Neurons
Neurons are the basic structural and functional unit of the nervous system. They are responsible for communicating and transporting information about a sensation to respond to a stimuli.
Location
Neurons are found densely packed in nervous tissue all around the body.
How A Signal is Created in a Neuron
To understand action potential and depolarization, it starts with first understanding the electrical events in the body. Overall, the human body is electrically neutral but certain areas are more positive or negative. We know that opposite charges attract, so barriers in the body keep these charges separate until the energy is needed. This idea is known as building potential. In neurons, there are positively and negatively charged ions around that have potential difference, and this energy is used is when a stimulus or action causes a chain of events to occur, using the potential. The potential difference can be measured in an electrical unit called voltage.
In order for a stimulus reaction to occur, the neuron starts at its resting state called the resting membrane potential. During this stage, there is a more negative charge on the inside of the axon, and it is surrounded by a positively charged environment. This means the neuron is polarized. The potential energy at this state is around 70 millivolts (MV). Inside the axon membrane there is a more negative charge because there are negatively charged proteins and other ions in the axon. On the outside, there are positive sodium ions. These ions come from something called the sodium potassium pump. This pump surrounds the membrane of a neuron. For every two potassium ions pumped into the cell, three sodium ions come out. This is why resting neurons are more negative on the inside than out.
The difference between the more positive outside than negative inside of the neuron is called an electrochemical gradient. But, nature wants to restore balance in the neuron (maintaining homeostasis); it doesn’t like the gradient. The only way for the gradient to balance out is for ions to pass across the membrane. This is done through ion channels which provide for passage across the membrane. Each channel opens for different purposes. Voltage-gated channels open and close in response to changes in the membrane potential. Ligand gated channels only open when a specific neurotransmitter latches onto its receptors. Mechanically gated channels open in response to physical stretching of membrane. Once these gates are open, the ions come in and even out the gradient; this is the key to all electrical events in neurons.
But, the voltage has to be great enough to open gates to cause the action potential, which allows the signal to pass onto the next neuron.This happens through depolarization: when a big enough change in the membrane potential triggers the gated channels to open. This big enough change can be measure though the voltage. It has to reach -55 mV to cause a change. When it reaches this level, sodium ions travel into the membrane making it positive causing an action potential; now the signals travels down to from the dendrites to the axon to the axon terminal then into the synoptic cleft, and then finally to the next neuron's dendrites to make a response.
The neuron now needs to reach its resting state again, this is through the process of repolarization. During repolarization, the sodium channels close and sodium potassium pump opens. This restores the negatively charged inside by pumping out three sodium ions and bringing into two potassium, bringing the neuron back to its resting state.
Neurotransmitters
Neurotransmitters can be defined as the molecules in a neuron that transmit messages to target cells. They either communicate with another neuron, muscle cell, or gland cell. Neurotransmitters live in the synaptic vesicles of a neuron. When neurotransmitters are ready to be released, the synaptic vesicles bind to the cell membrane, releasing the neurotransmitters where they cross into the synaptic cleft (gap between synapses between neurons). From there, they travel to receptor sites of another cell where they pass on the nerve impulse.
Neurotransmitters influence a neuron in one of three ways. Excitatory transmitters promote the generation of the action potential. Inhibitory transmitters stop an action potential. Neuromodulators regulate populations of neurons, being able to enhance an excitatory or inhibitory function.
Some common types of neurotransmitters include: acetylcholine which controls muscles and memory, serotonin which controls your mood and sleep, and dopamine which controls reward pathways/makes you feel happy.
Synapse Communication
Synapses are the communication links between neurons. Their job is to translate or convert signals from a neuron. There are trillions of synapses in your body, and they are what makes a neuron function. The two types of synapses are an electrical synapse and a chemical synapse. Electrical synapses are the faster of the two as they send an ion current directly to the next synapse. Chemical synapses are different; they use neurotransmitters. This makes their communication a little slower, but it has the advantage of being able to send more precise signals. The process starts with the presynaptic neuron sending down the signal to the presynaptic terminal. As described in the neurotransmitter section, the neurotransmitters are released into the synaptic cleft then bond onto the receptors of the postsynaptic neuron. During this process, the signal is transformed from an electrical to chemical then back to electrical signal.
