Organization of the Human Nervous System
High-Yield Summary
- 3 neuron types: sensory/afferent (into CNS), motor/efferent (out of CNS), interneurons (within CNS, most numerous, enable learning/memory/reasoning).
- CNS = brain + spinal cord (most processing). PNS = everything else, splits into somatic (voluntary, skeletal muscle) and autonomic (involuntary, organs/glands).
- Autonomic splits into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) — both are ALWAYS active; what changes is which dominates.
- Sympathetic: pupils dilate, heart rate/force up, airways widen, glycogenolysis (glycogen→glucose), digestion slows, sweating up, saliva down, bladder activity down.
- Parasympathetic: pupils constrict, heart rate/force down, airways return to resting, digestion up (saliva/enzymes/motility), bladder contracts + sphincters relax (urination).
- Epinephrine/norepinephrine (sympathetic response) are released from the adrenal medulla.
Key Terms
- Afferent (sensory) neuron
- Carries signals INTO the CNS from sensory receptors.
- Efferent (motor) neuron
- Carries signals OUT of the CNS to effectors (muscles, glands).
- Interneuron
- Connects/processes between neurons; most common type; supports learning, memory, reasoning.
- Somatic nervous system
- PNS branch for conscious sensation and voluntary movement (skeletal muscle).
- Autonomic nervous system
- PNS branch for involuntary control of heartbeat, breathing, digestion, blood vessels.
- Glycogenolysis
- Breakdown of stored glycogen into glucose during the sympathetic response, making fuel available to cells.
Sympathetic vs. Parasympathetic Effects
| Sympathetic (Fight-or-Flight) | Parasympathetic (Rest-and-Digest) |
|---|---|
| Pupils dilate | Pupils constrict |
| Heart rate/force increase | Heart rate/force decrease |
| Airways widen | Airways return to resting diameter |
| Glycogen → glucose (glycogenolysis); epinephrine/norepinephrine released | Energy conserved and replenished |
| Digestion slows | Digestion increases (saliva, enzymes, motility) |
| Sweating increases, saliva decreases | Not significant for sweat glands |
| Bladder activity decreases | Bladder wall contracts, sphincters relax (urination) |
Nervous System Organization (Top Down)
- 1Central nervous system (CNS) — brain + spinal cord, processes most information.
- 2Peripheral nervous system (PNS) — splits into somatic (voluntary, skeletal muscle) and autonomic (involuntary, organs/smooth muscle).
- 3Autonomic nervous system — splits into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest), both always active.
Common MCAT Trap
- Afferent = sensory = INTO the CNS; efferent = motor = OUT of the CNS.
- Sympathetic and parasympathetic aren't an on/off switch — both are always active; the question is which dominates at a given moment.
- Sweat glands and adrenal glands are driven almost entirely by the sympathetic division — parasympathetic has no major counteracting role there, unlike most other organs.
Quick Recall
Which neuron type is the most numerous and found entirely within the CNS?
Which division controls voluntary movement of skeletal muscle?
Where are epinephrine and norepinephrine released from during the sympathetic response?
What happens to digestion under sympathetic dominance vs. parasympathetic dominance?