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Biology Question - Essay Example

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Name Biology 21 October 2013 Biology Questions 1) How are graded potentials (for example, EPSPs and/or IPSPs) generated at dendrites and/or cell bodies of neurons? The brain comprises of two types of cells: neurons and glial cells. The neurons serve as the coordinators and recipients of information…
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Download file to see previous pages The neurons transmit information to each other through the dendrites. Action potentials are however comparably large in size. They signal strength of stimulus by frequency and not by amplitude. Graded potentials on the other hand are generated in dendrites and sensory receptors. Graded potentials sometimes generate action potentials and thus referred to as generator potentials. In case of an incoming signal, a pre synaptic excitatory neuron fires releasing neurotransmitters to the synaptic cleft. Consequently, if the transmitter binds to the post synaptic dendritic membrane, it elicits a transient depolarization called Excitatory Post Synaptic Potential (EPSPs) (Akers and Denbow 214). Inhibitory neurons on the other hand, elicit partial hyper-polarizations known as Inhibitory Post Synaptic Potentials (IPSPs) (Akers and Denbow 214). Summarily, EPSPs and IPSPs are both graded potentials. Finally, a number of pre synaptic neurons maybe firing simultaneously, hence impacting on the level of activity of post synaptic neurons. Hence, the total potential in the post synaptic neuron; that is, Grand Post Synaptic Potential (GPSP) is a summation of all EPSPs and IPSPs occurring approximately simultaneously (Dowling 242). 2) What is the functional significance of spatial and temporal summation being performed by axon hillocks of neurons? Axon hillock is the most excitable portion of a neuron. It is the region at the beginning of an axon. Action potentials are generated in this region. The occurrence of an action potential at the axon hillock is by either temporal summation or spatial summation. Action potentials usually occur in three stages, namely Resting, Depolarization and Repolarization phases. (Brown, Miller and Eason 223).Temporal summation simply refers to addition of a number of EPSPs that manifest close together in time, as a result of a subsequent firing of a single presynaptic neuron. Conversely, spatial summation refers to addition of EPSPs originating at the same time from several pre-synaptic neurons (Sherwood 107). This implies that the position of a synapse on the neuron is significant in determining the generation of an action potential. Summarily, synapses closer to the axon hillock have higher level of activity than those further away. Furthermore, the central integrative state of a neuron is also determined by temporal and spatial summation (Beck 135). 3) How are action potentials generated at axon hillocks and propagated along unmyelinated axons? Action potential is propagated along unmyelinated axon by a mechanism known as continuous propagation. The membrane is considered as a sequence of adjacent segments. The action potential starts at the initial segment. In the first step, the transmembrane potential becomes briefly positive, rather than negative. In the second step, a local current develops due to the influx of sodium ions into the cytosol and extracellular fluid, and in step three and four, the local current spreads in all directions hence depolarizing adjacent segments of the membrane. Usually, the axon hillock like the rest of the cell body, cannot respond with action potential because it lacks gated ion channels. However, when the initial portion of the axon is depolarized to threshold, an action potential develops there. The process thus proceeds in a chain reaction until distant portions of cell are afflicted. Every time a local current occurs, action potential moves forward, and not backward, since ...Download file to see next pagesRead More
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