Comparison of Brain Structure in Mammals, Amphibians, and Birds
The brain is a complex structure in all living species, even though they differ greatly in mammals, amphibians, and birds. It is the most important part of the body, because it maintains homeostasis and assists in every function. The brain structures of the three vertebrates, mammals, amphibians, and birds, have similar features and functions. There is still a great difference in the size, and importance of these structures in the brain. For instance, the bird brain is small in size, yet still very complex. The size and complexity of the cerebrum and cerebellum is the most notable difference in these vertebrates.
There are many similarities in the brain structure of the mammal, bird and amphibian. They have the several parts that are the same and perform the same function. For instance the medulla oblongata is the lowest portion of the brain (stem) and controls automatic functions. All three of these vertebrates have two hemispheres and different lobes. Each vertebrate also has a spinal cord that descends from the brain that carries nerve impulses to and from the body along with blood vessels. All vertebrates have spinal and cranial nerves associated with the brain and spinal cord. Also many systems are needed to maintain and control features of the body. For example the endocrine, autonomic, and peripheral systems are needed for full normal function of the animal. The ability to process information is very important. This involves the brain stem, many motor nerve cells, basal ganglia, and other important features. Even with these similar features there is still a large variety, even within the same species.
The mammalian brain is the most complex and the largest out of all the vertebrates. It has special features and characteristics that the others do not. “The unimpressive appearance of the human brain gives few hints of its remarkable abilities. It is about two good fistfuls of quivering pinkish gray tissue, wrinkled like a walnut, and somewhat the consistency of cold oatmeal” (Marieb 431). Since some features of the human brain are more developed in the human brain it allows the capability of memory, special senses, and intelligence. The cerebrum, which is the largest part of the brain, is split into two hemispheres that communicate with each other via the corpus callosum. These hemispheres are then separated into five different lobes that perform specific functions. The frontal lobe is located in the most anterior portion of the brain. This is the site of speech production, emotions, and voluntary movement. The two temporal lobes are located on the lower sides of the cerebrum. These areas are where the hearing and memory sites are located. The two parietal lobes are located between the frontal and occipital lobes. They are the middle lobes of each cerebral hemisphere. These lobes are the site for major senses for the body, such as touch, temperature, and pain. The most posterior lobe is the occipital. This is the area that receives and interprets visual images. There are many features involved in the separation of different parts of the brain. For instance the longitudinal fissure separates the left and right cerebral hemisphere. “Another large fissure, the transverse cerebral fissure, separates the cerebral hemispheres from the cerebellum” (Marieb 434). There are sutures that also separate the five different lobes. The coronal suture is an articulation between the parietal bones and the frontal bone, anteriorly. The sagittal suture is where the right and left parietal bones meet superiorly. Where the parietal bones and the occipital bone meet posterior is the lambdiod suture. The squamosal suture is where the parietal and temporal bones meet. The cerebrum is a heavy folded surface, which is different in each human. These folds are called sulci (shallow fissures) and fissures, which are deep grooves that separate larger regions of the brain. The elevated ridges on the brain’s surface are gyrus. During brain development “the continued growth of the cerebral hemispheres causes their surfaces to crease and fold, producing convolutions and increasing the surface area, which allows more neurons to occupy the limited space” (Marieb 433). Not all vertebrates have this folding of the brain. Compared the other vertebrates the human cerebrum is highly developed. The frontal lobe is much larger in the human while the midbrain and hind brain do not occupy as much room (compared to the amphibian brain). Below is an illustration from source of a developed human brain.
The whole human brain is mainly made of soft tissue. The cerebral cortex covers the entire cerebrum. It is an outer layer of gray matter, which is about 2-6 mm thick, and an inner layer of white matter that has spots of gray matter. The inner brain consists of the thalamus, the corpus callosum, the fornix, and the hypothalamus. The diencephalon consists of the “central core of the forebrain and surrounded by the cerebral hemispheres…and consists of three paired structures, the thalamus, hypothalamus, and epithalamus (Marieb 443). The thalamus is the information station and makes up most of the diencephalon. It receives sensory input from the spinal cord and midbrain, and then interprets and sorts these signals. This is a primary communication center and is also involved in learning and memory. The thalamus is surrounded by the limbic system, which controls survival behavior and emotions, and the hypothalamus, which controls the body’s automatic processes. The hypothalamus is located below the thalamus. It includes mammillary bodies that are relay stations. The major functions of the hypothalamus are autonomic control (involuntary), emotional response, body temperature regulation, regulation of food intake, regulation of water balance and thirst, regulation of sleep cycles, and control of endocrine system functioning (Marieb 445). The Autonomic System is composed of parts that tend to parallel each other. The sympathetic pathway will activate charge, and is mainly a stress response. The parasympathetic pathway controls the vegetative system, which is the ordinary operation of internal organs. “The epithalamus is the most dorsal portion of the diencephalon and forms the roof of the third ventricle. Extending from its posterior border and visible externally is the pineal gland” (Marieb 447). This gland is responsible for secreting melatonin, a hormone that induces sleep. Another major part of the inner brain is the corpus callosum. This section is made up of mainly white matter and is the largest of several bundles of nerve fibers (commissures). It connects specific areas of the two hemispheres for (structural and functional) communication of one side to the other. The brain is protected by the bony skull and it floats within this cavity in a clear cerebral fluid. “This fluid contains glucose, necessary to provide energy for cell function in the brain and spinal cord, as well as proteins and lymphocytes to guard against infection” (Clayman 66). There are two lateral ventricles (one located in each hemisphere) where the cerebral fluid is produced (choroid plexuses) and drains into the third ventricle, which is close to the thalamus. This fluid then flows to the fourth ventricle that is in front of the cerebellum. The book “The Human Body” gives a detailed explanation of menings in the brain: Three membranes known as the menings cover the brain. Lining the inside of the skull is the outermost membrane, the dura mater, which contains veins and arteries that nourish the cranial bones. The middle layer is known as the arachnoid (‘spiderlike’); this consists of a type of webbed and elastic connective tissue. Next to the surface of the cerebral cortex is the pia mater; between this delicate, innermost layer and the arachnoid is the subarachnoid space, which contains cerebrospinal fluid as well as blood vessels (69). The brain stem consists of the midbrain, pons, and the medualla oblongata. This portion of the brain controls behavior, movement, and most of the senses. The brain stem is covered in white matter that also surrounds the grey matter is inside. Its structure is like that of the spinal cord. “Brain stem centers produce the rigidly programmed, automatic behaviors necessary for survival. Positioned between the cerebrum and the spinal cord, the brain stem also provides a pathway for fiber tracts running between higher and lower neural centers” (Marieb 448). The pons controls breathing (indirectly) and carries impulses that originate in the cerebral cortex to the cerebellum. The medulla oblongata is in control of the autonomic reflexes of the heat beat and respiration. It is a major site for nerve pathways. There are sensory and motor neurons. It helps maintain posture and the awareness of body parts. Cranial nerves also originate here. These structures are within the brain and extend to certain structures. There are 12 cranial nerves in the human and there are three categories, sensory, motor and mixed. Another important feature is the cerebellum, or the little brain. “Neurons of the cerebellum link with other regions of the brain and the spinal cord, facilitating smooth, precise movement, and controlling balance and posture. It also plays a role in speech” (Clayman 66). It is formed out of the metencephalon and is separated into lobes. The surface is grey matter and the subsurface is white matter. The arbor vitae is present and has a tree-like appearance. The cerebellum maintains equilibrium and assists in coordination of movement, but does not contract any muscles only the cerebrum does. Many of these same features a seen in other vertebrates.
The amphibian brain is an elongated structure that is smaller than the human brain and not as complex. The frog brain is separated into three major sections, which are then separated more into smaller sections. The major regions are the forebrain, the midbrain, and the hindbrain. The most anterior part is the forebrain, which is divided into the telecephalon and diencephalon. The midbrain develops without further subdivision, and the hindbrain differentiates into the metencephalon and myelencephalon. The anterior-most telencephanlon bears two olfactory lobes and two cerebral hemispheres. The olfactory lobes terminate in the olfactory nerves, which carry impulses from the nasal cavities to the brain. The mesencephalon, immediately posterior to the diencephalon, bears two large optic lobes that serve to integrate nerve impulses from the eyes (Gillis). The optic lobes of the amphibian are large because vision is an important part of their survival.
Posterior to the mesencephalon is the metencephalon, which is represented by a narrow, transverse portion of the brain called the cerebellum; the cerebellum is involved in motor coordination in the frog. The most posterior portion of the brain is the myelencephalon, consisting of the medulla oblongata that tapers gradually into the spinal cord. A depression between the two sides of the medulla oblongata called the choroid plexus is partially responsible for the secretion of the lymph-like cerebrospinal fluid that fills spaces called ventricles in the brain and the central canal of the spinal cord (Gillis). Below is a labeled illustration of the developed frog brain.
The frog has two protective membranes that surround the central nervous system (Gilbert 49). The pia mater covers the brain and spinal cord. “Over the third and fourth ventricles the pia mater fuses with a thin layer of epithelium to form the choroid plexuses, vascularized tufts which hang down into the ventricles” (Gilbert 49). The pituitary gland in the frog “is one of the most important endocrine glands” because the hormones that it produces encourage growth and development, just like the other vertebrates (Gilbert 50). Compared to the human brain, the frog’s forebrain is underdeveloped, while the midbrain and hindbrain are quite larger. The important parts of the frog brain correspond to comparable parts in the human brain. The medulla regulates automatic functions such as digestion and respiration. Body posture and muscular co-ordination are controlled by the cerebrum. The cerebrum is very small in the frog. By comparison the human cerebrum is very large. Only ten cranial nerves originate in the frog’s brain. Man has 12. Similarly, the frog has only ten pairs of spinal nerves. Man has 30 pairs (www.lookd.com). The frog also has two small holes for nasal passage, but humans do not have these. Instead there are “complex valves” (www.lookd.com). If the cerebrum in the frog’s brain were to be removed, the animal would not have a huge response. It would behave and act normal. If the cerebrum was removed or did not develop in the human brain, there would be a big difference, “the individual is severely handicapped mentally” (Lee 30). The bird’s brain is smaller in size and is not as complex as the mammalian. “The brain of a bird weighs about 10 times as much as a brain of a reptile of the same weight, but slightly less than that of a mammal of the same weight.” Birds also “have a similar plan to their nervous system as the rest of the vertebrates. The central nervous system is made up of a brain, spinal cord, and nerves” (Ramel). Still many differences are present. Birds need to have a portion of the brain that controls flight, take-off, and landing. This is another reason for the significant difference in the brain structure.
A bird’s brain is different to a mammalian brain in that the complex folds found in the cerebral cortex of mammals are missing and the cerebral cortex itself is much smaller proportionally than in mammals. Instead the corpora striata, a more basic part of the cerebral hemispheres is proportionally larger and better developed. It is this portion of a bird’s brain which is used to control instinctive behavior-feeding, flying, reproduction etc. The midbrain is also well developed as this is part of the brain primarily concerned with sight, while the olfactory lobes are reduced as would be expected given that bird’s in general have little use of the sense of smell. (Ramel) The midbrain is important in maintaining posture and body temperature. Below is a labeled illustration of a developed bird brain.
“The bird’s skull is mostly occupied by eyes and the brain has to make do with what space it can find in a rather narrow cranium” (Ramel). The bird, just like mammals, will respond negatively if the cerebellum is removed. The cerebellum is highly developed in birds and is important in their sense of balance. For some days after removal of the cerebellum a bird can hardly fly, walk or stand; it falls readily to one side, but can eat and drink. The vestibular centers in the medulla are normally inhibited by the cerebellum. Stimulation of the cerebellum in birds inhibits postural tonus homolaterally, and destruction of the anterior lobe produces extensor spasticity (Prosser 635). Just like the mammals, birds also have an autonomic system. It consists of two sections, the sympathetic and parasympathetic. The sympathetic is used as the “fight or flight” system. The release or adrenaline and noradrenalin help the animal respond to its environment. This is extremely important for birds that may have many predators. The parasympathetic system helps the body recover from the stimulation of the sympathetic system. The bird also has 38 spinal nerves compared to the human’s 30 pairs. The large variations in size of the bird brains accounts for a large range of intelligence. The presence of motor and sensory neurons are also important. Motor neurons send impulses from the brain to the body, mainly the muscles. The sensory neurons send impulses from the sensory organs to the brain to interpret such senses as sound, taste, smell, etc.
The main parts of the brain, the cerebrum, medulla oblongata, the cerebellum, and brain stem are all seen in the brains of the mammal, amphibian, and bird, even though they very in size and importance. Still in all three vertebrate’s brain, the cerebrum is the most developed and largest portion. These extreme differences are the reason why animals, other than humans, do not have memory, cannot plan for the future, do not have emotions, or have a personal moral standard.