now we're going to talk about reflexes reflexes are among the most simple control systems that we're going to consider and that's because a reflex is an involuntary and rapid response to a stimulus how do reflexes work we're going to consider two examples both of which employ the same type of control system and that is feed forward control so in a previous lecture i explained this flow chart and the effects on a state variable in this case temperature on the right hand side and what feed forward control systems lack is feedback okay so there actually is no feedback uh that is there there is an initial stimulus that sets off a feed-forward response an efferent response but then once it has been initiated the state variable doesn't modulate or feed back onto that efferent response so this algorithm is all about initiating something and then not modulating it after that the efferent output in this case it was a furnace is independent of the afferent input the initiation has to do with the state variable exceeding some threshold in order to initiate it in this case to turn a furnace on so the two examples that we're going to consider employ different kinds of efferent pathways so the first is a somatic pathway somatic pathways end with skeletal muscles and so we're talking about musculoskeletal reflexes musculoskeletal reflexes include a somatic efferent pathway and we're all familiar with the example that i'm going to use because when you go to the doctor's office and they hit the hammer just below your kneecap then you know that sensation of your leg involuntarily kicking forward that's known as the patellar tendon reflex let's see how that works all right so here's a leg that's your kneecap right here uh that's the patella this is the patellar ligament which is where the little hammer is is hit and the patella has a tendon that connects to this muscle up here that's your quadriceps muscle so the tendon is right here this connective tissue um and then the quadriceps muscle has a muscle antagonist that is the muscle that serves to elongate the quadriceps when it's relaxed that's known as the hamstring muscle okay so when the patellar ligament is hit with the hammer boom like that it causes a very rapid elongation of the quadriceps muscle so the patella slides down and the quadriceps slides forward the speed of that action matters if you were to just put the hammer and push against the patellar ligament that would fail to trigger the reflex action so speed matters in this case the length of the muscle is detected we actually have two kinds of sensors embedded into our muscles that tell our central nervous system the state of the muscles the the only one we're going to talk about now is called a muscle spindle organ so it encodes muscle length by the frequency of action potentials the muscle spindle organ has an axon that terminates in the nerve cord we'll get to that in a second what we're going to do on the left hand side of this slide is to begin to construct a flow chart for our control diagram so the stimulus is picked up by the muscle spindle organ and what it's picking up on is the rapid extension or elongation or stretch of the quadriceps muscle that change in length is encoded along the afferent nerve is a train of action potentials okay so here we have the axon terminal for the muscle spindle organ by the way a nerve is a bundle of axons okay so that's the result is we get a kicking of the leg that's being driven by the quadriceps muscle so we know that the quadriceps muscle is going to be on the efferent side and we know that since it's a skeletal muscle that activation is going to be achieved through motor neurons okay which we see right there all right so that's our efferent pathway efferent pathway we've got a quadriceps muscle that is positively or actively or is activated by the motor neuron all right so we've got the afferent side of things in green that's the sensory side and the beginnings of the efferent side the quadriceps muscle in red what is the integration center here now note something what is missing here there's no brain we do not need the brain and in fact we don't use our brains for reflex actions instead all of the processing all the decision making occurs just with a very small number of neurons and in this case that decision making occurs in the gray matter of the spinal cord and it's there where we have our integration center the integration center emerges from both the afferent and efferent sides because the muscle spindle organ has an axon that's encoding the intensity of the stimulus by the frequency of action potentials that in turn is influencing the motor neuron directly through temporal summation and if you have multiple muscle spindle organs there's a potential for spatial summation as well so we've talked about spatial and temporal summation previously we know that if you've got a higher frequency of action potentials you're more likely to result in generating an action potential along an axon since this is a motor neuron that action potential is going to result in a twitch by the muscle and if there's a whole series of action potentials then you're going to have summation of force in the quadriceps muscle so we can add the integration center here now that we know that it's a combination of the muscle spindle organ and the motor neuron and we understand the behavior of this integration center because we i've already told you that it's a feed forward control system so there's some threshold stimulus that's required some rate of extension of that quadriceps muscle to exceed some threshold and the difference between state variable of the speed of stretching and that threshold once the state variable is exceeding that threshold then the quadriceps muscle will be activated now this is an example of a reflex arc that only has one synapse okay there is a little branch here of the muscle spindle organ axon i'll get to that in a minute but it's not necessary to explain the efferent response of the quadriceps muscle being activated and since there's only one synapse then we classify this as a monosynaptic reflex and later on we'll see an example of a polysynaptic reflex where there's more than one synapse now when you're classifying whether it's mono or polysynaptic what matters is how many synapses there are in series we see that with this branch here we've got another synapse that's in parallel that doesn't matter all that matters is how many synapses there are between the afferent and efferent sides and in this case there's only one okay we're ignoring the synapse that happens right at the muscle now in order for the quadriceps muscle to be effective in kicking your foot forward we wouldn't want its muscle antagonist to be activated at the same time so let's just say you were voluntarily trying to contract your hamstring muscle we wouldn't want that that would impede the reflex action so in order to prevent that from happening we have an inhibition of the motor neuron for the hamstring muscle and that's the function of that synapse that's in parallel to the one that's activating the quadriceps muscle and this is pretty typical of musculoskeletal reflexes so much so that we have a term for it which is reciprocal inhibition where a muscle antagonist for the muscle that is being activated is inhibited so in this case we look at the flow chart on the left that's the hamstring muscle we're inhibiting the hamstring muscle allowing the quadriceps muscle to contract that's causing the leg to move like that now this is uh a seemingly uh academic uh reflex action right because it's only happening in the in the doctor's office but actually if you walk around the world just you know walking you have small reflexes serving to maintain your equilibrium all the time are all of these little corrections that you make to stay upright and um and you're just sort of barely cognizant of their actions so as you're walking around certainly if there's a startle response if you um somebody leaps out at you and scares you or let's just say you step on attack then reflex actions respond to that startling stim stimulus with feed forward efferent actions that are similar to this in that you have reciprocal inhibition of muscle groups that cause you to spring into action now i've mentioned that you don't need your brain to work here and in fact we don't use the brain in order for this to work but you can still feel the action and that's because you have mechanosensory cells throughout the body and the muscle spindle organ itself can report its status to the brain so we do have this reporting occur in the brain and so there's an there's an ascending signal that goes through the thalamus and ends up in the cerebrum we don't talk about the cerebrum a lot in this class because it's very complicated it's more in the domain of a neurophysiology course but for now i'll just mention that it allows you or it includes a region that gives you a sense of your body and in particular the somatic sensory cortex is alerted when actions such as this occur so this allows you to feel the reflex action even though we don't need the brain to recruit the efferent response okay so that's our example of a somatic reflex action the autonomic nervous system can also be recruited in reflexes and we're going to consider an example that includes the parasympathetic pathways autonomic reflexes include both parasympathetic and sympathetic efferent pathways and the one that we're going to consider is the reflex that allows you to urinate okay so we're going to talk about the kidneys later the kidneys produce urine and that urine collects in your urinary bladder this is a muscular sphere that increases in diameter as it fills up with urine like that and you know that you feel that and you feel that because you have stretch receptors that also terminate in the spinal cord so this is much like this muscle spindle organ in that it's mechanosensory but these dendrites are woven into the walls of a smooth muscle lining and they're detecting the tension in the walls of the bladder so as it fills up the tension goes up now on the efferent side we have something kind of similar to the somatic pathway but this is going to be a parasympathetic pathway now in between we've have an interneuron okay that interneuron is not technically part of the autonomic pathway which is shown right here but it activates that pathway now whenever you see a synapse there is both an opportunity for processing and there's also the disadvantage which is that it takes some more time for that processing to occur synapses take about two milliseconds and so you there's pros and cons you know the more synapses you have the more opportunity there is for a complex network that can make decisions in a sophisticated way on the other hand tends to be slower when that's the case now the smooth muscle that lines the walls of the urinary bladder is not just a passive container for the urine but it also generates tension and drives your urination so on the efferent side we've got that smooth muscle to work with to generate or elevate pressure on the urine to expel it from your body now this pathway that we see on the right again we don't need the brain involved it's a reflex action this is an example of a polysynaptic reflex because we have two chemical synapses uh before the efferent pathway is uh recruited um so poly refers to the fact that we have an interneuron right here in that reflex action and that's going to have a positive effect so we've got we're tending to increase the activity of the smooth muscle that lines the urinary bladder so we can add that to our control system where we're going to have the same kind of integration center that i described for for both the feed forward thermoregulatory control system in them just a moment ago the somatic musculoskeletal control system so again the state variable compared to a threshold is what matters to set off the action in this case the state variable is bladder tension and that bladder attention is encoded by a train of action potentials that are sums by first the interneuron and then ultimately the parasympathetic neuron so the algorithm for our integration center which again is generated by just a small number of cells in this case we've got one interneuron between the afferent side and the efferent side and altogether the integration center the the sort of set of rules that this network that governs this network is that if the state variable the bladder tension exceeds the threshold then activate the smooth muscle increase the tension of the walls of the urinary bladder okay and that's going to tend to expel the urine from the bladder now there's a there's some more to the story here because we have a sphincter sphincter is essentially a closure of some vessel so here um there's a sphincter that that um is you can think of as a door that closes off the smooth muscle or maybe you can think of it as tying a water balloon that is skeletal muscle okay that's not going to be controlled by an autonomic pathway but instead by a motor neuron and that motor neuron also receives input from the stretch receptor that that functions to increase bladder tension through the parasympathetic pathway the way that that is achieved is that there's a branching of the axon of the stretch receptor we've got another interneuron involved but this interneuron is different because at its terminal it has a negative an inhibitory synapse and therefore a negative effect on the motor neuron for the skeletal muscle right there so the motor neuron that generates action potential so this motor neuron is constantly generating spikes you will see this sometimes is you'll have a motor neuron sometimes an autonomic neuron that just at rest is generating action potentials this inhibition from the stretch receptor so think okay the urinary bladder is full i am going to therefore have a negative effect on the frequency of action potentials for that skeletal muscle that's going to tend to relax the skeletal muscle so as a consequence just as the urinary bladder is increasing its tension the skeletal muscle is relaxing to allow the urine to escape so if we look to the left we add that to our control diagram we have a negative influence on skeletal muscle this is our second behavioral rule once we exceed the tension in the walls as detected by the stretch receptor then not only are we going to increase the tension in the walls the bladder but we're also going to deactivate the skeletal muscle that keeps the urine on the inside all right now when you were born that is the end of the story that is the complete control system soon as the bladder fills it gets expelled by contracting the walls and opening up the sphincter fortunately that's not the end of the story because we learn over time how to control this okay we can essentially override this reflex action with descending commands that come from the brain okay so the reflex is just as i've mapped it out but reflexes can be modulated and sometimes overridden by the central nervous system so for that we need to add a brain and the from the brain we have a descending command all right and that functions to inhibit the interneuron so here we can begin to see the value of having an interneuron because we've got more chemical synapses for other neurons to weigh in on in this case that extra minus that i've added there i'm going to magnify this because things are getting a little messy all right so we see that minus up above up top that's coming from the descending command okay so now we are going to inhibit that excitatory synapse on the parasympathetic efferent pathway okay that's for the smooth muscle so ins so we want to prevent this positive from happening and there's a second descending command which inhibits the inhibition all right so we are our interneuron in there this is the one on the right interneuron was inhibiting the motor neuron for the skeletal muscle now we're going to inhibit that inhibition so if we return to the flow chart on the left we're essentially creating a negative effect on both of those which means that the smooth muscle in the urinary bladder will not increase in tension it'll stay relaxed and the sphincter composed of skeletal muscles is going to maintain its tension by virtue of having a high frequency of action potentials along that motor neuron okay so it's possible to override reflex actions through descending commands and in this case in this example that is a learned ability that we achieve when we are potty trained early in life and hopefully we retain it forever not always the case all right so we posed this question of how do reflexes work we see two examples here the somatic pathways that include motor neurons skeletal muscles for musculoskeletal reflexes and then we have an autonomic pathway that's the smooth muscle we're combining that autonomic pathway along with the somatic pathway for the skeletal muscle sphincter that keeps the urine contained in the urinary bladder