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The simplest possible creatures have incredibly simple nervous systems made up of nothing but reflex pathways. For example, flatworms and invertebrates do not have a centralized brain. They have loose associations of neurons arranged in simple reflex pathways. Flatworms have neural nets, individual neurons linked together that form a net around the entire animal.

Most invertebrates (such as the lobster) have simple "brains" that consist of localized collections of neuronal cell bodies called ganglia. Each ganglion controls sensory and motor functions in its segment through reflex pathways, and the ganglia are linked together to form a simple nervous system. As nervous systems evolved, chains of ganglia evolved into more centralized simple brains.

Major Divisions of the Brain

Spinal cord
Diencephalon - thalamus, hypothalamus
Cerebral cortex

Brains evolved from ganglia of invertebrates. Regardless of the animal, brains have the following parts:

Brainstem - The brainstem consists of the medulla (an enlarged portion of the upper spinal cord), pons and midbrain (lower animals have only a medulla). The brainstem controls the reflexes and automatic functions (heart rate, blood pressure), limb movements and visceral functions (digestion, urination).

Cerebellum - The cerebellum integrates information from the vestibular system that indicates position and movement and uses this information to coordinate limb movements.

Hypothalamus and pituitary gland - These control visceral functions, body temperature and behavioral responses such as feeding, drinking, sexual response, aggression and pleasure.

Cerebrum (also called the cerebral cortex or just the cortex) - The cerebrum consists of the cortex, large fiber tracts (corpus callosum) and some deeper structures (basal ganglia, amygdala, hippocampus). It integrates information from all of the sense organs, initiates motor functions, controls emotions and holds memory and thought processes (emotional expression and thinking are more prevalent in higher mammals).

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