1. CNS MOTOR SYSTEMS Dr. YASIR M. KHALEEL M.Sc. , Ph.D
College of Medicine, University of Mosul
Department of Medical Physiology

2. Posture and movement
ORGANIZATION OF MOTOR FUNCTION BY THE SPINAL CORD Posture and movement depend on a combination of involuntary reflexes coordinated by the spinal cord and voluntary actions controlled by higher brain centers Activation and contraction of skeletal muscles is under the control of the motoneurons that innervate them. The motor system is designed to execute this coordinated response largely through reflexes integrated in the spinal cord.

3. Motor Units
A motor unit is defined as a single motoneuron and the muscle fibers that it innervates. The number of muscle fibers innervated can vary from a few fibers to thousands of fibers, depending on the nature of the motor activity. Thus, for eye movements requiring fine control, motoneurons innervate only a few muscle fibers. For postural muscles involved in large movements, motoneurons innervate thousands of muscle fibers. A motoneuron pool is the set of motoneurons innervating fibers within the same muscle.

4. The Force of Contraction of Muscle Is graded by recruitment of motor units (size principle) Small motoneurons have lowest threshold, so they fire first, but can generate small amount of force Large motoneurons have the highest thresholds to fire action potentials; thus, they fire last. Since large motoneurons innervate many muscle fibers, they can generate the greatest amounts of force The size principle states that as more motor units are recruited, progressively larger motoneurons are involved and greater tension will be generated

5. Types of Motoneurons
1- α Motoneurons innervate extrafusal skeletal muscle fibers. Action potentials in α motoneurons lead to contraction. 2- γ Motoneurons innervate specialized intrafusal muscle fibers, a component of the muscle spindles. The overall function of the muscle spindle is to sense muscle length. α Motoneurons and γ motoneurons are coactivated (activated simultaneously) so that muscle spindles remain sensitive to changes in muscle length even as the muscle contracts and shortens.

6. Types of Muscle Fibers
1- Extrafusal fibers constitute the majority of skeletal muscle, are innervated by α motoneurons, and are used to generate force. 2- Intrafusal fibers are specialized fibers that are innervated by γ motoneurons and are too small to generate significant force. Intrafusal fibers are encapsulated in sheaths, forming muscle spindles that run parallel to the extrafusal fibers.

7. MUSCLE SPINDLES
Muscle spindles are spindle-shaped organs composed of intrafusal muscle fibers and innervated by sensory and motor nerve fibers,
Muscle spindles are attached to connective tissue and arranged in parallel with the extrafusal muscle fibers.
γ Motoneurons are smaller and slower than the α motoneurons that innervate the extrafusal fibers.
The function of the γ motoneurons (either static or dynamic) is to regulate the sensitivity of the intrafusal muscle fibers they innervate

8. Function of Muscle Spindles
Muscle spindles are stretch receptors whose function is to correct for changes in muscle length when extrafusal muscle fibers are either shortened (by contraction) or lengthened (by stretch). Thus, muscle spindle reflexes operate to return muscle to its resting length after it has been shortened or lengthened.

9. The events that occur when a muscle is stretched (stretch reflex): 1- When a muscle (extrafusal muscle fibers) is stretched, Because of their parallel arrangement in the muscle, the intrafusal muscle fibers also are lengthened. 2- The increase in length of the intrafusal fibers is detected by the sensory afferent fibers innervating them. Thus, when the muscle is stretched, the increase in the length of the intrafusal fibers activates both group Ia and group II sensory afferent fibers. 3- Activation of group Ia afferent fibers stimulates α motoneurons in the spinal cord which innervate extrafusal fibers of the same muscle and cause the muscle to contract (i.e., to shorten). Thus, stretch (lengthening) is opposed by muscle contraction (shortening). γ Motoneurons are coactivated with the α motoneurons, ensuring that the muscle spindle will remain sensitive to changes in muscle length even during the contraction.

10. SPINAL CORD REFLEXES
Spinal cord reflexes are stereotypical motor responses to specific kinds of stimuli, such as stretch of the muscle.
The neuronal circuit that directs this motor response is called the Reflex Arc.
The reflex arc includes the sensory receptors; the sensory afferent nerves, the interneurons in the spinal cord; and the motoneurons, which direct the muscle to contract or relax

11. A- Stretch reflex is the simplest of all spinal cord reflexes, having only one synapse between sensory afferent nerves and motor efferent nerves
Example
knee-jerk reflex
1.When the muscle is stretched, group Ia afferent fibers in the muscle spindle are activated and their firing rate increases. These group Ia afferents enter the spinal cord and synapse directly on and activate α motoneurons ,
2.This will cause contraction of the muscle that was originally stretched.
When the muscle contracts, it shortens, thereby decreasing stretch on the muscle spindle, which returns to its original length, and the firing rate of the group Ia afferents returns to baseline.
3. Simultaneously, information is sent from the spinal cord to cause contraction of synergistic muscles and relaxation of antagonistic muscles.

12. B- Golgi Tendon Reflex
The Golgi tendon reflex is a disynaptic spinal cord reflex, which is also called inverse stretch reflex .
The Golgi tendon organ is a stretch receptor found in tendons, which senses contraction (shortening) of muscle and activates group Ib afferent nerves. Golgi tendon organs are arranged in series with the extrafusal muscle fibers.

13. The steps in the Golgi tendon reflex
1. When the muscle contracts, the extrafusal muscle fibers shorten, activating the Golgi tendon organs attached to them. In turn, the group Ib afferent fibers that synapse on inhibitory interneurons in the spinal cord are activated. These inhibitory interneurons synapse on the α motoneurons.
2. When the inhibitory interneurons are activated (i.e., activated to inhibit), they inhibit firing of the α motoneurons, producing relaxation of the homonymous muscle (the muscle that originally was contracted).
3. As the homonymous muscle relaxes, the reflex also causes synergistic muscles to relax and antagonistic muscles to contract.

14. C- Flexor-Withdrawal Reflex
The flexor-withdrawal reflex is a polysynaptic reflex that occurs in response to a painful or noxious stimulus.
Somatosensory and pain afferent fibers initiate a flexion reflex that causes withdrawal of the affected part of the body from the painful or noxious stimulus (e.g., touching a hand to a hot stove and then rapidly withdrawing the hand). The reflex produces flexion on the ipsilateral side (i.e., side of the stimulus) and extension on the contralateral side

15. The steps involved in the flexor-withdrawal reflex
1.When a limb touches a painful stimulus (e.g., hand touches a hot stove), afferent nerve fibers (groups II, III, and IV) are activated. These afferent fibers synapse on multiple interneurons in the spinal cord (polysynaptic reflex).
2.On the ipsilateral side of the pain stimulus, reflexes are activated that cause flexor muscles to contract and extensor muscles to relax, giving withdrawal of the hand from the painful stimulus.
3.On the contralateral side of the pain stimulus, reflexes are activated that cause extensor muscles to contract and flexor muscles to relax. This portion of the reflex produces extension on the contralateral side and is called the crossed-extension reflex. Thus, if the painful stimulus occurs on the left side, the left arm and leg will flex or withdraw and the right arm and leg will extend to maintain balance.
4.A persistent neural discharge, called an after-discharge, occurs in the polysynaptic reflex circuits. As a result of the after-discharge, the contracted muscles remain contracted for a period of time after the reflex is activated.

16. CONTROL OF POSTURE AND MOVEMENT BY THE BRAIN STEM
Descending motor pathways : those descending from the cerebral cortex and brain stem
Pyramidal & Extrapyramidal tracts
Pyramidal tracts
The pyramidal tracts are three tracts of common origin but separate destinations and these tracts are
1. The corticobulbar tract; which originates in the cerebral cortex and terminates on motor nuclei in the pons and medulla.
2. The corticospinal tracts; which originates in the cerebral cortex and terminates on the motor neurons of the spinal cord.
a- Lateral corticospinal tracts
b- Anterior (ventral) corticospinal tracts

18. The extrapyramidal system
It 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.

19. The Extrapyramidal Tracts
1. The rubrospinal tract originates in the red nucleus and projects to motoneurons in the lateral spinal cord. Stimulation of the red nucleus produces activation of flexor muscles and inhibition of extensor muscles.
2.The pontine reticulospinal tract originates in nuclei of the pons and projects to the ventromedial spinal cord. Stimulation has a generalized activating effect on both flexor and extensor muscles, with its predominant effect on extensors.
3.The medullary reticulospinal tract originates in the medullary reticular formation and projects to motoneurons in the spinal cord. Stimulation has a generalized inhibitory effect on both flexor and extensor muscles, with the predominant effect on extensors.
4.The lateral vestibulospinal tract originates in the lateral vestibular nucleus (Deiters' nucleus) and projects to ipsilateral motoneurons in the spinal cord. Stimulation produces activation of extensors and inhibition of flexors.
5.The tectospinal tract originates in the superior colliculus (tectum or "roof" of the brain stem) and projects to the cervical spinal cord. It is involved in control of neck muscles.

20. The Cerebellum
The cerebellum, or "little brain," regulates movement and posture and plays a role in certain kinds of motor learning. The cerebellum helps control the rate, range, force, and direction of movements (collectively known as synergy).
Damage to the cerebellum results in lack of coordination.
It is located in the posterior fossa just below the occipital lobe & it is connected to the brain stem by three cerebellar peduncles, which contain both afferent and efferent nerve fibers
There are three main divisions of the cerebellum:
Vestibulocerebellum is dominated by vestibular input and controls balance and eye movements.
Spinocerebellum is dominated by spinal cord input and controls synergy of movement.
Pontocerebellum is dominated by cerebral input, via pontine nuclei, and controls the planning and initiation of movements.

21. Layers of the Cerebellar Cortex
The cerebellar cortex has three layers, which are described in relation to its output cells, the Purkinje cells
The granular layer is the innermost layer. It contains granule cells, Golgi II cells, and glomeruli. In the glomeruli, axons of mossy fibers from the spinocerebellar and pontocerebellar tracts synapse on dendrites of granule and Golgi type II cells.
The Purkinje cell layer is the middle layer. It contains Purkinje cells, and its output is always inhibitory.
The molecular layer is the outermost layer. It contains outer stellate cells, basket cells, dendrites of Purkinje and Golgi II cells, and axons of granule cells. The axons of granule cells form parallel fibers, which synapse on the dendrites of Purkinje cells, basket cells, outer stellate cells, and Golgi type II cells

24. Input to the Cerebellar Cortex
Two systems provide excitatory input to the cerebellar cortex: the climbing fiber system and the mossy fiber system. Each system also sends collateral branches directly to deep cerebellar nuclei, in addition to their projections to the cerebellar cortex. Excitatory projections from the cerebellar cortex then activate secondary circuits, which modulate the output of the cerebellar nuclei via the Purkinje cells.
1. Climbing fibers originate in the inferior olive of the medulla and project directly onto Purkinje cells. These fibers make multiple synaptic connections along the dendrites of Purkinje cells, although each Purkinje cell receives input from only one climbing fiber. These synaptic connections are very powerful! A single action potential from a climbing fiber can elicit multiple excitatory bursts, called complex spikes, in the dendrites of the Purkinje cell. It is believed that climbing fibers "condition" the Purkinje cells and modulate their responses to mossy fiber input. Climbing fibers also may play a role in cerebellar learning.

25. 2. Mossy fibers constitute the majority of the cerebellar input
2. Mossy fibers constitute the majority of the cerebellar input. These fibers include vestibulocerebellar, spinocerebellar, and pontocerebellar afferents. Mossy fibers project to granule cells, which are excitatory interneurons located in collections of synapses called glomeruli. Axons from these granule cells then ascend to the molecular layer, where they bifurcate and give rise to parallel fibers. Parallel fibers from the granule cells contact the dendrites of many Purkinje cells, producing a "beam" of excitation along the row of Purkinje cells. The dendritic tree of each Purkinje cell may receive input from as many as 250,000 parallel fibers.
In contrast to the climbing fiber input to the Purkinje dendrites (which produce complex spikes), the mossy fiber input produces single action potentials called simple spikes. These parallel fibers also synapse on cerebellar interneurons (basket, stellate, and Golgi II).

26. Interneurons of the Cerebellum
The function of cerebellar interneurons is to modulate Purkinje cell output.
With the exception of granule cells, all of the cerebellar interneurons are inhibitory. Granule cells have excitatory input to basket cells, stellate cells, Golgi II cells, and Purkinje cells. Basket cells and stellate cells inhibit Purkinje cells (via parallel fibers). Golgi II cells inhibit granule cells, thereby reducing their excitatory effect on Purkinje cells.

27. Output of the Cerebellar Cortex
The only output of the cerebellar cortex is via axons of Purkinje cells. The output of the Purkinje cells is always inhibitory because the neurotransmitter released at these synapses is γ-amino butyric acid (GABA).
Axons of Purkinje cells project topographically to deep cerebellar nuclei and to lateral vestibular nuclei. This inhibitory output of the cerebellum regulates the rate, range, force, and direction of movement (synergy).

28. Disorders of the Cerebellum
Cerebellar lesions result in an abnormality of movement called ataxia. Cerebellar ataxia is a lack of coordination due to errors in rate, range, force, and direction of movement. Ataxia can be exhibited in one of several ways. There may be a delayed onset of movement or poor execution of the sequence of a movement, causing the movement to appear uncoordinated. A limb may overshoot its target or stop before reaching its target. Ataxia may be expressed as dysdiadochokinesia, in which a person is unable to perform rapid, alternating movements. Intention tremors may occur perpendicular to the direction of a voluntary movement, increasing near the end of the movement. (Intention tremors seen in cerebellar disease differ from the resting tremors seen in Parkinson's disease.) The rebound phenomenon is the inability to stop a movement; for example, if a person with cerebellar disease flexes his forearm against a resistance, he may be unable to stop the flexion when the resistance is removed.

29. Caudate nucleus, Putamen, Globus Pallidus, and Amygdala.
BASAL GANGLIA
The basal ganglia are the deep nuclei of the telencephalon:
Caudate nucleus, Putamen, Globus Pallidus, and Amygdala.
There also are associated nuclei, including
The ventral anterior and ventral lateral nuclei of the thalamus
The subthalamic nucleus of the diencephalon
The substantia nigra of the midbrain
The main function of the basal ganglia is to influence the motor cortex via pathways through the thalamus. The role of the basal ganglia is to aid in planning and execution of smooth movements. The basal ganglia also contribute to affective and cognitive functions.

31. The Basal Ganglia Pathways
The pathways into and out of the basal ganglia are complex.
Almost all areas of the cerebral cortex project topographically onto the striatum, including a critical input from the motor cortex.
The striatum then communicates with the thalamus and then back to the cortex via two different pathways.
1. Indirect pathway. In the indirect pathway, the striatum has inhibitory input to the external segment of the globus pallidus, which has inhibitory input to the subthalamic nuclei. The subthalamic nuclei project excitatory input to the internal segment of the globus pallidus and the pars reticulata of the substantia nigra, which send inhibitory input to the thalamus. The thalamus then sends excitatory input back to the motor cortex. In this pathway, the inhibitory neurotransmitter is GABA, and the excitatory neurotransmitter is glutamate. The overall output of the indirect pathway is inhibitory, as illustrated in the summary diagram at the bottom of the figure

32. 2. Direct pathway. In the direct pathway, the striatum sends inhibitory input to the internal segment of the globus pallidus and the pars reticulata of the substantia nigra, which send inhibitory input to the thalamus. As in the indirect pathway, the thalamus sends excitatory input back to the motor cortex. Again, the inhibitory neurotransmitter is GABA, and the excitatory neurotransmitter is glutamate. The overall output of the direct pathway is excitatory, as shown in the summary diagram at the bottom of the figure.

33. The outputs of the indirect and direct pathways from the basal ganglia to the motor cortex are opposite and carefully balanced:
The indirect path is inhibitory, and the direct path is excitatory.
A disturbance in one of the pathways will upset this balance of motor control, with either an increase or a decrease in motor activity. Such an imbalance is characteristic of diseases of the basal ganglia.
In addition to the basic circuitry of the indirect and direct pathways, there is an additional connection, back and forth, between the striatum and the pars compacta of the substantia nigra. The neurotransmitter for the connection back to the striatum is dopamine. This additional connection between the substantia nigra and the striatum means that dopamine will be inhibitory (via D2 receptors) in the indirect pathway and excitatory (via D1 receptors) in the direct pathway.

34. Diseases of the Basal Ganglia
Diseases of the basal ganglia include Parkinson's disease and Huntington's disease. In Parkinson's disease, cells of the pars compacta of the substantia nigra degenerate, reducing inhibition via the indirect pathway and reducing excitation via the direct pathway. The characteristics of Parkinson's disease are explainable by dysfunction of the basal ganglia: resting tremor, slowness and delay of movement, and shuffling gait. Treatment of Parkinson's disease includes replacement of dopamine by treatment with l-dopa (the precursor to dopamine) or administration of dopamine agonists such as bromocriptine. Huntington's disease is a hereditary disorder caused by destruction of striatal and cortical cholinergic neurons and inhibitory GABAergic neurons. The neurologic symptoms of Huntington's disease are choreic (writhing) movements and dementia. There is no cure.

35. MOTOR CORTEX
Voluntary movements are directed by the motor cortex, via descending pathways. The motivation and ideas necessary to produce voluntary motor activity are first organized in multiple associative areas of the cerebral cortex and then transmitted to the supplementary motor and premotor cortices for the development of a motor plan. The motor plan will identify the specific muscles that need to contract, how much they need to contract, and in what sequence. The plan then is transmitted to upper motoneurons in the primary motor cortex, which send it through descending pathways to lower motoneurons in the spinal cord. The planning and execution stages of the plan are also influenced by motor control systems in the cerebellum and basal ganglia.

36. The motor cortex consists of three areas: Primary motor cortex,
Supplementary motor cortex & Premotor cortex
1. Premotor cortex and supplementary motor cortex (area 6) are the regions of the motor cortex responsible for generating a plan of movement, which then is transferred to the primary motor cortex for execution.
The supplementary motor cortex programs complex motor sequences and is active during "mental rehearsal" of a movement, even in the absence of movement.
2. Primary motor cortex (area 4) is the region of the motor cortex responsible for execution of a movement. Programmed patterns of motoneurons are activated from the primary motor cortex. As upper motoneurons in the motor cortex are excited, this activity is transmitted to the brain stem and spinal cord, where lower motoneurons are activated and produce coordinated contraction of the appropriate muscles (i.e., the voluntary movement).