In-depth Study Notes on the Spinal Cord, Spinal Nerves, and Reflexes

Learning Outcomes

• Describe the basic structural and organizational characteristics of the nervous system.
• Discuss the structure and functions of the spinal cord, along with the three meningeal layers surrounding the central nervous system.
• Explain the roles of white matter and gray matter in processing and relaying sensory information and motor commands.
• Describe the major components of a spinal nerve and relate their distribution pattern to the regions they innervate.

Spinal Reflexes

• Definition: Rapid, automatic nerve responses triggered by specific stimuli.
• Controlled primarily by the spinal cord, not involving higher brain functions.

Gross Anatomy of the Spinal Cord

• Length: Approximately 18 inches (45 cm), width of about 1/2 inch (14 mm).
• Terminates between vertebrae L1 and L2.
• Exhibits bilateral symmetry with grooves:
  • Posterior Median Sulcus - on the posterior side.
  • Anterior Median Fissure - a deeper groove on the anterior side.

Spinal Cord Enlargements

• Enlargements due to the amount of gray matter related to sensory and motor involvement:
  • Cervical Enlargement - nerves for shoulders and upper limbs.
  • Lumbar Enlargement - nerves for pelvis and lower limbs.

Distal End Structures

• Conus Medullaris - a thin, conical structure below the lumbar enlargement.
• Filum Terminale - a fibrous thread at the end of the conus medullaris, attaching to the coccygeal ligament.
• Cauda Equina - nerve roots extending below the conus medullaris.

Spinal Cord Segments

• The spinal cord has 31 segments corresponding to the origins of spinal nerves:
  • Cervical Nerves - named inferior to the vertebrae.
  • Other Nerves - named superior to the vertebrae.

Roots of Spinal Nerves

• Each spinal nerve has two branches:
  • Ventral Root - contains axons from motor neurons.
  • Dorsal Root - contains axons from sensory neurons with dorsal root ganglia housing sensory neuron cell bodies.

Formation of Spinal Nerve

• The combination of dorsal and ventral roots forms a spinal nerve which is a mixed nerve carrying both afferent (sensory) and efferent (motor) fibers.

Meningeal Layers

• The spinal meninges consist of three layers:
  • Dura Mater (outer layer) - tough and fibrous, continuous with cranial dura.
  • Arachnoid Mater (middle layer) - contains a subarachnoid space filled with cerebrospinal fluid (CSF).
  • Pia Mater (inner layer) - adheres to the spinal cord surface.

Gray Matter and White Matter

• Gray Matter:
  • Surrounds the central canal; contains neuron cell bodies and neuroglia.
  • Organized into gray horns: anterior (motor nuclei), posterior (sensory nuclei), and lateral (visceral motor nuclei).
• White Matter:
  • Composed primarily of myelinated axons organized into columns (ascending and descending tracts).
  • Ascending Tracts (to brain) and Descending Tracts (to spinal cord).

Peripheral Nerves and Spinal Nerves

• Each spinal nerve is encased by:
  • Epineurium (outer layer) - dense network of collagen fibers.
  • Perineurium (middle layer) - surrounds fascicles.
  • Endoneurium (inner layer) - surrounds individual axons.

Peripheral Distribution of Spinal Nerves

• Formed lateral to the intervertebral foramen where roots unite, followed by branching pathways.
• Rami Communicantes:
  • White Ramus - carries visceral motor fibers to sympathetic ganglion.
  • Gray Ramus - contains unmyelinated nerves returning from the ganglion.
• Dorsal Ramus - innervates the back.
• Ventral Ramus - larger, innervates ventrolateral structures and limbs.

Sensory Nerves and Dermatomes

• Sensory information travels via dorsal, ventral, and white rami.
• Dermatomes - regions of skin monitored by specific spinal nerves.

Nerve Plexuses

• Interwoven networks formed from ventral rami; control limb and neck muscles:
  • Cervical Plexus (C1-C5) - innervates neck and diaphragm (e.g., Phrenic Nerve).
  • Brachial Plexus (C5-T1) - innervates the pectoral girdle and upper limbs (e.g., Musculocutaneous, Ulnar Nerves).
  • Lumbar Plexus (T12-L4) - major nerves include Genitofemoral and Femoral nerves.
  • Sacral Plexus (L4-S4) - includes the Sciatic nerve and its branches (Tibial and Fibular).

Reflexes

• Classification of reflexes based on:
  • Development (innate vs. acquired).
  • Motor Response (somatic vs. visceral).
  • Complexity (monosynaptic vs. polysynaptic).
  • Information Processing Site (spinal vs. cranial).

Reflex Arcs

• Components include:
  • Sensory receptor, sensory neuron, interneuron (for polysynaptic), motor neuron, effector.
• Example: Patellar Reflex - a stretch reflex with minimal delay between sensory input and motor output.

Five Steps in a Neural Reflex

1. Arrival of stimulus and activation of receptor.
2. Activation of sensory neuron.
3. Processing in the CNS (interneurons might be involved).
4. Activation of motor neuron.
5. Response of peripheral effector.