The term “conus” originates from the Latin word for cone, describing a tapered, conical structure found in two distinct locations within the human body. While both structures share a name due to their similar geometry, their functions are entirely separate, involving the central nervous system and the cardiovascular system. This article explores the specific anatomy and clinical importance of the Conus medullaris in the vertebral column and the Conus arteriosus within the heart.
The Spinal Cord’s Tapered End
The Conus medullaris is the terminal, cone-shaped end of the spinal cord proper. This structure marks the point where the long, continuous column of neural tissue concludes its descent within the vertebral canal. In the majority of adults, this tapering occurs at or near the level of the first and second lumbar vertebrae (L1 and L2).
This segment of the cord is neurologically dense, containing the sacral and coccygeal spinal segments, which range from S2 to S5. These segments are functionally responsible for coordinating several involuntary and voluntary actions. They control the motor innervation for parts of the lower extremities and provide sensory input from the perineal and genital regions.
The conus also houses the autonomic nerve pathways that govern functions like bladder, bowel, and sexual control. As the spinal cord ends, a thin, fibrous extension of the protective tissue layers, called the filum terminale, extends downward to anchor the conus to the coccyx. Immediately below the conus, the remaining lumbar and sacral nerve roots continue to stream downward in a loose bundle, which is descriptively named the cauda equina, or “horse’s tail”.
Clinical Significance of the Conus Medullaris
The precise anatomical location of the conus makes it a region of medical concern, as injury to this small area can have widespread effects on the lower body. Damage results in Conus Medullaris Syndrome (CMS), which presents with sudden, severe back pain and early onset of bladder and bowel dysfunction. CMS symptoms are generally symmetric, affecting both sides of the body equally, and may include a mix of upper and lower motor neuron signs, such as hyperreflexia in the lower extremities.
This differs distinctly from Cauda Equina Syndrome (CES), which involves injury to the nerve roots below the conus. CES symptoms are often asymmetric and are characterized by lower motor neuron signs like hyporeflexia and flaccid weakness. Understanding this difference is important for diagnosis and treatment, as both conditions can be caused by trauma, tumors, or disc herniations near the L1/L2 level.
The termination point of the conus is also a landmark for a lumbar puncture, or spinal tap. Since the spinal cord proper ends at L1/L2, a needle can be safely inserted into the lower lumbar spine, typically between the L3/L4 or L4/L5 vertebrae. At this lower level, the needle enters the space containing only the movable nerve roots of the cauda equina, which allows for the safe collection of cerebrospinal fluid for diagnostic testing.
The Heart’s Outflow Tract
In the heart, the second structure bearing the name conus is the Conus arteriosus, also known as the infundibulum. This smooth, funnel-shaped, muscular region is located in the upper part of the right ventricle and serves as the direct outflow tract for deoxygenated blood leaving the right side of the heart.
The function of the conus arteriosus is to smoothly channel blood from the right ventricle towards the pulmonary valve. The valve opens, allowing blood to enter the pulmonary artery for oxygenation in the lungs. The smooth lining of this region, unlike the textured muscle of the rest of the ventricle, assists in streamlining the blood flow.
Developmental irregularities in this tract are significant in congenital heart disease. Improper formation or narrowing of the conus arteriosus is a central feature of Tetralogy of Fallot (TOF). The resulting sub-pulmonary obstruction is one of the four defects defining this complex condition, hindering the proper flow of blood to the lungs.