Chapter 4: Activity of Neurons
The Neuron’s Structure
• Neurons are the information- conducting units of the nervous system. A neuron has many characteristics in common with other cells in the body, but
it also has special characteristics that allow it to send electrical impulses by using changes in chemical charges on its cell membrane.
• Perhaps the most prominent distinguishing features are the dendrites, whose presence greatly increases the cell’s surface area. The dendrites’ surface
area is further increased by many branches and by many small protrusions called dendritic spines that cover each branch.
• A neuron may have from 1 to 20 dendrites, each of which may have one or many branches, and the spines on the branches may number in the many
thousands. Because dendrites collect information from other cells, their surface areas determine how much information a neuron can gather. Because
the dendritic spines are the points of communication between neurons, the many thousands of spines provide some indication of how much
information a neuron may receive.
• Each neuron has a single axon, extending out of an expansion of the cell body known as the axon hillock (hillock means “ little hill”; Figure 4.1D).
The axon may have branches called axon collaterals, which usually emerge from it at right angles. Toward its end, the axon may divide into a number
of smaller branches called teleodendria (“ end branches”). At the end of each teleodendrion is a knob called an end foot or terminal button ( see Figure
• The terminal button sits very close to a dendritic spine on another neuron, although it does not touch that spine (see Figure 4.1C). This “ almost
connection,” consisting of the surface of the axon’s end foot, the corresponding surface of the neighbouring dendritic spine, and the space between the
two, is the synapse.
• The released chemical, a neurotransmitter, carries the message across the synapse to influence the electrical activity of the receiving cell, or target— to
excite it or inhibit it— and pass the message along.
The Cell As a Factory
• The cell is a miniature factory, with departments that cooperate to make, ship, and export proteins, the cell’s products. Proteins are
complex organic com-pounds, including enzymes, hormones, and antibodies, and they form the principal components of all cells as
• A factory has outer walls that separate it from the rest of the world and dis-courage unwanted intruders; a cell’s outer cell membrane
separates it from its surroundings and allows it to regulate the materials that enter and leave its domain.
• Unassisted, very few substances can enter or leave a cell, because the cell membrane presents an almost impenetrable barrier. Proteins
embedded in the cell membrane serve as the factory’s gates, allowing some substances to leave or enter and denying passage to the
• The nucleus, like the executive office of a factory, houses the blueprints— genes and chromosomes— where the cell’s proteins are
stored and copied. When needed, copies are sent to the factory floor, the part of the cell called the endoplasmic reticulum (ER). The
ER, an extension of the nuclear membrane, is where the cell’s protein products are assembled in accordance with the genes’
• The finished products are packed in a membrane and addressed in the Golgi bodies, which then pass them along to the cell’s transportation network, a
sys-tem of tubules that carries the packaged proteins to their final destinations (much like the factory’s interior system of conveyor belts and forklifts).
Microfilaments constitute the cell’s structural framework; microtubules contract and aid in the cell’s movements.
• Two other components of the cellular factory are important for our consideration: mitochondria are the cell’s power plants that supply its energy needs,
whereas lysosomes are saclike vesicles that not only transport incoming sup-plies but also move and store wastes. Interestingly, more lysosomes are
found in old cells than in young ones. Cells apparently have trouble disposing of their garbage just as we do.
The Cell Membrane: Barrier and Gatekeeper
• Neurons and glia are tightly packed together in the brain, but, like all cells, they are separated and cushioned by extracellular fluid. This fluid is com-
posed mainly of water in which salts and many other chemical substances are dissolved. Fluid is found inside a cell as well. This intracellular fluid, or
cytoplasm, also is made up mainly of water with dissolved salts and other chemicals, but the concentrations of dissolved substances inside and outside
the cell are very different. This difference helps explain the information- conducting ability of neurons.
• The cell membrane encases a cell and separates the intracellular from the extra-cellular fluid, allowing the cell to function as an independent unit. The
special, double- layer structure of the membrane makes this separation possible. The membrane bilayer also regulates the movement of substances
into and out of the cell. For example, if too much water enters a cell, the cell can burst, and, if too much water leaves, the cell can shrivel. The cell
membrane helps ensure that neither happens.
• The membrane bilayer is composed of a special kind of molecule called a phospholipid. The name comes from the molecule’s structure, which
features a “ head” that contains the element phosphorus ( P) and two “ tails” that are lipid, or fat, molecules. The head has a slight positive charge in
one location and a slight negative charge in another. The tails consist of hydrogen and carbon atoms bound tightly to one another, making them
• Quite literally, then, the head of a phospholipid molecule loves water and the tails hate it. These phospholipid molecules form a bilayer arranged so
that the heads of one layer are in contact with the intracellular fluid and the heads of the other layer are in contact with the extracellular fluid. The tails
of both layers point toward the inside of the bilayer, where they are hidden from water.
How the Cell Membrane Functions
• Only a few, small, nonpolar molecules, such as oxygen ( O 2 ), can pass freely through a phospholipid bilayer.
• Salts are molecules that separate into two parts when dissolved in water, with one part carrying a positive charge and the other part a negative charge.
These charged particles are collectively called ions. Ordinarily, the tightly packed polar surface of the phospholipid membrane prevents ions from
passing through the membrane, either by repelling them, binding to them, or blocking their passage if they are large.
• Proteins embedded in the cell membrane act as gates and transportation systems that allow selected substances to pass through the membrane.
Proteins are manufactured by the cell on instructions from the nucleus and different proteins enable the transport of different ions.
The Neuron’s Electrical Activity
• The neurons of most animals, including humans, are very tiny, on the order of 1 to 20 micrometers (lm) in diameter (1 lm one- millionth of a meter or one- thousandth of a millimeter).
• A giant axon could be removed from a live squid and kept functional in a bath of liquid designed to approximate the squid’s body fluids. In this way,
Hodgkin and Huxley determined how neurons send information and laid the foundation for what we now know about the electrical activity of
neurons. They discovered that differences in the concentration of ions on the two sides of a cell membrane create an electrical charge across the
membrane. They also discovered that the charge can travel along the surface of the membrane.
Recording from an Axon
• Hodgkin and Huxley’s experiments with the giant squid axon were made possible by the invention of the oscilloscope, an instrument that turns
electrical fluctuations into visible signals. You are familiar with one form of oscilloscope, an old- fashioned television set. An oscilloscope can also be
used as a sensitive voltmeter to measure the very small and rapid changes in electrical currents that come from an axon.
• Sensitivity is important because the duration and size of electrical charges are very small, on the order of milliseconds ms; 1 ms one- thousandth of a
second) and millivolts ( mV; 1 mV one- thousandth of a volt). Oscilloscopes are still used today for recording the activities of neurons, although the
job can also be— and frequently is— performed with the use of computers.
• Recordings from the axon are made with microelectrodes— insulated wires with very tiny, uninsulated tips. Microelectrodes were inserted into to the
subject’s temporal lobes in the single- cell recording described in the Portrait at the beginning of this chapter.
How the Movement of Ions Creates Electrical Charges
• Three factors influence the movement of ions into and out of cells: (1) a concentration gradient, ( 2) a
voltage gradient, and ( 3) the structure of the membrane.
• All molecules have an intrinsic kinetic energy called thermal motion, or heat: they move constantly.
Because of thermal motion, they spontaneously spread out from where they are more concentrated to
where they are less concentrated. This spreading out is called diffusion.
• When the substance is not evenly dispersed, the term concentration gradient describes the relative difference in the amount of a substance at different
locations in a container. As illustrated in Figure 4.13A, a little ink placed in water will start out concentrated at the site of first contact, but, even in the
absence of mechanical stirring, the ink will quickly spread away from that site.
• Because ions carry an electrical charge, we can describe their diffusion pattern not only by a concentration
gradient but also by a voltage gradient— the difference in charge between two regions that allows a flow of
current if the two regions are connected. The voltage gradient allows for measuring the relative
concentrations of positive and negative electrical charges in the current across the cell membrane.
• The intracellular and extracellular fluids of a neuron are filled with positively charged ions (cations) of both sodium, Na and potassium, K , as well as
negatively charged anions ( A ) of chlorine, Cl , or chloride ions. Recall that the neural fluids also contain protein anions— large, negatively charged
molecules. In Figure 4.13B, Na and Cl ions move down a voltage gradient from a highly charged area to an area of lower charge, just as they move
down a con-centration gradient from an area of high density to an area of lower density. When salt is dissolved in water, then, it diffuses either by
movement down a concentration gradient as shown in Figure 4.13A or by movement down a volt-age gradient as shown in Figure 4.13B.
• Chloride ions are now permitted to cross the membrane, as shown at the left in Figure 4.14B. The ions
will move down their concentration gradient from the side of the container where they are abundant to the
side of the container from which they were formerly excluded, as shown in the middle of Figure 4.14B.
The Na ions, in contrast, are still unable to cross the membrane. (Although Cl ions are larger than Na
ions, Na ions have a greater tendency to hold on to water molecules; as a result, the Na ions are bulkier
and unable to enter the chloride channels).
• If the only factor influencing the movement of Cl ions were the chloride concentration gradient, the efflux ( outward flow) of Cl ions from the salty to
the unsalty side of the container would continue until Cl ions were in equilibrium on both sides. But this equilibrium is not achieved. Because the Cl
ions carry a negative charge, they are attracted back toward the positively charged Na ions ( opposite charges attract). Consequently, the concentration
of Cl ions remains higher in the left half of the container than in the right half, as illustrated on the right in Figure 4.14B.
• ion channels, gates, and pumps play in five aspects of the cell membrane’s electrical activity: ( 1) the resting potential, ( 2) graded potentials, ( 3) the
action potential, ( 4) the nerve impulse, and ( 5) saltatory conduction.
The Resting Potential
• There is a limit on the number of K ions that accumulate inside the cell because, when the intracellular potassium concentration becomes higher than
the extracellular concentration, K ions start moving out of the cell, down its concentration gradient.
• The equilibrium of the potassium voltage gradient and the potassium con-centration gradient results in some K ions remaining outside the membrane.
Only a few K ions are needed outside the membrane to produce a relative negative charge on the inside of the membrane. As a result, K ions
contribute to the charge across the membrane.
• The high concentration of Na ions outside relative to inside the cell membrane is caused by the action of a sodium– potassium pump ( Na – K pump),
a protein molecule embedded in the membrane that shunts Na ions out of the cell and K ions into it. A neuron membrane’s many thousands of Na – K
pumps work continuously, each one exchanging three intracellular Na ions for two K ions with each pumping action.
• As summarized in Figure 4.15D, the unequal distribution of anions and cations leaves a neuron’s intracellular fluid negatively charged at about 70mV
relative to the fluid outside the cell. Three aspects of the semipermeable cell membrane contribute to this resting potential:
1. Large, negatively charged protein molecules remain inside the cell.
2. Gates keep out positively charged Na ions, and channels allow K and Cl ions to pass more freely.
3. Na – K pumps extrude Na from the intracellular fluid. Graded Potential
• Slight, sudden changes in the voltage of an axon’s membrane are graded potentials, highly localized and restricted to the vicinity on the axon where
they are produced. Just as a small wave produced in the middle of a large, smooth pond disappears before traveling much of a distance, graded
potentials produced on a membrane decay before traveling very far.
• If negative current is applied to the membrane, the membrane potential be-comes more negative by a few millivolts, increasing its polarity. As
illustrated in Figure 4.16A, it may change from the resting potential of 70 mV to a new, slightly higher potential of, say, 73 mV, a hyperpolarization.
Conversely, if the current applied to the membrane is positive, the membrane potential be-comes more positive by a few millivolts, decreasing its
polarity. As illustrated in Figure 4.16B, it may change from a resting potential of 70 mV to a new, slightly lower potential of, say, 65 mV, a
• In each case, electrical stimulation influences ion flow through the gates and channels in the membrane:
For the membrane to become hyperpolarized, the inside must become more negative, which can be accomplished with an efflux of K ions or an influx
of Cl ions.
For the membrane to become depolarized, the inside must become less negative, which can be accomplished by an influx of Na ions.
• Electrical stimulation of the cell membrane at resting potential produces localized graded potentials on the axon. An action potential, in contrast, is a
brief but extremely large flip in the polarity of an axon’s membrane, lasting about 1 ms.
• In an action potential, the voltage across the membrane suddenly reverses, making the inside positive relative to the outside, and then abruptly
reverses again, after which the resting potential is restored. This rapid change in the polarity of the membrane takes place when electrical stimulation
produces a large graded potential that causes the membrane’s potential to depolarize to threshold potential at about 50 mV. At this voltage level, the
membrane undergoes a remarkable change with no further stimulation.
• When threshold potential is reached, the resting voltage of the membrane suddenly drops to 0 mV and then continues to become more positive until
the charge on the inside of the membrane is as great as 30 mV— a total volt-age change of 100 mV. Then, almost as quickly, the membrane potential
re-verses again, returning to its resting potential and then bypassing it and becoming slightly hyperpolarized. This change is a reversal of a little more
than 100 mV. After this second reversal, the membrane gradually returns to its resting potential.
The Role of Voltage-Sensitive Ion Channels
• What cellular mechanisms underlie the movement of Na and K ion