Reticular Formation organization and division

Reticular formation is a diffused mass of neurons and nerve fibers, which form an ill-defined meshwork of

reticulum in central portion of the brainstem.

SITUATION OF RETICULAR FORMATION

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

ORGANIZATION OF RETICULAR FORMATION

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

RAPHE GROUP

Raphe group of nuclei are situated along the midline of the brainstem forming a continuous column. Raphe nuclei secrete serotonin (5-hydroxytryptamine), which is an inhibitory neurotransmitter.

PARAMEDIAN GROUP

Paramedian group includes nucleus reticularis paramedianus and pontine reticulotegmental nucleus.

These nuclei are concerned with motor functions.

LATERAL GROUP

Lateral group of nuclei are situated in the lateral one third of the tegmentum. It consists of nuclei with small (parvo cellular) cells. Neurons of these nuclei receive sensory sig nals from the cranial nerves, cerebellum and spinal cord.

MEDIAL GROUP

Medial group of nuclei are situated in the medial two third of the tegmentum. It consists of nuclei with small cells and giant (gigantocellular) cells. 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.

INTERMEDIATE GROUP

Intermediate group of nuclei are present only in the medulla. It is situated between the lateral and medial

groups of nuclei. These nuclei are concerned with auto nomic 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:

A. Medullary reticular formation

B. Pontine reticular formation

C. Midbrain reticular formation.

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

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.

NUCLEI OF PONTINE RETICULAR FORMATION

1. Nucleus reticularis pontis oralis

2. Nucleus reticularis pontis caudalis

3. Locus ceruleus nucleus

4. Subceruleus reticular nucleus

5. Tegmenti pontis reticular nucleus

6. Pedunculopontine reticular nucleus

7. Nucleus reticular cuneiformis.

NUCLEI OF MIDBRAIN RETICULAR FORMATION

1. Red nucleus

2. Nucleus tegmental pedunculopontis

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.

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, reticular formation along with its connections is divided into two systems:

A. Ascending reticular activating system

B. Descending reticular system.

ASCENDING RETICULAR ACTIVATING SYSTEM

Ascending reticular activating system (ARAS) begins in lower part of brainstem, extends upwards through pons, midbrain, thalamus and finally projects throughout the cere bral cortex. It projects into cerebral cortex in two ways:

1. Via subthalamus

2. Via thalamus.

 

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