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Neuromuscular Physiology
Published in Michael H. Stone, Timothy J. Suchomel, W. Guy Hornsby, John P. Wagle, Aaron J. Cunanan, Strength and Conditioning in Sports, 2023
Michael H. Stone, Timothy J. Suchomel, W. Guy Hornsby, John P. Wagle, Aaron J. Cunanan
Action potential propagation requires that a RMP threshold must be reached; typically this requires a positive increase of 10 to 15 mV in the RMP. In neurons, alterations in the RMP are initiated by increased Na+ permeability; however, if the influx of Na+ is small and the threshold value is not reached, no AP will occur. Physiologically, the threshold value can be reached by several mechanisms. For example, through temporal summation in which an increased frequency of impulses arrives from the same neuron (or a few), by spatial summation when a large number of separate neurons excite the effector cell simultaneously or both temporal and spatial summation.
Neuronal Firing Patterns and Models
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
These basic features of integrators and resonators are diagrammatically illustrated in Figure 8.2. In (a), a brief, subthreshold pulse is applied at t0. The membrane voltage of an integrator is shown returning smoothly, without oscillations, to the steady membrane voltage level, which may or may not be the resting level. A second pulse at t1 will take the membrane voltage beyond threshold and will cause firing, whereas a pulse at t2 will not. The effect is similar to that of temporal and spatial summation discussed in Section 7.2.1.
Neuronal Function
Published in Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal, Principles of Physiology for the Anaesthetist, 2020
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal
A synapse permits the transmission of a neural impulse in one direction from one nerve to another. A typical neuron in the CNS may receive inputs from several other neurons (convergence), or make synaptic contact with many other neurons (divergence). The postsynaptic potentials of neurons may be integrated by a process called summation. Temporal summation therefore occurs when a second postsynaptic potential (excitatory or inhibitory) arrives before the membrane has returned to its resting level (Figure 4.13). A typical postsynaptic potential may last about 15 ms and ion channels are open for less than 1 ms, and there is usually sufficient time for several channels to open over the course of a single postsynaptic potential. The effects of these two potentials are additive over time. Spatial summation occurs when a number of axon terminals over the surface of a neuron are active simultaneously and their combined postsynaptic potential is greater than any one individual potential (Figure 4.14). Commonly, the magnitude of a single EPSP may be 0.5–1 mV, far less than the 10–20 mV required to reach threshold. Spatial summation enables the combined EPSP to exceed threshold.
Cone pathway dysfunction in Jalili syndrome due to a novel familial variant of CNNM4 revealed by pupillometry and electrophysiologic investigations
Published in Ophthalmic Genetics, 2022
Robert A. Hyde, Evelina Kratunova, Jason C. Park, J. Jason McAnany
Differences in the cone-pathway-mediated measures may be attributed to differences in the spatial summation characteristics of these tests. That is, the ffERG is a mass electrical response pooled across the retina that requires a large area of intact cone function to produce a measurable response. In contrast, the full-field stimulus test is thought to be mediated by the retinal area with greatest sensitivity (27). Likewise, the critical area of spatial summation is thought to be relatively small for the cone-mediated PLR (less than approximately 16° of visual angle) (37). Of note, inferring retinal function based on the light-adapted ERGs alone would suggest that the two subjects in this study lack cone-pathway function. This, however, is clearly not the case: the subjects had measurable, although not necessarily normal, cone-mediated PLRs and luminance thresholds.
Photoaversion in inherited retinal diseases: clinical phenotypes, biological basis, and qualitative and quantitative assessment
Published in Ophthalmic Genetics, 2022
Serena Zaman, Thomas Kane, Mohamed Katta, Michalis Georgiou, Michel Michaelides
Stringham et al. describe in detail the spatial properties of photophobia (47). Photophobia obeys spatial summation when a stimulus is viewed centrally essentially via Piper’s law (radiance proportional to square root of stimulus area). It is also a function of eccentricity when viewed peripherally, due to decreased ganglion cell density and distribution of macular pigment. Point sources such as halogen bulbs (28,54,63,66,67) or an ophthalmoscope light (68) do not make for an ideal light stimulus as small deviations in fixation, or manipulations of the eyelids can greatly reduce the amount of light reaching specific parts of the retina such as the macula. A concave LED array such as that used on the Ocular Photosensitivity Analyser (55,69) and other sources that supply a large field evenly illuminated light source (47,48,65) ensure even delivery to the most sensitive areas of the retina. Aboshiha et al. use a uniform reflective sphere to deliver consistent illumination to the entire retina (17) and ensure consistency even with young paediatric patients who may find it difficult to maintain fixation.
Scene through the eyes of an apex predator: a comparative analysis of the shark visual system
Published in Clinical and Experimental Optometry, 2018
In most vertebrate retinae, there is a trade‐off between spatial resolving power and retinal sensitivity. While a large number of rods are typically connected to a single bipolar cell, cones tend to show less synaptic convergence. The high degree of rod summation by bipolar cells (and the subsequent convergence of many rod bipolars onto a single ganglion cell) results in higher scotopic sensitivity but reduced spatial resolving power. Given the relatively high number of rods in the shark retina, the levels of summation are high. This is exemplified in C. plumbeus and S. mitsukurii, where the degree of convergence occurring between the photoreceptor and the ganglion cell populations increases with retinal distance from the dominant visual axis. However, even in the area of highest spatial resolving power, there is a relatively high degree of spatial summation, that is, 48:1 in C. plumbeus and 40:1 in S. mitsukurii. The high spatial summation in sharks indicates that spatial pooling of visual information is an important mechanism used to increase sensitivity to light.1880 More information is required on interneuronal wiring in the shark retina to reliably indicate the level of summation occurring for each photoreceptor type.