Study Guide: Chapter 14 – Nervous System: Spinal Cord and Spinal Nerves

Chapter 14: Nervous System – Spinal Cord and Spinal Nerves

Overview

This chapter explores the structure and function of the spinal cord and spinal nerves, focusing on their anatomy, organization, and roles in sensory and motor pathways. Understanding these components is essential for grasping how the nervous system integrates and responds to stimuli.

Spinal Cord Structure

Gross Anatomy of the Spinal Cord

• Regions: Cervical, thoracic, lumbar, and sacral segments, each associated with specific spinal nerves.

• Enlargements: Cervical and lumbar enlargements correspond to limb innervation.

• Conus medullaris: The tapered, lower end of the spinal cord.

• Cauda equina: Bundle of spinal nerves below the conus medullaris.

• Filum terminale: Fibrous extension anchoring the spinal cord to the coccyx.

Spinal Cord Cross-Sectional Anatomy

• Gray matter: Central H-shaped region containing neuron cell bodies, dendrites, and synapses.

• White matter: Surrounds gray matter; contains myelinated axons organized into columns (funiculi).

• Central canal: Small channel in the center of the spinal cord containing cerebrospinal fluid (CSF).

Spinal Nerves

• Rootlets: Small bundles of axons emerging from the spinal cord.

• Ventral (anterior) root: Contains motor (efferent) fibers.

• Dorsal (posterior) root: Contains sensory (afferent) fibers; has a dorsal root ganglion with sensory neuron cell bodies.

• Spinal nerve: Formed by the union of dorsal and ventral roots; a mixed nerve carrying both sensory and motor fibers.

Spinal Reflex Arcs

Components of a Reflex Arc

• Sensory receptor: Detects stimulus.

• Sensory neuron: Transmits afferent impulses to the CNS.

• Integration center: One or more synapses in the CNS (may involve interneurons).

• Motor neuron: Conducts efferent impulses from the integration center to an effector.

• Effector: Muscle or gland that responds to the motor neuron impulse.

Types of Reflexes

• Monosynaptic reflex: Involves a single synapse (e.g., stretch reflex).

• Polysynaptic reflex: Involves one or more interneurons (e.g., withdrawal reflex).

Ascending and Descending Pathways

Major Sensory (Ascending) Pathways

• Dorsal column-medial lemniscal pathway: Fine touch, vibration, proprioception.

• Spinothalamic pathway: Pain, temperature, crude touch.

• Spinocerebellar pathway: Proprioceptive information to the cerebellum.

Major Motor (Descending) Pathways

• Corticospinal (pyramidal) tracts: Voluntary motor control of skeletal muscles.

• Extrapyramidal tracts: Involuntary and automatic control of muscles (e.g., posture, balance).

Upper vs. Lower Motor Neurons

• Upper motor neurons: Originate in the cerebral cortex or brainstem; synapse on lower motor neurons.

• Lower motor neurons: Originate in the spinal cord or brainstem; directly innervate skeletal muscles.

• Clinical significance: Lesions in upper vs. lower motor neurons produce different symptoms (e.g., spasticity vs. flaccid paralysis).

Spinal Cord Protection and Meninges

• Dura mater: Tough outer layer.

• Arachnoid mater: Middle, web-like layer.

• Pia mater: Delicate inner layer adhering to the spinal cord.

• Epidural space: Space between dura mater and vertebral wall; contains fat and blood vessels.

• Subarachnoid space: Between arachnoid and pia mater; contains CSF.

Clinical Considerations

• Spinal cord injuries: May result in loss of sensation and/or motor function below the level of injury.

• Reflex testing: Used to assess the integrity of the nervous system (e.g., patellar reflex, Babinski sign).

Summary Table: Key Features of Spinal Cord and Spinal Nerves

Feature	Description
Gray Matter	Neuron cell bodies, dendrites, synapses; forms H-shaped core
White Matter	Myelinated axons; organized into columns (funiculi)
Dorsal Root	Sensory (afferent) fibers; contains dorsal root ganglion
Ventral Root	Motor (efferent) fibers
Spinal Nerve	Mixed nerve; carries both sensory and motor fibers
Reflex Arc	Pathway mediating a reflex; includes receptor, sensory neuron, integration center, motor neuron, effector

Key Terms and Definitions

• Dermatome: Area of skin supplied by a single spinal nerve.

• Plexus: Network of intersecting nerves (e.g., brachial plexus).

• Monosynaptic reflex: Reflex involving a single synapse between sensory and motor neuron.

• Polysynaptic reflex: Reflex involving one or more interneurons.

Example: Patellar Reflex (Knee-Jerk Reflex)

• Stimulus: Tap to the patellar tendon stretches the quadriceps muscle.

• Sensory neuron transmits impulse to the spinal cord.

• Direct synapse with motor neuron causes quadriceps contraction.

• Result: Leg extends at the knee.

Additional info:

• For more details on specific tracts and clinical syndromes, refer to the main textbook or lecture materials.

• Understanding the organization of the spinal cord is essential for interpreting neurological deficits.