Overview
This lecture covers spinal cord reflexes, nerve anatomy, and the major nerve plexuses, highlighting their classification, structure, and clinical significance.
Reflexes: Definition and Classification
- A reflex is a rapid, predictable motor response to a stimulus.
- Reflexes can be classified as spinal (processed in the spinal cord) or cranial (processed in the brain).
- Innate reflexes are genetic; acquired reflexes are learned.
- Somatic reflexes involve skeletal muscles; visceral (autonomic) reflexes involve smooth muscle, cardiac muscle, or glands.
- Monosynaptic reflexes are fast and involve two neurons; polysynaptic reflexes are slower and involve more than two neurons.
- Notable examples: patellar (knee-jerk), withdrawal (pain response), pupillary (pupil constriction), consensual pupillary (both pupils constrict).
Reflex Arc Structure
- A reflex arc has five steps: stimulus activates receptor, sensory neuron sends info to CNS, processing occurs, motor neuron is activated, effector responds.
- Reflex arcs consist of a sensory receptor, sensory neuron, integration center (CNS), motor neuron, and effector (muscle or gland).
Nerve Anatomy
- There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
- Nerves are organized similarly to muscles: entire nerve (epineurium), fascicles (perineurium), and nerve fibers (endoneurium).
- Nerve fibers are axons of neurons, often covered by Schwann cells.
Nerve Plexuses
- Plexuses are networks of branching, interconnected nerves formed by ventral rami (except in the thoracic region).
- Cervical plexus: innervates neck and shoulders; key nerve is the phrenic nerve (controls diaphragm).
- Brachial plexus: innervates pectoral girdle and upper limb; key nerves include musculocutaneous, median, axillary, ulnar, and radial.
- Lumbar plexus: innervates lower abdomen and thigh; key nerves are femoral (hip flexors, knee extensors) and obturator (thigh adductors).
- Sacral plexus: innervates thigh, leg, and foot; major nerve is sciatic, branching into tibial and common fibular nerves.
- Thoracic nerves do not form a plexus and primarily innervate intercostal (respiratory) muscles.
Key Terms & Definitions
- Reflex — Rapid, predictable, involuntary motor response to a stimulus.
- Monosynaptic — Reflex with a single synapse between sensory and motor neuron.
- Polysynaptic — Reflex with two or more synapses.
- Plexus — Network of intersecting nerves.
- Phrenic Nerve — Nerve from cervical plexus controlling the diaphragm.
- Sciatic Nerve — Largest nerve from the sacral plexus, innervating most of the leg and foot.
Action Items / Next Steps
- Review diagrams of reflex arcs and nerve plexuses.
- Study the innervation regions for major nerves highlighted (phrenic, musculocutaneous, median, femoral, sciatic).
- Integrate lecture content with lab models and cross-sectional anatomy.