In the resting cell, the permeability of the membrane to potassium (P K ) is greater than its permeability to sodium (P Na ). Stimulation, like synaptic activity coming from other nerve cells, can depolarize (make less negative) the cell membrane. Sodium channels in the cell membrane are sensitive to membrane depolarization and they respond by opening, which increases the membrane’s permeability to sodium. If the depolarization reaches or exceeds a certain level (threshold), an action potential is produced. Action potentials develop because of a regenerative, positive feedback cycle. As the cell’s permeability to sodium increases, sodium conductance increases, and increased sodium conductance leads to greater depolarization of the membrane. As depolarization increases, sodium permeability increases again, and more voltage-sensitive channels open. With more channels open, sodium conductance and membrane depolarization increase until the membrane potential reaches the equilibrium potential for sodium. But, before the equilibrium potential for sodium is reached, two other events occur: the voltage-sensitive sodium channels close soon after they open, and the voltage-sensitive potassium channels open. With these channels open, potassium ions leave the cell and cause the membrane to repolarize (hyperpolarize) towards its resting level. This process of membrane hyperpolarization closes the voltage-sensitive potassium channels and re-primes the sodium channels so that they are ready to open once more. This is called the refractory period. Propagation of the action potential from the site of initiation to other locations along the nerve cell is caused by the positive charges in the cell leaking to an adjacent (unstimulated) region and depolarizing that region enough to create an action potential there. In this way, the signal moves from one region of the axon to the adjacent one, and ultimately to the end of the axon. Some axons are myelinated; the axon is covered with a series of Schwann cells, a type of glial cell which electrically insulates the axon. The spaces between adjacent Schwann cells are called the nodes of Ranvier, and they are the only regions along the axon where the membrane is exposed to the extracellular fluid. The myelin insulation prevents the currents associated with action potentials from leaking out of the membrane until they reach a node. So, action potentials take place only at the nodes in myelinated cells. In this laboratory you will record action potentials from the ventral nerve cord of an annelid, the earthworm Lumbricus terrestris. The ventral nerve cord of some invertebrates is a structure analogous to the dorsal Animal Nerve
nerve cord of vertebrates. The ventral nerve cord of an earthworm contains three giant neurons. The cord has one medial giant neuron with a lateral giant neuron on either side of the medial neuron. Because of their size, the three giant neurons in the earthworm nerve cord can generate action potentials with conduction velocities that permit the worms to have a fast escape reflex. When a large stimulus is delivered to the nerve cord, the neurons respond and action potentials from the medial and lateral neurons are seen. You will examine certain principles associated with neuronal activity:
• Neuron viability—determining the viability of the neurons in the nerve cord by observing action potentials from the medial and lateral giant neurons. • Thresholds of neurons—determining the stimulus amplitudes needed to generate action potentials from the medial and the lateral neurons.
• Conduction velocity—measuring the speed at which an
action potential propagates down the medial neuron.
• Effects of temperature—observing how cooling affects the conduction velocity of the medial neuron. • Stimulus strength and duration—observing how the stimulus amplitude needed to generate an action potential is related to the...