Human Anatomy - Kotsan I. Y. 2009

Neural pathways of the brain and spinal cord
Extrapiramidal pathways

Extrapiramidal pathways are phylogenetically older than pyramidal ones and feature extensive connections with the Brainstem and the Cerebral Cortex, which controls and regulates the extrapyramidal system.

The extrapyramidal system comprises the caudate and lentiform nuclei, claustrum, thalamus, substantia nigra, subthalamic Nucleus, red nucleus, reticular formation nuclei of the brainstem, inferior olivary Nucleus of the Medulla Oblongata, cerebellar nuclei, and White matter fibers that establish numerous interconnections among these nuclei.

Operating under the control of the cerebral cortex, the extrapyramidal system mediates higher unconditioned Reflexes rooted in species experience, unconsciously regulates Muscle activity, ensures involuntary automatic movements, maintains and redistributes muscle tone during movement, and prepares the motor apparatus for executing new, highly differentiated movements.

Unlike the pyramidal system, the extrapyramidal system involves significantly more synaptic relays. Damage to the extrapyramidal system alters motor activity and muscle tone. For instance, lesions of the caudate nucleus lead to choreic involuntary movements (choreic hyperkinesia), where sharp, involuntary movements shift from one body part to another, giving the patient the appearance of dancing.

Conversely, lesions of the globus pallidus produce the opposite effect, manifesting as motor rigidity.

Localization of a pathological process in the globus pallidus and substantia nigra disrupts plastic muscle tone, causing joint movements to become jerky, slowed, and inexpressive. The appearance of sweeping movements in the limb joints is a sign of subthalamic nucleus damage.

The nuclei of the extrapyramidal system maintain numerous connections not only with one another but also with the cerebral cortex.

Impulses from the extrapyramidal system travel via descending fibers to the motor Neurons of the anterior horns of the Spinal Cord within the following pathways: 1) rubrospinal tract, originating from the red nuclei; 2) vestibulospinal tract, from the vestibular nuclei of the Vestibulocochlear nerve; 3) tectospinal tract, from the superior and inferior colliculi of the Midbrain tectum; 4) reticulospinal tract, from the reticular Formation of the brainstem; and 5) olivospinal tract, from the inferior olivary nucleus, acting as an intermediate balance center.

Among these, however, the rubrospinal tract is the most robust.

The red nucleus is considered the primary coordination center of the extrapyramidal system. It communicates with the thalamus, striopallidal system, subthalamic region, cerebral cortex, and Cerebellum. Through the rubrospinal tract, impulses from the cerebellum reach the motor neurons in the anterior horns of the spinal cord, executing the so-called cerebellar correction that prevents inertia in a moving body.

Impulses originating in the basal nuclei and transmitted via the rubrospinal tract facilitate complex habitual movements (such as walking and running) while also maintaining Skeletal Muscle tone.

The rubrospinal tract (tractus rubrospinalis), or Monakow's tract, is an efferent, motor, two-neuron pathway responsible for automatic actions. It originates from the Cells of the red nucleus located in the tegmentum of the midbrain. The axons of large multipolar neurons emerge from the red nucleus and immediately cross to the opposite side within the midbrain tegmentum, forming the ventral tegmental decussation, or Forel's decussation, named after the Swiss neurologist August Forel. The fibers then descend into the spinal cord, gradually shifting laterally, pass through the reticular formation of the Pons and medulla oblongata into the lateral funiculi of the spinal cord, and lie slightly anterior to the lateral corticospinal (pyramidal) tract. This fiber bundle gradually thins as its axons terminate segmentally on the motor neurons of the ipsilateral anterior horns of the spinal cord, which serve as the second-order neurons. The axons of these second-order motor neurons travel within the anterior roots and subsequently the Spinal Nerves to reach the skeletal musculature.

The vestibulospinal tract (tractus vestibulospinalis) connects the vestibular nuclei to the anterior horns of the spinal cord, mediating postural adjustment reactions during loss of balance.

The first-order neuron resides in the vestibular nuclei of the VIII cranial nerve pair. These nuclei connect with the cerebellum and, via the medial longitudinal fasciculus, with the motor nuclei of the III, IV, and VI cranial nerve pairs. This connection with the oculomotor nuclei ensures stable eye positioning (maintaining the visual axis) during HEAD and Neck rotation. The axons of the primary vestibulospinal neurons descend within the anterior funiculus of the spinal cord, forming synapses on the motor cells of the anterior horns. Reticular formation neurons link the vestibulospinal tract with the basal nuclei.

The tectospinal tract (tractus tectospinalis) is a descending, motor, two-neuron pathway that mediates subconscious motor responses to auditory and visual stimuli. It originates in the region of the superior and inferior colliculi of the midbrain tectum.

The superior colliculi serve as subcortical visual centers, mediating orienting visual reflexes.

The inferior colliculi act as subcortical auditory centers, mediating orienting auditory reflexes. These Functions are enabled by neural connections between the superior colliculi of the midbrain tectum and the retina, and between the inferior colliculi and The Organ of Corti.

The superior and inferior colliculi together mediate the startle reflex—a protective alerting response characterized by turning the head and body toward an unexpected sound or flash of light. Simultaneously, flexor muscle tone increases, facilitating rapid postural changes.

Impulses traveling along the tectospinal tract prepare the Organism for a rapid reaction to sudden stimuli.

As one of the phylogenetically older pathways subordinated to the cerebral cortex, the tectospinal tract continues to mediate essential, subconscious reflex protective responses to sudden visual and auditory stimuli.

The Cell bodies of the first-order tectospinal neurons lie within the Gray matter of the superior and inferior colliculi of the midbrain tectum. Their axonal processes course ventrally, sweep anteriorly around the central gray matter, and form the dorsal tegmental decussation in the midbrain. Crossing to the opposite side, the fibers descend through the dorsal region of the pontine medulla oblongata before entering the spinal cord, where they run within the anterior funiculi slightly medial to the pyramidal tract, lying directly adjacent to the anterior median fissure. The tectospinal tract tapers as it descends, as its axons terminate segmentally on the motor neurons of the anterior horns of the spinal cord.

The axons of the second-order (motor) neurons of the anterior horns exit the spinal cord via the anterior roots and extend to the Muscles of the limbs, trunk, and partially the neck, terminating in effectors (motor endplates).

A smaller fraction of fibers running from the colliculi to the motor cranial nerve nuclei (pairs V, VII, XI, XII) constitutes the tectobulbar tract. The axons of these secondary neurons, whose cell bodies form the motor cranial nerve nuclei, travel within cranial nerve branches to the muscles of the head and neck. These muscles participate in executing protective responses to sudden auditory and visual stimuli.

The reticulospinal tract (tractus reticulospionalis) facilitates complex reflex responses that require the simultaneous engagement of multiple striated muscle groups. Impulses traveling along the reticulospinal fibers exert excitatory or inhibitory influences on spinal cord neurons.

The first-order neurons of the reticulospinal tract are located in the reticular formation of the brainstem. The axons of these large multipolar neurons descend, bundle together within the anterior funiculus of the spinal cord—occupying its lateral portion—and terminate segmentally on the motor neurons of the anterior horns. The axons of the second-order (motor) neurons leave the spinal cord via the anterior roots and spinal nerves to reach the skeletal musculature.

The olivospinal tract (tractus olivospinalis) serves to transmit coordination impulses from the intermediate center of equilibrium (the inferior olivary nucleus) to the motor neurons of the anterior horns of the spinal cord.

The first neurons of the olivospinal tract are located in the inferior olivary nucleus of the medulla oblongata. The axons of these neurons run in a descending direction, forming a bundle that makes up the anterolateral part of the lateral funiculus of the spinal cord. Here, the fibers of the olivospinal tract lie directly adjacent to the anterior funiculus of the spinal cord anteriorly, and to Gowers' tract posteriorly. Descending further, the fibers terminate segmentally on the neurons of the motor nuclei in the anterior horns of the spinal cord.

The axons of the second neurons, as part of the ventral roots and subsequently the spinal nerves, reach the skeletal musculature. The olivospinal tract can be traced at the level of the upper four cervical segments of the spinal cord.

The Descending Motor Pathways of the cerebellum, as well as the Descending Pathways from the cerebral cortex to the cerebellum, are associated with the functioning of the extrapyramidal system.