Spinal Cord and Peripheral Connections Practice Flashcards

Functions and Connectivity of the Spinal Cord

• The spinal cord is the first major component of the central nervous system (CNS) discussed, acting as the primary bridge between the brain and the peripheral nervous system (PNS).

• Primary functions include:

  • Spinal Reflexes: Through connections with spinal nerves, the cord links the body's periphery to the CNS, allowing for rapid involuntary responses.

  • Integration: The spinal cord is capable of processing information to inhibit or create excitatory impulses. Even basic reflexes require a form of integration within the cord.

  • Information Superhighway: It serves as a conduit for delivering sensory information upward to the brain and motor information downward from the brain towards the effectors (muscles and glands).

Extrinsic Anatomy and Landmarks

• Position and Length:

  • The spinal cord extends from the medulla of longata (a part of the brain) at the level of the foramen magnum.

  • It terminates distally at a variable region between $T_{12}$, $L_1$, or $L_2$.

• 31 Pairs of Spinal Nerves: The cord gives rise to approximately $31\,pairs$ of nerves arranged in a highly organized fashion.

• Specific Landmarks:

  • Cervical Enlargement: A proximal landmark where the cord expands to accommodate the neuronal connections required for the head, neck, and upper extremities.

  • Lumbar Enlargement (or Lumbosacral Enlargement): A distal landmark indicating the origin of nerves for the lower abdomen, groin, and lower extremities (via the lumbosacral plexus).

  • Conus Medullaris: The tapered, v-shaped distal end of the spinal cord proper.

  • Cauda Equina: A bundle of spinal nerves extending from the conus medullaris, named for its resemblance to a "tail of the horse."

  • Final Terminal: An extension of connective tissue (not a nerve) that anchors the distal end of the cord in place.

Protective Structures of the Spinal Cord

• The spinal cord is protected by a multi-layered system of bone, soft tissue, and fluid:

• Osseous Protection: The vertebral column (cervical, thoracic, and upper lumbar vertebrae) surrounds the cord.

  • Intervertebral Discs: Act as mechanical shock absorbers to withstand vertical compressive forces and prevent damage to the vertebrae or cord.

• Spinal Meninges: Three layers of protective soft tissue membranes:

  • Dura Mater: The outermost "tough mother" layer. It is a thick, durable casing.

  • Arachnoid Layer: The middle layer, described as "spider-like."

  • Pia Mater: The innermost "delicate" layer, which is directly adherent to the surface of the spinal cord.

• Spaces within the Protective Layers:

  • Epidural Space: Located between the internal surface of the vertebrae and the dura mater; it contains adipose tissue (fat) for cushioning against bumps and bruises.

  • Subdural Space: A "potential space" between the dura mater and the arachnoid layer, containing a minimal amount of fluid.

  • Subarachnoid Space: The most critical space, located between the arachnoid and pia mater. It contains Cerebrospinal Fluid (CSF).

Cerebrospinal Fluid (CSF)

• Production: CSF is produced by specialized ependymal cells.

• Functions:

  • Buoyancy: Allows the CNS to float, reducing the functional weight of the brain from approximately $3$ to $4\,pounds$ to about $2\,ounces$. This reduction in weight minimizes the force that could disrupt delicate tissues.

  • Shock Absorber: Provides protection against mechanical trauma.

  • Transportation: Circulates to deliver nutrients and remove metabolic waste products and hormones.

Intrinsic Anatomy: Gray and White Matter

• Sectioning the spinal cord reveals two distinct tissue regions:

• Gray Matter: Found in the center, shaped like the letter "H" or a butterfly.

  • It is primarily composed of cell bodies, axon terminals, and unmyelinated neurons.

  • It is divided into regions called Horns:

    • Posterior (Dorsal) Gray Horns: Receive sensory information. Sensory axons from reaching the limbs and trunk enter the cord here via the posterior root.

    • Anterior (Ventral) Gray Horns: Contain the cell bodies of motor neurons that exit the cord to innervate skeletal muscles.

    • Lateral Gray Horns: Contain autonomic motor neurons, particularly those of the sympathetic nervous system, to innervate the viscera.

• White Matter: Located on the outer aspect, surrounding the gray matter.

  • It is composed of myelinated axons, giving it a white appearance.

  • It is organized into Columns (or Funiculi):

    • Posterior White Columns

    • Anterior White Columns

    • Lateral White Columns

Tracts and Nerve Communication

• Tracts: Bundles of axons within the CNS. (Outside the CNS, these bundles are called nerves).

• Ascending Tracts: Sensory (afferent) pathways that carry information toward the brain.

• Descending Tracts: Motor (efferent) pathways that carry impulses away from the brain to effectors.

• Nomenclature: Tracts are named logically, specifying the origin and destination:

  • Spinocerebellar: Originates in the spine, ends in the cerebellum (ascending/sensory).

  • Spinothalamic: Originates in the spinal cord, ends in the thalamus (ascending/sensory).

  • Cortico Spinal: Originates in the cerebral cortex, ends in the spine (descending/motor).

  • Lubro Spinal: Originates in the red nucleus of the brain stem, ends in the spine (descending/motor).

  • Meticulo Spinal: Originates in the reticular formation of the brain stem, ends in the spine (descending/motor).

• Rule of Thumb: If the name starts with "spinal," it is sensory; if it ends with "spinal," it is motor.

Basic Reflex Arc Circuit

• The physical connection between the periphery and the CNS creates the functional reflex arc:

  1. Receptor: Detects a stimulus in the periphery.

  2. Sensory Neuron: Transmits the impulse through the spinal nerve into the posterior root and posterior gray horn.

  3. Integration Center: The spinal cord processes the information (can be standalone or involve the brain).

  4. Motor Neuron: Impulse is sent out through the anterior gray horn and anterior root.

  5. Effector: The impulse reaches a target, such as a skeletal muscle, the heart, or sweat glands.

Organization of Spinal Nerves

• Spinal nerves are classified by the region of the vertebral column from which they emerge:

  • Cervical, Thoracic, Lumbar, Sacral, and Coccygeal.

• Microscopic Structure:

  • Fascicles: Individual groups of axons within a nerve.

  • Epineurium: Connective tissue wrapping around the entire outer aspect of the nerve.

  • Perineurium: Connective tissue wrapping around each fascicle.

  • Endoneurium: Thin layer wrapping around the individual axon.

• Clinical Significance: Trauma or pressure can cause neuritis, leading to inflammation of the axons. Conditions like sciatica occur when chronic pressure on sensory axons triggers pain.

Nerve Plexuses

• A plexus is a functional group of spinal nerves working together to provide sensory and motor innervation to specific regions.

• Cervical Plexus: Serves the head, neck, and shoulders.

  • Phrenic Nerve: The most critical nerve, originating from $C_2$ through $C_6$. It innervates the diaphragm. Damage to the cord above $C_3$ requires a ventilator because the diaphragm cannot be activated.

• Brachial Plexus: Serves the shoulder, axillary region, and upper extremity.

  • Axillary Nerve: Deltoid muscle.

  • Musculoctaneous Nerve: Biceps brachii.

  • Radial Nerve: Triceps.

  • Ulnar and Median Nerves: Muscles of the hand.

• Lumbar Plexus: Serves the back, lower abdomen, and front of the lower extremity.

  • Femoral Nerve and Interarcterator (or Arterator): Innervate hip flexors and knee extensors (quadriceps and adductors).

• Sacral Plexus: Serves the back of the lower extremity.

  • Sciatic Nerve: The largest nerve in the body. It innervates the hamstrings and virtually all muscles below the knee and in the foot.

Dermatomes and Referred Pain

• Dermatome: A specific area of skin innervated by a single spinal nerve.

• Clinical Utility:

  • Referred Pain: Pain originating in an internal organ but felt on the skin. During a heart attack, ischemic pain in the heart is often felt in the $T_1$ and $T_2$ dermatomes, leading to sensations radiating down the arm and across the chest.

  • Shingles: Viral infections of nerves often follow a dermatomal distribution, with lesions appearing in a straight line relative to the affected nerve's territory.