PSYB65H3 Chapter Notes - Chapter 4: Hydrophile, Axon Terminal, Halle Berry

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21 Jul 2016
PSYB65 Chapter 4: The Structure and Electrical Activity of Neurons
The Neuron 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.
Overview of a Neuron:
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”).
The axon may have branches called axon collaterals, which usually emerge from it at right
Toward its end, the axon may divide into a number of smaller branches called teleodendria (“end
At the end of each teleodendrion is a knob called an end foot or terminal button.
The terminal button sits very close to a dendritic spine on another neuron, although it does not
touch that spine.
This “almost connection,” consisting of the surface of the axon’s end foot, the corresponding
surface of the neighboring dendritic spine, and the space between the two, is the synapse.
In contrast with the extensive information-gathering capacity of the dendrites and spines, the
single axon limits the neuron to only one output channel for communication.
The neuron’s cell wall encloses its contents, much as the banks of a river enclose its
The dendrites and the axon are simply fluid-filled extensions of the cell body. Information flows
from the dendrites to the cell body and axon, just as tributaries feed a river.
The axon’s dividing into teleodendria is analogous to the main river channel’s breaking up into a
number of smaller channels at the river delta before discharging its contents into the sea.
At each terminal button, information in the form of a chemical message is released onto a target.
Although information does flow from the dendrites to the cell body and then along the axon, a
neuron does not function simply like an unregulated river system, carrying all the input that it
receives to the delta that disgorges it into the sea.
Rather, a neuron is both an information-collecting and an information processing device. It
receives a great deal of information on its hundreds to thousands of dendritic spines, but it has
only one axon; so the message that it sends must be an averaged or summarized version of all the
incoming signals.
-Thus, the neuron can also be compared to a river system regulated by a dam located at the axon
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hillock. A dam can be opened or closed to allow more water flow at some times and less at
Information that travels through a neuron does not consist of a flow of liquid.
Instead, it travels on a flow of electrical current that begins on the dendrites and travels along
the axon to the terminals. In the axon, the summarized flow consists of discrete electrical
impulses. As each impulse reaches the terminal buttons, they release one or more chemicals.
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
A Cell= miniature factory, with departments that cooperate to make, ship,
and export proteins, the cell’s products.
Proteins are complex organic compounds, including enzymes, hormones, and antibodies, and they
form the principal components of all cells as well.
A factory has outer walls that separate it from the rest of the world and discourage 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.
The cell membrane envelops the cell body, the dendrites and their spines, and the axon and its terminals
and so forms a boundary around a continuous intracellular compartment.
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 rest.
This setup allows the cell to concentrate chemicals where they are needed and otherwise keep
them out of the way.
Prominent among the cell’s internal membranes is the nuclear membrane that surrounds the cell’s
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’ instructions.
Tubule: Tiny tube that transports molecules and helps give the cell its shape
Intracellular fluid: Fluid in which the cell’s internal structures are suspended
Mitochondrion: Structure that gathers, stores, and releases energy
Endoplasmic reticulum: Folded layers of membrane where proteins are assembled
Nuclear membrane: Membrane surrounding the nucleus
Nucleus: Structure containing the chromosomes and genes
Dendritic spine: Small protrusion on a dendrite that increases surface area
Cell membrane: Membrane surrounding the cell
Axon: Extension that transmits information from cell body to other cells
Golgi body: Membranous structure that packages protein molecules for transport
Lysosomes: Sacs containing enzymes that break down wastes
Microfilaments: Threadlike fibers making up much of the cell’s “skeleton”
Dendrite: Cell extension that collects information from other cells
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 system of tubules that carries the
packaged proteins to their final destinations
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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,
lysosomes are saclike vesicles that not only transport incoming supplies but also move and
store wastes.
-Interestingly, more lysosomes are found in old cells than in young ones.
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 composed mainly of water in which salts and many other chemical substances are
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.
Membrane Structure
The cell membrane encases a cell and separates the intracellular from the extracellular 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 cell membrane also regulates the concentrations of salts and other chemicals on either side,
because precise concentrations of chemicals within a cell are essential to its normal function.
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
electrically neutral.
the phospholipid molecules align to form a phospholipid bilayer, the double-layered cell
 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 differences in the electrical polarity of the head and tails of a phospholipid molecule
are the underlying reason why it can form membranes.
The head, being polar, is hydrophilic (from the Greek hydro, meaning “water,” and
philic meaning“love”: literally, “water loving”): it is attracted to water molecules because
they,too, are polar.
The charges on the molecules attract each other.
The nonpolar tails have no such attraction for water.
- they are hydrophobic, or “water hating” (from the Greek phobos, meaning “fear”).
*the head of a phospholipid molecule loves water and the tails hate it.
The tails of both layers point toward the inside of the bilayer, where they are hidden from water.
How the Cell Membrane Functions
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