Download Physiology Ch 55 p667-678 [4-25

Physiology Ch 55 p667-678
Cortical and Brain Stem Control of Motor Function
Motor Cortex and Corticospinal Tract – motor cortex in front of central sulcus, somatosensory
cortex behind central sulcus and feeds motor cortex signals to initiate motor activities
-motor cortex divided into primary motor cortex, premotor area, and supplementary motor area
Primary Motor Cortex – in the 1st convolution of frontal lobes anterior to central sulcus; begins
laterally in sylvian fissure and spreads superiorly to uppermost brain, into longitudinal fissure
-more than half of the primary motor cortex concerned with hands and muscles of speech
-point stimulation in hand motor areas most often causes a group of muscles to contract
-excitation of single neuron excites specific movement rather than specific muscle
Premotor Area – 1-3cm anterior to primary motor cortex  sylvian fissure inferiorly and 
longitudinal fissure superiorly where it abuts the supplemental motor area
-topography is the same as premotor cortex (mouth/face lateral handarmtrunk)
-signals in premotor area cause complex patterns of movement, such as shoulder positioning
and hand orientation for specific tasks
-premotor cortex first makes motor image of total muscle movement and sends signal to
primary motor cortex or to basal ganglia  thalamus  primary motor cortex
-special class of neurons called mirror neurons are active when person performs specific motor
task or when he/she observes same task performed by others
-activity of these neurons mirrors behavior of another person as though observer is
performing the movement
-mirror neurons are located in premotor cortex and inferior parietal cortex
Supplementary Motor Area – has another topographical organization for motor control
-lies inside longitudinal fissure but extends into superior frontal cortex
-stimulation here causes bilateral contraction (grasping movements
-this area functions in concert with premotor to provide body-wide attitudinal movements,
fixation movements of different body segments, positional movements of head/eyes
Specialized Areas of Motor Control in Human Motor Cortex –
1. Broca’s Area and Speech – there is a word formation named Broca’s area anterior to
primary motor cortex and above sylvian fissure
a. Damage makes it impossible to speak whole words as opposed to uncoordinated
utterances or simple yes/no
2. Voluntary Eye Movement Field – above Broca’s area in the premotor area is an area for
voluntary eye movements; damage will cause eye to lock involuntarily on specific objects
3. Head Rotation Area – higher in motor association area, stimulation causes head rotation
causing gaze to shift toward objects
4. Area for Hand Skills – in premotor area anterior to primary motor cortex is an area for hand
skills; destruction of which cause uncoordinated movements or motor apraxia
Transmission of Signals from Motor Cortex to Muscles – motor signals are transmitted to spinal
cord through corticospinal tract and other pathways involving basal ganglia, cerebellum, and
brainstem; direct pathways are concerned with detailed movements of distal limbs (hands and
fingers)
Corticospinal (pyramidal) Tract – originates 30% from primary motor cortex, 30%
premotor/supplementary motor cortex, and 40% somatosensory cortex
-passes through posterior limb of internal capsule  brainstem to form pyramids of medulla
(pyramidal decussation) and cross over to the other side to descend in lateral corticospinal
tracts to terminate on interneurons in intermediate regions of cord, some in dorsal horn, few in
ventral horn
-a few fibers DON’T cross and end up passing ipsilaterally through ventral corticospinal tracts
-most of these fibers cross in neck or thoracic region (control bilateral posture)
-largest fibers in corticospinal tract are large, myelinated fibers with 16um diameter, originate
from giant pyramidal cells (Betz cells) found only in primary motor cortex
-transmit velocities of 70m/s
Other Fiber Pathways in Motor Cortex:
1. Axons of Betz cells send short collaterals back to cortex itself for inhibition adjacent regions
of cortex to “sharpen” boundaries of excitatory signal
2. Large number of fibers pass from motor cortex to caudate nucleus and putamen and into
brain stem/spinal cord to control postural muscle contractions
3. Moderate number of fibers pass to red nuclei of midbrain and through rubrospinal tract
4. Few motor fibers deviate to reticular substance and vestibular nuclei of brain stem and go
to cord through reticulospinal and vestibulospinal tracts and others go to cerebellum
through reticulocerebellar and vestibulocerebellar tracts
5. Lots of motor fibers synapse in pontine nuclei, giving rise to pontocerebellar fibers
6. Collaterals can terinate in inferior olivary nuclei  olivocerebellar fibers to cerebellum
Incoming Sensory Fiber Pathways to Motor Cortex – some motor cortex signals controlled by
sensory systems such as somatosensory cortex and hearing/vision
1. Subcortical fibers from adjacent regions of cortex such as: somatosensory area, adjacent
areas of frontal cortex and visual/auditory cortices have fibers  motor cortex
2. Subcortical fibers arrive from corpus callosum from opposite hemisphere to connect 2 sides
3. Somatosensory fibers that arrive directly from ventrobasal complex of thalamus and relay
cutaneous tactile signals and joint/muscle signals from periphery
4. Tracts from ventrolateral and ventroanterior nuclei of thalamus, receiving signals from
cerebellum and basal ganglia  necessary for coordination and motor control functions
5. Fibers from intralaminar nuclei of thalamus; control general excitability of motor cortex
Red Nucleus is Alternative Pathway for Transmitting Cortical Signals to Spinal Cord– located in
mesencephalon, functions in association with corticospinal tract; receives direct fibers from
primary motor cortex through corticorubral tract and branching fibers from corticospinal tract
-fibers synapse in lower portion of red nucleus (magnocellular portion) containing large neurons
which give rise to rubrospinal tract that crosses to opposite side in lower brain stem and
courses adjacent and anterior to corticospinal tract into lateral columns of cord
-rubrospinal fibers terminate on interneurons in intermediate areas of gray matter, but some
terminate in ventral horn and has close conections with cerebellum
Function of Corticorubrospinal System – magnocellular portion of red nucleus has
somatographic representation of all muscles of the body; stimulation of a single point causes
contraction of a small group of fibers or a single muscle
-fineness representation of muscles is less developed in red nuclei than motor cortex
-corticorubrospinal pathway is an accessory route from cortex to spinal cord, and when
corticospinal fibers are destroyed, discrete movements can still occur through
corticorubrospinal tract, but fine movements like control of fingers is impaired; wrist functional
-rubrospinal tract lies in lateral columns of spinal cord and terminates on interneurons and
motor neurons controlling distal muscles of limb
-together with corticospinal system, they are called lateral motor system of the cord
-the vestibuloreticulospinal system is in middle of cord, called medial motor system
Extrapyramidal System – all portions of brain and brain stem contributing to motor control but
are not directly part of corticospinal system; including basal ganglia, reticular formation of brain
stem, vestibular nuclei, and red nuclei
Excitation of Spinal Cord Motor Control Areas by Primary Motor Cortex and Red Nucleus –
Vertical Columnar Arrangement of Neurons in Motor Cortex – cells in somatosensory and
motor cortices are arranged in vertical columns of cells, with each column functioning as unit
stimulating just a single muscle
-each column has 6 layers of cells, with layer 5 giving rise to pyramidal cells of corticospinal
tract, and layers 2-4 being the input signal entrances
Function of Each Column of Neurons – each column operates as a system, using multiple input
sources to determine output response, and each column can amplify/stimulate large numbers of
pyramidal fibers (50-100 pyramidal cells required to contracta muscle)
Dynamic and Static Signals are Transmitted by Pyramidal Neurons – if strong signal is sent to
muscle to cause initial rapid contraction, a much weaker signal can maintain the contraction for
longer period
-each column of cells excites 2 populations of pyramidal neurons: dynamic and static neurons
-dynamic neurons – excited at high rate for short period at beginning of contraction
-static neurons – fire at a much slower rate to maintain the force of contraction
-in the red nucleus, there is a greater percentage of dynamic neurons over static neurons, while
in cortex there is greater static neuron percentage
Somatosensory Feedback to Motor Cortex Helps Control Precision of Muscle Contraction –
when muscle contracts, somatosensory signals return from body to neurons in motor cortex
arising from muscle spindles, tendon organs, or tactile receptors of skin
-often cause positive feedback enhancement of muscle contraction
-for muscle spindle, fibers in spindles contract more than surrounding muscle and central
spindles become stretched/excited, which tells brain the muscle hasn’t contracted enough
-with tactile receptors, muscle contraction causes compression of skin against object (hand
grasp), which can cause further excitation of muscles to increase tightness of grasp
Stimulation of Spinal Motor Neurons – corticospinal and rubrospinal tracts entering cord in
dorsal portions of white columns and terminate on interneurons in intermediate gray area
-in the cervical enlargement where hands/fingers represented, large numbers of fibers
terminate directly on anterior motor neurons to provide direct route for contraction
Patterns of Movement Elicited by Spinal Cord Centers – stretch reflex is present at all times to
help dampen oscillations of motor movements initiated from brain
-when brain signal excites muscle, it is unnecessary to transmit inverse signal to antagonist
muscle, instead achieved by reciprocal innervation circuit always present in cord
-withdrawal, stepping, walking, scratching, can be activated on command
Effect of Lesions in Motor Cortex in Corticospinal Pathway (the stroke) – a ruptured blood
vessel that hemorrhages in brain or thrombosis of major artery supplying brain causes stroke,
which causes loss of supply to cortex/corticospinal tract
Removal of Primary Motor Cortex – removal of the area containing Betz pyramidal cells causes
various degrees of paralysis; if caudate nucleus and adjacent premotor and supplementary
nucleus are not damaged, gross postural and limb fixation movements can occur, but there is
loss of voluntary control of discrete movements of hands and fingers (distal limbs)
-hand can still contract, we just lose control over that ability
Muscle Spasticity Caused by Lesions that Damage Large Areas Adjacent to Motor Cortex –
primary motor cortex normally has continual tonic stimulatory effect on motor neurons of cord,
and when removed, hypotonia occurs; stroke causes involvement of primary motor cortex and
basal ganglia, and can cause muscle spasm on OPPOSITE side of body
-spasticity normally accompanies a stroke because there is a destruction of inhibitory pathways
from accessory tracts
Role of Brain Stem in Controlling Motor Function – brain stem has medulla, pons, and
mesencephalon. The brain stem controls: respiration, cardiovascular system, GI function,
stereotyped movements of body, equilibrium, and control of eye movements
-brain stem also serves as a way station for command signals from higher neural centers for
whole body movement and equilibrium; important structures are reticular nuclei and vestibular
nuclei
Support of Body Against Gravity – Roles of Reticular/Vestibular Nuclei –
Excitatory-Inhibitory Antagonism Between Pontine and Medullary Reticular Nuclei – reticular
nuclei are divided into two distinct groups: the pontine reticular nuclei (behind pons and into
mesencephalon, and the medullary reticular nuclei, extending through entire medulla
(ventral/medial to midline)
-these 2 nuclei function antagonistically to each other, with pontine nuclei exciting antigravity
muscle and medullary relaxing same muscles
Pontine Reticular System – pontine reticular nuclei transmit EXCITATORY signals down cord
through pontine reticulospinal tract in anterior column of cord and terminate on medial anterior
motor neurons to excite axial muscles of body (paraspinal muscles and extensor limb muscles)
to support body against gravity
-pontine nuclei have high degree of natural excitability and also receive excitatory
signals from vestibular nuclei and cerebellum
Medullary Reticular System – transmits INHIBITORY signals to same antigravity anterior motor
neurons by a different tract: medullary reticulospinal tract in the lateral column of the cord
-strong input collaterals from rubrospinal tract, corticospinal tract, and other pathways
-these activate reticular inhibitory system to counterbalance excitatory signals
from pontine reticular system
Role of Vestibular Nuclei to Excite Antigravity Muscles – all vestibular nuclei function with
pontine reticular nuclei to control antigravity muscles; they transmit strong excitatory signals to
antigravity muscles through lateral and medial vestibulospinal tracts in anterior columns of
spinal cord
-without support of vestibular nuclei, pontine reticular system would lose much excitation
-specific role of vestibular nuclei is to selectively control excitatory signals to different
antigravity muscles to maintain equilibrium in response to signals from vestibular apparatus
Decerebrate Rigidity – when brain stem is sectioned below midlevel of mesencephalon, but
pontine/medullary reticular systems + vestibular system is intact… rigidity occurs in antigravity
muscles (neck, back, and leg extensors)
-cause is blockage of input to medullar reticular nuclei from cerebral cortex, red nuclei,
and basal ganglia, so the medullary reticular inhibitor system is nonfunctional, causing
overactivity of pontine excitatory system
Vestibular Apparatus – sensory organ for detecting sensations of equilibrium found in bony
tubes and chambers in petrous portion of temporal bone (called bony labyrinth)
-this system has a system of membranous tubes called membranous labyrinth and is the
functional part of vestibular apparatus
-membranous labyrinth composed of cochlea, 3 semicircular canals, and 2 large chambers:
utricle and saccule
-cochlea is for hearing, the rest are for equilibrium
Maculae – Sensory Organs of Utricle and Saccule for Detecting Orientation of Head with
Respect to Gravity – inside the utricle and saccule, is a small sensory area called a macula
-Macula of utricle is in HORIZONTAL PLANE on inferior surface and is important for determining
orientation of head when head is upright
-Macula of saccule is in the VERTICAL PLANE and signals head orientation when person is lying
down
-each macula is covered by gelatinous layer which small CaCO3 crystals called statoconia are
embedded.
-statoconia have specific gravity 2-3x of surroundings + bends cilia towards gravitational pull
-macula also has thousands of hair cells, which project cilia into gelatinous layer
-bases and sides of hair cells synapse with sensory endings of the vestibular nerve
Directional Sensitivity of Hair Cells – Kinocilium – each hair cell has 50-70 small cilia called
stereocilia and one large one known as kinocilium, which is always located to one side
-filamentous attachment exists connecting the filaments and finally to kinocilium
-these attachments cause stereocilia to bend toward kinocilium and tug stereocilia away from
body which opens fluid channels in neuronal membrane to conduct positive ions and cause
receptor membrane depolarization
-bending in opposite direction reduces tension and closes ion channels to cuase
receptor hyperpolarization
-in each macula, each hair cells is oriented in a different direction so that some hair cells
stimulated when head bends forward, others when head bends backward, side-side, etc…
Semicircular Ducts – 3 ducts, the anterior, posterior, and lateral semicircular ducts are
arranged at right angles to one another and represent all 3 planes in space
-when head bent forward 30°, lateral semicircular ducts are horizontal, anterior ducts are in
vertical planes projecting forward and 45° outward, and posterior ducts are in vertical planes
projecting backward and 45° outward
-each semicircular duct has an enlargement at one end called the ampulla, and both ducts and
ampulla are filled with fluid called endolymph; flow of endolymph through ducts and ampulla
excites sensory organ in ampulla in the following manner:
-a crista ampullaris exists inside each ampulla; on top of crista exists a loose gelatinous
mass called the cupula
-when person rotates head in any direction, inertia of fluid in 1 or more ducts causes
fluid to remain stationary while semicircular duct rotates with head, causing fluid to
flow from duct and through ampulla to bend cupula to one side
-rotation of head to opposite side causes cupula to bend to opposite side
-hundreds of hair cells project into cupula from ampullary crest; all kinocilia are in same
direction in cupula, and bending of cupula in that direction causes depolarization of hair cells,
and bending in opposite direction causes hyperpolarization
-from these hair cells, signal travels through vestibular nerve to signal to CNS that head
has rotated
Function of Utricle and Saccule in Maintenance of Static Equilibrium – important for hair cells
to all be oriented in different directions in maculae of utricles and saccules so that with different
head positions, different hair cells become stimulated; patterns apprise CNS of position of head
with respect to pull of gravity  excite postural muscles through vestibular, cerebellar, and
reticular motor nerve systems
-utricle and saccule effective for maintaining equilibrium when head is near vertical position
Detection of Linear Acceleration by Utricle and Saccule Maculae – when body accelerates, the
statoconia, which have greater mass inertia than surrounding fluid, fall back on hair cell cilia and
information of disequilibrium is sent to CNS causing person to think they’re falling backward,
causing person to lean forward until resulting anterior shift of statoconia equals tendency to fall
backward due to acceleration
-Maculae do NOT detect linear velocity, only acceleration
Detection of Head Rotation by Semicircular Ducts – when head rotates in any direction
suddenly (angular acceleration), endolymph in semicircular ducts (because of inertia) tends to
remain stationary while semicircular ducts turn causing relative fluid flow in ducts in opposite
direction to rotation
-even when cupula is in resting position, hair cell emits tonic discharge, when animal rotates,
hairs bend to one side and rate of discharge increases, with continued rotation, excess discharge
decreases back to resting level
-this happens because after a few seconds, endolymph begins to move with semicircular
ducts
-when rotation suddenly stops, opposite takes place; endolymph rotates while semicircular duct
stops and cupula bends in opposite direction causing hair cell to stop discharging entirely
-thus, semicircular canals only transmit signals when head BEGINS and STOPS rotating
Predictive Function of Semicircular Duct System in Maintenance of Equilibrium – ducts do not
detect if body is off balance in forward, backward, or side direction and only detect head
beginning or stopping rotation
-yet, loss of function does cause person to have poor equilibrium when attempting to make
rapid, intricate changing body movements
-if person is running and falls turns to one side, he will fall off balance for a fraction of a second,
not enough time for utricle or saccule to detect off balance until after, but semicircular ducts DO
detect turning and send signal to CNS to make anticipatory correction
-semicircular ducts PREDICT when disequilibrium will occur and cause equilibrium centers to
make appropriate corrections
-removal of flocculonodular lobes of cerebellum prevents normal detection of duct canals but
has less effect on macular signals
Vestibular Mechanisms for Stabilizing Eyes – person constantly moving head around needs a
mechanism to control eyes gaze, which remain fixed on object long enough for clear image
-each time head is suddenly rotated, signals from semicircular ducts cause eyes to rotate in
equal/opposite direction to rotation of head resulting from reflexes transmitted through
vestibular nuclei and medial longitiduninal fasciculus to oculomotor nuclei
Neck Proprioceptors – detects orientation and movement of ONLY THE HEAD, so nervous
receptors need info on head form body, transmitted by proprioceptors on neck and body
directly to vestibular and reticular nuclei in brainstem and through cerebellum
-most important are joint receptors of neck; when head is leaned in one direction by bending
neck, impulses from neck proprioceptors keep signals originating in vestibular apparatus from
giving person sense of disequilibrium by transmitting signals that oppose those sent from
vestibular apparatus
-HOWEVER, when WHOLE BODY is leaned in one direction, vestibular apparatus signals
are NOT opposed by neck proprioceptors, and change IS perceived
Proprioceptive and Exteroceptive info From Other Parts of Body – pressure sensations from
footpads tell whether weight is distributed equally between 2 feet and whether weight on feet
is more forward or backward
-exteroceptive info needed for equilibrium when person is running; air pressure against front of
body signals that a force is opposing body in direction different from gravitational pull, so body
leans forward to oppose this
Importance of Visual Information in Maintenance of Equilibrium – if you destroy vestibular
apparatus, person can still use visual mechanisms for maintaining equilibrium, but only slowly
-when moving rapidly or when eyes are closed, equilibrium is immediately lost
Neural Connections of Vestibular Apparatus with CNS – most vestibular nerve fibers terminate
in brain stem in vestibular nuclei, located at junction of medulla and pons; some fibers pass
directly to reticular nuclei in brain stem without synapsing and also to cerebellar fastigial,
uvular, and flocculonodular lobe nuclei
-fibers that end in brain stem vestibular nuclei synapse with 2nd order neurons that also send
fibers to cerebellum, vestibulospinal tracts, medial longitudinal fasciculus and other areas
-PRIMARY pathway for equilibrium reflexes begins in vestibular nerves, where the nerves are
excited by vestibular apparatus
-pathway passes to vestibular nuclei and cerebellum  reticular nuclei of brain stem and spine
by way of vestibulospinal and reticulospinal tracts (control interplay between facilitation and
inhibition of antigravity muscles)
-flocculonodular lobes of cerebellum are concerned with dynamic equilibrium from semicircular
ducts; destruction of lobes causes loss of dynamic equilibrium during rapid changes in direction
of motion but does not seriously disturb equilibrium under static conditions
-signals transmitted upward in brain stem from vestibular nuclei and cerebellum through medial
longitudinal fasciculus causes corrective movements of eyes every time head rotates so that
eyes remain fixed on specific visual object
-signals also pass to cerebral cortex and terminate in primary cortical center for equilibrium in
parietal lobe deep in sylvian fissure on opposite side of auditory area of superior temporal
gyrus; these signals apprise psyche of equilibrium status of body
Function of Brain Stem Nuclei Controlling Subconscious, Stereotyped Movements – during
anencephaly, baby is born without a brain above mesencephalic region, but can still suckle,
extrusion of unpleasant food from mouth, and moving hands to mouth; can yawn and stretch
and cry and can follow objects with eyes
-many stereotyped movements of human are integrated into brain stem