Phys Chapter 55
Motor Cortex and Corticospinal Tract
 Primary motor cortex begins laterally in sylvian fissure and spreads over
the top to dip deep into longitudinal fissure
o More than half of entire primary motor cortex concerned with
controlling muscles of hands and muscles of speech
o Most often stimulation contracts group of muscles
 Premotor area extends inferiorly into sylvian fissure and superiorly into
longitudinal fissure; topographical organization roughly same as primary
motor cortex
o Nerve signals generated here cause much more complex
patterns of movement than discrete patterns generated in
primary motor cortex (i.e., position shoulders and arms so
hands properly oriented to perform specific tasks)
o Most anterior part of premotor area develops motor image of total muscle movement required, then in
posterior premotor cortex, image excites each successive pattern of muscle activity required to achieve
image; posterior part of premotor cortex sends signals either directly to primary motor cortex to excite
specific muscles or often by way of basal ganglia and thalamus back to primary motor cortex
o Mirror neurons become active when person performs specific motor task or when they observe same
task performed by others; activity of neurons mirrors behavior of other person as though observer
performing specific motor task
 Located in premotor cortex and inferior parietal cortex
 Transform sensory representations of acts heard or seen into motor representations of acts
 Important for understanding actions of other people and learning new skills by imitation
 Supplementary motor area lies mainly in longitudinal fissure but extends onto superior frontal cortex
o Contractions elicited by stimulating area often bilateral rather than unilateral
o Functions in concert with premotor area to provide body-wide attitudinal movements, fixation
movements of different segments of body, positional movements of head and eyes, etc., as background
for finer motor control of arms and hands by premotor area and primary motor cortex
 Damage to Broca’s region doesn’t prevent a person from
vocalizing but makes it impossible for person to speak whole
words rather than uncoordinated utterances or occasional
monosyllabic word
o Closely associated cortical area causes appropriate
respiratory function so respiratory activation of
vocal cords can occur simultaneously with
movements of mouth and tongue during speech
 Damage to voluntary eye movement area (yellow area)
prevents person from voluntarily moving eyes toward
different objects; eyes lock involuntarily onto specific
objects (controlled by occipital visual cortex); voluntary eye
movement area controls eyelid movements such as blinking
 When hand skills area destroyed by tumor or other lesion, hand movements become uncoordinated and
nonpurposeful (motor apraxia)
 Motor signals transmitted directly from cortex to spinal cord through corticospinal tract and indirectly through
multiple accessory pathways that involve basal ganglia, cerebellum, and various nuclei of brain stem
o Direct pathways concerned more with discrete and detailed movements, especially distal segments
 Corticospinal tract (pyramidal tract) originates roughly in thirds from primary motor cortex, premotor and
supplementary motor areas, and somatosensory areas posterior to central sulcus
o After leaving cortex, it passes through posterior limb of internal capsule (Between caudate nucleus and
putamen of basal ganglia) and then downward through brain stem, forming pyramids of medulla
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Majority of pyramidal fibers cross in lower medulla to opposite side and descend into lateral
corticospinal tracts of cord, terminating principally on interneurons in intermediate regions of cord gray
matter; few terminate on sensory relay neurons in dorsal horn
o Few fibers don’t cross in medulla; run down ipsilateral cord in ventral corticospinal tracts; most of these
eventually cross to opposite side of cord in neck or upper thoracic region; fibers concerned with control
of bilateral postural movements by supplementary motor cortex
o Large fibers originate from giant pyramidal cells (Betz cells) found only in primary motor cortex; fastest
rate of transmission of any signals from brain to cord
 Axons from giant Betz cells send short collaterals back to cortex itself; inhibit adjacent regions of
cortex when Betz cells discharge, sharpening boundaries of excitatory signal
Large number of fibers pass from motor cortex into caudate nucleus and putamen; from there, additional
pathways extend into brain stem and spinal cord, mainly to control body postural muscle contractions
Moderate number of motor fibers pass to red nuclei of midbrain; from these, additional fibers pass down cord
through rubrospinal tract
Moderate number of motor fibers deviate into reticular substance and vestibular nuclei of brain stem; from
there, signals go to cord by way of reticulospinal and vestibulospinal tracts, and others go to cerebellum by way
of reticulocerebellar and vestibulocerebellar tracts
Lots of motor fibers synapse in pontile nuclei, which give rise to pontocerebellar fibers, carrying signals into
cerebellar hemispheres
Collaterals terminate in inferior olivary nuclei, and from there, secondary olivocerebellar fibers transmit signals
to multiple areas of cerebellum
Once sensory info received, motor cortex operates in association with basal ganglia and cerebellum to excite
appropriate course of motor action
o Subcortical fibers from adjacent regions of cerebral cortex, especially from somatosensory areas of
parietal cortex, adjacent areas of frontal lobe anterior to motor cortex, and visual and auditory cortices
o Subcortical fibers that arrive through corpus callosum from opposite hemisphere; connect
corresponding areas of cortices in 2 sides of brain
o Somatosensory fibers that arrive directly from ventrobasal complex of thalamus; relay mainly cutaneous
tactile signals and joint and muscle signals from peripheral body
o Tracts from ventrolateral and ventroanterior nuclei of thalamus, which in turn receive signals from
cerebellum and basal ganglia; provide signals necessary for coordination among motor control functions
of motor cortex, basal ganglia, and cerebellum
o Fibers from intralaminar nuclei of thalamus; control general level of excitability of motor cortex
Red nucleus in mesencephalon functions in close association with
corticospinal tract
o Receives large number of direct fibers from primary motor cotex
through corticorubral tract, as well as branching fibers from
corticospinal tract as it passes through mesencephalon
o Fiber synapse in lower portion of red nucleus (magnocellar
portion), which contains large neurons that give rise to rubrospinal
tract, which crosses to opposite side in lower brain stem and
follows course immediately adjacent and anterior to corticospinal
tract into lateral columns of spinal cord
o Rubrospinal fibers terminate mostly on interneurons of
intermediate areas of cord gray matter, along with corticospinal
fibers; some rubrospinal fibers terminate directly on anterior
motor neurons, along with some corticospinal fibers
o Red nucleus has close connections with cerebellum
Magnocellar portion of red nucleus has somatogrpahic representation of all muscles of body; stimulation of
single point causes contraction of either single muscle or small group of muscles; fineness of representation of
different muscles far less developed than in motor cortex
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Corticorubrospinal pathway serves as accessory route for transmission of relatively discrete signals from motor
cortex to spinal cord; when corticospinal fibers destroyed but corticorubrospinal pathway intact, discrete
movements can still occur, except that movements for fine control of fingers and hands impaired
Rubrospinal tract lies in lateral columns of spinal cord, along with corticospinal tract, and terminates on
interneurons and motor neurons that control more distal muscles of limbs
Corticospinal and rubrospinal tracts together called lateral motor system of cord
Vestibuloreticulospinal system called medial motor system of cord
Extrapyramidal motor system – all portions of brain and brain stem that contribute to motor control but aren’t
part of direct corticospinal –pyramidal system
o Include pathways through basal ganglia, reticular formation of brainstem, vestibular nuclei, and often
red nuclei
o Pyramidal and extrapyramidal systems extensively interconnected and interact to control movement
Cells of motor cortex organized in vertical columns; each column of cells functions as unit, usually stimulating
group of synergistic muscles (sometimes single muscle)
o Each column has 6 distinct layers of cells
o Pyramidal cells that give rise to corticospinal fibers all lie in 5th layer of cells from cortical surface
o Input signals enter via layers 2-4
o 6th layer mainly gives rise to fibers that communicate with other regions of cerebral cortex itself
o Neurons of each column operate as integrative processing system, using info from multiple input
sources to determine output response from column
o Each column can function as amplifying system to stimulate large numbers of pyramidal fibers to same
muscle or to synergistic muscles; stimulation of single pyramidal cell seldom excites muscle
o If strong signal sent to muscle to cause initial rapid contraction, then much weaker continuing signal can
maintain contraction for long periods thereafter
 Each column of cells excites dynamic neurons and static neurons
 Dynamic neurons excited at high rate for short period at beginning of contraction,
causing initial rapid development of force
 Static neurons fire at much slower rate, but continue firing to maintain force
Red nucleus has dynamic and static characteristics; greater percentage of dynamic neurons in red nucleus and
greater percentage of static neurons in primary motor cortex
When muscle contracted, somatosensory signals return to motor cortex; often cause positive feedback
enhancement of muscle contraction
o If fusimotor muscle fibers in spindles contract more than large skeletal muscle fibers contract, central
portions of spindles become stretched and therefore excited; signals return rapidly to pyramidal cells in
motor cortex to signal them that large muscle fibers haven’t contracted enough
 Pyramidal cells further excite muscle, helping its contraction catch up with contraction of muscle
spindles
o If muscle contraction causes compression of skin against object, signals from skin receptors can, if
necessary, cause further excitation of muscles and therefore, increase muscle contraction (grip)
In cervical enlargement where hands and fingers represented, large
numbers of both corticospinal and rubrospinal fibers terminate directly on
anterior motor neurons, allowing direct route from brain to activate muscle
contraction
Many reflexes important when cord’s anterior motor neurons excited by
signals from brain (i.e., stretch reflex helps damp oscillations of motor
movements initiated from brain, providing part of motive power required
to cause muscle contractions when intrafusal fibers contract more than
skeletal muscles, eliciting servo-assist stimulation of muscle)
When brain signal excites muscle, usually unnecessary to transmit inverse
signal to relax antagonist muscle at same time; achieved by reciprocal
innervation circuit always present in cord for coordinating function of
antagonistic pairs of muscles
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Removal of portion of primary motor cortex (area that contains Betz pyramidal cells) causes varying degrees of
paralysis of represented muscles
o If sublying caudate nucleus and adjacent premotor and supplementary motor areas not damaged, gross
postural and limb fixation movements can still occur, but there is loss of voluntary control of discrete
movements of distal segments of limbs, especially hands and fingers
o Area pyramidalis essential for voluntary initiation of finely controlled movements
 Primary motor cortex normally exerts continual tonic stimulatory effect on motor neurons of spinal cord; when
stimulatory effect removed, hypotonia results
o Most lesions of motor cortex, especially stroke, involve primary motor cortex and adjacent parts of brain
such as basal ganglia; muscle spasm almost invariably occurs on opposite side as lesion
 Spasm results from damage to accessory pathways from nonpyramidal portions of motor cortex
o Pathways normally inhibit vestibular and reticular brain stem motor nuclei; when disinhibited, they
become spontaneously active and cause excessive spastic tone in involved muscles
Role of Brainstem in Controlling Motor Function
 Brainstem consists of medulla, pons, and mesencephalon; contains motor and sensory nuclei that perform
motor and sensory functions for face and head regions (spinal cord of face)
 Brainstem provides control of respiration, cardiovascular system, stereotyped movements of body, equilibrium,
eye movements, and partial control of GI function
 Brainstem serves as way station for command signals from higher neural
centers
 Pontine and medullary reticular nuclei function mainly antagonistically to
each other, with pontine exciting antigravity muscles and medullary relaxing
same muscles
 Pontine reticular nuclei transmit excitatory signals downward into cord
through pontine reticulospinal tract in anterior column of cord; fibers
terminate on medial anterior motor neurons that excite axial muscles of
body (muscles of vertebral column and extensor muscles of limbs)
o High degree of natural excitability; receive strong excitatory signals from vestibular nuclei as well as
from deep nuclei of cerebellum
o When pontine reticular excitatory system unopposed by medullary reticular system, it causes powerful
excitation of antigravity muscles throughout body
 Medullary reticular nuclei transmit inhibitory signals to same antigravity anterior motor neurons by way of
medullary reticulospinal tract in lateral column of cord
o Receive strong input collaterals from corticospinal tract, rubrospinal tract, and other motor pathways;
normally activate medullary reticular inhibitory system to counterbalance excitatory signals from
pontine reticular system
o Some signals from higher areas of brain can disinhibit medullary system
o Excitation of medullary reticular system can inhibit antigravity muscles in certain portions of body to
allow those portions to perform special motor activities
o Excitatory and inhibitory reticular nuclei constitute controllable system manipulated by motor signals
from cerebral cortex and elsewhere to provide necessary background muscle contractions for standing
against gravity and inhibit appropriate groups of muscles as needed so other functions can be done
 All vestibular nuclei function in association with pontine reticular nuclei to control antigravity muscles; transmit
strong excitatory signals by way of lateral and medial vestibulospinal tracts in anterior columns of spinal cord
o Without support of vestibular nuclei, pontine reticular
system loses much excitation of axial antigravity muscles
o Specific role of vestibular nuclei is to selectively control
excitatory signals to different antigravity muscles to
Solid lines excitatory
maintain equilibrium in response to signals from vestibular
Dashed lines inhibitory
apparatus
 When brain stem sectioned below midlevel of mesencephalon, but
pontine and medullary reticular systems and vestibular system left
intact, animal develops decerebrate rigidity; occurs in all antigravity muscles of neck, trunk, and legs
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Results from blockage of normally strong input to medullary reticular nuclei from cerebral cortex, red
nuclei, and basal ganglia; medullary reticular inhibitor system becomes nonfunctional
 Overactivity of pontine excitatory system causes rigidity to occur in lesions of basal ganglia and other
neuromotor diseases
Vestibular Sensations and Maintenance of Equilibrium
 Membranous labyrinth is functional part of vestibular apparatus
 Cochlea is major sensory organ for hearing; semicircular canals, utricle,
and saccule parts of equilibrium mechanism
 Macula of utricle lies mainly in horizontal plane on inferior surface of
utricle and plays important role in determining orientation of head
when head upright
 Macula of saccule located mainly in vertical plane and signals head
orientation when person lying down
 Each macula covered by gelatinous layer where many small CaCO3
crystals (statoconia) embedded; also have hair cells that project cilia
into gelatinous layer (bases and sides of hair cells synapse with sensory
endings of vestibular nerve)
o Statoconia have specific gravity more than that of surrounding fluid and tissues; weight of statoconia
bends cilia in direction of gravitational pull
o Hair cells have kinocilium (large cilia on one side) and lots of sterocilia; minute filamentous attachments
connect tip of each stereocilium to next one all the way to kinocilium
 Bending of stereocilia towards kinocilium opens fluid channels in neuronal cell membrane
around bases of sterocilia, conducting large numbers of cations in, causing depolarization
 Bending of stereocilia away from kinocilium reduces tension on attachments, closing ion
channels and causing hyperpolarization
o Under normal resting conditions, nerve fibers transmit continuous nerve impulses; when stereocilia
bent toward kinocilium, impulse traffic increases
o In each macula, each hair cell oriented in different direction, so bending head in different directions
bends hairs in different patterns, and pattern tells brain what head’s orientation is
 When head bent forward 30o, horizontal (lateral) semicircular duct horizontal with respect to surface of earth
o Anterior ducts in vertical planes that project forward and 45o outward
o Posterior ducts in vertical planes that project backward and 45o outward
o Each semicircular duct has enlargement at one end
(ampulla), and ducts and ampulla filled with endolymph;
flow of endolymph through one duct and ampulla excites
sensory organ of ampulla
 When person’s head begins to rotate, inertia of
fluid in semicircular ducts causes fluid to remain
stationary while semicircular duct rotates with
head; causes fluid to flow from duct and through
ampulla, bending cupula to one side (picture on
right; dotted line is where it normally rests)
 Cilia from hair cells on ampullary crest project into cupula; kinocilia all oriented same direction
 From hair cells, appropriate signals sent via vestibular nerve to apprise CNS of change in rotation
of head and rate of change in each of the 3 planes
 Vestibular, cerebellar, and reticular motor nerve systems of brain excite appropriate postural muscles to
maintain proper equilibrium with signals from utricle/saccule system
o Maculae operate to maintain equilibrium during linear acceleration in same manner
o Maculae don’t operate for detection of linear velocity, just acceleration; velocity detected by pressure
end-organs in skin detecting air resistance
o Maculae can’t detect that person is off balance until after they are already off balance
 Semicircular ducts detect angular acceleration, not velocity; loss of function causes person to have poor
equilibrium when attempting to perform rapid, intricate changing body movements
o Detect body turning and informs CNS person will fall unless some anticipatory correction made
 Removal of flocculonodular lobes of cerebellum prevents normal detection of semicircular duct signals but has
less effect on detecting macular signals
 Each time head is suddenly rotated, signals from semicircular ducts cause eyes to rotate in direction equal and
opposite to rotation of head; results from reflexes transmitted through vestibular nuclei and medial longitudinal
fasciculus to oculomotor nuclei
 Vestibular apparatus detects orientation and movement of head only; info about orientation of head with
respect to body transmitted from proprioceptors of neck and body directly to vestibular and reticular nuclei in
brainstem and indirectly via cerebellum
o When head tilted but body still upright, neck proprioceptors exactly oppose signals from vestibular
apparatus, so person doesn’t feel sense of disequilibrium
 Pressure sensations from footpads tell whether weight distributed equally between feet and whether weight on
feet is more forward or backward
 After destruction of vestibular apparatus and most proprioceptive info from body, person can still use visual
mechanisms reasonably effectively for maintaining equilibrium
 Most vestibular nerve fibers terminate in brainstem in
vestibular nuclei (junction of medulla and pons); some fibers
pass directly to brainstem reticular and cerebellar fastigial,
uvular, and flocculomotor lobe nuclei without synapsing
o Fibers that end in brainstem vestibular nuclei synapse
with second-order neurons that send fibers into
cerebellum, vestibulospinal tracts, medial longitudinal
fasciculus, and others of brainstem (reticular nuclei)
o Primary pathway for equilibrium reflexes begins in
vestibular nerves (excited by vestibular apparatus);
pathway passes to vestibular nuclei and cerebellum;
signals sent into reticular nuclei of brainstem and down
spinal cord by vestibulospinal and reticulospinal tracts;
signals to cord control interplay between facilitation and inhibition of antigravity muscles
o Flocculonodular lobes of cerebellum concerned with dynamic equilibrium signals from semicircular
ducts; destruction of these lobes results in loss of dynamic equilibrium during rapid changes in direction
but doesn’t seriously disturb equilibrium under static conditions
o Signals transmitted upward in brain stem from both vestibular nuclei and cerebellum via medial
longitudinal fasciculus cause corrective movements of eyes every time head rotates
o Signals pass upward either through medial longitudinal fasciculus or reticular tracts to cerebral cortex,
terminating in primary cortical center for equilibrium in parietal lobe
Functions of Brainstem Nuclei in Controlling Subconscious, Stereotyped Movements
 Some anencephalic (nothing above mesencephalic region) babies kept alive for many months; able to perform
stereotyped movements for feeding (suckling, extrusion of unpleasant food from mouth, and moving hands to
mouth to suck fingers)
 Can yawn and stretch, cry, follow objects with movements of eyes and head
 Placing pressure on upper anterior parts of legs causes them to pull to sitting position