Overview of neuron function

In this video, I want to provide an overview of neuron function, which I think of sort of like how a gun functions. And we'll go into a lot more detail on how a neuron functions in later videos. But in this video. I just want to give a bird's eye overview of it. The function of neurons is to process and transmit information. Without input, most neurons have a stable electrical charge difference across their cell membrane, where it's more negative inside the cell membrane and more positive outside the cell membrane. And we call this the resting membrane potential or just resting potential for short. And this resting potential is really how the neuron is going to be able to be excitable and respond to input. And I think of this as similar to loading a gun by putting a bullet in it. Neurons receive excitatory or inhibitory input from other cells or from physical stimuli like odorant molecules in the nose. Input information usually comes in through the dendrites. Although less often, it'll come in through the soma or the axon. The information from the inputs is transmitted through dendrites or the soma to the axon with membrane potential changes called graded potentials. These graded potentials are changes to the membrane potential away from the resting potential, which are small in size and brief in duration, and which travel fairly short distances. The size and the duration of a graded potential is proportional to the size and the duration of the input. Summation, or an adding together of all the excitatory and inhibitory graded potentials at any moment in time occurs at the trigger zone, the axon initial segment right here. This summation of graded potentials is the way neurons process information from their inputs. If the membrane potential at the trigger zone crosses a value called the threshold potential, information will then be fired down the axon. So I like to think of this process of summation of the excitatory and inhibitory graded potentials at the trigger zone as analogous to the trigger of a gun. In fact, that's why it's called the trigger zone. I think of the graded potentials as being like the finger on the gun, that may be squeezing a little harder or relaxing. But once the trigger of the gun is pulled back past a certain threshold distance, a bullet will be fired down the barrel of the gun, just like if the membrane potential of the trigger zone crosses a threshold value, information will be fired down the axon. The way information is fired down the axon is with a different kind of change to the membrane potential called an action potential. An action potential is usually large in size and brief in duration. But it's usually conducted the entire length of the axon, no matter how long it is, so that it can travel a very long distance, just like a bullet usually has no trouble making it down the barrel of the gun. And like a bullet traveling through the barrel of a gun, action potentials tend to travel very quickly down the length of the axon. Action potentials are different than graded potentials because they're usually the same size and duration for any particular neuron, as opposed to the graded potentials, whose size and duration depends on the size and the duration of the inputs. Action potentials are conducted faster along larger axons, axons with a larger diameter, and along axons that have a myelin sheath, that I've drawn in yellow here. When an action potential reaches the axon terminals at the end of the axon, information will then cross, usually a small gap, to the target cell of the neuron. And the way this happens for most synapses where an axon terminal makes contact with the target cell is by release of molecules called neurotransmitters that bind to receptors on the target cell and which may change its behavior. Neurotransmitter is then removed from the synapse. So it's reset to transmit more information. And I think of this part as similar to the bullet leaving the gun, to hit the target. The input information that was converted into the size and the duration of graded potentials is then converted into the temporal pattern of firing of action potentials down the axon. And this information is then converted to the amount and the temporal pattern of neurotransmitter release at the synapse. These steps are how neurons transmit information, often over long distances. This is the general way that neurons usually function. But there are multiple functional types of neurons. So let's take a look at some of those. Here I've drawn a few different neurons, with their somas in red, their axons in green, and their dendrites in blue. And I've drawn a line here to separate between the central nervous system on this side-- so I'll just write CNS for short-- and the peripheral nervous system on this side-- so I'll just write PNS for short. And there's some different ways we can categorize functional types of neurons. The first way is the direction of information flow between the CNS and the PNS. If a neuron like this pseudounipolar neuron right here brings information from the periphery in toward the central nervous system, we call that an afferent neuron. Afferent, meaning it's bringing information into the central nervous system. We can also call this type of neuron a sensory neuron because the information it's bringing into the central nervous system involves information about a stimulus. And a stimulus is anything that can be sensed in the internal or external environment, which is to say anything inside the body or anything outside the body. These neurons are carrying information away from the central nervous system out into the periphery. So instead of calling them afferent neurons, we call them efferent neurons. And there are two main kinds of efferent neurons. The first we call motor neurons. Motor, which means movement. These are efferent neurons that control skeletal muscle, the main type of muscle that's attached to our skeleton, that moves us around. These motor neurons are also called somatomotor neurons or neurons of the somatic nervous system. The other type of the efferent neurons are called autonomic neurons. And these neurons control smooth muscle, like the muscle around our blood vessels; cardiac muscle, the muscle of our heart; and gland cells, the cells of our glands that secrete hormones into the bloodstream. These autonomic neurons are also called visceromotor neurons or neurons of the autonomic nervous system. Most neurons of the central nervous system aren't any of these types of neurons, however. They're like this neuron, in that they connect other neurons together. So these are called interneurons, neurons between neurons. And there are many interneurons in the central nervous system, forming very complex pathways for information to travel. So that while an individual neuron is processing and transmitting information, these complex networks of neurons in the central nervous system are doing even more complex processing and transmitting of information.