BRAIN STEM FUNCTIONS

CNS BLOCK

Objectives

• Describe the components of Brain stem.• Enumerate different functions of brainstem. • Know the reticular formation and its activating

& inhibiting system• Role of brainstem in brain death.

Brain Stem• Critical connecting link between spinal cord and

higher brain regions• Oldest region of the brain• Continuous with spinal cord• All incoming and outgoing fibers traversing

between periphery and higher brain centers must pass Brain Stem

• Consists of– Medulla– Pons – Midbrain

BRAINSTEM

• Most of the cranial nerves come from the brainstem. The brainstem is the pathway for all fiber tracts passing up and down from peripheral nerves and spinal cord to the highest parts of the brain.

Cranial Nerves

Cranial NervesNo. Cranial Nerve Major Functions

I Olfactory smell

II Optic vision

III Oculomotor eyelid and eyeball movement

IV Trochlear innervates superior oblique turns eye downward and laterally

V Trigeminal chewing face & mouth touch & pain

VI Abducens turns eye laterally

VII Facialcontrols most facial expressions secretion of tears & saliva taste

Cranial NervesNo. Cranial Nerve Major Functions

VIIIVestibulocochlear (auditory)

hearing equilibrium sensation

IX Glossopharyngeal taste senses carotid blood pressure

X Vagussenses aortic blood pressure slows heart rate stimulates digestive organs taste

XI Spinal Accessory controls trapezius & sternocleidomastoid controls swallowing movements

XII Hypoglossal controls tongue movements

The brainstem controls several important functions of the body including:

• Alertness• Arousal• Breathing• Blood Pressure• Digestion• Heart Rate• Other Autonomic Functions• Relays Information Between the Peripheral Nerves

and Spinal Cord to the Upper Parts of the Brain

MID BRAIN

• Visual reflex center• Auditory reflex center • eye movement, and • body movement.

Pons

1. It is involved in motor control and sensory analysis... for example, information from the ear first enters the brain in the pons.

2. It has parts that are important for the level of consciousness and for sleep.

3. Some structures within the pons are linked to the cerebellum, these are involved in movement and posture.

Pons

4. The pons is a bridge-like structure which links different parts of the brain and serves as a relay station from the medulla to the higher cortical structures of the brain. It contains the respiratory center.

Medulla Oblongata

• The medulla oblongata functions primarily as a relay station for the crossing of motor tracts between the spinal cord and the brain. It also contains the respiratory, vasomotor and cardiac centers, as well as many mechanisms for controlling reflex activities such as coughing, gagging, swallowing and vomiting.

Respiratory Centers• Dorsal respiratory group (DRG)

– Contains inspiratory center– Functions in every respiratory cycle

• Ventral respiratory group (VRG)– Contains expiratory center– Used only during forced breathing

• Pneumotaxic center in pons, play a role in switching between inspiration and expiration

• Apneustic center in pons, promotes inspiration by stimulating the inspiratory neurons in the medulla.

Respiratory Center

Chemoreceptor Reflexes

• Respond to chemical changes in blood, CSF• Centers:

– Carotid bodies and aortic bodies (peripheral)• Sensitive to pH, PCO2, PO2 in arterial blood

– Medulla (central chemoreceptor)• Sensitive to pH and PCO2 in CSF• Stimulation

• Stimulation: increase in depth and rate of respiration

Chemoreceptor Reflexes

• Much more sensitive to changes in PCO2 than PO2 – Due to small increase in PCO2 stimulates receptors

(regulates under normal conditions)• PO2 generally does not decline enough to stimulate

receptor

– Cannot hold your breath

VASOMOTOR CENTER

VASOMOTOR CENTER

THE DEGLUTITION CENTER

• The deglutition (swallowing) center are areas in the medulla and pons that respond to the incoming impulses by sending motor signals back to the pharynx and the upper esophagus (swallowing reflex). These motor impulses travel via the cranial nerves (V, IX, X, XII) and a few superior cervical nerves.

The Reticular Formation

• it can influence the heart rate, respiration, posture and equilibrium.

• The so-called reticular activating system plays a critical role in sustaining consciousness. It regulates sleep and wakefulness.

• The mechanism that triggers REM sleep is located in the pontine reticular formation.

Activating Systems of the Brain

• Cerebrum requires a constant input to remain active.

• Signals from the brainstem activate wide areas of the cortex (background activation) or specific areas to perform discrete tasks.

Excitatory Signals from the Brainstem

• Bulboreticular facilitory area- sends excitatory signals to the antigravity

muscles- sends excitatory signals to the thalamus and

from here they are distributed to widespread areas of the cortex

• Bulboreticular area is excited by signals from the periphery, especially pain signals and also signals from the cortex (positive feedback).

Inhibitory Signals from the Brainstem

• reticular inhibitory area– sends inhibitory signals to the bulboreticular

area– when the inhibitory area is excited, it will

decrease the activity of the excitatory area and decrease the activity of the cortex

Location of excitatory

and inhibitory areasof the brain

Brain stem and essential functions for life

• The brain stem regulates most of the automatic functions of the body that are essential for life. Examples are:

• breathing • heartbeat • blood pressure

• The brain stem relays information to and from the brain to the rest of the body.

• It plays a very important role in the brain’s core functions, such as consciousness, awareness and movement.

• Once brain death has occurred, there is no possibility for consciousness

Brain stem death

Brain death occurs when a person no longer has any activity in their brain stem and no potential for consciousness

• Brain death can occur when the blood and oxygen supply to the brain is stopped for example due to cardiac arrest, cerebrovascular accident etc

• When brain stem function is permanently and irreversibly lost, the person will be confirmed dead

• A ventilator machine may maintain ventilation and oxygenation and keep the heart beating and oxygen circulating through the blood

• But once the brain stem has permanently irreversibly stopped functioning the heart will eventually stop beating even if a ventilator has been used

• Brain death is considered as a clinical syndrome charcterized by the absence of reflexes that have pathways through the brain stem in a deeply comatose, ventilator-dependent, patient.

Brain stem death

• The diagnosis of brain death may be considered (but needs to be confirmed) when:

• A person fails to respond to any outside stimulation.

• The person is unconscious.

• The person's heartbeat and breathing can only be maintained using a ventilator.

• There is clear evidence that serious damage to the brain has occurred, and it cannot be cured.

• Reversible causes and factors such as drug overdose, severe hypothermia, severe hyperglycaemia etc have been excluded

Brain stem death

• Rarely, there are some conditions that can make it appear as though someone is brain dead, such as: drug overdose, severe hypothermia, severe hyperglycaemia. These may be reversible.

• So various tests can be carried out to find out whether brain death has actually occurred for example: pupils reaction to light, cold calorics test, corneal reflex, gag response etc.

Summary• Functions

• Most of cranial nerves arise from brain stem• Neuronal clusters within brain stem control heart and blood

vessel function, respiration, and many digestive functions• Plays role in regulating muscle reflexes involved in equilibrium

and posture• Reticular formation within brain stem receives and integrates

all incoming sensory synaptic input • Reticular activating & inhibiting system activate or inhibit the

activity of higher centers• Centers that govern sleep are in brain stem (evidence suggests

center promoting slow-wave sleep lies in hypothalamus) • Brain stem death due to vital centers in mid brain