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Nervous System: Organization and Reflexes – Biological Bases of Behavior (PSY, BIO)

by Tarry Ahuja, MD
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    00:01 Okay. Now, let’s talk about how the nervous system is actually organized.

    00:04 So we know that the nervous system is going to receive a lot of different information.

    00:08 So that’s sensory information.

    00:10 That information needs to then go on and get passed on to the brain or the central nervous system to figure out what to do with it.

    00:16 And then we need to have an action.

    00:18 So the brain is going to say, “I need to do something about this.” So send a signal to muscles or glands to act upon the decision.

    00:25 So the analogy is you put your hand on a hot stove, that’s nervous information coming in.

    00:31 Sorry, information going into the nervous system, saying, “Ouch” burning.

    00:35 And so that signal of activating the thermal receptors on your hand, in the dermal layer, sends a signal up to your brain, and the brain says, “Oh my God, this is hot, move your hand, start yelling.” And then it’s going to send a signal back to your hand to do this.

    00:52 And I keep kind of coming back to do this example because I think we’ve all done this before, grab a hot pot, and how quickly do you move your hand? And thank God we do because it would be terrible if you’re like, “Hmm, I think this pot is burning my hand.” And your hand would be completely charred.

    01:07 But instead we have this action.

    01:09 So think how quickly that happens and I hope you kind of appreciate that.

    01:13 Now, let’s take a look at the different components.

    01:16 So, all that sensory information that’s coming in is going to be down through the peripheral nervous system.

    01:21 Figuring out what we need to do, the process of understanding perceiving the response and having a response to the information coming in would be the central nervous system and then we go back to the peripheral nervous system in order to have an action.

    01:35 But this time it’s done through the motor function side as opposed to the sensory side.

    01:41 Now, let’s take a look at how this sort of process works live when it’s all pieced together.

    01:46 So, motor neurons carry information from the nervous system towards the muscles or glands or organs.

    01:53 Another -- glands is another way to talk about an organ.

    01:57 So, the motor signal is telling it what to do.

    02:00 So these organs are known as the effectors because they’re having an effect on our response to the sensory information that came in.

    02:09 Motor neurons carry the signal from the central nervous system, so from the brain to the effectors via efferent neurons.

    02:15 So you should know the difference between efferent and afferent.

    02:19 Afferent means information coming in from the receptor, so say your skin.

    02:24 And efferent would be information going out to the muscles to do something.

    02:29 Okay? Now, the simplest sort of -- the simplest example of this motor -- sorry, sensory to motor, it would be a reflex.

    02:43 And reflex is a directive motor response to sensory input without conscious thought.

    02:47 So, in English we’re saying, a reflex is you moving in response to some type of sensory input without you even happening to think about it.

    02:55 And that’s why you hear in the -- when people just talk in layman’s terms, we say, “Well, that’s just a reflex reaction.” Meaning I wasn’t thinking about it.

    03:03 And that’s what they’re referring to is that it was something you do without even thinking.

    03:08 So a reflex is exactly that.

    03:09 It’s your muscles responding to some type of input without actually thinking about it.

    03:15 So, the muscle stretch reflex is a great example of that.

    03:18 So there’s a sensory neuron that detects stretching of a muscle and it transmits this signal via motor neuron.

    03:24 And the reflex involving is only two neurons and one synapse and this is known as a monosynaptic reflex arc.

    03:30 An example of that is the quad and thigh and the knee reflex of your tendon.

    03:37 And you know, we’ve all been to the doctor and he pulls out that magical little rubber hammer and he does the little hit on your knee and you’re wondering what does this mean and why is he doing this.

    03:45 What he’s actually looking for here.

    03:46 She’s looking for is that your reflex is working; now you have good conduction between your sensory and motor neurons and that your muscles are working okay.

    03:56 So they just want to make sure that the whole process of signal transmission is working smoothly.

    04:01 And you know, if all goes well at the MCAT and you become a doctor, you might get one of those magical little rubber hammers.

    04:06 Now, here’s a blowup looking at the specific steps.

    04:10 And you know, we’re not going to go through every little box here, but the point is you can see the hammer, you can see the tendon that we’re hitting.

    04:17 And that is, once you hit it, it actually causes a stretching of that neuron which activates that neuron and that’s how we get the contraction of the quadriceps muscle or your thigh.

    04:30 And then, we have also another concurrent activation of the hamstring muscle which is at the back of your leg.

    04:38 So it’s actually a really great example of sort of two things happening at once.

    04:43 So, we’ll walk through the steps here.

    04:46 So, sensory neuron synapses with the inhibitory interneuron which goes toward the hamstring.

    04:51 So, it’s doing two things which is really cool.

    04:55 So this is an example of an interneuron illustrating the integrative role of the nervous system.

    04:59 So the interneuron is, like the name implies, something that’s kind of in the middle.

    05:04 And it’s a neuron that’s saying, “Hey, the quadriceps muscle just contracted. And so, for it to be able to do that, I’m going to have to relax the opposing hamstring muscle.” And so, by activating one, you’re actually inhibiting or relaxing the other and that’s what allows you to sort of kick your leg when you get that little hammer hit.

    05:22 So, concurrent relaxation and contraction is an example of reciprocal inhibition.

    05:28 Okay? So you should understand that that reflex arc.

    05:31 You should understand the fact that you’re activating one and you’re inhibiting the other and this process is quite integrative which is done by the interneurons and we call this reciprocal inhibition.


    About the Lecture

    The lecture Nervous System: Organization and Reflexes – Biological Bases of Behavior (PSY, BIO) by Tarry Ahuja, MD is from the course Individual Influences on Behavior. It contains the following chapters:

    • Organization
    • Reflexes

    Included Quiz Questions

    1. A reflex involving two neurons and one synapse.
    2. A direct motor response to sensory input without conscious thought.
    3. A sensory neuron detecting stretch.
    4. Propagation of action potentials via Nodes of Ranvier.
    5. A stimulus or event capable of producing physical or emotional stress.

    Author of lecture Nervous System: Organization and Reflexes – Biological Bases of Behavior (PSY, BIO)

     Tarry Ahuja, MD

    Tarry Ahuja, MD


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