pyramidal pyramidal And AndExtrapyramidal tracts tracts By: Dr. Khaled Ibrahim

THE DESCENDING MOTOR SYSTEMS  For voluntary movements to occur, one needs 2 neurons: 1- Upper motor neurons 1- Upper motor neurons  whose cell bodies lie in the higher motor centers in the brain and brain stem, and their axons constitute the descending motor pathways. 2- Lower motor neurons  whose cell bodies lie in the spinal ventral horns or the corresponding cranial motor nuclei, and include both  -and  -MNs. Axons of the lower motor neurons proceed through the peripheral somatic nerves to innervate skeletal muscles.

 The descending motor pathways have commonly been divided into “pyramidal” and “extrapyramidal” tracts.  This classification is based on the finding that the motor tract which originates from the cerebral cortex and descends to the spinal cord (the corticospinal tract) passes through the pyramids of the medulla, and therefore has been called the “the pyramidal tract”.  The rest of the descending motor pathways do not travel through the medullary pyramids, and are therefore collectively gathered under the heading: “the extrapyramidal tracts”. Classification of descending motor systems

 An alternative classification of the descending motor tracts is based on the sites of termination of these tracts in the spinal cord.  On this basis, the descending motor pathways can be classified, with respect to their spinal sites of termination, into:  composed of tracts which terminate primarily on the dorsolateral neurons (or their associated interneurons)  innervate the distal muscles of the limbs.  more concerned with controlling fine voluntary movements of the extremities.  composed of tracts which terminate primarily on the ventromedial neurons (or their associated interneurons)  innervate the trunk (axial) muscles and the proximal (girdle) muscles of the limbs  more concerned with postural control. Lateral system Medial system

 There are five important sets of descending motor tracts, named according to the origin of their cell bodies and their final destination: 1)the corticobulbospinal tract, (= Pyramidal tract) 2)the rubrospinal tract, 3)the reticulospinal tracts, 4)the vestibulospinal tracts, and 5)the tectospinal tract. Extrapyramidal tracts

The Corticobulbospinal Tract Origin 1)30% of the tract fibers come from the primary motor area (area 4), 2)30% come from the premotor area and the supplemental motor area (both of which constitute area 6). 3)The remaining 40% of the CBS tract fibers come from the somatic sensory areas of the cerebral cortex. Divisions: 1)The Corticobulbar Division 2)The Corticospinal Division

The Corticobulbar Division Course and Termination  Axons originate from the head and face regions of the cortical areas (4, 6, 3, 2, 1, 5, 7).  The corticobulbar tract Terminate in the motor nuclei of : 3rd and 4th cranial nerves in the midbrain 5th, 6th, and 7th cranial nerves in the pons 9th, 10th, 11th, and 12th cranial nerves in the medulla  Corticobulbar tract from one side of the brain terminates mostly in the cranial motor nuclei of both sides of the brain stem. Except, the lower part of the facial nerve nucleus, and the hypoglossal nerve nucleus receive only contralateral innervation from the cerebral cortex.

The Corticospinal Division (= Pyramidal tract):

Course and Termination fibers of the CBS tract descend from the cerebral cortex collect together and descend through the posterior limb of the internal capsule through the middle portion of the cerebral peduncles of the midbrain The fibers are separated by transverse pontine fibers in the pons In the upper medulla oblongata where they form the pyramids of the medulla About 80% to 90% cross to the opposite side of the spinal cord (contralaterally) and continue as the “lateral corticospinal tract”. fibers which do not decussate in medulla (about 10%) descend on the same side of the spinal cord (ipsilaterally) as the “ventral corticospinal tract”. In the lower region of the medulla, most of the fibers cross to the opposite site forming the “medullary decussation”

Ventral corticospinal tract Lateral corticospinal tract  Remaining 10% of fibers  Descends ipsilaterally in the ventral column on the same side.  Till the mid-thorathic region only  Terminate gradually by crossing at various levels of the spinal cord to terminate on the medially- situated neurons directly or more common indirectly through interneurons.  It is classified as a part of “medial motor system”.  80-90% of fibers  Descends contralaterally in the lateral column of the opposite side)  Along the whole length of the spinal cord.  Terminate gradually on the laterally situated neurons on the ventral horn directly or more common indirectly through interneurons.  It is classified as a part of “lateral motor system”.

Functions of the Corticobulbospinal Tract: 1) Initiation of Voluntary Movements  This is particularly mediated by the CBS tract fibers that join the lateral motor system. These fibers include: 1)The corticobulbar tract fibers that end in: (i) The lower part of the facial nerve motor nucleus which supplies the muscles of the lower part of the face, and (ii) The hypoglossal nucleus that innervate the tongue muscles. These facial and tongue muscles are concerned with highly skilled movements such as those involved in the act of speech. 2)The lateral corticospinal tract fibers that descend in the spinal cord for control of muscles of the distal parts of the limbs, especially the hand and digits muscles, which subserve fine skilled movements used in manipulation by hand and fingers, and other accurate motor actions done by the limbs.

2) Role in Automatic and Postural Movements:  This is mediated by small portion of the CBS tract fibers that join the medial motor system. These fibers include: 1)Part of these fibers innervates some cranial nerve motor nuclei which supply muscles of the head involved in such activities as closure of the eye lids, chewing, swallowing, and phonation which often occur automatically. 2)Another part descends in the ventral corticospinal tract which exerts some control on axial and girdle muscles, that are concerned mainly with postural adjustments. 3) Facilitation of the muscle tone:  CBS facilitates motor neurons, especially those innervating the distal flexor muscles of the limbs.

The Rubrospinal Tract Origin red nucleus of the midbrain Afferent receives afferent connections from: 1)Ipsilateral cortical motor areas (corticorubral pathway), 2)Contralateral side of the cerebellum, 3)Basal ganglia. Course and Termination  On leaving the red nucleus, the fibers of the rubrospinal tract cross to the opposite side > descend contralaterally through the brain stem and the lateral column of the spinal white matter, very close to the lateral corticospinal tract.

 in the brain stem, some fibers of the rubrospinal tract terminate in various nuclei of the brain stem, as well as the brain stem reticular formation.  in the spinal cord, fibers leave it and terminate on the more laterally situated motor neurons and their associated interneurons in the cord gray matter, similar in this respect to the lateral corticospinal tract fibers. Functions of the Rubrospinal Tract: 1)an additional pathway for transmission of cerebral cortical motor commands to the lower motor neurons similar to those of the corticospinal tract. 2)facilitatory to the  - and  -MNs of the distal flexor muscles, but they are inhibitory to extensor muscles.

The Reticulospinal Tracts Divisions: 1) Medial reticulospinal tract. 2) Lateral reticulospinal tract. Lateral reticulospinal tract Medial reticulospinal tract  arises from neurons in the “medullary reticular formation”  its fibers descend to all levels of the spinal cord  synapse with interneurons that inhibit the  - and  -MNs of antigravity and extensor muscles, but they facilitate the  - and  -MNs of flexor muscles.  arises from neurons of the “pontine reticular formation”  descends to all levels of the spinal cord  terminate mainly on interneurons in the spinal gray matter which excite the medially situated  - and  -MNs innervating the antigravity muscles, that is, the muscles of the vertebral column and the extensor muscles of the lower limbs.

Lateral reticulospinal tract Medial reticulospinal tract The medullary reticular formation receives afferent signals from: (i) the premotor area of cerebral cortex, (ii) the paleocerebellum, and (iii) red nucleus.  Functions: activate the medullary inhibitory system, which can thus counter- balance the facilitatory effect of the pontine reticular formation on the antigravity muscles.  The pontine reticular formation has a high degree of natural excitability.  In addition, it receives strong excitatory signals from the vestibular nuclei and the neocerebellum.  Functions: the medial (or pontine) reticulospinal tract exerts a strong facilitatory effect on the motor neurons of the antigravity and extensor muscles to support the body posture against gravity.

 Both the facilitatory (pontine) and inhibitory (medullary) reticular formations constitute together a controllable system that is regulated by signals from the cerebral cortex, cerebellum, and other motor centers to adjust the level of muscle tone in various muscles under different postural conditions.

The Vestibulospinal Tracts Divisions: 1) Lateral Vestibulospinal tract. 2) Medial Vestibulospinal tract. Medial Vestibulospinal tract Lateral Vestibulospinal tract  originates from the medial vestibular nucleus  descends to the cervical and upper thoracic regions only.  synapses on interneurons associated with the  - and  -MNs innervating muscles of the neck which regulate the head position.  originates from the lateral vestibular nucleus,  descends to all levels of the spinal cord  synapses on interneurons associated with the  - and  -MNs of antigravity and extensormuscles, which are medially situated in the spinal ventral horn.  synapses on interneurons associated with the  - and  -MNs of antigravity and extensor muscles, which are medially situated in the spinal ventral horn.

Functions Change of head position in relation to the earth’s gravity Exposure to acceleratory forces Vestibular nuclei Vestibulospinal Tract 1) adjusting the tone and contraction of antigravity muscles to maintain the body posture and equilibrium by LVST. 2) regulating the position of the head and upper limbs during exposure to acceleration by MVST.

The Tectospinal Tract Origin Arises mainly from superior colliculi & to less extent from inferior colliculi. Course and Termination The fibers of the tract cross the midline -----> descend only to the cervical segments of the spinal cord > terminate on interneurons associated with the motor neurons innervating the neck muscles. Functions Orienting responses that initiate reflex turning of the head in response to visual or auditory stimuli Example: turning the head to look at the source of a sudden visual stimulus

Finally, one can conclude that: Lateral corticospinal tract + rubrospinal tract forms the “Lateral Motor System” Whereas; Medial corticospinal tract + reticulospinal tract + vestibulospinal tracts + tectospinal tracts forms the “Medial Motor System”

The Primary Motor Area Location in the precentral gyrus of the frontal lobe, and corresponds to Brodmann’s area 4. Body Representation 1) contralateral and inverted. However, several facial muscles are represented bilaterally. 2) organized in a somatotopic manner with the feet at the upper medial region of the gyrus and the face at the lower lateral region 3) Area of representation is proportional with the complexity of function done by the muscle. So, muscles of hands and tongue occupies 50% of this area

Neural Connections A) Afferents 1)Thalamus and Somatic Sensory Area sensory feed-back input from the muscle and joint proprioceptors which provides the motor cortex with information about the actual motor performance 2) The premotor and supplemental motor of the same side providing higher control of its activity. 3) The basal ganglia and cerebellum regulate and coordinate its motor activity. 4)The visual and auditory cortices providing it with information about the spatial relations of the body to the external environment.

5) The prefrontal region appropriate course of motor action suitable with the surrounding environment. 6) The motor areas of the opposite hemisphere coordinating bilateral motor activities performed by both sides of the body. B) Efferents 1) About 30% of the axons of the corticobulbospinal tract. 2) large number of fibers that project onto the basal ganglia, establishing a neural pathway for planning and programming of motor actions.

4) moderate number of fibers that pass to the red nucleus in the midbrain, and also to the reticular formation in the brain stem for controlling their activity 3) tremendous number of fibers which project to the cerebellum, establishing pathways between the motor cortex and the cerebellum for coordination and regulation of movements. Role in Movements 1) discharges the descending motor commands that produce voluntary movements. It controls both “distal” and “proximal” muscles 2) facilitatory to the tone of distal muscles, particularly flexor muscles.

Premotor Area Location  The premotor area lies immediately anterior to the lateral regions of the primary motor area  It occupies a large portion of area 6, and is bounded superiorly by the supplemental motor area. Neural Connections With the Primary & supplemental motor areas: With the Cerebellum With the Basal Ganglia

Role in Movements 1) Enhancing the primary motor area to commence its activity. 2) Adjusting posture during performance of voluntary movements. 3) In association with the supplemental motor area, establishing the motor programs necessary for execution of complex movements. 4) Inhibit grasp reflex 5) Influence autonomic activity as heart rate & arterial blood pressure 6) Inhibit muscle tone

7) A few highly specialized motor centers have been found in the premotor areas of the human cerebral cortex : Broca’s Area for Speech The Frontal Eye Movements Area located above Broca’s area in the frontal lobe located above Broca’s area in the frontal lobe controls voluntary movements of the eyes toward different objects in the visual field. controls voluntary movements of the eyes toward different objects in the visual field. Head Rotation Area  located just above the eye movement area in the motor cortex.  directing the head toward different visual objects. Area for Hand Skills (Exner’s Area)