Overview
This lecture reviews the difference between afferent (sensory) and efferent (motor) functions of the peripheral nervous system (PNS) and explains their pathways through the spinal cord and cranial nerves.
Structure of the Spinal Cord and Nerves
- The spinal cord is part of the central nervous system (CNS) and acts as an integration center.
- Gray matter includes the dorsal horn, lateral horn, and ventral horn.
- White matter contains fiber tracts carrying signals.
- The dorsal and ventral roots combine to form a typical spinal nerve emerging from the vertebral column.
Afferent (Sensory) vs Efferent (Motor) Function
- Efferent (motor) signals exit the CNS via ventral horn to innervate muscle fibers, causing contraction (e.g., via alpha motor neuron and acetylcholine).
- Afferent (sensory) signals enter the CNS from sensory receptors, travel through the dorsal root ganglia to the dorsal horn, and ascend to the brain.
- Efferent means "going out" from the CNS to effector organs; afferent means "coming in" to the CNS from sensory receptors.
Cranial Nerves and Their Functions
- Most cranial nerves arise from the brainstem and can be sensory, motor, or both (mixed).
- There are twelve cranial nerves, each with a specific function.
- Mnemonic: "Some Say Merry Money But My Brother Says Big Brains Matter Most" helps classify each nerve as sensory, motor, or both.
- Sensory (afferent): Cranial nerves I (Olfactory), II (Optic), VIII (Vestibulocochlear).
- Motor (efferent): Cranial nerves III (Oculomotor), IV (Trochlear), VI (Abducens), XI (Accessory), XII (Hypoglossal).
- Mixed (sensory & motor): Cranial nerves V (Trigeminal), VII (Facial), IX (Glossopharyngeal), X (Vagus).
Key Terms & Definitions
- Peripheral Nervous System (PNS) — Nerves outside the brain and spinal cord, including sensory and motor pathways.
- Central Nervous System (CNS) — Comprises the brain and spinal cord; main integration center.
- Afferent — Sensory signals entering the CNS from the body.
- Efferent — Motor signals exiting the CNS to target effectors (muscles, glands).
- Alpha Motor Neuron — Nerve cell that stimulates muscle contraction.
- Dorsal Root — Carries afferent sensory fibers into the spinal cord.
- Ventral Root — Carries efferent motor fibers out from the spinal cord.
Action Items / Next Steps
- Practice matching each cranial nerve with its function using the mnemonic.
- Review spinal cord diagrams to reinforce root and horn locations.