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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