Reading: In your textbook, read the "SPINAL CORD" section (p. 172-180) and pages 286-288.

1. Be able to explain the parts (and the functions) of a reflex arc.

2. Understand Reflex vs. Reaction.

3. Know clinical significance of reflexes.

4. Become familiar enough with the Reflex Lab computer program (how we measure reflexes in this lab) that you are able to acquire and interpret the reflex data.

5. Be familiar with the concept of a transducer.


A reflex is a response to a perturbing stimulus that acts to return the body to homeostasis. This may be subconscious as in the regulation of blood sugar by the pancreatic hormones, may be somewhat noticeable as in shivering in response to a drop in body temperature; or may be quite obvious as in stepping on a nail and immediately withdrawing your foot.

A reflex arc refers to the neural pathway that a nerve impulse follows. The reflex arc typically consists of five components (3):

1. The receptor at the end of a sensory neuron reacts to a stimulus.

2. The sensory (afferent) neuron conducts nerve impulses along an afferent pathway towards the central nervous system (CNS).

3. The integration center consists of one or more synapses in the CNS.

4. A motor (efferent) neuron conducts a nerve impulse along an efferent pathway from the integration center to an effector.

5. An effector responds to the efferent impulses by contracting (if the effector is a muscle fiber) or secreting a product (if the effector is a gland).

Reflexes require a minimum of two neurons, an sensory neuron (input) and a motor neuron (output) (see Figure 1) The sensory neuron (such as a pain receptor in the skin) detects the stimuli and sends a signal towards the CNS. This sensory neuron synapses with a motor neuron which innervates the effector tissue (such as skeletal muscle to pull away from the painful stimuli). This type of reflex is the "withdrawal" reflex and is monosynaptic, meaning only one synapse has to be crossed between the sensory neuron and the motor neuron. It is the simplest reflex arc and the integration center is the synapse itself. Polysynaptic reflexes are more complex and more common. They involve interneurons which are found in the CNS. More complex reflexes may have their integration center in the spinal cord, in the brainstem, or in the cerebrum where conscious thoughts are initiated.

Many people conider only the simplest types of responses as "reflexes", those that are always identical and do not allow conscious actions. We must not confuse these with "reactions", which are different from reflexes in that they are voluntary responses to a stimulus from the environment. For example, while the body has various subconscious physiological responses to mitigate cold, as humans we can simply choose to put on more clothes. This is a conscious order made by the cerebrum, not an involuntary response to a stimulus. This is a very complex response involving millions of neurons and some time to process the voluntary response. In contrast, spinal reflexes occur much faster, not only because they involve fewer neurons, but also becuase the electrical signal does not have to travel to the brain and back. Spinal reflexes only travel to the spinal cord and back which is a much shorter distance. Because of this and the complexity of conscious reactions, they take more time to complete than a reflex. On average, humans have a reaction time of 0.25 seconds to a visual stimulus, 0.17 for an audio stimulus, and 0.15 seconds for a touch stimulus (2). Reaction times vary from individual to individual. Because of the higher degree of neural processing, reaction times can be influenced by a variety of factors. Reaction times can decrease with practice, as often times athletes have faster reaction times than non-athletes. Sleepiness, emotional distress, or consumption of alcohol can also impact reaction time.



Figure 1 : Neural connections in a spinal reflex (1).

Stretch reflexes are a special type of muscle reflex which protect the muscle against increases in length which may tear or damage muscle fibers. Stretch reflexes are very important in maintaining your upright posture in humans. Consider the patellar reflex -- a very familiar reflex for those of us who have visited the doctor's office over the years. The primary purpose of the patellar reflex, which is the stretch reflex of the quadriceps femoris muscle in your anterior thigh, is to prevent the stretching of the quadriceps. The patellar reflex is illustrated in Figure 2.


The patellar tendon attaches the quadriceps muscle to the tibia bone of the lower leg. The quadriceps is an extensor muscle because when it contracts it extends the angle of the knee joint by raising the lower leg. Tapping the patellar tendon stretches the quadriceps muscle and causes the sensory receptor of the muscle, called a spindle fiber, to send a signal along the afferent neuron to the spinal cord. This causes the efferent neuron to return a signal to the quadriceps muscle to contract and lift the lower leg. This action resists the initial stretch and is a classic example of negative feedback.

To consider more carefully the events which results of in the "sensation" of muscle stetch, we have to define a few more muscle fiber types. Muscle spindles, called intrafusal fibers, are innervated by sensory neurons and are arranged in parallel to normal muscle fibers (extrafusal). Intrafusal fibers respond to tension by depolarizing a sensory neuron. The sensory neuron synapses with a motor neuron in the spinal cord that innervates extrafusal fibers. The contraction of the extrafusal fibers releases tension on the intrafusal fibers, decreasing stimulation to neuron. The extensor muscle of the knee (quadriceps femoris) is attached just below the knee to the anterior part of the tibia by the patellar tendon. Hitting the patellar tendon with a rubber mallet stretches the spindles in the quadriceps muscle, causing contraction (Sherwood, 2001).

Figure 2: The neural pathway of the knee-jerk reflex (1)

Neural activity at other sites in the body may influence the patellar reflex response. Some evidence suggests that the magnitude of the patellar reflex can be increased when the contractile tone of the quadriceps muscle is increased. Mental activity, which increases muscle tone, may increase the magnitude of the response. Decreased mental activity (sleep, restfulness) can decrease the magnitude of the patellar reflex while physical activity and mental stress may make the reflex less sensitive.

Reflex testing is of clinical value. The presence of the patellar response indicates:

1. The muscle spindle, afferent (sensory) and efferent (motor) neurons, neuromuscular junctions, and the muscle are working appropriately.

2. An appropriate balance of excitatory and inhibitory inputs from the higher brain levels.

3. The integrity of the L2-L4 vertebral segments of the spinal cord.

Tests for simple muscle reflexes, such as the patellar reflex, are basic to any physical exam when motor nerve or spinal damage is suspected. These tests can help locate the damage, because motor nerves above the damage aren't affected, but nerves that originate at or below the injury will produce abnormal reflexes. Your doctor may also test similar stretch rflexes that exist in the triceps muscle and also the Achilles tendon.



The hardware has already been connected for you. The transducer (a goniometer), which converts a biological event to an electrical signal, is constructed as two levers set an an angle to each other. Changes in the angle can be read by the computer. A hammer is connected to the goniometer and acts as a signal to the computer to begin gathering data when it impacts the patellar tendon.


1. Have the subject sit on the edge of the table with right leg dangling freely. With the straps provided, attach the transducer on the outer side of the right knee with the box facing outwards; make sure the transducer hinge coincides with the knee joint, and that the levers are parallel to the leg bones. The goniometer should be as close to 90 degrees with the legs dangling freely.

2. Open "Reflex Lab VI". The computer screen should look something like this:

Figure 3: LabView interface for the Reflex program.

3. You should first practice obtaining the patellar reflex using the rubber reflex mallet provided at your lab station: hit the patellar ligament just below the knee-cap with the pointed end of the mallet. You may mark the area on the knee as a guide for subsequent data collection.

4. Practice getting the hand of a good involuntary knee jerk using the white hammer. When you are ready to obtain data, press the white RUN arrow in the top left corner of the screen. Data collection will begin only after the button on the end of the white hammer is depressed. Strike the patellar ligament with the button end of the hammer. You will then see your knee jerks on the bottom left screen. When you are ready to take measurements of the latent period (the time it takes between hammer blow and movement) and magnitude of the knee jerk (angle change) you just recorded, press the STOP DATA button. The knee jerk trace will then be transferred to the upper right hand screen. Your instructor will show you how to take measurements for latent period and magnitude of the knee jerk in class.

5. Take the data from five good involuntary knee jerks and five "experimental" knee jerks.

For each of the following exercises, you will examine two basic features of the reflex: maximum rotation and latent period. Consult the following figure below to make your maximum rotation and latent period measurements.

Figure 4: Making the measurements for Latent period. Measure the time (in seconds) it takes for the reflex to begin by starting where the trace levels off (the trace moves up the y axis when the mallet is hit) to the time when the trace goes down. Magnitude (measured in degrees) is measure as the dip in the trace as indicated on the graph.


Each group will do parts A-E. This will allow us to compare B,C,D & E against the control response seen in A. Record the data as you perform the experiments, then enter the data into the excel spreadsheet on the instructor's computer when you have collected all of your trials. The data will be pooled from all the lab sections and posted on e-mailed to you or posted on Blackboard.

A. Involutary reflex: Obtain data from three normal knee jerks from one subject who is looking away from the tester. You should get similar results each time if you hit the knee correctly. Record latent period and maximum rotation each time.

B. Voluntary knee movement: Obtain data from three voluntary knee jerks from one subject. The subject should again look away but this time voluntarily jerk the knee after hearing the hammer tap on a soft surface. The tester should minimize the effect of anticipation by not being predictable about when the hammer is tapped.

C. Involuntary patellar with mental distraction: Prepare several addition problems consisting of ten three-digit numbers. One lab member should read these problems one at a time to the subject and the subject should try to solve the addition problems in their head with no paper or pencil (mental math). Obtain data from three knee jerks of this type from one subject.

D. Involuntary patellar with muscle-straining: Obtain data from three knee-jerks while the subject grasps their hands in front of them and strains to pull them apart.

E. Involuntary patellar with iced quadriceps muscle: After completing A-D, unstrap the subject and ice the right quadriceps muscle for 20 minutes. WRAP ICE PACK in TOWEL - do not place ice directly on skin. Restrap towards the end of "icing" then retest the involuntary reflex.

Link to write-up


1. Winter, Sharon."What's the connection?" from The Neural Sciences Activity manual. National Association of Biology Teachers, nlca/nlcapdf/05NLCAchp3.pdf, date accessed, 2/3/03.

2. Columbus Ohio Local Outreach team. "Neural Networks" from the ‚"Frontiers in Physiology Project" The American Physiological Society. 1997.

3. Jensen, Murray. "GC1135 Human Anatomy & Physiology. Human Reflex lab.", date accessed 2/3/03