Neuronal signaling and the structure of the nervous system and muscle

Neurobiology

Resting

potential

If the sodium/potassium pump were not working, an equilibrium both of charge and of Na+ a K+ ions would eventually (though gradually, due to the limited space available for permeating the membrane) be reached on both sides of the membrane. There would be no ability to form an action potential in such a situation, as there would not be any mass migration of molecules across the membrane due to an imbalanced charge or polarity. This implies that the quantity of ions that crosses the membrane during an action potential is directly related to the strength and possibly the speed fo the impulse.

The essential ion in causing the exocytosis of neurotransmitters from a presynaptic cell is Ca2+. The depolarization of the nerve terminal causes Ca2+ selective channels to open, and as there is a much higher concentration of calcium outside the cell, the influx of Ca2+ ions creates a measurable current. These Ca2+ ions are thought to bind to certain proteins that exist along the surface of synaptic vesicles; these vesicles re pushed out to create fusion. These vesicles are then drawn back into the cell and recycled for future use.

3)

The action potential's arrival at the axon terminal causes the opening of the Ca2+ selective ion channels. The influx of Ca2+ ions then trigger the extension of the synaptic vesicles, which fuse to the cell membrane of the motor neuron and release the neurotransmitter acetylcholine into the synaptic cleft. The acetylcholine then diffuses along the membrane and binds to nicotonic acetylcholine receptors on the motor end plate. These receptors are ligand-gated ion channels, and acetylcholine causes them to open. These channels funnel sodium into and potassium out of the muscle cells, causing a depolarization which triggers the release of Ca2+ in the muscle, which in trun initiates muscle function.

4)

After activation has occurred and opened a voltage-gated sodium ion channel, the influx of sodium actually pulls another gate inside the channel closed until activation occurs again. This activation cannot occur until the refractory period has passed, allowing enough action potential to build up and create a useful flow of ions that produces a significant current. The pulling shut of this gate is what prevents an action potential from occurring in the opposite direction; the inward flow of ions is never allowed to take place to the degree that the cellular environment can become positive for extended periods of time, which could then possibly trigger such an action potential were it allowed to occur.

5)

though sensitization is an opposite process of habituation, and sensitization is an adaptive process, habituation could not properly be considered a maladaptive trait. it, too, is an adapted response that is useful and even necessary in certain circumstances. Just as sensitization would not be considered an advantageous adaptation in every synaptic response -- everything would then carry the same level of heightened response -- habituation is not always or even often a bad thing. It is necessary in aiding the body to distinguish between those events that need strong responses and those that don't. Habituation reduces synaptic response for stimulus that is unimportant, which frees attention to stimuli that are more worthy of noting and/or truly demand immediate attention.

6)

In addition to habituation and sensitization, and indeed in part contributing to these adaptive changes, synapses can go through physical or morphological changes that contribute to the processes of learning and memory. Synapses can both lengthen and thicken through use, enabling more connections to be formed and larger amounts of neurotransmitters to be delivered, when needed. The new connections thus formed are what facilitate learning, memory, and future access of stored information. Synaptic morphological changes are one of the only physical changes that truly explain the process of learning.

7)

The terms "fight" and "flight" refer to the responses of the sympathetic nervous system to perceived threats of an immediate nature. this nervous system originates in the spinal chord, and directs the automatic functions of the body such as heart beat, digestion, etc. When a threat is perceived, unnecessary processes such as digestion are stopped, while heart rate and adrenaline production are increased to allow for a burst of energy -- either in fending off the threat or fleeing from it. When watching a scary movie alone at night, this system is likely to become engaged due to the perception of a threat; sudden noises are likely to cause an involuntary flight reaction that, of course, subsides after a moment.

8)

Temporal summation in a nerve cell occurs when the length of time over which successive activation potentials occur is sufficiently long enough to allow for the potentials to continue to the point where they begin to overlap. When this occurs, a new activation starts to begin before the climax of the preceding action potential has been reached. This action potential essentially ends prematurely, or summates, as it begins the rise into the next action potential, which ends up being larger in magnitude than the constituent action potentials. Summation of active potential in muscle fibers allows for similarly larger action potentials, which can increase the strength of the fiber contractions.

9)

There are two basic types of skeletal muscle fiber, Type I and Type II, or fast and slow. Type II fibers can be further subdivided by both size and speed. Basically, the larger a muscle fiber is, the faster its "twitch" or activation response time (i.e. The faster it contracts in response to a given amount of stimulus), and the faster it fatigues. Type I muscle fibers are slow twitch; they are slow to fatigue but also slow to move and do not produce as much power. Type II muscle fibers are larger and faster, producing more power but tiring faster as well.

10)

Peripheral fatigue involves the physical failure of muscles to carry out their typical functions, usually through a combination of muscle tissue and neurotransmitter inhibitors at the site of the muscle. Thus, periphery fatigue can be thought of as fatigue that occurs at the muscle site, and is usually the first type of fatigue to occur during extended muscle use. Initial muscle failures of muscle ability during a workout with multiple repetitions are an example of muscle fatigue; a depletion and/or imbalance of neurotransmitters is at least partially to blame for the inability of the muscle to function, and a resting period is needed to restore full resting potential. Central fatigue occurs along the central nervous system, and results from a similar depletion/imbalance of neurotransmitters and the ions that create resting and action potentials. This takes longer to occur, however; the eventual fatigue and overall weakness at the end of a workout is related more to central fatigue than simply to peripheral fatigue, and takes longer to "reset."