Neurons & the Nervous System
The human nervous system consists of billions of nerve cells (or neurons)plus supporting (neuroglial) cells. Neurons are able to respond to stimuli (such as touch, sound, light, and so on), conduct impulses, and communicate with each other (and with other types of cells like muscle cells).
The nucleus of a neuron is located in the cell body. Extending out from the cell body are processes called dendrites and axons. These processes vary in number & relative length but always serve to conduct impulses (with dendrites conducting impulses toward the cell body and axons conducting impulses away from the cell body).
Neurons can respond to stimuli and conduct impulses because a membrane potential is established across the cell membrane. In other words, there is an unequal distribution of ions (charged atoms) on the two sides of a nerve cell membrane. This can be illustrated with a voltmeter:
With one electrode placed inside a neuron and the other outside, the voltmeter is 'measuring' the difference in the distribution of ions on the inside versus the outside. And, in this example, the voltmeter reads -70 mV (mV = millivolts). In other words, the inside of the neuron is slightly negative relative to the outside. This difference is referred to as the Resting Membrane Potential. How is this potential established?
The membranes of all nerve cells have a potential difference across them, with the cell interior negative with respect to the exterior (a). In neurons, stimuli can alter this potential difference by opening sodium channels in the membrane. For example, neurotransmitters interact specifically with sodium channels (or gates). So sodium ions flow into the cell, reducing the voltage across the membrane.
Once the potential difference reaches a threshold voltage, the reduced voltage causes hundreds of sodium gates in that region of the membrane to open briefly. Sodium ions flood into the cell, completely depolarizing the membrane (b). This opens more voltage-gated ion channels in the adjacent membrane, and so a wave of depolarization courses along the cell — the action potential.
As the action potential nears its peak, the sodium gates close, and potassium gates open, allowing ions to flow out of the cell to restore the normal potential of the membrane (c) (Gutkin and Ermentrout 2006).
Establishment of the Resting Membrane Potential
Membranes are polarized or, in other words, exhibit a RESTING MEMBRANE POTENTIAL. This means that there is an unequal distribution of ions (atoms with a positive or negative charge) on the two sides of the nerve cell membrane. This POTENTIAL generally measures about 70 millivolts (with the INSIDE of the membrane negative with respect to the outside). So, the RESTING MEMBRANE POTENTIAL is expressed as -70 mV, and the minus means that the inside is negative relative to (or compared to) the outside. It is called a RESTING potential because it occurs when a membrane is not being stimulated or conducting impulses (in other words, it's resting).
What factors contribute to this membrane potential?
Two ions are responsible: sodium (Na+) and potassium (K+). An unequal distribution of these two ions occurs on the two sides of a nerve cell membrane because carriers actively transport these two ions: sodium from the inside to the outside and potassium from the outside to the inside. AS A RESULT of this active transport mechanism (commonly referred to as the SODIUM - POTASSIUM PUMP), there is a higher concentration of sodium on the outside than the inside and a higher concentration of potassium on the inside than the outside (Animation: How the Sodium-Potassium Pump Works).
The nerve cell membrane also contains special passageways for these two ions that are commonly referred to as GATES or CHANNELS. Thus, there are SODIUM GATES and POTASSIUM GATES. These gates represent the only way that these ions can diffuse through a nerve cell membrane. IN A RESTING NERVE CELL MEMBRANE, all the sodium gates are closed and some of the potassium gates are open. AS A RESULT, sodium cannot diffuse through the membrane & largely remains outside the membrane. HOWEVER, some potassium ions are able to diffuse out.
OVERALL, therefore, there are lots of positively charged potassium
just inside the membrane and lots of positively charged sodium ions
some potassium ions on the outside. THIS MEANS THAT THERE ARE MORE
CHARGES ON THE OUTSIDE THAN ON THE INSIDE. In other words, there is an
unequal distribution of ions or a resting membrane potential. This
will be maintained until the membrane is disturbed or stimulated. Then,
if it's a sufficiently strong stimulus, an action potential will occur.
Voltage sensing in a potassium ion channel. a, The control of ion flow through voltage-gated channels is very sensitive to the voltage
across the cell membrane. By comparison, an electronic device such as a transistor is much less sensitive to applied voltage.
b, MacKinnon and colleagues (Zhou et al. 2001) have found that the voltage sensors in a bacterial potassium channel are charged 'paddles'
that move through the fluid membrane interior. Four voltage sensors (two of which are shown here) are linked mechanically to
the channel's 'gate'. Each voltage sensor has four tethered positive charges (amino acids); the high sensitivity of
channel gating results from the transport of so many charges, 16 in all, most of the way across the membrane (From: Sigworth 2003).
In a cross section view of the voltage-dependent potassium channel,
two of the four paddles move up and down, opening and closing the
central pore through which potassium ions flow out of the cell, restoring the
cell's normal negative inside, positive outside polarity.
An action potential is a very rapid change in membrane potential that occurs when a nerve cell membrane is stimulated. Specifically, the membrane potential goes from the resting potential (typically -70 mV) to some positive value (typically about +30 mV) in a very short period of time (just a few milliseconds).
What causes this change in potential to occur? The stimulus causes the sodium gates (or channels) to open and, because there's more sodium on the outside than the inside of the membrane, sodium then diffuses rapidly into the nerve cell. All these positively-charged sodiums rushing in causes the membrane potential to become positive (the inside of the membrane is now positive relative to the outside). The sodium channels open only briefly, then close again.
The potassium channels then open, and, because there is more potassium inside the membrane than outside, positively-charged potassium ions diffuse out. As these positive ions go out, the inside of the membrane once again becomes negative with respect to the outside (Animation: Voltage-gated channels) .
Threshold stimulus & potential
All-or-None Law - action potentials occur maximally or not at
all. In other words, there's no such thing as a partial or weak action
potential. Either the threshold potential is reached and an action
occurs, or it isn't reached and no action potential occurs.
The absolute refractory period places a limit on the rate at which a neuron can conduct impulses, and the relative refractory period permits variation in the rate at which a neuron conducts impulses. Such variation is important because it is one of the ways by which our nervous system recognizes differences in stimulus strength, e.g., dim light = retinal cells conduct fewer impulses per second vs. brighter light = retinal cells conduct more impulses per second.
How does the relative refractory period permit variation in rate of impulse conduction? Let's assume that the relative refractory period of a neuron is 20 milliseconds long and, further, that the threshold stimulus for that neuron (as determined, for example, in a lab experiment with that neuron) is 0.5 volt. If that neuron is continuously stimulated at a level of 0.5 volt, then an action potential (and impulse) will be generated every 20 milliseconds (because once an action potential has been generated with a threshold stimulus [and ignoring the absolute refractory period], another action potential cannot occur until the relative refractory period is over). So, in this example, the rate of stimulation (and impulse conduction) would be 50 per second (1 sec = 1000 ms; 1000 ms divided by 20 ms = 50).
If we increase the stimulus (e.g., from 0.5 volt to 1 volt), what happens to the rate at which action potentials (and impulses) occur? Because 1 volt is an above-threshold stimulus, it means that, once an actional potential has been generated, another one will occur in less than 20 ms or, in other words, before the end of the relative refractory period. Thus, in our example, the increased stimulus will increase the rate of impulse conduction above 50 per second. Without more information, it's not possible to calculate the exact rate. However, it's sufficient that you understand that increasing stimulus strength will result in an increase in the rate of impulse conduction.
Impulse conduction - an impulse is simply the movement of action potentials along a nerve cell. Action potentials are localized (only affect a small area of nerve cell membrane). So, when one occurs, only a small area of membrane depolarizes (or 'reverses' potential). As a result, for a split second, areas of membrane adjacent to each other have opposite charges (the depolarized membrane is negative on the outside & positive on the inside, while the adjacent areas are still positive on the outside and negative on the inside). An electrical circuit (or 'mini-circuit') develops between these oppositely-charged areas (or, in other words, electrons flow between these areas). This 'mini-circuit' stimulates the adjacent area and, therefore, an action potential occurs. This process repeats itself and action potentials move down the nerve cell membrane. This 'movement' of action potentials is called an impulse.
Myelination, the process by which glial cells ensheath the axons of neurons in layers of myelin, ensures the rapid conduction of electrical impulses in the nervous system. The formation of myelin sheaths is one of the most spectacular examples of cell-cell interaction and coordination in nature. Myelin sheaths are formed by the vast membranous extensions of glial cells: Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS). The axon is wrapped many times (like a Swiss roll) by these sheetlike membrane extensions to form the final myelin sheath, or internode. Internodes can be as long as 1 mm and are separated from their neighbors by a short gap (the node of Ranvier) of 1 micrometer. The concentration of voltage-dependent sodium channels in the axon membrane at the node, and the high electrical resistance of the multilayered myelin sheath, ensure that action potentials jump from node to node (a process termed "saltatory conduction") (ffrench-Constant 2004).
Schwann cells (or oligodendrocytes) are located at regular intervals along the process (axons and, for some neurons, dendrites) & so a section of a myelinated axon would look like this:
Between areas of myelin are non-myelinated areas called the nodes of Ranvier. Because fat (myelin) acts as an insulator, membrane coated with myelin will not conduct an impulse. So, in a myelinated neuron, action potentials only occur along the nodes and, therefore, impulses 'jump' over the areas of myelin - going from node to node in a process called saltatory conduction (with the word saltatory meaning 'jumping'):
Because the impulse 'jumps' over areas of myelin, an impulse travels much faster along a myelinated neuron than along a non-myelinated neuron.
Types of Neurons - the three main
of neurons are:
Multipolar neurons are so-named because they have many (multi-) processes that extend from the cell body: lots of dendrites plus a single axon. Functionally, these neurons are either motor (conducting impulses that will cause activity such as the contraction of muscles) or association (conducting impulses and permitting 'communication' between neurons within the central nervous system).
Unipolar neurons have but one process from the cell body. However, that single, very short, process splits into longer processes (a dendrite plus an axon). Unipolar neurons are sensory neurons - conducting impulses into the central nervous system.
Bipolar neurons have two processes - one axon & one dendrite. These neurons are also sensory. For example, biopolar neurons can be found in the retina of the eye.
Neuroglial, or glial, cells - general functions include:
1 - forming myelin sheaths
2 - protecting neurons (via phagocytosis)
3 - regulating the internal environment of neurons
in the central nervous system
Synapses usually occur between the axon of a pre-synaptic neuron & a dendrite or cell body of a post-synaptic neuron. At a synapse, the end of the axon is 'swollen' and referred to as an end bulb or synaptic knob. Within the end bulb are found lots of synaptic vesicles (which contain neurotransmitter chemicals) and mitochondria (which provide ATP to make more neurotransmitter). Between the end bulb and the dendrite (or cell body) of the post-synaptic neuron, there is a gap commonly referred to as the synaptic cleft. So, pre- and post-synaptic membranes do not actually come in contact. That means that the impulse cannot be transmitted directly. Rather, the impulse is transmitted by the release of chemicals called chemical transmitters (or neurotransmitters).
Micrograph of a synapse (Schikorski and Stevens 2001).
Post-synaptic membrane receptors
When an impulse arrives at the end bulb, the end bulb membrane becomes more permeable to calcium. Calcium diffuses into the end bulb & activates enzymes that cause the synaptic vesicles to move toward the synaptic cleft. Some vesicles fuse with the membrane and release their neurotransmitter (a good example of exocytosis). The neurotransmitter molecules diffuse across the cleft and fit into receptor sites in the postsynaptic membrane. When these sites are filled, sodium channels open & permit an inward diffusion of sodium ions. This, of course, causes the membrane potential to become less negative (or, in other words, to approach the threshold potential). If enough neurotransmitter is released, and enough sodium channels are opened, then the membrane potential will reach threshold. If so, an action potential occurs and spreads along the membrane of the post-synaptic neuron (in other words, the impulse will be transmitted). Of course, if insufficient neurotransmitter is released, the impulse will not be transmitted.
Impulse transmission - The nerve impulse (action potential) travels down the presynaptic axon towards the synapse, where it activates voltage-gated calcium channels leading to calcium influx, which triggers the simultaneous release of neurotransmitter molecules from many synaptic vesicles by fusing the membranes of the vesicles to that of the nerve terminal. The neurotransmitter molecules diffuse across the synaptic cleft, bind briefly to receptors on the postsynaptic neuron to activate them, causing physiological responses that may be excitatory or inhibitory depending on the receptor. The neurotransmitter molecules are then either quickly pumped back into the presynaptic nerve terminal via transporters, are destroyed by enzymes near the receptors (e.g. breakdown of acetylcholine by cholinesterase), or diffuse into the surrounding area.
This describes what happens when an 'excitatory' neurotransmitter is
released at a synapse. However, not all neurotransmitters are
Types of neurotransmitters:
2 - Inhibitory - neurotransmitters that make membrane potential
negative (via increased permeability of the membrane to potassium) &,
therefore, tend to 'inhibit' (or make less likely) the transmission of
an impulse. One example of an inhibitory neurotransmitter is gamma
acid (GABA; shown below). Medically, GABA has been used to treat both
and hypertension. Another example of an inhibitory neurotransmitter is
beta-endorphin, which results in decreased pain perception by the CNS.
Neurotransmitters (acetylcholine described starting at about 2:55)
Used by permission of John W. Kimball
2 - Spatial summation - transmission of an impulse by simultaneous
nearly simultaneous stimulation of two or more pre-synaptic neurons
Used by permission of John W. Kimball
Neurons & the Nervous System II
Related & useful links:
The Physical Factors Behind the Action Potential
Explore the Brain and Spinal Cord
The Animated Brain
ffrench-Constant, C., H. Colognato, and R. J. M. Franklin. 2004. Neuroscience: the mysteries of myelin unwrapped. Science 304:688-689.
Goodlett, C.R., and K. H. Horn. 2001. Mechanisms of alcohol-induced damage to the developing nervous system. Alcohol Research & Health 25:175–184.
Gutkin, B. and G. B. Ermentrout. 2006. Neuroscience: spikes too kinky in the cortex? Nature 440: 999-1000.
Schikorski, T. and C. F. Stevens. 2001. Morphological correlates of functionally defined synaptic vesicle populations. Nature Neuroscience 4: 391-395.
Sigworth, F. J. 2003. Structural biology: life's transistors. Nature 423:21-22.
Zhou, M., João H. Morais-Cabral, Sabine Mann and Roderick MacKinnon. 2001. Potassium channel receptor site for the inactivation gate and quaternary amine inhibitors. Nature 411:657-661.
to BIO 301 syllabus
Lecture Notes 1 - Cell Structure & Metabolism
Lecture Notes 2b - Neurons & the Nervous System II
Lecture Notes 3 - Muscle
Lecture Notes 4 - Blood and Body Defenses I
Lecture Notes 4b - Blood and Body Defenses II
Lecture Notes 5 - Cardiovascular System
Lecture Notes 6 - Respiratory System