Substantia Nigra

The motor system controls the timing, direction, amplitude, and force of movement through the coordinated opposing actions of agonist and antagonist muscles. It also keeps the body in a stable position through postural and righting reflexes. Reflex movements are involuntary, stereotyped responses to stimuli. Rhythmic movements have both reflex and voluntary components. Voluntary movements are performed at will.

Reflex Movements

Withdrawing a foot from a noxious stimulus or spreading the arms when falling are examples of reflex movements. Intrinsic muscle reflexes regulate muscle tone and elasticity and are impor-.O tant for postural control and coordination of u muscle groups. Specific functions such as joint § stabilization or adjustment of contraction ^ strength are achieved with the aid of inhibitory ,2 spinal interneurons. Extrinsic reflexes include S protective reflexes (flexor response to noxious stimulus, corneal reflex) and postural reflexes (extensor reflex, neck reflex).

Rhythmic Movements

Walking, breathing, and riding a bicycle are rhythmic movements. They are subserved both by spinal reflex arcs and by supraspinal influence from the brain stem, cerebellum, basal ganglia, and motor cortex.

the cortex through thalamic relay nuclei. Fine motor control thus depends on the continuous interaction of multiple centers responsible for the planning (efferent copy) and execution of movement.

Motor cortex (p. 25). Voluntary movements are planned in the motor areas of the cerebral cortex. The primary motor area (area 4) regulates the force of muscle contraction and the goal-oriented direction of movement; it mainly controls distal muscle groups. The supplementary motor area (medial area 6) plays an important role in complex motor planning. The premotor area (lateral area 6) receives nerve impulses from the posterior parietal cortex and is concerned with the visual and somatosensory control of movement; it mainly controls trunk and proximal limb movement. Cerebellum (p. 54). The cerebellum coordinates limb and eye movements and plays an important role in the maintenance of balance and the regulation of muscle tone. Basal ganglia (p. 210). The basal ganglia have a close anatomic and functional connection to the motor cortex and participate in the coordination of limb and eye movement.

Voluntary Movements

Voluntary movements depend on a sequence of contractions of numerous different muscles that is planned to achieve a desired result (motor program). Hence different parts of the body are able to carry out similar movements (motor equivalence) more or less skillfully, e. g., simultaneous rotation of the big toe, foot, lower leg, leg, pelvis and trunk. Voluntary movements incorporate elements of the basic reflex and rhythmic movement patterns; their smooth execution depends on afferent feedback from the visual, vestibular, and proprioceptive systems to motor centers in the spinal cord, brain stem, and 42 cerebral cortex. Further modulation of voluntary movements is provided by the cerebellum and basal ganglia, whose neural output reaches

Supplementary motor cortex

Area 8-

Premotor cortex

Substantia Nigra Cortex

- Cerebellum

Cortical motor areas, afferent connections (visual, vestibular, somatosensory)

Centromedian nucleus

Ventral lateral nucleus

Thalamus -

Corticofugal-

pathways (execution of movement)

Substantia nigra pars compacta

Substantia nigra pars reticularis

Red nucleus, — pars magnocel-lularis

Red nucleus, pars parvocel-lularis

Vestibular nuclei

Cortical motor areas, afferent connections (visual, vestibular, somatosensory)

Centromedian nucleus

Ventral lateral nucleus

Thalamus -

Corticofugal-

pathways (execution of movement)

Substantia nigra pars compacta

Substantia nigra pars reticularis

Red nucleus, — pars magnocel-lularis

Red nucleus, pars parvocel-lularis

Vestibular nuclei

Nucleus Reticularis Thalami

Caudate nucleus

Putamen

Globus pallidus externus

Globus pallidus internus Subthalamic nucleus

Fastigial nucleus Cerebellum

Dentate nucleus

Globose and emboliform nuclei

Caudate nucleus

Putamen

Globus pallidus externus

Globus pallidus internus Subthalamic nucleus

Fastigial nucleus Cerebellum

Dentate nucleus

Globose and emboliform nuclei

Motor pathways

(cortex, basal ganglia, thalamus, brain stem, cerebellum, spinal cord)

Pyramidal Tract

Each fiber of the pyramidal tract originates in the first or upper motor neuron, whose cell body is located in the primary motor area (area 4), primary sensory areas (areas 1-3), the supplementary motor area, or the premotor area (area 6). The fibers descend through the posterior portion of the internal capsule through the cerebral peduncle, pons, and medulla, forming a small bulge (pyramid) on the anterior surface of the medulla. Most of the fibers cross the midline in the decussation of the pyramids and then descend through the spinal cord in the lateral corticospinal tract. Among the minority of fibers n that do not cross in the pyramidal decussation, .O most continue in the ipsilateral anterior corti-u cospinal tract, crossing the midline in the ante-§ rior spinal commissure only once they reach the ^ level of their target motor neurons. The py-,2 ramidal tract mainly innervates distal muscle groups in the limbs. In the brain stem, the pyramidal tract gives off fibers to the motor nuclei of the cranial nerves (corticopontine and corti-cobulbar tracts). Fibers from the frontal eye fields (area 8) reach the nuclei subserving eye movement (cranial nerves III, IV, VI) through the pyramidal tract. The motor nuclei of cranial nerves III, IV, VI, and VII (lower two-thirds of the face) are innervated only by the contralateral cerebral cortex; thus, unilateral interruption of the pyramidal tract causes contralateral paralysis of the corresponding muscles. In contrast, the motor nuclei of cranial nerves V (portio minor), VII (frontal branch only), IX, X, XI, and XII receive bilateral cortical innervation, so that unilateral interruption of the pyramidal tract causes no paralysis of the corresponding muscles.

Nonpyramidal Motor Tracts

Other motor tracts lead from the cerebral cortex via the pons to the cerebellum, and from the cerebral cortex to the striatum (caudate nucleus and putamen), thalamus, substantia nigra, red nucleus, and brain stem reticular formation. These fiber pathways are adjacent to the py-44 ramidal tract. Fibers arising from the premotor and supplementary motor areas (p. 43) project ipsilaterally and contralaterally to innervate the muscles of the trunk and proximal portions of the limbs that maintain the erect body posture. Because of the bilateral innervation, paresis due to interruption of these pathways recovers more readily than distal paresis due to a pyramidal lesion. Lesions of the pyramidal tract usually involve the adjacent nonpyramidal tracts as well and cause spastic paralysis; the rare isolated pyramidal lesions cause flaccid paralysis (p. 46). Corticopontine fibers. Corticopontine fibers originate in the frontal, temporal, parietal, and occipital cortex and descend in the internal capsule near the pyramidal tract. The pontine nuclei project to the cerebellum (p. 54). Other functionally important tracts. The ru-brospinal tract originates in the red nucleus, decussates immediately, forms synapses with in-terneurons in the brain stem, and descends in the spinal cord to terminate in the anterior horn. Rubrospinal impulses activate flexors and inhibit extensors, as do impulses conducted in the medullary portion of the reticulospinal tract. On the other hand, impulses conducted in the pon-tine portion of the reticulospinal tract and in the vestibulospinal tract activate extensors and inhibit flexors.

Motor Unit

A motor unit is the functional unit consisting of a motor neuron and the muscle fibers innervated by it. The motor neurons are located in the brain stem (motor nuclei of cranial nerves) and spinal cord (anterior horn). The innervation ratio is the mean number of muscle fibers innervated by a single motor neuron. The action potentials arising from the cell body of a motor neuron are relayed along its axon to the neuromuscular synapses (motor end plates) of the muscle fibers. The force of muscle contraction depends on the number of motor units activated and on the frequency of action potentials. Innervation ratios vary from 3 for the extraocular muscles and 100 for the small muscles of the hand to 2000 for the gastrocnemius. The smaller the innervation ratio, the finer the gradation of force. The muscle fibers of a motor unit do not lie side by side but are distributed over a region of muscle with a cross-sectional diameter of 5-11 mm.

Somatotopic organization of motor cortex

Pontocerebellar fibers Spinocerebellum

Pontocerebellar fibers Spinocerebellum

Fronto Ponto Cerebellar Tract

Somatotopic organization of motor cortex

Pontocerebellum

Motor neurons

Muscle fiber, motor end plate region

Pyramidal tract

Three motor units

Pontocerebellum

Motor neurons

Muscle fiber, motor end plate region

Pyramidal tract

Three motor units c o o c

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Responses

  • calleigh
    How does the cerebellum interact with the motor cortex?
    6 years ago
  • bertha
    Where is the pyramidal sinus?
    6 years ago
  • Rita
    What do the motor neurons in the pyramidal pathway control?
    6 years ago
  • wendy
    Does the motor cortex control glands?
    5 years ago

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