Spinal Cord & Spinal Nerves

CHAPTER 18

SPINAL CORD & SPINAL NERVES

Introduction

• The spinal cord is nervous tissue that is continuous with the brain.

• It exits the skull and enters through the foramen magnum.

• Functions of the spinal cord:

  • Spinal reflexes: involuntary responses to stimuli.

  • Integration center: processing center for sensory and motor information.

  • Pathway for sensory impulses: transmits sensory information to the brain.

  • Pathway for motor impulses: conveys motor commands from the brain to the body.

Spinal Cord Anatomy

Protections and Coverings

• Vertebral column:

  • The vertebral foramen of vertebrae forms an open "tube" that surrounds the spinal cord as it passes through.

• Meninges:

  • A connective tissue covering that surrounds the brain and spinal cord.

  • Layers and spaces:

  • Epidural space:

    • Between dura mater and vertebrae.

    • Contains some fat and connective tissue.

    • Site for administering epidural anesthetics. Only around the spinal cord, not the brain.

  • Dura mater: (known as "tough mother")

    • This layer is composed of dense, irregular connective tissue.

  • Subdural space:

    • Located between the dura and arachnoid mater, it contains interstitial fluid.

Meninges Continued

• Arachnoid mater:

  • A delicate, cobweb-like layer covering the spinal cord.

• Subarachnoid space:

  • Located between the arachnoid and pia mater, it contains cerebrospinal fluid (CSF).

  • Important for spinal anesthesia (administered at L4-5) and for spinal taps.

• Pia mater:

  • A thin, delicate layer that adheres closely to the spinal cord and brain.

Meningitis

• Meningitis is the inflammation of the meninges which can lead to serious health issues.

• Spinal tap (lumbar puncture):

  • A procedure where a sample of CSF is removed from the subarachnoid space using a needle.

  • It is performed below the level of L4 to avoid damaging the spinal cord.

External Anatomy of the Spinal Cord

• Cervical enlargement:

  • A bulge found in the region from C4 to T1, contributing to spinal nerves that innervate the upper limbs.

• Lumbar enlargement:

  • Located in the lower thoracic area from T9 to T12, contributing to spinal nerves for the lower limbs.

• Termination of the spinal cord:

  • In adults, it typically ends at L1-L2, while in children, it extends to L3-L4.

• Cauda equina:

  • Refers to lumbar, sacral, and coccygeal nerves that continue through the vertebral foramen, exiting from lumbar and sacral intervertebral foramina even though the cord has ended.

• Filum terminale:

  • An extension of pia mater that continues down to the coccyx.

Roots of Spinal Nerves

• Dorsal (posterior) root:

  • Contains sensory fibers.

  • The dorsal root ganglion houses the cell bodies of sensory neurons, which are unipolar.

• Ventral (anterior) root:

  • Contains motor fibers and is made up of axons of neurons whose cell bodies are located in the gray matter.

  • The anterior horn is responsible for skeletal muscle, while the lateral horn is related to cardiac and smooth muscle.

Internal Anatomy of the Spinal Cord

Grooves

• Anterior median fissure:

  • A deep, wide groove on the anterior side of the spinal cord.

• Posterior median sulcus:

  • A shallower and narrower groove located on the posterior side of the spinal cord.

Gray Matter

• Composed of unmyelinated nerve fibers, containing many cell bodies and unmyelinated axons.

• Structurally resembles an “H.”

  • Gray commissure: the crossbar of the H, containing neurons that cross from one side of the cord to the other.

  • Central canal: a small opening in the center of the cord, containing CSF and continuous with the 4th ventricle of the brain.

  • Gray horns:

  • Contain many cell bodies:

    • Anterior horn: motor functions (somatic).

    • Posterior horn: sensory functions.

    • Lateral horn: autonomic functions.

White Matter

• Composed of myelinated fibers surrounding the gray matter.

• Anterior white commissure: connects the white matter on the right and left sides of the cord.

• White columns:

  • Comprised of anterior, posterior, and lateral white columns.

  • Contains tracts which are bundles of neurons traveling to or from a common location.

  • Ascending tracts: carry sensory information.

  • Descending tracts: convey motor commands.

Spinal Cord Functions

• Sensory tracts include:

  • Spinothalamic tract

  • Spinocerebellar tract

• Motor tracts include:

  • Corticospinal tract

• Naming of tracts:

  • Based on their position and direction of signal flow. For example, the anterior spinothalamic tract carries impulses from the spinal cord towards the brain (specifically the thalamus) and is found in the anterior part of the spinal cord.

Spinal Reflexes

• Defined as automatic responses to changes in the environment.

• The integration center for spinal reflexes lies in the gray matter of the spinal cord.

• Examples of spinal reflexes:

  • Somatic reflexes: lead to skeletal muscle contractions.

  • Autonomic (visceral) reflexes: involve smooth and cardiac muscles and glands, affecting processes like heart rate, respiration, digestion, and urination.

• Note: Cranial reflexes have connections through cranial nerves, while spinal reflexes primarily use spinal pathways.

Spinal Reflex Arc

• Describes the path taken by nerve impulses during a reflex action. It consists of five components:

  1. Receptor: detects the stimulus.

  2. Sensory neuron: transmits signals toward the central nervous system.

  3. Integrating center: resides in the spinal cord gray matter where processing occurs.

  4. Motor neuron: carries commands away from the central nervous system.

  5. Effector: muscle or gland that produces the response.

• The spinal reflex arc enables rapid, unconscious, and involuntary responses to stimuli, often providing protection (e.g., the knee jerk reflex).

Illustration of Tendon Reflex

• Steps in the tendon reflex mechanism:

  1. Tension in the tendon stimulates the sensory receptor (tendon organ).

  2. The sensory neuron is excited and transmits the signal to the spinal cord.

  3. Within the integrating center (spinal cord), the sensory neuron activates an inhibitory interneuron.

  4. The motor neuron responsible for the muscle attached to the same tendon is inhibited, resulting in relaxation of that muscle.

  5. Antagonistic muscles contract, producing the reflex response.

Spinal Nerves

• Spinal nerves exit the spinal cord through the intervertebral foramen and are part of the peripheral nervous system (PNS).

• A total of 31 pairs of spinal nerves:

  • 8 cervical

  • 12 thoracic

  • 5 lumbar

  • 5 sacral

  • 1 coccygeal

• These nerves are mixed nerves: they contain both sensory and motor fibers.

Structure & Connective Tissue Coverings

• Endoneurium: covers individual neurons.

• Perineurium: covers fascicles (bundles of neurons).

• Epineurium: covers the entire nerve.

Branching of Spinal Nerves

• A branch of spinal nerves is known as a ramus (plural: rami).

  • Dorsal ramus: branches to the skin and deep back muscles.

  • Ventral ramus: branches to limbs and ventral body.

  • Rami communicantes: part of the autonomic nervous system (ANS).

Plexus

• Spinal nerves branch and form networks known as plexuses (which means "braid").

• Nerve fibers in anterior rami from different levels can join together and exit plexus at different levels than they originated from the spinal cord.

• Types of plexuses include:

  • Cervical plexus

  • Brachial plexus

  • Lumbosacral plexus

• Note: There is no distinct thoracic plexus.

Brachial Plexus

• The brachial plexus is formed from the anterior rami of spinal nerves C4-T1. It innervates the upper limbs and is highly complex, consisting of multiple branches and trunks (superior trunk, middle trunk, inferior trunk) that split into anterior and posterior divisions.

Dermatomes

• Dermatomes refer to areas of skin that are supplied by specific spinal nerves and have sensory functions.

• They often overlap slightly and are clinically significant for mapping sites of potential cord injury or disease.