Lab 9: Speed of Nerve Transmission
Lab 9: Conduction velocity in the nervous system
Objectives
- To calculate the conduction velocity of the sciatic nerve in the reflex arc from the Achilles tendon to the gastrocnemius muscle in humans.
- To measure conduction velocity in the sciatic nerve of a frog by stimulating the nerve and measuring the response through external recording electrodes.
- To measure threshold values, conduction velocity, and refractory period in earthworm giant nerve fibers
Lab 9 Overview
Conduction Velocity in Nerves: Background
A neuron is a cell that is specialized for the transmission of nervous impulses. The axon is the part of the neuron that conducts impulses; the axon is usually a long outgrowth, or process, that carries impulses away from the cell body of a neuron toward target cells.
A nerve impulse, also called an action potential, is the signal that is transmitted along an axon that enables nerve cells to communicate and to activate many different systems in an organism. Action potentials may originate in the brain and result in a deliberate movement or they may be involved in a reflex arc that is independent of the brain. An action potential may be transmitted to a muscle where it spreads throughout the muscle causing contraction.
Neurons have the property of being able to generate action potentials. The action potential is caused by a change in the neuron membrane permeability. This change in permeability results in a change in distribution of ions across the membrane. The change in distribution of ions leads to a change in electrical charge (potential) across the membrane. Changes in electrical potential can be experimentally detected as the action potential passes along the axon of the neuron.
Changes in electrical potential of the axon can be detected and displayed on a recording device in the laboratory by one of two basic methods:
A. Intracellular Recording: Two electrodes are placed on either side of the membrane of the neuron, one inside the cell and one outside. As the ions move into and out of the cell a change in potential difference is recorded between the electrodes. It is from this type of experiment that the information on the ion movement during an action potential has been obtained. Most of the diagrams of action potentials that are illustrated in textbooks and that you studied in lecture originate from this type of experiment. This technique is performed on large, isolated neurons.
B. Extracellular Recording: A pair of electrodes is placed on the outside of the neuron. As the action potential passes along the neuron, a change in potential between the electrodes may be measured and recorded as a biphasic AP. This is a result of the net difference in potential as the action potential passes first one electrode and then the other electrode. This method does not measure ion flow but the passage of the action potential between the two electrodes. This method has the distinct advantage that it can be used to record the passage of an action potential (as in a muscle) from the surface of the body. It is also useful as a tool to record action potentials from whole nerves (in contrast to having to puncture individual neurons).
In today's laboratory you will be using method B above; you will be recording from a nerve, which is a bundle of neurons, rather than from a single neuron. This method not only enables you to visualize the action potential but also allows you to determine the speed at which the action potential travels along a nerve.
Background Information on Experimental Method. Speed or velocity is determined by measuring the distance traveled and dividing by the time required to travel the distance (speed = distance/time). Therefore, in order to estimate the speed of conduction of an action potential, it is necessary to measure both the length of the nerve and the time that it takes the action potential to travel along the measured distance.
Measurement of the distance is relatively straightforward. It can be done using a ruler or a tape measure.
The measurement of time is more complicated. Action potentials travel very quickly; therefore, the times to be measured are very small and require more sophisticated instrumentation. The computer with PowerLab, like the oscilloscope, is ideally suited to measure events that happen in a very short amount of time.
Powerlab will act as a digital 2-channel oscilloscope. Time will be recorded on the X axis and voltage on the Y. Time and sensitivity can be adjusted on each channel.
A useful feature of PowerLab is that the operator can initiate a sweep of the screen (i.e. the computer starts sampling). This is known as the TRIGGER. The trigger allows you to capture the time period immediately after an event. It is possible to "trigger" the computer, to begin to collect data (to sweep), at the same time as the stimulus is applied. Thus a record of the stimulating event and the time immediately after the stimulus, when the response occurs, is generated. In this application of the trigger, the computer is set to generate a single sweep upon stimulation of the nerve. PowerLab sweeps the screen once and displays the data. It will not collect any more data until it is re-triggered. Time can be measured on the X axis.
This week you will be using both Channel 1 where the trigger will be displayed and Channel 3 where the responses will be recorded.
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Conduction Velocity in Mammalian NervesTo determine the speed of conduction in a mammalian nerve a reflex is initiated by tapping the Achilles tendon. This will cause the gastrocnemius (calf) muscle to contract. The distance that the action potential travels is measured. The time between the tapping of the tendon and the contraction of the muscle can be measured on the computer screen. Hence speed of conduction can be calculated. (See fig. 9.1.)
The Reflex Arc: When the Achilles tendon is stretched by tapping with a hammer, a contraction occurs in the gastrocnemius muscle. A reflex arc is initiated by stretching the tendon, an action that stimulates stretch receptors in the muscle. Those stretch receptors respond by initiating an action potential in sensory neurons. The action potential travels through those sensory neurons to the spinal cord where they synapse directly with motor neurons. The excitation travels back to the gastrocnemius muscle where it causes contraction of the muscle. Thus the tendon that was initially stretched is returned to its original length through contraction, completing the reflex arc.
The function of this type of reflex arc is to maintain posture. Muscles are continually stretching and returning to their original length without the intervention of the brain. Note that this response is monosynaptic. The sensory neuron synapses directly with the motor neuron in the spinal cord; there is no interneuron involved.
The Electromyogram (EMG): is a recording of a muscle contraction that can be taken from the skin above a muscle. An action potential travels down a nerve, through a nerve/muscle junction and into a muscle. In the muscle the action potential spreads throughout the muscle causing contraction of the muscle fibers. The passage of the action potentials can be sensed by electrodes placed on the skin above the muscle, which when amplified (as in the ECG) can be displayed on a computer screen.
The Reflex Hammer: is a percussion hammer used to test reflexes. The hammer that you will use has been modified so that when it hits the tendon, the hammer closes a circuit and generates a small signal. This signal is used to trigger a sweep by the computer.
