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INTRODUCTION TO THE BRAIN
&
BRAINSTEM
28. 04. 2014
Kaan Yücel
M.D., Ph.D.
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Dr.Kaan Yücel
yeditepeanatomyfhs122.wordpress.com
Introduction to the brain & Brainstem
The brain (encephalon; Greek, within the head) is divided into three major divisions.
1) Hindbrain (Rhombencephalon)
I. Medulla oblongata
II. Pons
III. Cerebellum
Pons and cerebellum are called as metencephalon.
2) Midbrain (Mesencephalon)
3) Forebrain (Prosencephalon)
I. Telencephalon (Cerebrum)
II. Diencephalon (between brain)
The brainstem (a collective term for the medulla oblongata, pons, and midbrain) is that part of the brain that remains
after the cerebral hemispheres and cerebellum are removed.
The brainstem is the oldest part of the CNS. The brainstem is made up of the medulla oblongata, the pons, and the
midbrain and occupies the posterior cranial fossa of the skull. It is stalklike in shape and connects the narrow spinal
cord with the expanded forebrain.
The brain stem contains 10 cranial nerves, and most of the motor and sensory systems pass through this important
region. It is a relatively small region (approximately 7 cm long) that links the forebrain (i.e., cerebral cortex) and the
spinal cord and all messages going between the two areas must go through the brain stem.
The brainstem has three broad functions: (1) it serves as a conduit for the ascending tracts and descending tracts
connecting the spinal cord to the different parts of the higher centers in the forebrain; (2) it contains important reflex
centers associated with the control of respiration and the cardiovascular system and with the control of
consciousness; and (3) it contains the important nuclei of cranial nerves III through XII.
The midbrain measures about 0.8 inch (2 cm) in length and connects the pons and cerebellum with the forebrain.
The cerebral hemispheres are connected to the brainstem by two large fiber tracts , the cerebral peduncles. The
narrow cavity of the midbrain is the cerebral aqueduct, which connects the third and fourth ventricles.
The dorsal aspect of the midbrain is known as the tectum (L., roof] and incorporates the paired superior and inferior
colluculi (singular, colliculus). The superior and inferior colliculi are also known as corpora quadrigemina. These
are rounded eminences that are divided into superior and inferior pairs by a vertical and a transverse groove. The
superior colliculi are centers for visual reflexes, and the inferior colliculi are lower auditory centers.
The pons is anterior to the cerebellum and connects the medulla oblongata to the midbrain. It is about 1 inch (2.5
cm) long and owes its name to the appearance presented on the anterior surface, which is that of a bridge connecting
the right and left cerebellar hemispheres.
The medulla oblongata is situated in the posterior cranial fossa, lying beneath the tentorium cerebelli and above the
foramen magnum. It is related anteriorly to the basal portion of the occipital bone and the upper part of the odontoid
process of the axis and posteriorly to the cerebellum.The medulla oblongata not only contains many cranial nerve
nuclei that are concerned with vital functions (e.g., regulation of heart rate and respiration), but it also serves as a
conduit for the passage of ascending and descending tracts connecting the spinal cord to the higher centers of the
nervous system.
The 12 pairs of cranial nerves are part of the peripheral nervous system (PNS) and pass through foramina or fissures
in the cranial cavity. In addition to having similar somatic and visceral components as spinal nerves, some cranial
nerves also contain special sensory (such as hearing, seeing, smelling, balancing, and tasting). The special sensory
components are associated with.
Nuclei of 12 cranial nerves: 10 of them in the brainstem
Midbrain Of the IV & III
Pons
Of the other 4 - VIII,VII,VI,V
Medulla Of the last 4 - XII,XI,X, IX
The reticular formation (L. reticulum, “little net”) consists of various distinct populations of cells embed in a
network of cell processes occupying the central core of the brainstem. From an evolutionary perspective, the
reticular formation is phylogenetically an ancient neural complex that is closely associated with two other ancient
neural systems, the olfactory system and the limbic system.
Parasympathetic System in the Brainstem
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•
Edinger-Westfall nucleus in the
midbrain (mediates the diameter of the pupil in response to light)
•
Superior and inferior salivatory nuclei in the pons and medulla
•
Dorsal motor nucleus of the vagus nerve in the medulla.
Dr.Kaan Yücel
yeditepeanatomyfhs122.wordpress.com
Introduction to the brain & Brainstem
INTRODUCTION TO THE BRAIN
The brain (encephalon; Greek, within the head) is divided into three major divisions. These are, in
ascending order from the spinal cord;
1) Hindbrain (Rhombencephalon)
I. Medulla oblongata
II. Pons
III. Cerebellum
Pons and cerebellum are called as metencephalon.
2) Midbrain (Mesencephalon)
3) Forebrain (Prosencephalon)
I. Telencephalon (Cerebrum)
II. Diencephalon (between brain)
The brainstem (a collective term for the medulla oblongata, pons, and midbrain) is that part of the brain
that remains after the cerebral hemispheres and cerebellum are removed.
1. Telencephalon (Cerebrum): Telencephalon by far forms the largest region of the brain. They consist
of the cerebral hemispheres, basal ganglia and ventricles. The basal ganglia participate in regulating motor
performance.
2. Diencephalon: Diencephalon or between-brain is so called because it lies between the cerebral
hemispheres and the midbrain. The main structures of the diencephalon are the thalamus and hypothalamus.
The thalamus processes most of the information reaching the cerebral cortex from the rest of the central
nervous system. Hypothalamus regulates autonomic, endocrine, and visceral function.
3. Mesencephalon (Midbrain): is the smallest brain stem component and lies anterior to pons. Several
regions of the midbrain play a dominant role in the direct control of eye movements, whereas others are
involved in motor control of skeletal muscles. The midbrain is also involved with the coordination of visual and
auditory reflexes.
4. Metencephalon [Pons+Cerebellum]
Pons (Latin, bridge), which lies above the medulla, conveys information about movement from the
cerebral hemisphere to the cerebellum.
Cerebellum, lies behind the pons and is connected to the brain stem by several major fiber tracts called
peduncles. The cerebellum modulates the force and range of movement and is involved in learning of motor
skills.
5. Medulla oblongata (medulla): which lies directly above the spinal cord, includes several centers
responsible for such vital autonomic functions as digestion, breathing, and the control of heart rate.
BRAINSTEM
The brainstem is the oldest part of the CNS. The brainstem is made up of the medulla oblongata, the pons, and
the midbrain and occupies the posterior cranial fossa of the skull. It is stalklike in shape and connects the
narrow spinal cord with the expanded forebrain.
The brain stem contains 10 cranial nerves, and most of the motor and sensory systems pass through this
important region. It is a relatively small region (approximately 7 cm long) that links the forebrain (i.e., cerebral
cortex) and the spinal cord and all messages going between the two areas must go through the brain stem.
The brainstem has three broad functions: (1) it serves as a conduit for the ascending tracts and descending tracts
connecting the spinal cord to the different parts of the higher centers in the forebrain; (2) it contains important
reflex centers associated with the control of respiration and the cardiovascular system and with the control of
consciousness; and (3) it contains the important nuclei of cranial nerves III through XII.
Midbrain [Mesencephalon]
The midbrain measures about 0.8 inch (2 cm) in length and connects the pons and cerebellum with the
forebrain. The cerebral hemispheres are connected to the brainstem by two large fiber tracts , the cerebral
peduncles. The narrow cavity of the midbrain is the cerebral aqueduct, which connects the third and fourth
ventricles.
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Introduction to the brain & Brainstem
The dorsal aspect of the midbrain is known as the tectum (L., roof] and incorporates the paired superior and
inferior colluculi (singular, colliculus). The superior and inferior colliculi are also known as corpora
quadrigemina. These are rounded eminences that are divided into superior and inferior pairs by a vertical and
a transverse groove. The superior colliculi are centers for visual reflexes, and the inferior colliculi are lower
auditory centers.
The region of the mesencephalon below the cerebral aqueduct is known as the midbrain (mesencephalic)
tegmentum (L., cover).
The midbrain comprises two lateral halves, called the cerebral peduncles; each of these is divided into an
anterior part, the crus cerebri, and a posterior part, the tegmentum, by a pigmented band of gray matter, the
substantia nigra.
The substantia nigra is a large motor nucleus situated between the tegmentum, and the crus cerebri and is
found throughout the midbrain. The substantia nigra is concerned with muscle tone and is connected to the
cerebral cortex, spinal cord, hypothalamus, and basal nuclei.
The crus cerebri contains important descending tracts and is separated from the tegmentum by the substantia
nigra. These descending tracts connect the cerebral cortex to the anterior gray column cells of the spinal cord,
the cranial nerve nuclei, the pons, and the cerebellum.
Pons
The pons is anterior to the cerebellum and connects the medulla oblongata to the midbrain. It is about 1 inch
(2.5 cm) long and owes its name to the appearance presented on the anterior surface, which is that of a bridge
connecting the right and left cerebellar hemispheres.
The anterior surface is convex from side to side and shows many transverse fibers that converge on each side to
form the middle cerebellar peduncle.
Pons has a convex anterior surface marked by transversely running fibers which laterally forms a bundle called
middle cerebellar peduncle.
Main Features
- The trigeminal nerve emerges from the anterior surface at its junction with middle cerebellar peduncle.
Presents a basilar sulcus in the midline which lodges basilar artery
- In the groove between Pons and the medulla oblongata, there emerge, from medial to lateral, abducent,
facial and vestibulocochlear nerves.
Posterior surface of the pons is limited laterally by superior cerebellar peduncle and forms the upper part of the
floor of the 4th ventricle.
Main Features:
The floor is divided into symmetrical halves by a median sulcus.
Lateral to this sulcus is an elongated elevation, the medial eminence, which is bounded laterally by a
sulcus limitans.
- Inferior end of medial eminence is slightly expanded to form facial colliculus, which is produced by facial
nerve
- The upper end of sulcus limitans presents a bluish-gray coloration and the area is called substantia
ferruginosa.
Area vestibule lies lateral to sulcus limitans
Parts of the Pons
·
a posterior part, the tegmentum, and
·
an anterior basilar part
Medulla Oblongata
The medulla oblongata is situated in the posterior cranial fossa, lying beneath the tentorium cerebelli
and above the foramen magnum. It is related anteriorly to the basal portion of the occipital bone and the upper
part of the odontoid process of the axis and posteriorly to the cerebellum.
The medulla oblongata not only contains many cranial nerve nuclei that are concerned with vital functions
(e.g., regulation of heart rate and respiration), but it also serves as a conduit for the passage of ascending and
descending tracts connecting the spinal cord to the higher centers of the nervous system.
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Introduction to the brain & Brainstem
The medulla oblongata is conical in shape. Its broad part joins the pons above and narrow part becomes
continuous with the spinal cord. The junction between medulla and spinal cord coincides with the level of the
upper border of atlas (first cervical vertebra).
Its length is about 3 cm and its width is about 2cm at its upper end.
It is divided into
1. A lower closed part with central canal and
2. An upper open part posteriorly which is related to the lower part of the 4th ventricle
Features on the anterior surface of Medulla Oblongata
Anterior median fissure, is an upward continuation of similar fissure present on the spinal cord
Anterolateral sulcus, on each side, is in line with the ventral roots of spinal cord
- Gives attachment to the rootlets of the hypoglossal nerve
Pyramid is an elevation on each side of the midline between anterior median fissure and anterolateral sulcus.
- Composed of bundles of nerve fibers of corticospinal tract that descends from the cerebral cortex to the spinal
cord
- Tapers inferiorly where the majority of fibers cross over to the opposite side, obliterating the medulla. These
crossing fibers constitute the decussation of the pyramid.
Olive is a prominent, elongated oval swelling that lies in the upper part of medulla posterolateral to the pyramid
separated by anterolateral sulcus.The elevation is produced by the underlying inferior olivary nucleus.
Features on posterior surface of the medulla oblongata
Posterior median sulcus is upward continuation of the similar fissure on the spinal cord.
Posterolateral sulcus lies in line with the dorsal roots of spinal nerves.
- Gives attachment to the rootlets of 9th, 10th and 11th cranial nerves.
Between the posterior median sulcus and posterolateral sulcus, the medulla contains tracts (asccending) that
enter it from the posterior funiculus of the spinal cord.
- Fasciculus gracilis lies medially and fasciculus cuneatus lies laterally
- Both fasciculi end in rounded elevations called gracile tubercle (nucleus gracilis) and cuneate tubercle
(nucleus cuneatus) respectively.
Just above these tubercles, medulla is occupied by a triangular fossa which forms the lower part of the 4th
ventricle. This fossa is bounded on each side by inferior cerebellar peduncle which connect the medulla to
cerebellum.
Features on the posterior part of the medulla that forms the floor of the 4th ventricle:
Presents median sulcus, on each side of which there is a longitudinal elevation called the median eminence
(continuous above in the pontine part of the floor of 4th ventricle). The eminence is bounded laterally by sulcus
limitans.
The sulcus limitans is marked by a depression called inferior fovea. The part of the medulla below fovea
presents hypoglossal triangle medially and vagal triangle laterally.
The inferior angle where the lateral margins of the floor meet is called obex.
CRANIAL NERVES
The 12 pairs of cranial nerves are part of the peripheral nervous system (PNS) and pass through
foramina or fissures in the cranial cavity. All nerves except one, the accessory nerve [XI], originate from the
brain. In addition to having similar somatic and visceral components as spinal nerves, some cranial nerves
also contain special sensory (such as hearing, seeing, smelling, balancing, and tasting). The special sensory
components are associated with.
Nuclei of 12 cranial nerves
10 of them in the brainstem
Midbrain Of the IV & III
Pons
Of the other 4 - VIII,VII,VI,V
Medulla Of the last 4 - XII,XI,X, IX
I
Olfactory
Purely sensory
Telencephalon
Smelling
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Introduction to the brain & Brainstem
II
Optic Sensory
Retinal ganglion cells
Seeing
III
Oculomotor Mainly motor
Midbrain
Eye movements & pupillary reflex
IV
Trochlear
Motor
Midbrain
Intorts the eyeball.
V
Trigeminal Both sensory and motor
Pons
Receives sensation from the face and innervates the muscles of mastication
VI
Abducens
Mainly motor Pons
Abducts the eye.
VII
Facial Both sensory and motor
Pons
Provides motor innervation to the muscles of facial expression. Also receives the special sense of taste
from the anterior 2/3 of the tongue and provides secretomotor innervation to the salivary glands (except
parotid) and the lacrimal gland.
VIII
Vestibulocochlear Mostly sensory Pons
Hearing and balance
IX
Glossopharyngeal Both sensory and motor
Medulla
Receives taste from the posterior 1/3 of the tongue, provides secretomotor innervation to the parotid
gland, and provides motor innervation to the stylopharyngeus. Some sensation is also relayed to the brain
from the palatine tonsils.
X
Vagus Both sensory and motor
Medulla
Supplies branchiomotor innervation to most laryngeal and pharyngeal muscles (except the
stylopharyngeus, which is innervated by the glossopharyngeal). Also provides parasympathetic fibers to nearly
all thoracic and abdominal viscera till the proximal two-thirds of the transverse colon. Receives the special
sense of taste from the epiglottis. A major function: controls muscles for voice and resonance and the soft
palate.
XI
Accessory (often separated into the cranial accessory and spinal accessory nerves) Medulla
Mainly motor Cranial and Spinal Roots
Controls the sternocleidomastoid and trapezius muscles, and overlaps with functions of the vagus
nerve (CN X). Symptoms of damage: inability to shrug, weak head movement.
XII
Hypoglossal mainly motor Medulla
Provides motor innervation to the muscles of the tongue (except for the palatoglossus, which is
innervated by the vagus nerve) and other glossal muscles. Important for swallowing (bolus formation) and
speech articulation.
Reticular formation
The reticular formation (L. reticulum, “little net”) consists of various distinct populations of cells embed in a
network of cell processes occupying the central core of the brainstem. From an evolutionary perspective, the
reticular formation is phylogenetically an ancient neural complex that is closely associated with two other
ancient neural systems, the olfactory system which mediates the visceral sense of smell, and the limbic system
which functions in the visceral and behavioral responses to emotions.
The reticular formation consists of an intricate mixture of neuronal cell bodies and fascicles of axons running in
small bundles, which are oriented in many different directions. Thus, as its name suggest, it is like a network
extending from the spinal cord through the medulla, the pons, the midbrain, the subthalamus, hypothalamus and
the thalamus.
The reticular formation and the olfactory and limbic systems are interrelated as a result of their participation in
visceral functions and behavioral responses. The reticular formation is continually informed of activity
occurring in almost all areas of the nervous system and responds by influencing the following: skeletal muscle
motor activity; somatic and visceral sensation; autonomic nervous system; endocrine functions; biological
rhythms, via reciprocal connections to the hypothalamus, and the level of consciousness.
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Introduction to the brain & Brainstem
More than 100 nuclei scattered throughout the tegmentum of the midbrain, pons and medulla have been
identified as being part of the brainstem reticular formation. Most of the nuclei of the reticular formation are
not clearly as defined as are other nuclei of the CNS. Although the nuclei of the reticular formation have a
number of diverse functions, they are classified according to the following four general functions:
1- The regulation of the level of consciousness, and ultimately cortical alertness
2- The control of somatic motor movements
3- The regulation of visceral motor or autonomic functions
4- The control of sensory information
Autonomic Nervous System
The autonomic nervous system is distributed throughout the central and peripheral nervous systems. It is
divided into two parts, the sympathetic and the parasympathetic and, consists of both afferent and efferent
fibers. This division between sympathetic and parasympathetic is made on the basis of anatomical differences,
differences in the neurotransmitters, and differences in the physiologic effects.
The autonomic nervous system and the endocrine system control the internal environment of the body. It is the autonomic
nervous system that provides a fine discrete control over the functions of many organs and tissues, including heart
muscle, smooth muscle, and the exocrine glands.
The autonomic nervous system functions for the most part at the subconscious level. We are not aware, for example, that
our pupils are dilating or that our arteries are constricting. The system should not be regarded as an isolated
portion of the nervous system, for it is known that it can play a role with somatic activity in expressing emotion and that
certain autonomic activities, such as micturition, can be brought under voluntary control. The various activities of the
autonomic and endocrine systems are integrated within the hypothalamus.
The sympathetic part of the autonomic system has the efferent fibers originating from the spinal cord. The function of
the sympathetic system is to prepare the body for an emergency. The heart rate is increased, arterioles of the skin and
intestine are constricted, arterioles of skeletal muscle are dilated, and the blood pressure is raised. There is a redistribution
of blood; thus, it leaves the skin and gastrointestinal tract and passes to the brain, heart, and skeletal muscle. In addition,
the sympathetic nerves dilate the pupils; inhibit smooth muscle of the bronchi, intestine, and bladder wall; and close the
sphincters. The hair is made to stand on end, and sweating occurs.
The activities of the parasympathetic part of the autonomic system are directed toward conserving and
restoring energy. The heart rate is slowed, pupils are constricted, peristalsis and glandular activity is increased,
sphincters are opened, and the bladder wall is contracted.
The connector nerve cells of the parasympathetic part of the autonomic nervous system are located in the brainstem and
the sacral segments of the spinal cord.
Parasympathetic System in the Brainstem
 Edinger-Westfall nucleus in the midbrain (mediates the diameter of the pupil in response to light)
 Superior and inferior salivatory nuclei in the pons and medulla (mediate salivary secretion and the production of tears)
 Dorsal motor nucleus of the vagus nerve in the medulla. The parasympathetic system controls the motor responses of
the heart, lungs, and gut elicited by the vagus nerve (e.g., slowing of the heart rate and constriction of the bronchioles).
The sympathetic and parasympathetic components of the autonomic system cooperate in maintaining the stability of the
internal environment. The sympathetic part prepares and mobilizes the body in an emergency, when there is sudden
severe exercise, fear, or rage.
The parasympathetic part aims at conserving and storing energy, for example, in the promotion of digestion and the
absorption of food by increasing the secretions of the glands of the gastrointestinal tract and stimulating peristalsis.
The sympathetic and parasympathetic parts of the autonomic system usually have antagonistic control over a viscus. For
example, the sympathetic activity will increase the heart rate, whereas the parasympathetic activity will slow the heart
rate. The sympathetic activity will make the bronchial smooth muscle relax, but the muscle is contracted by
parasympathetic activity.
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