neuroanatomy Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defi ...

, the medullary pyramids are paired

white matter White matter refers to areas of the central nervous system that are mainly made up of myelinated axons, also called Nerve tract, tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distr ...

structures of the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...

that contain motor fibers of the corticospinal and corticobulbar tracts – known together as the

pyramidal tracts The pyramidal tracts include both the corticobulbar tract and the corticospinal tract. These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (''cort ...

. The lower limit of the pyramids is marked when the fibers cross (

decussate Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. . Similarly, the anatomical term chiasma is named aft ...

Structure

The

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

portion of the

contains the medullary pyramids. These two ridge-like structures travel along the length of the medulla oblongata and are bordered medially by the anterior median fissure. They each have an anterolateral sulcus along their lateral borders, where the

hypoglossal nerve The hypoglossal nerve, also known as the twelfth cranial nerve, cranial nerve XII, or simply CN XII, is a cranial nerve that innervates all the extrinsic and intrinsic muscles of the tongue except for the palatoglossus, which is innervated b ...

emerges from. Also at the side of each pyramid there is a pronounced bulge known as an

olive The olive, botanical name ''Olea europaea'' ("European olive"), is a species of Subtropics, subtropical evergreen tree in the Family (biology), family Oleaceae. Originating in Anatolia, Asia Minor, it is abundant throughout the Mediterranean ...

. Fibers of the posterior column, which transmit sensory and

proprioceptive Proprioception ( ) is the sense of self-movement, force, and body position. Proprioception is mediated by proprioceptors, a type of sensory receptor, located within muscles, tendons, and joints. Most animals possess multiple subtypes of propri ...

information, are located behind the pyramids on the medulla oblongata. The medullary pyramids contain motor fibers that are known as the corticobulbar and corticospinal tracts. The corticospinal tracts are on the anterior surface of the pyramids. These tracts transport motor signals that originated in the precentral gyrus and travelled through the internal capsule to the medulla oblongata and pyramids. Extrapyramidal tracts are those motor tracts that do not traverse the medullary pyramids. At the pyramids' most caudal end, the corticospinal axons

(or cross over) the midline and continue down the spinal cord on the contralateral side. The fibers that decussated will go down the lateral corticospinal tract while the fibers that did not decussate will travel down the

anterior corticospinal tract The anterior corticospinal tract (also called the ventral corticospinal tract, medial corticospinal tract, direct pyramidal tract, or anterior cerebrospinal fasciculus) is a small bundle of Descending neuron, descending fibers that connect the cer ...

. Nearly 90 percent of the fibers decussate and travel down the lateral corticospinal tract while the other 10 percent travels down the

Decussation

The two pyramids contain the motor fibers that pass from the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

to the

and

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

. These are the corticobulbar and corticospinal fibers that make up the

pyramidal tract The pyramidal tracts include both the corticobulbar tract and the corticospinal tract. These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (''cort ...

s. About 90% of these fibers leave the pyramids in successive bundles and

(cross over) in the anterior median fissure of the medulla oblongata as the ''pyramidal decussation'' or ''motor decussation''. Having crossed over at the middle line, they pass down in the posterior part of the

lateral funiculus The most lateral of the bundles of the anterior nerve roots is generally taken as a dividing line that separates the anterolateral system into two parts. These are the anterior funiculus, between the anterior median fissure and the most lateral ...

as the lateral corticospinal tract. The other 10% of the fibers stay uncrossed in the

. The pyramidal decussation marks the border between the spinal cord and the medulla oblongata.

Function

The medullary pyramids contain the motor fibers of the

– the corticospinal and corticobulbar tracts.

Clinical significance

Whiplash as a result of a car accident can lead to brainstem injuries that affect the pyramids at the medulla oblongata. These pyramid injuries are usually a result of a dislocation at the occiput or spinal level C1. Injuries to the pyramids of the medulla oblongata can also be caused by the quick hyperextension of the neck (cervical region of the spine). Hyperextension of the neck can pull and tear the pyramids, leading to a variety of symptoms such as weakness in all four limbs, difficulty swallowing, and difficulty speaking. A bilateral infarction in the pyramids of the medulla can result in motor quadriplegia. This is rare, however, as there have only been four cases of this reported.

Additional images

File:Gray679.png, Medulla oblongata and

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other mammals, lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of ...

. Anterior surface. (Pyramid visible at center.) File:Gray684.png, Sketch of the brainstem, with the

visible in red, and pyramidal decussation labeled at lower right. File:Gray688.png, Section of the medulla oblongata at the level of the decussation of the pyramids.

References

Saladin, Kenneth (2012). Anatomy & Physiology: The Unity of Form and Function, Sixth Edition. New York: McGraw-Hill.

External links

Diagram at csus.edu

Whiplash Injuries: The Cervical Acceleration/Deceleration Syndrome book

Neurology Journal article

{{Authority control Brainstem