The Spinal Cord & Spinal Nerves
Chapter 13: The Spinal Cord & Spinal Nerves
Overview of the Spinal Cord
Definition: The spinal cord, along with the brain, forms the central nervous system (CNS).
Functions of the Spinal Cord:
Spinal Cord Reflexes: Automatic responses to stimuli that do not require direct involvement of the brain.
Integration: The spinal cord integrates (sums up) inhibitory and excitatory nerve impulses.
Conduit for Information Travel: Acts as a highway for the upward and downward transmission of sensory and motor data between the body and the brain.
Protection of the Spinal Cord
Connective Tissue Coverings: The spinal cord is safeguarded by:
Meninges: Three protective membranes enveloping the spinal cord and brain.
Vertebrae: The bony structures that encase the spinal cord.
Cerebrospinal Fluid (CSF): A cushioning fluid surrounding the spinal cord.
Meninges
Layers of Meninges:
Dura Mater: Outermost and thickest layer providing significant protection.
Arachnoid Mater: The middle layer composed of a web of collagen fibers.
Pia Mater: The innermost layer, thin and transparent, closely adheres to the surface of the spinal cord.
Structures Covering the Spinal Cord
Vertebrae: Bony encasement of the spinal cord.
Epidural Space: Area filled with fat, providing cushioning.
Spaces Between Meninges:
Subdural Space: Filled with interstitial fluid, located between dura mater and arachnoid mater.
Subarachnoid Space: Contains CSF, positioned between arachnoid mater and pia mater.
Denticulate Ligaments: Help anchor the spinal cord in place, arising from the pia mater.
Clinical Applications Related to Meninges
Meningitis: Inflammation of the meninges, typically a result of infection.
Spinal Tap (Lumbar Puncture): Procedure to remove CSF from the subarachnoid space for diagnostic or therapeutic purposes (e.g., delivering antibiotics, contrast agents, anesthetics).
External Anatomy of the Spinal Cord
Length: Approximately 16-18 inches long; diameter about 3/4 inch.
Termination: In adults, the spinal cord typically ends at the L2 vertebra; in newborns, it ends at L4.
Growth: The spinal cord growth ceases around age 5.
Enlargements: Specific enlargements occur in the cervical and lumbar regions that correspond to the origins of nerves supplying upper and lower limbs respectively.
Inferior End of the Spinal Cord
Conus Medullaris: The cone-shaped distal end of the spinal cord.
Filum Terminale: Thread-like extension of pia mater anchoring the spinal cord within the vertebral canal.
Cauda Equina: Bundle of nerve roots (dorsal and ventral) from the lower spinal nerves, often referred to as the 'horse's tail'.
Spinal Nerves
Total Number: There are 31 pairs of spinal nerves, categorized by their region of emergence:
Cervical Nerves: 8 pairs (C1-C8)
Thoracic Nerves: 12 pairs (T1-T12)
Lumbar Nerves: 5 pairs (L1-L5)
Sacral Nerves: 5 pairs (S1-S5)
Coccygeal Nerves: 1 pair
Function: These nerves are essential for communication between the spinal cord and the majority of the body, conveying both sensory and motor information.
Structure of Spinal Nerves
Roots of Spinal Nerves:
Dorsal Root: Comprised of incoming sensory fibers; contains the dorsal root ganglion where sensory neuron cell bodies reside.
Ventral (Anterior) Root: Contains outgoing motor fibers transferring impulses away from the spinal cord.
Gray Matter of the Spinal Cord
Structure: Divided by the anterior median fissure and posterior median sulcus, which bisect the white matter into right and left halves.
Shape: Gray matter appears like the letter H or a butterfly, housing neuron cell bodies and dendrites.
Components:
Dorsal Gray Horns: Paired structures in the posterior section.
Ventral Gray Horns: Paired structures in the anterior section.
Lateral Horns: Present only in the thoracic spinal cord.
Gray Commissure: Connects both halves at the midline; central canal present, filled with CSF.
Internal Anatomy of the Spinal Cord
White Matter: Lies externally to the gray matter, divided into multiple columns (anterior, lateral, and posterior).
Columns and Tracts: Bundles of nerve axons that share a common origin or destination; known as tracts carrying similar information.
Anterior White Commissure: Positioned anteriorly to the gray commissure, connecting the left and right white matter of the spinal cord.
Spinal Cord Physiology
Functions of the Spinal Cord:
White Matter Tracts: Operate as highways facilitating nerve impulse conduction to and from the brain.
Gray Matter Integration: Processes and integrates incoming and outgoing signals.
Tracts of the Spinal Cord
Function of Spinal Tracts: Serve as conduits for sensory (ascending) and motor (descending) information.
Sensory Tracts: Ascend toward the brain.
Motor Tracts: Descend from the brain.
Naming of Tracts: Typically indicates their anatomical position and the direction of nerve impulses. For example:
Anterior Spinothalamic Tract: This tract is located in the anterior portion of the spinal cord, conveying sensory impulses from the spinal cord to the brain (specifically the thalamus).
Types of Spinal Tracts
Spinothalamic Tract: Transmits information related to pain, temperature, deep pressure, and crude touch.
Posterior Columns: Responsible for sensing proprioception, discriminative touch, two-point discrimination, pressure, and vibration.
Direct Pathways: Include corticospinal and corticobulbar tracts, controlling precise voluntary movements.
Indirect Pathways: Include rubrospinal and vestibulospinal tracts, involved in programming automatic movements, posture, muscle tone, and coordination of visual reflexes.
Reflexes and Reflex Arcs
Role of Spinal Cord: Acts as an integrating center for reflex actions, located within the gray matter.
Reflexes: Defined as quick, predictable, automatic responses to environmental changes, crucial for maintaining homeostasis. Types include spinal and cranial, as well as somatic and autonomic reflexes.
Reflex Arc Components
Structure of a Reflex Arc: Composed of five essential components:
Receptor: Senses stimulus/change.
Sensory Neuron: Transmits signal to the spinal cord.
Integrating Center: Processes the signal (usually within the spinal cord).
Motor Neuron: Conducts output signal to effector.
Effector: Muscle or gland that elicits the response.
Example: Stretch Reflex (Patellar Reflex)
Mechanism: Serves as a feedback mechanism to regulate muscle length by inducing muscle contraction upon stretching. This prevents injury by activating muscle contraction when over-stretched.
Nature: Monosynaptic and ipsilateral reflex arc (involves one synapse and occurs on the same side).
Events in Stretch Reflex:
Sensory Receptor Activation: Muscle spindle detects stretching of muscle.
Motor Neuron Activation: The activated motor neuron results in muscle contraction in response to extension.
Inhibition of Antagonistic Muscles: Motor neuron is inhibited to antagonistic muscles, allowing them to relax.
Clinical Considerations
Reflex Testing: Assessing reflexes can be beneficial in diagnosing potential disorders or injuries.
Plantar Flexion Reflex: Assess by stroking the lateral margin of the sole; normal response is curling of the toes. In contrast, upward fanning of toes (Babinski sign) is normal in infants under 18 months due to incomplete myelination and abnormal in adults.
Overview of Spinal Nerves
Composition: Each of the 31 pairs of spinal nerves are classified and numbered based on their origin.
Cervical: 8 pairs (C1-C8)
Thoracic: 12 pairs (T1-T12)
Lumbar: 5 pairs (L1-L5)
Sacral: 5 pairs (S1-S5)
Coccygeal: 1 pair
Spinal nerves are described as mixed nerves containing both sensory and motor fibers.
Connective Tissue Coverings of Spinal Nerves
Structure Hierarchy:
Fiber: A singular axon, enclosed in an endoneurium.
Fascicle: A collection of fibers grouped within a perineurium.
Nerve: A bundle of fascicles encapsulated within an epineurium.
Branching of Spinal Nerves
Formation: Spinal nerves emerge from dorsal and ventral roots.
Rami Branching: After exiting the vertebral column, spinal nerves bifurcate into:
Dorsal Rami: Supply muscles and skin over the back.
Ventral Rami: Form plexuses and supply the anterior trunk and limbs.
Nerve Plexus Formation
Function: A nerve plexus is formed by the merging of ventral rami from several spinal nerves into networks (plexuses), essential in the neck, arm, lower back, and sacral regions.
Caveat: There is no nerve plexus noted in the thoracic region.
Cervical Plexus
Origin: Formed from the ventral rami of spinal nerves C1 to C5.
Role: Supplies sensory and motor innervation to parts of the head, neck, and shoulders.
Phrenic Nerve (C3-C5): Innervates the diaphragm; damage to the spinal cord above C3 can result in respiratory failure.
Sacral Plexus
Supply Area: Encompasses the buttocks, perineum, and parts of the lower extremities.
Sciatic Nerve: Largest nerve in the body arising from this plexus.
Injury Risks: Injury may lead to sciatica, characterized by pain radiating from the buttocks down the back of the leg due to:
Herniated discs
Dislocated hip
Osteoarthritis of lumbar spine
Uterine pressure during pregnancy
Improper gluteal injections.
Branches of Sacral Plexus
Components:
Roots: Comprise anterior and posterior divisions.
Major branches: Include superior gluteal, inferior gluteal, tibial, common fibular, and various other smaller nerves serving muscles and skin in the region.
Dermatomes
Definition: The skin across the body is innervated by spinal nerves carrying somatic sensory impulses to the spinal cord. Every spinal nerve (excluding C1) innervates a specific segment of skin known as a dermatome.
Clinical Relevance: Understanding dermatomes assists physicians in localizing spinal cord injuries or malfunction based on sensory loss or numbness patterns.
Shingles
Etiology: Results from an infection of peripheral nerves by the varicella zoster virus (chickenpox).
Symptoms: Leads to pain, skin discoloration, and a distinctive line of blisters along the affected dermatome, indicating the corresponding nerve involvement.