A Basic Overview of Neuron Cell Signaling

By Jonathan Qu

Paper Length: 1396 words and 7010 characters (no spaces)

Intro

Cells inside and out of our bodies are constantly communicating, and just like how there are all kinds of people there are all kinds of cells. The way these different cells send and receive signals can be vastly different or almost identical, but in this paper, I’ll only be discussing nerve cell signaling.

Nervous System

The nervous system is the body’s command center, it controls all conscious decisions you make like opening the fridge and all the unconscious decisions like flinching. It consists of 2 main parts, the central nervous system and the peripheral nervous system. The central nervous system is your brain and spinal cord. Your peripheral nervous system consists of the nerves that carry signals from the spinal cord to the rest of the body (Office of Communications).

Brain

The brain is what makes us sentient. Overall, it manages our decisions, experiences, feelings, memories, and much more.

Cerebral Cortex

The cortex is the “cap” of the brain, it is the outermost layer of gray matter. The “cerebral cortex is organized into distinct functional areas made up of sensory, motor and association areas. It has a broad range of functions including perception and awareness of sensory information, planning, and initiation of motor activity. It also plays a key role in higher cognitive functions, such as decision making, motivation, attention, learning, memory, problem-solving, and conceptual thinking” (Ocran).

Thalamus

The thalamus intercepts sensory signals from our neurons and designates which part of the cortex the signal will go to be processed (Cyr)

Hypothalamus
The hypothalamus connects the central nervous system to the endocrine system. It is responsible for heart rate, blood pressure, appetite, thirst, temperature, and the release of various hormones” (Thau) to maintain homeostasis.

Spine

The spine “send(s) motor commands from the brain to the peripheral body as well as to relay sensory information from the sensory organs to the brain” (Thau).

Neurons

These long thread-like cells are what transmit messages from the brain to the rest of the body and from the body to the brain. At the “head,” there are the branchlike dendrites that receive neurotransmitter molecules or ligands from a previous neuron. The soma is the cell’s body, it contains the nucleus of the cell and its main organelles. (Bangalore) The long thin middle is the axon, led by the axon hillock, the wire like axon can be surrounded by a myelin sheath. The myelin sheath inhibits the change in ion concentrations inside and outside the cell at certain points to hasten the movement of the signal as an electrical charge to the axon terminals. At the terminals, these charges are turned back into chemical signals and through synaptic transmission the signal is passed on (Osmosis). While neurons cannot control what they say, or the number of neurotransmitters sent, they can control the frequency of the signal.

Synapses

The small gap between one neuron and the next. In a chemical synapse the neurons are not connecting, the neurons close enough to easily send signals to each other at a fast rate through chemical signals (Synaptic signaling) (Bangalore). While in electric synapses the neurons are connected by gap junctions.

Myelin

A fatty protein that coats the axon of some neurons like insulation for a wire. It's produced by Schwann cells. Myelin leaves gaps in the coat called nodes of Ranvier (Bangalore) to improve efficiency. Myelin drastically speeds up the transfer of messages in these neurons by creating a “high-resistance, low-capacitance" environment (Britannica). When myelin is present, the current is unable to normally travel down the axon. Instead, the myelin provides a place for the positive current inside the axon to “jump” down the length of the myelin through a process called saltatory conduction.

Nodes of Ravier

These kinks in the myelin sheath are where ion channels allow for the free flow of ions to change the charge in the axon (Britannica).

Figure 1 A neuron’s structure and parts

"File:Neuron1.jpg" by No machine-readable author provided. NickGorton~commonswiki assumed (based on copyright claims). is licensed under CC BY-SA 3.0.

Figure 2 The Differences between the two types of Synapses.

Image was found from Nature Reviews Neuroscience

Chemical Synapse Signaling

In response to the brain sending a signal for the body to do a voluntary action such as picking up the remote, the neurons in our brains use the chemical synapses to relay the message to your arm to pick up the remote. First our eyes will send a message to the pulvinar nucleus portion of the thalamus that sends visual stimuli of the remote to the correct portion of the cortex to be processed. Next the cortex would determine that we need to pick up that remote and change the channel (Neuroscientifically Challenged). Our neurons would fire up and send a chemical signal down into our arms and tense the muscular system to grab the remote (Osmosis.)

Only showing the start and result, we miss the vital middle portion that is the effect of the start and the cause of the start. How do the neurons amplify the signal to go from one neuron to being able to affect every muscle in our arm? How does the neuron transfer signal from neuron to neuron? What must occur for the result to be produced?

When the brain sends the first signal, it will release a neurotransmitter through the first neurons terminal to the correct neuron pathway. With enough EPSPs occurring, the membrane potential (the overall voltage of the cell) will cross the threshold value of the next neuron, this value differs from tissue to tissue. When this threshold value is reached, at the axon hillock Na+ voltage-gated channels begin to open, leading to more Na+ channels opening from the change in membrane potential further and further down the axon. This is referred to as the action potential. When this depolarization process finishes the cell’s charge is about +40mV. The Na+ channels will inactivate, a mode different from being closed. The inactivation gate blocks further ion movement. When the Na+ channels are inactivated, K+ channels open. However, because the K+ channels do not have the inactivation gate that the Na+ channels have, it takes longer for the K+ channels to close. Another pump that the cell uses is the Na+/K+ pump, an active transport channel that takes 3 Na+ out of the cell and in turn takes in 2 K+. The K+ channels and the active pump together led to an IPSP, and the overall charge in the cell is hyperpolarized at about –75mV. This is ARP. This repolarizing phase is ARP. Next phase is RRP, here the over repolarization is corrected as K+ channels start to close and the Na+ channels fully close, leaving the cell back at its original –65m. As the charge moves into the axon terminal, it causes vesicles of neurotransmitters to be sent to the postsynaptic neurons through the cells up to more than 100000 synapses (Osmosis). The signal is finally terminated by the ligands diffusing, enzymes breaking them down, or being sucked back into the presynaptic (Khan Academy).

Electrical Synapse Signaling

When our body makes an involuntary action, such as flinching from stubbing a toe, our body uses electrical synapses. An example of a neuron that could use electric signaling is an “A-fiber nociceptors whose axons are myelinated and support conduction velocities of approximately 5–30 m/s” (Dubin). In the toe stubbing case, first our nociceptors, pain receptors that sense temperatures and pressure extreme enough to damage tissue, would send an electrical signal to our lower motor neurons in our spine, where the signal would be sent for our leg to contract thus pulling it away from the potentially dangerous stimuli (Fetterman). Then the signal would be sent to our brain for it to be processed as pain. In electrical signaling, there is still the pre- and post-synaptic neuron. However, instead of the synaptic cleft, there is a gap junction that connects the cells. When stubbing your toes, the pain receptors would send a large amount of ions to the lower motor neurons in the spine that control our legs and then the motor neurons would send ions down to our leg to tell it to contract. At the same time, our legs would send a signal up the spine to our brain for it to process pain. This process is noticeably faster than the chemical signaling due to a few reasons

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