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?
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.
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 pass through the 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 ions
just inside the membrane and lots of positively charged sodium ions PLUS
some potassium ions on the outside. THIS MEANS THAT THERE ARE MORE POSITIVE
CHARGES ON THE OUTSIDE THAN ON THE INSIDE. In other words, there is an
unequal distribution of ions or a resting membrane potential. This potential
will be maintained until the membrane is disturbed or stimulated. Then,
if it's a sufficiently strong stimulus, an action potential will occur.
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.
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 potential
occurs, or it isn't reached and no action potential occurs.
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.
Schwann cells 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 types
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:
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).
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 (also called, as in the figure above, chemically gated ion 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.
This describes what happens when an 'excitatory' neurotransmitter is
released at a synapse. However, not all neurotransmitters are 'excitatory':
Types of neurotransmitters:
2 - Inhibitory - neurotransmitters that make membrane potential more
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 aminobutyric
acid (GABA; shown below). Medically, GABA has been used to treat both epilepsy
and hypertension. Another example of an inhibitory neurotransmitter is
beta-endorphin, which results in decreased pain perception by the CNS.
Used by permission of John W. Kimball
2 - Spatial summation - transmission of an impulse by simultaneous or
nearly simultaneous stimulation of two or more pre-synaptic neurons
Neurons & the Nervous System II
Related & useful links:
Membrane Transport and Bioelectric Activity
Development of Transmembrane Resting Potential
The Physical Factors Behind the Action Potential
Nerve Action Potentials
Saltatory Conduction of Action Potentials
Neurons: Our Internal Galaxy
The Nervous System
Explore the Brain and Spinal Cord
The Animated Brain
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