Animal nervous systems allow bodies to sense the environment, process information, and produce coordinated responses. Across animal evolution, nervous systems generally became more centralized, with nerve cells grouped into cords, ganglia, and brains. This trend matters because it helps explain how animals gained faster movement, sharper senses, and more flexible behavior.
Comparing simple and complex nervous systems shows how structure supports function.
Key Facts
- A nerve net is a diffuse web of neurons with no central brain, common in cnidarians such as jellyfish and hydra.
- Bilateral symmetry often supports cephalization, the concentration of sensory organs and nervous tissue near the head.
- Ganglia are clusters of neuron cell bodies that process local information and coordinate body regions.
- Centralization means more neurons are concentrated in major processing centers such as a brain or nerve cord.
- Signal speed can be estimated by v = d/t, where v is speed, d is distance, and t is time.
- A reflex arc can produce a rapid response through sensory neuron to interneuron to motor neuron pathways, sometimes before the brain fully processes the stimulus.
Vocabulary
- Neuron
- A neuron is a specialized nerve cell that carries information using electrical impulses and chemical signals.
- Nerve net
- A nerve net is a loose network of neurons spread through the body without a single central control organ.
- Ganglion
- A ganglion is a cluster of neuron cell bodies that acts as a small processing center.
- Cephalization
- Cephalization is the evolutionary concentration of sensory structures and nervous tissue at the front end of an animal.
- Central nervous system
- The central nervous system is the main processing region of an animal nervous system, usually including a brain and nerve cord or spinal cord.
Common Mistakes to Avoid
- Calling every animal nervous system a brain, which is wrong because many animals have nerve nets or ganglia instead of a centralized brain.
- Assuming evolution always moves in a straight line from simple to complex, which is wrong because nervous systems adapt to specific lifestyles and environments.
- Confusing ganglia with glands, which is wrong because ganglia process nerve signals while glands release substances such as hormones or enzymes.
- Thinking reflexes always require conscious thought, which is wrong because many reflex pathways are processed quickly through local circuits or the spinal cord.
Practice Questions
- 1 A nerve signal travels 1.2 m from a sensory receptor to a processing center in 0.040 s. What is the signal speed in m/s?
- 2 An earthworm has 21 body segments, and each segment contains one pair of ganglia. How many ganglia are present in those segments?
- 3 Explain why an active predator with a distinct head and forward-facing sensory organs would benefit from a more centralized nervous system than a stationary animal with a nerve net.