8 3 Brains We have looked at many specific examples of how neurons work but how are all of the neurons in an animal organized Neurons must be wired together to fire together or form an electrical circuit in order to operate as a complex system that controls output This output could be motor output and behavior but a lot of the function of a nervous system is to maintain normal body functions including chemical balance fluid pressure pH and supplying all the cells of the body with the oxygen and other small molecules they need to stay alive and perform their specialized roles Recall that at its simplest a nervous system consists of sensory afferents interneurons and efferents The interneurons the cells that integrate environmental input and distribute the response comprise the central nervous system of an animal The neurons that act as environmental sensors and the neurons that innervate the effector organs comprise the peripheral nervous system Central nervous systems all serve this same basic purpose and are all made of the same fundamental pieces However there is an astonishing diversity of how these pieces are put together and a corresponding complexity of outputs that they are capable of We will address this complexity along an evolutionary timeline from least complex to most As always if we want to understand the physiology of animal nervous systems we have to incorporate physics chemistry and many sub disciplines of biology from molecules to evolution of species over billions of years Because nervous systems can differ so greatly from animal to animal it can be difficult to compare one to another and unequivocally identify similarities and differences Some of the layers of biological organization that we consider are shown below in a framework adapted from a paper by Pollen and Hofmann https utexas instructure com courses 1099761 files 35069909 download verifier fHirkFywOjTjyGP3qxhDO29BSO7uh0GaVM3CqqN2 wrap 1 https utexas instructure com courses 1099761 files 35069909 download verifier fHirkFywOjTjyGP3qxhDO29BSO7uh0GaVM3CqqN2 wrap 1 https utexas instructure com courses 1099761 files 35069909 download verifier fHirkFywOjTjyGP3qxhDO29BSO7uh0GaVM3CqqN2 wrap 1 2008 The earliest branching event in animal evolution occurred when one group developed two cell layers during embryonic development This separated sponges which develop from only one cell layer and thus show little cell specialization as mature animals from all other animals Sponges represent the most basal lineage of animals like present day fossils as they are our best living representation of what animals were like to begin with Sponges are the only animals that do not have nervous systems All other animals develop from at least two cell layers during embryogenesis which allows cells to differentiate into two sets of tissues early on Placozoans marine disc like creatures that consist of 1000 3000 cells Ctenophores comb jellies and Cnidarians anemones corals jellyfish are the only animals with nervous systems that are not organized into afferents and efferents Recall that these radially symmetric animals have diffuse nets of neurons that are all interconnected like a web and can fire action potentials in all directions Their synapses are not directional they are functionally bipolar so a sensory stimulus on any part of the body will radiate throughout the body in every direction Although the nervous systems of radially symmetric animals are fairly disorganized these animals are able to respond to their environment and sometimes exhibit series of behaviors Even coral which is sessile immobile responds to changes in water temperature by coordinating massive releases of gametes facilitating fertilization in the surrounding water The sea anemone Calliactis parasitica is able to climb over shells of nearby molluscs by performing a somersault onto the shell Most animals are bilaterally symmetrical They are divided in half by a midline that runs from the head anterior end to the tail posterior end creating a left and right side During the evolution of bilateral symmetry many of the sensory afferents became clustered near the anterior end of the animal a process referred to as cephalization from Greek kephal meaning head These sensory afferents became grouped together as sense organs located in the head of the animal Most bilaterally symmetrical animals have other clusters of neurons in their body called ganglia which behave as simple integrating centers Ganglia in the head are often grouped together and organized forming a brain Ganglia in the brain are referred to as nuclei and axons of these nuclei are grouped together to form tracts Axons of neurons in the peripheral nervous system are also often grouped together which are colloquially referred to as nerves A series of animals from earliest in evolution to most recent is shown below to illustrate how the degree of organization has changed with evolutionary time In addition to cephalization there have been trends of further specialization within the nervous system and regionalization as the brain has become larger and more complex and divided into areas that perform special functions Note that these are examples from several animal groups and each example may not represent all of the group s members For example flatworms the simplest bilaterally symmetric animals include the non parasitic Planarians which have brains as shown and parasitic groups which have several ganglia Roundworms are more developed with bodies that are segmented which allows different parts along the length of the body to have specialized roles Segmentation was a major event in animal evolution that has independently evolved twice leading to annelids and arthropods and again leading to vertebrates without dividing the body into zones along the anterior posterior axis there is less anatomical and functional specialization into different body parts Within each animal lineage the size of the brain scales with the size of the species Larger species of a particular group have more neurons which make larger brains For example an elephant has a proportionally larger brain than a smaller mammal like a shrew However brain and body size do not scale across groups In particular birds mammals and octopuses have unusually large brains for their body sizes much larger than comparably sized reptiles The allometric scaling of brain and body size measured as weight within groups is shown in the graph below where each
View Full Document
Unlocking...