 This video will cover an introduction to the anatomy of the nervous system. This material mostly comes from chapters 12 and 14 of Open Stax Anatomy and Physiology, which is freely available online. And some additional content comes from Grey's Anatomy, the edition from 1918, which is in the public domain and also freely available online. Some information comes from a review article by Larry Squire published in the journal Neuron. And there's also an image that comes from Wikipedia, and so all of this information is either covered by a Creative Commons license or is in the public domain, and so it's all freely available online. As we go, we will cover the following study objectives, define central and peripheral nervous system, and describe the general functions of each, list the components of a reflex arc, describe the major functional divisions of the peripheral nervous system, describe the structures of a typical multipolar neuron, list examples of where bipolar and unipolar, also known as pseudo-unipolar neurons are found in the human body, describe the functions of the major types of glial, or also known as neuroglial cells, in the central nervous system, the astrocytes, oligodendrocytes, microglia, and epindymal cells, in the peripheral nervous system, the satellite cells and Schwann cells. Describe how information is transmitted through the nervous system, explain how action potentials are generated and propagated, and describe the structure and function of a synapse. The central nervous system is composed of the brain and spinal cord, all nervous tissue outside the central nervous system is part of the peripheral nervous system. The major components of the peripheral nervous system are called nerves and ganglia. Ganglia are collections of neurons located outside of the central nervous system, and nerves contain long extensions of neurons, which are called axons, that rapidly transmit information. The function of the peripheral nervous system is to transmit information in and out of the central nervous system. The central nervous system contains control centers that integrate sensory information coming from the peripheral nervous system and sends out motor commands to regulate other organs. Much of our understanding of the functions of the nervous system and the brain in particular has come from the study of unfortunate individuals that have suffered damage such as a trauma to the brain. A famous example is Phineas Gage, shown in the image here, holding a large iron rod that large iron rod impaled Phineas Gage through the skull while he was working on the railroad in 1848. The rod severely damaged a large region of the brain on the anterior known as the prefrontal cortex. Surprisingly, Gage survived, and witnesses reported that his memory and general intelligence seemed unimpaired after the accident. However, it was reported that his personality changed drastically. Friends described him as no longer acting like himself. Before the accident, Gage was remembered as a hard-working, amiable man, and afterward, he turned into an irritable, temperamental, and by some accounts, lazy person. Though his life was changed dramatically by this accident, he was able to become a functioning stagecoach driver later in life, suggesting that the brain has the ability to recover even from major trauma such as this. Another famous example is the case of Henry Mollison, best known as patient HM, who suffered from epilepsy, a condition in which the brain becomes overactive causing seizures. Despite high doses of anti-convulsant medication, HM could not work or lead a normal life. Then, a neurosurgeon named William Scoville performed an experimental procedure in an attempt to treat his epilepsy. Scoville removed large portions of the brain on the lateral sides, including a structure known as the hippocampus. You can see the hippocampus in the illustration from Gray's Anatomy on the left here, shown in light blue. The surgery was successful at preventing epilepsy, but the unintended result of this surgery was a profound memory impairment. At first, it seemed that HM was incapable of forming any new memories. After the surgery, he forgot daily events nearly as fast as they occurred. But this memory deficit was in the absence of any general intellectual loss. However, then a neuropsychologist named Brenda Milner carefully studied HM and was able to demonstrate that he was capable of some forms of learning. A famous test that Milner used to demonstrate a distinct type of memory is called the mirror drawing task. In this test, HM was asked to draw between the lines within a star pattern without directly looking at his hand, but instead using a mirror to look at a reflection of his hand drawing on the paper. This task is difficult, but with practice HM was able to improve. While he was able to learn this skill by forming what we now call a procedural memory, he never formed a memory of the event, and he would remark with surprise after performing well on the task having no memory of his practice. And so that was just to give us a little insight into the functions of the brain, a major component of the central nervous system. The spinal cord is also a major component of the central nervous system. Both the brain and the spinal cord can function as control centers for reflexes. So it reflexes a stereotypical response to a stimulus, and there are five components of a reflex arc. The example shown here is a withdrawal reflex in response to a painful stimulus. So the stimulus is something in the external environment. In this example, the stimulus in the external environment is a source of heat. We could also respond to a stimulus that's in our body, an internal stimulus. But for this example, the stimulus is something very hot. And if you touch something very hot with your fingers, there are thermoreceptors, receptors that are in your skin that detect the heat. And so the first component of a reflex arc is always a receptor. In this example, the receptor is a thermoreceptor measuring the temperature of the skin and the information will be carried from the receptor through an afferent pathway down a sensory neuron. That afferent pathway carries information from the receptor to the control center in the central nervous system. So in this simple example, the control center is in the spinal cord. And the control center receives sensory information coming from the receptor and then integrates that information and sends out commands. And so commands travel out from the control center through an efferent pathway of a motor neuron. And so the efferent pathway carries commands from the control center out to an organ called an effector. In this case, the organ is a skeletal muscle. And so the skeletal muscle will contract in order to withdraw the hand from the painful stimulus of the heat. And so the effector is the organ or cells that are producing a response to the commands that come down the efferent pathway from the control center. And so the control centers are found within the central nervous system. And there are information coming in from the peripheral nervous system and information that travels out through the peripheral nervous system to reach the effector organs. So the peripheral nervous system can be subdivided into major functional divisions. The efferent fibers of the peripheral nervous system are carrying sensory information from receptors into the control centers of the central nervous system. We have somatic efferent fibers that carry sensory information from the skin, muscles, and joints. And so these are detecting the sense of touch as well as the sense of the position of the body. And so somatic comes from the word soma, the Greek word for body. And so the somatic efferent fibers are carrying sensory information from the body. And that would include the sensory information coming from the thermoreceptors in our skin, detecting the temperature of our skin. That information travels from the receptor to the control center on efferent fibers, that is axons within nerves that carry information from the receptors in towards the central nervous system. So those are sensory neurons that have their axons traveling through nerves of the peripheral nervous system. There are also visceral efferent fibers. The visceral fibers are carrying sensory information from our visceral organs, that is the organs in the ventral body cavities, like the heart and the lungs, the organs of the digestive system, the blood vessels. These are all examples of visceral organs and visceral efferent fibers carry information from receptors located in our visceral organs and carries that information into the control centers. So these are also sensory neurons that are carrying the information in from receptors in the ventral body cavities. Now in response to that information the central nervous system will send out commands through efferent fibers of the peripheral nervous system. That is the axons of motor neurons that are traveling through nerves carry commands out from the central nervous system in order to regulate the activity of our efferent organs. If the efferent organs are skeletal muscles then the fibers are somatic efferent fibers. So there are motor neurons that extend their axons out in order to excite the contraction of skeletal muscles. Those axons traveling through nerves in the peripheral nervous system are called somatic efferent fibers. There are also autonomic efferent fibers. So the autonomic division of the peripheral nervous system regulates smooth muscle, cardiac muscle, and glands. And the autonomic nervous system is under involuntary control. And so while the somatic branch of the peripheral nervous system, the somatic efferent information is under voluntary control. You can consciously decide to move your body and send signals out through somatic efferent fibers. You do not have the ability to consciously send signals out through autonomic efferent fibers. Instead, these fibers are responding to involuntary reflexes where information often coming through visceral efferent fibers is processed by control centers that send out commands automatically without any voluntary control in order to maintain the vital functions of our heart and lungs and digestive organs. And so these autonomic fibers are coordinating the activities of our visceral organs. Many of these reflexes are essential to life. And as we go on to study the peripheral nervous system in more detail, we'll see there's two major divisions within the autonomic nervous system. There are sympathetic efferent pathways and parasympathetic efferent pathways. The sympathetic branch stimulates the mobilization of energy in order to deal with a stressful situation. The sympathetic branch is the fight or flight division of the autonomic nervous system that would increase the activity of the heart and would also decrease the activity of the digestive system. In contrast, the parasympathetic division is commonly known as the rest and digest division of the autonomic nervous system. Its functions are opposed, they're very much opposite to the sympathetic branch. That is, the parasympathetic branch will slow down heart rate in order to decrease the activity of the heart. The parasympathetic branch will stimulate increased activity of the digestive organs in order to help bring nutrients into the body during the rest and digest phase. Now we're going to start going through a little more detail of the structural organization of the nervous system. There is gray matter and white matter within the nervous system. We see here a picture of the brain with a section removed to show the gray matter and white matter within the cerebrum. There is gray matter found superficially in what we call the cerebral cortex. So the cortex of the cerebrum is made of gray matter. Gray matter contains the neurons, contains what we call the cell bodies of neurons, as well as some extensions from those neurons called dendrites that are forming connections with other neurons called synapses. So the gray matter contains neuronal cell bodies, dendrites, and synapses. The white matter contains the axons of neurons, that is the long extensions that carry information rapidly from one part of the nervous system to another. So the white matter in the central nervous system forms what we call tracts. Tracks are bundles of axons that are rapidly transmitting information from one place to another. The gray matter can also be called a nucleus. If it's deep inside the brain or deep inside the spinal cord we will see in the central nervous system collections of gray matter that are called nuclei. And so the word nucleus has several different meanings in anatomy and physiology. In the brain we see a nucleus is a collection of gray matter found deep within the brain or spinal cord. When we studied the anatomy of cells we learned that a nucleus inside of a cell is the region that contains the DNA where the processes of DNA replication for cell division occur and where the processes of transcription where the process of transcription occurs where DNA is read in order to produce messenger RNAs that can then be translated by ribosomes in the cytoplasm. And then at an even smaller level so the nucleus is an organelle at the cellular level the organelle level. If we go even further in down to the chemical level the nucleus of an atom is the region inside of an atom that contains the protons and neutrons. And so an atom contains protons, neutrons, and electrons. The electrons are surrounding the nucleus but in the center of an atom are protons and neutrons forming a nucleus of an atom. And so we won't talk very much about the chemical level of using the term nucleus in this class but as we're studying the nervous system we're going to be focused on the nuclei in the brain and spinal cord and we may also talk somewhat about the nucleus inside cells. For example neurons have a nucleus inside of the cell body and glial cells, neuroglial cells, the other major cell type in the nervous system. These cells also have a nucleus inside of their cytoplasm. Here we see a diagram showing the major components of the nervous system. The brain and spinal cord are the structures of the central nervous system that receive sensory information and process that information and then develop the commands to be sent out to regulate organs throughout our body. The brain is responsible for the conscious perception of sensory information. The brain also forms memories and is important for regulating our decisions, our motor commands, producing what we call our personality and the brain sends out motor responses, motor commands. Some of these are voluntary where we can decide what we want to do. However the brain also performs lots of unconscious processing and can coordinate involuntary responses by regulating the autonomic nervous system. There are many involuntary processes that help keep us alive by regulating our visceral organs like our heart and our lungs and our digestive system. The spinal cord contains some control centers for simple reflexes and the spinal cord also contains the pathways for sensory and motor information to travel in and out of the brain. So we'll see the tracks are the bundles of axons, the fibers that are traveling, carrying information rapidly through the central nervous system and large collections of tracks running up and down the spinal cord form columns. On the posterior we have posterior white columns carrying the sensory information, ascending sensory information up towards the brain and then on the lateral and anterior sides are white columns that contain the descending tracks where motor commands are sent down from the brain through the spinal cord. The spinal cord connects to spinal nerves and the brain connects to cranial nerves. These nerves are the white matter of the peripheral nervous system and then there are ganglia in the peripheral nervous system. This is the gray matter of the peripheral nervous system. The dorsal root ganglion contain the cell bodies of sensory neurons and then there are autonomic ganglia that contain the cell bodies of motor neurons that extend their axons out in order to regulate the activities of our visceral organs, our heart, the smooth muscle in our blood vessels, the smooth muscle in our digestive organs and then also glands that are regulated by the autonomic efferents. Then the digestive tract contains an enteric nervous system. The enteric nervous system is a subdivision of the peripheral nervous system because it's not found in the brain or spinal cord. However, it's different than the rest of the peripheral nervous system because the enteric nervous system does contain some control centers making it capable of operating independently. In the central nervous system the bundles of axons that are carrying information rapidly from one place to another are called tracts and in the peripheral nervous system those bundles of axons that rapidly transmit information are instead called nerves. In this diagram here we can see the visual pathway and the example of the optic nerve which is a cranial nerve and the cranial nerves are all numbered one through twelve. This is cranial nerve number two, the optic nerve that carries visual information from the eye and carries that information into the central nervous system. Now in the central nervous system the same axons are instead called the optic tract and the optic tract will carry that information into a region within the brain called the thalamus. And so the bundles of axons that are the extensions of neurons that rapidly transmit information can be called nerves in the peripheral nervous system and tracts in the central nervous system. In this illustration we can see major structures of white and gray matter in the central and peripheral nervous system specifically the spinal cord and spinal nerves. The gray matter of the spinal cord is found deep underneath white matter so the region in the center is the gray matter that forms horns in the spinal cord what we call the posterior gray horn, lateral gray horn and anterior gray horn. Now remember that gray matter contains the cell bodies of neurons as well as the synapses where neurons connect to one another. The anterior gray horn contains the cell bodies of motor neurons that extend out to excite skeletal muscles so these are somatic motor neurons and the axons that extend out travel through the ventral root and are called somatic efferent fibers. The autonomic motor neurons have their cell bodies in the lateral gray horn they also extend their axons out through the ventral root then on the posterior of the spinal cord is the posterior gray horn. Sensory information will travel into the posterior gray horn and inside of the posterior gray horn are some neurons that will relay that information or perform the integration of a control center that will then regulate the efferent motor neurons and so the sensory neurons that carry information into the posterior gray horn have their cell bodies in the peripheral nervous system in the structure known as the dorsal root ganglion. So a ganglion is a collection of neuronal cell bodies in the peripheral nervous system the ganglion are the gray matter of the peripheral nervous system the axons of the sensory neurons travel through the spinal nerves and then travel into the spinal cord on the dorsal side through a structure known as the dorsal root and so we can see the spinal nerves here are the white matter of the peripheral nervous system the ganglion are the gray matter of the peripheral nervous system then in the central nervous system the spinal cord contains nuclei that are clustered together in large groups known as the gray horns and in the spinal cord surrounding the gray matter is white matter containing tracks bundles of axons that carry information up and down the spinal cord those tracks are bundled together into larger structures known as white columns we have posterior white column that relay sensory information from the posterior gray horn up towards the brain and what we call ascending tracks through the posterior white column then the lateral white columns and anterior white columns contain most of the descending tracks where motor information coming from the brain travels rapidly down the spinal cord and there is there are some sensory tracks some ascending tracks that are also mixed into the lateral and anterior white columns here we see an fmri or functional magnetic resonance image that is measuring activity in the brain the fmri actually measures changes in blood flow in regions of the brain and this blood flow is associated with an increase in the signaling the information processing that's happening in a region of the brain and here we see an increase activity in the regions of the brain that are processing visual information as the subject is being shown a visual stimulus the visual pathway will increase its activity and so more blood will flow to that area of the brain that's processing the visual information here we see an illustration of a neuron this is a typical neuron called a multipolar neuron so the multipolar neuron has many different extensions coming out of the cell body so the cell body also known as the soma is the large region of a neuron that surrounds the nucleus then there are many processes or extensions coming off of the cell body the numerous processes extending from the cell body of a multipolar neuron are called dendrites and the dendrites function as a receptive region forming synapses with the axons of other neurons in order to receive the information coming from the axon the dendrite will have receptors to detect neurotransmitters released from the the axon and so the axon is the large process that comes out of the cell body and the axon can travel a very long distance for example there are axons that extend from your spinal cord all the way down to the tip of your toe and the longest example is a sensory neuron axon that can travel all the way from the tip of your toe up into your spinal cord up through your spinal cord in the the tracts of the dorsal white column or posterior white column all the way up to the brain and so a axon can be about as long as a person is tall however not all axons are that long many axons for much shorter connections in order to perform processing within a region of the brain and so the axon connects to the cell body at a region known as the axon hillock and the axon will carry information rapidly from that connection of the axon hillock all the way down to the end of the axon known as the axon terminals and those axon terminals form synapses where they communicate with other cells and so a synapse is a small space where the axon terminal releases a chemical message called a neurotransmitter in order to send information to another cell and this could be another neuron or it could be a muscle or a gland that's being regulated by this neuron now the axon is coded in some cases by a myelin sheath we see here the myelin sheath is created by an oligodendrocyte an oligodendrocyte is a type of glial cell or supporting cell in nervous tissue the oligodendrocytes are specifically found in the central nervous system where they function to produce the myelin sheath the myelin sheath is important for increasing the speed of action potentials that are electrochemical signals that rapidly travel down the axon because the myelin sheath insulates the axon it can speed up the conduction that is how fast an action potential travels from the axon hillock where it attaches to the cell body to the axon terminal where it forms a synapse with another cell not all neurons are a multipolar neurons the other major types of neurons found in the human body are called bipolar neurons or unipolar neurons and a true unipolar neuron is not found in in the human body instead what we have is called a pseudo unipolar neuron and so while the multipolar neurons are the most numerous the most common type of neuron and most of the neurons in the central nervous system are multipolar as well as most of the motor neurons that extend their axons out through the peripheral nervous system there are some bipolar neurons that are important for carrying sensory information in and these are found in the eye in the retina of the eye is one place you can find bipolar neurons and you can see the shape of a bipolar neuron is a neuron that has only two processes extending out from the cell body one of those processes will be attached to dendrites that carry information in and the other process that extends out would be the axon that carries the information out to another synapse to relay information to another neuron and so one example of a bipolar neuron is the bipolar cells of the retina in the eye that relay information from the photoreceptors in the retina to another type of neuron in the retina called the retinal ganglion cell then there are also bipolar neurons found in the inner ear within a region of the inner ear called the spiral ganglion and these bipolar cells are relaying information important for the sense of hearing there are also bipolar neurons found in the olfactory epithelium that is within the superior nasal cavity these bipolar neurons are important for detecting chemicals that are dissolved in our mucus of the superior nasal cavity that information is what we use in order to detect the sense of smell and so the olfactory neurons the olfactory receptor cells are bipolar neurons and the the dendrites of those neurons extend down into the nasal cavity whereas the axon will extend up into the central nervous system to form a synapse with neurons inside of the olfactory bulb on the inferior surface of the brain now the pseudo unipolar neuron has its cell body inside of the dorsal root ganglion the most common pseudo unipolar neurons are the sensory neurons that have their cell bodies in the dorsal root ganglion that are carrying sensory information in through the afferent fibers either the somatic afferent fibers as well as the visceral afferent fibers travel into the dorsal root ganglion and then their axons will continue through the dorsal root into the spinal cord where they can synapse with other neurons inside of the posterior gray horn or extend up through the ascending pathways the ascending tracks of the white matter in the spinal cord in the white columns like the posterior white column so the pseudo unipolar neuron is found in the dorsal root ganglion and it's a sensory neuron it's the most common sensory neuron has this structure a pseudo unipolar and the structure you can see only one process is extending out of the cell body of a pseudo unipolar or unipolar neuron and that that one process would extend out in a true unipolar neuron and in a pseudo unipolar one process will extend out and then branch in two large branches and that entire structure that we see attached to the cell body of the pseudo unipolar neuron is an axon here we see an illustration showing on the top two examples of pseudo unipolar neurons that are sensory neurons that have their cell bodies found in the peripheral nervous system within ganglia the dorsal root ganglion located very close to the spinal cord but just outside of the spinal cord these neurons have dendrites that directly attached to the axon then the axon travels as an afferent fiber in a spinal nerve all the way to the cell body and then will continue past the cell body in the dorsal root in order to enter the spinal cord on the top we see the dendrites are free nerve endings an example of a receptor like a thermal receptor that detects temperature or the pain receptors that can detect chemical stimuli released from damaged tissues these types of receptors are the free nerve endings or the dendrites of a pseudo unipolar neuron a sensory neuron there are other types of sensory receptors in which a special ending is surrounding the dendrites forming a capsule so an encapsulated nerve ending is present in some of our touch receptors an example would be the pacinian corpuscle found deep in the dermis and hypodermis that detects deep pressure and vibration the pacinian corpuscle has an encapsulated nerve ending surrounding dendrites and then information travels in still through an afferent fiber that then connects to the dorsal root in order to travel into the spinal cord so those are examples of pseudo unipolar neurons that have their cell bodies found in the dorsal root ganglion and are carrying sensory information in just below that we see an example of a specialized receptor cell called a photoreceptor a rod a specific type of photoreceptor cell found in the retina of the eye and so the rod detects light and when it detects light it can then send the signal onto a bipolar cell a neuron that has two extensions coming out of its cell body here we see an illustration showing a little more detail of the structure of the retina inside of the eye where the photoreceptor cells called the rods and cones are detecting light and then sending the information onto bipolar cells the dendrites of bipolar cells receive the information coming from the photoreceptor cells and all come together to form one process attached to the cell body then on the opposite side of the cell body the axon extends out to relay information from the bipolar cell to another neuron called a retinal ganglion cell glial cells are the other major type of cell found in nervous tissue so the neurons are the cells that are responsible for carrying information and performing the calculations inside of the nervous system glial cells support the functions of neurons the astrocytes found in the central nervous system are important for regulating the chemical environment surrounding neurons they can do this by regulating the transport of chemicals from the blood into the central nervous system and they can also perform chemical reactions to break down neurotransmitters and other waste products that come from neurons oligodendrocytes are the glial cells that create the myelin sheath surrounding axons in the central nervous system in order to insulate the axons and increase the speed that an action potential travels down the axon microglial cells are related to the leukocytes of the immune system while leukocytes are not able to cross into the central nervous system the microglial cells replace the functions of leukocytes by performing phagocytosis microglial cells can clean up cellular debris as well as pathogens like bacteria that find their way into the central nervous system epindymal cells are found lining spaces inside of the central nervous system and epindymal cells are important for moving fluid through those spaces the fluid inside the spaces of the central nervous system are called the fluid it's called cerebrospinal fluid and it fills spaces called ventricles inside of the brain and a central canal through the spinal cord epindymal cells have cilia that can beat back and forth in order to help move cerebrospinal fluid epindymal cells are also found lining capillaries inside of the ventricles of the brain and they help to regulate the movement of chemicals from the blood to form cerebrospinal fluid there are also glial cells in the peripheral nervous system here we see satellite cells surrounding the cell body of a pseudo-unipolar neuron inside of a ganglion like the dorsal root ganglion so the satellite cells have functions similar to astrocytes in the peripheral nervous system the satellite cells regulate the chemical environment surrounding the cell bodies of neurons inside of ganglia then Schwann cells are found in nerves where they create a protective layer surrounding fibers inside of the nerve and these Schwann cells are responsible for creating the myelin sheath on axons that are myelinated in nerves although not all of the axons will be myelinated there will be Schwann cells surrounding axons even if they are not myelinated and so what is the difference between being myelinated and just being supported and protected by a Schwann cell well myelin involves the Schwann cell or oligodendrocyte wrapping its plasma membrane around the axon many times so here we can see the process of myelination where a glial cell surrounds an axon and at first just having that glial cell surrounding the axon provides some protection to the axon and an unmyelinated axon within a nerve can be surrounded by a Schwann cell in that fashion however in order to increase the speed of the action potential the glial cell can wrap its plasma membrane around the axon many times squeezing all the cytoplasm out of the inner region and this is what we call myelin the inner sheath formed by a glial cell having wrapped the plasma membrane around the axon many times leaving only the numerous layers of plasma membrane and the plasma membrane is formed from phospholipids and so this phospholipid layer creates an insulating barrier surrounding the axon that helps speed up the movement of the action potential the increases the conduction velocity of the action potential this diagram summarizes the functions of the peripheral and central nervous system working together to coordinate a simple task so here we have the sensory receptors found in the skin thermal receptors that are detecting the temperature of the water in the shower as you put your hand into the shower and detect that that water is very hot the sensory receptors respond by sending information in through an afferent fiber that afferent fiber travels through a nerve in the peripheral nervous system and the cell body of this sensory neuron is found in the dorsal root ganglia then the afferent fiber continues in through the dorsal root into the spinal cord and then travels up the spinal cord through the posterior white column through tracks in the central nervous system and there can be several different neurons in the central nervous system that relay information up and perform the processing function of the control center to detect how intense is the stimulus is it very hot or is it just warm and then calculate the appropriate response and so the brain will interpret the sensory information and if it interprets that the water is too hot then commands will be sent out through a descending pathway where motor neuron found in the brain in the cerebral cortex of the brain extends its axon down through tracks in the brain that then connect to tracks in the spinal cord and these tracks would be on the anterior in the anterior white columns and then traveling down the anterior white columns to form a synapse with a motor neuron a lower motor neuron that has its cell body in the anterior gray horn then that neuron will send the signal out with its axon traveling through the ventral root into the spinal nerve then out to excite the skeletal muscles that are needed in order to contract and withdraw your hand from the water and so this process can occur very rapidly and it needs to be very rapid in order for us to withdraw our hand for its severely burnt from the hot water the way that information travels rapidly through the nervous system is in what we call action potentials so an action potential is a brief reversal of the membrane potential of an axon this is an electrochemical signal that travels down the axon now we're going to zoom in and focus on the electrochemical basis of the action potential so to start off with we'll have to discuss the trans membrane proteins that allow chemicals to move across the plasma membrane the chemicals that we're going to be focused on are chemicals that have an electrical charge called ions ions that have a negative charge are called anions but we're going to focus on sodium and potassium for understanding the action potential sodium and potassium both have a positive charge so they are called cat ions there is a transmembrane protein called the sodium potassium pump that moves sodium out of the cell and potassium inside the cell uses ATP as an energy source in order to do this because sodium is being concentrated outside the cell and potassium is being concentrated inside the cell they are both moving against the force of diffusion from their concentration gradients now there are also ion channels that allow sodium and potassium to move across the plasma membrane and when sodium moves into the cell it's bringing a positive charge into the cell with it when potassium moves out of the cell it's moving a positive charge out of the cell the membrane potential is the separation of charges across the plasma membrane of a cell the forces that govern the movement of an ion are both the force of diffusion from the concentration gradient as well as an electrical force if the electrical force is opposite to the concentration gradient then it's possible for the electrical force and the concentration gradient force to balance out the membrane potential at which the electrical force balances with the force of diffusion is called the equilibrium potential for an ion for sodium because there's a high concentration of sodium outside of the cell and diffusion forces sodium into the cell the equilibrium potential is a positive charge that the value is around positive 60 millivolts the equilibrium potential for potassium is a negative value because there's a high concentration of potassium inside the cell the cell has to have a negative charge in order to oppose the force of diffusion and have the balance of the equilibrium potential now the membrane potential of the cell at any moment results from a combined effect of all of the equilibrium potentials of all of the ions that are inside and outside of the cell however sodium and potassium have the largest influence on the membrane potential and the resting membrane potential when a neuron is not processing information is not firing an action potential the resting membrane potential is approximately negative 70 millivolts and so negative 70 is much closer to the equilibrium potential of potassium than it is to the equilibrium potential of sodium because at rest there's a greater permeability of the membrane to potassium than to sodium the resting membrane potential is the electrochemical potential across the plasma membrane when the neuron is inactive at rest the inside of the cell is more negatively charged than the outside of the cell this charge can be measured with electrodes and millivolts are the unit most commonly used to quantify the membrane potential the membrane potential is the charge within the cell relative to the charge outside this potential can change as the neuron receives and transmits information the membrane potential at rest or the resting membrane potential is around negative 70 millivolts resulting from a higher concentration of negatively charged chemicals called anions inside of the cytosol relative to the concentration of charged chemicals outside of the cell and so there's a higher concentration of negative charges in the cell than there are outside of the cell or there's less positive charges inside the cell than there are outside of the cell creating a separation of charge or membrane potential at rest that is negative 70 millivolts. If we place an electrode inside the cell and another electrode outside the cell to compare we can measure this voltage and as you can see in the illustration here with measuring the membrane potential at rest the value is approximately negative 70 millivolts. Now there are channels that can be regulated by chemical signals in order to open and change the membrane potential. Ligand gated channels are ion channels that can be opened by a chemical stimulus like a neurotransmitter. The example shown here is a nicotinic acetylcholine receptor this is the type of receptor that detects the neurotransmitter acetylcholine which is released from motor neurons to excite skeletal muscles at the neuromuscular junction. In the resting state when acetylcholine is not bound the channel is closed however when acetylcholine binds to the receptor the channel opens increasing the permeability of the membrane to sodium ions and sodium will rush into the cell bringing positive charges into the cell. As positive charges come into the cell the membrane potential will become less negative what we call depolarizing or depolarization of the membrane potential. In contrast if we were to have an ion channel open that would allow potassium to move out of the cell then that would have an effect of making the membrane potential more negative and making the membrane more negative is called hyperpolarization. Another type of ion channel is called a voltage gated ion channel which is open in response to a change in the membrane potential. When the membrane potential becomes depolarized this can lead to opening of a voltage gated channel and that voltage gated channel can then cause further depolarization of the membrane potential and this is how the action potential works. When voltage gated sodium channels open sodium will be allowed to rush into the cell. That sodium rushes into the cell depolarizing the plasma membrane potential which opens more voltage gated sodium channels causing more sodium to rush in and this process will continue triggering more and more voltage gated sodium channels along the axon so that the action potential will spread down the axon. There are also voltage gated potassium channels that are triggered by depolarization. They're triggered a little bit later than the voltage gated sodium channels and so the threshold for voltage gated sodium channels is around negative 60 millivolts whereas it takes a positive membrane potential around positive 30 millivolts to open the voltage gated potassium channels. When a voltage gated potassium channel opens then potassium will rush out of the cell and that will cause the membrane potential to become more negative. An action potential begins with the opening of voltage gated sodium channels producing depolarization. This occurs until the voltage gated sodium channels close. When they're triggered they open for about a millisecond and then they close and they open long enough to reach the threshold to open voltage gated potassium channels. When the voltage gated potassium channels open potassium rushes out of the cell causing repolarization back to the resting membrane potential of negative 70 and then further hyperpolarization making the membrane potential even more negative than when at rest. Eventually those voltage gated potassium channels close and the membrane potential will return to the resting membrane potential around negative 70 millivolts. When sodium channels like the nicotinic acetylcholine receptor open increasing the permeability of the membrane to sodium the membrane potential changes from the resting membrane potential becomes closer to the equilibrium potential for sodium. In other words when the sodium channel opens sodium rushes into the cytoplasm causing the membrane potential to become less negative. This change in membrane potential is called depolarization and when there's a change in the membrane potential caused by a neurotransmitter at a synapse we call it a postsynaptic potential because depolarization brings the membrane potential closer to threshold for voltage gated sodium channels to open causing an action potential. We call this type of potential an excitatory postsynaptic potential or EPSP. One EPSP may not be enough to bring the membrane potential to threshold but multiple EPSPs can add together in a process called summation. Some neurotransmitters open ion channels that cause the membrane to become more negative producing potentials known as inhibitory postsynaptic potentials. Both EPSPs and IPSPs are also known as graded potentials because they get weaker as they spread out over time and space. Multiple graded potentials overlap in summation and if there are enough EPSPs overlapping to reach threshold for voltage gated sodium channels to open then an action potential is triggered and in contrast to a graded potential that becomes weaker over time and space an action potential is an all or nothing response. The action potential doesn't get weaker it's always the same size there's not large or small action potentials all action potentials are the same size there. We'll have a depolarization from threshold of negative 55 or negative 60 all the way up to a positive membrane potential around 30 or 40 millivolts and then repolarize back hyperpolarization until the potassium channels close and we return to resting membrane potential. These action potentials are initiated at the axon hillock because the voltage gated channels are concentrated on the axon. If voltage gated sodium channels are triggered at the axon hillock an action potential will spread from the axon hillock down to the axon terminal. At the axon terminals the action potential triggers the release of neurotransmitters at the synapse. The synapse is a specialized structure where the axon terminal of a neuron releases neurotransmitter for communication with another cell. The presynaptic neuron is the neuron that's releasing neurotransmitters from its axon terminals. The synaptic cleft is the small space in between the axon terminal and the cell that is receiving the neurotransmitter known as the postsynaptic cell. Neurons can form synapses with other neurons so the postsynaptic cell could be a postsynaptic neuron. There are other types of synapses for example a neuron can form a synapse with a muscle cell as we call a neuromuscular junction or a synapse can form between a neuron and a gland forming a neuro glandular junction.