In "Tracing Heads," Shubin briefly explains the development of nervous systems from simpler ancestral parts, like a notochord (94).
Although Shubin focuses primarily on the specific trend toward cephalization in this section, his discussion is based on the assumption that our vertebrate nervous system has evolved over time.
How exactly does this nervous system work? How has it changed from our ancestors to vertebrates? What selective pressures in our evolutionary history have favored this development?
- Vincent Fiorentini
(vincent@panatechcomputer.com)
The vertebrate nervous system is a very complicated network of cells called neurons that are used to coordinate the actions of an animal and transmit signals between different parts of its body. The vertebrate nervous system is split into two major parts: the central nervous system and the peripheral nervous system. The central nervous system contains the brain, spinal cord, and retina. The peripheral nervous system is made up of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system. The way this system works is that the neurons send signals to other cells as electrochemical waves travelling along thin fibers called axons. These signals cause chemicals called neurotransmitters to be released at junctions called synapses between the neurons. Two types of neurons seen in the nervous system are sensory neurons and motor neurons. Sensory neurons are neurons that send signals that inform the central nervous system of the state of the body and the external environment. In other words, they “are responsible for converting external stimuli from the organism's environment into internal electrical impulses” (ScienceDaily). Motor neurons connect the nervous system to muscles or other effector organs. These neurons can facilitate muscle contraction and allows the muscles to move.
ReplyDeleteIn the more primitive or unicellular organisms, nervous systems were not seen; like a sponge does not have a nervous system. However, they did have genes that were able to function as a sort of nervous system that allowed them to have a form of locomotion. Other animals that do not have nervous systems are placozoans and mesozoans. In radially symmetric animals, the nervous system consists of a nerve net. Nerve nets consist of interconnected neurons. This type of nervous system lacks a brain or any form of cephalization. With this form of nervous system, animals are able to respond to physical contact. However, this is not a selective advantage for these animals because even though they can respond to their environment, they have trouble knowing where the stimulus is coming from. In vertebrates and other bilaterally symmetrical organisms, with the formation of a brain and the use of cephalization, the animals were able to know where contact was coming from and therefore respond accordingly. This could be helpful for animals that are being attacked and need to defend themselves because they have to be able to know where their attacker is coming from in order to do this.
Sources:
http://icb.oxfordjournals.org/content/47/5/712.full
http://www.sciencedaily.com/articles/s/sensory_neuron.htm
http://www.sciencedaily.com/articles/m/motor_neuron.htm
http://rstb.royalsocietypublishing.org/content/363/1496/1523.full
Danielle Webb (dwebb456@gmail.com)
TWO PART RESPONSE:PLEASE READ BOTH OF MY POSTS DUE TO CHARACTER LIMIT-
ReplyDeleteShubin explains the similarities of the invertebrate Amphioxus species, that shares many of the features vertebrates (animals with backbone) have. Although Amphioxus lack backbone, they contain a nerve cord running along its back similar to vertebrates. A rod (notochord-filled with jelly-like substance-provides support) runs parallel to the nerve chord. Unlike Amphioxus, our notochord is differentiated and becomes part of the disks that lie between our vertebrae. The Amphioxus worm reveals something of the origin and nervous system of our bodies, and the transition of the head goes back from the history of worms. The simplest animals to display cephalization were the flatworms.
What makes the head so vital in advanced organisms?
Shubin comments on the nature of the transition of headless to headed organisms by a process known as cephalization, which is an evolutionary process in which the sensory organs move towards the anterior region. Cephalization provides us with an evolutionary advantage, as having sensory organs facing our direction of movement allow us to have a more focused and accurate assessment of the nature of our environment. Cephalization has also led organisms in the direction of bilateral symmetry (as appose to radial).
As Danielle Webb has mentioned the complexity of the nervous system, cephalization is advantageously associated with it, in that it allows the concentration of neurons into a body region. A neuron is an excitable cell specialized in electrical signaling over long distances. These receive input from sensory sells/other neurons and send output to muscles/other neurons. Sensory organs have also been made into a body region (eyes, nose mouth, ears). Cephalization has also led to to the arising of the central nervous system (pertaining to the brain) and peripheral nerves. The advancement of the brain has played a vital role in the development of the nervous system.
The general function of the nervous system is to act as the control-freak of the body. It sends and receives information by communicating throughout the body. The main control center that is affiliated with the nervous system is the brain, which is made up of many parts including-cerebrum, cerebellum, and brain stem. The cerebrum controls senses and vision. The cerebellum controls coordination. The brain stem controls breathing and digestion (Also links to the spinal cord). Neurons carry impulses from the sensory receptors to the central nervous system. The central nervous system includes the brain and spinal cord. Information from nerve impulses go to the spinal cord. The peripheral nervous system includes all the nerves in the body, and sends messages from sensors to the Central Nervous System. Environmental stimulus is received by receptors which stimulate sensor neurons to send signals to the Central Nervous System. The Central Nervous System sends signals to motor neurons to send signals to effectors to elicit response. Sensory and motor neurons are part of the peripheral nervous system, while the Central Nervous System contains interneurons.
In the ancestors of vertebrates, simplest nervous systems were found in cnidarians such as hydra. These had a loosely organized system of nerves with no central control, and stimulation at any point spread to cause movemnt of entire body. The jellyfish, is a cnidarian that has exhibited basic nerve centralization. The nervous system forms an indifferentiated network to serve and coordinate swimming motions. This basic nervous system served simple communications so parts open and contract at the same rates. Bilateral symmetry, along with cephalization, has led to advancement in the nervous system. The enlargement of the anterior ganglia (receives sensory input and control feeindg) gave rise to brains. An anterior brain connected to a nerve cord is the basic structure for organisms with CNS.
Soon enough, from invertebrates to vertebrates, brains became larger and more complex, and composed a series of swellings of the anterior end of the spinal cord (serve as two-way path of communication). Evolutionary history has favored this form of nervous system development as it has allowed organisms to function smoother and be alert to their physical environment. The nervous system has a plethora of benefits, including the most vital ones to be communication, defense, and self-awareness.
ReplyDeleteSources:
http://library.thinkquest.org/5777/ner1.html
http://en.wikipedia.org/wiki/Neurulation
http://www.sciencedaily.com/articles/health_medicine/nervous_system/
http://serendip.brynmawr.edu/bb/kinser/Structure1.html
Campbell
Your Inner Fish
Kyle Kim (piece847@gmail.com)