Chapter 14: The Nervous System – Spinal Cord and Spinal Nerves

Introduction to the Central Nervous System (CNS)

Overview

The central nervous system (CNS) is composed of the spinal cord and the brain. Both structures are responsible for integrating and processing information, serving as the main control centers for the body’s activities.

• Spinal cord: Integrates and processes information from the body and relays it to the brain.

• Brain: Integrates and processes information, coordinating higher-level functions.

Gross Anatomy of the Spinal Cord

Key Features

The spinal cord is a cylindrical structure with distinct anatomical regions and landmarks.

• Cervical enlargement: Region where nerves serving the upper limbs arise.

• Lumbosacral enlargement: Region where nerves serving the lower limbs arise.

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

• Filum terminale: Fibrous extension from the conus medullaris, forming part of the coccygeal ligament.

• Cauda equina: Bundle of spinal nerve roots extending from the lower end of the spinal cord.

• Posterior median sulcus: Shallow groove along the dorsal midline.

• Anterior median fissure: Deep groove along the ventral midline.

Spinal Cord Segments

• 31 spinal segments: Each segment gives rise to a pair of spinal nerves.

• Each segment consists of:

  • Dorsal roots (sensory fibers)

  • Dorsal root ganglia (cell bodies of sensory neurons)

  • Ventral roots (motor fibers)

  • Spinal nerve: Mixed nerve containing both afferent (sensory) and efferent (motor) fibers

Spinal Meninges

Structure and Function

Spinal meninges are specialized membranes that protect the spinal cord, provide physical stability, and absorb shock. They are continuous with the cranial meninges and consist of three layers:

• Dura mater: Tough, outermost layer; stabilizes the spinal cord within the vertebral canal.

  • Epidural space: Space between dura mater and vertebral bones, contains adipose tissue.

• Arachnoid mater: Middle layer; separated from the pia mater by the subarachnoid space.

  • Cerebrospinal fluid (CSF): Flows within the subarachnoid space, providing cushioning and nutrient transport.

• Pia mater: Innermost layer; adheres to the surface of the spinal cord and brain tissue.

  • Extensions called dentate (denticular) ligaments anchor the spinal cord to the dura mater.

Sectional Anatomy of the Spinal Cord

Gray Matter

Gray matter surrounds the central canal and is organized into horns due to its shape. It consists of neuron cell bodies (somas) and glial cells.

• Posterior (dorsal) horns: Contain somatic sensory and visceral nuclei.

• Lateral horns: Contain visceral motor nuclei (present in thoracic and upper lumbar regions).

• Anterior (ventral) horns: Contain somatic motor nuclei.

• Gray commissure: Connects right and left halves of the spinal cord.

White Matter

White matter is located outside the gray matter and consists of myelinated and unmyelinated axons organized into tracts or columns.

• Posterior white column

• Anterior white column

• Lateral white column

• Columns convey:

  • Sensory tracts (ascending tracts): Carry sensory information to the brain.

  • Motor tracts (descending tracts): Carry motor commands from the brain to the body.

Spinal Nerves

Classification and Structure

There are 31 pairs of spinal nerves:

• 8 cervical nerves

• 12 thoracic nerves

• 5 lumbar nerves

• 5 sacral nerves

• 1 coccygeal nerve

Connective Tissue Layers

• Epineurium: Outer layer, continuous with dura mater.

• Perineurium: Surrounds a fascicle (bundle of axons).

• Endoneurium: Surrounds a single axon.

Peripheral Distribution

• Spinal nerves form two main branches:

  • Dorsal (posterior) ramus: Innervates muscles and skin of the back.

  • Ventral (anterior) ramus: Innervates muscles and skin of the limbs and anterior trunk.

• Spinal nerves T1 to L2 have additional branches:

  • White ramus: Contains myelinated fibers to the autonomic ganglion.

  • Gray ramus: Contains unmyelinated fibers from the autonomic ganglion.

• Dermatomes: Regions of skin supplied by specific spinal nerves; useful for diagnosing nerve or root damage.

Nerve Plexuses

Overview

A nerve plexus is an interwoven network of nerves. There are four major plexuses:

• Cervical plexus

• Brachial plexus

• Lumbar plexus

• Sacral plexus (sometimes combined with lumbar as lumbosacral plexus)

Cervical Plexus (C1–C5)

• Composed of ventral rami of C1–C5.

• Contains cutaneous and muscular branches.

• Sensory input from skin of chest, shoulder, neck, and ear.

• Motor output to infrahyoid and anterior neck muscles.

• Phrenic nerve (C3–C5): Innervates the diaphragm, essential for breathing.

Brachial Plexus (C4–T1)

• Extends from C4–T1; supplies upper limb and pectoral girdle.

• Axillary nerve: Deltoid and teres minor muscles.

• Musculocutaneous nerve: Elbow flexors.

• Median nerve: Wrist and finger flexors.

• Ulnar nerve: Wrist and finger flexors.

• Radial nerve: Extensors of elbows, wrists, and fingers.

Lumbar Plexus (T12–L5)

• Iliohypogastric and ilio-inguinal nerves: Innervate external and internal oblique and transverse abdominis muscles.

• Lateral femoral cutaneous nerve: Sensory from anterior, lateral, and posterior thigh.

• Genitofemoral nerve: Sensory from anteromedial thigh and external genitals.

• Femoral nerve: Innervates quadriceps, sartorius, pectineus, and iliopsoas.

• Obturator nerve: Innervates gracilis, obturator externus, and adductor magnus.

Sacral Plexus (L4–S4)

• Superior and inferior gluteal nerves: Innervate gluteus muscles.

• Sciatic nerve (tibial and common fibular branches): Innervates semimembranosus, semitendinosus, and adductor magnus.

• Pudendal nerve: Innervates perineum, external anal sphincter, and urethral sphincter.

Reflexes

Definition and Arc

A reflex is an immediate, involuntary motor response to a stimulus. The reflex arc is the neural pathway involved in a reflex action.

• Components of a reflex arc:

  • Stimulation and activation of receptor

  • Activation of sensory neuron

  • Information processing in CNS

  • Activation of motor neuron

  • Response by effector

Classification of Reflexes

Development	Response	Complexity of Circuit	Processing Site
Innate Reflexes (genetically determined) Acquired Reflexes (learned)	Somatic Reflexes (skeletal muscle contraction) Visceral (autonomic) Reflexes (smooth/cardiac muscle, glands, adipose tissue)	Monosynaptic (one synapse) Polysynaptic (multiple synapses)	Spinal Reflexes (processing in spinal cord) Cranial Reflexes (processing in brain)

• Monosynaptic reflex: Single synapse between afferent and efferent neurons (e.g., stretch reflex).

• Polysynaptic reflex: Two or more synapses; involves interneurons (e.g., withdrawal reflex).

Example: Stretch Reflex

The patellar reflex is a classic example of a monosynaptic reflex, where tapping the patellar tendon leads to contraction of the quadriceps muscle.

Additional info:

• Reflexes are essential for rapid protective responses and maintaining homeostasis.

• Dermatomes and nerve plexuses are clinically important for diagnosing nerve injuries and planning surgical interventions.