Organization of the Human Nervous System
High-Yield Summary
- 3 neuron types: sensory/afferent (into CNS), motor/efferent (out of CNS to effectors), interneurons (within CNS, connect sensory↔motor, most numerous).
- CNS = brain + spinal cord. PNS = everything else, split into somatic (voluntary, skeletal muscle) and autonomic (involuntary, organs/smooth muscle).
- Autonomic splits into parasympathetic ("rest and digest") and sympathetic ("fight or flight") — largely opposing organ-specific effects, both maintain homeostasis.
- Reflex arcs bypass the brain: monosynaptic (sensory→motor directly, e.g. knee-jerk) vs. polysynaptic (sensory→interneuron(s)→motor, e.g. withdrawal reflex).
- Conscious awareness of a reflex stimulus arrives AFTER the reflex has already occurred (interneurons relay info to the brain in parallel).
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
- CNS-only neuron connecting sensory and motor neurons; most numerous type; key for reflexes, integration, learning/memory.
- Effector
- A muscle or gland that receives motor neuron instructions.
- Reflex arc
- Simple neural circuit producing an automatic response, often bypassing the brain.
Parasympathetic vs. Sympathetic Effects
| Parasympathetic (Rest & Digest) | Sympathetic (Fight or Flight) |
|---|---|
| Pupils constrict | Pupils dilate |
| Heart rate slows | Heart rate increases |
| Airways constrict | Airways dilate |
| Stimulates bile release | Glycogen → glucose (raises blood sugar) |
| Stimulates digestion | Inhibits digestion |
| Not significant for sweat glands | Stimulates sweating |
| Not significant for adrenal glands | Stimulates adrenaline release |
| Relaxes bladder, promotes urination | Relaxes bladder, prevents urination under stress |
Monosynaptic vs. Polysynaptic Reflex Arcs
| Monosynaptic | Polysynaptic |
|---|---|
| Sensory neuron synapses directly on motor neuron | Sensory → one or more interneurons → motor neuron(s) |
| Fastest possible response (e.g. knee-jerk reflex) | More complex/coordinated (e.g. withdrawal reflex) |
Nervous System Organization (Top Down)
- 1Central nervous system (CNS) — brain + spinal cord.
- 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).
Common MCAT Trap
- Afferent = sensory = INTO the CNS; efferent = motor = OUT of the CNS — a classic mix-up under time pressure.
- Sympathetic and parasympathetic don't fully oppose everywhere — e.g. neither significantly drives sweat glands or adrenal glands EXCEPT sympathetic (parasympathetic has no major role there).
- You become consciously aware of a reflex stimulus only AFTER the reflex has already fired — the reflex arc doesn't wait for the brain.
Quick Recall
Which neuron type carries signals out of the CNS to effectors?
What is the classic example of a monosynaptic reflex arc?
Which autonomic branch increases heart rate and dilates airways?
What is the most numerous neuron type, and where is it found?