The following are two lists of

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s ordered by the size of their

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...

. The first list shows number of

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

s in their entire nervous system, indicating their overall neural complexity. The second list shows the number of neurons in the structure that has been found to be representative of animal intelligence. The human brain contains 86 billion neurons, with 16 billion neurons in the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting of ...

Overview

Neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

s are the cells that transmit

information Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random ...

in an

so that it can

sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...

stimuli A stimulus is something that causes a physiological response. It may refer to: * Stimulation ** Stimulus (physiology), something external that influences an activity ** Stimulus (psychology), a concept in behaviorism and perception * Stimulus (eco ...

from its environment and behave accordingly. Not all animals have neurons; ''

Trichoplax ''Trichoplax adhaerens'' is one of the three named species in the phylum Placozoa. The others are '' Hoilungia hongkongensis'' and '' Polyplacotoma mediterranea''. The Placozoa is a basal group of multicellular animals (metazoa). ''Trichoplax'' ...

'' and

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

s lack

nerve cell A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. No ...

s altogether. Neurons may be packed to form structures such as the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s or the neural ganglions of

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...

s. The number of neurons and their relative abundance in different parts of the brain is a determinant of neural function and, consequently, of behavior.

Whole nervous system

All numbers for neurons (except Caenorhabditis and Ciona), and all numbers for synapses (except Ciona) are estimations.

List of animal species by forebrain (cerebrum or pallium) neuron number

The question of what physical characteristic of an animal makes an animal intelligent has varied over the centuries. One early speculation was brain size (or weight, which provides the same ordering.) A second proposal was brain-to-body-mass ratio, and a third was

encephalization quotient Encephalization quotient (EQ), encephalization level (EL), or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regressi ...

, sometimes referred to as EQ. The current best predictor is number of neurons in the forebrain, based on Herculano-Houzel's improved neuron counts. It accounts most accurately for variations in dependence on the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

. The elephant depends on its exceptionally large cerebellum, while birds make do with a much smaller one. Differing methods have been used to count neurons and these may differ in degree of reliability. These are the optical fractionator, an application of

stereology Stereology is the three-dimensional interpretation of two-dimensional cross section (geometry), cross sections of materials or tissues. It provides practical techniques for extracting quantitative information about a three-dimensional material from ...

and the isotropic fractionator, a recent methodological innovation. Most numbers in the list are the result of studies using the newer isotropic fractionator. A variation of the optical fractionator was responsible for the previous total human brain neuron count of 100,000,000,000 neurons, which has been revised down to 86,000,000,000 by the use of the isotropic fractionator. This is in part why it may be considered to be less reliable. Finally, some numbers are the result of estimations based on correlations observed between number of cortical neurons and brain mass within closely related taxa. The following table gives information on the number of neurons estimated to be in the sensory-associative structure: the

(aka

pallium The pallium (derived from the Roman ''pallium'' or ''palla'', a woolen cloak; : ''pallia'') is an ecclesiastical vestment in the Catholic Church, originally peculiar to the pope, but for many centuries bestowed by the Holy See upon metropolit ...

) for mammals, the dorsal ventricular ridge ("DVR" or "hypopallium") of the pallium for birds, and the corpora pedunculata ("mushroom bodies") for insects.

Notes

References

{{animal cognition

Neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

Animal nervous system Animals by number of neurons Number of neurons

Overview

Whole nervous system

List of animal species by forebrain (cerebrum or pallium) neuron number

See also

Notes

References