PHYSIOLOGY
Dr. Basim Mohamad Alwan Lecture 9THE MOTER TRACTS
The motor tracts (also called the descending tracts) are divided into two groups of tracts; the pyramidal and the extrapyramidal.
THE PYRAMIDAL TRACTS
The pyramidal tracts are three tracts of common origin but separate destinations (fig. 9-1). These tracts are:The corticonuclear tract; which originates in the cerebral cortex and terminates on motor nuclei in the midbrain and pons.
The corticobulbar tract; which originates in the cerebral cortex and terminates on motor nuclei in the pons and medulla.
The corticospinal tract; which originates in the cerebral cortex and terminates on the motor neurons of the spinal cord.
THE ORIGIN OF THE PYRAMIDAL TRACTS
The pyramidal tracts originate from:The primary motor area: 30% of the fibers.
The premotor and supplementary motor areas: 30% of the fibers.
3. The somatic sensory areas: 40% of the fibers.
The pyramidal tracts arise from lamina V of the cerebral cortex which contains the pyramidal cells. There are two types of fibers in the pyramidal tracts:
a. Thick fibers (16 um in diameter). These arise from the giant pyramidal cells (Betz cells) found only in the primary motor area. They constitute only 3% of the total number of fibers.
b. Thin fibers (less than 4 um in diameter). These arise from small pyramidal cells found in different parts of the cortex. They constitute the remaining 97% of the fibers.
THE CORTICONUCLEAR TRACT
ORIGIN:
From the eye field area in the frontal lobe (area 8) and the related areas in the motor and the somatosensory areas.
COURSE AND DESTINATION:
Fibers descend down through the genu of the internal capsule to the brainstem. They terminate on the nuclei of cranial nerves III, IV in the midbrain and VI in the pons on both sides.
FUNCTIONS
Voluntary conjugate movements of the eye to look at different objects.
Facilitate the stretch reflex of the external ocular muscles.
THE CORT1COBULBAR TRACT
origin:From the lower part of the motor and sensory" areas of the cerebral cortex.
COURSE AND DESTINATION:
Fibers descend down through the genu of the internal capsule to the pons and medulla oblongata. They cross to the opposite side to terminate on the nuclei of the cranial nerves V, VII, IX, XI and XII.
FUNCTIONS
1. Voluntary movement of muscles in the head and neck.
2. Facilitation of stretch reflex of these muscles
Figure 9 -1: The origin, course, and destinations of the pyramidal tracts.
THE CORTICOSPINAL TRACT
ORIGIN:
From the motor, premotor, supplementary motor and somatic sensory areas of the cerebral cortex.
COURSE AND DESTINATIONS:
Fibers descend in the corona radiata, then through the anterior two-thirds of the posterior limb of the internal capsule down to the brainstem. In the medulla, fibers collect together to form the medullary "pyramid". In the lower medulla, pyramidal fibers take one of three courses:
90% of the fibers cross to the opposite side in the motor decussation and descend in the posterolateral column of the spinal white matter as the "lateral corticospinal tract". They terminate on the ventral horn cells mainly through intemeurons, but some fibers terminate directly on ventral horn cells.
8% of the fibers descend directly in the ventral column of the spinal white matter of the same side as the "ventral corticospinal tract". They cross gradually as they descend in the cervical and upper thoracic segments of the spinal cord to terminate on the ventral horn cells of the opposite side.
2% of the fibers descend directly in the posterolateral column of the spinal white matter as the "uncrossed corticospinal tract". They terminate on the ventral horn cells of the same side.
FUNCTIONS
The crossed (lateral and ventral) corticospinal tracts have the following functions:1. Production of fine voluntary movements of the distal parts of the body e.g. fingers and hands.
2. Facilitation of lower motor neurons and stretch reflex.
The uncrossed corticospinal tract has the following functions:
1. Provide bilateral innervation of some muscles as the respiratory and abdominal muscles.
2. Gross positioning movements controlled by the supplementary motor area.
3. Help partial recovery of movements after injury of the crossedcorticospinal tracts.
THE EXTRA PYRAMIDAL SYSTEM
The extrapyramidal system (fig. 9-2) includes all parts of the nervous system other than the pyramidal tracts that contribute to the control of skeletal muscle activity. These parts include the basal ganglia, the red nucleus, the reticular formation of the brainstem, the vestibular nuclei, and the tectum of the midbrain and the inferior olive of the medulla. All these parts send signals down to the spinal motor nuclei in the descending extrapyramidal tracts.THE BASAL GANGLIA
It receives projection fibers from the motor cortex to corpus striatum and then to globus pallidus then to several nuclei in the brainstem. These nuclei include the subthalamus, substantia nigra, red nucleus, tectum of the midbrain, reticular formation, vestibular nucleus and inferior olive.
THE RED NUCLEUS
It is located in the midbrain and receives projection fibers from the motor cortex (the corticorubral tract) and collaterals from the corticospinal tract as it passes through the midbrain. It also receives projection fibers from the globus pallidus of the basal ganglia. All these fibers synapse in the lower part of the red nucleus which contains giant pyramidal neurons similar to Betz cells. Rubrospinal tract originates from these neurons, cross to the opposite side and descend in the spinal cord very closely anterior to the lateral corticospinal tract. Fibers terminate on the ventral horn cells either directly or through interneurons.
FUNCTIONS OF THE RED NUCLEUS
1. It is a relay station in the corticorubrospinal pathway which acts as an accessory pathway for the corticospinal tracts. This pathway can initiate gross movements.
2. It is inhibitory to the motor neurons and stretch reflex through stimulation of the inhibitory reticular formation of the brainstem.
Lesions of the pyramidal tracts usually involve the corticorubrospinal pathway. If the latter is spared, a good deal of gross movements is retained. Only the fine movements of the fingers and hands are considerably impaired.
THE TECTUM OF THE MIDBRAIN
It receives projection fibers from the globus pallidus of the basal ganglia, and gives origin to two descending extrapyramidal tracts:The lateral tectospinal tract: Originates from the superior colliculus (the center of visual reflexes), crosses to the opposite side and terminates in the cervical segments of the spinal cord. It is concerned with directing the eye and turning the head towards a light source (visuospinal reflexes).
The ventral tectospinal tract: Originates from the inferior colliculus (the center of auditory reflexes), crosses to the opposite side and terminates in the cervical segments of the spinal cord. It is concerned with turning the head to direct the ears towards a sound source (audiospinal reflexes).
THE RETICULAR FORMATION OF THE BRAINSTEM
It receives projection fibers from the globus pallidus of the basal ganglia, and gives origin to two descending extrapyramidal tracts:The lateral reticulospinal tract: Originates from the inhibitory reticular formation of the medulla. Some fibers cross to the opposite side, but most fibers descend in the same side of the spinal cord. It inhibits the gamma motor neurons, thus inhibiting the stretch reflex and skeletal muscle tone.
The ventral reticulospinal tract: Originates from the facilitatory reticular formation of the pons. Fibers descend without crossing to terminate on the gamma motor neurons of the ipsilateral side of the spinal cord. It facilitates the gamma motor neurons, thus facilitating the stretch reflex and the skeletal muscle tone.
Figure 9 - 2: A schematic diagram of the extrapyramidal system.
THE VESTIBULAR NUCLEUS OF THE MEDULLA
It receives projection fibers from the globus pallidus of the basal ganglia, and gives origin to two descending extrapyramidal tracts;
• The lateral vestibulospinal tract: Originates from the vestibularnucleus, descends uncrossed to terminate on the alpha and gamma motor neurons.
• The ventral vestibulospinal tract: Originates from the vestibularnucleus, descends on both sides of the spinal cord to terminate onthe alpha and gamma motor neurons.
The vestibulospinal tracts facilitate the stretch reflex and skeletal muscle tone. They mediate some postural reflexes.
THE INFERIOR OLIVE OF THE MEDULLA
The inferior olive receives input fibers from the motor cortex, the globus pallidus of the basal ganglia and the spinal cord. It sends output fibers to the cerebellum. It projects the fibers of the olivospinal tract which descend in the spinal cord to terminate on the ventral horn cells of the same side. The olivospinal tract is facilitatory to the stretch reflex and the skeletal muscle tone.The inferior olive works in close association with the cerebellum to correct any deviation of the muscle contraction from the preset plan of movement (the servocomparator function of the cerebellum).
GENERAL FUNCTIONS OF THE EXTRAPYRAMIDAL SYSTEM
Mediation of gross movements which involve a group of large muscles.Provides a weaker alternative to the pyramidal system for mediation of some discrete movements.
Mediation of fixation and positioning movements which accompany other fine movements.
Adjustment of the skeletal muscle tone through facilitation or inhibition.
Adjustment of muscle movements to match preset plans to reach a certain target.
THE UPPER AND LOWER MOTOR NEURONS
To do a voluntary movement, signals start in the motor neurons of the cerebral cortex and reach the skeletal muscles through two orders of neurons:1. Upper motor neurons:
These are the neurons of the pyramidal and extrapyramidal tracts in the CNS. They extend from the cerebral cortex and the extrapyramidal nuclei down to the motor neuron pool of the brainstem and spinal cord. Neurons of the motor neuron pool themselves are not included in the upper motor neurons.
2. Lower motor neurons:
These are the neurons of the motor neuron pool and their axons which form the motor nerves to the skeletal muscles.