The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. The brainstem is the structure that connects the cerebrum of the brain to the spinal cord and cerebellum. The brainstem is a stalk-like projection of the brain extending caudally from the base of the cerebrum. It is one of the most ancient parts of the brain.

The brainstem is very small, making up around only 2.6 percent of the brain's total weight. It has the critical roles of regulating heart and respiratory function, helping to control heart rate and breathing rate. It also provides the main motor and sensory nerve supply to the face and neck via the cranial nerves. Ten pairs of cranial nerves come from the brainstem. Other roles include the regulation of the central nervous system and the body's sleep cycle. It is also of prime importance in the conveyance of motor and sensory pathways from the rest of the brain to the body, and from the body back to the brain. These pathways include the corticospinal tract (motor function), the dorsal column-medial lemniscus pathway (fine touch, vibration sensation, and proprioception), and the spinothalamic tract (pain, temperature, itch, and crude touch).

It is composed of three sections in descending order: the midbrain, pons, and medulla oblongata.

Midbrain

The midbrain is the topmost part of the brainstem, the connection central between the brain and the spinal cord.

The midbrain is further subdivided into three parts: tectum, tegmentum, and the ventral tegmental area. The tectum forms the ceiling. The tectum comprises the paired structure of the superior and inferior colliculi and is the dorsal covering of the cerebral aqueduct.

The tegmentum which forms the floor of the midbrain, is ventral to the cerebral aqueduct. Several nuclei, tracts, and the reticular formation are contained here.

The ventral tegmental area (VTA) is composed of paired cerebral peduncles. These transmit axons of upper motor neurons.

Pons

The pons lies between the midbrain and the medulla oblongata. It is separated from the midbrain by the superior pontine sulcus, and from the medulla by the inferior pontine sulcus. It contains tracts that carry signals from the cerebrum to the medulla and to the cerebellum and also tracts that carry sensory signals to the thalamus. The pons is connected to the cerebellum by the cerebellar peduncles. The pons houses the respiratory pneumotaxic center and apneustic center that make up the pontine respiratory group in the respiratory center. The pons co-ordinates activities of the cerebellar hemispheres. The pons and medulla oblongata are parts of the hindbrain that form much of the brainstem.

Medulla Oblongata

The medulla oblongata, often just referred to as the medulla, is the lower half of the brainstem continuous with the spinal cord. Its upper part is continuous with the pons. The medulla contains the cardiac, dorsal and ventral respiratory groups, and vasomotor centres, dealing with heart rate, breathing and blood pressure. Another important medullary structure is the area postrema whose functions include the control of vomiting.