PHYSIOLOGY OF THE

BRAIN STEM

Dr Syed Shahid Habib

Professor & Consultant Clinical Neurophysiology
Dept. of Physiology
College of Medicine & KKUH
King Saud University
 OBJECTIVES
At the end of this lecture the student should
be able to :
Enumarate Components of Brain stem
List Important structures in brain stem
DescribeFunctions of the Brain Stem
Describe Signs & Symptoms of brain stem
lesion
Understand brain stem function tests
 A sample Case
A 58 y/o was referred to you because of recent
onset of left hemiparesis, left-sided loss of
propioception and right-sided tongue
deviation. What CNS structures are affected?

Explain the symptoms with regards to

structures affected.
Where is the lesion?
FIGURE 8A: Brainstem 3 — Cranial Nerves Nuclei — Motor
FIGURE 8B: Brainstem 4 — Cranial Nerves Nuclei — Sensory
 Functions of the Brain Stem

Though it is small, brain stem is an extremely

important part of the brain:
1. A Conduct structure (way station) .
2. Provides the origin of the cranial nerves
(CN III-XII).
3. Conjugate eye movement motor coordination of the
eyes that allows for bilateral fixation on any object.

4. Many many Integrative functions.

 Role of the Brain Stem in
Controlling Motor Functions

1. Control of respiration
2. Control of the cardiovascular system
3. Partial control of gastrointestinal function
4. Control of many stereotyped movements
of the body
5. Control of equilibrium
6. Control of eye movements
 Brain Stem Motor Functions (cont.)
•Brain stem serves as a way station for “command signals” from higher
neural centers.

•The autonomic nervous system is activated mainly by centers

located in the spinal cord, brain stem, and hypothalamus (Cardiovascular
Gastrointestinal Autonomic Reflexes.)

•Functions of Brain Stem Nuclei in Controlling Subconscious, Stereotyped

Movements (anencephaly)
•Motor branch of the fifth cranial nerve, and the chewing process is controlled by nuclei
in the brain stem and also swallowing, salivary secretion, vomiting
(chemoreceptor trigger zone)

•The actual mechanics of feeding are controlled by centers in the brain stem.

•Vasomotor center for CV control (Baroreceptors) in medulla

•Brain stem Neurohormonal Systems in the human brain for activating four
neurohormonal systems

•Many of the behavioral functions elicited from the hypothalamus and other limbic
structures are also mediated through the reticular nuclei in the brain stem and their
associated nuclei.
 Brain Stem Motor Functions (cont.)
• Although the micturition reflex is an autonomic spinal cord reflex, it can
also be inhibited or facilitated by centers in the cerebral cortex or brain stem
in pons

• Accommodation Is Controlled by Parasympathetic Nerves by 3rd CN

• Neural Pathways for Control of Eye Movements. also shows brain stem
nuclei for the third, fourth, and sixth cranial nerves by medial longitudinal
fasciculus

• Auditory Nervous Pathways → superior olivary nucleus

• Nucleus of tractus solitarious→ Taste pathway→Sup & Inf Salivatory
Nuclei

• Bulboreticular facilitatory area of brain stem for gamma efferent system

(stabilizes joints)
• Control of Cerebral Activity by Continuous Excitatory Signals from the Brain
Stem (Reticular Excitatory Area of the Brain Stem→bulboreticular
facilitory area→it is the same brain stem reticular area that transmits
facilitorysignals downward to the spinal cord to maintain tone in the
antigravity muscles and to control levels of activity of the spinal cord reflexes.
 1. Conduct functions (way station)
All information related from the body to the
cerebrum and cerebellum and vice versa,
must traverse the brain stem.
a) The ascending sensory pathways coming from the body to the brain includes:
The spinothalamin tract for pain and temperature sensation.
The dorsal column, fasciculus gracilis, and cuneatus for touch,
proprioceptive and pressure sensation.
b) Descending tracts
The corticospinal tract (UMN): runs through the crus cerebri, the basal
part of the pons and the medullary pyramids; 70-90 % of fibers cross in
the pyramidal decussation to form the lateral corticospinal tract,
destined to synapse on lower motor neurons in the ventral horn of the
spinal cord.
Upper motor neurons that originate in the brain stem's
vestibular, red, and reticular nuclei, which also descend and
synapse in the spinal cord.
• CN III (oculomotor) • CN IX (glossopharyngeal):
• CN IV (trochlear) Tastes, salivates, swallows,
monitors carotid body and
Both moves eyes; CN III
sinus.
constricts the pupils,
accommodates. • CN X (vagus): Tastes, swallows,
lifts palate, talks,
communication to and from
thoraco-abdominal viscera.
• CN V (trigeminal): Chews and
• CN XI (accessory): Turns head,
feels front of the head.
lifts shoulder.
• CN VI (abducens): Moves eyes.
• CN XII (hypoglossal): Moves
• CN VII (facial): Moves the face, tongue.
tastes, salivates, cries.
• CN VIII (acoustic): Hears,
regulates balance.
 INTEGRATIVE FUNCTIONS
• It controls consciousness & sleep cycle (alertness and arousal) through
reticular formation.
• It has got center for cardiovascular, respiratory & autonomic nervous
system.
• It has centers for cough, gag, swallowing and vomiting.
• Sense of body balance (Vestibular functions)
• Substantia nigra which is a part of the basal ganglia is present in
midbrain and is involved in control of movements.
• Midbrain also contain red nucleus which regulate the motor activity
through cerebellum.
• Inferior and superior colliculi are situated on the dorsal surface of the
midbrain and is involved in auditory & visual processing required for head
movements.
• Pain sensitivity control: Periaqueductal grey matter of mesencephalon
is an area which is rich in endogenous opioid and is important in
modulation of painful stimuli.
o Ventral layer of brainstem is motor in function.
o Middle layer is sensory in function & contains medial
lemniscus which conveys sensory information from
dorsal column.

Basic
Structure
of Brain
Stem
1. Roof Plate
2. Tegmentum
3. Basal Portion
 Functions of Midbrain
• Nerve pathway to cerebral hemispheres.
• Auditory and Visual reflex centers.
• Cranial Nerves:
• CN III - Oculomotor [motor]. (Related to eye movement).
• CN IV - Trochlear [motor]. (Superior oblique muscle of
the eye which rotates the eye down and out).
Signs & Symptoms of midbrain lesion
CN (Cranial Nerve) Deficits: Ipsilateral CN III, CN IV palsy and ptosis (drooping).
Pupils: Size: Midposition to dilated.
Reactivity: Sluggish to fixed.
Movement: Abnormal extensor.
Respiratory: Hyperventilating.
Loss of consciousness (LOC): Varies
 Functions of pons
• Respiratory Center.
• Cranial Nerves:
CN V - Trigeminal [motor and sensory]. (Skin of face, tongue,
teeth; muscle of mastication).
CN VI - Abducens [motor]. (Lateral rectus muscle of eye which
moves eye laterally).
CN VII - Facial [motor and sensory]. (Muscles of expression).
CN VIII - Acoustic [sensory]. (Hearing)

Signs & Symptoms of pons lesion

• Pupils size: Pinpoint
• LOC: Semi-coma
• Movement: Abnormal extensor.
• Respiratory: -Apneustic (Abnormal respiration marked by sustained
inhalation). -Hyperventilation.
• CN Deficits: CN V, CN VI, CN VII, CN VIII.
 Functions of medulla oblongata
• Crossing of motor tracts.
• Cardiac Center.
• Respiratory Center.
• Vasomotor Center (nerves having muscular control of
the blood vessel walls)
• Centers for cough, gag, swallow, and vomit.
• Cranial Nerves:
• CN IX - Glossopharyneal [mixed]. (Muscles & mucous membranes of
pharynx, the constricted openings from the mouth & the oral pharynx
and the posterior third of tongue).
• CN X - Vagus [mixed]. (Pharynx, larynx, heart, lungs, stomach).
• CN XI - Accessory [motor]. (Rotation of the head and shoulder).
• CN XII - Hypoglossal [motor]. (Intrinsic muscles of the tongue).
 Signs and symptoms of lesion in medulla
• Movement: Ipsilateral paralysis.
• Pupils: Size: Dilated. Reactivity: Fixed.
• Respiratory: Abnormal breathing patterns
• CN Palsies: Inability to control movement. Absent
cough & gag reflex.
• LOC: Comatose.
To test reticular formation
Alertness, Consciousness & Sleep.
Corticospinal tract
Motor power, reflexes
Pain response
Facial grimacing on firm pressure over the supra
orbital ridge.
To test respiratory center
look for the normal pattern of respiration
To test cardiovascular functions
Look for normal circulatory function
To test brainstem reflexes:
• Pupilary and corneal reflexes.
• Vestibulo-ocular reflex: Injection of iced
water into the ear will produce eyes
movement.
• Oculo-cephalic reflex: Eyes will be fixed
when head is moved in one or another
directions.
• Gag reflex.
• Cough reflex
 Excitation of the Excitatory Area by
Peripheral Sensory Signals.

A Reticular Inhibitory Area Located in the Lower Brain Stem

Neurohormonal Systems in the Human Brain. Brain stem areas in the
human brain for activating four neurohormonal systems,

1. The locus ceruleus and the

norepinephrine system. important role in
causing dreaming, thus leading
to a type of sleep called rapid eye
movement sleep (REM sleep).

2. The substantia nigra and the dopamine

system.

3. The raphe nuclei and the serotonin

system. At Cord supress pain and in higher
centers cause normal sleep

4. The gigantocellular neurons of the

reticular excitatory area and the
acetylcholine system.
Activation of these acetylcholine neurons
leads to an acutely awake and excited
nervous system.