Study Guide: Anatomy and Physiology of the Spinal Cord and Reflexes

13-2 – Gross Anatomy of the Spinal Cord

Structures Enclosing and Protecting the Spinal Cord

• Vertebral Bones

  • Each vertebra has an opening; when aligned, these form the vertebral canal containing the spinal cord.

• Spinal Meninges

  • Continuous with cranial meninges.

  • Protect the spinal cord by securing, cushioning, and isolating it from its surroundings.

  • Meningitis: Inflammation of the meninges.

  • Three layers:

    1. Dura mater (outermost):

       • Tough, dense with collagen fibers.

       • Forms the dural sac, tapers and joins the filum terminale at the coccygeal ligament.

       • Epidural space: Between dura mater and vertebral bone, filled with fat and blood vessels. Site for epidural anesthesia.

       • Subdural space: Between dura mater and arachnoid mater.

    2. Arachnoid mater (middle):

       • Subarachnoid space: Between arachnoid mater and pia mater, filled with cerebrospinal fluid (CSF).

       • Site for lumbar puncture (spinal tap) to analyze CSF.

    3. Pia mater (innermost):

       • Thin, adheres tightly to the spinal cord surface.

       • Anchored to dura mater via filum terminale and denticulate ligaments.

Longitudinal Anatomy of the Spinal Cord

• The spinal cord extends from the foramen magnum to the level of L1-L2 vertebrae.

• Enlargements:

  • Cervical enlargement: Supplies nerves to the upper limbs.

  • Lumbar enlargement: Supplies nerves to the lower limbs.

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

• Cauda equina: Bundle of nerve roots extending below the conus medullaris.

• The spinal cord is divided into segments, each giving rise to a pair of spinal nerves.

• Number of spinal cord segments:

  • 8 cervical

  • 12 thoracic

  • 5 lumbar

  • 5 sacral

  • 1 coccygeal

• Each segment has a pair of spinal nerves and two median depressions:

  • Anterior median fissure (ventral side)

  • Posterior median sulcus (dorsal side)

13-3 – Gray Matter and White Matter of the Spinal Cord

Organization of Gray Matter

• Gray matter is organized into horns, each containing nuclei (groups of neuron cell bodies).

• Types of nuclei:

  • Sensory nuclei: Receive information from sensory receptors.

  • Motor nuclei: Send commands to effectors.

• Horns of the spinal cord:

  • Anterior (ventral) gray horns: Somatic motor nuclei (skeletal muscle control).

  • Lateral gray horns: Visceral motor nuclei (autonomic control; present in thoracic and upper lumbar segments).

  • Posterior (dorsal) gray horns: Somatic and visceral sensory nuclei.

• Gray commissures: Axons crossing from one side of the cord to the other.

Organization of White Matter

• White matter consists of myelinated and unmyelinated axons organized into columns (funiculi).

• Columns:

  • Anterior/ventral white columns

  • Lateral white columns

  • Posterior/dorsal white columns

• Each column contains tracts (bundles of axons):

  • Ascending tracts: Carry sensory information to the brain.

  • Descending tracts: Carry motor commands from the brain.

13-4 – Spinal Nerves and Nerve Plexuses

Spinal Nerves

• Spinal nerves are part of the peripheral nervous system (PNS).

• Each spinal nerve is formed by the union of dorsal and ventral roots.

• Spinal nerves are mixed nerves (contain both sensory and motor fibers).

• Named according to the spinal cord segment they attach to.

• Enclosed in three connective tissue layers:

  1. Epineurium: Outermost, dense collagen fibers.

  2. Perineurium: Middle, surrounds bundles of axons (fascicles).

  3. Endoneurium: Innermost, surrounds individual axons.

Peripheral Distribution of Spinal Nerves

• After exiting the vertebral column, each spinal nerve divides into rami:

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

  • Ventral ramus: Innervates ventrolateral body wall and limbs.

  • Rami communicantes: Contain autonomic (visceral) fibers.

Dermatomes

• A dermatome is a specific region of skin supplied by a single pair of spinal nerves.

• Clinically important for diagnosing nerve or spinal cord injuries.

• Example: Shingles (herpes zoster) causes a painful rash in the dermatome of the affected nerve.

Nerve Plexuses

• Nerve plexuses are networks of intersecting nerves formed by ventral rami.

• Allow multiple spinal nerves to supply the same structures.

• Major plexuses:

  1. Cervical plexus: C1–C5; innervates neck, thoracic cavity, diaphragm (phrenic nerve).

  2. Brachial plexus: C5–T1; innervates pectoral girdle and upper limbs.

     • Axillary nerve: Shoulder muscles.

     • Musculocutaneous nerve: Flexor muscles of arm.

     • Radial nerve: Extensor muscles of arm and forearm.

     • Median nerve: Flexor muscles of forearm and digits; carpal tunnel syndrome.

     • Ulnar nerve: Flexor muscles of forearm and digits.

  3. Lumbar plexus: T12–L4; innervates lower abdomen, anterior and medial thigh.

     • Femoral nerve: Anterior thigh muscles.

     • Obturator nerve: Adductor muscles of thigh.

  4. Sacral plexus: L4–S4; innervates posterior thigh, most of lower leg and foot.

     • Sciatic nerve: Largest nerve, branches into tibial and fibular nerves.

13-5 – Neuronal Pools

Organization and Circuit Patterns

• The nervous system contains about 10 million motor neurons, 10 million sensory neurons, and 20 billion interneurons.

• Interneurons are organized into neuronal pools (functional groups of interconnected neurons).

• Neuronal pools may stimulate or depress activity in other parts of the spinal cord.

• Circuit patterns:

  1. Divergence: Spreads stimulation to many neurons (e.g., visual information from the eye delivered to different brain areas).

  2. Convergence: Many sources synapse on one neuron (e.g., subconscious and conscious control of breathing).

  3. Serial processing: Information relayed in a stepwise sequence (e.g., pain signals).

  4. Parallel processing: Several neurons process the same information simultaneously (e.g., withdrawal reflex).

  5. Reverberation: Collateral branches stimulate earlier neurons, creating a feedback loop.

13-6 – Reflexes

Overview of Reflexes

• Reflexes are rapid, automatic responses to specific stimuli.

• Each reflex produces the same motor response every time.

• Reflex arc: The neural circuit producing a reflex.

• Five steps in a simple reflex arc:

  1. Arrival of stimulus and activation of receptor

  2. Activation of sensory neuron

  3. Information processing by interneuron(s)

  4. Activation of motor neuron

  5. Response by effector

• Types of reflexes:

  • By development:

    • Innate (genetically programmed, e.g., withdrawal reflex)

    • Acquired (learned, e.g., braking a car)

  • By response:

    • Somatic (skeletal muscle)

    • Visceral (autonomic, smooth/cardiac muscle, glands)

  • By complexity:

    • Monosynaptic (one synapse, e.g., stretch reflex)

    • Polysynaptic (multiple synapses, e.g., withdrawal reflex)

  • By processing site:

    • Cranial (processed in brain)

    • Spinal (processed in spinal cord)

13-7 – Monosynaptic and Polysynaptic Reflexes

Monosynaptic Reflexes

• Stretch reflex: Regulates muscle length; triggered by sudden stretch of tendon (e.g., patellar "knee jerk" reflex).

• Steps:

  1. Stimulus stretches muscle.

  2. Distortion of receptor sends action potential through sensory neuron.

  3. Sensory neuron synapses with motor neuron in spinal cord.

  4. Motor neuron activates muscle fibers, causing contraction.

• Some stretch reflexes are postural reflexes (help maintain posture).

Polysynaptic Reflexes

• Involve interneurons; can control more than one muscle group.

• Interneurons may stimulate or inhibit muscle groups.

• Tendon reflex: Monitors tension in tendons; prevents muscle damage from excessive tension.

Additional info:

• Reflexes are essential for survival, providing rapid responses to potentially harmful stimuli.

• Clinical testing of reflexes is used to assess the integrity of the nervous system.