Spinal Cord Overview

Overview of the Spinal Cord

• The spinal cord is an elongated cylindrical structure located within the vertebral canal.
• Surrounded by meninges and cerebrospinal fluid (CSF), it extends from the foramen magnum to the second lumbar vertebra.
• Continuity with the medulla oblongata at the superior end and tapering to form the conus medullaris at the inferior end.
• The filum terminale connects the conus medullaris to the coccyx.
• Comprises 31 pairs of spinal nerves and is segmented into cervical, thoracic, lumbar, and sacral regions.
• Features cervical and lumbosacral enlargements for upper and lower limb innervation, respectively.

Cross-Section of the Spinal Cord

• Divided into anterior and posterior sections by a median fissure and posterior median sulcus.
• Central gray matter surrounded by white matter, which is arranged in columns.
• Spinal nerves emerge as rootlets, forming dorsal (sensory) and ventral (motor) roots.

Gray Matter

• Composed of nerve cell bodies, processes, neuroglia, and blood vessels.
• Contains multipolar neurons of three main types:
  • Sensory Neurons: Located in the dorsal horn; receive peripheral body impulses.
  • Lower Motor Neurons: Found in the ventral horn; transmit impulses to skeletal muscles.
  • Interneurons: Connect sensory and motor neurons, forming reflex arcs.

Dorsal Horn Nuclear Groups

• Substantia Gelatinosa (Rexed lamina II): Apical apex of the horn; processes pain, temperature, and touch.
• Nucleus Proprius (Rexed lamina IV): Anterior to substantia gelatinosa; senses position and movement.
• Nucleus Dorsalis (Rexed lamina VII): Located at the base of the dorsal horn; involved with muscle spindle information.
• Visceral Afferent Nucleus (Rexed lamina VII): Processes visceral afferents from T1 to L3 segments.

Ventral Horn Motor Neurons

• Also known as lower motor neurons; influence by descending pathways.
• Composed of alpha and gamma motor neurons, which innervate extrafusal and intrafusal muscle fibers.
• Organized into groups based on innervation location:
  • Medial Group: Innervates neck and trunk muscles.
  • Central Group: Smaller, located in cervical and lumbosacral segments.
  • Lateral Group: Innervates limb muscles, located in cervical and lumbosacral segments.

Lateral Horn Neuronal Groups

• Present from T1 to L2-3 segments; gives rise to preganglionic sympathetic fibers.
• Present from S2-4 segments; gives rise to preganglionic parasympathetic fibers.

White Matter

• Comprised of myelinated nerve fibers; arranged in anterior, posterior, and lateral columns.
• Tracts consist of sensory ascending fibers and motor descending fibers, named based on origin and destination (e.g., spinothalamic, corticospinal).

Spinal Nerves

• 31 pairs of spinal nerves formed from dorsal and ventral roots, merging laterally.
• Dorsal root contains sensory neuron ganglia.
• Each spinal nerve splits into dorsal and ventral rami, with distinct roles for innervation.
  • Dorsal ramus innervates trunk muscles and skin.
  • Ventral ramus forms plexuses (e.g., cervical, brachial, lumbar, sacral, coccygeal).

Dermatomes

• Areas of skin innervated by a single spinal nerve; useful in localizing spinal cord lesions.
• Notable dermatomes include:
  • C2: Posterior skull
  • T4: Nipples
  • L1: Inguinal region
  • S2-S4: Penile and anal regions.

Spinal Meninges

• Connective tissue membranes surrounding the spinal cord, continuous with cranial meninges:
  • Dura Mater: Outermost layer, continuous with epineurium of spinal nerves.
  • Arachnoid Mater: Middle layer, thin and wispy.
  • Pia Mater: Innermost layer, adheres tightly to the spinal cord.
• Spaces include epidural (blood vessels and fat), subdural (serous fluid), and subarachnoid (CSF).

Blood Supply

• Supplied by branches of vertebral and segmental arteries; includes anterior and posterior spinal arteries.
• Segmental arteries provide 31 pairs of radicular branches to supply spinal nerve roots.
• Veins parallel the arteries, draining into radicular veins and the vertebral venous plexus.

Overview of the Spinal Cord

• Elongated and cylindrical, situated in the vertebral canal.
• Extends from foramen magnum to the second lumbar vertebra.
• Includes the conus medullaris at its tapered end, connected to the coccyx via the filum terminale.
• Gives rise to 31 pairs of spinal nerves, forming the cauda equina.
• Divided into four segments: cervical, thoracic, lumbar, and sacral.
• Exhibits cervical (upper limbs) and lumbosacral (lower limbs) enlargements.

Gray Matter

• Composed of nerve cell bodies, neuroglia, and blood vessels.
• Organized into dorsal and ventral horns, with commissures connecting left and right halves.
• Contains multipolar neurons categorized into three types:
  • Sensory neurons in the dorsal horn receiving impulses from the body.
  • Lower motor neurons in the ventral horn transmitting signals to skeletal muscles.
  • Interneurons linking sensory and motor neurons within spinal reflex arcs.
• Dorsal horn subdivided into four groups: substantia gelatinosa, nucleus proprius, nucleus dorsalis, visceral afferent nucleus.

Dorsal Horn Neuron Groups

• Substantia Gelatinosa: Processes pain, temperature, and touch signals; extends throughout the spinal cord.
• Nucleus Proprius: Processes proprioception and movement; located anterior to substantia gelatinosa.
• Nucleus Dorsalis: Receives input from muscle spindles and tendons; found in C8 to L3-4 segments.
• Visceral Afferent Nucleus: Involved in visceral sensory input; extends from T1 to L3.

Ventral Horn Motor Neurons

• Organized into medial, central, and lateral groups.
• Medial group innervates neck and trunk muscles.
• Central group includes phrenic and spinal accessory neurons for diaphragm and SCM.
• Lateral group innervates limb muscles; flexor neurons are located dorsal to extensor neurons.

White Matter

• Contains myelinated nerve fibers arranged in anterior, posterior, and lateral columns.
• Tracts formed by nerve fibers sharing a common origin and termination.
• Includes ascending sensory and descending motor pathways; named based on origin and destination.

Spinal Nerves

• Comprises 31 pairs; emerges as rootlets that form dorsal (sensory) and ventral (motor) roots.
• Each spinal nerve splits into dorsal and ventral rami.
• Ventral ramus gives rise to plexuses (cervical, brachial, lumbar, sacral, coccygeal) for limb peripheral innervation.

Dermatomes

• Skin areas supplied by individual spinal nerves, useful for localizing spinal lesions.
• Each spinal level covers distinct skin zones with some overlap; examples include:
  • C2: Posterior skull,
  • T4: Nipples level,
  • L10: Umbilicus,
  • S2-S4: Anal and genital regions.

Spinal Meninges

• Three connective tissue layers encasing the spinal cord: dura mater (outer), arachnoid mater (middle), and pia mater (inner).
• Epidural space contains blood vessels and fat, while subarachnoid space contains CSF and supports the spinal cord.

Blood Supply

• Supplied by branches of the vertebral and segmental arteries, including anterior and posterior spinal arteries.
• Veins correspond to artery naming conventions; drain into the vertebral venous plexus, emptying into neck and trunk veins.

Dive into the anatomy and function of the spinal cord with this quiz. Explore topics such as gray matter, white matter, spinal nerves, and blood supply. Test your knowledge on the segmentation and the covering of the spinal cord.