on 17-Aug-2020 (Mon)

Isolated lesions of only tectospinal and reticulospinal fibers are essentially never seen.

Tectospinal fibers project to upper cervical levels where they influence reflex movement of the head and neck. Such move- ments may be diminished or slowed in patients with damage to these fibers.

Pontoreticulospinal (medial reticulospinal) fibers are excitatory to extensor motor neurons and to neurons innervating axial musculature; some of these fibers also may inhibit flexor motor neurons.

In contrast, some bulboreticulospinal (lateral reticulospinal) fibers are primarily inhibitory to extensor motor neurons and neurons innervating muscles of the neck and back; these fibers also may excite flexor motor neurons via interneurons

Reticulospinal (and vestibulospinal) fibers contribute to the spasticity that develops in patients having lesions of corticospinal fibers.

These fibers, particularly reticulospinal fibers, also contribute to the tonic extension of the arms and legs seen in decerebrate rigidity when spinal motor neurons are released from descending cortical control.

The sudden increase in extensor rigidity, seen in decerebrate patients when a noxious stimulus is applied to, for example, the skin between the toes, is mediated via spinoreticular fibers (traveling in the ALS) that end on reticulospinal neurons whose axons descend to increase the level of excitation to extensor motor neurons

Medial vestibulospinal fibers primarily inhibit motor neurons innervating extensors and neurons serving muscles of the back and neck.

Lateral vestibulospinal fibers may inhibit some flexor motor neurons, but they mainly facilitate spinal reflexes via their excitatory influence on spinal motor neurons innervating extensors.

Vestibulospinal and reticulospinal (see Figure 8-17 on pp. 222–223) fibers contribute to the spasticity seen in patients with damage to corticospinal fibers or to the tonic extension of the extremities in patients with decerebrate rigidity.

In the case of decerebrate rigidity, the descending influences on spinal flexor motor neurons (corticospinal, rubrospinal) are removed; the descending brainstem influence on spinal extensor motor neurons predominates; this is augmented by excitatory spinoreticular input (via ALS) to some of the centers giving rise to reticulospinal fibers (see also Figure 8-15 on pp. 218–219). See Figure 8-17 for lesions that influence the activity of rubrospinal and reticulospinal fibers.

The primary motor cortex and premotor cortical areas contain the cell bodies of upper motor neurons whose axons descend along with axons from the primary somato- sensory cortex as the corticonuclear tract to the brainstem and the corticospinal tract to the spinal cord, sending signals about voluntary movement of the head and body, respectively

A 45-year-old man complains of left foot drop, which started 5 months ago. This was very subtle at first, but has been getting progressively worse. The patient can now barely lift his left foot up. He walks with difficulty. More recently he has noticed some clumsiness of his left hand. Upon further questioning he admits to frequent “twitching” of the muscles in his distal lower limbs (particularly in the calf), forearms, and shoulders. His family thinks he has lost some weight. There is no numbness, tingling, pain, or any other neurologic symptom noted. Family history is negative for any neurologic disorder. Examination shows severe weakness and flaccidity of left ankle dorsiflexion and mild weakness of the intrinsic left hand muscles and left finger extension. Otherwise, strength seems adequate. There is atrophy noted in the muscles of the left hand and the left pretibial muscles. Fasciculations are noted in several muscle groups of the legs, arms, and back. Deep tendon reflexes are pathologically brisk in the arms and legs bilaterally. A Babinski response is present bilaterally. The rest of the neurologic exam was unremarkable.

Premotor cortical areas

The premotor cortical areas consist of four regions: the premotor cortex, supplementary motor cortex, frontal eye field, and the cingulate motor areas, all of which reside in the frontal lobe. The premotor cortical areas function in the programing of complex motor activity, which is then relayed to the primary motor cortex where the execution of motor activity is initiated

Pennsylvania

Ballots must be received by Nov. 3.

Approximately two-thirds of the corticospinal tract fibers (Fig. 13.5) originate from the pyramidal cell layer of the frontal cortex ; that is, one-third arise from Brodmann’s area 4, and one-third from area 6 (mostly from the supplementary motor area, with some fibers from the premotor cortex).

The other third of the corticospinal tract fibers arise from the parietal cortex; specifically from the posterior parietal area (Brodmann’s areas 5 and 7) and the somatosensory cortex (Brodmann’s areas 3, 1, and 2)

The pyramidal cells (pyramidal neurons) are referred to as the upper moto- neurons (UMNs) of the descending motor pathways. Their axons descend to synapse in the spinal cord gray matter with: • interneurons in laminae V–VII, which in turn synapse with motoneurons; • alpha motoneurons, which innervate skeletal muscle fibers; or • gamma motoneurons, which innervate the contractile portion of muscle spindles (muscle stretch receptors)

The corticospinal tract consists of a mix of large-diameter, heavily-myelinated, fast-conducting axons and small-diameter, lightly-myelinated and unmyelinated axons.

Unlike the upper motoneurons, the fibers arising from neurons residing in the somatosensory cortex descend to synapse with second order sensory neurons in the somatic relay nuclei of the ascending sensory pathways (and thus are not considered to be upper motoneurons). There they influence motor activity by modulating the transmission of sensory information to higher brain centers (see later)

As the fibers continue their descent, they gradually shift in a posterior direction and come to occupy the posterior half (or posterior third) of the posterior limb of the internal capsule.

When the internal capsule is viewed on a coronal section of the cere- brum, the corticospinal fibers are somatotopically organized so that the fibers terminating in more rostral levels of the spinal cord (innervating the upper limb) are located more rostrally, whereas the fibers that terminate in lower spinal cord levels (innervating the lower limb) are located more caudally

When the fibers reach the midbrain, they are located in the middle third of the crus cerebri (basis pedunculi) of the cerebral peduncle. The corticospinal fibers are somatotopically orga- nized at this level also. The fibers innervating the upper limb are located medially, next to the corticonuclear tract, whereas the fibers that innervate the lower limb are located laterally.

The corticospinal tract continues inferiorly through the brain- stem where it disperses into bundles in the basal pons and then reassembles to form a distinct bundle known as the pyramid (a protuberance on the ventral aspect of the medulla)

In the medulla, the fibers maintain the same somatotopic organization as in the crus cerebri: that is, the fibers that innervate the upper limb are located medially, whereas the fibers that innervate the lower limb are located laterally.

The axons that emerge from the neurons residing in the lateral surface of the primary motor cortex controlling the upper limb, decussate in the rostral aspect of the pyramidal decussation, whereas the axons that emerge from the neurons residing in the medial surface of the primary motor cortex controlling the lower limb decussate in the caudal aspect of the pyramidal decussation. These axons then descend in the lateral funiculus (L., “cord”) of the entire length of the spinal cord, as the lateral corticospinal tract, with fibers exiting the tract to synapse in the cervical spinal cord located medially, and the fibers that synapse in the lumbar and sacral levels are located laterally.

Figure 13.5 ● The origin, course, and termination of the corticospinal tracts. The lateral corticospinal tract synapses with lower motoneurons that innervate the upper and lower limb muscles, whereas the anterior corticospinal tract synapses with the lower motoneurons that innervate the muscles of the neck, shoulder, and trunk, as well as the proximal upper limb (girdle) musculature.

The lateral corticospinal tract mediates the execution of rapid, skilled, voluntary movements of the distal musculature of the upper and lower limbs, especially the intrinsic muscles of the hand

The lateral corticospinal tract contains mostly crossed fibers; however, a small percentage of its fibers are uncrossed. This tract contains among its fibers a group of large-diameter myelinated axons arising from the giant Betz cells residing in the primary motor cortex

Most of the lateral corticospinal tract upper motoneurons (whose cell bodies reside in the motor cortex in the frontal lobe) projecting to the cervical and lumbosacral spinal cord, end in laminae VII–IX in the lateral intermediate zone gray matter (a region of the spinal cord gray matter that is populated by interneurons, and is thus similar to the reticular formation of the brainstem, which also contains interneurons), as well as the anterior (ventral) horn gray matter (containing interneu- rons and lower motoneurons), where they form excitatory syn- apses with interneurons

The first order afferent fibers transmitting sensory input from the muscle spindles also establish synaptic contacts with the in- hibitory interneurons (receiving input from the corticospinal fibers) in the spinal cord, mediating reflex activity

In contrast, a small number of the lateral corticospinal tract upper mo- toneurons project to the anterior (motor) horns of the spinal cord, where they form excitatory synapses directly with lower motoneurons. Many of these monosynaptic connections are with lower motoneurons innervating muscles of the hand

The Betz cell fibers descend to terminate in the lumbar and sacral spinal cord levels where they synapse directly with the lower motoneu- rons that innervate the musculature of the lower limb. These monosynaptic connections of Betz cells with the lower moto- neurons innervating the muscles of the lower limb are actu- ally fewer in number than the monosynaptic connections to lower motoneurons innervating the hand

Although the lateral corticospinal tract contains mostly crossed fibers, it also contains a minor group of fibers (approximately 2–3%), that do not cross at the pyramidal decussation, but instead descend ipsilaterally. Thus, the lat- eral corticospinal tract contains both crossed and uncrossed fibers. At their termination, the uncrossed fibers synapse with the spinal cord interneurons mediating movement of the axial (trunk) and proximal limb (girdle) musculature. The uncrossed fibers are associated with the maintenance of upright posture and general orientation of the limbs

Approximately, one-third of the lateral corticospinal tract axons (whose cell bodies reside in the parietal lobe) project to laminae IV–VI of the posterior horn gray matter.

The upper motoneurons of the anterior corticospinal tract do not decussate in the caudal medulla. Instead, they de- scend in the anterior funiculus of the spinal cord to termi- nate mainly in the anterior horn gray matter of the cervical (mainly) and upper thoracic spinal cord levels. Near their termination, these fibers decussate to the opposite side of the spinal cord via the anterior white commissure to synapse with interneurons that in turn synapse with lower motoneu- rons. Other fibers of the anterior corticospinal tract decussate to the opposite side near their termination and instead syn- apse directly with lower motoneurons, innervating the axial musculature (such as those of the neck, shoulder, and trunk) as well as the proximal upper limb (girdle) musculature

Approximately 55% of all of the corticospinal fibers termi- nate at cervical levels of the spinal cord, 20% terminate at thoracic levels, and the remaining 25% terminate at lumbar and sacral levels of the spinal cord (Fig. 13.8). The large num- ber of corticospinal fibers terminating at cervical spinal cord levels is an indication of the extensive influence they exert on neurons affecting the motor activity of the upper limb mus- culature, especially that of the hand

Although the fibers of the corticospinal tracts terminate principally in the spinal cord, they distribute collaterals to widespread regions of the nervous system in their descent. Collaterals terminate in the ventral nuclei of the thalamus, the basal nuclei, the reticular formation, the red nucleus, the pontine nuclei, the posterior column nuclei, the posterior (sensory) horn of the spinal cord, and the intermediolateral (lateral) cell column (sympathetic) of the spinal cord. Thus, the corticospinal tract probably has more than one function at various levels of the neuronal axis

The primary somatosensory area, located in the postcen- tral gyrus (Brodmann’s areas 3, 1, and 2), also projects fibers involved in motor activity to the brainstem and spinal cord.

Instead, they [The descending somatosensory fibers] synapse with the second order sensory neurons in the somatic sensory relay nu- clei of the sensory ascending pathways (the nucleus gracilis and the nucleus cuneatus of the brainstem), and the sensory nuclei in the dorsal horn of the spinal cord. Their function there is to influence motor activity by modulating the sen- sory information that is relayed to the brainstem and spinal cord from peripheral structures; thus they are not considered to be upper motoneurons.