Neural Pathways

Pyramidal system Direct Activation Pathway. All of the motor impulses that orginate at the cortical level (in cortex of brain) travel through this tract. This pathway supplies the voluntary muscles of the head, neck, and limbs. Neurons of this tract originate in the post-central gyrus or primary motor cortex.

The pyramidal system has two tracts: corticospinal and corticobulbar. The nerve fibers of the corticospinal tract travel down through the internal capsule down to the medulla. At the inferior level of the medulla, 70-90% of the nerve fibers on each side will deccusate, or cross over to the other side. Those that cross over continue down the spinal cord and are collectively known as the lateral corticospinal tract, and these fibers exert contralateral control. The remaining 10-30% travel down the ipsilateral side and are collectively known as the ventral corticospinal tract, and these fibers exert ipsilateral control. These will continue to travel down the spinal cord until they reach the level of the muscles they will serve, where they will then synapse with the nuclei of lower motor neurons.

The nerve fibers of the corticobulbar tract initially follow the same pathway as the corticospinal tracts. However, these will begin to synapse with the motor nuclei of the cranial nerves beginning at the level of the upper pons. These fibers will deccusate at various levels of the brainstem. Of interest is that the nuclei of the facial nerve receive bilateral innervation for some muscles and unilateral representation for others. Generally speaking, the upper face is more bilaterally innervated, and the bottom half is more unilaterally innervated (but by contralateral fibers).


Extrapyramidal system Indirect Activation Pathway

The pyramidal system was the primary pathway for voluntary movement. The extrapyramidal system is another motor system that is important for control of movements. Neuronal activity for this motor system begins in the cerebral cortex and ultimately exerts an influence on the lower motor neurons. The pathways are indirect, as opposed to the direct pathways of the pyramidal system. 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. The extrapyramidal system, however, is polysynaptic.

The major extrapyramidal nuclei are the basal ganglia. Remember that the basal ganglia is composed of the Globus Pallidus, Putamen, and Caudate Nucleus. Together, the Globus Pallidus and Putamen are called the Lenticular Nucleus. Together, all three are called the Corpus Striatum. Other structures related to the extrapyramidal system include the substantia nigra, red nucleus, subthalamic nucleus, and reticular formation of the mesencephalon. The cerebellum is also thought of as contributing to the extrapyramidal system.

The extrapyramidal system works by modifying neural impulses that originate in the cerebral cortex. Impulses generated at the primary motor strip are sent via the extrapyramidal fibers to the basal ganglia. In a complex network of pathways, the structures of the basal ganglia modify impulses and send information to each other. Some fibers will then be directed down to synapse with the lower motor neurons. Other fibers are routed through the thalamus and back up to the cortex.

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.

Damage to the extrapyramidal system, but especially damage to the basal ganglia, will result in movement disorders known as dyskinesias. Different types of dyskinesias include:

Myoclonus - characterized by involuntary single or repetitive jerks of a body part. If the jerks are repetitive, they can be rhythmic or non-rythmic. They can be isolated to one muscle group or a number of muscles at the same time. These movements can occur spontaneously, but also to stimuli (visual, tactile or auditory). Hiccups are a form of myoclonus (brief spasm of diaphragm.).

Tics - these are rapid, repeatingly coordinated or patterned movemetns that are under partial control by the affected person. Often, the person will relate that they have an irresistible urge to perform the movements. They can often supress the movements temporarily. Simple tics may appear similar to dystonia or myoclonus. Complex tics are coordinated and can involve jumping, noises, lip smacking, and other rapid, repeated movements.

Chorea - characterized by rapid, involuntary, random, purposless movements of a body part. Can be present at rest, during sustained postures, and during movement.

Can be subtle or obvious. 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.

Ballism - characterized by gross, abrupt contractions of axial and proximal muscles of the extremities that can produce flailing.

Athetosis - a relatively slow, writhing, purposless movement of a body part. Athetosis and choreaic movements often combine with eachother, and called choreoathetosis. Athetosis is a major category of the effects of Cerebal Palsey.

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.

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.

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. Action tremor occurs during movement. Terminal tremor occurs as the body part nears a target. Can be caused by cerebellar circuit problems.