The human brain receives millions of gigabytes of information every day, and we perform thousands of actions as reactions to these data. We receive information from the external world via our five senses. For example, when we are driving a car, we experience our surroundings primarily through our eyes, which keeps us on the road without hitting anything.
The brain is able to receive this information integrated with existing information, and direct specific behavioral output. So, how is this possible? The human brain contains billions of neurons, each connected to several thousand other neurons.
These neurons communicate with each other through electrical and chemical signals. Each neuron consists of the same basic structure, the cell body which contains the nucleus, and important genetic information. the dendrites that receive information from other neurons, and an axon which sends information down to its multiple terminals, or end regions, which are known as the terminal buttons.
The part where the cell body connects with the axon is termed the axon hillock. Each neuron holds an electrical charge. When a neuron is in a resting state, the charge is about minus 70 millivolts.
This is mainly due to the difference in concentration of ions, essentially, positively or negatively charged particles, inside and outside the cell. The outside of the cell has a high concentration of positively charged sodium ions, as well as negatively charged chloride ions, more commonly known as salt or sodium chloride. The inside of the cell has a smaller amount of positively charged potassium ions. and a larger quantity of negatively charged protein ions. Embedded in the surface of the cell membrane, which separates the inside of the cell from the outside, are protein structures known as ion channels.
These channels act as gates by both permitting and hindering the flow of ions into and out of the cell. Normally, these ion channels are held closed. maintaining the electrical potential of the cell.
However, they can be opened if the right message is received from a neighboring neuron. Each neuron can receive inputs from thousands of other neurons, and the place where one neuron makes a connection with another neuron is called the synapse. When a neuron releases a chemical signal known as a neurotransmitter, NT, into the synapse, The chemical will bind to another type of protein embedded in the membrane of the receiving neuron, known as a receptor. Once the neurotransmitter binds to the receptor, much like a key into a lock, this leads to a number of other biochemical events.
One possibility is that this receptor binding will open an ion channel. This binding may also lead to a sodium channel opening, allowing the passage of positive sodium ions into the cell. As these ions are positively charged, the area around the channel will also become more positively charged. This small change in the membrane is called an excitatory postsynaptic potential, EPSPs.
Similarly, it is also possible that part of the cell becomes more negative, for example, by influx of negatively charged ion. This is called an inhibitory postsynaptic potential. If enough positively charged ions enter the cell, thereby resulting in more excitatory postsynaptic potentials, and the membrane potential reaches a threshold of excitation at the axon hillock, this will trigger an action potential.
The action potential is essentially a brief increase in the permeability of the membrane to sodium, immediately followed by a brief increase in the permeability of the membrane to potassium. Once an action potential is generated, it propagates all the way down the axon to the terminal buttons. In this sense, action potentials fire in an all-or-nothing fashion. Once triggered, it will continue all the way down the length of the axon to the terminal buttons.
It is important to note that while EPSPs are small, local, and contain graded changes in the membrane potential from more negative to more positive, an action potential is a brief but larger reversal in the membrane polarity. Once the action potential reaches the terminal buttons this change The change in membrane polarity causes calcium channels, normally held closed, to open and allow calcium into the cell. Calcium then acts as a signal to release the neurotransmitter substance, which is docked at the presynaptic membrane via exocytosis.
The neurotransmitter then binds to the postsynaptic receptor, and then the cycle begins again. So the cells in our eyes receive input and send a message to the cells in our motor cortex, to send yet another signal to the cells connected to our leg muscles that will hit the brakes.