BIOL 22000 Lecture Notes - Lecture 4: Bronchus, The Reflex, Preganglionic Nerve Fibers
8 Jun 2018
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Chapter 4 The Nervous System
4.1 Cells of the Nervous System
• Neurons are specialized cells capable of transmitting electrical impulses and then
translating those electrical impulses to chemical signals
NEURONS
• The nucleus is located in
the cell body, also called
the soma
o Endoplasmic
reticulum and
ribosomes are
located in the soma
• Dendrites receive
incoming messages from other cells
o Emanating directly from the soma
• The information received from the dendrites is transmitted through the cell body and
reach the axon hillock, which combine the incoming signals
• The axon hillock plays an important role in action potentials, transmission of electrical
impulses down the axon
• Signal received by dendrites can either be excitatory or inhibitory
o If it’s excitatory enough, it will initiate an action potential
• The axon is a long appendage that terminates in close proximity to a target structure
• Myelin prevent signal loss or crossing of signal and increases the speed of conduction
• Myelin sheath maintains the electric signal within one neuron
• Myelin is produced by oligodendrocytes in the central nervous system and Schwann
cells in the peripheral nervous system
• The small breaks in the myelin sheath with exposed area of axon membrane is called
nodes of Ranvier
o nodes of Ranvier are critical for rapid signal conduction
• At the end of the axon is the nerve terminal or synaptic bouton
o Enlarged and flattened to maximize neurotransmission to the next neuron and
ensure proper release of neurotransmitters, the chemicals that transmit
information between neurons
• Neurons are not physically connected to each other, there is a small space between them,
so the terminal of the axon can release neurotransmitter
o The space is called synaptic clef
o Together, the nerve terminal, synaptic clef, and postsynaptic membrane is called
synapse
• Multiple neurons can bundle together to form a nerve in peripheral nervous system
o These nerves can be sensory, motor or mixed (mixed nerve carry both sensory
and motor)
• Multiple neurons can bundle together to form a tract in central nervous system
o Tract only carry one type of information
o The cell bodies of neurons in the same tract are grouped into nuclei
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OTHER CELLS IN THE NERVOUS SYSTEM
• Glial cells, or neuroglia play both structural and supportive roles like myelin
• Astrocytes nourish neurons and form the blood–brain barrier, which controls the
transmission of solutes from the bloodstream into nervous tissue
• Ependymal cells line the ventricles of the brain and produce cerebrospinal fluid, which
physically supports the brain and serves as a shock absorber
• Microglia are phagocytic cells that ingest and break down waste products and pathogens
in the central nervous system
• Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin around axons
4.2 Transmission of Neural Impulses
THE ACTION POTENTIAL
• Neurons use all-or-nothing messages called action potentials to relay electrical impulses
down the axon to the synaptic bouton
• Action potentials release neurotransmitters into the synaptic cleft
Resting Potential
• All neurons exhibit a resting membrane potential
• Potential difference between the
inside of the neuron and the
extracellular space is about -70 mV,
with the inside of the neuron being
more negative
• Neurons use selective permeability
to ions and the Na /K ATPase to
maintain this negative internal
environment
• Like other cell, neuronal plasma
membrane is fairly impermeable to
charged species, because the
membrane contains a thick nonpolar
barrier (fatty acid tails) that is not
favorable for ions to cross
• Inside the neuron, K+ is high and
Na+ is low, and vice versa for the
outside
• The negative resting potential is generated by both negatively charged proteins within the
cell and the relatively greater permeability of the membrane to K+ compared with Na+
• Since K+ is more permeable and it its concentration is greater inside, K+ will diffuse
down its gradient out of the cell
o K is positively charged, so its movement out of the cell results in a cell interior
that is negative
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• The Na+ /K+ ATPase is important for restoring this gradient after action potentials have
been fired
o It transports three Na+ out of the cell for every two K+ into the cell using one
ATP
o ATP is required because both Na+ and K+ is moving against their gradient, aka
primary active transport
o Each time the pump work, the cell become more negative because only two
positive charges are moved in for every three moved out
The Axon Hillock
• Excitatory input causes depolarization (raising the membrane potential, Vm, from its
resting potential) and thus makes the neuron more likely to fire an action potential
• Inhibitory input causes hyperpolarization (lowering the membrane potential from its
resting potential) and thus makes the neuron less likely to fire an action potential.
• If axon hillock receives enough excitatory input to be depolarized, an action potential will
be triggered
o The threshold value is usually in the range of –55 to –40 mV
• Not every stimulus generates a response, and a small excitatory signal maybe not reach
the threshold for a stimulus
• Postsynaptic neuron may receive several excitatory and inhibitory from different
presynaptic neurons
o The additive effects of multiple signals are known as summation
• There are two types of summation: temporal and spatial
o In temporal summation, multiple signals are integrated during a relatively short
period of time from a single presynaptic
▪ Several small excitatory signals firing at nearly the same moment could
bring a postsynaptic cell to threshold, enabling an action potential
o In spatial summation, the additive effects are based on the number and location
of the incoming signals from multiple presynaptic
Ion Channels and Membrane Potential
• If the cell is brought to threshold, voltage-gated sodium channels open in the membrane
• There is a strong electrochemical gradient that promotes the migration of sodium into
the cell
o From an electric standpoint, the interior of the cell is more negative than the
exterior of the cell, which favors the movement of positively charged sodium
cations into the cell
o From a chemical standpoint, there is a higher concentration of sodium outside the
cell than inside, which also favors the movement of sodium into the cell
• As sodium passes through these ion channels, the membrane potential becomes more
positive, so the cell rapidly depolarizes
• Sodium channels open in response to changes in membrane potential, and inactivated by
them
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