Brainstem is the part of brain formed by medulla oblongata, pons and midbrain. Brainstem contains ascending and descending tracts between brain and spinal cord. It also contains many centers for regulation of vital functions in the body.
MEDULLA OBLONGATA
Medulla oblongata or medulla is the lowermost part of brain. It is situated below pons and is continued downwards as spinal cord. Medulla forms the main pathway for ascending and descending tracts of the
spinal cord. It also has
many important centers which control the vital functions.
1. Respiratory
Centers
Dorsal and ventral group
of neurons form the medullary respiratory centers, which maintain normal
rhythmic respiration.
2. Vasomotor
Center
Vasomotor center controls
blood pressure and heart rate.
3. Deglutition
Center
Deglutition center
regulates the pharyngeal and esophageal stages of deglutition.
4. Vomiting Center
Vomiting center induces vomiting during irritation or inflammation of gastrointestinal (GI) tract.
5. Superior and
Inferior Salivatory Nuclei
Salivatory nuclei control
the secretion of saliva.
6. Cranial Nerve
Nuclei
Nuclei of 12th, 11th, 10th
and some nuclei of 8th and 5th cranial nerves are located in the
medulla
oblongata.
12th cranial (hypoglossal) nerve controls the movements of tongue. 11th cranial (accessory) nerve controls the movements of shoulder and 10th
cranial (vagus) nerve controls almost all the vital
functions in the body, viz. cardiovascular
system, respiratory system, GI system, etc. 8th cranial
nerve (the cochlear division of this nerve), which has the relay in medulla oblongata,
is concerned with the auditory function.
7. Vestibular
Nuclei
Vestibular nuclei contain
the second order neurons of vestibular nerve. There are four vestibular nuclei,
situated in the rostral
part of medulla and caudal part of pons, namely superior, medial, lateral and
inferior
vestibular nuclei. Medial
and inferior vestibular nuclei extend into medulla. are controlled by higher
centers, situated in cerebral cortex and hypothalamus.
PONS
Pons forms a bridge
between medulla and midbrain.
Functions of Pons
1. Axons of pontine nuclei
join to form the middle cerebellar peduncle or the brachium pontis. Pons
forms the pathway that
connects cerebellum with cerebral cortex.
2. Pyramidal tracts pass
through the pons
3. Medial
lemniscus is joined by the fibers of 10th, 9th, 7th and 5th cranial nerves
in pons
4. Nuclei
of 8th, 7th, 6th and 5th cranial nerves are located in pons
5. Pons contains the
pneumotaxic and apneustic centers for regulation of respiration
6. It also
contains the vestibular nuclei, which are already mentioned in medulla
oblongata.
MIDBRAIN
Midbrain lies between pons
and diencephalon. It consists of two parts:
A. Tectum
B. Cerebral peduncles.
TECTUM
Tectum is formed by two
structures:
1. Superior colliculus
2. Inferior colliculus.
1. Superior
Colliculus
Superior colliculus is a
small structure and is an important center for
reflexes. Through tectospinal
tract, superior colliculus
controls the movements of the eyes, head, trunk and limbs, in
response to visual
impulses.
Efferent fibers from superior colliculus going to the nucleus of III cranial
(oculomotor) nerve cause constriction of pupil during light reflex. Thus, it forms
the center for light reflex. Superior
colliculus also receives afferents from optic
tract, which helps in the integration of optical and postural reflexes.
2. Inferior
Colliculus
Inferior colliculus
consists of single layer of neurons to which the lateral
lemniscus (auditory fibers) synapses.
2. Nucleus
parvocellularis, which is formed by smaller cells. Fibers from this form mainly the
rubroreticular tract.
Connections
of red nucleus
Afferent
connections: Red nucleus receives fibers from:
1. Nucleus
parvocellularis, which receives fibers from motor cortex (area 6) –
corticorubral fibers
2. Nucleus
magnocellularis, which receives fibers from motor cortex (area 6) –
pallidorubral fibers
3. Nucleus
magnocellularis, which receives fibers from dentate nucleus (of opposite side) –
cerebellorubral or dentatorubral tract.
Efferent connections: Red
nucleus sends efferent fibers to various parts of brain and
spinal cord:
1.
Rubrospinal tract to spinal cord
2. Rubrobulbar tract to
medulla
3.
Rubroreticular fibers to reticular formation
4.
Rubrothalamic tract to lateral ventral nucleus of thalamus
5. Rubroolivary tract to
inferior olivary nucleus
6. Fibers
to nuclei of 3rd, 4th and 6th cranial nerves.
Functions
of red nucleus
1. Control of muscle
tone: Because of its connections with cerebellum, vestibular apparatus and
skeletal
muscle, the red nucleus
plays an important role in facilitating the muscle tone.
2. Control of complex
muscular movements:
Red nucleus controls the
complex muscular movements. It plays an important role in the
integration of various
impulses received from many important areas of brain.
3. Control of righting reflexes: Red nucleus is the center for all righting reflexes except optical righting
reflexes.
4. Control
of movements of eyeball: Through its efferent
connections with nuclei of 3rd, 4th and 6th
cranial nerves, red nucleus
plays an important role in the control of ocular movements.
5. Control of skilled
movements: Red nucleus plays an important role in controlling the skilled
muscular
movements by its
connections with spinal cord and cerebral cortex.
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