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Nervous system maintains coordination through the use of electrical and chemical processes. There are three aspects: sensory, motor, and integrative, which we will discuss throughout the system. The nervous system has a set of unique characteristics 1. Excitability: Nervous cells can be stimulated or excited. All cells can do this, but neurons are the most efficient. 2. Conductivity: Nervous cells respond to stimuli by producing electrical signals that can be conducted out to other neurons or to muscle or to a gland. 3. Secretion: Nervous cells have the ability to secret a neurotransmitter that can stimulate other cells. (Neurotransmitter is a chemical which the nerve cells use to communicate with other cells) The nervous system has two parts: 1. The Central Nervous system and the Peripheral Nervous System. The CNS includes the brain and spinal cord and functions in processing and integration. The PNS includes the nervous tissue including nerves in the rest of the body and is responsible for detection/sensation, stimulates change in other systems, and are the doers. 2. The second division is the Somatic vs. Autonomic Systems 2
The Somatic System is voluntary and is largely involved with skeletal muscle The Autonomic System is involuntary, and is responsible for secretions, sensations, and control of cardiac and smooth muscle and glands The big questions to determine which system you have is to ask Do I or don t I have control? Most tissues receive nerve fibers from both systems. The divisions are not necessarily mutually exclusive (especially the somatic and autonomic). The Systems of both divisions work together to maintain control and homeostasis. 2
Neurons are the functional cell of the nervous system. If one characteristic of the nervous cells is that they are excitable, neurons take the cake. They are extremely excitable. It is not enough that they are excited, but they have to make other cells excited too. They are like 3 year olds. Once they are really excited, they have to tell everyone. So they are excitable and they conduct impulses. There are three types of neurons. 1. Sensory or afferent neurons detect stimuli found in the PNS and transmits them to CNS. Think of them like an Inbox. 2. Motor or efferent send signals to the muscles and gland cells from the CNS. Motor neurons begin in the spinal cord, but most of the cell is in the PNS. They carry out going signals. Think of them like an Outbox. 3. Interneurons or association neurons are entirely in the CNS. Their function is to process, store, and retrieve info received from sensory neurons and pass on a response to the motor neurons. These cells are like some street lanes, traffic only goes one way. You can t use the same neuron to send messages in and out, like you can t drive east on a westbound lane. 3
You can follow the path of the neural signal. The sensory neurons detect a stimulus in your finger and send that information to the CNS. The interneurons determine that you finger needs to move and send an appropriate response to the motor neuron. The motor neuron then send the message to the muscle that will move your finger. 4
The neuron has a unique anatomy. The following structures are typical of a motor neuron. Sensory neurons have a slightly different shape. A Soma is the cell body and is the processing part of cell. This is where the normal cell stuff and functions occur. Within the soma we will find Nissl bodies. Nissl bodies are rough ER, this is unique to the neuron. Dendrites are extensions of the soma that are used to detect stimulus. They are the receiving portion of a neuron. They are usually short and highly branched. The Axon hillock is considered the trigger zone or where an impulse is generated. Once a signal is generated in the axon hillock, it is sent down the Axon. The single axon is a long thick extension used to send signals to other cells. The Terminal branches are where the axon splits, so it can branch out towards multiple cells. Compared to axons, they do not have myelin (we will get to the details of myelin shortly.) Finally, Synaptic Knobs are the very ends of the branches and interacts with other cells (think of your neuromusclular junction). They also contain neurotransmitters. 5
This is the structural classification of neurons. 6
We have two types of cells in nervous system: neurons and neuroglial cells or glial cells. Neuroglia are helper cells in the nervous system. Oligodendrocytes make a myelin sheath, but only for the cells in the CNS. They are octopus shape with branches that reach out and wrap around neurons. Whereas the schwann cells (in PNS) only wrap around one cell, the branches of oligodendrocytes wrap around multiple neurons. Ependymal cells resemble cuboidal epithelium and are found in the CNS and produce cerebrospinal fluid. Microglial cells wonder around the CNS and remove dead nervous tissue, microorganisms and other foreign matter. Clusters of microglia can be used to find damaged areas. Astrocytes are the most abundant glial cells in the CNS and have a star shape. They also have the most diverse functions: they provide a supportive framework, form the blood-brain barrier (more on that shortly), supply nourishment, secrete nerve growth factors, and influence signaling. Satellite are only found in the PNS. Little is known of their actual function, but they surround neuron cell bodies. 7
Schawnn cells are on the next slide 7
Schwann cells are a type of neuroglia that forms myelin sheath around the neurons in the PNS. They wrap around the axon over and over again. The inner layers of the wrap are called the myelin sheath. In this sheath, a substance called myelin is released. Myelin is a fatty protein that insulates the axons which provides protection for the electrical signal. By doing so, myelin helps speed up the signal. This is analogous to the plastic coating around an electrical cord. The myelin prevents the electrical signal form dispersing and flowing in multiple directions and keeps it going straight down the axon. As the schwann cells wrap around the axon, the cytoplasm and nucleus gets pushed to the outside layer. This outside layer is called the neurilemma. The Neurilemma be can used as a guide to help a cut axon grow back down its original path. It provides a regeneration tube to help the axon heal. Schwann cells are too small to coat the entire axon, which is really long, so it takes multiple cells. The spaces between the cells are called the nodes of ranvier. They are NOT myelinated. As an impulse travels down the axon, the signal gets weaker the further it travels, the nodes allow the signal to recharge. In brain or spinal cord, some regions look white and some other look grey. The grey matter contains mostly unmyelinated axons where as white matter is made up of 8
mostly myelinated axons, which looks white. 8
Here is our schwann cell. The first layers wrap really tightly and pack together. This is where the myelin will be produced. You can also see the neurilemma with the schwann cell nucleus. 9
We can see in this diagram how a regeneration tube can form. You do not need to know the steps of regeneration, but be familiar with the concept as a role of the neurilemma. 10
Here is a diagram of the neuroglia cells in relation to neurons. The ones shown here are in the CNS. 11
One of the key characteristics of a neuron is that ability to send an electrical current. Before we go through the steps of how a neuron sends an impulse (or an electrical current), there are a few terms and concepts we need to discuss. Membrane potential or Electrical Potential This is a difference in the concentration of charged particles across the membrane. It creates a potential for energy (via current or flow of charged particles). If a cell has potential it is called polarized. Resting Membrane potential This is when a nerve is not conducting an impulse. 2 critical ions, Na+ and K+, are required to establish resting potential. There are more Na+ ions outside the cell than there are K+ inside the cell, the inside of the cell is negatively charged with respect to the outside of the cell. Resting Potential is usually measured around -70 millivolts (mv). A stimulus is anything that changes the resting potential. It will cause the cell membrane to be leaky. This results in more positive ions moving into the cell causing there to be less of difference between the inside and outside of the cell. The of movement of the ions causes the potential to move closer to equilibrium 12
Threshold potential is the critical voltage required to send in impulse. Neurons receive constant stimulus, and as previously mentioned, this causes the membrane to leak. A stimulus is considered strong enough to pass on, when enough ions have moved to cause a significant change in voltage. This is the threshold potential. It is usually -55 mv. 12
To continue with our terminology: Depolarization If threshold is reached, this opens more gates and allows more Na+ to flood into the cell. The neuron becomes more and more permeable to Na+. The membrane potential shifts to a less negative value. Repolarization After depolarization the membrane becomes more permeable to K+, and allows it to leave the cell quickly. Repolarization returns the cell to its original state. An Action Potential is a full cycle of depolarization and repolarization due to Na+ coming in and K+ going out in response to a really, really strong stimulus. A neuron can have 1000 action potentials in 1 sec, this allows the nervous system to be very, very fast. Hyperpolarization is the opposite of depolarization and creates a greater difference between the inside and outside of the cell. It makes the membrane more negative and it becomes harder for an action potential to occur. 13
Now that we are familiar with some of the terms, lets start to walk through the steps of an action potential. 1. An adequate stimulus is applied to the neuron. Causing Na to leak into the cell. Enough Na leaks in that threshold potential (-55mV) is reached. 2. Once threshold potential is reached, sodium channels open and Na flows into the axon causing the membrane to depolarize. 3. As the depolarizing occurs, more Na channels open and more Na flows in and the membrane depolarizes even more. 4. At a certain voltage (+35mV) the Na channels start closing. 14
5. Once +35mV is reached, there is a reversal of polarity and repolarization begins. 6. Potassium channels fully open and K+ rushes out of the cell. This counter acts the movement of Na. 7. The inside of the cell again becomes less positive and is referred to as repolarization. 8. Repolarization is aided by Na/K pumps. The pump removes 3 Na from the cell and brings in 2 K. Thus, returning the ions to their original locations. This is an active process as the ions are moving against their concentration gradients and requires ATP. 9. Hyperpolarization occurs. This is when the cell goes slightly pass its resting potential of -70mV because too many potassium ions have left the inside of the cell. 10. Finally go back to resting state with the aid of the Na/K pump. 15
There are a few key points about action potentials that need to be mentioned and recalled. First, action potentials do not happen all at once along the entire axon. There is a cascade effect to action potentials. The first action potentials start at the axon hillock and then stimulate action potentials in the next region. There is a cascade effect with action potentials, one will trigger another one to happen near by and so on down the line, like a domino effect. As mentioned, action potential starts at the axon hillock. There are 350-500 gates per square micro unit in the hillock compared to 50-75 elsewhere. Therefore, there is a greater chance of reaching threshold quicker here. This is an all or nothing event. Either threshold is reached or it isn t. Once it starts, it is not reversible. Sodium and Potassium are the key players and cause the change. Also while Na+/K+ are flowing through the gates, the Na+/K+ pump is shuttling 3 Na+ out and 2 K+ in to put the concentrations of each ion back to resting. This pump requires ATP. The key voltages to keep in mind are -70mV, -55mV, and +35mV 16
You may need to review the steps of the action potential a few times in order for it to sink in. This animation should help. 17
This diagram represents the change in voltage as the membrane moves through the action potential. Step one is the local potential (we ll define this momentarily) and leads to threshold potential which is step 2. Step 3 is depolarization followed by the critical voltage of +35 as step 4. We then switch to repolarization in step 5. Step 6 is the slight hyperpolarization. And step 7 is the return to resting. Notice that once you hit threshold potential, depolarization and repolarization happen quite quickly. 18
A Local Potential is a short range change in membrane potential and is reversible. If a stimulus is not strong enough, the change in potential will not start an action potential, and the membrane will go back to normal. A refractory period is a period of time when the neuron resists re-stimulation. There are two stages, an absolute and relative refractory period. The absolute refractory period is a portion of the action potential where no stimulus, regardless of how strong can trigger a new action potential to start in that location. The relative period is a segment of time when a very, very strong stimulus can cause can cause another action potential to happen at that location. 19
In this diagram, we can see the absolute and relative refractory periods. The absolute refractory covers the course of depolarization and repolarization. Regardless of how strong a stimulus is, the membrane cannot start another action potential. The relative refractory period occurs during hyperpolarization. During this time, an extremely strong stimulus can kick off another action potential. It will just take more movement of sodium to reach threshold than when a cell is at resting potential. 20
Saltatory Conduction is conduction via skipping and jumping of an impulse down the axon. The impulse flowing down the axon is fast, but slows down (runs out of juice), luckily at the Nodes of Ranvier the action potential is recharged. This diagram shows how the impulse travels down the axon and is recharged at the nodes. It is almost as if the impulse jumps from node to node. 21
A synapse is a region where a neuron carries info toward another structure like a muscle or a gland. There are 3 components: an axon (or pre-synaptic structure), synaptic cleft, and a post-synaptic structure (either another neuron or a different form of cell). Neurotransmitters are stored and released from vesicles in the presynaptic structure and travel across the cleft to the post-synaptic structure. This is how a an impulse can be passed on from cell to cell. There are 60 different kinds of neurotransmitters in our body. 22
A diagram of the synapse. Recall the neuromuscular junction. 23
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