Spinal Cord & Spinal Nerves

Chapter 12: Spinal Cord & Spinal Nerves

Overview of the Nervous System

  • Discussion of the central nervous system (CNS) differs in arrangement compared to OpenStax text.

  • Recommendations for using the ebook:

    • Utilize the index to clarify confusing topics in the notes.

Components of the Central Nervous System (CNS)

  • Composed of:

    • Brain

    • Spinal Cord

  • Entire CNS is encased by three membranes known as the meninges:

    • Dura Mater:

    • Outermost layer.

    • Thickest and toughest membrane.

    • Arachnoid Mater:

    • Located beneath the dura mater.

    • Pia Mater:

    • Innermost layer covering the surface of the spinal cord.

    • Subarachnoid Space:

    • Contains cerebrospinal fluid (CSF), located between the arachnoid and pia mater.

Cerebrospinal Fluid (CSF)

  • Fluid in the subarachnoid space and interior cavities of the brain/spinal cord.

  • Production:

    • Formed in the ventricles of the brain.

Structure of the Spinal Cord

  • The spinal cord is located in the vertebral canal.

  • Extends from the foramen magnum to the region between the 1st and 2nd lumbar vertebrae (or to the 2nd lumbar if following Seeley).

  • Conus Medullaris:

    • Blunt end of the spinal cord where it tapers.

  • Filum Terminale:

    • Fibrous extension of pia mater anchoring to the coccyx.

  • Cauda Equina:

    • Collection of nerves resembling a horse's tail, found inferior to the conus medullaris.

  • Enlargements:

    • Cervical Enlargement: Area where nerves of the arm enter/exit.

    • Lumbar Enlargement: Area where nerves of the leg are attached.

Anatomy of the Spinal Cord

  • Surface features:

    • Anterior Median Fissure:

    • Located on the ventral aspect.

    • Posterior Median Sulcus:

    • Located on the dorsal surface.

  • White Matter:

    • Outer portion composed of myelinated fibers, forming columns or funiculi.

  • Gray Matter:

    • Inner part composed of cell bodies; includes:

    • Gray Commissure: Crosses the midline.

    • Central Canal: Contains CSF, continuous with the fourth ventricle of the brain.

Spinal Reflexes

  • Definition: Rapid, automatic responses to stimuli.

  • Reflex Arc:

    • Components include:

    • Receptor: Senses stimulus.

    • Afferent Neuron: Sensory neuron conducting impulse to CNS.

    • Integrating Center: Found in the brain or spinal cord.

    • Efferent Neuron: Motor neuron conducting impulse to effector.

    • Effector: Muscle or gland responding to the impulse.

Types of Reflexes

  • Somatic Reflex:

    • Involves skeletal muscle.

  • Autonomic (Visceral) Reflex:

    • Involves smooth muscle, cardiac muscle, or glands.

  • Classification by Neurons in the Reflex Arc:

    • Monosynaptic Reflex:

    • Consists of one sensory and one motor neuron.

    • Direct synapse between sensory and motor neuron in CNS.

    • Polysynaptic Reflex:

    • Involves one or more interneurons in addition to sensory and motor neurons.

    • Example: Illustration provided showing polysynaptic reflex with one interneuron.

Composition of Spinal Nerves

  • Description: Bundles of fibers from the peripheral nervous system attached to the spinal cord.

  • Mixed Nerves:

    • Contain both sensory and motor fibers.

  • Components of a Nerve:

    • Epineurium: Surrounds entire nerve.

    • Perineurium: Surrounds bundles of fibers (fascicles).

    • Endoneurium: Encloses individual nerve fibers.

Number of Spinal Nerves

  • Total: 31 pairs of spinal nerves:

    • 8 pairs: Cervical

    • 12 pairs: Thoracic (Intercostal)

    • 5 pairs: Lumbar

    • 5 pairs: Sacral

    • 1 pair: Coccygeal

  • Each spinal nerve forms:

    • Dorsal (Posterior) Root: Contains sensory (afferent) fibers, often referred to as the sensory root.

    • Ventral (Anterior) Root: Contains motor (efferent) neurons, often called the motor root.

    • Dorsal root has an enlargement known as dorsal root ganglion containing the cell bodies of sensory neurons.

Dermatomes

  • Definition: A segment of skin innervated by a single spinal nerve.

  • Could also be defined as the area innervated by the dorsal root of that spinal nerve.

  • An illustration shows the dermatomes of the body, facilitating understanding of sensory input regions.

Lab Component of the Course

  • Specific nerves will be identified in the laboratory component, enhancing practical understanding of spinal nerves and reflexes.

Quiz Information

  • Quiz #9: Covers material related to spinal cord and spinal nerves up to this point.

  • This marks the end of Test 3 Material.