Your text here!     
THE EXTRAPYRAMIDAL SYSTEM

Home | THE BRAIN STEM | THE CEREBELLUM | SITE MAP

 
NEUROEMBRIOLOGY
MEMBRANE POTENTIAL
THE SYNAPSE
THE MUSCLE TONE
MOTOR UNIT
DESCENDING MOTOPATHS
RECEPTORS
SENSORY PATHWAYS
AUTONOMIC NERVOUS SYSTEM
HYPOTHALAMUS AND THALAMUS
THE TELENCEPHALON
NEUROCHEMISTRY
 
 
  Related sites

neuro.science
neurosurgery.tv
neurosurgery.me
e-neurosurgery.com
neurosurgery.mx
neurophysiology.ws
microneuroanatomy
diencephalon.info
mesencephalon.org
medullaoblongata.info
neurooncology.ws
pituitaryadenomas.com
pituitaryadenoma.net
meningiomas.org
meningiomas.info
glioma.ws
vestibularschwannomas
acousticschwannoma
craniopharyngiomas
craniopharyngiomas
pinealomas.com
ependymoma.info
skullbase.surgery
spine.surgery
spinesurgeries.org
spondylolisthesis.info
vascularneurosurgery
vascularneurosurgery
e-neuroradiology.com
cns.clinic
cns-clinic.net

 
 

 

The extrapyramidal system is composed of motor fibers which do not pass through the medullary pyramids but which nevertheless exert a measure of control over bodily movements. The system is difficult to describe, partly because of the complexity of pathways and feedback loops which compose it. Nevertheless, the extrapyramidal system can be divided into three controlling systems: the cortically originating indirect pathways, the feedback loops, and the auditory-visual-vestibular descending pathways.

Cortically Originating Indirect Descending Pathways At the same time signals are being transmitted over the pyramidal system to produce a specific movement, additional signals relative to the movement are also relayed to the basal nuclei, red nucleus, and brainstem reticular formation. The basal nuclei evaluate the command signal sent down the pyramidal pathways and may con­tribute to the establishment of needed background muscle tone for the move­ment. The nuclei are able to do this in part by projecting to the red nuclei, which influence spinal cord alpha and gamma motor neurons via rubrospinal tracts. Similar indirect routing to the spinal cord is achieved through corticore­ticulospinal and corticorubrospinal pathways (Fig. 16-5).

The function of these indirect pathways to the spinal cord motor neurons may include more than providing background muscle tone for movements directed by the motor cortex. Recall from Chap. 6 that ablation studies in which the rubrospinal tracts are experimentally cut have shown that the cor­ticospinal and rubrospinal tracts have somewhat similar effects on spinal motor neurons. When the rubrospinal tracts of monkeys were damaged along with earlier pyramidal tract sections, the loss of skilled control in distal muscles became even more severe and yet there was little or no loss of proximal muscle control. Even so, because the red nucleus receives input from the basal and cerebellar nuclei as well as direct input from the cerebral cortex, its function may include modifying or "fine tuning" the motor neurons which innervate the muscles involved in a given movement.Feedback Loops The feedback loops described here include neural cir­cuits in which a signal sample is fed back to a "comparator," which is in a posi­tion to compare the signal with some desired condition and subsequently take steps to "adjust" or "modify" it. The extrapyramidal system includes two such feedback systems: the cortically originating extrapyramidal system feedback loops (COEPS feedback loops) and the proprioceptor originating extra­pyramidal system feedback loops (POEPS feedback loops).

The CO EPS feedback loops are composed of fibers originating in the motor cortex which synapse in subcortical centers. After integrating and evaluating the signals, the centers project fibers back to the cortical source for modifica­tion. Three such loops are illustrated in Fig. 16-6. In loop A the signal is "tapped off" to the corpus striatum (caudate and putamen), which in turn pro­ject to the globus pallidus. Pallidothalamic fibers then project to the thalamus, which completes the loop by projecting back to the cortical source. Somewhere in this loop the original signal sent down the pyramidal tracts is compared and evaluated with other input relative to the movement. After appropriate integra­tion, modifying feedback signals are returned to the cortex via the thalamocor­tical fibers. In loop B the sample signal is sent to pontine nuclei for subsequent referral to the cerebellum, where it is probably compared to proprioceptive input coming from muscles, tendons, and joints involved in the movement. This input probably includes such things as the current state of muscle tone and the relative position and movement of the limb involved. Following integration of this input, the cerebellum then projects its output to the thalamus (via denta­tothalamic tracts) which then completes the loop by sending fibers back to the cortical source through thalamocortical projections. In loop C. the sample sig­nal is sent to the substantia nigra. which projects in turn to the corpus striatum. From here the feedback circuit is identical to that illustrated in loop A. The im­portance of these feedback loops to normal motor control can be most clearly seen by an examination of the clinical signs associated with dysfunction of the basal nuclei and their related brain stem areas, which we will examine later.

The other feedback loop system which is included in the extrapyramidal system is composed of the POEPS feedback loops. In this system the modifica­tion is not directed back toward the cortical source (as are the COEPS loops), but to the spinal cord motor neurons instead. The principal loop involves the relay of muscle, tendon. and joint proprioceptive information to the cerebellum via the spinocerebellar tracts. The signals are integrated in the cerebellum and probably compared with the intended signals sampled by corticopon­tocerebellar pathways. In this way the cerebellum might compare the intended movement with the instantaneous performance of that movement as sampled by the proprioceptors of the spinocerebellar tracts. It could then direct modifi­cation through its projections to the vestibular. reticular, and rubral nuclei and their respective descending tracts to the appropriate motor neurons of the spi­nal cord.

Auditory Visual Vestibular Descending Pathways Postural adjustments in response to auditory, visual. and vestibular signals is an additional way to regu­late the activity of spinal motor neurons. Auditory and visual input to the tectal nuclei of the midbrain may be responsible for producing reflex movements of the body in response to a sudden sound or bright light. Similarly. input from the vestibular apparatus to the vestibular nuclei and cerebellum no doubt plays a role in reflex postural adjustments through the vestibulospinal and other tracts.

It should be pointed out here that because of the complex nature of the neural circuits which effect motor control through routes other than the pyramidal system, a precise and universally agreed upon definition of the ex­trapyramidal pathways has never been achieved.

Clinical Signs of Basal Nuclei and Related Brainstem Dysfunction

 

Certain disease conditions relating to motor control appear to be positively linked to dysfunction of the basal nuclei and those structures functionally related to them including the thalamus, subthalamus, and substantia nigra.

Chorea is a condition characterized by uncontrolled random movements of the body often accompanied by facial grimaces. Evidence indicates that the condition is often associated with dysfunction of the corpus striatum. It is often seen as a complication of rheumatic fever in children. Recovery from this childhood form of the disease, Sydenham's chorea, is usually complete with no subsequent lingering effects. A more severe form, Huntington's chorea, is a he­reditary disease which becomes progressively worse and often leads to severe mental debilitation. A thetosis is a condition characterized by slow wormlike movements of the peripheral appendages, and is also associated with damage to the corpus striatum and lateral parts of the globus pallidus. Voluntary move­ments in the affected appendages are often impaired. Violent flinging of a limb or limbs is a rare condition called ballismus. If one limb is involved the condi­tion is called monoballismus, and if both limbs on a single side are affected the term is hemiballismus. It is generally associated with damage to the subthal­amus and can occur spontaneously or be brought on by the initiation of a vol­untary movement involving the affected limb.

Perhaps the most familiar disease condition in this group is Parkinson's disease (paralysis agitans). It is characterized by an increasing tremor during rest. Also observed are a "pill-rolling" action of the fingers, a poverty of move­ment expressed by difficulty in initiating voluntary movements such as getting up from a chair and walking, a plastic or deathlike rigidity often demonstrated by a "cog-wheeling" phenomenon when a limb is passively moved, and an in­creasing masklike fixed expression to the face.

The cog-wheeling phenomenon that occurs as a limb is passively moved is tentatively explained by the following mechanism. Initial resistance is due to muscle tone as the limb is moved. Release comes when group Ib afferents from Golgi tendon organs inhibit homonymous alpha motor neurons. Then as the passive movement of the limb continues, tension again develops until the threshold of the Golgi tendon organs is once again reached, causing a second release. This rachetlike movement continues as the limb is passively moved. Parkinson's disease is usually associated with dysfunction of the basal nuclei and the substantia nigra.

Feedback loops in electronic systems must be finely tuned in order to prevent oscillations. In physiological systems the feedback loops must also be working properly in order to prevent oscillations in muscle systems. In Parkin­son's disease, the fine tuning is lost and oscillating signals to motor neurons produce tremors. It appears that the principal, site of malfunction lies in the dopamine-releasing fibers of the nigrostriatal pathway. There are both excita­tory cholinergic nigrostriatal fibers and inhibitory doparninergic nigrostriatal fibers. Fine tuning seems to require the complete integrity of both types. In Parkinson's disease, the feedback system becomes "untuned" by the inability of the inhibitory dopaminergic neurons to produce and release dopamine. Some success has been achieved in the treatment of this condition by the adminstra­tion of i.-dopa, a dopamine precursor which is taken up by dopaminergic nigrostriatal fibers and converted to dopamine. With this subsequent "replace­ment" of the missing transmitter, some degree of fine tuning is restored and the severity of symptoms is often reduced

bullet Source Files

1) Photoshop 7 file for the complete layout.
• solid_business_v2.psd

2) Flash MX files:
• flash_image_01.fla, flash_text_01.fla – Home, About Us, Contact, Sitemap.
• flash_image_02.fla, flash_text_02.fla – News, Tutorial.
• flash_image_03.fla, flash_text_03.fla – Catalog, Product1, Product2, New Page.
• flash_image_04.fla, flash_text_04.fla – Getting Started.
 
bullet Images Resource

The images contained in this product were purchased from:

Clipart.com

The images and/or photos used in this template are the copyrighted property of Jupiterimages and are being used with permission under license. Images may only be used within this template. Images can not be removed in any way for other uses. All copyrights remains with the original licensor.
 
bullet SWFObject: Javascript Flash embed script

This template uses the SWFObject (v1.4) Flash detection and embed script provided by Geoff Stearns at http://blog.deconcept.com/swfobject/. This script detects whether or not the user has the Flash player, and if it does, it loads in the Flash movie for them to view. If they don't have the Flash player, it displays your alternate content. The alternate content is what you see in your web editor (a static image or text).
The SWFObject script also resolves the IE update issue where embedded Flash movies require an extra click for interaction; using the script will allow viewers to interact with the Flash movie directly. The SWFObject script also allows your web site to have fully validated HTML.
Please be aware that the script may be updated occasionally, and it is your responsibility to visit the SWFObject site to check for updates and implement them.
 
bullet Included page

Included Page function is used in this template. Once you modify the area where the Included Page function is used in the associated file, your changes will automatically appear on all pages.

1. A copyright at the bottom of each page. -> includes/copyright.htm
2. Icons in each page -> includes/icons.htm
3. Information box in the left in each page. -> includes/info_box.htm
4. Company Logo in each page. -> includes/logo.htm
5. Bottom menu in each page. -> includes/menu_bottom.htm
6. Global menu in each page. -> includes/menu_global.htm
7. Main menu in each page. -> includes/menu_main.htm
8. News in the right in each page. -> includes/news.htm
9. Quick menu in each page -> includes/quick_menu.htm
 
bullet How to make a new web

1. Unzip the downloaded product.
2. Copy the unzipped web folder to your My Webs directory.
3. On the File menu, go to Open Web or Site.
4. Browse to your My Webs directory and find the web folder you just copied.
5. Select that folder, click open and edit.

Or, you can copy the web folder to the following directories depending on your FrontPage version and then use it as a FrontPage web template if you wish.

** FrontPage 2000/2002 **
FrontPage Web Template folder:
C:\Program Files\Microsoft Office\Templates\1033\Webs\<folder_name.tem>

** FrontPage 2003 **
FrontPage Web Template folder:
C:\Documents and Settings\<username>\Application Data\Microsoft\FrontPage\Webs\<folder_name.tem>
 
bullet To apply a New Web Template in FrontPage 2000/ 2002

1. On the File menu, select New, and then click Web
2. In the Web Site list, choose the web for your new web.
3. Type or select a name for your new web in the Specify the location of the new web dropdown box.
4. Click Ok to apply the web template to your new web.
5. Open the "index.htm" file and read the instructions. The new web also includes a detail tutorial on how to use the web.
 
bullet To apply a New Web Template in FrontPage 2003

1. On the File menu, Select New.
2. Click "More web site templates" in the right column.
3. Choose the "TestDrive" template. (This may also be in your "My Templates" folder.)
4. Click OK.

 


Prof. Munir Elias

Our brain is a mystery and to understand it, you need to be a neurosurgeon, neuroanatomist and neurophysiologist.

neurosurgery.tv

Please visit this site, where daily neurosurgical activities are going on.


Inomed ISIS IOM System

Home | NEUROEMBRIOLOGY | MEMBRANE POTENTIALS | THE SYNAPSE | MUSCLE TONE - SPINAL REFELXES | SKELETAL MUSCLE CONTRACTION AND THE MOTOR UNIT | DESCENDING MOTOR PATHWAYS | Bottom Menu | Bottom Menu | RECEPTORS | SENSORY PATHWAYS | AUTONOMIC NERVOUS SYSTEM | DIENCEPHALON | TELENCEPHALON | NEUROCHEMISTRY

Copyright [20016] [CNS Clinic-Jordan]. All rights reserved