Membrane rest potential. Generation and radiation action potential.

Action Potentials

Start level of knowledge:

o    Solids and elastic moduli

o    Fluids: pressure and Pascal's principle

o    Buoyancy and Archimedes principle

o    Fluid dynamics and Bernoulli's equation

o     Bonding and Molecular Structure

Cell Membrane Potentials

Cell membranes in general, and membranes of nerve cells

If the cell membranes were simply permeable to these ions, they would approach an equilibrium with equal concentrations on each side of the membrane, and hence no voltage difference. This makes it clear that the processes which produce the membrane potential are not simply diffusion and osmosis. In the selectively permeable cell membranes are ion channels which allow K⁺ ions to pass to the interior of the cell, but block Na⁺ ions. Negatively charged proteins on the interior of the cell are also denied passage across the membrane. In addition, there are active transport mechanisms at work. There is a process which utilizes ATP to pump out three Na⁺ ions and pump in two K⁺ ions. The collective action of these mechanisms leaves the interior of the membrane about -70 mV with respect to the outside. For the nerve cell, this equilibrium is disturbed by the arrival of a suitable stimulus. The dynamic changes in the membrane potential in response to the stimulus is called an action potential. After the action potential the mechanisms described above bring the cell membrane back to its resting state.

Diffusion

Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. If the partition is removed as in the lower illustration, the gases will mix because of the random velocities of their molecules. In time a uniform mixture of A and B molecules will be produced in the container. The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles.

Membrane Transport

The transport of water and other types of molecules across membranes is the key to many processes in living organisms. Many of these transport processes proceed by diffusion through membranes, which are selectively permeable, allowing small molecules to pass, but blocking larger ones. These processes, including osmosis and dialysis, are sometimes called passive transport since they do not require any active role for the membrane. Other types of transport, called active transport, involve properties of the membrane to selectively "pump" certain types of molecules across the membrane.

The transport of gases across membranes depends upon diffusion and the solubility of the gases involved. In life science applications such transport is characterized by Graham's Law and Fick's Law.

Nerve Cell

Many nerve cells are of the basic type illustrated above. Some kind of stimulus triggers an electric discharge of the cell which is analogous to the discharge of a capacitor. This produces an electrical pulse on the order of 50-70 millivolts called an action potential. The electrical impulse propagates down the fiber-like extension of the nerve cell (the axon). The speed of transmission depends upon the size of the fiber, but is on the order of tens of meters per second - not the speed of light transmission that occurs with electrical signals on wires. Once the signal reaches the axon terminal bundle, it may be transmitted to a neighboring nerve cell with the action of a chemical neurotransmitter.

The dendrites serve as the stimulus receptors for the neuron, but they respond to a number of different types of stimuli. The neurons in the optic nerve respond to electrical stimuli sent by the cells of the retina. Other types of receptors respond to chemical neurotransmitters.

The cell body contains the necessary structures for keeping the neuron functional. That includes the nucleus, mitochondria, and other organelles. Extending from the opposite side of the cell body is the long tubular extension called the axon. Surrounding the axon is the myelin sheath, which plays an important role in the rate of electrical transmission. At the terminal end of the axon is a branched structure with ends called synaptic knobs. From this structure chemical signals can be sent to neighboring neurons.

Chemical Neurotransmitters

Data source: http://ifcsun1.ifisiol.unam.mx/Brain/trnsmt.htm

The nerve cells which are used for the perception of external events will, upon being excited by the proper stimulus, transmit an action potential down their axons. When the electrical signal reaches the axon terminal bundle, it interacts with structures called synaptic knobs. It stimulates an influx of calcium (Ca²⁺) through voltage-gated Ca²⁺ gates. This caused the movement of vescicles toward the membranes of the synaptic knobs. Inside these vescicles are neurotransmitter chemicals. The neurotransmitters are manufactured in the cell body and travel down t he axon to be stored in vescicles associated with the synaptic knobs. When a vesicle reaches the cell membrane of the synaptic knob, it fuses with the cell membrane and releases its neurotransmitter into the synaptic region. On the postsynaptic neuron are receptors that will specifically bind these neuotransmitters. The neurotransmitter will either excite or inhibit the firing of the postsynaptic neuron.

One mechanism for inhibition of the firing of the post-synaptic neuron is to cause hyperpolarization like that which follows the pulse of an action potential. This would raise the threshold for firing of the neuron.

Note that the pre- and post-synaptic neurons have been drawn identically above, but that is just out of ignorance of what the structural differences are.

Transmission of a nerve impulse along an axon

A nerve cell is like a receiver, transmitter and transmission line with the task of passing a signal along from its dendrites to the axon terminal bundle.

The stimulus triggers an action potential in the cell membrane of the nerve cell, and that action potential provides the stimulus for a neighboring segment of the cell membrane. When the propagating action potential reaches the axon, it proceeds down that "transmission line" by successive excitation of segments of the axon membrane.

Just the successive stimulation of action potentials would result in slow signal transmission down the axon. The propagation speed is considerably increased by the action of the myelin sheath.

The myelin sheath around the axon prevents the gates on that part of the axon from opening and exchanging their ions with the outside environment. There are gaps between the myelin sheath cells known as the Nodes of Ranvier. At those uncovered areas of the axon membrane, the ion exchange necessary for the production of an action potential can take place. The action potential at one node is sufficient to excite a response at the next node, so the nerve signal can propagate faster by these discrete jumps than by the continuous propagation of depolarization/repolarization along the membrane. This enhanced signal transmission is called salutatory conduction.

Action Potentials

Action potential from a giant squid axon.                                                  

In response to the appropriate stimulus, the cell membrane of a nerve cell goes through a sequence of depolarization from its rest state followed by repolarization to that rest state. In the sequence, it actually reverses its normal polarity for a brief period before reestablishing the rest potential.

The above example of the squid action potential was patterned after a measured action potential shown in West's Medical Physics. The approximate time intervals shown were scaled from time markers on the experimental trace. The times seem very short to me. I thought the recovery time to rest potential was more like 100 msec.

The action potential sequence is essential for neural communication. The simplest action in response to thought requires many such action potentials for its communication and performance. For modeling the action potential for a human nerve cell, a nominal rest potential of -70 mV will be used. The process involves several steps:

1.    A stimulus is received by the dendrites of a nerve cell. This causes the Na⁺ channels to open. If the opening is sufficient to drive the interior potential from -70 mV up to -55 mV, the process continues.

2.    Having reached the action threshold, more Na⁺ channels (sometimes called voltage-gated channels) open. The Na⁺ influx drives the interior of the cell membrane up to about +30 mV. The process to this point is called depolarization.

3.    The Na⁺ channels close and the K⁺ channels open. Since the K⁺ channels are much slower to open, the depolarization has time to be completed. Having both Na⁺ and K⁺ channels open at the same time would drive the system toward neutrality and prevent the creation of the action potential.

4.    With the K⁺ channels open, the membrance begins to repolarize back toward its rest potential.

5.    The repolarization typically overshoots the rest potential to about -90 mV. This is called hyperpolarization and would seem to be counterproductive, but it is actually important in the transmission of information. Hyperpolarization prevents the neuron from receiving another stimulus during this time, or at least raises the threshold for any new stimulus. Part of the importance of hyperpolarization is in preventing any stimulus already sent up an axon from triggering another action potential in the opposite direction. In other words, hyperpolarization assures that the signal is proceeding in one direction.

6.    After hyperpolarization, the Na⁺/K⁺ pumps eventually bring the membrane back to its resting state of -70 mV .

Control question:

1.    What is Diffusion?

2.    Explain the Cell Membrane Potentials .

3.    How work Membrane Transport?

4.    From what consist of Nerve Cell?

5.    How work Chemical Neurotransmitters

6.    Transmission of a nerve impulse along an axon

7.    Explain the Action Potentials