act primarily on the flexor motor system.
cortico-rubro-rubrospinal pathway
___ neurons excite muscles that act against gravity and that are involved in the fine control of movement
corticospinal and rubrospinal
- arises from the superior colliculus of the midbrain and soon crosses over to the contralateral side in the dorsal tegmental decussation
- descend to cervical levels of the spinal cord
- serves to produce postural changes in response to visual stimuli that reach the superior colliculus
tectospinal tract
- result of disruption of central motor pathways that arise from the cerebral cortex
- lesion may occur in the cortex, internal capsule, crus cerebri, lower medulla, or even lateral funiculus of the spinal cord
upper motor neuron syndrome
classic signs of upper motor neuron syndrome
(1) paralysis or weakness (paresis) of movement of the affected muscles;
(2) a marked increase in muscle tone (hypertonia) coupled with spasticity ;
(3) the presence of abnormal reflexes, such as the Babinski sign, after stroking along the sole of the foot with a sharp object; and
(4) the diminution of some reflexes such as the abdominal reflexes.
a marked increase in muscle tone
hypertonia
resistance to movement in a single direction
spasticity
- extension and fanning out of the big toe
- also referred to as an extensor plantar response
babinski sign
The myotatic reflexes reappear and are highly pronounced
hyperreflexia
sequencing of discharge patterns of different groups of ventral horn cells at a given time for a purposeful movement to occur requires command signals from corticospinal fibers
paralysis of movement
corticospinal fibers make contact with both interneurons that excite motor horn cells as well as several types of inhibitory interneurons.
spasticity and hypertonia
- inhibitory interneuron
- produces recurrent inhibition of motor neurons innervating a specific muscle group
renshaw cell
inhibitory interneuron that receives inputs from a 1b sensory fiber and which makes synapse with a homonymous motor neuron
second type
of inhibitory interneuron is one which receives input from a 1a sensory afferent fiber and which makes synaptic contact with a motor neuron that supplies the antagonistic muscle
third type
facilitatory to spinal reflexes and the gamma motor system
pontine reticulospinal system
A 67-year-old man was taken to the emergency room after collapsing in a movie theater. The patient regained consciousness and, several days later, displayed a right-side paralysis with pronounced spasticity. The neurologist concluded that the stroke might have involved
Internal capsule
A patient presented with paralysis of the left side of the limbs and left side of the lower face and deviation of the tongue to the left with no atrophy and with no loss of taste sensation. This constellation of deficits most likely resulted from a lesion of the:
Right internal capsule
A 35-year-old man suffered a stroke that did not cause paralysis. However, he discovered that he was unable to perform complex learned movements. The region of the cerebral cortex most likely affected by the stroke was the:
Premotor cortex
- results in a paralysis or paresis of the contralateral limb(s), Babinski sign, spasticity, and hypertonicity
- motor regions of the cerebral cortex, internal capsule, and other regions that contain the descending fibers of the corticospinal and corticobulbar tracts
upper motor neuron paralysis
flaccid paralysis of the affected limbs and results from a lesion of the ventral horn cells that innervate the muscles in question directly
lower motor neuron paralysis
contains corticospinal and corticobulbar fibers that project to the contralateral spinal cord and to the brainstem.
right internal capsule
result in paralysis of the limbs on the left side, a deviation of the tongue to the left side and paralysis of the right side of the lower face
lesion of the right internal capsule
extremely important in providing sequencing or programming mechanism for learned movements
premotor area
- signals the appropriate groups of neurons in the spinal cord (and/or brainstem) to respond in a particular set of sequences
premotor cortex
- damage to this region would result in loss of the sequencing mechanism that is so necessary for the occurrence of complex learned movements
apraxia
Damage to the ___ will produce a somatosensory loss
postcentral gyrus
Damage to the ___ will produce an upper motor neuron paralysis
precentral gyrus
Damage to the ___ will produce an auditory loss
temporal cortex
damage to the ___ will produce different kinds of intellectual deficits,
prefrontal cortex
An investigator designed an experiment to characterize how neurons in the lower brainstem respond to administration of a specific neurotoxic substance. To effectively carry out this study, it was necessary to use a decerebrate preparation in which the region of the brain rostral to the pons was disconnected from the brainstem and spinal cord. Following this surgery, the investigator noted that the animal displayed marked rigidity of the limbs, which was most pronounced in the hind limbs. When the student of this investigator asked why there was such pronounced rigidity, the investigator should have provided which of the following reasons?
There is preservation of the lateral vestibulospinal tract with loss of cortical inputs to reticular formation.
During what appeared to be routine surgery for a torn ligament, a middle-aged man suffered a stroke. After a few days, the patient showed some recovery because he was able to walk with some difficulty, and, in addition, sensory functions seemed normal. However, a neurologic evaluation revealed a weakness in muscles that regulate breathing, speech, swallowing, and facial expression. A subsequent magnetic resonance imaging scan indicated that the stroke was limited but primarily affected the:
Genu of internal capsule
- gives rise to the lateral vestibulospinal tract, which projects to all levels of the spinal cord
- passes in the ventral funiculus of the cord and innervates alpha and gamma motor neurons of extensors and provides powerful excitation of these neurons
lateral vestibular nucleus
- weakness in the muscles of the head and face
- involves corticobulbar pathways that innervate, in part, cranial nerve motor nuclei
pseudobulbar palsy
Corticospinal tract origin
Area 4 (30%);
area 6 (30%);
areas 3, 1, 2 (40%)
Corticospinal tract origin
All levels of spinal cord but mainly to cervical and lumbar levels
- Planning of response
- provides muscle contraction of individual muscles
primary motor cortex
Provides sensory input to dorsal horn and dorsal column nuclei that filters out irrelevant stimuli from these nuclei that normally pass to somatosensory cortex
primary somatosensory cortex
- Receive major input from posterior parietal cortex; coordinate voluntary movements;
- can activate groups of muscles, elicit postural adjustments, and control opening and closing of hands and body orientation
supplementary and premotor areas
- Paralysis or weakness of movement, mainly of distal musculature;
- marked increase in muscle tone (hypertonia), spasticity, and pres- ence of abnormal reflexes (Babinski, extensor plantar reflex);
- lesions of supplementary and premotor cortices can result in various forms of apraxia
corticospinal tract
origin of corticobulbar tracts
Lateral aspects of the
primary motor and somatosensory cortices
distribution of corticobulbar tracts
Bilateral to most cranial nerves except contralateral to lower part of CN VII, CN XI, and CN XII
Voluntary control over cranial nerve functions, especially motor and autonomic functions of cranial nerves
corticobulbar tracts
Generally, weakness in muscles innervated by cranial nerves (pseudobulbar palsy), except for CN VII and CN XII where effects are more pronounced (i.e., jaw and tongue deviate to opposite side of lesion)
corticobulbar tract
origin of lateral reticulospinal tract
Large-celled region of
medulla
distribution of lateral reticulospinal tract
Fibers from each tract descends to all levels of spinal cord, synapsing upon alpha and gamma motor neurons and interneurons
Inhibits voluntary movements and reduces muscle tone
lateral reticulospinal tract
Selective lesions limited to the reticulospinal tracts in humans have not been reported, but lesions, especially of the lateral reticulospinal tract, are believed to contribute to hypertonia and spasticity
lateral reticulospinal tract
origin of medial reticulospinal tract
Large-celled regions of
pons
- Facilitates voluntary or cortically induced movements;
- increases muscle tone
medial reticulospinal tract
origin of lateral vestibulospinal tract
Lateral vestibular
nucleus
distribution of lateral vestibulospinal tract
Projects to all levels of spinal cord
Facilitates alpha and gamma motor neurons of extensors and facilitates extensor reflexes
lateral vestibulospinal tract
origin of medial vestibulospinal tract
Medial vestibular
nucleus
distribution of medial vestibulospinal tract
Projects to cervical levels, bilaterally
Activates lower motor neurons associated with spinal accessory nerve, assisting in the lifting and rotation of the head and shoulder blade
medial vestibulospinal tract
origin of rubrospinal tract
Red nucleus
distribution of rubrospinal tract
Pathway decussates near level of origin and descends mainly to cervical levels of spinal cord and indirectly to cerebellum
Facilitates alpha motor neurons of flexor motor neurons and flexor reflexes
rubrospinal tract
result in tremor and movement disorders
lesions in red nucleus
