The nervous system and the brain have several diverse forms in different types of organisms. From simple nerve nets to complex systematically layered structures, the nervous system comes in various sizes as well as organizational morphologies. The range and structural differences between brains are indicative of the variety of environment and ecological niches organisms inhabit, as well as their different (Powers, 2014). Therefore, whether simple or elaborate, the nervous system and the brain are adaptive. As articulated in Elphick, (2013), Holland et al. (2013), DeFelipe (2013), Holland (2015), and Torday and Miller (2016), both the central and peripheral nervous system have evolved depending on the selective environmental pressures to which organisms have been subjected. Consequently, in investigating the evolution of the different parts of the brain of different vertebrates, some general morphological and functional features are apparent. For example, in certain reptiles (snakes and lizards) and some mammals (dogs), neural structures such as the olfactory bulb are more advanced and of considerable size (Tattersall, 2006). These features are less prominent in other organisms such as the herbivores and humans.
The same trend is apparent when moving from early primitive life forms to more evolved organisms. This is particularly true in the function of the lower brainstem as well as cerebral cortex (Torday and Miller, 2016). In most vertebrates, therefore, including mammals and reptiles, the underlying architecture of brains, particularly the brainstem, can be recognized across organisms despite the subtle differences that might exist among them (Rehkämper and Zilles, 1991). Correlated with the development of the brain is the capacity to learn. Mammals, birds, and mollusks show a higher learning capacity than others. (Ghysen, A., 2003). This article compares the Central Nervous System (CNS) of reptiles and mammals. The (CNS) comprised of the brain and the spinal cord (Messé et al., 2014). The CNS also contains most of the neurons (nerve cells) in the body that includes the peripheral nervous system.
Similarities in the CNS
The nervous system in both mammals and reptiles is organized into discernible regions based on structure and intended function. For example, both groups have similar subdivisions of the brain: the forebrain (divided into the telencephalon and diencephalon), the midbrain, and the hindbrain (Ghysen, 2003). In mammals, the structure of the hindbrain remains more conserved than the forebrain does (Ghysen, 2003). Reptiles and mammals share similar parts of the brain. These parts are the basal ganglia, brain stem, and cerebellum. These brain parts handle identical body functions such as the coordination of movement, breathing, balance, and the simple urges of survival like defense, flight, mating, and feeding (Elphick et al., 2013; Steinhausen et al., 2016. In both organisms, the cerebellum has various segments that include the archicerebellum, paleocerebellum, and the neocerebellum. Collectively, the cerebellum is concerned with a host of functions including balance and equilibrium as well (skilled) movements (Naumann et al., 2015). In reptiles, the paleocerebellum forms the greatest mass of the cerebellum while in mammals the development of the entire cerebellum is linked with the cerebral cortex (Messé et al., 2014).
Differences in the CNS
Compared to mammals, the brain of reptiles is significantly smaller. The reptilian cortex also consists of far fewer subdivisions than that of mammals, particularly when compared to primates, canidae, and rodents (Laurent et al., 2016). Unlike mammals, reptiles also have a variety of mechanoreceptors, which they use to not only sense vibrations but also sound and other forms of mechanical disturbance (Siminoff and Kruger, 1968). This is true especially in snakes. Reptiles have muscle spindles spread across their body that help detect and control the stretch of their body muscles (Siminoff and Kruger, 1968).
Moreover, thgroughout the mammalian evolutionary history, the mammilkian hindbrain retained much of the initial recognizable structure (Ghysen, 2003) as an enlarged anterior segment of the spinal cord. It encloses a network of dorsal and ventral sensory and motor nerve endings. In contrast, the reptilian hind brain is just a longitudinal continuation of the spinal cord (Naumann et al., 2015). Very little is known, whether this spinal cord-brain continuity formation in reptilians houses some of the cranial nerves as it does in mammals. In mammals, 25 cranial nerves can be observed; innervating the muscles of the head region and specialized sense organs, including taste buds on the surface of the body. Moreover, the telencephalon in mammals consists of a dorsal pallial area that differentiates into the cerebral cortex region (Kaas, 2011).
Similarities in the function of CNS
Reptiles and mammals harbor a diverse set of sensory organs that use the brain as the central processing hub (Naumann et al., 2015). According to Naumann et al. (2015) the retina of both mammals and reptiles, captures visual information and relays it for processing in the pallium section of the brain via the tectum and the thalamus. Correspondingly, the olfactory cues from the nose first pass through the olfactory bulb before reaching the pallium. Both reptiles and mammals have a clear three-layered cerebral cortex that is structurally identical to that of the mammalian allocortex (Naumann et al., 2015). The ventral pallium of reptiles also forms the dorsal ventricular ridge, a structure whose equivalent in mammals is still a question of debate among anatomists (Naumann et al., 2015).
Differences in function of CNS
The differences between the nervous systems of reptiles and mammals are enunciated by differences in the perception/sensing of chemical cues. Generally, receptors that perceive chemical stimuli are described as taste (or gustatory) if they sense dissolved molecules. They are however olfactory (smell) when they detect airborne stimuli. While most mammals tend to rely on their taste buds located in the mouth to discern cues, most reptiles, especially the snakes, rely more on their sense of smell than on taste (Siminoff and Kruger, 1968). In reptiles, these smell detectors are varied and are located in the nasal passages, where they can also undertake sniffing functions, similar to that of our canine counterparts. Most reptiles, particularly the snakes, and certain lizards also have the Jacobson’s Organ, which they use to taste odors directly from their buccal cavities (Noble and Kumpf, 1936).
Mammals and reptiles further exhibit significant dissimilarity in the manner in which they perceive and regulate thermal energy (Tattersall, Cadena, and Skinner, 2006). While both organisms have thermal sensory organs located on their skin, reptiles have other special organs that are more sensitive to ambient temperature variation than mammals do. For example, certain types of snakes such as pythons and rattlesnakes have pit organs with a remarkable capacity to sense changes in temperature. The sensitivity of this organ allows snakes to detect and trace radiant heat source (Tattersall, Cadena, and Skinner, 2006).
Moreover, unlike mammals, reptiles are ectothermic organisms. They are therefore behaviorally as well as physiologically adapted to fluctuating temperatures. This adaptation also makes their brain accustomed to extreme thermal conditions; an attribute that is missing in mammals. Additionally, certain reptiles like the freshwater turtle (Chrysemys picta), has evolved nervous and brain adaptations that enable it to survive in anoxic conditions for protracted periods (Naumann et al., 2015). Though this ability to survive hypoxic environments is not yet clearly understood, it is believed the mechanism underlying this adaptation can be exploited for the treatment of brain injury caused by ischemia (Naumann et al., 2015).
Another significant difference between the brain and nervous system of mammals and reptiles is that the region of the brain where emotions are processed is more developed mammals. This region is associated with the limb system to interpret these emotion (Naumann et al., 2015).
Therefore, it is the seat of value judgments and hence exerts a strong influence on our behavioral patterns. The mammalian brain also has an outer brain layer known as the cortex. This region helps mammals analyze and make complex decisions as well as take control of their emotions (DeFelipe, 2013). On the other hand, the reptilian brain is subdivided into a lateral cortex (equivalent to the piriform in mammals) and a medial cortex that sandwiches the dorsal cortex in between. The dorsal cortex serves to receive multi-modal inputs such as visual inputs in turtles (Laurent et al., 2016). Comparative anatomists have also found little evidence of the presence of somatosensory areas in the cortex region of the reptilian brain (Willemet, 2012).
This comparative study has revealed that the vertebrate brain and spinal cord configuration has a seemingly identical general plan. Regarding function, the peripheral nervous system comprises of a host of sensory and motor nerves. These nerves carry signals back and forth between the CNS and other parts of the body. The spinal cord is confined within the vertebral column. Its primary function is to provide reflex reactions independent of the brain. It also receives inputs from the brain’s higher centers. While this structural configuration reflects a general basic outline in function, it has also been shown that these brains are different – both structurally and functionally. Therefore, although the CNS of reptiles and mammals have similar structure and function, they exhibit some fundamental differences such as kin, and temperature regulation. These differences not only reflect their evolutionary history but also reveals their respective adaptations to their environments.
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