Objectives

     This laboratory has two objectives.  First, you will learn a little about nervous control and the differences between voluntary and involuntary reflexes.  Second, and more importantly, you will become familiar with electronic measurement.  Before you leave this lab, you should understand what each piece of equipment does, and you should know how to operate it.  You will use this equipment for most of the experiments through the rest of the semester.
 

Background

     The knee jerk is the most commonly tested reflex.  It is a spinal reflex, meaning that the neural circuit only goes up to the spinal cord, not the brain.  Figure 1 shows a schematic diagram of this reflex.  There are sensory receptors in your muscles that detect when they are being stretched (smacking the tendon provides a nice, sharp little stretch).  These receptors trigger nerve impulses which travel up the sensory neuron to the spinal cord.  In the spinal cord, the impulses cross over to a motor neuron and travel down to the same muscle, causing it to contract.
 The reason doctors test knee jerk reflexes is that if the response is slow, it may indicate some defect in nerve conduction.  Most of the muscles moving major joints have similar reflexes, but the knee jerk is the easiest to test.

 

     The advantage of a spinal reflex is that it is fast.  Nerve impulses only have to go through two neurons, so the pathway is short and there is only one spot where an impulse has to jump over to another neuron.  Since crossing from one neuron to the next is relatively slow, having only one such crossing is a pretty good arrangement.  By contrast, if you were to consciously decide to move your leg in response to a stimulus, the nerve impulse would have to travel to the spinal cord, jump to another neuron to travel up the spinal cord, then zip around through a bunch of neurons in your brain as you recognize what is happening and decide what to do, then it has to go back down the spinal cord and out a motor neuron.  By the time the impulses have travelled this route, it's probably too late to respond.

     So what is the point of a knee-jerk reflex?  The point of the reflex is 
that if a sudden weight dropped on you (Figure 2), the buckling of your
knees would stretch your thigh muscles, automatically causing the muscles
to contract harder via the spinal reflex.  Thus, whenever the muscle is
suddenly loaded, it reflexively contracts to support that load, so you don't
collapse.  This is quite useful for walking.  Every time you put weight on
your foot, your muscles contract to support you.  Without spinal reflexes,
people would look silly staggering around campus, having to consciously
think about working their muscles for each step, but with the muscles
reacting too hopelessly late to be useful.  Chewing gum at the same time
would be out of the question.
 
 

Methods

Equipment
 In order to measure the knee jerk reflex, we need a way to tell when the stimulus was applied, and when the leg responds.  We also need a measuring system that is fast enough to detect a response that occurs in about a tenth of a second.  That's why we use electronics.  The rubber hammers are equipped with a button on the business end.  When that button strikes anything, it completes a circuit and sends a 5 volt signal to channel ain2 (analog input #2).  The force transducer responds mostly to bending forces, so you should connect your leg to the transducer in such a way that when your leg moves, it creates a slight bending load on the transducer.  This creates a voltage that will be sent to channel ain0 (analog input #0).  Then you can look on the display and measure the time between stimulus (hammer) and movement.

The Superscope instrument
 Use the instrument called "knee-jerk" (Fig. 3).  The start button begins the recording.  It will automatically record for two minutes, but you will probably want to stop (punch the stop button) before that so that you can measure the time intervals.  The computer will not allow you to do much calculating while it is busy collecting data.  The marker dials control the position of the marker.  Above each marker, the display shows the x and y values at that point.  The indicator "delta x" automatically gives you the time difference between the two marked points.

Procedure
 For every person in the group, measure the response time of the knee jerk reflex.  Have them sit so that their feet dangle unsupported and make sure they relax their leg muscles.  Connect the force transducer in such a way that moving the subject's leg pulls on it.  Start Superscope and whap their patellar tendon with the hammer.  This tendon is the one that connects the top of the shinbone to the bottom of the kneecap.  In addition to the involuntary response, measure the response time of a voluntary reflex (you can strike the person on the back and have them kick when they feel the hammer, or strike the table and have them kick when they hear the hammer).  Compare the involuntary and voluntary response times.  How much variation is there within your group?
 

Fig. 3

What you should know by the end of this lab:
 How a spinal reflex works
 The pathway and processing of electrical signals in the data acquisition system
  - system components, channels
 How to use the Superscope interface
  - understanding the output
  - changing axis scales and scrolling
  - getting quantitative data
 How to work an amplifier
  - gain, zero adjust
 How to find and clearly display an electrical signal
 Knowing that the "power" button must always be on for an electrical device to work
  - and it must be plugged in

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