hello everybody my name is Iman welcome back to my YouTube channel today we're covering chapter 4 for MCAT biology and this chapter is going to cover the nervous system now it's through the nervous system that organisms sense pain temperature and all aspects of their environment it also serves to coordinate sensory information and respond to stimuli specifically the nervous system is really responsible for the control of muscular movement neuromuscular reflexes and glandular secretions and in addition to all that the nervous system is also responsible for higher level thinking and mental function now despite all of its complex functions the nervous system operates through basic electrical and chemical signals so what are we exactly going to cover in this chapter we're going to explore the following three main objectives first we cover the cells of the nervous system here we're going to discuss neurons and other cells in the nervous system like glial cells second we're gonna cover transmission of neural impulses this objective is dense but super important since we discuss action potential and the synapse last we're gonna have an overview on the organization of the human nervous system this is going to include covering the Central and peripheral nervous systems talking about the autonomic nervous system and finally reflexes hopefully by the end of the chapter we'll have we'll we'll have covered everything we need to know for the MCAT biology nervous system section all right so let's go ahead and get started with the first objective cells of the nervous system now understanding the nervous system is really centered around understanding neurons because they are the cells responsible for transmitting electrical and chemical signals throughout the body these signals are really crucial for coordinating sensory information for responding to stimulus and controlling muscular movements as well as other functions so what are neurons neurons are specialized cells capable of transmitting electrical impulses and then translating those electrical impulses to chemical signals each neuron has a shape that matches its function dictated by other cells which that neuron interacts with there are a variety variety of different neurons in the body but they all share some specific features that are noted here in this image we're going to cover each of these features Define what they do and their roles now like other cells besides of course mature red blood cells neurons have a nucleus this nucleus is located in the cell body also called the Soma all right the Soma is also the location of the endoplasmic reticulum and the ribosomes now the cell also has many appendages emanating directly from the Soma these tree barge Branch looking things are called dendrites and they receive incoming messages from other cells the information information received from the dendrites is then transmitted through the cell body all right until it reaches the axon hillocks all right which integrates that incoming signal the axon hillox plays an important role in action potentials or the transmission of electrical impulses down the Axon all right signals arriving from the dendrites all right can be either excitatory or inhibitory and the axon hillocks will sum these signals and if the result is excitatory enough it's going to initiate an action potential and we're going to discuss what an action potential actually um means in the next objective but for now all right if the result is excitatory enough it's going to initiate an action potential now this axon is a long appendage that terminates in close productivity to a Target structure whether that be a muscle a gland or another neuron now most mammalian nerve fibers are going to be insulated by myelin to prevent signal loss or crossing of signals so just like the insulate just like insulation prevents wires that are next to each other from accidentally dis urging each other the myelin sheath maintains the electric signal within one neuron in addition it increases the speed of conduction in the axon myelin is produced by oligodendrocytes in the central nervous system and Shawan cells in the peripheral nervous system now at certain levels along the axon there are going to be small breaks in the myelin sheath with exposed areas of axon member these are called nodes of ranvir all right nodes of rain we are critical for Rapid signal conduction and then finally at the end of the axon is the axon terminal this structure is enlarged and flat and to maximize neurotransmission to the next neuron and it ensures proper release of neurotransmitter transmitters which are the chemicals that will transmit information between neurons now although neurons kind of look a little complicated their design is actually quite signal as simple and the neuron is broken down into two major regions we have the region for receiving and processing information and we have a region for conducting and transmitting information to other cells the type of information that's received processed and transmitted by a neuron really depends on its location in the nervous system for example neurons located in the occipital lobe are going to process visual information while neurons in the motor pathways will obviously process and transmit information that controls the movement of muscles however regardless of the type of information all neurons have the same basic anatomical structure let's repeat some of those important structures remember dendrites receive signals from neighboring neurons like a radio antenna axon transmits signals over a long distance like a telephone wire axon terminal transmits signals to other neuron dendrites or tissues like a radio transmitter and that myelin sheath that speeds up the signal transmission along the Axon remember axons carry neural signals away from the Soma dendrites carry signals toward the Soma now important to know but neurons they're not physically connected to each other between neurons there is a small space into which a terminal the in there is a small space into which the terminal portion of the axon releases neurotransmitters which are going to bind to the dendrites of the postsynaptic neuron this space is known as the synaptic cleft together the nerve terminal the synaptic cleft and the postsynaptic membrane are known as the synapse neurotransmitters released from the axon terminal are going to trans Traverse the synaptic cleft and bind to receptors on the postsynaptic neuron now you can have multiple neurons that are bonded together bundled together I should say and they form a nerve in the peripheral nervous system these nerves they can be sensory motor or mixed which refers to the types of information they carry and mixer mixed nerves are going to carry both sensory and motor information all right the cell bodies of neurons of the same type can be clustered together into an a ganglia now in the center nervous system axons or axons may be bundled together to form tracts unlike nerves tracts only carry one type of information the cell bodies of neurons in the same tract are grouped into nuclei with that we've learned about neurons but neurons are not the only cells in the nervous system neurons can be supported by other cells these cells are often called glial cells we don't need to know too much about glial cells for the MCAT all right the only thing we need to know are the a couple of types of glial cells and the descriptions and their descriptions so we're going to cover those all right to the extent that we may be tested on on the MCAT nothing more all right first kind of glial cell are astrocytes they nourish neurons and they form the blood-brain barrier which controls the transmission of solutes from the bloodstream into the nervous tissue all right that's astrocytes then we have epidemal cells they align the ventricles of the brain and they produce cerebrospinal fluid which physically supports the brain and serves as a shock absorber then we have microglia these are phagocyte site cytic cells that ingest and break down waste products and pathogens in the central nervous system and last we have oligodendrocytes and shaoin cells they produce myelin around axons and with that we can actually move into our second objective which will cover the transmission of neural impulses we want to start by discussing the action potential all right the action potential is an All or Nothing message used by neurons to relay electrical impulses down the axon to the synoptic to the terminal Axon it's important because it's going to allow for the rapid and efficient transmission of signals throughout the nervous system it's going to be initiated at the axon hillocks where incoming cells from the dendrites are going to be integrated and of course if the sum of those signals is excitatory enough it's going to initiate an action potential the action potential travels down the axon towards the terminal axon where it triggers the release of neurotransmitters to be transmitted to the next neuron all right now we want to get into more details so we're going to start at the very beginning all neurons exhibit a resting membrane potential this means that there is an electrical potential difference or voltage between the inside of the neuron and the extracellular space usually this is about minus 70 millivolts with the inside of the neuron being negative relative to the outside neurons use selective permeability to ions and the sodium potassium atpase to maintain this negative internal environment all right and we can see that in this figure here all right let's go over this all right the sodium potassium atpase it maintains a resting membrane potential of minus 70 millivolts by moving three sodium ions out of the cell for every two potassium ions it moves into the cell all right let's say this again the sodium potassium atpase is important for restoring this gradient after Action potentials have been fired it transports three sodium ions out of the cell for every two potassium ions into the cell of course at the expense of ATP ATP is necessary because both sodium and potassium are moved against their gradients by this process and this qualifies as primary active transport each time the pump works it results in the inside of the cell becoming relatively more negative since only two positive charges are moved in for every three that's moved out and once again all right this sodium potassium atpase helps maintain negative inter the negative internal environment um and it helps restore this gradient after Action potentials have been fired we're going to discuss this even further here shortly all right but I want to restate and remind you right we made this we made this statement earlier neurons can receive both excitatory and inhibitory input all right excitatory input causes depolarization okay that means it raises the membrane potential from its resting potential and makes the neuron more likely to fire an action potential inhibitory input causes hyperpolarization all right which is the lowering of the membrane potential from resting potential and it makes the neuron less likely to fire an action potential if the axon hillox receives enough excitatory input to be depolarized to the threshold value which is usually in the range of minus 55 to minus 40 millivolts and action potential will be triggered okay what does this imply this implies something very important to hear that not every stimulus necessarily generates a respond a small excitatory signal may not be sufficient to bring the axon hillocks to that threshold all right in addition a post-synaptic neuron may receive information from several different presynaptic neurons some of which are excitatory some of which are inhibitory and the additive effects of these multiple signals is known as a summation and it is that additive effect of multiple signals that we're going to look at to determine whether a action potential will be triggered all right but there are two types of summations actually we have temporal and spatial in temporal summation multiple signals are integrated during a relatively short period of time whereas in spatial summation the additive effects are based on the number and location of those incoming cells now let's go back to action potentials now all right let's talk about the main events that take place during an action potential all right so let's say a triggering event occurs that depolarizes the cell body all right and the signal comes from other cells connecting to the neuron and it causes positively charged ions to flow into the cell body what is going to happen and how can we visualize this in a membrane potential versus time figure so what I'm going to do is I'm going to move to these two figures here and we're going to really talk about um Action potentials using these figures all right so we know we talked about our resting potential being around minus 70 millivolts all right so that's just one our cell is is resting all right this is our resting membrane potential all right what happens when a triggering event happens all right so first thing that's going to happen is depolarization all right and depolarization is going to continue to happen until we initiate an action potential all right so we're going to discuss what that means what these two steps mean all right we're going to see this effect right here on our graph all right and let's talk about it so a triggering event like a signal from other cells is going to happen and it's going to cause positively charged ions to flow into the cell body all right these ions are going to pass through channels that are open all right with when a specific chemical like a neurotransmitter binds to that channel this influx of positive ions is going to bring the cell membranes potential closer to zero which is known as depolarization so what's going to happen is we're going to start pumping in positive ions into the cell membrane thereby making this um membrane potential more and more positive all right so that's the beginning steps of depolarization we're pumping positive ions into the cell they're coming in through open channels now if the cell body becomes positive enough it's going to trigger voltage-gated sodium channels found in the axon to also open allowing more positively charged anions to continue to be pumped into the cell all right so these channels open allowing positively charged sodium ions to flow into the negatively charged accent what does this do this depolarizes the surrounding axon and sets off a chain reaction of sodium channels opening down the axon all right this is still depolarization all right but now the neuron actually is going to keep getting more and more positively charged all right and as it becomes positively charged your action potential is going to pass through all right so this effect of positively charged ions moving into the cell is called depolarization and that means our resting potential is going to become more and more positive all right and our action and that's going to initiate an action potential all right so action potential initiated after depolarization and the action potential has been initiated we're going to have repolarization all right repolarization so after depolarization the inactivation Gates of the sodium channels are going to close that's going to stop any inward Rush of positive ions at the same time potassium channels are going to open and there and since there's more potassium inside the cell potassium ions are going to begin to exit the neuron all right so we're going to lose positive charges inside our cell and this loss of positively charged ions returns this cell back towards its resting state slowly and this is known as repolarization all right so this part is called repolarization all right so first we had depolarization we had sodium influx into the cell until we met our until the action potential was initiated and then we had a potassium efflux so potassium positive ions started leaving the the the cell all right and so we can slowly start to return to our resting potential okay but before we truly reach our resting potential and stay there hyperpolarization is going to happen all right hyperpolarization okay so as the action potential passes through these potassium channels they stay open for a little longer continuing to let positive ions exit the neuron this temporary exit of positive ions actually will cause that cell to hyper polarize making it more negative then it's resting state all right so in hyperpolarization we just have the exit of positive ions from the cell so much so that it becomes more negative than its resting state but last and not least we're going to restore the resting state restore the resting state how do we do this what does this all right that's going to be our sodium potassium atpase pump all right so the potassium channels will eventually close and then the sodium potassium pump is going to come into action this pump actively transports sodium ions out of the cell potassium ions back into the cell using energy from ATP and it's going to work to reestablish the resting membrane potential and ion concentrations all right and that's going to prepare the neuron for whatever the next action potential happens all right this happens all right every time all right an action a triggering event causes the the the depolarization of the cell body all right every time a signal coming from another neuron um triggers and initiates an action potential all right so we had depolarization that initiates an action potential then repolarization and then hyperpolarization that makes the the cell a little more negative than its resting potential but then our sodium potassium pump comes into play to restore the resting the the resting state all right now to this point it's important to understand a few things all right action potentials are an all or none um process meaning that they are either triggered or not triggered like flipping a switch the maximum frequency at which a neuron can send action potentials is going to be determined by its refractory period all right the absolute refractory period is a time when it's impossible to send another action potential here your sodium channels are locked shut and that's going to prevent depolarization and subsequent Action potentials right because those are two important steps too to start the process the relative refractory period is a time when it's difficult to send an action potential but not impossible the style is still hyper polarized requiring a larger than usual trigger to reach its depolarization threshold um so it becomes difficult to send an action potential these refractory periods really help direct the action potential down the axon and indicate the intensity of a stimulus in addition these refractory periods also allow the neuron to replenish its supply of neurotransmitters at the axon terminal ensuring it can continue transmitting messages now this helps us move into discussing this the synapse right we previously said right neurons are not actually in direct physical contact there is a small space between neurons called the synaptic cleft where um neurotransmitters are secreted now the root neurons the neuron that comes before um the the synaptic cleft is called the presynaptic neuron while the neuron after the synaptic cleft is called the postsynaptic neuron if the postsynaptic cell is a gland or muscle it's termed an effector now in most cases synapse synapses work through chemical transmission this involves the use of a small molecule called neurotransmitters to relay messages from one cell to the next it is critical to understand the difference between electrical and chemical transmission here all right so within a neuron electricity is used to pass this signal down the length of the axon all right that's elect that's electrical all right between neurons all right between neurons chemical foreign ERS are used to pass signals to the subsequent neuron so this is chemical all right it's really important to understand the difference between electrical and chemical transmission within the neuron electrical transmission between neurons chemical transmission now once the neurotransmitters are released into the synapse neurotransmitter molecules will diffuse across the synaptic cleft and they're going to bind to receptors on the postsynaptic membrane this binding process is going to trigger a response in the postsynaptic neuron allowing the signal to be transmitted further now to ensure proper regulation neurotransmission requires control all right constant signaling to the postsynaptic cell is typically under undesirable so neurotransmitters must be removed from the synaptic cleft and must be regulated sometimes right three main mechanisms accomplish this goal all right firstly all right neurotransmitters can be broken down through enzymatic reactions effectively deactivating them that's one type of Regulation second uptake uh re-uptake carrier transport neurotransmitters back into the synap presynaptic neuron all right so again re-uptake carriers can transport those neurotransmitters back into the presynaptic neuron that's another type of Regulation and finally certain neurotransmitters May simply diffuse out of the synaptic cleft right and so that's the third kind of way for neurotransmitter regulation to occur these mechanisms of neurotransmitter removal they play a crucial role in maintaining the precise and controlled nature of neural communication they ensure that signals are properly transmitted preventing over stimulation or constant activation of the postsynaptic cell all right with that we move into our final objective organization of the human nervous system now the nervous system is a remarkable collection of cells that govern both involuntary and voluntary Behavior while also maintaining homeostasis now the function of the nervous system includes many things all right they can include sensation and perception motor function cognition executive function and planning language comprehension memory emotional expression balance regulation of endocrine organs and the regulation of things like heartbeat and breathing rate Etc all right the human nervous system is really a complex web of over 100 billion cells that communicate coordinate and regulate signals from the rest of the body mental and physical action occurs when the body can react to external stimuli using the nervous system and so in this section we're going to look at the nervous system and it's a basic organization and the first thing we want to talk about all right are three kinds of nerve cells in the nervous system these are sensory neurons motor neurons and interneurons Sensory neurons also known as afferent neurons they transmit motor information from the brain and the spinal cord to sorry Sensory neurons afferent neurons they transmit sensory information from receptors to the spinal cord and brain I misspoke I apologize all right let me repeat that sensory neurons transmit sensory information from receptors to the spinal cord and brain all right then motor neurons known as efferent neurons they transmit motor information from the brain and spinal cord to the muscles and glands and then interneurons are found between other neurons they are the most numerous of these three types they're located predominantly in their brain and spinal cord and they're often linked to reflexive Behavior awesome now with that let's turn to the overall structure of the human nervous system which is diagrammed very nicely here the nervous system can be broadly divided into two primary components the central nervous system and the peripheral nervous system all right the central nervous system is composed of the brain and the spinal cord so on the brain the brain consists of both white and gray matter the white matter consists of axons encased in myelin sheaths the gray matter consists of unmyelinated cell bodies and dendrites in the brain the white matter lies deeper than than the gray matter and at the base of the brain is the brain stem which is largely responsible for the basic functions like breathing all right so the brain receives and processes sensory information initiates responses stores memory and generates thoughts and emotions then we have the spinal cord which is also still part of the central nervous system it conducts signals to and from the brain and it controls reflexive activities uh the spinal cord extends downward from the brain stem and it can be divided into four divisions cervical thoracic Lumbar and sacral almost all of the structures below the neck receive sensory and motor intervention from the spinal cord the spinal cord is also protected by the vertebral uh vertebral column which transmits nerves at the space between adjacent vertebrae now like the brain the spinal cord also consists of both white and gray matter and the white matter lies on the outside of the cord and the gray matter is deep within so that's the central nervous system then we have the peripheral nervous system in contrast it's made out of nerve tissue and fibers outside of the brain and spinal cord the peripheral nervous system connects the central nervous system to the rest of the body and itself can be subdivided into two categories the somatic nervous system and the autonomic nervous system uh so like we said the peripheral nervous system has and includes both motor and sensory neurons within the motor neurons of the peripheral central system um we have somatic nervous system and the autonomic nervous system now the somatic nervous system consists of sensory and motor neurons distributed throughout the skin joints and muscles Sensory neurons transmit information through afferent fibers motor impulses and contrast travel through efferent fibers all right and then we have the autonomic nervous system that's generally regulates heartbeat respiration digestion and glandular secretions in other words the autonomic nervous system manages the involuntary muscles associated with many internal organs and glands it also helps regulate body temperature by activating sweat and sweating or Pilot erection depending on whether we are too hot or too cold and the main thing to understand about these functions is that they're very automatic all right hence why it's called the autonomic nervous system those two words are very coolest to each other now the autonomic nervous system has two subdivisions that's the uh sympathetic and the parasympathetic divisions all right these two branches often act in opposition to one another meaning they're kind of antagonistic for example the sympathetic nervous system acts to accelerate heart rate and inhibit digestion while the parasympathetic nervous system in contrast decelerates heart rate and increases digestion the main role of the parasympathetic parasympathetic nervous system is to conserve energy It's associated with sleep and rest um and so usually the the catchphrase that helps you remember the parasympathetic is rest or digest in contrast the sympathetic nervous system is activated by stress okay this includes anything from mild stress to emergencies okay the sympathetic nervous system is closely associated with rage and fear reactions and the catchphrase to remember the sympathetic division is fight or flight all right it increases heart rate it redistributes blood to muscles of locomotion it increases blood glucose dilates the eyes releases epinephrine Etc all right so remember sympathetic fight or flight parasympathetic rest and digest and we can see here all the functions that are taken care of by the parasympathetic and the sympathetic nervous systems all right constriction slow heart beating stimulates bile release blood vessels are constricted digestive system is activated uterus is relaxed urinary system increases the urinary um output that's parasympathetic and for sympathetic your pupils dilate your heart rate increases your Airways dilate your sweat glands are stimulated your digestive system decreases activity your adrenal glands stimulate the production of adrenaline all right so on and so forth so that is parasympathetic compared to sympathetic fantastic the last and final thing we want to cover here in this chapter are reflexes all right neural circuits called reflex arcs control reflex reflexive behavior for example consider what occurs when someone steps on a nail so The receptors in your foot will detect pain and the pain signal is transmitted by Sensory neurons up to the spinal cord at the at that point the sensory neurons connect with interneurons which can then relay pain impulses up to the brain but rather for the rather than wait for the brain to send out a signal interneurons in the spinal cord can also send signals to the muscles of both legs directly causing the individual obviously to withdraw the foot with with pain while supporting with the other foot now the original sensory information still makes its way up to the brain however by the time it arrives there the muscles have already responded to the pain thanks to the flux of Arc there are two types of reflexive arcs monosyn synaptic and polysynaptic in a monosynaptic reflex arc there is a single synapse between the sensory neuron that receives the stimulus and the motor neuron that responds to it in contrast polysynaptic reflex arc there is at least one interneuron between the sensory and motor neurons all right and with that we've covered everything we need to know for the nervous system all right let me know if you have any questions comments concerns down below other than that good luck happy studying and have a beautiful beautiful day future doctors