MCB 32 Introductory Human Physiology
Notes for Thursday, September 7
Textbook: Chapter 3, pps. 51-67
NOTE: The first quiz will be given this coming week in your discussion section. It will cover Chapters 1, 2 and the material covered in today’s lecture from Chapter 3. The quiz will be taken only from material covered in your textbook.
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OUTLINE FOR LECTURE
l. Cell Communication
ll. Physical properties of the cell membrane
lll. Neuron structure and function
lV. Action potential
l. Cell Communication
Although all cells perform many of the basic functions we talked about in lecture 1 and 2, most cells perform specialized functions as well.
Specialized cells benefit the organism as a whole only if cells communicate with each other.
In complex organisms, communication between cells is accomplished either by electrical currents or by chemicals.
Most electrical currents are transmitted by specialized cells called neurons.
Electrical signals can be transmitted very rapidly.
Chemical forms of communication are considerably slower than electrical communication.
Chemical communication is ideally suited for situations where communication must be sustained for long periods of time and for communicating with many different tissues and organs at once.
ll. Physical properties of the cell membrane
To understand how the electrical communication system works we must first understand how an electrical charge is generated and maintained across the cell membrane of all cells.
Membrane Potential
Compared to the extracellular fluid, the cytosol (the fluid inside the cell), has more negative charges and less positive charges; thus, an electrical charge exists across the cell membrane.
This difference in charge is called the membrane potential because energy was required to separate these charges; thus, the separated charges have the potential to do work.
Membrane potentials are expressed with the fluid outside the cell defined as a reference point, having zero electrical potential.
In most cells the membrane potential is about -70 millivolts (a millivolt, abbreviated mV, is a thousandth of a volt) with the inside of the cell more negative than the outside of the cell.
Diffusion Potential
In the previous lecture we stated that because of random motion, molecules tend to diffuse across a membrane from an area of high concentration toward an area of lower concentration, provided that the membrane is permeable to the molecule in question.
For neutral molecules, diffusion ceases when the concentration of the particles are equal on both sides of the membrane.
For ions, which carry a net positive or negative charge, the situation is somewhat more complex.
Movement of ions is influenced by the electrical properties of the two fluids as well as the differences in concentration.
Therefore, ions have both electrical and chemical gradients, combined they produce an electrochemical gradient.
Ions move down their electrochemical gradient, which takes both chemical concentration and electrical charge into account.
Example 1.
Two fluids separated by a semipermeable membrane; on one side is pure water and on the other side is a solution of (Na+) which can cross the membrane and an equal number of negatively charged ions, which cannot. Both solutions are electrically neutral, meaning they contain an equal number of positive and negative charges.
Initially, Na+ diffuses across the membrane because of the difference in concentration, but as Na+ diffuses, a difference in electrical charge develops.
This electrical charge due to the movement of Na+ begins to oppose further diffusion of positively charge Na+ ions, because like charges repel each other and unlike charges attract.
The electrical charge that develops across a selectively premeable membrane when ions diffuse is called a diffusion potential because it comes about as the result of diffusion.
Because movement of ions is influenced by both concentration and charge, an ion can actually be made to move against a concentration gradient if there is sufficient electrical charge across the membrane.
Example 2.
A semipermeable membrane separates two solutions containing equal concentrations of Na+ and a negatively charged molecule that cannot diffuse. An electrical charge is then introduced across the membrane.
Although Na+ is initially at equal concentrations on both sides of the membrane, the introduction of charge produces an electrical force that causes net movement of Na+ toward the negatively charge side.
The net movement of Na+ will stop when the concentration of Na+ becomes sufficiently high that diffusion due to the concentration difference exactly equals the movement caused by the electrical attraction.
How the negative cell membrane potential comes about and how is it maintained?
The negative membrane potential is the result of: 1) the net negative charges on large molecules, such as proteins, which remain are trapped within the cell because they cannot diffuse across the cell membrane 2) the greater permeability of the cell membrane to K+ than Na+ and 3) the activity of the membrane Na+-K+ pump.
lll. Neuron structure and function
Electrical Activity of Nerve Cells
Nerve cells (neurons) are excitable because they have evolved the capacity to alter their membrane potential under physiological conditions.
Neurons have evolved specifically for the purpose of communication.
In effect, neurons function as tiny living electrical cables, actually producing electrical impulses and transmitting these impulses over long distances.
Anatomy of a typical neuron
The neuron has a cell body, which contains a nucleus and other organelles typical of all cells.
Projecting from the cell body are extensions of the cell body, known as dendrites, and at least one much longer projection, the nerve axon, or nerve fiber.
The axon is specialized for transmitting electrical information over long distances.
The dendrites are specialized for receiving incoming information from other neurons and transmitting it only short distances, generally to the cell body.
The region where the cell bodies narrows to form the axon is called the axon hillock.
Side branches called axon collaterals may extend from the main axon.
At the end of the axon and its collaterals are the axon terminals, which are regions of the neuron specialized for transmitting information to another cell.
Although they share the ability to transmit information electrically, neurons come in a variety of sizes and shapes depending on their location and function.
General Function of Neurons
The transmission of information by a neuron can be divided into three phases:
Phase 1 is the generation of a graded potential in the dendrites and the cell body.
A graded potential is a small change in the membrane potential in a localized region of a dendrite or the cell body. Its magnitude varies depending on the strength of the signal that generated it.
Phase 2 is the generation of an action potential at the axon hillock, which travels down the axon to the axon terminals. An action potential is a large and rapid change in the membrane potential that, once initiated, cannot be stopped, sweeping down the entire length of the axon.
Phase 3 is synaptic transmission, the process whereby information provided by the action potential is transmitted from the neuron to another cell.
All cells have a negative membrane potential. Neurons have the special property of being able to rapidly alter their membrane potential.
What is different about neurons, such that their membrane potentials can be changed so quickly?
The answer is that neurons have a large number of gated channels in the cell membrane.
Gated channels are defined both by the signal that causes them to open and close, chemically gated or voltage gated, and by the primary ion that they let pass through, Na+, K+ or in some cases Ca+2.
The opening or closing of gated channels for a particular ion alters the membrane permeability to that ion, thereby either allowing the ions to diffuse rapidly or restricting their diffusion.
Rapid changes in ion movements alter the membrane potential.
Graded Potentials
The dendrites and the cell body of a typical neuron have chemically gated types of ion channels.
The binding of a particular chemical to a receptor associated with the channel causes the channel to open, thereby making the membrane more permeable to a particular ion.
A typical channel in the dendrite of a neuron would be a chemically gated Na+ channel.
Because there is a substantial electrochemical gradient favoring diffusion into the cell, Na+ enters through the temporarily opened channels, making the membrane potential less negative in that local region.
The magnitude and even the duration of a graded potential can vary depending on how many gated channels are opened and how long they remain open.
There may be hundreds of these local regions of altered permeability on a single neuron at any given time, each covering only a small local region and lasting only a short time.
However, these small potentials can undergo summation; that is, they can add onto each other in both space and time.
Graded potentials are short-lived, disappearing over time and with distance from the stimulus.
When Na+ enters the cell and the inside of the cell becomes less negative, we say that the membrane potential has been reduced, or that partial depolarization has occurred.
Conversely, if the membrane potential were to become more negative than usual, we say the the cell membrane has become hyperpolarized.
Finally, return of the membrane potential to the normal resting level is known as repolarization.
Action Potentials
Graded potentials in the region of the dendrites and cell body are local events that are not transmitted over great distances.
Something difference happens at the axon hillock.
If the graded potentials summate sufficiently to produce a depolarization to about
-55mV in the region of the axon hillock, an action potential may be generated that sweeps down the axon at constant speed.
Once it is initiated it cannot be stopped until it reaches the axon terminal.
An action potential comes about because the axon hillock and the rest of the axon have voltage-gated channels rather than chemically gated ones.
Thus, the opening and closing of ion channels is affected by the membrane potential itself.
The membrane potential at which an action potential is generated is called the threshold.
Once voltage-gated channels are activated at the axon hillock, the change in membrane potential spreads to adjacent areas of the axon, activating voltage gated channels there as well.
For this reason, the activation of voltage-gated channels tends to be self-propagating as it proceeds down the axon.
Note that the propagation of an action potential down the axon is an example of a positive feedback system; the opening of voltage-gated channels causes the membrane potential to change even further, causing the opening of voltage-gated channels in adjacent regions.
This process is terminated only when the action potential reaches the axon terminal and thus has nowhere to go.
Getting the action potential started is only the first phase of the action potential, however, for eventually the membrane potential must be returned to its original resting state. Two additional events occur to make this happen.
First, the voltage gated Na+ channels do not remain open indefinitely, even if the membrane were to stay depolarized; they automatically close after a time delay.
Second, the axon hillock and axon also contain voltage-gated K+ channels. These were also activated by depolarization of the membrane to threshold, but they are so slow to open that they are just beginning to open as the Na+ channels are closing.
The combination of closing the Na+ channels and opening K+ channels repolarizes the membrane, because now there is a net movement of positively charged ions out of the cell. Na+ stops diffusing in and K+ begins diffusing out.
Finally, the K+ channels close when the membrane potential returns to near the resting level.
Action potentials travel at a constant speed and amplitude and under normal conditions cannot be stopped. Thus, they are said to be self-propagating, like ripples on a pond.
The constant amplitude and rate of travel is maintained by the diffusion of charged particles in the region of the action potential.
Because opposite charges attract, the positive charges in the local region of the nerve impulse can spread by diffusion to more negative regions nearby, slightly depolarizing the adjacent region and bringing it closer to threshold.
These charges are not crossing the membrane, but simply diffusing to a nearby region either inside or outside the axon.
Once this local diffusion causes threshold to be reached in a nearby region, the Na+ channels open and Na+ diffuses inward, perpetuating the action potential.
This whole process is a continuous one; thus, we can think of the nerve impulse, as sliding smoothly down the axon as each adjacent region is brought to threshold by local diffusion of charged particles.
Refractory Period
If a neuron is stimulated weakly, only a few action potentials are generated.
A more intense stimulus generates more action potentials per unit time.
Nevertheless, each action potential is a separate event, alike in amplitude and shape.
You either get an action potential in response to the graded potential or you don’t. The identical nature of each action potential is known as the all-or-none rule.
Why doesn’t a larger stimulus generate a larger action potential?
The explanation is that during the time that a segment of axon is producing one action potential, it is incapable of producing another.
The period of time during which the axon cannot respond at all is known as the absolute refractory period.
Toward the end of the action potential, when the membrane potential has returned almost to the resting level and the Na+ channels are closed, there is a relatively refractory period during which another action potential can be initiated, but only if the stimulus is stronger than usual.
These refractory periods ensure that each action potential is a discrete event.
Speed of Nerve Impulse Conduction
Conduction velocity depends on temperature, the diameter of the nerve axon, and also on whether the axon is coated with a sheath of an insulating material called myelin;
such axons are called myelinated axons.
Nerve impulse speed increases if a tissue is warmed above normal temperature amd decreases if the tissue is cooled.
Local cooling of a nerve can completely interrupt transmission of action potentials.
Nerve impulse speeds increase with increasing diameter of the axon.
Nerve impulse speed varies from 1 mile per hour to over 260 miles per hour.
Conduction speed increases if the axon is insulated with myelin. This is because myelin does not conduct electrical current and thus acts as an insulator similar to the insulation surrounding an electrical wire.
Myelin is produced by non-neural cells that surround the axon. The myelin sheath is not continuous, but is interrupted at regular intervals.
The point where the sheath is interrupted are called nodes of Ranvier.
Because myelin prevents the inward flow of current across the axon membrane during an action potential at a nearby node, the only flow of current in the region of the myelin sheath is by local current flow either inside or outside the cell.
At the next node, however, this local current flow causes threshold to be reached, generating an action potential, which in turn causes local current flow beneath the myelin sheath to yet another node, and so on.
In effect, a new action potential is created at each node.
Impulse conduction from node to node in this fashion is called saltatory conduction .
Saltatory conduction in myelinated nerves is much faster than conduction of nerve impulses in unmyelinated fibers because conduction by local current is much faster than conduction that requires the opening and closing of channels.
The advantage of myelination is that it allows increased speed of information transmittal without having to increase axon diameter.