Reticular Formation Notes

Reticular Formation Definition

Reticular formation is a diffused mass of neurons and nerve fibers forming an ill-defined meshwork of the reticulum in the central portion of the brainstem.

Situation Of Reticular Formation

The reticular formation is situated in brainstem. It extends downwards into the spinal cord and upwards up to thalamus and subthalamus.

Organization Of Reticular Formation

The reticular formation is constituted by 5 groups of nuclei. All these nuclei are structurally and functionally distinct.

1. Raphe Group: Raphe group of nuclei are situated along the midline of the brainstem forming a continuous column. Raphe nuclei secrete serotonin (5-HT) which is an inhibitory neurotransmitter.
2. Paramedian Group: Paramedian group includes nucleus reticularis para-medians and pontine reticulotegmental nucleus. These nuclei are concerned with motor functions.
3. Lateral Group: Lateral group of nuclei is situated in the lateral 1/3 of the tegmentum. It consists of nuclei with small
   (parvocellular) cells. The neurons of these nuclei receive sensory signals from the cranial nerves, cerebellum, and spinal cord.
4. Medial Group: It is situated in the medial 2/3 of the tegmentum. It consists of nuclei with small cells and giant (giganto-cellular) cells. The nuclei of this group form the major output of the reticular formation and send fibers to the hypothalamus, thalamus, and spinal cord. These nuclei are associated with motor functions.
5. Intermediate Group: This group of nuclei is present only in the medulla. It is situated between the lateral and medial groups of nuclei. These nuclei are concerned with the autonomic regulation of respiration, heart rate, and blood pressure.

Divisions Of Reticular Formation

Reticular formation is divided into three divisions based on the location in brainstem:

1. Medullary reticular formation
2. Pontine reticular formation
3. Midbrain reticular formation.

Each division of reticular formation has its own collection of nuclei.

1. Nuclei Of Medullary Reticular Formation
   1. Lateral reticular nucleus
   2. Ventral reticular nucleus
   3. Dorsal reticular nucleus
   4. Gigantocellular reticular nucleus
   5. Paragigantocellular reticular nucleus
   6. Paramedian reticular nucleus
   7. Parvocellular reticular nucleus
   8. Magnocellular reticular nucleus
2. Nuclei Of Pontine Reticular Formation
   1. Nucleus reticular pontis oralis
   2. Nucleus reticular pontis caudalis
   3. Locus cereleus nucleus
   4. Subcereleus reticular nucleus
   5. Tegmenti pontis reticular nucleus
   6. Peduncucopontis reticular nucleus
   7. Nucleus reticular cuneiformis
3. Nuclei Of Midbrain Reticular Formation
   1. Red nucleus
   2. Nucleus tegmenti pedunculopontine
   3. Nucleus reticular subcuneiformis

Connections Of Reticular Formation

• Afferent Connections: Reticular formation receives collaterals from almost all the ascending sensory pathways. It also receives fibers from different parts of the brain:
  1. Optic pathway
  2. Olfactory pathway
  3. Auditory pathway
  4. Taste pathway
  5. Spinal and trigeminal pathways carrying touch sensation
  6. Pathways for pain, temperature, vibration, and kinesthetic sensations
  7. Cerebral cortex
  8. Cerebellum
  9. Corpus striatum
  10. Thalamic nuclei.

Read And Learn More: Medical Physiology Notes

• Efferent Connections: Reticular formation sends fibers to the following parts of central nervous system:
  1. Cerebral cortex
  2. Diencephalon – thalamus, hypothalamus, and subthalamus
  3. Midbrain – red nucleus, tectum, and substantia nigra
  4. Cerebellum
  5. Spinal cord.

Functions Of Reticular Formation

Based on functions, the reticular formation along with its connections is divided into two systems.

1. Ascending reticular activating system
2. Descending reticular system.

1. Ascending Reticular Activating System (Aras): ARAS begins in lower part of the brainstem, extends upwards through the pons, midbrain, thalamus, and finally projects throughout the cerebral cortex. It projects into the cerebral cortex in two ways:

1. Via subthalamus
2. Via thalamus.
   • The ARAS receives fibers from the sensory pathways via long ascending spinal tracts.

Functions of ARAS

1. ARAS is concerned with arousal phenomenon, alertness, maintenance of attention, and wakefulness. Hence, it is called ascending reticular activating system.
2. Stimulation of midbrain reticular formation produces wakefulness by generalized activation of the entire brain including cerebral cortex, thalamus, basal ganglia, and brainstem.
3. Any type of sensory impulses such as impulses of proprioception, pain, auditory, visual, taste, and olfactory sensations cause sudden activation of the ARAS-producing arousal phenomenon in animals and human beings.
4. Even the impulses of visceral sensations activate this system. The sympathetic stimulation and adrenaline cause arousal by affecting midbrain.
5. ARAS also causes emotional reactions
6. ARAS plays an important role in regulating the loafing processes and the development of conditioned reflexes.

Mechanism of Action of ARAS: The impulses of all the sensations reach the cerebral cortex through two channels

1. Classical or specific sensory pathways
   • Classical sensory pathways are the pathways that transmit sensory impulses from receptors to cerebral cortex via the thalamus. Some of the pathways carry impulses of a particular sensation only.
   • For example, the auditory stimulus transmitted by the auditory pathway reaches the auditory cortex via the thalamus and causes the perception of sound. Such classical sensory pathways are called specific sensory pathways.
2. Ascending reticular activating system or nonspecific sensory pathway
   • All the sensory pathways send collaterals to ARAS, which is a multisynaptic relay system. These collaterals project in diffused areas of ARAS.
   • So the sensory impulses transmitted via the collaterals reach different parts of ARAS. It also receives afferents from the spinal cord directly in the form of the spinoreticular tract.
   • ARAS in turn sends the impulses to almost all the areas of cerebral cortex and other parts of the brain. Hence, this pathway is called the nonspecific sensory pathway.
   • The nonspecific projection of ARAS into the cortex is responsible for the arousal, alertness, and wakefulness.
   • The sensory impulses transmitted directly to the cortex via the classical pathway cause the perception of only the particular sensation.
   • Whereas, the impulses transmitted to cortex via ARAS do not cause the perception of any particular sensation but cause the generalized activation of almost all the areas of the cerebral cortex and other parts of brain.
   • This leads to reactions of arousal, alertness, and wakefulness.
   • The ARAS is in turn controlled by the feedback signals from cerebral cortex. Also, an inhibitory system controls the activities of ARAS.
   • The inhibitory system involves posterior hypothalamus, intralaminar and anterior thalamic nuclei, and medullary area at the level of tractus solitarius.
   • The tumor or lesion in ARAS leads to sleeping sickness or coma. The impact of head injury on ARAS also causes coma.

2. Descending Reticular System

• The descending reticular system includes reticular formation in brainstem, the reticulospinal tract, and reticular formation in spiral cord.
• It modifies the activities of spinal motor neurons. Functionally, descending reticular system is divided into two subdivisions:
  1. Descending facilitatory reticular system
  2. Descending inhibitory reticular system.

1. Descending Facilitatory Reticular System: Descending facilitatory reticular system is present in upper and lateral reticular formation. Its functions are:
   1. Facilitation of somatomotor activities:
      1. Descending facilitatory reticular system maintains muscle tone by exciting the gamma motor neurons in spinal cord. Stimulation of this area causes increased muscle tone
      2. It facilitates the movements of the body. Stimulation of this part of reticular system causes exaggerated movements
      3. It plays a role in wakefulness and alertness by activating the ARAS.
   2. Facilitation of vegetative functions:: Descending facilitatory reticular system is the center for facilitation of the autonomic functions such as cardiac function, blood pressure, respiration, gastrointestinal function, and body temperature.
2. Descending Inhibitory Reticular System: Descending inhibitory reticular system is located in a small area in the lower and medial reticular formation. Its functions are:
   1. Control of somatomotor activities:
      1. The descending inhibitory reticular system plays an important role in the control of muscle tone. By receiving signals from the basal ganglia, it inhibits the gamma motor neurons of the spinal cord and decreases muscle tone. Stimulation of this area causes decreased muscle tone
      2. It is responsible for the smoothness and accuracy of voluntary movements. It controls muscular activity by inhibiting the motor neurons of the spinal cord
      3. It also controls the reflex movements.
   2. Control of vegetative functions:: The descending inhibitory reticular system is the center for inhibition of several autonomic functions such as cardiac function, blood pressure, respiration, gastrointestinal function, and body temperature.