brainstem

 Brain stem includes-

Mid brain

Pons

Medulla oblongata

Reticular formation

1. Mid brain/mesencephalon

Mid brain  is area of brain situated around cerebral aqueduct between cerebral above and pons below.

It consists of nuclei and nerve fibers (tracts) which connect cerebrum with lower parts of brain and with spinal cord.

Nerve cells in mid brain act as relay station for ascending and descending nerve fibers.

2. Pons

Pons is situated in front of cerebellum, below the mid brain and above the medulla oblongata.

It consist mainly nerve fibers (white mater)  that form a bridge between two hemisphere of cerebellum and of fibers passing between higher levels of brain and spinal cord.

There are nuclei within pons that act as relay station, other nuclei in pons are pneumotaxic centre and apneustic center. These areas help to control breathing along with medullary respiratory area.

Pons also have nuclei associated with cranial nerves.

The anatomical structure of pons differs from that of cerebrum, in that cell bodies (grey mater) lie deeply and nerve fibers are on surface.

3. Medulla oblongata

Lower part of mid brain. Length; 2'5cm

Medulla oblongata extends from pons above and is continuous with spinal cord below and extends from foramen magnum to the pons.

The central canal of spinal cord continuous upward into medulla.

Both medulla and pons help form ventricle wall of fourth ventricle.

The outer aspect is composed of white mater, which passes between brain and spinal cord and grey mater, which lies centrally.

Network of grey mater and white mater called reticular formation.

Medulla oblongata

1. External features

Anterior 

posterior

2. Internal features

Pyramid decussation; crossing

Sensory decussation

Olive

Reticular formation

Medulla oblongata has special features;

1. Decussation of pyramids

Control motor activity; cerebrum to spinal cord

In medulla, motor nerves descending from the motor area in cerebrum to spinal cord in pyramidal tracts cross from oneside to another .

This means processing of impulse from right side to left side or from left side to right side occurs in medulla. That's why left hemisphere of cerebellum control the right half of body and right hemisphere of cerebrum control the left half of body.

These tracts are main pathway to skeletal (voluntary muscle)

2. Sensory decussation

Control sensory activity; spinal cord to cerebrum

Some of sensory nerves ascending to cerebrum from spinal cord cross from one side to the other in medulla.

3. Olive

Situated lateral to each pyramid is an oval shaped swelling called Olive.

In medulla oblongata vital centers, consisting group of cell bodies (nuclei) associated with autonomic reflex center, these are-

Cardiovascular center

Respiratory center

Reflex center

Cardiovascular center

The cardiovascular center in medulla oblongata control the rate of cardiac contraction. It also control blood pressure.

Respiratory center

The respiratory centre in medulla oblongata control the rate and depth of breathing and maintain respiratory rhythm.

Reflex center

Activate reflex in response of foreign bodies.

Connection made between afferent and efferent neuron.

Protect body from harm, irritants present in stomach and respiratory track stimulate the medulla oblongata, activating the reflex center. The vomiting, coughing, sneezing, reflex start to expel the irritants outside.

Reticular formation

Reticular formation is a collection of neurons in core of brainstem, surrounded by neural pathway that conduct ascending ( sensory) and descending ( motor) nerves impulse between brain and spinal cord.

Reticular formation is set of interconnected nuclei that are located central core of medulla oblongata, pons, and mid brain.

Function of reticular formation

Reticular formation is involved in-

Coordination of skeletal muscle activity associated with voluntary motor movement and maintenance of balance.

Coordination of activity controlled by autonomic nervous system. For example, cardiovascular, respiratory and gastrointestinal activity.

Reticular formation and its connections system called reticular activating system (RAS) which is responsible for regulating arousal and sleep-wake transition. RAS passes sensory information to cerebral cortex.

Reticular formation passes pain signal from lower body to cerebral cortex.

RAS input from touch, pain, pressure, temperature receptor to cerebral cortex. Ascending reticular formation. Descending motor signal to spinal cord.

Reticular formation nuclei that modulate activity to cerebral cortex are called RAS.

For an example; Brain learn to ignore repetitive meaningless stimuli while remaining sensitive to other.  

Person can sleep loud traffic place, but awakened promptly due to alarm or crying baby.