Spinal cord

Introduction

The spinal cord is a cylindrical structure of nervous tissue that extends from the brainstem down the vertebral canal. It serves as a vital communication pathway between the brain and the rest of the body, transmitting sensory and motor information. Additionally, the spinal cord is essential for coordinating reflexes and certain autonomic functions.

Anatomy of the Spinal Cord

Gross Anatomy

• Length and Width: The spinal cord typically measures about 42-45 cm in adults and varies in width, being thickest in the cervical and lumbar regions.
• Segments: It is divided into cervical, thoracic, lumbar, sacral, and coccygeal regions, each giving rise to spinal nerves.
• Conus Medullaris: The tapered lower end of the spinal cord, located near the first lumbar vertebra.
• Filum Terminale: A fibrous extension anchoring the spinal cord to the coccyx.

External Features

• Anterior Median Fissure: A deep groove along the anterior surface separating the two halves of the cord.
• Posterior Median Sulcus: A shallow groove along the posterior surface.
• Denticulate Ligaments: Lateral extensions of pia mater that stabilize the cord within the vertebral canal.
• Spinal Nerves and Rootlets: Thirty-one pairs of spinal nerves emerge from the cord, formed by dorsal (sensory) and ventral (motor) rootlets.

Internal Structure

• Gray Matter: Contains neuronal cell bodies organized into dorsal (sensory), ventral (motor), and lateral (autonomic) horns.
• White Matter: Composed of myelinated axons arranged in ascending sensory and descending motor tracts.
• Central Canal: A narrow channel filled with cerebrospinal fluid, running longitudinally through the center of the cord.

Histology of the Spinal Cord

• Neurons and Glial Cells: The spinal cord contains multipolar neurons responsible for transmitting sensory and motor signals, supported by glial cells such as astrocytes, oligodendrocytes, and microglia which provide structural support, myelination, and immune defense.
• Organization of Gray and White Matter: Gray matter forms an H-shaped or butterfly-shaped core in the center, surrounded by white matter. The gray matter contains neuronal cell bodies and interneurons, while the white matter consists of myelinated axons organized into columns or funiculi for signal conduction.
• Blood Supply and Microvasculature: The spinal cord receives blood through the anterior and posterior spinal arteries, supplemented by segmental medullary arteries. Capillary networks penetrate both gray and white matter to provide oxygen and nutrients.

Spinal Cord Tracts

Ascending Tracts (Sensory)

• Spinothalamic Tract: Transmits pain, temperature, and crude touch sensations from the body to the thalamus.
• Dorsal Column-Medial Lemniscal Pathway: Carries fine touch, vibration, and proprioceptive information to the brain.
• Spinocerebellar Tracts: Conveys unconscious proprioceptive information to the cerebellum for coordination of movement.

Descending Tracts (Motor)

• Corticospinal Tract: Controls voluntary movements, particularly fine motor control of distal muscles.
• Rubrospinal Tract: Facilitates flexor muscle tone and motor coordination.
• Vestibulospinal and Reticulospinal Tracts: Regulate posture, balance, and automatic movements.

Function of the Spinal Cord

• Conduction of Sensory and Motor Signals: The spinal cord serves as a major highway for transmitting sensory information from peripheral receptors to the brain and motor commands from the brain to muscles and glands.
• Reflex Activity and Reflex Arcs: It mediates rapid, involuntary responses to stimuli through reflex arcs, which involve sensory input, interneurons in the gray matter, and motor output without requiring cortical involvement.
• Integration of Autonomic Functions: The spinal cord participates in autonomic control, regulating functions such as heart rate, blood pressure, and digestion through connections with sympathetic and parasympathetic neurons.

Spinal Cord Development

• Embryological Origins: The spinal cord develops from the caudal part of the neural tube during early embryogenesis, with neuroepithelial cells differentiating into neurons and glial cells.
• Development of Gray and White Matter: Gray matter forms from the basal and alar plates, which give rise to motor and sensory neurons respectively, while white matter develops as axons extend and become myelinated.
• Formation of Spinal Nerves: Spinal nerves arise from the fusion of dorsal (sensory) and ventral (motor) rootlets, establishing segmental innervation of the body.

Blood Supply and Lymphatics

• Anterior and Posterior Spinal Arteries: The anterior spinal artery runs along the anterior median fissure and supplies the anterior two-thirds of the cord, while paired posterior spinal arteries supply the posterior one-third.
• Segmental Medullary Arteries: These arteries, such as the artery of Adamkiewicz, reinforce the longitudinal spinal arteries and provide additional blood flow to the thoracolumbar regions.
• Venous Drainage: The spinal cord is drained by a network of anterior and posterior spinal veins that communicate with internal vertebral venous plexuses.
• Lymphatic Drainage: While the spinal cord itself lacks conventional lymphatic vessels, cerebrospinal fluid clearance is facilitated by perivascular spaces and meningeal lymphatic channels.

Clinical Correlations

Spinal Cord Injuries

• Complete vs Incomplete Lesions: Complete lesions result in total loss of motor and sensory function below the injury, whereas incomplete lesions preserve partial function.
• Quadriplegia and Paraplegia: Cervical injuries can cause paralysis of all four limbs (quadriplegia), while thoracic or lumbar injuries typically result in paralysis of the lower limbs (paraplegia).
• Spinal Shock and Recovery: Immediately after injury, spinal shock may cause temporary loss of reflexes, followed by gradual recovery depending on the extent of damage.

Diseases of the Spinal Cord

• Multiple Sclerosis: Demyelinating disease affecting central nervous system tracts, leading to sensory and motor deficits.
• Transverse Myelitis: Inflammation across one or more spinal segments, resulting in motor, sensory, and autonomic disturbances.
• Spinal Cord Tumors: Primary or metastatic tumors can compress neural structures, causing pain, weakness, or sensory loss.

Diagnostic Techniques

• MRI and CT Imaging: Non-invasive imaging to evaluate cord integrity, lesions, or compression.
• Electrophysiological Studies: Electromyography and nerve conduction studies assess functional status of spinal nerves and motor pathways.
• Lumbar Puncture and CSF Analysis: Allows examination of cerebrospinal fluid for infection, inflammation, or neoplastic cells.

References

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