the mixing of impulses is prevented in nerves. give reason
Answers
Explanation:
A rapid, reversible enhancement of synaptic transmission from a sensory neuron is reported and explained by impulses that reverse direction, or reflect, at axon branch points. In leech mechanosensory neurons, where one can detect reflection because it is possible simultaneously to study electrical activity in separate branches, action potentials reflected from branch points within the central nervous system under physiological conditions. Synapses adjacent to these branch points were activated twice in rapid succession, first by an impulse arriving from the periphery and then by its reflection. This fast double-firing facilitated synaptic transmission, increasing it to more than twice its normal level. Reflection occurred within a range of resting membrane potentials, and electrical activity produced by mechanical stimulation changed membrane potential so as to produce and cease reflection. A compartmental model was used to investigate how branch-point morphology and electrical activity contribute to produce reflection. The model shows that mechanisms that hyperpolarize the membrane so as to impair action potential propagation can increase the range of structures that can produce reflection. This suggests that reflection is more likely to occur in other structures where impulses fail, such as in axons and dendrites in the mammalian brain. In leech sensory neurons, reflection increased transmission from central synapses only in those axon branches that innervate the edges of the receptive field in the skin, thereby sharpening spatial contrast. Reflection thus allows a neuron to amplify synaptic transmission from a selected group of its branches in a way that can be regulated by electrical activity.
Changes in synaptic transmission are crucial to the function of many neurons. Most studies of synaptic plasticity concern changes that are located at the synapse, including long-term potentiation, long-term depression, and short-term synaptic depression (1). Transmission, however, can also be influenced within the axon before the synapse, and a neuron’s branching pattern can produce changes in transmission over time. Conduction of action potentials may fail at axon branch points, which can reduce synaptic transmission by decreasing the number of synaptic terminals activated (2–4). Branch points can also act as frequency filters, allowing separate branches of an axon to activate their synapses at different frequencies (5, 6).
Another way that a neuron’s branching pattern can affect impulse propagation is by delaying an impulse as it travels through a branch point. This delay can outlast the refractory period of the axon region entering the branch point, causing that region to fire a second time, thereby reflecting the impulse (7, 8). Reflection is related to action potential failure, or conduction block, in that it occurs when an action potential is near failure.
For this study, synaptic transmission was measured from leech mechanosensory neurons. Stimulating the soma of these neurons activates the entire set of presynaptic terminals, which was presumed to produce the maximal level of transmission for a single impulse (2). While recording from a postsynaptic cell, it was noticed that stimulating one region of the presynaptic neuron increased transmission to a level much higher than this presumed maximal level. This report describes the mechanism of this increase in transmission.
Electrical activity in leech sensory neurons is known to hyperpolarize the membrane and to produce conduction block (9). Previous theoretical work has concluded that reflection occurs for a range of branch-point morphologies (7, 8, 16). A compartmental model was used to examine whether this range increases when the effects of electrical activity are considered.