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Overview of Motor System
What’s the motor system?• Parts of CNS and PNS specialized for control
of limb, trunk, and eye movements• Also holds us together• From simple reflexes (knee jerk) to voluntary
movements (96mph fast ball)
• Remarkable: Muscles only contract
Plan
• Components of the motor systems• Basic principle of movement control
– What is helpful for understanding basic motor system organization
• Motor programs• Descending motor pathways
Key concepts
1.The contraction of skeletal muscle produces movement by acting on the skeleton.
2. Motor neurons activate the skeletal muscles.
3. Sensory feedback from muscles is important for precise control of contraction.
4. The output of sensory receptors like the muscle spindle can be adjusted.
5. The spinal cord is the source of reflexes for initiation and control of movement.
6. Spinal cord function is influenced by higher centers in the brainstem.
7. The highest level of motor control is the cerebral cortex.
8. The basal ganglia and the cerebellum provide feedback to the motor control areas of the cerebral cortex and brainstem.
• Somatic motor activity depends upon the patternrate of discharge
• of the spinal motor neurons and • neurons in the motor nuclei of the cranial
nerves.
• This is the final common pathway(spinal motor neurones muscles)
• The neurons supplying skeletal muscles have their cell bodies located in the ventral horn of the spinal cord.
• The neurons which are of the Aα type (hence called the α-motoneuron) receive and integrate inputs from various parts of the brain as well from sensory receptors. Hence, they serve as the final common pathway to the muscle.
• The final common pathway receives two types of inputs:
Inputs through sensory nerves. from receptors in the muscle (the muscle spindle), the tendon (Golgi tendon organ) and skin (nociceptors).
As these sensory neurons relay on the a-motoneuron, they constitute a reflex arc and subserve important spinal reflexes like the stretch reflex, negative stretch reflex and the withdrawal reflex.
(2
Descending motor pathways from supraspinal centers. These descend from various parts of the brain like the cerebral cortex, basal ganglia, cerebellum and descending reticular formation.
Functional Hierarchy of Motor Paths
Motor execution: force & direction
• Inputs to spinal cordfrom the same spinal segment Supra -segmental inputs other spinal segments, the brain stem, the cerebral cortex.
• Inputs to spinal cordend directly on the α motor neurons, via interneurons via the γ efferent system to the muscle spindles and back through the Ia afferent fibers to the spinal cord.
• Integrated activity regulates the posture of the body and makes coordinated movement possible.
The inputs converging on the motor neurons subserve three functions:
• voluntary activity
• adjust body posture to provide a stable background for movement
• coordinate the action of the various muscles to make movements smooth and precise.
• The patterns of voluntary activity are planned within the brain, and the commands are sent to the muscles primarily via the
• corticospinal and
• corticobulbar systems.
• Posture is continually adjusted before and during movement by posture-regulating systems.
• Movement is smoothed and coordinated by the medial and intermediate portions of the cerebellum (spinocerebellum) and its connections.
• The basal ganglia and the lateral portions of the cerebellum (neocerebellum)
• are part of a feedback circuit to the premotor and motor cortex
• that is concerned with planning and organizing voluntary movement.
Organization
• Motor output is of two types: involuntary, voluntary.
• Involuntary movements also involve rhythmic responses such as
swallowing, chewing, scratching, walking,
• largely involuntary but subject to voluntary adjustment and control.
• To move a limb the brain must plan a movement, arrange appropriate motion at many different joints at the same time, adjust the motion by comparing plan with performance.
• The motor system "learns by doing," • Performance improves with repetition. • This involves synaptic plasticity.
• Commands for voluntary movement originate in cortical association areas.
• The movements are planned in the cortex basal ganglia cerebellar hemispheres,
• The basal ganglia and cerebellum both funnel information to the
premotor and motor cortex
• by way of the thalamus.
• Motor commands from the motor cortex are relayed in the corticospinal tracts and corticobulbar tracts to motor neurons in the brain stem.
• However, collaterals project to motor neurons in the brain stem and spinal cord.
• These indirect pathways can also mediate voluntary movement.
• Movement sets up alterations in sensory input from the
special senses muscles, tendons, joints, skin.
• This feedback information, which adjusts and smoothes movement, is relayed directly to the motor cortex and to the spinocerebellum.
• The spinocerebellum projects in turn to the brain stem.
• The main brain stem pathways that are concerned with posture and coordination are the
rubrospinal, reticulospinal, tectospinal, and vestibulospinal tracts
• and corresponding projections to motor neurons in the brain stem.
Reticulospinal, tectospinal and vestibulospinal tracts
Rubrospinal tract
Control of Axial & Distal Muscles • Medial or ventral pathways are concerned with
the control of muscles of the trunk and proximal portions of the limbs (axial muscles)
• Lateral pathways are concerned with the control of muscles in the distal portions of the limbs.
• The axial muscles are concerned with postural adjustments and gross movements,
• The distal limb muscles are those that mediate fine, skilled movements.
• Thus, the neurons in the medial portion of the ventral horn innervate the proximal limb muscles, particularly the flexors,
• the lateral ventral horn neurons innervate the distal limb muscles.
• Similarly, the ventral corticospinal tract and the medial descending paths from the brain stem (the tectospinal, reticulospinal, and vestibulospinal tracts) are concerned with adjustments of proximal muscles and posture
• The lateral corticospinal tract and the rubrospinal tract are concerned with distal limb muscles and, particularly in the case of the lateral corticospinal tract, with skilled voluntary movements.
• Phylogenetically, the medial pathways are old, whereas the lateral pathways are new.
• Because the fibers of the lateral corticospinal tract form the pyramids in the medulla, the corticospinal pathways have often been referred to as the pyramidal system.
• The rest of the descending brain stem and spinal pathways that do not pass through the pyramids and are concerned with postural control have been called the extrapyramidal system.
• the motor system has often been divided into upper and lower motor neurons.
• Lesions of the lower motor neurone are associated with
flaccid paralysis, muscular atrophy, absence of reflex responses.
• Destruction of the "upper motor neurons” causesspastic paralysis hyperactive stretch reflexes absence of muscle atrophy
However, there are three types of "upper motor neurons"
• Lesions in the posture-regulating pathways cause spastic paralysis
• lesions limited to the corticospinal and corticobulbar tracts produce weakness (paresis) rather than paralysis, and the affected musculature is generally hypotonic
Cerebellar lesions produce incoordination.
CORTICOSPINAL & CORTICOBULBAR SYSTEM
• The fibers that pass from the motor cortex to the cranial nerve nuclei form the corticobulbar tract.
• The fibers that cross the midline in the medullary pyramids and form the lateral corticospinal tract make up about 80% of the fibers in the corticospinal pathway.
• The lateral corticospinal tract is concerned with skilled movements, and in humans its fibers end directly on the motor neurons.
• The remaining 20% make up the anterior or ventral corticospinal tract,
• does not cross the midline until it reaches the level of the muscles it controls.
Cortical Motor Areas
• motor cortex (M1) in the precentral gyrus
• supplementary motor area on and above the superior bank of the cingulate sulcus
• premotor cortex on the lateral surface of the brain
• somatic sensory area I in the postcentral gyrus
• somatic sensory area II in the wall of the sylvian fissure
• 30% of the fibers making up the corticospinal and corticobulbar tracts come from the motor cortex
• 30% come from the premotor cortex
• 40% from the parietal lobe, especially the somatic sensory area.
• The various parts of the body are represented in the precentral gyrus, with the feet at the top of the gyrus and the face at the bottom.
• The facial area is represented bilaterally,
• but the rest of the representation is unilateral, the cortical motor area controlling the musculature on the opposite side of the body.
• The cortical representation of each body part is proportionate in size to the skill with which the part is used in fine, voluntary movement.
• The areas involved in speech and hand movements are especially large in the cortex;
• use of the pharynx, lips, and tongue to form words and of the fingers and apposable thumbs to manipulate the environment are activities in which humans are especially skilled.
Supplementary Motor Area
• The supplementary motor area projects to the motor cortex.
• It appears to be involved primarily in programming motor sequences.
• Lesions of this area in monkeys produce awkwardness in performing complex activities and difficulty with bimanual coordination.
Premotor Cortex
• projects to the brain stem areas concerned with postural control the motor cortex providing part of the corticospinal and corticobulbar output.
• it may be concerned with setting posture at the start of a planned movement and with getting the individual ready to perform
• Damage to the lateral corticospinal tract in humans produces the Babinski sign:
• dorsiflexion of the great toe and fanning of the other toes when the lateral aspect of the sole of the foot is scratched.
• Except in infancy, the normal response to this stimulation is plantar flexion in all the toes. The Babinski sign is believed to be a flexor.
• withdrawal reflex that is normally held in check by the lateral corticospinal system.
• It is of value in the localization of disease processes, but its physiologic significance is unknown
The End
