bioa02 chapter 44

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26 Jun 2011

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Chapter 44
44.1 – What cells are unique to the nervous system?
-nervous systems are composed of two unique categories of cells
onerve cells or neurons and glial cells or glia
-neurons are excitable
ocan generate and propagate electrical signals, which are known as nerve
impulses, or action potentials
ohave long extensions called axons that enable them to conduct action
potentials over long distances
-Glial cells do not conduct action potentials
oThey support neurons physically, immunologically, and metabolically
-A nerve is a bundle of axons that come from many different neurons
-Afferent neurons carry sensory information into the nervous system
oThat information comes from specialized sensory neurons that transducer
(convert) various kinds of sensory input into action potentials
-Efferent neurons carry commands to physiological and behavioral effectors
such as muscles and glands
-Interneurons integrate and store information and facilitate communication
between sensors and effectors
-Numbers of neurons organized into clusters called ganglia
oGanglia serving different functions may be distributed around the body
-Animals that are bilaterally symmetrical, ganglia frequently come in pairs, one on
each side of the body
oOne pair of ganglia is larger than the others, and is therefore given the
designation of brain
-In vertebrates, most cells f the nervous system are found in the brain and the
spinal cord the sites of most information processing, storage and retrieval
oThe brain and the spinal cord are called central nervous system (CNS)
-Information is transmitted from sensory cells to the CNS and from the CNS to
effectors via neurons that extend or reside outside of the brain and the spinal cord
oThese neurons and their supporting cells are called peripheral nervous
system (PNS)
-The human nervous system contains an estimated 10^11 neurons
-Information is passed from one neuron to another where they come into close
proximity at structures called synapses
othe cell that sends the message is the presynaptic neuron
othe cell that receives it is the postsynaptic neuron
-the human brain must contain up to 10^14 synapses, which can be highly plastic,
strengthening with use and weakening with disuse
-neurons behave similarly in animals as different as squids and humans
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-the transmission of action potentials can be rapid, up to 100 meters per second or
-most neurons have four regions
ocell body contains the nucleus and most of the cell’s organelles
many projections may sprout from the cells body
most of these projections are shrub like dendrites
odendrites bring information from other neurons or sensory cells to the
cell body
oaxons usually carry information away form the cell body
length differs among different types of neurons
are the “telephone lines of the nervous system
oat the tip of each of these tiny nerve endings is a swelling called axon
terminal, that comes very close to the target cell to form a synapse
-an action potential arriving at an axon terminal causes the release of chemical
messenger molecules called neurotransmitters, from the axon terminal
othey diffuse across the space and bind to receptors on the plasma
membrane of the postsynaptic or target cell
-There are many more glial cells than neurons in the human brain
-Glial cells are not excitable, and do not transmit electrical signals
oSome physically support and orient the neurons and help them make the
right contract during embryonic development
oOther supply neurons with nutrients, maintain the extracellular
environment, consume foreign particles and cellular debris, or insulate
-The in CNS, some glial cells are called oligodendrocytes, which wrap around
the axons of neurons, covering them with concentric layers of insulating plasma
-In the PNS, glia called Schwann cells perform this function
-Myelin is the covering produced by oligodendrocytes and Schwann cells, and
it gives many parts of the nervous system a glistening white appearance
-Not all axons are myelinated, but those that are can conduct action potential more
rapidly than those axons that are not myelinated
-Glia called astrocytes contribute to the blood-brain barrier, which protects the
brain from toxic chemicals in the blood
oHelp form the blood-brain barrier by surrounding the smallest, most
permeable blood vessels in the brain
oSince the barrier consists of plasma membranes, it is permeable to fat-
soluble substances such as anesthetic and alcohol, which explains why
these substances have such rapid and marked effects on the nervous
44.2 – How do neurons generate and conduct signals?
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-action potentials are generated when ion channels in the plasma membranes of
neurons open for a short time and permit ions to move across the membrane
othe movement of these charged molecules is driven by differences in their
concentration gradients and by electrical charge differences on the two
sides of the membrane
-at rest, the inside of the neuron is electrically negative compared to the outside
-any difference in electric potential across the plasma membrane is membrane
potential, which is measured in millivolts
-when the neuron is resting and not firing action potentials, the membrane
potential is called resting potential
-voltage is a force that causes electrically charged particles to move between
two points
ovoltage is to the flow of electrically charged particles as pressure is to the
flow of water
oin wires, electric current is carried by electrons, but in solutions and across
cell membranes, electric current is carried by ions
othe major ions are Na+, K+, Ca2+, and Cl-
-if one electrode is places inside the plasma membrane of an axon, and another
electrode is placed just outside of the axon, the difference in voltage can be
-the typical resting potential is usually between 60 and 70 millivolts (mV)
-The resting potential provides a means for neurons to respond to a stimulus
-Because of the voltage difference across the membrane, ions would cross the
membrane if they could
oSince the inside of a resting cell is negative, positively charged ions such
as Na+ would enter if they could
oTherefore any chemical or physical stimulus that changes the permeability
of the plasma membrane to ions will produce a change in the cell’s
membrane potential
-The most extreme change in membrane potential is the action potential
oFor 1 or 2 milliseconds, positively charged ions flow into the cell, making
the inside of the cell more positive than the outside
-Ion pumps and channels are responsible for the distribution of charges across the
membrane that create resting and action potentials
-Ion pumps require energy to move ions or other molecules against the
concentration or electrical gradients
-A major ion pump is the sodium-potassium pump, so called because it actively
expels Na+ from inside the cell, exchanging it for K+ from outside the cell
-The Na+-K+ pumps is also known as sodium-potassium ATPase, a term
emphasizing it as an enzyme complex requiring ATP to do its work
-The sodium-potassium pump keeps the concentration of K+ inside the cell greater
than that of the extracellular fluid, and the concentration of Na+ inside the cell
less than that of the extracellular fluid
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