The net change in postsynaptic membrane voltage determines whether the postsynaptic cell has reached its threshold of excitation needed to fire an action potential.
If the neuron only receives excitatory impulses, it will also generate an action potential. However, if the neuron receives as many inhibitory as excitatory impulses, the inhibition cancels out the excitation and the nerve impulse will stop there. One technology, for example, allows patients to type out sentences by twitching their cheek. These sentences can then be read aloud by a computer. A relatively new line of research for helping paralyzed patients, including those with ALS, to communicate and retain a degree of self-sufficiency is called brain-computer interface BCI technology and is illustrated in Figure 2.
This technology sounds like something out of science fiction: it allows paralyzed patients to control a computer using only their thoughts. There are several forms of BCI. Some forms use EEG recordings from electrodes taped onto the skull. These recordings contain information from large populations of neurons that can be decoded by a computer. Other forms of BCI require the implantation of an array of electrodes smaller than a postage stamp in the arm and hand area of the motor cortex.
Origin of Cells 6. Cell Division 2: Molecular Biology 1. Metabolic Molecules 2. Water 3. Protein 5. Enzymes 6.
Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2. Chromosomes 3. Meiosis 4. Inheritance 5. The stronger the stimulus, the greater number of fibers are used, and the signal is transmitted rapidly. This phenomenon by which a stronger stimulus spreads to multiple nerve fibers is also termed as spatial summation. It is the process by which rapid discharges from a single presynaptic terminal are added to generate a combined effect. It is the process by which successive discharges of the presynaptic terminal are added together.
When a presynaptic terminal releases neurotransmitters, the ion channels on the postsynaptic neuron remain open for only one or a few milliseconds. However, the potential generated by the opening of these channels stays for around 15 milliseconds after the channels have closed.
Thus, the opening of the same ion channel within this time can increase the postsynaptic potential to a greater level. Thus, the rapid the discharge, the greater is the potential generated. Temporal summation is increased when the time constant of the nerve fiber is increased. The time constant is the time for which the response to a particular input signal is present. If the time constant is increased, the rapid discharges during this time will get added and generate a cumulative effect by the process of temporal summation.
In this way, rapid discharges will lead to the generation of an action potential in the postsynaptic neuron, using a single presynaptic terminal. Temporal summation is also applied for conducting signals of increasing strength by nerve fibers.
It is done by increasing the frequency of nerve impulse generation in each fiber. A strong stimulus causes nerve impulse generation at a rapid rate in sensory fibers. The rapidly generated high-frequency nerve impulses are carried to the CNS where they are perceived as a strong stimulus. Sometimes, a neuron is receiving stimulatory and inhibitory signals at the same time. The stimulatory signals tend to increase to depolarize the cell while the inhibitory signals hyperpolarize it.
In this case, the simultaneous summation of inhibitory and excitatory potentials takes place. In this way, if a neuron is excited by a source, an inhibitory signal from another source will reduce the potential below the threshold level, turning off the activity of neurons. Sometimes, the summated postsynaptic potential is excitatory but is below the threshold level. In this case, the summated potential is unable to generate an action potential but makes the membrane potential close to the threshold.
Such a neuron is said to be a facilitated neuron. A facilitated neuron has a membrane potential nearer to the threshold for firing but is not at the firing level.
Any additional excitatory potential will take the membrane potential to the firing level and an action potential will develop. In this section, we will talk about the properties of neurons that make the neuronal summation possible.
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