Brainstem

The brainstem is a vital, stalk-like structure located at the posterior of the brain.

It serves as the essential bridge connecting the cerebrum to the spinal cord. Despite its critical importance, the brainstem is quite small in scale. It accounts for only approximately 2.6 percent of the total brain weight. This small area manages life-sustaining functions like heart rate and breathing. It also regulates the body's sleep cycle and levels of consciousness.

The brainstem functions as a major conduction pathway for the nervous system. It carries motor and sensory signals between the brain and the rest of the body. For example, the corticospinal tract transmits motor functions downward. The dorsal column-medial lemniscus pathway carries sensations like fine touch, vibration, and proprioception. Additionally, the spinothalamic tract carries signals for pain, temperature, itch, and crude touch. Ten pairs of cranial nerves also emerge from the brainstem. These nerves provide the primary motor and sensory supply to the face and neck.

The structure is divided into three distinct anatomical regions. The uppermost section is the midbrain, which connects to the thalamus.

Below the midbrain lies the pons, which sits between the midbrain and the medulla. The lowest part is the medulla oblongata, which is continuous with the spinal cord. The pons is connected to the cerebellum by the cerebellar peduncles. The midbrain is further organized into the tectum, the tegmentum, and the ventral tegmental area.

The midbrain contains several specialized nuclei and pathways. The tectum acts as a ceiling and includes the superior and inferior colliculi. The superior colliculus is involved in vision, while the inferior colliculus is a principal nucleus for the auditory pathway. Below the tectum is the tegmentum, which contains various nuclei and the reticular formation. The substantia nigra pars compacta is a concentration of neurons in the ventral midbrain. This area uses the neurotransmitter dopamine to regulate motor function and emotion. Dysfunction in this specific area is implicated in Parkinson's disease.

The pons serves as a critical relay and control center. It contains tracts that carry signals from the cerebrum to the medulla and the cerebellum. It also carries sensory signals upward to the thalamus. Within the pons, the respiratory pneumotaxic center and the apneustic center are located. These two areas make up the pontine respiratory group. This group helps coordinate the breathing process. The pons also helps coordinate the activities of the cerebellar hemispheres.

The medulla oblongata manages several involuntary bodily processes. It contains the cardiac, dorsal, and ventral respiratory groups. It also houses vasomotor centers that regulate blood pressure and heart rate. A specific structure called the area postrema is located here to control vomiting. In the medulla, the pyramids contain fibers of the corticospinal tract. These fibers travel downward to synapse with lower motor neurons in the spinal cord. The medulla also features the olives, which are swellings containing various nuclei.

Developmentally, the brainstem emerges from the neural tube. The midbrain originates from the mesencephalon, which is a primary brain vesicle. The rest of the brainstem comes from the rhombencephalon, or the hindbrain. The rhombencephalon differentiates into the metencephalon and the myelencephalon. The metencephalon eventually becomes the pons and the cerebellum. The myelencephalon becomes the medulla oblongata.

Blood supply is essential for maintaining brainstem function. The main supply comes from the vertebral arteries and the basilar arteries.

There is often variability in how these arteries connect. This variability can lead to different syndromes if specific vessels are excluded from the blood supply. The brainstem relies on this constant flow to regulate consciousness, attention, and autonomic reflexes.