Cortical and Brain Stem Control of Motor Function

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Primary Motor Cortex

-located in the FRONTAL LOBE within the GYRUS immediately anterior to the central sulcus, called the PRECENTRAL GYRUS or BRODMANN'S AREA 4

-In the PRECENTRAL GYRUS
1) stimulation of the LATERAL-MOST portion --> activation of head and neck muscles
2) Middle portion --> movement in the hand, are or shoulder
3) medial portion --> activation of the trunk and lower extremity muscles

**at some STIMULATION points individual muscles were activated, whereas at others a group of muscles were activated

Premotor Area

$$Immediately anterior to the lateral portion of the primary motor cortex
-this cortex forms a PORTION of Brodmann's area 6 and contains a somatotopically organized map of the body MUSCULATURE
**Stimulation in this cortex --> movements that involve groups of muscles (ie the arm and shoulder to place the hand in position to perform a certain task)

Supplementary Motor Area

-located in the medial portion of area 6 on the dorsal convexity and medial wall of the hemisphere just anterior to the lower extremity portion of the precentral gyrus

**Stimulation here requires GREATER INTENSITY and typically --> BILATERAL MUSCLE ACTIVATION (usually involving the UPPER EXTREMITIES)

Broca's area

-motor speech area
-premotor area
-lies just anterior to the face portion of the primary motor cortex near the sylvian fissure
-activity here engages the musculature needed to convert simple vocal utterances into whole words and complete sentences

The frontal eye field (Brodmann's area 8)

-also lies just anterior to the precentral gyrus but somewhat more DORSAL than Broca's Area
-this cortical region controls the conjugate eye movements required to SHIFT GAZE from one object to another
-in the premotor area

Head rotation area

-associated with the frontal eye field
-functionally linked to area 8
-serves to enable movements of the head CORRELATED with eye movement
-slightly higher in the motor association area

Control of fine movements of the hand

-located in the premotor cortex just anterior to the hand region of area 4
**when this area is damaged, the muscles of the hand are NOT PARALYZED, but certain hand movements are lost
-called MOTOR APRAXIA

Corticospinal (pyramidal) tract
-output pathway from the motor cortex

-the corticospinal tract originates from the primary motor cortex (30%), and the premotor cortex (30%), the remainder is divided among several other areas; including the primary somatosensory cortex, parietal lobe areas, and portions of the cingulate gyrus

1) After leaving the cortex, axons of this tract enter the POSTERIOR LIMB OF THE INTERNAL CAPSULE and pass CAUDALLY through the brain stem to the ventral surface of the MEDULLA where they are contained in the MEDULLARY PYRAMIDS
-At the junction of the medulla and spinal cord, most of the fibers cross the midline to enter the LATERAL FUNICULUS of the spinal cord and form the LATERAL CORTICOSPINAL TRACT, which extends throughout the length of the cord
**the fibers that do NOT cross the midline continue as far as the thoracic spinal cord in the VENTRAL CORTICOSPINAL TRACT
$$ some go LATERAL and some go VENTRAL

The largest fibers in the pyramidal tract are ___

-16 um in diameter and are believed to originate from the GIANT CELLS of BETZ found in the precentral gyrus
**There are ~34,000 Betz cells, and the total number of fibers in the corticospinal tract is ~ 1 million (Betz cells = 3%)

Other fiber pathways from the motor cortex

1) axons from the Giant Betz cells to the cortex itself (inhibitory) to sharpern boundaries

2) Caudate nucleus and Putamen - from here additional pathways extend into the brain stem and spinal cord mainly to control BODY POSTURAL MUSCLE CONTRACTIONS

3) Red nuclei of midbrain - from these, additional fibers pass down the cord through the RUBROSPINAL TRACT

4) moderate number of motor neurons deviate into the reticular substance and vestibular nuclei of the brain stem
-from there, signals go to the cord by way of reticulospinal and vestibulospinal tracts and others go to the cerebellum by way of the reticulocerebellar and vestibulocerebellar tracts

5) tremendous amount synapse in the PONTILE NUCLEI which give rise to PONTOCERBELLAR FIBERS, carrying signals into the cerebellar hemispheres

6) Collaterals also terminate in the inferior olivary nuclei, and from there, secondary olivocerebellar fibers transmit signals to multiple areas of the cerebellum

Incoming Sensory fiber pathways to the motor cortex

-somatosensory cortex as well as fibers from a variety of THALAMIC NUCLEI that carry info from ascending somatosensory pathways, cerebellum, basal ganglia, and reticular activating system

Like neurons in the visual cortex, those in the motor cortex are organized into ____

-Vertical modules
-each vertical unit may control the activity of a synergistic group of muscles or an individual muscle
-it is estimated that 50-100 pyramidal neurons must be activated simulatenously or in rapid succession to cause muscle contraction

Often, if a strong signal is needed to causes INITIAL muscle activation, a weaker signal is able to maintain the contraction for longer periods therafter....

-the substrate for this function may involve two populations of corticospinal neurons:

1) Dynamic neurons - produce high output for short time periods. initiator

2) static - fire a less intense signal at a slower rate to maintain the force of contraction

Red nucleus

-located in the mesencephalon, functions in close association with the corticospinal tract
**it receives a large number of direct fibers from the primary motor cortex through the CORTICORUBRAL TRACT
--> Rubrospinal tract (starting at magnocellular portion of red nucleus) --> crosses to opposite side in lower brain, follows a course immediately adjacent and anterior to the corticospinal tract into the lateral columns of the spinal cord
-terminate mostly on the interneurons of the intermediate areas of the cord

-same, but less fine tuned function as motor cortex

Stimulation of Spinal motor neurons

-Most of the cortical input is focused on the pool of spinal interneurons, but apparently some corticospinal axons synapse directly with ventral horn motor neurons
**the corticospinal system may carry "command signals" that activate patterns of movement whose composition is determined by aggregates of spinal INTERNEURONS

Stroke

-caused by a ruptured blood vessel that bleeds into the brain or by THROMBOSIS of a vessel that produces local ischemia in neighboring brain tissue
**when wither event involves the primary motor cortex (origin of the corticospinal tract), the resulting motor deficits are characterized by the loss of voluntary control of discrete movements involving the distal portions of extremities, particularly the fingers and hands (not paralysis)

-When the tissue damage extends beyond the primary cortex and involves the neurons that project to the caudate, putamen, or reticular formation, characteristic symptoms such as hyperreflexia, hypertonia, and spasticity occur

Supporting body against gravity

-The pontine and medullary areas of the reticular formation function in opposition to one another

1) The pontine levels - tend to excite antigravity muscles
-strongly activated by ascending somatosensory fibers, vestibular nuclei, and cerebellar nuclei, and when unopposed by medullary levels, the excitation of antigravity muscles is sufficiently strong to support the body

2) medullary levels - inhibit antigravity muscles
-the inhibitory influence derived from the medullary reticulospinal fibers is strongly influenced by input from the cerebral cortex and the red nucleus

Role of vestibular nuclei to excite antigravity muscles

-the LATERAL VESTIBULAR NUCLEUS transmits excitatory signals (mainly by way of the lateral vestibulospinal tract) that strongly excite antigravity muscles
**This system is influenced most strongly by the vestibular sensory sensory apparatus and uses the antigravity muscles to maintain balance

decerebate rigidity

-when the brain stem is sectioned at about mid-collicular levels, leaving the reticulospinal and vestibulospinal tracts intact

**characterized by hyperactivity in the antigravity muscles, primarily the neck, trunk and extremities
$$Activation of the antigravity muscles is unopposed b/c the CORTICOSPINAL and RUBROSPINAL tracts have been sectioned, along with cortical activation of the medullary reticulospinal fibers

$$ Although the cortical drive on pontine reticulospinal system has also been interrupted, there is sufficient activation remaining from other excitatory inputs such as the ascending somatosensory pathways and cerebellar nuclei

antigravity muscles have ____ stretch reflexes

-greatly enhanced
-exhibit spasticity
**It is believed that the descending influence from the pontine reticulospinal fibers affects primarily the GAMMA MOTOR NEURONS (if they are activated the stretch reflex becomes greatly enhanced/more fine-tuned) 1a primary afferent fibers

Vestibular Apparatus

-the sensory organs for the vestibular sense are located in a system of bony chambers in the petrous portion of the temporal bone
-each BONY ENCLOSURE houses a membranous chamber of tubular structure that contains the SENSORY HAIR CELLS and the terminal ends of the primary sensory fibers of the 8th cranial nerve that lead into the brain

The membranous structures include:
1) three semicircular canals or ducts
Larger chambers:
2) utricle
3) saccule

within each utricle and saccule is a small specialized structure called _____

The surface of each macula is covered by ____

-the macula - flattened area ~ 2mm in diameter that lies in the horizontal plane on the inferior surface of the utricle and in the vertical plane in the saccule

- a gelatinous layer in which calcium carbonate crystals called STATOCONIA are embedded

**the macula contains supporting cells and sensory hair cells WITH CILIA that protrude upward into the gelatinous layer
-each cell has 50-70 stereocilia and ONE large KINOCILIUM (tallest cilium) that is positioned off to the side of apical surface of the cell
-the stereocilia become progressively shorter toward the opposite
side of the KNICILIUM
-minute filaments connect the tip of each cilium to the next adjacent one and serve to open ion channels in the cilia membrane, which is bathed in endolymphatic fluid

When the stereocilia are bent TOWARD the kinocilium -->___

-ion channels OPEN, ions enter the cell from the endolymph, and the cell is DEPOLARIZED

**opposite is true

The utricle and saccule are sensitive to ____

-linear acceleration
**When the head accelerates in any plane of gravity, the statoconia shift and displace hair cell CILIA in a specific direction, which depolarizes or hyperpolarizes others

Anterior, posterior, lateral canals

-Anterior and posterior - both in the vertical plane (both angled 45 degrees forward and backward respectively)

The sensory epithelium in each canal is formed by an ____

-ampulla composed of ciliated sensory hair cells capped by a small crest called the CRISTA AMPULLARIS - which protrudes into an overlying gelatinous mass, the CUPULA

-each canal contains endolymph, which is free to move with rotation of the head; as it does, the cupula is deflected, along with the cilia that protrude into it from the hair cells
**Movement in one direction is depolarizing; movement in the opposite direction is hyperpolarizing

Head rotation (angular acceleration)

-the endolymph canals, b/c of its inertia tends to remain stationary and produces relative endolymph flow opposite that of head rotation
-the CUPULA is deflected, the cilia are displaced, and the hair cells are depolarized or hyperpolarized, depending on the direction of CUPULA DEFLECTION

Persistent rotation of head - the endolymph attains the same direction and velocity as the head rotation, the cupula is no longer deflected, and the hair cells are not stimulated

When the rotation stops: there is again endolymph flow relative to the cupula (in the direction of rotation)

Primary function of semicircular canals

-do not serve to maintain equlibrium
-signal the beginning (or end) of head rotation; they are thus PREDICTIVE in function

The activation of motor responses (Vestibular reflex actions) is achieved by ____

-projections from the vestibular nuclei to the lateral vestibulospinal tract

When head orientation changes, the eyes must be moved to maintain a stable image on the retina. How?

-through connections from the semicircular canals to the vestibular nuclei, which then control motor neurons of the third, fourth and sixth cranial nerves (eye movements) via projections through the MEDIAL LONGITUDINAL FASCICULUS

Sensation of dysequilibrium when the neck is BENT

-Proprioceptors in muscles and joints of the neck provide input to the vestibular nuclei which counteracts this sensation

Medial longitudinal fasciculus

-component of the brain stem reticular formation
-function to maintain the eyes ON TARGET WHEN HEAD ORIENTATION CHANGES
-interconnected with vestibular nuclei

The perception of head and body movement is achieved through ____

-vestibular input to the THALAMUS, which then projects to the cerebral cortex

Vestibular system and cerebellum

-maintains extensive projections to, and receives projections from, the cerebellum

**the cerebellar FLOCCULONODULAR LOBE is related to semicircular canal function and, when affected by lesions --> causes a loss of equilibrium during rapid changes in direction of the head motion

**the UVULA of the cerebellum plays a similar role in regard to static equilibrium

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