Stages of Brain Development - Essay Example

1. Describe the stages of brain development. The stages of brain development are cell birth, cell migration, cell differentiation, cell maturation, synaptogenesis, cell death and synaptic pruning, and myelogenesis. Cell birth happens during the formation of the embryonic disc, or the primitive body, 15 days after a sperm fertilizes an egg cell. At day 21, the neural plate, or primitive neural tissue, occupies the external layer of embryonic cells. The neural plate folds to create the neural groove, with the latter curling to produce the neural tube. After cell birth is cell migration, which takes place when cells move to create different cell populations. Next to cell migration is cell differentiation that begins with gene expression, wherein previously dormant genes undergo activation, and which ends up with the cell producing a certain protein. Neuroblasts, at this point, turn into particular types of neurons. After cell differentiation is cell maturation, which happens 20 weeks after conception and continues after birth. It takes place through dendritic and axonal growth. Maturing neurons build dendrites to give surface area for synapses with other cells and to expand their axons to proper targets, which will launch synapse formation. Axonal growth relies on a particular process, which begins with the growth cone at the end of the axon. As these growth cones branch out, they create shoots or filopodia that, after they reach certain targets, other filopodia follow them. Next to cell maturation is synaptogenesis, or the creation of synaptic connections. Synapses reach 1014 or 100,000 trillion. After synaptogenesis is cell death and synaptic pruning. The brain chisels out unneeded neurons and prunes synapses. Neural Darwinism explains that the brain removes excess neurons through competition among them. Neurons die because target cells create neurotrophic factors that axon terminals absorb and which create messages that seem to tell some neurons to die in a programmed process called apoptosis. The last stage of brain development is myelogenesis or production of myelin. Myelination demonstrates cerebral maturation. 2. Contrast the different ways in which rods and cones contribute to our visual experiences (i.e. color perception and specificity). Describe the visual pathways from the retina to the brain. Light energy travels through the pupil and into the light-sensitive surface of the eye called the retina. This activates the photoreceptor cells on the retina, which results to the creation of people’s visual worlds. The photoreceptor cells and neurons generate visual precision and enable people to see different colors, including dimness and brightness. Photoreceptors change light energy to chemical energy, which activates neural activity. Rods and cones are two kinds of photoreceptors. They are structurally different because rods are longer and have cylindrical ends, while cones are shorter and have tapered ends. Rods are specialized in functioning at low light levels, while cones are specialized for color and highly responsive to bright light levels. Cones participate in attaining color vision and visual acuity (which is the ability to see fine details). Furthermore, rods have the same pigment, while cones have three different kinds of pigments: red, green, and blue. The presence of these cone receptors and their relative numbers and distribution affect people’s color perceptions. Red and green cones have almost the same numbers, while blue cones are fewer, which explain why people are not as sensitive to blue wavelengths and why they are more sensitive to red and green ones. 3. How is location of objects in the environment coded in the eye and the brain? The eye and brain work together in determining the location of objects in the environment. Every ganglion cell reacts to stimulation on a small part of the retina, which is the former’s receptive field. The receptive field is the part where it can potentially affect a cell’s firing. The whole visual field is composed of thousands of ganglion cell’s receptive fields. Connections from the retina to the lateral geniculate nucleus (LGN) can represent location. Every LGN cell has a receptive field too, the part of the retina that controls its activity. If two adjacent retinal ganglion cells (RGC) synapse on a single LGN cell, the receptive field of the latter will be the product of two RGCs, so the receptive fields of LGNs are larger than RGCs. The LGN projects to the striate cortex (region VI), which preserves spatial information. For every LGN cell that projects to region VI, it creates a topographic map, or a spatially arranged neural representation, which is basically a map of what people see. The center position of the visual field is represented at the brain’s rear side, while the periphery is represented at the anterior part. The upper part of the visual field is seen below region VI, while the lower dimension is at the top of VI. The rest of the regions have the same topographic maps as VI. VI neurons must project to other regions in an organized way to produce a clear position of objects. 4. What is a biological clock and how is it related to both body functions and the environment? Describe evidence for the neural basis of a biological clock. A biological clock pertains to neural systems that produce a time pattern for behaviors. Biorhythms refer to the internal timing mechanisms that control or start different biological processes and are connected to the cycles of days and seasons of the Earth. Curt Richer (1965) learned that animals that experience damage to their hypothalamus lose their circadian rhythms. In addition, experimenters observed that the suprachiasmatic nucleus (SCN) of the hypothalamus serves as the master biological clock. Some of the evidence for the role of the SCN in circadian rhythms are the following: (1) damage to the SCN does not stop animals from eating, drinking, and sleeping, but they begin performing these activities haphazardly; (2) SCN cells are more active during the day than the night based on mice studies; and (3) SCN cells that are taken from the brain and cultured in a dish maintain their periodic rhythms. Besides the SCN, the neural structures of the intergeniculate leaflet and the pineal gland also have clocklike behaviors. Almost each body cell also has its own clocklike activities. In the brain, SCN cells link with each other through inhibitory GABA synapses. GABA is the primary inhibitory neurotransmitter of the central nervous system. These connections in the GABA enable SCN cells to behave with synchrony, but they also react to external inputs, such as receiving light signals from the rods and cones of the eyes. 5. How does experience with the environment change the structure of the brain? What specific evidence (cite studies to support claims) do we have that experience changes the structure of brain? Neuroplasticity believes that the brain is flexible and can be changed into different forms at microscopic levels through being exposed to different environmental conditions and changes (e.g. cultural practices) and internal events (e.g. impacts of hormones, injury, and genetic mutations). Experience can change the structure of the brain. The experiment of Donald Hebb in 1947 used two groups of rats: the home rats which freely grew in the kitchen and the control-group rats which grew up in laboratory cages. Hebb’s wife also usually chased these home rats. Hebb then gave these rats intelligence tests, where the home rats performed better than caged rats. Hebb concluded that experience can improve intelligence. Stimulating environments can lead to maximum brain development than stifling ones. Other studies showed that the brains of animals that grew up in complex environments were bigger and had more synapses than those who were reared in cages. Several studies further showed that having three weeks of tactile stimulation expanded synapse numbers in different parts of the cortex in adults. Increasing number of synapses may come from increased sensory processing due to complex and stimulating environments. Other examples show that exposure to specific music and cultures can make other musical genres and cultural beliefs peculiar, which suggests that experiences impact how people think of and react to novel experiences. Different experiences, thus, can change the brain differently. Read More