Motor Control: Pathways, Neurons, and Muscle Function

Muscle spindles stimulate stretch, while Golgi tendon organs stimulate muscle contraction. Cerebellar tremor is characterized by low frequency. Motor pathways begin in the cerebral cortex and brainstem. Lower spinal cord motoneurons innervate skeletal muscles, including extrafusal and intrafusal fibers. Stimulation of muscle spindles or Golgi tendon organs is accomplished through the reticulospinal tract.

Muscle Tone and Movement Disorders

• Hypotonia: Decreased muscle tone
• Dyskinesia: Atypical, unconscious muscle movement
• Hypertonia: Increased muscle tone
• Spasticity: Hyper-contraction of muscles
• Akinesia: Atypical unconscious posture

Cortical Control of Movement

Layer V contains pyramidal neurons. The primary motor cortex contains smaller and fewer pyramidal neurons than the premotor cortex. The premotor cortex stores motor memory and controls complex or sequential muscle movements. The supplementary motor area helps ensure appropriate muscle action. The association cortex allows input from the somatosensory cortex.

Upper and Lower Motor Neuron Lesions

Lower Motor Neuron Lesions: Atrophy, hypotonia

Upper Motor Neuron Lesions: Paralysis, positive Babinski sign, hypertonia, spasticity

Lower motor neurons originate in the ventral horn of the spinal cord. They are arranged somatotopically, with the medial area innervating axial muscles and the lateral area innervating appendicular muscles. The larger the area devoted to motor control, the more precise the control. Pyramidal neurons are not located in the cerebellum. The premotor cortex fires before the primary motor cortex. The posterior parietal association cortex receives input from the somatosensory cortex. All upper motor neurons originate in areas of the motor cortex.

The ventral corticospinal tracts decussate at the ventral horn at the specific level of its spinal cord innervation. Upper motor neurons originate in the red nucleus of the midbrain (rubrospinal tract), exert control over the autonomic nervous system (reticulospinal tract), receive input from the inner ear and cerebellum (vestibulospinal tract), primarily innervate muscles associated with flexion (rubrospinal tract), receive spinal input from optic nerves (tectospinal tract), and are ipsilateral (vestibulospinal tract, reticulospinal tract).

Cerebellar Pathways

The correct order of input to output to and from the cerebellum is: mossy fibers, Purkinje cells, deep cerebellar nuclei.

Damage to the medial area of the ventral horn of C5 would affect the trapezius muscle. Taste is not affected by damage to the corticobulbar tract. Lower motor neurons originate in the ventral horns of the spinal cord. Upper motor neurons of the motor pathways originate in all areas of the brain except the basal nuclei. The basal nuclei prevent excessive, unnecessary muscle movements.

Direct and Indirect Pathways

The direct pathway of muscle movement involves the striatum releasing GABA, which inhibits the internal globus pallidus. This reduces inhibition to the thalamus, which then increases excitation to the motor cortex, resulting in movement. The main cells of the cerebellum that carry information into the cerebellum are mossy fibers. Upper motor neurons of all direct pyramidal tracts originate in the motor cortex. The corticobulbar tract synapses with all cranial nerves except CN VI. Ventral corticospinal tracts control axial skeletal muscles. The lower motor neurons of the medial vestibulospinal and tectospinal tracts originate in the ventral horns of the cervical spinal cord. The rubrospinal tract is associated with muscles involved in flexion.

Muscle Spindles and Neurological Conditions

Extrafusal muscle fibers are not contained within muscle spindles.

Neurological Conditions:

• Basal Nuclei: Huntington's disease
• Cerebellum: Cerebellar ataxia
• Lower Motor Neurons: Guillain-Barré syndrome
• Upper Motor Neurons: Paraplegia

A 42-year-old male with amyotrophic lateral sclerosis (ALS) would experience a combination of upper and lower motor neuron symptoms.