Overview
This lecture covers the anatomy and functional organization of the spinal cord, spinal nerves, and peripheral nerve plexuses, emphasizing their structure, pathways, and integration in reflexes.
Central Nervous System Structure
- The central nervous system (CNS) includes the brain (cerebrum, diencephalon, brainstem, cerebellum) and the spinal cord.
- The brain is protected by the skull; the spinal cord exits the skull through the foramen magnum and passes through the vertebral column.
- In the spinal cord, gray matter (neurons, unmyelinated fibers) is central; white matter (myelinated fibers) is peripheral, opposite of the cerebral cortex arrangement.
- Dorsal (posterior) spinal cord is sensory; ventral (anterior) is motor.
Spinal Cord Anatomy and Organization
- Spinal cord has a central canal containing cerebrospinal fluid (CSF).
- Spinal nerves enter dorsally (sensory) and exit ventrally (motor).
- Ascending tracts in dorsal/lateral columns carry sensory information; descending tracts in anterior/lateral columns carry motor information.
- Spinal tracts: if the name starts with “spino-” it's ascending (sensory); ends in “-spinal,” it's descending (motor).
Gray Matter and Neuron Types in Spinal Cord
- Gray matter is butterfly-shaped with dorsal (sensory), lateral (autonomic motor), and ventral (somatic motor) horns.
- Somatic motor neurons (multipolar) are in anterior (ventral) horns; their axons exit via the ventral root to skeletal muscle.
- Sensory neuron cell bodies are in the dorsal root ganglion (unipolar neurons).
- Autonomic motor neurons are in the lateral horns and target smooth muscle, cardiac muscle, glands, adipose tissue.
Peripheral Nervous System and Spinal Nerves
- Spinal nerves are part of the peripheral nervous system (PNS) and are mixed (sensory + motor).
- 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
- Each nerve passes through its associated vertebral level.
- Spinal nerves branch into roots and form plexuses: networks that create major peripheral nerves.
Major Nerve Plexuses and Key Nerves
- Cervical plexus (C1–C5): supplies neck and diaphragm (phrenic nerve: C3–C5).
- Brachial plexus (C4/T1): supplies shoulder and upper limbs (axillary, median, radial, ulnar nerves).
- Thoracic nerves don’t form plexuses; they follow the ribs as individual nerves.
- Lumbar plexus (T12–L4): supplies abdominal, pelvic regions, and anterior thigh (femoral nerve: L2–L4).
- Sacral plexus (L4–S3): supplies gluteal region, lower limb (sciatic nerve: L4–S3).
- The safest location for gluteal injections is the upper outer quadrant to avoid the sciatic nerve.
Reflex Arcs and Spinal Cord Integration
- Reflex arc components: receptor, afferent (sensory) pathway, integration (gray matter), efferent (motor) pathway, effector (muscle/gland).
- Example: Patellar tendon (stretch) reflex is monosynaptic, causing quadriceps contraction.
- Withdrawal reflex (e.g., from pain) involves multiple synapses, excitatory to flexor muscle, inhibitory (via interneuron) to extensor muscle.
- Both types of reflexes test nervous system components and are key to physical exams.
Key Terms & Definitions
- Gray Matter — CNS tissue with neuron cell bodies and unmyelinated fibers.
- White Matter — CNS tissue with myelinated nerve fibers.
- Dorsal Root Ganglion — Cluster of sensory neuron cell bodies outside the spinal cord.
- Plexus — Network of intersecting spinal nerve branches forming peripheral nerves.
- Phrenic Nerve — Nerve controlling diaphragm, essential for breathing.
- Monosynaptic Reflex — Reflex involving a single synapse between sensory and motor neuron.
- Withdrawal Reflex — Polysynaptic reflex causing a body part to pull away from pain.
Action Items / Next Steps
- Review spinal cord cross-section and identify gray/white matter, roots, horns.
- Memorize the 31 spinal nerve pairs and major plexuses and nerves.
- Practice drawing and labeling reflex arc components.
- Prepare for Test #3: includes spinal cord anatomy, spinal nerves, nerve plexuses, and spinal reflexes.