Nervous Systems And Sensory Organs
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You take a needle and prick your finger, your response is probably to get your finger away from the needlepoint quickly and maybe yell ouch. You take the same needle and poke a protist, sponge, cnidarian, or worm with it and they will all probably exhibit a similar response, at least they will all retreat. But how can this be possible? Protists are unicellular organisms that don't have a system to communicate after coming in contact with a stimulus. Similarly, sponges just sit on a rock all day, they also don't have any nervous functions. Cnidarians on the other hand have a nerve net, but how do they coordinate responses without a brain? All of these questions, along with how body plans relate to nervous system evolution, will be answered as this paper explores the evolution of the nervous system and sense organs from protists to the vertebrates.
The classification of protozoans has been changed a lot through time. Cavalier-Smith (1993) came up with what he called the simplest definition of the kingdom Protozoa. They are eukaryotes, other than those that primitively lack mitochondria and peroxisomes, which lack the shared derived characters that define the higher derived kingdoms of Animalia, Fungi, and Plantae (Cavalier-Smith, 1993). Even though protozoans are simple unicellular organisms, they can still respond to many of the same stimuli higher order organisms respond too.
Take for example that science class most students have. You put paramecium under a microscope and try to touch them with a probe, or watch their response to the light from the microscope. Most of the time when the anterior membrane of Paramecium is mechanically stimulated the ciliary power stroke reorients so the cell swims backwards, or retreats (Ogura & Machemer, 1980). If the posterior membrane is stimulated the cilia beat towards the rear, causing the organism to move forward (Ogura & Machemer, 1980). The light from the microscope can affect both the photoreceptors and thermoreceptors of the protists. The unicellular alga Euglena shows two regions of peak sensitivity to light during photokinesis at 465 nm and then again near 630 nm and during phototaxis 490-500 nm (Leys et al., 2002). Euglena is phototactic and its system consists of locomotory flagellum, an eyespot, and a photoreceptor (Gualtieri, 2001). As the organism moves, the eyespot senses the amount of light that reaches it and therefore pushes the Euglena in the direction of more light (Gualtieri, 2001). But moving towards light also means a change in temperature, especially if the light source is close to the organism. Paramecium cells are themo-sensitive and tend to accumulate at temperatures they were cultured at (Toyoda et al., 2009). They become used to their membrane fluidity at this temperature, and small temperatures changes drastically change this fluidity (Toyoda et al., 2001). If the temperatures change too much the Paramecium will retreat away from the heat in order to survive (Hennessey, Saimi, & Kung, 1983).
Protists also have chemosensory responses to certain odorants and tastes. Rodgers, Markle, and Hennessey (2008) found G-protein coupled receptors in the Paramecium. They tested whether Paramecium and Tetrahymena could respond to the common higher order organisms' odorants and tastants (Rodgers, Markle, & Hennessey, 2008). If they are affected by the odorants or tastants they will do an "avoiding reaction," which can be seen when the organism is leaving an attractant or enters a repellent (Valentine, Yano, & Van Houten, 2008). The Tetrahymena was more sensitive and could detect all of the tastants sampled, while Paramecium only detected four or the ten (Rodgers, Markle, & Hennessey, 2008). Since Paramecium feed on bacteria Valentine, Yano, and Van Houten (2008) showed that they are attracted to bacterial metabolites such as, folate, acetate, glutamate, cyclic AMP, Biotin, and Ammonium. So an organism without a nervous system or sense organs has the ability to respond to many of the same environmental factors that higher order organisms respond too.
Similar to the protozoans, sponges lack definite body symmetry and also lack nerves and cell junctions, allowing no communication between cells (Leys et al., 2002). Sponges do respond to both light and mechanical stimuli. Recently, some sponges have been found to respond to light by contracting their cilia (Leys et al., 2002). Most larvae, via their cilia, are sensitive to light near 440 nm and again at 600 nm causing them to respond by straightening and bending (Leys et al., 2002). Although sponges lack neurons they are sedimentary feeders and therefore need ways of dealing with excessive particulates in their feeding currents (Tompkins-MacDonald & Leys, 2008). Cellular sponges have the ability to close the openings to their incurrent canals, constrict the size of their intake canals, and even carry out a series of slow contractions that expel unwanted material (Tompkins-MacDonald & Leys, 2008). The syncytial tissues of glass sponges allow action potentials initiated at single or multiple sites to propagate through the entire animal, stopping the feeding current (Tompkins-MacDonald & Leys, 2008). When Tompkins-MacDonald and Leys (2008) tested this response they found that by probing the interal body wall, allowing light to touch the outer body wall, or by knocking on the outer body wall pumping was stopped. This shows that a sponge, although not having an nerves or cell junctions can still respond to its environment.
Cnidaria and Ctenophora
Cnidaria and Ctenophora are the most basally branching lineages with specialized sense organs. The Cnidaria are radially symmetrical and have a nerve net where the sensor and ganglionic neurons and their processes are interspersed among the epithelial cells of both layers (Watanabe, Fujisawa, & Holstein, 2009). Sensory structures that form part of epidermis are found in all animal phyla. Cnidarian neurons do not cluster to form a central nervous system or ganglia, which is why the nerve net is considered the simplest nervous system (Sarnat & Netsky, 2002). In cnidarians sensory structures consist of naked sensory neurons whose dendrite is formed by a modified cilium (Jacobs et al., 2007). Sensilla are individual sensory neurons, or small groupings of sensory neurons, that typically function in one of the following; light detection, mechanoreception, and chemoreception (Jacobs et al., 2007). Photoreception and chemoreception involve G protein-coupled receptors (GPCRs) and membrane ion channels, similar to what was observed in the protists (Jacobs et al., 2007). Jacobs et al. (2007) believes that sense organs and kidneys in bilterians may have evolved from groupings of choanocytes in sponges. Cnidarian sense organs are usually associated with the free swimming form that resembles a jellyfish (Jacobs et al., 2007).
Neural regionalization is most evident in the medusozoans that have rhopalia , an eye system with lenses (Watanabe, Fujisawa, & Holstein, 2009). Other cnidarians contain simple eyes. A statocyst is a dense array of mechanosensory cells that serve as a touch plate (Jacob et al., 2007). In most cnidarians the rhopalia, sense organ, alternate with tentacles, appendages, similar to how vertebrates have organs associated with appendages (Jacobs et al., 2007). Photoreceptors responsible for contractions in Hydra in response to blue light at 470 nm, are consistent in spectral location and shape with a rhodopsin-based photoreceptive system (Leys et al., 2002).
One of the newest findings deal with coral larvae and their exterior cilia being able to detect and respond to underwater sound fields (Vermeij et al., 2010). Vermeij et al. (2010) setup six chambers directed towards underwater speakers playing day and night reef sounds. Free-swimming coral larvae moved predominately towards the speakers independent of chamber orientation (Vermeij et al., 2010). This study was done because fish larvae used it as well.
The flatworms have true bilateral symmetry (Reuter & Gustafsson, 1995). Some flatworms have a nerve net like Cnidarians. Others have a central nervous system that consists of anterior ganglia, the brain, and one or several pairs of longitudinal nerve cords that are connected in a ladder-like configuration (Reuter & Gustafsson, 1995). The peripheral nervous system is just a meshwork of nerves that are interconnected to the central nervous system (Reuter & Gustafsson, 1995). Platyhelminthes has eyes, a light sensing organ, on the dorsal side of the body composed of two cell types: pigment cells and photoreceptor cells (Inoue et al., 2004). The pigment cells form an eye-cup while the visual neurons are located outside the eye-cup (Inoue et al., 2004). The eyes do not allow Planarians to see distinct images, but allows them to be repulsed by bright light, a condition known as negative phototrophism (Hyde, 2003). Not only does the head bear a pair of eyes, but a also a pair of ear-like lobes called auricles. Auricles have nothing to do with hearing; instead they are involved in mechanoreception, chemoreception, and pressure reception (Hyde, 2003).
Caenorhabditis elegans (C. elegans) has been established as a genetic and genomic model organism (Zhang, 2008). C. elegans does not have a visual or auditory system so it depends on chemosensation to detect bacteria to feed on (Zhang, 2008). C. elegans has exactly 302 neurons, 32 of which are chemosensory because they have ciliated endings that are directly exposed to their external environment (Troemel et al., 1995). Free-living nematodes use amphids and phasmids as sensory structures to seek food and avoid harmful situations, while parasitic nematodes use amphids to actively or passively see a host (Srinivasan, Durak, Sternberg, 2008). Amphids are either enclosed in the amphid sheath or exposed to the environment (Zhang, 2008). Ascaris lumbricoides (A. lumbricoides), a larger nematode, contains 298 neurons (Srinivasan, Durak, Sternberg, 2008). Each group of neurons reacts to certain stimuli. For example, some respond to salt (ASE chemosensory neurons), others respond to volatile aldehydes, ketones, and alcohols (AWC olfactory neurons), and yet others respond to chemical, mechanical, and osmotic stimuli (ASH neurons) (Srinivasan, Durak, Sternberg, 2008; Troemel et al., 1995).
Mollusks, Annelids, and Arthropods
There are three different nervous systems seen in mollusks alone. Bivalves tend to have no cephalization, while slowing moving mollusks have some cephalization, primary to connect senses and motor information while moving through the environment (Gregory, 2006). The cephalopods require complex sense organs and so they are highly cephalized Gregory, 2006). The cephalopods are known for their well-developed eye, that functions almost exactly like the human eye, which is why they such good eye sight (Oceanic Research Group, 2007).
Annelids and Arthropods have repeating segments and an anterior brain. Each segment contains its own ganglion, which controls the muscles of that segment (Gregory, 2006). The nerve cord of both phylum runs directly through all of the segments (Gregory, 2006). These two were grouped together because they are very similar in function for being two different phyla.
Amphioxous, part of the phylum chordata, are only capable of a few reflexive responses. They do not have the ability to recognize tactile stimuli, so all stimuli are interpreted as a threat and the organism curls away in defense (Sarnat & Netsky, 2002). This reaction demonstrates how neurons that feel the stimuli on one side of the body are transmitted and affect motor neurons on the other side of the body (Sarnat & Netsky, 2002). The neuron that served this function was known as the decussating interneuron (Sarnat & Netsky, 2002). This is the underlying groundwork of the vertebrate nervous systems.
Vertebrates have bilateral symmetry, complex sense organs and complex behaviors, requiring a very cephalized, complex nervous system. Vertebrates consist of two nervous systems, the Central and Peripheral. The central nervous system (CNS) contains the brain and spinal cord, while the peripheral nervous system (PNS) is composed of the nerves running through the body. The CNS has been conservative in its evolution, especially when looking at the senses of the vertebrates (Hodos & Butler, 1997). The receptor types are either monopolar or pseudomonopolar neurons, each consist of parallel pathways connecting the receptors to the primary central neurons, which are located inside the sense organs where the stimuli is processed (Hodos & Butler, 1997).
Nerves are bundles of neurons, without cells bodies (Gregory, 2006). Most nerves contain both sensory and motor abilities (Gregory, 2006). There are both cranial and spinal nerves. There are multiple cranial nerves in all vertebrates, with humans having 12, and they are responsible for both sensory and motor information (Brown, 2003). The nerves are numbered using roman numerals from 1 to 12 (Brown, 2003). Brown (2003) described all of the following cranial nerves. Cranial nerve I is the olfactory nerve and it carries the sense of smell to the olfactory bulb of the brain. Cranial nerve II is the optic nerve and it carries visual information to the brain. Cranial nerve III is the oculomotor nerve and it provides motor ability to the four-extrinisic eye muscles, muscles of the upper eyelid, and intrinsic eye muscles. Cranial nerve IV is the trochlear nerve and it gives motor ability to the superior oblique eye muscle. Cranial nerve V is the trigeminal nerve and it provides sensory information from the face, forehead, nasal cavity, tongue, gums and teeth. Cranial nerve VI is the abducens nerve gives motor ability to the lateral rectus muscle of the extrinisic eye. VII is the Facial nerve that provides humans with facial expressions. VIII is the vestibulocochlear nerve and it innervates the hair cell receptors of the inner ear. IX or the glossopharyngeal nerve moves the pharynx, soft palate, and posterior region of the tongue. X is the vagus nerve, it is the longest nerve, and provides sense transports from the ear to the taste buds to the throat. Cranial nerve XI is the spinal accessory nerve and it is involved in swallowing and powering muscle movement for the upper shoulders, head, and neck. Lastly, XII is the hypoglossal nerve and it moves the muscles of the tongue (Brown, 2003). As you can see the ability for control of all of these senses and movements makes for a very complex nervous, something that was never seen in earlier organisms. The spinal nerves are connected directly to the spinal cord by two roots, the dorsal (strictly sensory) and ventral (strictly motor) (Gregory, 2006). All of the above are seen in the PNS, which is then subdivided into the Somatic and Autonomic Nervous System.
The Somatic Nervous System is the voluntary system, including all of the nerves that serve the skeletal muscles and exterior sense organs (Gregory, 2006). Reflexes are also seen in this nervous system. Just like in the lower organisms, vertebrates respond to stimuli. Except vertebrates have a much more complex stimulus-reflex system. Remember how a Paramecium came in contact with a stimulus and retreated until adaptation occurred? Vertebrates have the ability to think about coming in contact with a stimulus, they don't have to come in contact with everything they see because they have the somatic nervous system (Gregory, 2006). The other subdivision is the autonomic nervous system that is responsible for actions without conscious control; examples are heart beating and smooth muscle actions (Gregory, 2006).
Everything in the PNS needs a place to send its information too and that is why there is the central nervous system. In the more primitive animals the brain, or ganglia, was there to simply send out reflexes to external stimuli (Gregory, 2006). The vertebrates have evolved a very complex brain because they have the ability to respond to reflexes, hold memory, learn, and think (Gregory, 2006). The brain has three divisions, the hindbrain, midbrain, and forebrain. The important portion when talking about senses is the midbrain because it receives sensory information and sends it to the forebrain to be processed (Gregory, 2006). In fishes and amphibians it is geared towards reflexes associated with visual input (Gregory, 2006). The cerebrum in reptiles, birds, and mammals receives sensory information and coordinates motor responses (Gregory, 2006). There are four lobes the frontal (motor functions), parietal (sensory receptors from skin), occipital (vision), and temporal (hearing and smelling) (Gregory, 2006). Similar to decussating interneuron in Amphixous, vertebrates have the corpus callosum that contains neurons that cross from one side of the brain to the other, allowing communication between both sides (Gregory, 2006).
Bioluminescence is when luciferase catalyzes the oxidation of luciferin to excited oxyluciferin that then relaxes to produce a visible photon (Akilesh, 2000). The most common colors for bioluminescence are blue and green, although red and violet can be seen as well (Akilesh, 2000). Bioluminescence was developed in order to protect the organism. For example dinoflagellates flash their light during night or dark cycles to distract predators and reveal their predator to higher predators (Akilesh, 2000). Another example is the cookie-cutter shark, whose belly lights up, and is covered during the day by another organ. When the shark swims up in the waters its neck does not have the bioluminescence and so it appears to be a small fish and when bigger predators get close it attacks (Akilesh, 2000). Bioluminescence is seen more in aquatic organisms, probably because it is more beneficial to them in the mercy waters.
This paper talked about the different phylum from protists all the way to vertebrates. Protists being unicellular and having no nervous system still had the ability to respond to many different kinds of environmental stimuli. Sponges have no symmetry and they are also able to respond to environmental stimuli, although usually larvae respond to more. Cnidarians have radial symmetry and therefore a corresponding nerve net, which helps them respond to environmental stimuli. Platelyhelminthes are the first to have bilateral symmetry and to go along with that their nervous system extends the length of its body, with centralization in the head, or anterior end. They also have to ganglia at the end of each nerve cord and the nerve cords are connected to allow both sides of the body to move together. The mollusks have a wide range of diversity in their nervous systems, ranging from nerve nets to highly cephalized cephalopods. The segmentation of annelids and arthropods allows each segment to be controlled individually but the nerve cord still runs through each segment. The vertebrates have the most evolved nervous system. This is because they have the ability to respond to stimuli, hold memory, learn, and think. This means that we don't have to come in contact with everything in the environment to understand it. The more complex a body plan becomes the more complex the corresponding nervous system becomes as well. A simple body plan doesn't have the room to house a huge brain, and therefore simple or no nervous systems are seen.
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