Spinal Cord, Nerves, and Reflexes Notes

Chapter 12: Spinal Cord, Nerves, Reflexes

Objectives

• Spinal Cord Functionality: The spinal cord can operate independently of the brain to execute reflex actions.
• Anatomical Features: Understand the structure of the spinal cord and its segments.
• Meningeal Layers: Identify the three layers of meninges surrounding the spinal cord.
• Gray Matter vs. White Matter: Distinguish the roles of gray and white matter in sensory and motor information processing.
• Spinal Nerve Components: Understand the anatomical parts that constitute a spinal nerve.
• Rami associated with Spinal Nerves: Describe the rami and their functions.
• Distribution Patterns: Relate spinal nerve distribution to their innervation area.
• Cervical Plexus: Describe its structure and function.
• Brachial Plexus: Explain the significance of its distribution pattern.
• Lumbar and Sacral Plexuses: Describe the function of these nerve plexuses.
• Reflex Steps: Outline the steps involved in a reflex action.

The Brain and Spinal Cord (Module 12.1)

• Both the brain and spinal cord receive sensory input via receptors, host reflex centers, and send motor outputs to effectors.
• Reflex: A rapid, automatic response triggered by specific stimuli.
• Spinal Reflexes: Controlled within the spinal cord without direct involvement from the brain.

Introduction to the Spinal Cord

• The spinal cord is composed of approximately 100 million neurons and a higher number of neuroglia.
• Reflex Circuits: Facilitate quick reactions to environmental changes.
• Gray Matter: Site of integration for postsynaptic potentials (IPSPs and EPSPs).
• White Matter: Contains sensory and motor tracts that communicate with the brain.

Functions: Spinal Cord and Spinal Nerves

• Conduction: Relay information between the brain and the body.
• Locomotion: Involve coordinated actions of muscle groups, controlling flexors and extensors.
• Reflexes: Involuntary stereotyped responses to various stimuli.

Spinal Cord Structure (Module 12.2)

• Length: Approximately 45 cm (18 in.), ending at L1–L2.
• Segments: 31 total (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal).

Key Anatomical Features

• Conus Medullaris: Cone-shaped termination of the spinal cord at L1–L2.
• Filum Terminale: Connective tissue anchoring the spinal cord to the S2 region, composed of pia mater.
• Cauda Equina: Nerve roots extending from the lower spinal cord to exit at various intervertebral foramina.

Cross Sectional Anatomy

• White Matter: Comprises both myelinated and unmyelinated axons.
• Gray Matter: Contains neuron cell bodies, neuroglia, and unmyelinated axons.
• Central Canal: Contains cerebrospinal fluid (CSF) for shock absorption.

Meninges (Module 12.3)

• Spinal Meninges: Three protective membranes around the spinal cord and brain:
  • Dura Mater: Tough outer layer.
  • Arachnoid Mater: Middle layer resembling a spider web.
  • Pia Mater: Delicate inner layer adhering closely to the spinal cord's surface.

Spaces Associated with Meninges

• Subarachnoid Space: Contains CSF, providing shock absorption and nutrient diffusion.
• Epidural Space: Between the dura mater and vertebrae, housing fat and blood vessels.

White and Gray Matter (Module 12.4)

• Gray Matter Structure: Organized into specific horns (posterior, anterior, lateral) based on neuron type.
• Nuclei: Functional groupings of neuron cell bodies.
• Gray Commissures: Axons that cross from one side of the spinal cord to the other.
• White Matter Organization: Comprised of three columns (posterior, lateral, anterior), organizing various ascending and descending tracts.

Spinal Nerve Organization (Module 12.5)

• Connective Tissue Layers: Surrounding spinal nerves, including epineurium, perineurium, and endoneurium.
• Blood Supply: Arteries and veins travel through these connective layers to supply the nerve.
• Rami: Each spinal nerve splits into two major branches:
  • Posterior Ramus: Innervates muscles, joints, and skin of the back.
  • Anterior Ramus: Serves the lateral/anterior trunk and limbs.
  • Rami Communicantes: Connect spinal nerves T1–L2 with the autonomic nervous system (ANS).

Dermatomes and Shingles

• Dermatome: Skin area innervated by a specific spinal nerve (not including the face).
• Shingles: Caused by the varicella-zoster virus leading to painful rashes, primarily affecting sensory neurons.

Path of Sensory and Motor Information

• Sensory Pathway: Sensory information travels through posterior roots and is processed in the spinal cord.
• Motor Pathway: Initiates from the anterior root, travels through spinal nerves, and distributes to target muscles or glands.

Spinal Nerves Form Nerve Plexuses (Module 12.7)

• Nerve Plexus: Formed by anterior rami of adjacent spinal nerves; consist of four major plexuses:
  1. Cervical Plexus: Innervates neck and diaphragm.
  2. Brachial Plexus: Innervates pectoral girdle and upper limbs.
  3. Lumbar Plexus: Supplies pelvic girdle and lower limbs.
  4. Sacral Plexus: Also serves pelvic girdle and lower limbs.

Reflexes (Module 12.12)

• Definition and Importance: Reflexes are rapid, automatic responses critical for maintaining homeostasis.
• Steps in a Reflex Arc:
  1. Activation of Receptor: Triggered by stimuli (e.g., stepping on a tack).
  2. Sensory Neuron Activation: Initiates graded potentials leading to action potentials.
  3. CNS Information Processing: Sensory neurons create EPSPs at interneurons.
  4. Motor Neuron Activation: Interneurons activate motor neurons for response.
  5. Peripheral Effector Response: Responding effector executes the reflex action (e.g., muscle contraction).

Classification of Reflexes

• By Development:
  • Innate Reflexes: Present at birth.
  • Acquired Reflexes: Learned behaviors.
• By Nature of Response: Somatic (skeletal muscle control) vs. Visceral (involuntary control).
• By Circuit Complexity: Monosynaptic (single synapse) vs. Polysynaptic (multiple interneurons).
• Processing Site: Spinal reflexes (spinal cord) vs. Cranial reflexes (brain).