this is lecture five in lecture four we went over crossbridge cycling so how sarir shorten and talked about the length tension relationship in particular active tension uh which is about uh actin mice and crossbridge formation and we talked about a few not all right but a few explanations for Force development in this lecture will cover the nervous systems recruitment of skeletal muscle beginning in the brain in the motor cortex and ending at the neuromuscular Junction uh next lecture we'll jump that Junction and complete the contraction but today it's the motor cortices right cortexes uh what motor units are and how they behave and we'll get into action potentials how those work uh let's start with the nervous system in general uh this is your brain and nerves is a very complex Network that sends signals from the brain all over the body and vice first it it transmits messages from the periphery back to the brain for interpretation and we categorize the parts of the nervous system as either Central or peripheral central is your brain and spinal cord uh so your central nervous system lives inside of your bones inside of your skull inside of your vertebrae it's pretty well protected your peripheral nervous system is not so well protected it's outside of the brain and spine outside of the skull and spinal cord um vertebrae and it goes pretty much everywhere else uh I mean there's no like nerves in your hair but but pretty much everywhere else in your body uh neurons are are going and the purpose of the peripheral nervous system is to relay information back and forth between the central nervous system and your peripheral meat right your your limbs and organs and other tissues um within the peripheral branch there's the autonomic nervous system this is automatic control of stuff blood pressure heart rate urination uh fight ORF flight things and breathing digestion a bunch of feedback loops lots of regulatory things both sympathetic and parasympathetic that's the autonomic nervous system and then there's a somatic nervous system this is distinct from the autonomic system in that somatic isn't un conscious it's voluntary for the for the most part it's voluntary we'll talk about involuntary is things like reflex arcs uh those are in the somatic nervous system but this is the branch of the peripheral nervous system that's responsible for muscle contraction voluntary muscle contraction happens here and categorizing it as as peripheral doesn't mean the brain isn't involved impulses for voluntary contraction originate in in the motor cortex which is in the brain it's in the cerebral cortex both hemispheres right and left uh and it's a it's a strip that's sort of like where headphones would go not the earbuds but if you have headphones where the headband arches over your head that's where the primary motor cortex lives in your brain uh but there isn't just a primary motor cortex there are other cores involved in motor action but we'll start with the primary motor cortex it's the most important one in blue here it's at the back end of the frontal lobe um and there are neurons in this strip called bets cells betz that are the largest neurons in your brain uh up to 100 micrometers in diameter which is pretty Lush for human hair again a sarir is just over two microns and the average hair might be 50 or so and so when you're seeing this this diameter of a 100 micrometers in these B cells that's pretty huge um and again you don't need to know you know specifically B cells but but you do need to know upper motor neurons and lower motor neurons and Alpha and Gamma motor neurons but we'll get to those later for now it's upper motor neurons and lower and upper that's in your central nervous system and then the lower motor neurons those are in your peripheral nervous system now the interaction between upper motor neuron and lower motor neuron happens at the ventral horn or the anterior horn or vental root it's called a bunch of things you'll see um but that's it's just a front uh the upper and lower motor neurons they synapse at the front of your spine and then those lower motor uh neurons they leave the spine and head out toward a tissue this is why it's considered the peripheral nervous system uh and the the lower motor neurons are in the periphery right they're relaying uh the message from the CNS from an upper motor neuron out toward in this case an arm or a leg an AB right some sort of muscle this is muscle physiology we're going to talk about muscle uh Recruitment and there isn't a single neuron the ACT if it's a single muscle fiber motor control isn't that segregated each motor neuron controls a lot of fibers uh and when one neuron fires lots of fibers are called to action uh some of those actions are very isolated but there's teamwork in your neurons they're integrated in a way that you do like reach and grab as one coordinated neural activity and this is where some of those other motor areas can assist with planning and execution uh but the primary motor cortex is the most important slab of brain for muscle action you'll see maps uh of where particular body areas are controlled uh pictures like this are called the motor homunculus homunculus means little person in in Latin it's not a perfect representation though or even a good representation in part if someone experiences damage that causes paralysis or if there's some amputation you can see shifting of motor areas but beyond that it's way more scattered than these Maps indicate um it's almost like what people say about taste buds if you if you've heard this map described you know this is the you know part of your mouth where you taste sweet uh over here is sour and you bitter is at the back of the tongue and salty is at the tip or all whatever just all nonsense uh the homunculus map has a comparable amount of sense it's real um but it's a lot more scattered and sloppy than the diagram suggests and there's overlap in areas too getting into the premotor cortex this is in the frontal lobe just in front of the primary motor cortex uh it's a little bit bigger than it than it looks like in this map but the the neurons aren't as big there's no huge bet cells in here um lots of important roles though preparation of movement uh and responses to external stimula you know sensory guided uh motions so like if people have lesions here what really seems to be impaired is their planning of movement which happens before the motion so activity in the premotor cortex tends to precede activity in the primary motor cortex um the premotor cortex might fire you know a tenth of a second before the action Begins the supplementary motor area this green space here um also planning of movement there's overlap between these brain areas and the supplementary motor area has roles in motor learning uh in addition to that the posterior parietal complex this is in the parietal lobe uh top of the brain behind the frontal lobe uh it seems to help with sensory motor uh like visual motor transformation so like if I say catch and I and I throw something in your direction this area of the brain is going to be activated uh so those are the brain regions the most important one for you to know is the primary motor cortex just also know that it's supported by other other areas uh and and getting back to the nerves remember that there are upper motor neurons and lower motor neurons the upper neurons are in the um cortex and brain stem that's where those live uh but just think of them as as being in the primary motor cortex and then their axons um descend down the spine and they synapse with a lower motor neuron or an inch neuron but let's keep it simple and just say that upper motor neurons begin in your brain travel down the spine and synapse with a lower motor neuron um and then the upper motor neurons activate the lower neurons using glutamate that's the neurotransmitter they use uh to to depolarize to activate the uh lower motor neurons and those lower motor uh neurons those depolarized muscles with with acetyl choline not glutamate so upper to lower that's glutamate lower to muscle that's acetyl choline but we'll talk about that in the next lecture uh so Alpha motor nerves these are are voluntary lower motor nerves their cell bodies that are actually in the CNS the central nervous system but their axons inate extra fusil fibers meaning the fibers you can voluntarily contract we'll talk more about that but for now um extrafusal as opposed to intrafusal muscle fibers which are inovated by um gamma motor neurons and Gamma motor neurons have diameters of about five micrometers they're tiny just a couple of sarom in diameter tiny tiny nerves um but we'll talk more about extra and intrafusal fibers in a future lecture um but for now extrafusal is voluntary in intrafusal is involuntary and you have beta motor nerves too which inate both intrafusal and extrafusal but not many of these and for the sake of this class we'll just ignore them uh now the peripheral nervous system I is not just like receiving messages from the CNS it's transmitting messages too um so so eer or motor information exits from the ventral horn or ventral rout and aerid or sensory information enters the dorsal horn or dorsal route uh here you can see uh some motor neurons exiting the vental route and and extending toward muscle fibers uh before we talk about that how that interation Works um we have a couple of terms to Define first is a motor unit this is a motor neuron and every single muscle fiber that neuron innervates both of those that's a motor unit um one neuron might activate a thousand fibers uh you know each muscle fiber is interated uh by a single motor neuron not several but each motor neuron inates multiple muscle fibers and the muscle fibers that belong to one motor unit aren't often immediate neighbors uh as you can see in the diagram on the right the the fibers activated by a motor neuron aren't usually adjacent they're scattered throughout the muscle girth um and the second term is the all or none principle what this means is that if a motor neuron fires all the fibers in that motor unit will contract uh not just some of them but all of them and they'll contract as hard as they possibly can a huge stimulus does not make them contract harder right either 100% of the fibers in that motor unit contract maximally or 0% of them contract at all those are the only two options it's not like a light switch that has a dimmer where you can control the brightness um it's either on or it's off that's how motor units work uh now nerve excitement is an electrical event um is controlled by the movement of ions an ion is just an atom it could be multiple atoms uh that has a charg right positive or negative an annion is negatively charged a cation is positively charged and the manipulation of charges is how neurons work it's also how batteries work the manipulation of charges that's how batteries work um if you've ever put batteries into a remote or any other device you you notice that they have a plus sign and a minus sign and when you insert the batteries you have to orient them according to those positive and negative terminals um because inside the batteries there's a separation of charges and the batteries die when the negative charge is done moving neurons have separations of charges too inside the neuron and outside uh in the extracellular fluid there are differences in concentrations of ions sodium and potassium being the two that we're interested in now at the moment we'll get to some other ions later but uh sodium is really high outside the neuron and pottassium is really high inside uh and it takes active transport it takes ATP uh to move ions against their concentration gradients there's an ATP a enzyme here uh that's powering the The Exchange that's shoving the sodium outside and the potassium pulling it inside uh so that's how uh we can recharge our batteries our our you know neurons um now these are both positively charged sodium potassium are both cat I uh but they're stored in different concentrations and that uneven distribution results in a separation of charges uh so in other words there's a voltage difference inside and outside of the neuron and an action potential which is the firing of that neuron is the um inverting of that voltage uh the the inside becomes positively charged relative to the outside that's an action potential now this is the classic action potential diagram for a neuron resting membrane potential is shown to be7 molts uh which means a charge inside of the cell at rest is about 70 molts lower than the charge outside of the cell um if some sodium which is positively charged comes in and all other properties are held constant uh the negative charge of that cell will diminish a bit uh this is depolarization that's what it's called Uh returning to its resting potential is called repolarization and making it even more negative is called hyperpolarization this is all in reference to the cell being quote polarized uh meaning there's a difference in potential across the cell's membrane the voltage is lower inside so it's polarized if you depolarize you eliminate that repolarize you reestablish polarization hyperpolarize make it even more uh polarized now the values shown here um are about neurons but not really motor neurons these values aren't consistent across all neuron types um pretty much all animal cells uh regardless of whether they're excitable have a resting membrane potential it's not just neurons your red blood cells have a tiny negative charge 101 molts or so um smooth muscle cells are -50 to -60 molts um skeletal muscle cells much more negative 85 toga 95 uh molts um and some cell types resting potential is sort of a poor term because they're constantly changing in others you can get a pretty good uh measurement neurons um tend to be you know a bit more negative than smooth muscle you know 65 to70 on average somewhere around there but it's not consistent across all neurons um that said let's just use this diagram and so if you get a small excitement a small influx of sodium uh that influx if if it reaches 55 molts um 15 molt jump the voltage gated ion channels open and these channels are membrane proteins uh that are sensitive to charge gradients um and at at resting potential they're closed uh when they open sodium rushes in uh and and moving according to its concentration gradient right sodium is positively charged so that makes it positive inside the neuron and that electrical Spike that positivity that is your action potential uh but you have to reach that gate threshold for this to happen uh lots of tiny stimuli tiny excitements aren't sufficient to depolarize the nerve think of the nerve as like it's being whispered to if the whisper isn't loud enough it doesn't interpret the message uh but if the whisper gets louder and louder and louder eventually it'll hear it and it will make sense of that message and be inspired into action um so it's like an Audiology test a set of instructions but it's super quiet and then less quiet and then less quiet and then less quiet until you can finally hear it and once you can you leap into action you leap into action potential um uh but following that action potential there's a refractory period a refractory period in the same way that your hearing would be terrible briefly after a really loud noise um during that period if someone wants to get your attention they need to whisper really loud and so that's sort of how a refractory period uh works and this is what those voltage gated sodium channels look like kind of they're they're they're embedded in the cell membrane and when the channels open they're activated and then very quickly uh they switch to their inactivated shape and when they're inactivated they're unable to be reopened uh no matter how excitatory the stimulus they cannot do it they cannot be reopened um this absolute inability to reopen um is when the potassium channels open and potass potassium rushes out of the cell and that repolarize it turning to its negative charge uh and then the voltage gated sodium channels transition to their closed potentially active State uh there's a difference between the absolute refractory period and a relative refractory period the absolute refractory period is while the neuron is positively charged right the sodium channels will not reopen uh but then enough potassium leaks out and you can refire the nerve uh at that point it's just harder uh because too much pottassium has leaked out so it's even more negative than its resting potential um the excitatory stimulus to to reach that gate threshold it's no longer you know 15 or so Mill volts maybe now it's 30 or 35 molts you need that big of a jump um a bigger louder stimulus is required to activate them so that's the relative refractory period uh but eventually it recovers uh it gets back to its normal resting potential uh those sodium potassium pumps do their job and it gets back to that let's just call it negative 70 uh this is a look at the timing of action potential sodium rushes in quickly uh within a millisecond the neurons charge is positive um potassium conductance this isn't as quick I mean it's fast but it's not as fast um so the absolute refractory period that's going to be between one and two milliseconds um but the muscle contraction that follows that might last 15 to 100 milliseconds um so the contraction period is much longer than the refractory period if you fire a motor neuron every 3 to four milliseconds you're not going to see like a strobe light effect in motor control it's not going to be quivering at a high frequency you'll get a steady contraction uh because that effect lingers longer than the nerve impulse um as long as you're getting those impulses to the muscle steadily enough you'll contract smoothly uh it it's not going to relax between each stimulus now the heart behaves uh differently uh even if the relative refractory period in skeletal muscle let's say it lasts five milliseconds before it gets recruited again that's nothing compared to the heart uh which has it's about a quarter of a second or so the the refractory period um so once you have an action potential it gets sent to the Target tissue in this case skeletal muscle and it's transmitted through myelinated nerves uh this is called saltatory induction uh the signal leaps from node to node they're called nodes of ronier ran whatever um gaps between the milein sheaths right little patches where it's not insulated and this is where Action potentials can be generated uh the thickness of the myin sheath varies uh between nerves uh the large ger the diameter of the myin the faster the conduction the nerve conduction and this is something we'll talk about in future lectures uh both with eer uh and afferent signals for now though uh just know that some nerves send their impulses at the pace of walking and others go at the speed of Ferraris and the thickness of the milein sheath is what determines those differences in in uh velocity uh now schan cells are what form the myelin uh each Swan Schwan cell covers about a hundred or so micrometers of an axon um so a long nerve will have thousands of these uh and there are lots of conditions that are going to impair Schwan cells including like leprosy and and if these conditions will affect nerve transmission because myin is critical um to to conduction velocity and now recurring theme in all of these lectures is variance nerves vary in thickness uh both axon diameter and myin layers both how layered is the myin and how thick is the axon both of these things vary so here's a glimpse at some diameters right Alpha fibers on the left gamma fibers on the right uh nothing you need to memorize here but but if you look at the diameter in micrometers of an alpha fiber uh at the node of ronier uh you'll see an average of about three um in the peripheral nervous system and about 3.3 in the central nervous system compared to a gamma fiber which is just under half of that thickness at each of those sites so nerves vary uh without memorizing any numbers those differences in general will become important in future lectures uh in lecture four last lecture we talked about where your strength comes from and you know what explains it we're going to add a couple of additional explanations um so in the last lecture we said the number of muscle fibers you recruit partly determines Force output um but what determines the number of muscle fibers is the number of motor neurons that are excited U so that's sort of the same explanation uh although it would get a little bit more complicated in in upcoming lectures but another Factor uh that determines Force output is called rate coding and this is the frequency of achieving an action potential uh the duration of inter Spike intervals uh so if you increase the load on the muscle uh you'll increase the firing rate of the motor neurons um the number of action potentials in a second or a you know given duration uh so with a refractory period of 1 to two milliseconds the behavior of the muscle you has not sufficiently relaxed when the next stimulus comes in um you haven't put all the calcium away and Unbound the cross Bridges yet that takes time and the more rapid your action potentials are that results in more a a a stronger muscle contraction uh but uh there are are some more steps between the depolarization of the motor neuron the action potential and the motor neuron and the contraction of the muscle fiber itself there's a neuromuscular Junction where the nerve meets the muscle and that's what we'll talk about in the next lecture but for this one here are the questions that you should be able to answer if you review these and think about what we talked about you should be able to answer these and in lecture six uh we'll finish what's called excitation contraction coupling I will see you and then