Reticular Formation

You may have wondered which part of the brain is responsible for keeping a person awake and regulating the level of consciousness. The answer lies in reticular formation. Reticular formation, as the name suggests, is a network of neurons and nerve fibers, present in the brain.

Earlier, no particular function was known to be associated with the reticular formation. Today, the reticular formation is considered to play a very important role in different activities of the brain and the nervous system.

In this article, we will go through the anatomical features such as location, structure, connections and blood supply of the reticular formation. We will also talk about the various functions associated with the reticular formation. In the end, we will discuss some of the pathologies and clinical significance of the reticular formation.

Anatomy

Location

The reticular formation is located in the brain stem. It extends throughout the length of the brainstem, along the central axis, from the spinal cord to the thalamus.

It occupies the anterior portions of medulla, pons, midbrain, hypothalamus, and thalamus. The reticular formation is strategically placed among the important nuclei and the nerve fibers crossing the brainstem that is crucial for its various functions.

Structure

The reticular formation resembles a net made up of nerve fibers and nerve cells. It is a deeply placed diffuse network of fibers and nuclei. This network can be diffusely divided into three longitudinal columns; median column, medial column, and the lateral column.

The median column occupies the median plane. It
consists of the intermediate-size neurons.

The lateral column is located lateral to the median column. This column occupies mainly small neurons.

The medial column is located medial to the median
column. It mainly consists of large-size neurons.

All these neurons are diffusely packed. It is
difficult to differentiate between them and classify them into groups. However,
all the neurons are monoaminergic and secrete important neurotransmitters that
influence the activities of the entire central nervous system.

Afferent Fibers

Different pathways from the entire central nervous system project onto the reticular formation. Here we will discuss some important afferent pathways.

Three tracts from the spinal cord project onto the
reticular formation. These include:

• Spinothalamic tract
• Spinoreticular tract
• Medial lemniscus

 The ascending
afferent tracts of the sensory cranial nerve also project to the reticular
formation. These include:

• Vestibular pathway
• Visual pathway
• Acoustic pathway

The reticular nuclei also receive afferent fibers
cerebellum as cerebelloreticular pathway.

In addition, it receives afferent from the subthalamus, hypothalamus, and thalamus. The nuclei in the corpus striatum and the limbic system as well as the neurons of the primary motor cortex and the somatosensory cortex also send afferent fibers to the reticular formation.

Efferent Fibers

The nuclei of the reticular formation send efferent fibers to different areas of the CNS. They continue as reticulobulbar tract in the brainstem and reach the motor nuclei of the cranial nerves. The efferent fibers of the reticular nuclei continue as reticulospinal tract, for the motor nuclei present in the anterior horn of the spinal cord.

Other efferent fibers extend to the sympathetic and the parasympathetic outflow. They also reach the nuclei of corpus striatum, cerebellum, red nuclei, thalamus, substantia nigra and tectum of midbrain.

Blood Supply

The blood supply of reticular formation is derived from the branches of vertebral arteries and the basilar artery. The blood supply is the same as for the part of brainstem containing the reticular formation.

For example, the part of reticular formation in medulla oblongata obtains blood from the same blood vessels that provide blood to other areas of medulla.

Physiology

In this section, we will discuss the important functions of the reticular system in detail.

Control of skeletal muscle

The reticular formation plays an important role in regulating the activity of skeletal muscles. It does so by influencing the activity of the alpha and gamma motor neurons through the reticulospinal and reticulobulbar tracts.

In this way, the reticular formation can modulate muscle tone and reflex activity. It also brings about reciprocal inhibition; for example, when the flexor muscles contract, the antagonistic relaxation of extensors is due to reticular formation.

Control of Muscle Tone
and Balance

Along with the vestibular
apparatus of the inner ear and the vestibular spinal tract, the reticular
formation plays an important role in maintaining the tone of the antigravity
muscles when standing.

Control of Respiratory
Muscles

The respiratory centers
of the brainstem that control the respiratory muscles are also considered to be
a part of the reticular formation.

Control of Facial Expressions

The reticular formation also plays a role in controlling the muscles of facial expression when associated with emotion. For example, when you smile or laugh in response to a joke, the motor control to your facial muscles is provided by the reticular formation on both sides of the brain.

Control of Somatic and Visceral
Sensations

Because of its strategic
central location in the cerebrospinal axis, the reticular formation can influence
all ascending pathways that pass to higher levels. This influence can be
facilitative or inhibitory. In particular, the

reticular formation is
considered to have a key role in the gating mechanism, a mechanism for the
control of pain perception.

Control of the Autonomic Nervous System

It is considered that the higher control of the autonomic nervous system, from the cerebral cortex, hypothalamus, and other subcortical nuclei, can be exerted by the reticulobulbar and reticulospinal tracts, which descend to the sympathetic

outflow and the
parasympathetic craniosacral outflow.

Control of the Endocrine Nervous
System

The reticular formation is considered to influence the synthesis or release of releasing or release-inhibiting factors. In this way, it controls the activity of the hypophysis cerebri. The reticular formation can do this either directly or indirectly through the hypothalamic nuclei.

Influence on the Biologic
Clocks

The reticular formation
also influences the biologic rhythms of the body by means of its afferent and
efferent pathways to the hypothalamus.

The Reticular Activating System

The level of consciousness and arousal are controlled by the reticular formation. The ascending pathways carrying the sensory information to the higher centers are channeled through the reticular formation. The reticular formation, in turn, projects this information to different parts of the cerebral cortex. This causes a sleeping person to awaken.

It is considered that
the state of consciousness is dependent on the continuous projection of sensory
information to the cortex. Different degrees of wakefulness also depend on the
degree of activity of the reticular formation.

The activity of the reticular formation is strongly increased by the incoming pain sensations. This greatly excites the cerebral cortex.

Clinical Significance

As the main function of reticular formation is to
control the level of wakefulness and consciousness, any abnormality of the
reticular system results in the following manifestations.

Narcolepsy

It is a sleep disorder which causes excess sleep, day-time sleep, sleep paralysis, and hallucinations. It can also cause partial or complete loss of muscle tone associated with some emotions. This condition is called cataplexy.

Injury or damage to the reticular formation is
considered to be a major cause of narcolepsy and cataplexy.

Loss of Consciousness

It has been demonstrated in the experimental animals that damage to the reticular formation causes persistent unconsciousness.

Pathologic lesions of the reticular formation in humans can also result in loss of consciousness and even coma. It has been considered that the loss of consciousness in epilepsy may be due to the inhibition of the activity of the reticular formation in the upper part of the diencephalon.

Conclusion/Summary

The reticular formation is a network of neurons and nerve fibers extending throughout the length of the brainstem.

It is divided into three columns;

• Median column with intermediate
  size neurons
• Medial column with large neurons
• Lateral column with small neurons

The nuclei receive afferent and efferent fibers from
the rest of the CNS.

The important afferent fibers come from the spinal
cord, cerebellum, motor cortex, and the vestibular system.

The most important efferent fibers are the reticulobulbar and reticulospinal tracts.

The reticular formation performs very important
functions. These include:

• Controlling the contractions of
  skeletal muscles via reticulobulbar and reticulospinal tracts
• Control of facial expressions
  associated with emotions
• Control of skeletal muscle tone and
  balance
• Controlling the respiratory muscles
• Controlling the level of
  consciousness and wakefulness
• Endocrine control

The clinical conditions associated with the lesions of
the reticular system are narcolepsy and loss of consciousness.

References

1. Gray, Henry. "FIG. 701: Henry Gray (1825-1861). Anatomy of the Human Body. 1918". Bartleby.com. Archived from the original on 2018-04-21. Retrieved 2019-09-12.
2.  Purves, Dale (2011). Neuroscience (5. ed.). Sunderland, Mass.: Sinauer. pp. 390–395. ISBN 978-0-87893-695-3.
3.  Iwańczuk W, Guźniczak P (2015). "Neurophysiological foundations of sleep, arousal, awareness and consciousness phenomena. Part 1". Anaesthesiol Intensive Ther. 47 (2): 162–167. doi:10.5603/AIT.2015.0015. PMID 25940332
4. Augustine JR (2016). "Chapter 9: The Reticular Formation". Human Neuroanatomy (2nd ed.). John Wiley & Sons. pp. 141–153. ISBN 9781119073994. Archived from the original on 4 May 2018. Retrieved 4 September 2017.
5. "the definition of reticular activating system". Dictionary.com. Archived from the original on 2017-02-05.