so we have covered neural Pathways that run from a receptor up the spinal cord to the brain and then back down the spinal cord out to an affector but we also have other forms of neural Pathways which involve a sensory receptor and motor component but the pathway doesn't actually reach the brain this type of neural pathway is called a reflex or a reflex arc and in this learning objective we're going to cover the components of a reflex arc how we classify reflexes and some specific examples so a reflex is a rapid involuntary response to a stimulus that occurs automatically without conscious thought when we consider the makeup of a reflex it looks similar to or contains the same components as any other neural pathway but the major difference is that the integration site of the sensory input is in the spinal cord and not the brain the benefit of having the integration Center in the spinal cord means that our reactions or outputs can happen quicker and no conscious thought needs to be involved when we consider many of our reflexes serve as protective functions for example how we reflexively pull our hand away from something that's hot it's beneficial for this action to happen as quickly as possible and without the need for us to be actively thinking about it so a typical reflex arc will consist of a sensory receptor which responds to a particular stimuli a sensory neuron which conducts that information towards the integrating Center the integrating Center which will sit within the spinal cord now that can be an interneuron like we've got here so an interneuron is just the neuron in between a sensory neuron and a motor neuron or the integrating Center can be the sinapse directly between a sensory neuron and a motor neuron we then have the motor neuron which propagates the information or the motor output towards an affector and then our affector which is typically a muscle or a gland that carries out the response now because we like to classify everything in anatomy and physiology there are a number of ways in which you can classify everything reflex for example you can classify The Reflex based on whether it's an innate reflex in that you're born with it or an acquired reflex which you do learn but it becomes reflexive over time so as an example of an innate reflex there are many reflexes in babies for example the moro reflex which is where the babies will fling their arms out to the side if they feel like or they actually are being dropped or the rooting reflex where a baby will turn its head and open its mouth if you stroke its cheek so this helps with the early stages of feeding an acquired reflex is something like riding a bike whereby once you have learned how to ride the bike and you have experience riding a bike it's actually very difficult to unlearn this skill and when you're sitting on top of a bike you start to ride it without really actually thinking about it we can then classify reflexes based on where the integr Center sits so we can have cranial reflexes which is when the integrating Center sits in the brain stem so this is involved in reflexive movements of the eyes for example that happen as you're reading or a spinal reflex which is where the integrating Center sits in the spinal cord and usually involves structures lower down in the body such as your arms and your legs we can classify reflexes based on the nature of the response so for example a visc reflex will involve an organ or a structure that we cannot consciously control such as when the pupil of the eye constricts or get smaller when you shine a bright light or a sematic reflex which will involve a sceletal muscle and includes things like the stretch reflex which is when the doctor Taps just below your knee on your patal tendon and your leg will kick out reflexes can be classified based on whether they involve one side or both sides of the body so an Ipsy lateral reflex involves a sensory receptor and a motor neuron and fector on the same side of the body a contralateral reflex will involve a sensory receptor and sensory neuron on one side of the body and a motor neuron and an affector on the other side and lastly we can also classify reflexes based on the number of neurons and therefore the number of synapses involved in a reflex arc so a mono synaptic reflex with mono meaning one occurs when a sensory neuron synapses directly with a motor neuron so there's only one syapse in that reflex arc a poly synaptic reflex with poly meaning many or more than one is when there might be an interneuron so you have at least two synapses in that reflex arc or when one sensory neuron might syapse with more than one motor neuron now two specific reflexes that we're going to walk through now and I want you to be aware of due to their relationship to some of the sensory receptors that we talked about earlier are the stretch reflex and the tendon reflex so a stretch reflex causes contraction of a sceletal muscle which is our affector in response to the stretching of the muscle which is the stimulus and is detected by a muscle spin which is the receptor so we will have a go at this in class but stretch reflexes can be elicited by tapping on tendons attached to the muscles at the elbow the wrist the knee and the ankle joints this is because when you tap on those tendons it kind of bends that tendon in and it will stretch the muscle that it's attached to now the most common example of a stretch reflex is the patella reflex or the knee-jerk reflex and so I'll walk you through that process in this image now so we've got a reflex hammer and we're tapping on this patal tendon here when we tap on it it will push that tendon in and it will stretch this quadricep muscle which contains our muscle spindles so remember our muscle spindles are proprioceptors that detect change in muscle length when that muscle is stretched it will send this sensory information along the sensory neuron into the spinal cord now this sensory neuron can syapse directly with the motor neuron that motor neuron carries the motor information back down to the same muscle and what it tells that muscle to do is actually contract so when a muscle contracts it gets shorter and it's like the opposite of stretching so it relieves that stretching so this part here can be one of our reflex arcs however depending on where this stretch reflex takes place that sensory neuron can actually syapse with either another modor neuron or an inter neuron and then a modor neuron and in this example here it's innovating the hamstring muscles and here it's telling our hamstring muscle to relax so if you think you want to contract your quads you need your hamstring muscle to relax to actually allow for that contraction so this reflex can either be a simple mono synaptic reflex or a poly synaptic reflex depending on where in the body it takes place we then have our tendon reflex and our tendon reflex Works to protect our muscles and our tendons by forcing that muscle to relax before the muscle tension or the muscle Force becomes so great that we're actually going to do damage to that muscle of the tendon so in this example the sensory receptors are our Gogi tendon organs and remember these receptors detect and respond to changes in muscle tension so we've got our GGI tendon organ here sitting in the tendon that attaches our quadricep muscles to the batella and let's say we're doing a really strong leg extension so we're Contracting this Muscle really hard it's causing a change in tension in the tendon and this is detected by our gold tendon organ this sensory information is then transmitted into our integrating Center in the spinal cord here we're connecting to an inter neuron and then a motor neuron and this mon neuron is going back to innovate that same quadricep muscle and telling it to relax so when that muscle relaxes the tension and the force going through the muscle and therefore our gold you tendon organ is reduced but again because of the location of this reflex in the body it's involving the knee joint and our knee joint is uh controlled by both our quadricep and our hamstring muscles that sensory neuron can also syapse with another interneuron and another motor neuron which now innovates our hamstring muscle and it tells it to contract so if our hamstring muscle contracts it forces our quadricep muscle to relax and stretch if our muscle is relaxed and stretched it's reducing that amount of tension or force in our tendon reflex H sorry in our GGI tendon organ so our muscle spindles detect stretch so in a stretch reflex the muscle will contract to relieve the stretch in a tendon reflex our GOI tendon organs detect changes in tension or in force the tendon reflex will cause that muscle to relax to counteract that increased tension or force in the muscle the last example I wanted to provide you was was our crossed extensor reflex and this is a contra lateral reflex so involves both sides of the body and I'll let you walk through this diagram in your own time as there's lots going on and I think it might be better for you to have a bit of a read while I'm not talking but this reflex is triggered as an example when you stand on something sharp or hot in response to this painful or damaging stimuli you immediately withdraw that leg and foot but to ensure that you don't fall over your other leg must extend so you lift this leg off the ground but so that you don't fall over this leg now has to extend so you can balance on that foot so we call this the crossed extensor reflex because one leg must Flex to lift up the foot while the other leg extends so that you can put it down and not all over this reflex is an example of a contra lateral reflex because it involves both sides of the body a poly synaptic reflex because it involves many neurons and synapses with all the flexing and the extending of muscles and also a spinal reflex because the integrating Center would sit in the spinal cord we lastly Al also call it a sematic reflex because it involves sceletal muscle which usually we vol unily control and not a visceral organ in which we have no control over