BRAIN STEM

● The brain stem is the lower part of the brain, adjoining and structurally
continuous with the spinal cord.

● The brainstem is a stalk-like projection of the brain extending caudally

from the base of the cerebrum. It functions to bridge communication
between the cerebrum with the cerebellum, and spinal cord.

● The brainstem is made of three sections: midbrain, pons, and medulla

oblongata.
● It's role is maintaining functions necessary for life, such as (1) breathing, (2)
consciousness, (3) maintaining blood pressure, (4) heart rate, and (5) sleep
regulation.

● The brainstem contains both collections of white and grey matter:

● The grey matter within the brainstem consist of nerve cell bodies and
include important brainstem nuclei, for example, ten of the twelve cranial
nerves nuclei originate from the brainstem.
● The white matter tracts of the brainstem involve neuron axons traveling
between the cerebrum, cerebellum, and spinal cord to the peripheral
nervous system. These tracts carry information to the brain (afferent
pathways, such as the somatosensory pathways) and from the brain
(efferent pathways such as the corticospinal tract).

● The brainstem is located in the posterior cranial fossa of the skull [4]. The
brainstem can be divided into three regions or components: (1) the
midbrain, (2) the pons, and (3) the medulla.

● The nervous system is a continuous neuronal network and functions in an

continuous manner. However, anatomical distinctions and boundaries of
the brainstem include:

● superiorly (or ventrally): the midbrain is continuous with the cerebral

hemisphere
● inferiorly (or caudally): the medulla is continuous with the spinal cord
● posteriorly: the pons and medulla are separated from the cerebellum by
the fourth ventricle.
 A. Medulla Oblongata

● The brain majorly can be classified into the forebrain, midbrain and the
hindbrain. This lower part of the brain, hindbrain, further can be
distinguished into the Cerebellum, Medulla Oblongata and Pons Varolii.

● The word Medulla Oblongata has its origins from Latin, where “medis”
corresponds to middle and “oblongata” corresponds to long. Hence, the
medulla is a long-stem like structure, piriform, conic resembling. It is found
in the lowest region of the brain chiefly regulating the autonomic functions
such as breathing, heartbeat and digestion. It is critically important as it
connects the spinal cord, pons and the cerebral cortex. Additionally, it
assists to maintain body posture and governs our reflexes.

● The medulla oblongata develops from the myelencephalon. It is the most

caudal brainstem structure situated in the posterior-most part of the
brainstem merging with the spinal cord.

● It is located anterior to the cerebellum in the brain stem, inferior to the

pons. It is the lowest section of the hindbrain. It is situated in the anterior
section of the posterior cranial fossa, stretching down to the foramen
magnum.

Key components of the Medulla oblongata – Anatomy of Medulla Oblongata

Medulla is divided into two major parts –

● Ventral medulla – the front portion

● Dorsal medulla – rear portion. It is also known as the tegmentum

Components:

1. Median fissures

Along the posterior and anterior portions of the medulla, shallow grooves are
found, these are the median fissures.
2. Pyramids

The upper part of the medulla ventrally, is deeply grooved in the midline along
with a bold complexity on either side, these are the pyramids. It is as a result of
the contained cortico-spinal fibres.

3. Olivary bodies

In the upper part of the medulla, the area between the posterolateral and
anterolateral sulcus is distinguished by a swelling, a pair of oval structures
known as the olivary bodies or olives. It contains nerve fibres linking the
cerebellum and pons with the medulla. The swelling is as a result of the large
mass of gray matter, the inferior olivary nucleus.

4. Fasciculus gracilis

The rear end of the medulla, these structures typically are the continuation of the
bundle of nerve fiber tracts extending from the spinal cord to the medulla.

Medulla Oblongata Function

● The medulla is critical in performing some important body functions

pertaining to the regulation of mental, motor and sensory processing. It is
responsible to regulate the autonomic functions of the body while it
connects the higher levels of the brain to that of the 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.
It regules body posture and balance.

● All the medullary centers and nuclei of cranial nerves are controlled by
higher centers, situated in cerebral cortex and hypothalamus.

B. Pons

● The pons is the largest part of the brainstem, located above the medulla and
below the midbrain. It is a group of nerves that function as a connection
between the cerebrum and cerebellum (pons is Latin for bridge).

● The pons develops from the embryonic metencephalon (part of the hindbrain,
 developed from the rhombencephalon), alongside the cerebellum.

● The pons is a horseshoe-shaped collection of nerve fibres located in the

anterior part of the posterior cranial fossa.

● The largest segment of the brainstem is the pons. It connects the descending
and ascending tracts between the spinal cord and brain.

Anterior Surface

● The anterior or ventral surface of the pons is marked by a bulging formed by

the transverse pontocerebellar fibres.

● The pontomedullary junction is an important anatomical landmark defined

by the angle between the lower border of the pons and the superior border of
the medulla.

Several cranial nerves originate from the ventral surface of the pons:

● Cranial nerve V: trigeminal – originates from the lateral aspect of mid pons
● Cranial nerve VI: abducens – originates from the pontomedullary junction,
close to the midline
● Cranial nerve VII: facial – originates from the cerebellopontine angle, the
more lateral aspect of the pontomedullary junction.
● Cranial nerve VIII: vestibulocochlear – originates laterally to the facial nerve.

Posterior Surface

The pons is intimately related to the cerebellum and is connected to it by the

middle cerebellar peduncles.

Structure

The pons is comprised of two major components – the ventral pons and the
tegmentum.

The ventral pons contains the pontine nuclei, which are responsible for
coordinating movement. Fibres from the pontine nuclei cross the midline and
form the middle cerebellar peduncles on their way to the cerebellum.

The tegmentum is the evolutionarily older part of the pons which forms part of
the reticular formation – a set of nuclei found throughout the brainstem that are
responsible for arousal and attentiveness. Damage to this part of the pons may
result in anosognosia for hemiplegia, where patients are unaware of their
paralysis.

The rest of the pons is made up of tracts passing through the pons including:

● Descending corticospinal tracts – responsible for voluntary motor control

of the body.
● Descending corticobulbar tracts – responsible for voluntary motor control
of face, head and neck.
● Ascending medial lemniscus tracts – responsible for fine touch, vibration
and proprioception.
● Ascending spinothalamic tracts – responsible for pain and temperature
sensation.

Functions

● The primary function of the pons is to transmit signals between your

forebrain and cerebellum.

● The pons is essential in sending information to your body, giving you

sensory cues and motor information. This means the pons can help you
taste, touch, and communicate.
● Regulates your breathing. The pneumotaxic center is a bundle of nerve
cells in your pons. They play an essential role in regulating your breathing.
This bundle of nerves handles how much air you breathe and how often
you take another breath. The pons ensures you get enough oxygen whether
sitting, walking, or running.
● Regulates your sleep and wake cycles. Deep sleep is a component of your
sleep and wake cycles regulated by the pons.
● Experiencing sensory input. Since the pons helps send information from
the right and left parts of your brain, it also affects your sensory input and
function. Consequently, injury to the pons can negatively impact your
sensory function and movement.
C. Midbrain

● The midbrain is a part of the central nervous system, located below the
cerebral cortex and at the topmost part of the brainstem.
● Midbrain lies between pons and diencephalon, (derived from the
developmental mesencephalon), which serves as a vital connection point
between the other major regions of the brain - the forebrain and the
hindbrain.
● It is a channel for the spinal cord transmitting stimuli (sensory) from the
head and body to the direct brain.

Divisions

When viewed in cross-section, the midbrain can be divided into three portions:

● Tectum (posterior)
● Tegmentum
● Cerebral peduncles (anterior)

Tectum

● Situated posteriorly, the tectum (Latin for "roof" or "covering") is

composed of the tectal plate and superior and inferior colliculi.
● Coliculli, (collectively the corpora quadrigemina) which sit directly inferior
to the pineal gland.
● The colliculi are separated by the cruciform sulcus; there are two superior
and two inferior colliculi.
● Thus, the 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.

2. Inferior Colliculus
Inferior colliculus consists of single layer of neurons to which the lateral
lemniscus (auditory fibers) synapses. Inferior colliculus is the center for auditory
reflexes. Stimulation of this also produces reflex vocalization.

Tegmentum

The tegmentum is the phylogenetically-old part of the brainstem and runs

through the pons and medulla oblongata. Its contents include:

● A variety of ascending and descending tracts pass through the midbrain (eg
the medial lemniscus and the anterolateral tracts)
● Reticular formation: This highly diverse and integrative area contains a
network of nuclei responsible for many vital functions including arousal,
consciousness, sleep-wake cycles, coordination of certain movements, and
cardiovascular control.
● Spinothalamic tract: This major nerve pathway carries information about
pain and temperature sensation from the body to the thalamus of the brain.
● Corticospinal tract: This major nerve pathway carries movement-related
information from the brain to the spinal cord.
● The red nucleus is one of the brainstem nuclei, part of the extrapyramidal
system, and is a portion of the motor system specifically dedicated to the
modulation and regulation of movement.
● Nuclei for two cranial nerves: cranial nerve III (oculomotor nerve) and
cranial nerve IV (trochlear nerve).
● Part of the raphe nuclei (clusters of serotonin-producing neurons found in
the brainstem that send serotonin throughout the central nervous system);
● Substantia nigra: crescent-shaped mass of nerve cells in the midbrain with
a dynamic role. Although it is the smallest portion of the midbrain, it is
involved in the regulation of gathering auditory and visual sensory
information, motor control (primarily informed by the production and
distribution of dopamine), reward-based learning patterns, and the
Circadian rhythm.
● Ventral tegmental area (VTA): This structure contains dopamine-producing
 cell bodies and plays a key role in the reward system.

Cerebral Peduncles

● Cerebral peduncles: Anterior to the tegmentum are the cerebral peduncles

which are composed of the large ascending and descending tracts that run
to and from the cerebrum.

● The cerebral peduncles contain part of the substantia nigrae, which (like
the ventral tegmental area) contain large collections of dopamine-
producing neurons.
● Periaqueductal gray (PAG) matter: This area plays a primary role in
processing pain signals, autonomic function, and behavioral responses to
fear and anxiety. Recently, this structure has been linked to controlling the
defensive reactions associated with post-traumatic stress disorder (PTSD).

D. Reticular Formation

● The brainstem reticular formation (RF) represents the archaic core of those
pathways connecting the spinal cord and the brain.
● The RF is a complex network of brainstem nuclei and neurons that serve as
a major integration and relay center for many vital brain systems to
coordinate functions necessary for survival.
● The RF forms a net-like connection of nuclei and neurons which work
together with other regions of the central nervous system to allow complex
tasks eg the regulation of our state of consciousness, emotion processing,
visual coordination, cardiovascular control, and posture. Interneurons make
up the vast majority of the neurons in the RF allow this vast connectivity,
receiving approximately 100,000,000 nervous impulses every single second.
● The reticular formation does not contain distinct boundaries, and the many
nuclei included in this structure do not have precise delineations of territory.

Ascending and Descending Pathways

The reticular formation has two systems, an ascending and a descending.

 ● Ascending reticular activating system (ARAS) brings sensory messages
from the RF to the brain cortex and vice versa. The ARAS is the regulator of
consciousness and arousal and other information eg respiration rate,
cough response, cardiac rhythm, and mastication (chewing).
● Descending reticular system (DRS) that brings messages to and from the
motor neurons of the spinal cord, the reticulospinal tracts . These tracts are
the major descending pathways from the reticular formation and act on
many levels of the spinal cord to coordinate movements and autonomic
functions. The reticulospinal tracts project to spinal cord motor neurons
and help to modulate tone, balance, posture, and coordination of body
movements with the assistance of other sensory stimuli, such as visual,
auditory, vestibular, and proprioceptive information.
● Both systems are part of a single parallel system as they work at the same
time with ther RS modulating how many messages are processed.

Functions Include:

● Pain Modulation
The reticular formation is one means by which pain signals from the lower body
reach the cerebral cortex. It is also the origin of the descending analgesic
pathways. The nerve fibers in these pathways act in the spinal cord to block the
transmission of some pain signals to the brain, activating the endogenous opioid
system.

● Muscle Activity
The reticulobulbar and reticulospinal tracts also allow the reticular formation to
have a wide spread impact on skeletal muscles: eg

● Coordinates the activity of the respiratory centres that control the muscles
of respiration.
● Modifies reflex activity and muscle tone
● Aids in the process of standing by working alongside the vestibular
apparatus to preserve muscle tone in the antigravity muscles (antigravity
muscles help maintain an upright, balanced posture)

● Consciousness
The RS is the primary regulator of arousal and consciousness, suppresses the
individual’s level of consciousness during sleep. Efferent fibers from the reticular
formation can convey sensory information to the cortex of a sleeping individual,
which would awaken that person. With injury or pathological insult to areas of
the RF, periods of unconsciousness may result. Bilateral damage to the reticular
formation of the midbrain may lead to coma or death.

● Habituation
Habituation is a process in which the brain learns to ignore repetitive,
meaningless stimuli while remaining sensitive to others. For example, when a
person can sleep through loud traffic in a large city, but is awakened promptly by
the sound of a crying baby. The reticular activating system contains RF nuclei
that modulate activity of the cerebral cortex, thus responsible for habituation.

● Autonomic and Endocrine System

The autonomic and endocrine nervous system is subject to regulation by the
reticular formation.

● The descending reticulospinal and reticulobulbar fibers are involved with

the craniosacral (parasympathetic) and thoracolumbar (sympathetic)
outflows.
● The reticular formation indirectly regulates the endocrine nervous system
by acting on the hypothalamus to regulate hormonal release.

E. Cerebellum

● The cerebellum, which stands for ‘little brain,’ is a hindbrain structure that
controls balance, coordination, movement, and motor skills, and it is
thought to be important in processing some types of memory.
● In psychology, the cerebellum is often defined as a brain region
responsible for coordinating and refining motor movements, ensuring
balance and posture, and facilitating procedural learning.
● The cerebellum is also one of the few mammalian brain structures where
adult neurogenesis (the development of new neurons) has been confirmed.

Location

● The cerebellum is located at the back of the brain, behind the brainstem,
below the temporal and occipital lobes, and beneath the back of the
cerebrum.
 ● The cerebellum is divided into two hemispheres.

Structure

● The cerebellum consists of the cerebellar cortex, the outer layer, containing
folder brain tissue, filled with most of the cerebellum’s neurons. It plays a
key role in processing and integrating information sent to the cerebellum.
● There is also a fluid-filled ventricle and cerebellar nuclei, which is the
innermost part, containing neurons that communicate information from
the cerebellum to other areas of the brain.
● The cerebellum’s afferent and efferent nerve fibres make up the white
matter. The cerebellar nuclei, which are grey masses, are found in the
white matter.

Within the cerebellum, there are thought to be three anatomical lobes, which are
divided by two fissures (large furrows)– the primary fissure and the
posterolateral fissure:

● Anterior lobe (anterior meaning ‘to the front’): Primarily involved in the
coordination of limb movements.
● Posterior lobe (posterior meaning ‘to the back’): The largest part and plays
a significant role in planning, initiating, and timing movements.
● Flocculonodular lobe: The oldest part of the brain in terms of evolution.
This part of the cerebellum is responsible for balance and spatial attention,
as well as receiving visual input.

The cerebellum can also be divided into three functional areas:

● Cerebrocerebellum – the largest division, formed by the lateral

hemispheres. It is involved in planning movements and motor learning. It
receives inputs from the cerebral cortex and pontine nuclei, and sends
outputs to the thalamus and red nucleus. This area also regulates
coordination of muscle activation and is important in visually guided
movements.

● Spinocerebellum – Spinocerebellum is connected with the spinal cord and

hence the name. Body and limb motions are regulated by the
spinocerebellum. It gets proprioceptive information from the trigeminal
 nerve, the dorsal columns of the backbone, the visual and auditory systems.
It transmits fibres to the brain stem and cerebral cortex from deep
cerebellar nuclei in order to modulate descending motor systems. This area
functions in regulating body movements by allowing for error corrections.

● Vestibulocerebellum – the functional equivalent to the flocculonodular lobe.

It is involved in controlling balance and ocular reflexes, mainly fixation on
a target. It receives inputs from the vestibular system, and sends outputs
back to the vestibular nuclei.

Functions

● The cerebellum, located at the base of the brain, is responsible for

coordinating voluntary movements, maintaining posture, balance, and
equilibrium, as well as refining motor movements to be smooth and
precise. It also plays a role in some cognitive functions, such as attention
and language processing.

● The cerebellum’s main role is to monitor and regulate motor behavior

without any need for conscious awareness.
● The cerebellum also aids in balance and posture. It monitors information
regarding balance and posture to ensure that when we are standing or
walking, we are not falling down and we are able to keep ourselves steady.
● In terms of motor learning, the cerebellum is vital when learning a new
skill. For instance, if someone were learning to ride a bike for the first time,
you may expect they would typically start off making mistakes and falling
off the bike.
● The cerebellum helps to fine-tune the motor skills required to ride a bike
until they get to a point where the action can be completed seamlessly and
almost automatically.