Overview
This lecture covers the parasympathetic nervous system, its comparison to the sympathetic system, and details the twelve cranial nerves, including their functions and mnemonic devices for memorization.
Parasympathetic vs Sympathetic Nervous System
- The heart beats about 60 bpm at rest under autonomic nervous system control.
- Disconnecting the heart from autonomic input increases its rate to about 100 bpm at rest.
- The parasympathetic nervous system (PNS) calms the body and conserves energy.
- The sympathetic system prepares the body for immediate action (fight or flight).
- PNS is responsible for digestion, reproduction, excretion, and immune response.
- Proper balance (homeostasis) requires both parasympathetic and sympathetic inputs.
Structural Differences
- Sympathetic ganglia are near the spinal cord; parasympathetic ganglia are near target organs.
- Both systems use acetylcholine (ACh) in preganglionic synapses.
- Postganglionic parasympathetic neurons release ACh; sympathetic neurons typically release norepinephrine.
- Sympathetic nerves come from the thoracolumbar (midsection) spinal cord.
- Parasympathetic nerves are craniosacral, mostly leaving the brain directly as cranial nerves.
Cranial Nerves Overview
- There are 12 cranial nerves with varying sensory, motor, or mixed functions.
- Some cranial nerves handle voluntary movement, others sensory input, or both.
- The names and functions of the cranial nerves must be memorized for anatomy.
Names and Functions of Cranial Nerves
- Cranial nerves (by number and name): Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Auditory (Vestibulocochlear), Glossopharyngeal, Vagus, Spinal Accessory, Hypoglossal.
- Mnemonics help remember their order and type (e.g., "On Old Olympus’ Towering Top...").
- Vagus nerve (X) is both sensory and motor, key for visceral organ control.
Parasympathetic Responses and Homeostasis
- Vagus nerve carries signals to and from organs, regulating heart rate, digestion, and relaxation.
- Parasympathetic and sympathetic systems work together for balance, crucial for homeostasis.
- Parasympathetic tone usually dominates, keeping the body's functions calm and steady.
Sex and the Autonomic System
- Sexual function requires both parasympathetic (arousal, blood flow) and sympathetic (excitement, climax) input.
- Balance of both systems is necessary for normal function.
Key Terms & Definitions
- Parasympathetic nervous system — division that calms the body and promotes routine maintenance.
- Sympathetic nervous system — division that prepares the body for stressful or emergency situations.
- Cranial nerves — twelve nerves emerging directly from the brain, serving sensory and motor functions.
- Acetylcholine (ACh) — neurotransmitter used in parasympathetic nerve signaling.
- Norepinephrine — neurotransmitter mainly used by sympathetic neurons.
- Vagus nerve — tenth cranial nerve with both sensory and motor roles, vital for organ regulation.
- Homeostasis — the body's ability to maintain internal balance.
Action Items / Next Steps
- Memorize the names, numbers, and functions (sensory, motor, both) of the twelve cranial nerves.
- Learn and choose a mnemonic device to aid cranial nerve memorization.
- Review the structural differences between the sympathetic and parasympathetic systems.