13. The Spinal Cord & Spinal Nerves

Chapter 13: The Spinal Cord & Spinal Nerves

Overview of Spinal Cord

  • Definition: The spinal cord, together with the brain, forms the Central Nervous System (CNS).

  • Functions:

    • Spinal Cord Reflexes: Automatic responses to stimuli that do not require input from the brain.

    • Integration: The summation and processing of both inhibitory and excitatory nerve impulses.

    • Communication Pathway: Acts as a highway for the upward and downward travel of sensory and motor information.

Protection of the Spinal Cord

  • The spinal cord is safeguarded by:

    • Meninges: Three layers of connective tissue that encase the spinal cord and brain.

    • Vertebrae: Bony structures that form the spinal column.

    • Cerebrospinal Fluid (CSF): Cushions the spinal cord within the vertebral canal.

Meninges

  • Composition: Three protective layers surrounding the spinal cord:

    • Dura Mater: Outermost, tough layer.

    • Arachnoid Mater: Middle layer resembling a web of collagen fibers.

    • Pia Mater: Innermost layer, thin and transparent, directly adheres to the spinal cord.

Structures Covering the Spinal Cord

  • Vertebrae: Protect the spinal cord.

  • Epidural Space: Filled with adipose tissue, providing cushioning.

  • CSF: Located in the subarachnoid space, essential for cushioning and nutrient transport.

    • Dura Mater: Thick and durable with a subdural space containing interstitial fluid.

    • Arachnoid Mater: Contains CSF in the subarachnoid space.

    • Pia Mater: Contains denticulate ligaments that anchor the spinal cord in place.

Clinical Applications

  • Meningitis: Inflammation of the meninges.

  • Lumbar Puncture (Spinal Tap): A medical procedure to remove cerebrospinal fluid from the subarachnoid space for diagnostic or therapeutic reasons.

External Anatomy of the Spinal Cord

  • Dimensions: Approximately 16-18 inches long and 3/4 inch in diameter.

  • Termination:

    • In adults: Ends at lumbar vertebra L2.

    • In newborns: Ends at L4.

  • Growth: The spinal cord stops growing by age 5.

  • Enlargements: Cervical and lumbar enlargements are present, serving as the origin points for nerves to the upper and lower limbs.

Inferior End of Spinal Cord

  • Conus Medullaris: Cone-shaped end of the spinal cord.

  • Filum Terminale: A threadlike extension of the pia mater that anchors the spinal cord within the canal.

  • Cauda Equina: A bundle of dorsal and ventral roots of the lowest spinal nerves, resembling a horse's tail.

Spinal Nerves

  • Definition: 31 pairs of mixed nerves that convey communication between the spinal cord and the body.

  • Categorization: Named according to the region of the spinal cord from which they emerge:

    • 8 pairs of Cervical Nerves (C1-C8)

    • 12 pairs of Thoracic Nerves (T1-T12)

    • 5 pairs of Lumbar Nerves (L1-L5)

    • 5 pairs of Sacral Nerves (S1-S5)

    • 1 pair of Coccygeal Nerves

Anatomy of Spinal Nerves

  • Roots of Spinal Nerves:

    • Dorsal Root: Contains incoming sensory fibers; includes the dorsal root ganglion (swelling) with cell bodies of sensory neurons.

    • Ventral (Anterior) Root: Contains outgoing motor fibers.

Gray Matter of the Spinal Cord

  • Structure:

    • Divided into two parts by the anterior median fissure and the posterior median sulcus, penetrating the white matter.

    • Shaped like an "H" or butterfly and contains neuron cell bodies and dendrites.

    • Horns: Paired dorsal (posterior) and ventral (anterior) gray horns; lateral horns are found only in the thoracic spinal cord.

  • Gray Commissure: Crosses the midline of the spinal cord.

  • Central Canal: Contains cerebrospinal fluid.

Internal Anatomy of the Spinal Cord

  • White Matter: Surrounds the gray matter, divided into anterior, lateral, and posterior white columns.

  • Columns: Each column contains distinct bundles of nerve axons, called tracts, that have a common origin or destination and convey similar information.

Physiology of the Spinal Cord

  • Functions:

    • White Matter Tracts: Serve as highways for nerve impulse conduction to and from the brain.

    • Gray Matter: Receives and integrates incoming and outgoing signals.

Spinal Cord Tracts

  • Function: Tracts carry sensory and motor information.

    • Sensory Tracts: Ascend to the brain.

    • Motor Tracts: Descend from the brain.

  • Naming of Tracts: Indicates position and direction of the signal.

    • Example: The Anterior Spinothalamic Tract is located in the anterior part of the spinal cord, with impulses traveling from the spinal cord toward the brain (thalamus).

Function of Spinal Tracts

  • Spinothalamic Tract: Responsible for sensations of pain, temperature, deep pressure, and crude touch.

  • Posterior Columns: Carry information regarding proprioception, discriminative touch, pressure, vibration, and two-point discrimination.

  • Direct Pathways: Includes corticospinal and corticobulbar tracts, facilitating precise voluntary movements.

  • Indirect Pathways: Such as rubrospinal and vestibulospinal tracts, involved in programming automatic movements, posture, muscle tone, equilibrium, and coordination of reflexes.

Reflexes and Reflex Arcs

  • Spinal Reflexes: The spinal cord serves as the integrating center; reflexes are rapid, predictable, and automatic responses to environmental changes that maintain homeostasis.

  • Types of Reflexes: Can be spinal or cranial, somatic or autonomic.

Reflex Arc Components

  • Pathway: Includes 5 components:

    • Receptor: Senses the stimulus.

    • Sensory Neuron: Carries information to the spinal cord.

    • Integrating Center: Processes the information (located in the spinal cord).

    • Motor Neuron: Transmits the response signal.

    • Effector: Muscle or gland that executes the response.

Stretch Reflex (e.g., Patellar Reflex)

  • Mechanism: Acts as a feedback mechanism to control muscle length through contraction, preventing injury from overstretching.

  • Nature: Monosynaptic, ipsilateral reflex arc involving the following events:

    • Muscle spindle receptors signal the muscle's stretch.

    • Activated motor neuron causes muscle contraction.

    • Antagonistic muscles relax.

Clinical Considerations

  • Reflex Testing: Important diagnostic tool for detecting disorders or injury.

    • Example: Plantar Flexion Reflex: Involves stroking the lateral margin of the sole. Normal response: curling under the toes; abnormal response: upward fanning of toes (Babinski sign) seen in adults due to injury or disease.

    • Normal fanning occurs in children under 18 months due to incomplete myelination.

Anatomy of Spinal Nerves

  • Comprised of 31 Pairs of spinal nerves, each categorized by their origin level:

    • 8 pairs of cervical (C1-C8)

    • 12 pairs of thoracic (T1-T12)

    • 5 pairs of lumbar (L1-L5)

    • 5 pairs of sacral (S1-S5)

    • 1 pair of coccygeal nerves.

  • Function: Mixed sensory and motor nerves.

Connective Tissue Coverings

  • Presentation of Spinal Nerve Axons:

    • Fiber: A single axon within an endoneurium.

    • Fascicle: A bundle of fibers within a perineurium.

    • Nerve: A bundle of fascicles within an epineurium.

Branching of Spinal Nerve

  • Formation: Spinal nerves are branches from dorsal and ventral roots.

  • Rami: Spinal nerves branch into dorsal and ventral rami:

    • Dorsal Rami: Supply skin and muscles of the back.

    • Ventral Rami: Form plexuses and supply the anterior trunk and limbs.

Nerve Plexus

  • Description: Networks formed by the joining of ventral rami of spinal nerves. Found in the neck, arms, lower back, and sacral regions; not found in the thoracic region.

Cervical Plexus

  • Composition: Formed from the ventral rami of spinal nerves C1 to C5.

  • Supply Areas: Parts of head, neck, and shoulders:

    • Phrenic Nerve (C3-C5): Responsible for innervating the diaphragm.

    • Impact: Damage to the spinal cord above C3 can lead to respiratory arrest.

Sacral Plexus

  • Supply Areas: Provides innervation to the buttocks, perineum, and part of the lower extremities.

  • Sciatic Nerve: Largest nerve in the body, arising from the sacral plexus.

    • Clinical Note: Injury to the sciatic nerve can cause sciatica, characterized by pain from the buttocks down the back of the leg.

    • Potential Causes of Injury: Can arise from herniated discs, dislocated hips, osteoarthritis of the lumbosacral spine, pressure from the uterus during pregnancy, or improperly administered gluteal injections.

Branches of the Sacral Plexus

  • Detailed descriptions of nerve origins and their respective contributions to the plexus (L4 contributions to the femoral nerve, etc.).

Dermatomes

  • Function: Areas of skin innervated by specific spinal nerves, crucial for diagnosing spinal cord or spinal nerve issues.

    • All spinal nerves except C1 innervate specific and constant skin segments.

  • Clinical Relevance: Understanding dermatomes assists clinicians in identifying malfunctioning spinal cord segments. The skin on the face receives sensory innervation from Cranial Nerve V.

Clinical Implications of Dermatomes

  • Patterns of Numbness: Can indicate damaged regions in the spinal cord.

  • Spinal Cord Transection: A complete severing of the spinal cord results in a loss of sensation and motor control below the injury site.

Shingles

  • Condition: A viral infection causing pain, skin discoloration, and blistering derived from infection in peripheral nerves by the chickenpox virus. Produces symptoms along affected dermatomes.