Lateral Zone
A. Reticular Formation
relationship of the cerebral peduncle to the medial temporal lobe demonstrates that the cerebral peduncles are adjacent to the medial temporal lobe and that herniation of the temporal lobe will affect the cerebral peduncle signs in the III cranial nerves and descending pathways.
Figure 4.10 is a myelin-stained section through the cerebral peduncles demon-strating the relationship between the peduncles and the III cranial nerve.
neuronal stain, for the Golgi type I cells with long axons, neurons are shown with their dendrites extending transversely and the axons bifurcating into ascending and descending branches, which run throughout the system. Each neuron receives input from at least 1000 neurons, and each neuron connects to as many as 10,000 neurons in the reticular formation. The reticular formation of the brain stem blends inferi-orly into lamina VII of the cord, and superiinferi-orly, it is continuous with the hypotha-lamus and dorsal thahypotha-lamus of the diencephalon.
1. Nuclei
Many nuclei have been identified in the reticular formation. Functionally, the nuclei can be divided into cerebellar and noncerebellar nuclei (Table 4.5). The cer-ebellar portion of the reticular formation includes primarily the lateral and parame-dian nuclei of the medulla and the tegmental nucleus of the pons.
The noncerebellar portion nuclei are divided anatomically into three columns:
the raphe, central, and lateral groupings. Most of the nuclei in the reticular formation consist of large cells with ascending and descending axons.
Functionally, the reticular formation in the medulla is especially important because the descending reticulospinal fibers and much of the ascending reticular system originate there.
2. Descending Reticulospinal System (Table 4.6)
The nucleus reticularis gigantocellularis is found at the rostral medullary levels, dorsal and medial to the inferior olive. This nucleus gives origin to much of the lateral reticulospinal tract, which is primarily an ipsilateral tract, running in the lat-eral funiculus of the spinal cord in all levels and terminating on internuncial neurons.
The axons from the nucleus reticularis pontis oralis and caudalis form much of the medial reticulospinal tract, which runs in the anterior funiculus in all levels and terminates on internuncial neurons.
The lateral reticular nucleus is located in the lateral margin of the reticular formation, dorsal to the inferior olive, whereas the ventral reticular nucleus is found in the caudal end of the medulla, dorsal to the inferior olive. These two nuclei, in conjunction with the nuclei in the pons and midbrain, form much of the ascending reticular fibers in the central tegmental tract, distributing to neurons in the thalamus (intralaminar and reticular), hypothalamus, and corpus striatum. The
Table 4.5 Functional groupings of nuclei in the reticular formation
Noncerebellar Cerebellar related
Raphe, central, and lateral in the tegmentum of brain stem
Lateral and paramedian nuclei of medulla and tegmental nucleus of the pons
Table 4.6 Major pathways of the reticular formation
Tract Origin Termination of pathway
Lateral reticulospinal Nucleus gigantocellularis Spinal cord and autonomic interneurons Medial reticulospinal Pontis oralis and caudalis Spinal cord and autonomic interneurons Central reticulospinal Lateral and ventral Thalamic–intralaminar and reticular,
hypothalamus, corpus striatum, substantia nigra
paramedian reticular nucleus is found near the midline at midolivary levels, dorsal to the inferior olive, and provides direct input into the anterior lobe of the cerebel-lar vermis.
3. Input to Reticular Formation
The reticular formation receives information via the ascending spinal tracts (spino-tectal, spinoreticular, spinothalamics, and spinocerebellar) from the brain stem itself (olivoreticular, cerebelloreticular, and vestibulospinal), from the cerebral hemispheres (corticoreticular), and from the basal ganglia and hypothalamus. The cranial nerves are another important source, especially nerves I, II, V, VII, VIII, and X, for sensory information that appears to project most heavily onto the central nuclei. The hypothalamus and striatum also project to the reticular system via the dorsal longitudinal fasciculus on the floor of the cerebral aqueduct and fourth ven-tricle and the more diffuse descending fiber systems in the core of the reticular formation.
4. Output of Reticular System
The medial lemniscus is a specific point-to-point relay system with few synapses (a closed system), while the fiber tracts of the reticular system are a multisynaptic nonspecific system (an open system)…
a. Central Tegmental Tract
The central tegmental tract is the principal fiber tract of the reticular formation. Its descending portions are located in the medial tegmentum, and its ascending por-tions are located in the lateral tegmentum. The ascending system projects to the thalamic intralaminar and reticular nuclei, hypothalamus, basal ganglia, substantia nigra, and red nucleus. The reticular system is functionally important in controlling our “posture” by its reflex relationship to the position of our body in space and in controlling our internal milieu by maintaining the stability of our viscera. The
descending system synapses via the reticulospinal tract onto interneurons that mediate their effects through alpha and gamma motor neurons in the spinal cord and via autonomic pathways and cranial nerves onto visceral neurons.
b. Role of Ascending Reticular System
This fiber system is the structural and functional substrate for maintaining con-sciousness. It also receives proprioceptive, tactile, thermal, visual, auditory, and nociceptive information via the spinal and cranial nerves. Many sensations (cutane-ous, nociceptive, and erotic) activate the system. The sensory information ascends via the central tegmental tract into the limbic midbrain area from which information can be more directly passed into the thalamus and hypothalamus. This midbrain to diencephalons to telencephalon circuit seems especially important in setting our level of consciousness.
c. Role of Descending Systems
The reticular formation has many roles and is especially important in control-ling posture and orientation in space. Stimulation of the nucleus reticularis gigartocellularis of the medulla inhibits the knee jerk. Stimulation of reticularis oralis/caudalis of the pons facilitates the knee jerk. The lateral part of the retic-ular formation is the receptor area, whereas the medial portion is the effector zone and the origin of the central tegmental and reticulospinal tracts. Many of the functions vital to the maintenance of the organism are found in the medulla.
d. Neurochemically Defined Nuclei in the Reticular Formation Affecting Consciousness
Cholinergic Nuclei. The cholinergic nuclei are located in the dorsal tegmentum of the pons and midbrain, in the mesopontine nuclei, and in the basal forebrain region and they project diffusely to the cerebral cortex through the thalamus and have a modulating influence on cerebral cortical activity and wakefulness.
Monoamine Nuclei. In the reticular formation, we find cells containing nore-pinephrine and serotonin. The norepenore-pinephrine-containing cells are found in the locus ceruleus (blue staining) in the upper pons and midbrain and they project widely upon nuclei in the spinal cord, brain stem, thalamus, hypothala-mus, and corpus striatum, which are important for maintaining attention and wakefulness.
Serotinergic Nuclei are found in the raphe of the medial tegmental zone in the medulla, pons, and midbrain. The nuclei in the pons and medulla project onto the
spinal cord and brain stem, whereas the nuclei in the upper pons and midbrain project onto the thalamus, hypothalamus, corpus striatum, and cerebral cortex. The serotinergic system facilitates sleep.
An area outside of the reticular formation, the histamine-containing area of the posterior hypothalamus, is also important in maintaining wakefulness.