Medulla oblongata, or simply medulla, is present at the lowermost portion of the brainstem. It connects pons to the midbrain, is continuous with the spinal cord posteriorly, and it merges with the foramen magnum at the base of the skull.
It is a tube or funnel shaped structure that is wide at the top and connected to pons. The primary function of the medulla is to connect the spinal cord with higher parts of the brain and control autonomic activities such as breathing, blood pressure and heart rate.
Anatomy
Our brain is divided into three major parts: forebrain, midbrain and hindbrain. The forebrain is composed of cerebral hemispheres, the midbrain is composed of the tectum and tegmentum, and lastly, the hindbrain is composed of medulla oblongata, pons and cerebellum.
The pons is a portion of the brainstem that connects the medulla to the cerebellum. It functions to transmit sensory information and motor impulses to and from the brain to the facial regions. The cerebellum, on the other hand, is situated below the cerebral hemispheres and above the medulla oblongata. It functions to coordinate sensory information with muscular responses.
The medulla oblongata can be further divided into two main parts –
- The ventral medulla: It is composed of two triangular structures called the pyramids, where the pyramidal tracts are found. The pyramidal tracts are made up of the corticobulbar tract, which extends from the motor cortex of the frontal lobe to the cranial nerves in the brainstem, and the corticospinal tract, which extends from the cerebral cortex to the spinal cord.
The corticospinal tracts cross while descending through the lower portion of the medulla, forming a point that has been termed the decussation of pyramids. The ventral medulla also contains a set of paired structures called olivary bodies or simply olives that help in relaying information from the cerebral cortex, brainstem and diencephalon to the cerebellum. They are located laterally in the pyramids.
- The dorsal medulla: The brain is divided into four cerebral ventricles that are interconnected to each other. The dorsal medulla forms the lower portion of the fourth ventricle that is continuous with the central canal of the spinal cord and allows the circulation of cerebrospinal fluid.
The fourth ventricle is surrounded by white matter on the inside and grey matter on the outside, similar to the spinal cord. It is at the dorsal medulla where the last seven cranial nerves arise, and these nerves exit the medulla ventrally.
The medulla has both unmyelinated (grey matter) and myelinated (white matter) nerve fibres. The white matter of the medulla is not totally separated from the grey matter, but both remain intermingled (as seen in the brainstem) and form part of the reticular formation. The neurons found in the reticular formation play important roles in the transfer of sensory and motor information.
Functions
The medulla oblongata connects the spinal cord to the higher parts of the brain and hence controls many important autonomic nervous system functions. It is the reflex centre for coughing, sneezing, swallowing and vomiting. It regulates heart rate and blood pressure, which is controlled by the release of stress hormones from the endocrine glands. It is the primary centre for respiratory and cardiovascular activities.
The cranial nerves found in the medulla are important for the digestion of food, speech and movement of the shoulder. It also transfers sensory information between the central and peripheral nervous systems.
Clinical Significance
- Lateral Medullary Syndrome (Wallenberg Syndrome): It is a stroke to the medulla that is caused by blockage of the vertebral arteries or the posterior inferior cerebellar arteries. It is accompanied by vertigo, headache and impaired facial sensation on the face.
- Medial Medullary Syndrome (Dejerine Syndrome): The blood flow to the pyramidal decussation is blocked by a lesion which results in contralateral arms and leg paralysis.
- Bilateral Medial Medullary Syndrome: It affects the bilateral medial medulla and is presented by flaccid quadriplegia and respiratory failure.
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