Nervous System Lecture Notes

Nervous System Lecture Outline

General Functions of the Spinal Cord & Nerves

• Structural and Functional Link
  • Carries sensory input from the body to the brain.
  • Sends motor commands from the brain to the rest of the body.
• Spinal Reflexes
  • Fast reactions that do not involve the brain.
  • Enables quick responses to stimuli for protection and survival.

Anatomy of the Spinal Cord

• Location and Structure
  • Extends from the medulla oblongata to the vertebral canal.
  • Tapers at the inferior end, forming the conus medullaris.
  • Approximately ¾ inch in diameter and 16-18 inches in length, shorter than the vertebral column.
• Regions
  • Divided into cervical, thoracic, lumbar, sacral regions.
  • Contains two widened areas for increased neuron capacity:
  • Cervical Enlargement: Neurons innervate the upper limbs.
  • Lumbar Enlargement: Neurons innervate the lower limbs.

Gross Anatomy of the Spinal Cord and Spinal Nerves

• Spinal Nerves
  • 31 pairs total, each named according to the region of the spinal cord.
  • Identified by a letter (region) and number (level) (e.g., T7).
• Nerve Structure
  • Made of axon bundles, wrapped in connective tissue:
  • Epineurium: Surrounding the entire nerve.
  • Perineurium: Around the fascicles.
  • Endoneurium: Around individual axons.

Spinal Nerve Identification and Gross Anatomy

• Rootlets and Roots
  • Rootlets merge to form roots:
  • Posterior Root: Contains sensory neurons and their ganglia.
  • Anterior Root: Contains motor neurons.
  • Each spinal nerve is a mixed nerve, possessing both sensory and motor properties.
• Cauda Equina
  • Nerve collection at the spinal cord's end resembling a horse's tail; starts at L1 vertebra.

Protection and Support of the Spinal Cord

• Protective Structures
  • Bone: Vertebral column houses the spinal cord.
  • Meninges: Three-layered protective membranes covering the cord:
  • Pia Mater: Delicate layer directly on the cord.
  • Arachnoid Mater: Web-like layer, houses CSF.
  • Dura Mater: Tough outer layer providing structural support.
• Cerebrospinal Fluid (CSF)
  • Fills the subarachnoid space and provides cushioning.

Distribution of Gray and White Matter in the Spinal Cord

• Gray Matter
  • Comprised of neuron cell bodies and unmyelinated axons.
  • Contains:
  • Posterior Horns: Sensory neuron axons, interneuron cell bodies.
  • Anterior Horns: Somatic motor neuron cell bodies.
  • Lateral Horns: Autonomic motor neuron cell bodies (T1-L2 only).
• White Matter
  • Myelinated axons forming ascending and descending tracts:
  • Posterior Funiculus: Contains sensory tracts.
  • Lateral Funiculus: Mixed sensory and motor tracts.
  • Anterior Funiculus: Also contains ascending and descending tracts.

Conduction Pathways in the Spinal Cord

• Pathway Characteristics
  • Paired: Each side has corresponding pathways.
  • Decussation: Most pathways cross over to the opposite side.
  • Generally composed of multiple neurons in a chain.
• Sensory Pathways
  • Carry signals from somatic and visceral receptors.
  • Includes somatosensory pathways for skin, muscle, and viscera.
• Motor Pathways
  • Consist of upper and lower motor neurons:
  • Upper Motor Neurons: Originate in the brain.
  • Lower Motor Neurons: In the spinal cord or brainstem, innervate muscles.

Spinal Nerve Branches and Plexuses

• Distribution
  • Each spinal nerve splits into:
  • Posterior Ramus: Innervates back.
  • Anterior Ramus: Larger branch innervating the trunk, limbs.
• Dermatomes
  • Segments of skin supplied by single spinal nerves; useful in localizing nerve damage.
• Nerve Plexuses
  • Networks of interweaving rami of spinal nerves:
  • Cervical Plexus (C1-C5): Innervates neck region and diaphragm (phrenic nerve).
  • Brachial Plexus (C5-T1): Innervates upper limbs.
  • Lumbar Plexus (L1-L4): Innervates lower abdomen, thighs.
  • Sacral Plexus (L4-S4): Contains major nerves like sciatic.

Reflexes and Reflex Arcs

• Components of a Reflex Arc
  • Reflexes involve:
  • Receptor: Detects stimulus.
  • Sensory Neuron: Carries signal to CNS.
  • Integration Center: Processes input (usually in the spinal cord).
  • Motor Neuron: Carries output signal to effector.
  • Effector: Muscle or gland that responds to the stimulus.
• Types of Reflexes
  • Monosynaptic Reflex: Direct communication (e.g., stretch reflex).
  • Polysynaptic Reflex: Involves one or more interneurons (e.g., withdrawal reflex).

Clinical Applications

• Understanding spinal cord anatomy and nerve function is crucial for diagnosing nerve injuries and planning treatment strategies:
  • Spinal Cord Injuries: May require prompt medical intervention to preserve function.
  • Nerve Block Procedures: Target specific regions for pain relief or intervention.