The brainstem (Latin: truncus encephali) lies in the posterior cranial fossa and is the distal part of the brain. The brainstem faces the clivus and dorsum sellae ventrally, while dorsally – the cisterna magna, fourth ventricle, and cerebellum. The brainstem is 6 -7.5 cm long and 3-4 cm wide. 

The brainstem works as a connecting passage between the spinal cord and the cerebrum, between the cerebrum and cerebellum, and between the spinal cord and the cerebellum. The brainstem is divided into the medulla oblongata, pons, and midbrain. All parts of the brainstem regulate heart rate, respiration, blood pressure, and other essential functions. The brainstem contains many cranial nerve nuclei, tracts, and neural pathways traveling through. Between the brainstem’s tracts and cranial nuclei is the reticular formation consisting of neurons.

The medulla oblongata is the lower part of the brain that appears as a swelling at the upper end of the spinal cord. Besides being a conduit for fibers running between the spinal cord and higher brain regions, it contains control centers for involuntary functions such as blood pressure, breathing, swallowing, and vomiting.

The medulla oblongata is the end part of the brainstem. The medulla oblongata lies in the posterior cranial fossa. The upper part of the medulla oblongata continues as the pons, while the lower part – as the spinal cord. The medulla oblongata has two surfaces – ventricular (anterior) and dorsal (posterior) surface.

The pons is the middle segment of the brainstem located above the medulla oblongata. The pons lies in the posterior cranial fossa. Like medulla oblongata, also the pons houses nuclei and has tracts passing through them. Tracts within the pons not only travel to the medulla oblongata but also connect the cerebrum with the cerebellum. 

An angle between the medulla oblongata and pons is known as the pontomedullary junction. Another angle between the pons, cerebellum, and medulla oblongata is seen – the cerebellopontine angle. The functions of the pons are different, like sleep, swallowing, respiration, hearing, taste.

The midbrain or mesencephalon is the uppermost part of the brainstem. The midbrain lies between the pons below and the thalamus above. The midbrain takes part in regulating eye movement, visual and auditory functions, temperature regulation. The midbrain has two parts separated by the cerebral aqueduct. These parts are the anterior part and the posterior part.

The reticular formation is made of neurons spread between the spinal cord and the thalamus and branching connections also to the brainstem. The nuclei of the reticular formation can be found in the brainstem. These nuclei are divided into three groups: lateral, medial, and median.