DESCRIBE IN DETAIL ABOUT EXTRAPYRAMIDAL SYSTEM ?
A 1 INTRODUCTION
1 The extrapyramidal system is the part of the motor system involved in modulation and regulation of movement.
2 As its name suggests, it is distinct from the motor fibres that are relayed through the pyramids of the medulla oblongata (corticospinal and corticobulbar tracts).
3 It is composed of nerve cells, nerve tracts and pathways that connect the cerebral cortex, basal ganglia, thalamus, cerebellum, reticular formation, and spinal neurons.
4 Brainstem nuclei of the extrapyramical system include the red nucleus and substantia nigra.
Extrapyramidal system – An Indirect Activation Pathway
1 The pyramidal system was the primary pathway for voluntary movement.
2 The extrapyramidal system is another motor system that is important for control of movements.
3 Neuronal activity for this motor system begins in the cerebral cortex and ultimately exerts an influence on the lower motor neurons.
4 The pathways are indirect, as opposed to the direct pathways of the pyramidal system.
5 The long axons of the corticospinal tract and corticobulbar tract make only one synapse with the lower motor neuron, so the pyramidal system is called monosynaptic ( v imp - viva q )
6 The extrapyramidal system, however, is polysynaptic.
7 The major extrapyramidal nuclei are the basal ganglia ( is composed of the Globus Pallidus, Putamen, and Caudate Nucleus )
8 Together, the Globus Pallidus and Putamen are called the Lenticular Nucleus.
9 Together, all three are called the Corpus Striatum.
10 Other structures related to the extrapyramidal system include the substantia nigra, red nucleus, subthalamic nucleus, and reticular formation of the mesencephalon.
11 The cerebellum is also thought of as contributing to the extrapyramidal system.
12 The extrapyramidal system works by modifying neural impulses that originate in the cerebral cortex.
13 Impulses generated at the primary motor strip are sent via the extrapyramidal fibers to the basal ganglia.
14 In a complex network of pathways, the structures of the basal ganglia modify impulses and send information to each other.
Why it is called polysynaptic ?
15 Some fibers will then be directed down to synapse with the lower motor neurons.
16 Other fibers are routed through the thalamus and back up to the cortex.
17 The role of the extrapyramidal system includes the following: (1) selective activation of movements and supression of others (2) Initiation of movements (3) setting rate and force of movements (4) coordinating movements.
18 Damage to the extrapyramidal system, but especially damage to the basal ganglia, will result in movement disorders known as dyskinesia
Types of dyskinesias
A ) Myoclonus - characterized by involuntary single or repetitive jerks of a body part.
A1 If the jerks are repetitive, they can be rhythmic or non-rythmic.
A2 They can be isolated to one muscle group or a number of muscles at the same time.
A3 These movements can occur spontaneously, but also to stimuli (visual, tactile or auditory).
A4 Hiccups are a form of myoclonus (brief spasm of diaphragm.).
B ) Tics - these are rapid, repeatingly coordinated or patterned movemetns that are under partial control by the affected person.
B1 Often, the person will relate that they have an irresistible urge to perform the movements.
B2 They can often supress the movements temporarily.
B3 Simple tics may appear similar to dystonia or myoclonus.
B4 Complex tics are coordinated and can involve jumping, noises, lip smacking, and other rapid, repeated movements.
C ) Chorea - characterized by rapid, involuntary, random, purposless movements of a body part.
C1 Can be present at rest, during sustained postures, and during movement.
C2 Can be subtle or obvious.
C3 These movements can often be modified by the person after initial onset so that they are made to appear intentional in order to cover them up.
D ) Ballism - characterized by gross, abrupt contractions of axial and proximal muscles of the extremities that can produce flailing.
E ) Athetosis - a relatively slow, writhing, purposless movement of a body part.
F ) Athetosis and choreaic movements often combine with eachother, and called choreoathetosis.
G ) Athetosis is a major category of the effects of Cerebal Palsey.
H ) Dystonia - a slow form of hyperkinesia characterized by involuntary abnormal postures resulting from excessive co-contraction of antagonistic muscles. Writers cramp is a form of this.
I ) Spasm - a general term that designates a variety of muscular contractions. Tonic spasms are prolonged. Clonic spasms are repetitive, have a rapid onset, and are brief.
J ) Tremor - Rhythmic (periodic) movement of a body part. Resting tremors occur when a body part is at rest. Postural tremor occurs when the body part is maintained against gravity.
J1 Action tremor occurs during movement.
J2 Terminal tremor occurs as the body part nears a target.
J3 Can be caused by cerebellar circuit problems.
LOCATION
1 Extrapyramidal tracts are chiefly found in the reticular formation of the pons and medulla, and target neurons in the spinal cord involved in reflexes, locomotion, complex movements, and postural control.
2 These tracts are in turn modulated by various parts of the central nervous system, including the nigrostriatal pathway, the basal ganglia, the cerebellum, the vestibular nuclei, and different sensory areas of the cerebral cortex.
3 All of these regulatory components can be considered part of the extrapyramidal system, in that they modulate motor activity without directly innervating motor neurons.
The extrapyramidal tracts include parts of the following
1 rubrospinal tract
2 pontine reticulospinal tract
3 medullary reticulospinal tract
4 lateral vestibulospinal tract
5 tectospinal tract
RUBROSPINAL TRACT
1 In the midbrain, it originates in the magnocellular red nucleus, crosses to the other side of the midbrain, and descends in the lateral part of the brainstem tegmentum.
2 In the spinal cord, it travels through the lateral funiculus of the spinal cord in the company of the lateral corticospinal tract.
Function
1 In humans, the rubrospinal tract is one of several major motor control pathways.
2 It is smaller and has fewer axons than the corticospinal tract, suggesting that it is less important in motor control.
3 It is one of the pathways for the mediation of voluntary movement.
4 The tract is responsible for large muscle movement as well as fine motor control, and it terminates primarily in the cervical spinal cord, suggesting that it functions in upper limb but not in lower limb control.
5 It primarily facilitates flexion in the upper extremities.
6 It is small and rudimentary in humans.
7 In some other primates, however, experiments have shown that over time, the rubrospinal tract can assume almost all the duties of the corticospinal tract when the corticospinal tract is lesioned.
RETICULOSPINAL TRACT
1 The reticulospinal tract (or anterior reticulospinal tract) is an extrapyramidal motor tract that descends from the reticular formation in two tracts to act on the motor neurons supplying the trunk and proximal limb muscles.
2 It is involved mainly in locomotion and postural control, although it does have other effects as well.
Functions
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Integrates information from the motor systems to coordinate automatic movements of locomotion and posture
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Facilitates and inhibits voluntary movement; influences muscle tone
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Mediates autonomic functions
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Modulates pain impulses
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Influences blood flow to lateral geniculate nucleus of the thalamus.
Components
1 The tract is divided into two parts, the medial (or pontine) and lateral (or medullary) reticulospinal tracts (MRST and LRST).
2 The MRST is responsible for exciting anti-gravity, extensor muscles.
2A The fibers of this tract arise from the caudal pontine reticular nucleus and the oral pontine reticular nucleus and project to the lamina VII and lamina VIII of the spinal cord
3 The LRST is responsible for inhibiting excitatory axial extensor muscles of movement.
4 The fibers of this tract arise from the medullary reticular formation, mostly from the gigantocellular nucleus, and descend the length of the spinal cord in the anterior part of the lateral column.
5 The tract terminates in lamina VII mostly with some fibers terminating in lamina IX of the spinal cord.
6 The sensory tract conveying information in the opposite direction is known as the “spinoreticular tract”.
Clinical significance
1 Mostly inhibited by the corticospinal tract, if damage occurs at the level of or below the red nucleus (e.g. to the superior colliculus) it is called decerebration and causes decerebrate rigidity, an unopposed extension of the head and limbs.
2 The reticulospinal tracts also provide a pathway by which the hypothalamus can control sympathetic thoracolumbar outflow and parasympathetic sacral outflow.
III The lateral vestibulospinal tract is one of the descending spinal tracts of the ventromedial funiculus.
A ) The lateral part of the vestibulospinal tract is the major portion and is composed of fibers originating in the lateral, superior, and inferior vestibular nuclei (primarily the lateral).
B ) It projects ipsilaterally down to the lumbar region of the spinal cord. There it helps to maintain an upright and balanced posture by stimulating extensor motor neurons in the legs.
C ) It also innervates muscles of the trunk, thus additionally aiding in body posture.
D ) The lateral vestibular nuclei receive input from cerebellum, particularly the vestibulocerebellum, or the flocculi and nodulus. The cerebellum aids in coordinating postural adjustments.
IV Tectospinal tract
A ) In humans, the tectospinal tract (also known as colliculospinal tract) is a nerve pathway that coordinates head and eye movements.
B ) This neural tract is part of the indirect extrapyramidal tract. To be specific, the tectospinal tract connects the midbrain tectum and the spinal cord.
C ) It is responsible for motor impulses that arise from one side of the midbrain to muscles on the opposite side of the body.
D ) The function of the tectospinal tract is to mediate reflex postural movements of the head in response to visual and auditory stimuli.
E ) The portion of the midbrain from where this tract originates is the superior colliculus, which receives afferents from the visual nuclei (primarily the oculomotor nuclei complex), then projects to the contralateral (decussating ventral to the mesencephalic duct) and ipsilateral portion of the first cervical neuromeres of the spinal cord, the oculomotor and trochlear nuclei in the midbrain and the abducens nucleus in the caudal portion of the pons.
F ) The tract descends to the cervical spinal cord to terminate in Rexed laminae VI, VII, and VIII to coordinate head, neck, and eye movements, primarily in response to visual stimuli.