Psychology - Neuron Structure & Action Potentials

In this video, we're learning about neuron structure and action potentials. So we'll cover the structure of a neuron, the types of neurons and how they work together, and finally how action potentials work. Let's start by looking at the structure of a neuron. A neuron is a type of specialized cell that transmits electrical signals throughout our nervous system. There are five main parts of a neuron to cover. Starting with the dendrites, these are branch-like extensions that receive signals from other neurons. We can think of them as the receiving end of the neuron because they pick up incoming signals and carry that information towards the cell body. You might also see the cell body called the soma. It contains the nucleus and is essentially where the cell maintains itself, produces proteins, and releases the energy it needs to function. For the third part, we've got the axon, which is a long, thin fiber that carries electrical signals towards other neurons, muscles, or glands. Many axons are covered by a myelin sheath, which is a fatty layer that insulates the axon, and this basically just means that it helps signals travel faster. The myelin sheath isn't continuous, though, as it contains these small gaps between sections of myelin, which are called nodes of renvier, and they play an important role in speeding up signal transmission, as we'll see later on in the video. Finally, at the end of the axon, we find the axon terminals, which are these small branches that form connections with other neurons. The way they do this is by releasing chemical messengers called neurotransmitters, and these neurotransmitters pass the signal onto the next neuron. Next, let's look at the three types of neurons in our nervous system. Sensory neurons, relay neurons, and motor neurons. These three types of neurons work together in what we call a reflex arc, which is the pathway that allows us to respond quickly to potentially harmful stimuli, like when we touch something sharp. So let's go through each type of neuron in the context of the reflex arc. The first step in the reflex arc involves sensory neurons, which are also known as afferent neurons. These neurons send information from our sensory receptors, which are specialized cells that detect stimuli like this sharp pin, towards the CNS, which is the central nervous system, consisting of the brain and spinal cord. In the reflex arc specifically, sensory neurons send signals directly to the spinal cord, bypassing the brain, which is what allows the response to be so fast and means you don't need to think about the action before you do it. The second step involves relay neurons, which you might also hear be called interneurons. These act as connectors between sensory and motor neurons within the central nervous system. In the reflex arc, relay neurons in the spinal cord process the information received from sensory neurons and quickly pass it on to motor neurons. The third and final step involves motor neurons, which are also called efferent neurons. These carry signals from the central nervous system out to our effectors, which are muscles and glands that produce a response. So in our example, motor neurons carry the instruction from the relay neurons to the muscles in your arm, which would be the effector. Triggering a rapid, automatic response that pulls your hand away from the sharp object. This all happens without conscious thought, which is why reflexes are so fast. Finally, let's explore how action potentials work, and we use this term to describe the electrical signal that travels along a neuron. First, we need to understand what a neuron is like in its resting state, which is when there is no action potential being transmitted. In this state, the space inside the neuron has a negative electrical charge compared to the space outside the neuron. We call this difference in charge the resting potential, and it's maintained by the movement of ions across the neuron's membrane. When a neuron is stimulated, channels open in its membrane, allowing positively charged ions to rush in, making the inside become more positively charged than the outside. If the inside becomes positive enough to reach a certain level, which we call the threshold potential, an action potential is generated and can travel along the axon of the neuron. An important feature of action potentials is the all-or-nothing principle. What this means is that if the threshold potential is reached, the neuron generates an action potential at full strength, but if the threshold potential is not reached, the neuron doesn't generate an action potential at all. The key point here is that there's no such thing as a partial action potential, it is either all or nothing. If a neuron generates an action potential, it enters what we call a refractory period, a brief recovery time when the neuron can't generate another action potential. This ensures that signals travel in only one direction along the axon, towards the axon terminals. As we mentioned earlier, in neurons with a myelin sheath, action potentials travel much faster, and this happens through a process called saltatory conduction. When we say that the myelin insulates the axon, this is because ions can't travel through the neuron's membrane at points where the myelin sheath is, which means the action potential essentially jumps from one node of Ranvier to the next, rather than traveling continuously along the whole length of the axon. This makes transmission much faster and more efficient. Much of what we know about action potentials comes from research by Hodgkin and Huxley in 1952. They conducted experiments using the giant axons of squid, which were large enough to insert electrodes into and measure the electrical changes directly. Their experiments showed that, at rest, the inside of the neuron was negatively charged compared to the outside, but during an action potential, this reversed and the inside became positively charged compared to the outside. This helped establish the fundamental mechanisms of how action potentials are generated and travel along neurons. 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