Neuron Firing Mechanisms and Neurotransmitter Dynamics
Understanding the firing mechanisms of neurons in response to neurotransmitters is a fundamental aspect of neuroscience. This article delves into the complexities of whether a neuron fires once or repeatedly, and whether it needs to receive neurotransmitters all at once or can detect a slow accumulation.
Introduction
The behavior of neurons in response to neurotransmitters can vary widely, influenced by the amount of neurotransmitter released, the location of its effect, and the specific physiology of the neuron. This article aims to provide a comprehensive understanding of these mechanisms, offering insights based on historical experiments and contemporary knowledge.
Historical Insights: Katz Experiments
Alfred L. Zaharoff Katz, a neurophysiologist, conducted seminal experiments on neuromuscular junctions, leading to the discovery of the concept of quantal release of neurotransmitters. According to Katz’s experiments, small end-plate excitatory potentials can cumulate through temporal and spatial summation, leading to a single depolarization action potential. However, this is not the usual response. Most neurons exhibit a volley of responses due to supra-threshold neurotransmitter release.
Complexity of Neuronal Responses
Neurons are not uniform in their functions; some can generate plateau potentials and repeated firing in response to brief stimuli, while others are specialized to generate a few action potentials in a phasic manner. The neurotransmitter’s effect is not solely determined by the quantity but also by where it lands on the neuron. Most neurons have multiple input sites, and the location of these sites is crucial in determining the travel of the signal to the spike-initiating zone.
Quantal Release and EPSP Accumulation
The process of neurotransmitter release is often described in terms of quantal events, where each synaptic vesicle releases a specific amount of neurotransmitter. For example, an excitatory neurotransmitter such as acetylcholine might raise the resting potential of the postsynaptic cell by approximately 0.5 mV, creating an excitatory postsynaptic potential (EPSP). This EPSP lasts only 15 milliseconds and fades away, returning to the resting potential unless immediately followed by a new burst of neurotransmitter.
To actually trigger a neuron to fire, at least 30 EPSPs must occur in quick succession. This is because the resting potential of a neuron is typically around -65 mV, and each EPSP brings it closer to its threshold for firing, which is around -55 mV. Thus, the pool of EPSPs needs to accumulate to a significant level to surpass the threshold and induce an action potential.
Spontaneous Neuron Activation
Neurons can spontaneously fire even without external stimulation, a phenomenon known as resting discharge or background activity. This means that neurotransmitters do not merely turn neurons “on” or “off,” but rather they modulate the frequency of firing. A slow buildup of neurotransmitters does not directly trigger a postsynaptic cell to fire, as each quantum of neurotransmitter just slightly increases the resting potential.
However, a prolonged but small increase in neurotransmitter could eventually lead to the summation of EPSPs to the threshold level, causing the neuron to fire. It is important to note that this process is highly dependent on the rate of neurotransmitter release and the resting potential of the neuron.
Conclusion
Neurons exhibit a dynamic response to neurotransmitters, which can vary from a single firing to repeated volleys. Understanding the mechanisms underlying these responses is crucial for comprehending neural communication and functionality. While a slow buildup of neurotransmitters may not directly trigger an action potential, the cumulative effect can eventually lead to firing if the threshold is reached.