Midbrain Anatomy and Function

Brainstem Structure

• The brainstem consists of midbrain, pons, and medulla oblongata
• The superior colliculi are part of the visual pathway, controlling reflex movements of the eyes, head, and neck in response to visual stimuli
• The inferior colliculi are part of the auditory pathway, controlling reflex movements of the head, neck, and trunk in response to auditory stimuli

Midbrain

• The midbrain is the shortest brain stem segment, approximately 2 cm in length
• It connects the pons and cerebellum with the forebrain
• The midbrain comprises two lateral halves, the cerebral peduncles, each divided into an anterior part (crus cerebri) and a posterior part (tegmentum) by the substantia nigra
• The tegmentum contains the red nucleus, which receives information from the cerebrum and cerebellum and issues subconscious motor commands concerned with muscle tone and posture
• The substantia nigra secretes dopamine to inhibit the excitatory neurons of the basal nuclei

Pons

• The pons is anterior to the cerebellum and connects the medulla oblongata to the midbrain
• It is approximately 2.5 cm long and appears as a bridge connecting the right and left cerebellar hemispheres
• The anterior surface is convex and shows many transverse fibers that converge on each side to form the middle cerebellar peduncle
• The basilar groove is a shallow groove in the midline that lodges the basilar artery

Medulla Oblongata

• The medulla oblongata connects the pons superiorly with the spinal cord inferiorly
• The junction of the medulla and spinal cord is at the origin of the anterior and posterior roots of the first cervical spinal nerve
• The medulla oblongata is conical in shape, with its broad extremity directed superiorly
• The central canal of the spinal cord continues upward into the lower half of the medulla, expanding as the cavity of the fourth ventricle

Functions

• The brainstem serves as a conduit for the ascending tracts and descending tracts connecting the spinal cord to the higher centers in the forebrain
• It contains important reflex centers associated with the control of respiratory and cardiovascular functions and consciousness
• It contains the important nuclei of cranial nerves III through XII