USMLE Guide: Spinal Cord Structure and Functions

Introduction

The spinal cord is a vital component of the central nervous system (CNS) responsible for transmitting sensory and motor signals between the brain and the rest of the body. Understanding the structure and functions of the spinal cord is crucial for medical professionals preparing for the United States Medical Licensing Examination (USMLE). This guide aims to provide a comprehensive overview of the spinal cord's anatomy and its various functions.

Anatomy of the Spinal Cord

• Location: The spinal cord extends from the foramen magnum at the base of the skull to the level of the first or second lumbar vertebra.
• Length: In adults, the spinal cord typically measures around 16 to 18 inches in length.
• Segments: The spinal cord consists of 31 segments, each associated with a pair of spinal nerves. These segments are categorized into five regions: cervical, thoracic, lumbar, sacral, and coccygeal.
• Grey Matter: The central region of the spinal cord contains grey matter, which consists of neuronal cell bodies, dendrites, and synapses. It is divided into anterior, lateral, and posterior horns.
• White Matter: Surrounding the grey matter, the spinal cord contains white matter composed of myelinated axons organized into tracts or columns. These tracts transmit signals up and down the spinal cord.

Functions of the Spinal Cord

1. Sensory Functions:

   • Ascending Tracts: Sensory information from the body is transmitted through ascending tracts to the brain for interpretation. The dorsal column-medial lemniscus pathway is responsible for transmitting fine touch, proprioception, and vibration sensations. The spinothalamic tract carries pain and temperature sensations.
   • Reflex Arcs: The spinal cord plays a vital role in reflex actions, which are automatic responses to stimuli. Examples include the patellar reflex and withdrawal reflex.

2. Motor Functions:

   • Descending Tracts: Motor commands originate in the brain and travel down the spinal cord through descending tracts, which synapse with motor neurons in the grey matter. The corticospinal tract is the major pathway responsible for voluntary motor control.
   • Spinal Reflexes: Reflexes also involve motor functions. The spinal cord controls various reflexes, such as the stretch reflex, withdrawal reflex, and crossed-extensor reflex.

3. Autonomic Functions:

   • Sympathetic and Parasympathetic Control: The spinal cord plays a crucial role in coordinating autonomic functions of the body. Sympathetic preganglionic neurons originate from the thoracic and lumbar segments of the spinal cord, while parasympathetic preganglionic neurons are located in the sacral segments.

4. Coordination of Movement:

   • The spinal cord integrates sensory information and coordinates motor outputs, enabling smooth and coordinated movements.

Clinical Relevance

• Spinal Cord Injuries: Damage to the spinal cord can result in severe consequences, such as paralysis or loss of sensation below the level of injury. Understanding the anatomy and functions of the spinal cord is crucial for diagnosing and managing spinal cord injuries.
• Neurological Disorders: Various neurological disorders can affect the spinal cord, including spinal cord tumors, multiple sclerosis, and syringomyelia. A comprehensive understanding of spinal cord structure and functions is essential for diagnosing and treating these conditions.

Remember to review additional resources and practice questions to reinforce your knowledge of spinal cord structure and functions for the USMLE.