Annelids and arthropods have segmentally arranged clusters of neurons called ganglia.
Nervous system organization usually correlates with lifestyle.
Sessile molluscs (e.g., clams and chitons) have simple systems, whereas more complex molluscs (e.g., octopuses and squids) have more sophisticated systems.
The CNS is composed of the brain and spinal cord.
The peripheral nervous system (PNS) is composed of nerves and ganglia.
The spinal cord conveys information from the brain to the PNS.
The spinal cord also produces reflexes independently of the brain.
A reflex is the body’s automatic response to a stimulus.
For example, a doctor uses a mallet to trigger a knee-jerk reflex.
Cell body of
sensory neuron in
Invertebrates usually have a ventral nerve cord while vertebrates have a dorsal spinal cord.
The spinal cord and brain develop from the embryonic nerve cord.
Vertebrate Nervous System
Ventricles, gray matter, and white matter
The central canal of the spinal cord and the ventricles of the brain are hollow and filled with cerebrospinal fluid.
The cerebrospinal fluid is filtered from blood and functions to cushion the brain and spinal cord.