23 Mar 2015
A synapse happens when the electrical activity in the pre-synaptic neuron influences the post-synaptic neuron. There are two types of synapses in the body, the electrical (gap junctions) and chemical. Electrical synapses occur in pre and post synaptic neurons that are joined via gap junctions. A chemical synapse releases a neurotransmitter molecule that is triggered by an action potential. The neurotransmitter is then released into the synaptic cleft. The neurotransmitter is diffused across the cleft and binds to receptors on the post-synaptic neuron and can trigger a new action potential.
In further detail, when an action potential begins in a neuron, it travels down the axon, when the action potential reaches the axon terminal, calcium channels open, and calcium ions rush into the neuron. The neuron then makes and stores neurotransmitter in vesicles. When calcium binds to the vesicles, the vesicles carry neurotransmitter toward the presynaptic membrane. When the vesicles contact the axon terminal membrane, the neurotransmitter is released into the synaptic cleft. The action potentials arriving at the presynaptic terminal cause voltage-gated calcium ion channels to open. Calcium ions (Ca2+) diffuse into the cell and cause synaptic vesicles to release acetylcholine, a neurotransmitter molecule. After the release, the neurotransmitter is still in the cleft, which then can be removed by diffusion, re-uptake, and deactivation. A reuptake is when a transporter moves the neurotransmitter back into presynaptic neuron which uses energy. Deactivation is when the enzyme breaks neurotransmitter down into parts. The acetylcholine is broken with the acetyl cholinesterase. This happens in the synaptic cleft. Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic neuron. The postsynaptic neuron receptors are activated. In this case, these receptors allow Sodium in the neuron by facilitated diffusion, causing an action potential to start in the postsynaptic membrane. Neurotransmitters are released from receptors and diffuse back to the synaptic cleft. Vesicles recycle some neurotransmitter to prepare the neuron for its next action potential. Acetylcholine molecules diffuse from the presynaptic terminal across the synaptic cleft and bind to their receptor sites on the ligand-gated sodium ion (Na+) channels. This causes the ligand-gated sodium ion channels to open and sodium ions diffuse into the cell, making the membrane potential more positive. If the membrane potential reaches threshold level, an action potential will be produced.
There is a specific neuron for each colour, shape, when we think of two different things; two neurons make a connection, these results in thinking, learning and our behaviours. Learning involves neurons in the brain and the synapses between them. Neurons ad synapses make connections in order for us to think and learn new things. Signals are sent from one neuron to another by jumping across the synapse. In the process of sending signals, it involves the presynaptic neuron, the neuron sending the signal, the neurotransmitter which is a chemical released by neurons at the synapse for the purpose of relaying information to other neurons via receptors. The synaptic cleft is the space across which a nerve impulse passes from an axon terminal to a neuron or effector cell. The receptor molecule is a protein that recognizes a specific 3-dimensional shape which then goes into the postsynaptic neuron, the neuron which receives the signal.
Every time we interact with our environment and other people, our thoughts and behaviour change depending on our surrounding. When we sense or touch the surrounding, there are billions of cells in our nervous system that make these movements possible. Our experience from the past is stored and retrieved with neurons in the brain. Our thoughts and behaviour change due to our experience and interactions because drugs, stress, emotions which releases certain chemicals in the synapse that then mediates our interaction with other people. There are certain neurons that determine the way we feel and react. Based on our experience we build new neurons that then connect to other neurons for us to make sense of the world. Mirror neurons affect our feelings.
The foundation of the types of memory discussed and the brain structures responsible for specific aspects of memory is the neural network of the brain. To understand memory storage and retrieval, it is necessary to look at the neurophysiology of the brain. Information that enters and travels through specific parts of the brain is passed along this neural web. Nerve impulses arrive and travel through a neuron to the synapse. The impulses then travel across the synapse of one neuron via electrical activity to another neuron. Through the travelling of impulses across the synapses, information is passed from neuron to neuron. The junctions of a single nerve fibre can number up into the hundreds. The nerve impulses arriving at a neuron form a pattern similar to a microstructure of wave forms. These wave forms interact with similar, overlapping wave forms of functioning neurons. The interaction of these wave forms together causes a new pattern of nerve impulses to result. This impulse pattern effects protein and other chemical molecules at the synaptic junctions of neurons.
Memory and memory storage is based on the firing and travelling of impulses across patterns of nerve cells. The activity of neurons that are affected by an experience is altered in this process in order for this experience to be remembered and behaviour to be modified. Thus memory consists of changes and alterations of effected neurons left from a past activity or experience. Multiple patterns of these traces are left in effected neurons and these patterns make up a memory. Thus memories are stored and reinforced by stimulation of a neuron from arriving impulses. The initial experience leaves a memory trace or pattern in effected neurons. Repetition and recall of a memory further reinforces a memory trace, making it stronger and easier to access. Further recall and response based on this memory trace becomes larger than it had previously been. This phenomenon is known as long-term potentiation, or LTP. The duration of this effect differs from hours to weeks or longer. The duration depends on the age, health, and experience of the person, as well as the properties and strength of the stimuli. The best analogy for this is a river that constantly flows and cuts a deeper, more defined impression in its channel bed. Thus changes occur at the synapses between neurons after impulses arrive and these changes effect subsequent impulses, reinforcing the memory trace left. The changes occur in the structure of the synapse and the chemical molecules released here. Thus memory storage is based on a web of neurons that are altered to leave a memory trace. This memory trace differs based on space and time. That is, the neural network in different areas of the brain is responsible for holding different types of memories for varying amounts of time.
The complexity and beauty of this system of neurons is its flexibility and plasticity. The connections that exist between neurons are constantly changing. Throughout a lifetime, the structure of neurons is constantly changing as neurons die, knowledge is accumulated, information changes, and disease, old age, and other misfortunes destroy neurons and their connections. Constant modifications are being made of the neuron webs. Furthermore, the structure and connections of neurons differs from person to person based on unique experiences and environments. Thus the architecture of every person's brain is slightly different. These differences in the brain architecture at the neuron level create a unique individual who is constantly undergoing changes; changes to better suit new demands or experiences. This plasticity of the neuronal structure is necessary for storing memories. The strengths and structure of connections among neurons determines the strength and location of a saved memory.
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