Download CHAPTER 4 STRUCTURE AND CELL BIOLOGY OF THE NEURON

CHAPTER 4
STRUCTURE AND CELL BIOLOGY OF THE NEURON
4.1. THE STRUCTURE OF NEURONS
A neuron is a specialized cell, found in the brain, ganglia (groups of neurons located
outside the central nervous system), and some sensory organs. Neurons come in many different
shapes and sizes, but they all have some common features.
4.1.1. Common features of neurons.
Nearly all neurons have a cell body (or soma), an axon, and one or more dendrites.
Figure 4-1. A hypothetical neuron as it might appear viewed under a microscope. The neuron has many dendrites,
but only one axon. However, after it leaves the cell body, the axon may branch extensively to send information to
other neurons.
The cell body contains the nucleus and other organelles. It is the maintenance center of
the neuron. It contains the cell's genetic material as well as the molecular machinery for
synthesizing different chemical substances used for information transfer to other neurons, for
maintenance and repair of the cell, for taking in and generating energy to run the cell's
physiological processes, and for excreting waste products from the cell.
The axon is like a telephone line; it conducts electrical impulses generated at the cell
body and transmits information to other neurons.
The dendrite is like a mailbox; it receives information in the form of chemicals released
by the axon terminals of other neurons, often from many different sources.
4.1.2. Relation of structure to function.
Different neurons perform different functions. Their chemical makeup and morphological
structure varies according to what their task is. For example, a neuron that receives information
from thousands of different sources but sends information to only one place may have a large
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and highly branched dendritic tree, but one small, unbranched axon. A neuron that receives
information from one or a few sources may have a small, minimally branched dendrite.
Figure 4-2. Neurons take many different shapes, but all have the same fundamental structure. The examples shown
here (left to right) include a bipolar cell in the retina with only one dendrite and one simple axon; a motor neuron
with many symmetrical dendrites and a branched axon; a cortical pyramidal cell with one large complex dendrite,
several smaller dendrites and a branched axon; a specialized type of neuron in the cerebellum with an extremely
large dendritic tree but a relatively simple axon.
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Thought question: A neuron in the cochlear nucleus (the first central processing station in the
auditory system) receives strong input from one sensory fiber, but projects to many different
neurons in different parts of the brain. What do you think it might look like?
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4.2. THE ELECTRICAL ACTIVITY OF NEURONS
4.2.1. Properties of the neuron at rest.
When a neuron is not receiving any input there is a potential difference (voltage) between
the inside and outside of the cell. This voltage is just like the voltage you would measure
between two poles of a flashlight battery, but tiny, only a few thousandths of a volt (i.e., a few
millivolts). The potential difference measured when the neuron is inactive is called the resting
membrane potential.
The resting membrane potential is due to several factors:
There are ions (electrically charged chemical substances) both inside and outside the
neuron. An ion is an electrically charged chemical substance such as sodium (Na+), potassium
(K+), calcium (Ca++), or chloride (Cl-). Ions are present in the extracellular space outside the
neuron and in the intracellular space inside the neuron.
The cell membrane is less permeable to some ions than to others, i.e., it is
semipermeable.
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Because of the characteristics of the cell membrane, the concentrations of chloride and
sodium are higher outside the cell, and the concentrations of potassium and organic anions (large
negatively charged molecules such as proteins) are greater inside the cell. Overall, there are more
negatively charged ions inside the cell than outside.
The resting membrane potential is, therefore, negative; it is approximately -70 millivolts
(mV).
Figure 4-3. Schematic diagram to illustrate the distribution of ions inside and outside a neuron. The mushroom-like
structure on the cell membrane is meant to represent an ion channel, a pore in the membrane that lets a certain kind
of ion through under certain circumstances, but closes under other circumstances. Na+ = sodium; Cl- = chloride; K+
= potassium; A- = large negatively charged molecule. The passive ion channel is always open. It allows ions to
diffuse along their concentration gradient (from an area of high concentration to an area of low concentration) or
their electrical gradient (positive ion to an area of negative charge, or vice versa)
An active (energy-driven) mechanism, the sodium-potassium pump, removes three
sodium ions from inside the cell in exchange for every two potassium ions that are brought in
from outside of the cell.
4.2.2. Membrane properties and ion channels.
The cell membrane of a neuron is semipermeable. This means that some ions can go
through but others cannot. The cell membrane contains specialized pores or ion channels that
allow specific ions (e.g., sodium [Na+], potassium [K+], chloride [Cl-], or calcium [Ca++] to
pass through under certain conditions.
There are several different types of ion channels:
Passive ion channels allow ions to pass through the membrane along their concentration
gradient (i.e., from an area of higher concentration to an area of lower concentration).
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Ligand-gated channels (also called chemical-gated channels) open only when another
molecule binds (attaches) to them. They are most commonly opened by the binding of
neurotransmitters released by the axon terminals of other neurons.
Figure 4-4. Ligand-gated ion channels have a binding site for a particular neurotransmitter. When a
neurotransmitter molecule (triangle) binds to the site (open V-shaped slot on channel), it causes the shape of the
channel to change, so that the central pore goes from a closed state to an open state. Once the channel is open, a
specific type of ion can pass through.
Figure 4-5. Voltage-gated ion channels are sensitive to the potential difference between the inside and outside of
the neuron. For the channel illustrated, the pore is closed at resting potential (left). When depolarization occurs
(inside less negative than under resting conditions), the shape of the channel changes and it opens, letting a specific
type of ion pass through (right). If the inside becomes too positive relative to the outside, another shape change
occurs, and the channel closes again.
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Thought question: If neuron A released equal amounts of the same neurotransmitter onto two
other neurons (B and C) when stimulated, how could you design a cell membrane (using
different types of ion channels) that would cause neuron B to become depolarized (more positive
inside), but cause neuron C to become hyperpolarized (more negative inside) whenever neuron A
is active?
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