Reticular Formation and Limbic System

Reticular Formation and Limbic System

CHAPTER OBJECTIVE

· To provide a brief overview of the structure and function of the reticular formation

A twenty-four-year-old medical student is rushed by ambulance to the emergency department after an accident on his motorcycle. On examination, he is found to be unconscious and shows evidence of severe injury to the right side of his head. He fails to respond when spoken to, and he does not make any response to deep painful pressure applied over his supraorbital nerve. The plantar reflex is extension, and the corneal, tendon, and pupillary reflexes are absent. The patient is clearly in a deep coma. Further neurologic examination reveals nothing that might add to the diagnosis. A computed tomography scan shows a large depressed fracture of the right parietal bone of the skull.

After a week in the intensive care unit, the patient's condition changes. He suddenly shows signs of being awake but not aware of his environment or inner needs. To the delight of his family, he follows them with his eyes and responds in a limited manner to primitive postural and reflex movements; he does not, however, speak and does not respond to commands. Although he has sleep-wake

RETICULAR FORMATION

The reticular formation, as its name suggests, resembles a net (reticular) that is made up of nerve cells and nerve fibers. The net extends up through the axis of the central nervous system from the spinal cord to the cerebrum. It is strategically placed among the important nerve tracts and nuclei. It receives input from most of the sensory systems and has efferent fibers that descend and influence nerve cells at all central nervous system levels. The exceptionally long dendrites of the neurons of the reticular formation permit input from widely placed ascending and descending pathways. Through its many connections, it can influence skeletal muscle activity, somatic and visceral sensations, the autonomic and endocrine systems, and even the level of consciousness.

· To present the parts of the limbic system and its functions cycles, he does not respond appropriately to pain. The patient's neurologic condition is unchanged six months later.

The neurologist determines that the patient is awake but not aware of his surroundings. He explains to the family that the part of the brain referred to as the reticular formation in the brainstem had survived the accident and is responsible for the patient apparently being awake and able to breathe without assistance. However, because his cerebral cortex is dead, the patient will remain in this vegetative state.

Not very long ago, the reticular system was believed to be a vague network of nerve cells and fibers occupying the central core of the brainstem with no particular function. Today, it is known to play a key role in many important activities of the nervous system.

Limbic system was a term loosely used to describe the part of the brain between the cerebral cortex and the hypothalamus, a little understood area of the brain. Today, it is known to play a vital role in emotion, behavior, drive, and memory.

The reticular formation consists of a deeply placed continuous network of nerve cells and fibers that extend from the spinal cord through the medulla, the pons, the midbrain, the subthalamus, the hypothalamus, and the thalamus. The diffuse network may be divided into three longitudinal columns: the first occupying the median plane, called the median column, and consisting of intermediate-size neurons; the second, called the medial column, containing large neurons; and the third, or lateral column, containing mainly small neurons.

With classic neuronal staining techniques, the groups of neurons are poorly defined; therefore, tracing an anatomical pathway through the network is difficult. However, with the new techniques of neurochemistry and cytochemical localization, the reticular formation is shown to contain highly organized groups of transmitter-specific cells that can influence functions in specific central nervous system areas. The monoaminergic groups of cells, for example, are located in well-defined areas throughout the reticular formation.

Polysynaptic pathways exist, and both crossed and uncrossed ascending and descending pathways are present, involving many neurons that serve both somatic and visceral functions.

Inferiorly, the reticular formation is continuous with the interneurons of the gray matter of the spinal cord, while superiorly, impulses are relayed to the cerebral cortex; a substantial projection of fibers also leaves the reticular formation to enter the cerebellum.