Transcript Diversity in neural signaling - Evans Laboratory: Environmental

ANIMAL
PHYSIOLOGY
BIOL 3151:
Principles of Animal
Physiology
Dr. Tyler Evans
Email: [email protected]
Phone: 510-885-3475
Office Hours: M,W 10:30-12:00 or appointment
Website: http://evanslabcsueb.weebly.com/
LAST
LECTURE
TRANSMISSION AT THE NEUROMUSCULAR JUNCTION
• when an action potential reaches the axon terminal of the neuromuscular
junction it triggers calcium (Ca+2) channels to open
• the concentration of Ca+2 inside the neuron is much lower than outside, so
Ca+2 moves into the neuron along its concentration gradient
• this increase in internal Ca+2 concentration triggers the release of SYNAPTIC
VESICLES, synaptic vesicles contain neurotransmitters, which are then
released across the synapse
Textbook
Fig 4.16
pg 162
LAST LECTURE
Diversity in Neurophysiology
• although all neurons have the same basic components, each of these
components has been modified by evolution to better perform specific tasks
• all neurons have DENDRITES, a CELL BODY (SOMA) and an AXON, but details of
each structure are variable
EXAMPLES OF NEURON DIVERSITY
textbook Fig 4.18 pg 166
LAST LECTURE
Diversity in Neurophysiology
• OHM’S LAW describes the speed of an action potential traveling
down an axon
• speed or CURRENT (I) of the signal depends on two variables:
voltage and resistance
V= IR
voltage
current
or
V
I=
R
resistance
• essentially, the strength of the signal along an axon (current) is
greatest when voltage (input energy) is high and resistance is low
LAST LECTURE
Diversity in Neurophysiology
Altering myelination and axon diameter
• fastest conduction occurs in either large diameter axons or myelinated axons
textbook Table 4.3 pg. 172
LAST LECTURE
Diversity in Neurophysiology
Altering myelination and axon diameter
• squid giant axons can obtain rapid conduction speed in the absence of
myelination by increasing the diameter of its axons
• squid giant axons are
hundreds of times larger in
diameter than mammalian
axons
• squid axons can be 1 mm
in diameter, while most
mammalian axons are 5µm
in diameter
Diversity in Neurophysiology
Altering myelination and axon diameter
• in squid, giant axons are found in the neurons that stimulate muscle
contraction around the mantle cavity
• squid mantle cavity is used for jet propulsion locomotion
• squid can expand and contract the mantle, drawing water into the mantle
cavity and rapidly expelling it through the siphon
water intake
mantle cavity
Siphon (output)
NEUROPHYSIOLOGY: Diversity in neural signaling
Altering myelination and axon diameter
• this form of locomotion allows the squid to move very fast, but in order for this
jet propulsion to work properly, muscle fibers throughout the entire mantle
must contract at THE SAME TIME!
• however some parts of the mantle are much further away from the squid’s
central nervous system than others
CNS
• in order for signals to reach all parts of the
mantle at the same time, action potentials
must be conducted faster in order to reach
distal parts of the mantle at the same time
as action potentials controlling the mantle
muscles close to the CNS
NEUROPHYSIOLOGY: Diversity in neural signaling
How do squid ensure that signals needing to travel
only a short distance reach target muscles AT THE
SAME TIME as those traveling longer distances
CNS
NEUROPHYSIOLOGY: Diversity in neural signaling
ANSWER:
• axons that activate muscles at the far end of the mantle have VERY LARGE
DIAMETERS
• axons that activate muscles in the region of the mantle closest to the central
nervous system have smaller diameters
• combining axons of various diameters allows the near-simultaneous contraction
of the mantle
Mantle Contraction in the squid Loligo
Textbook Fig 4.24 pg 177
NEUROPHYSIOLOGY: Diversity in neural signaling
Why do large diameter axons conduct signals faster?
• the effects of RESISTANCE explain why larger diameter axons, including squid
giant axons, conduct signals more rapidly that small diameter axons
• recall from last lecture:
• OHM’S LAW describes the relationship between current, voltage and
resistance
V= IR
voltage
current
or
V
I=
R
resistance
• essentially, the strength of the signal along an axon (current) is greatest when
voltage (input energy) is high and RESISTANCE IS LOW
NEUROPHYSIOLOGY: Diversity in neural signaling
• membrane resistance and intracellular resistance both decrease as
axon diameter increases (inversely proportional)
• so large diameter axons conduct signals faster by reducing resistance
• resistance is caused by ions “leaking” across membranes and less
“leaking” occurs in large diameter axons.
V
I=
R
• by keeping R low, large
diameter axons increase
conduction speed
NEUROPHYSIOLOGY: Diversity in neural signaling
Consequences of Increasing Axon diameter
• although increasing axon diameter provides faster conduction
velocity, there are two main disadvantages to using large axons to
increase conduction velocity:
1. Large axons take up more space and
this may limit the number of neurons
that can be packed into the nervous
system
• mammals have nervous
systems densely packed with
neurons and accessory cells
and cannot afford to lose
additional space
textbook Fig 4.19 pg. 168
NEUROPHYSIOLOGY: Diversity in neural signaling
Consequences of Increasing Axon diameter
• although increasing axon diameter provides faster conduction
velocity, there are two main disadvantages to using large axons to
increase conduction velocity:
2. Large diameter axons
have a much larger
volume of cytoplasm
per unit length, making
them energetically
expensive to make and
maintain.
• cytoplasm contains all of the organelles (e.g. mitochondria) and
many energy demanding processes occur in the cytoplasm
textbook Fig 4.25 pg. 179
NEUROPHYSIOLOGY: Diversity in neural signaling
Consequences of Increasing Axon diameter
• as a result, giant axons are typically used when extremely high speed
conduction is necessary for survival.
• in squid the axons control the rapid contraction of the mantle used
during escape and prey capture behaviors
• Not all muscles in
the mantle will
contract during
basic swimming
NEUROPHYSIOLOGY: Diversity in neural signaling
Myelinated Axons
• myelinated neurons are found only in vertebrates, though lampreys
and hagfish (very old lineage) lack myelin sheaths
• although some invertebrates have “wrappings” that act similar to
vertebrate myelin sheaths, these membrane wrappings are not as
effective in signal conduction
NEUROPHYSIOLOGY: Diversity in neural signaling
Myelinated Axons
•
•
•
•
true myelin sheaths were an important evolutionary innovation
allowed rapid signal conduction in a small amount of space
helped to provide conditions for complex vertebrate nervous systems
more complex nervous systems allowed animals to evolve more complex
behavior, physiology, social systems, etc.
Mammals
(lots of myelin)
Lampreys/Hagfish
(no myelin)
NEUROPHYSIOLOGY: Diversity in neural signaling
Myelinated Axons
• nervous systems that rely on large diameter axons and less effective axon
wrappings are simple relative to complex nervous systems seen in vertebrates
• invertebrates have simple nervous systems that typically lack a central
processing center like a brain
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• recall that an action potential in a neuromuscular axon terminates by triggering
the release of the neurotransmitter ACETYLCHOLINE, which ultimately triggers
muscle contraction.
Textbook
Fig 4.17
pg 163
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• however, SYNAPTIC TRANSMISSION is incredibly diverse
• animals use different types of transmission to accomplish
different tasks
• some neurons don’t even use
neurotransmitters in signaling, but are
instead are directly connected to their
target cells via GAP JUNCTIONS
• synapses where pre- and
post-synaptic cells are
connected by gap junctions
are called ELECTRICAL
SYNAPSES
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• in ELECTRICAL SYNAPSES, signals are sent directly from the presynaptic cell to
the postsynaptic cell via GAP JUNCTIONS
• in CHEMICAL SYNPASES, the signal is transmitted via chemical neurotransmitters,
which cross the SYNAPTIC CLEFT and bind to a receptor on the postsynaptic cell
to induce a response.
textbook Fig 4.26 pg 181
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• ELECTRICAL SYNAPSES and CHEMICAL SYNAPSES differ in a number of ways:
1. DIRECTION OF FLOW OF INFORMATION
• in chemical synapses, information flows in a single direction, that is from
the presynaptic cell to the post-synaptic cell
e.g. vertebrate
motor neuron
(Fig 4.2 pg. 145)
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• ELECTRICAL SYNAPSES and CHEMICAL SYNAPSES differ in a number of ways:
1. DIRECTION OF FLOW OF INFORMATION
• in electrical synapses, information can flow in BOTH DIRECTIONS
• because pre- and post-synaptic cells are directly connected
textbook Fig 4.26 pg 181
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• ELECTRICAL SYNAPSES and CHEMICAL SYNAPSES differ in a number of ways:
2. SPEEED OF TRANSMISSION
• electrical synapses are fast
• chemical synapses are slower because of delays associated with the
diffusion of neurotransmitters across the synaptic cleft and their binding
to receptors on the post-synaptic neuron
textbook Fig 4.26 pg 181
What type of signal would be involved in
reflexes?
Chemical OR Electrical
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• there is large diversity in the types of neurotransmitters (see Table 4.4 pg 184)
• collectively, these features allow for more complex signaling events to occur
• the proportions of electrical to chemical synapses differs among organisms
What type of
synaptic
transmission
would you
expect
mammals to use
most often?
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• major benefit to the use of chemical signals is the diversity of signals that can
be conducted:
• there is a lot of variation in both the structure of chemical synapses and in the
types of neurotransmitters
TYPES OF SYNAPSE
• axon terminal, axon vericosities, en passant synapse and spine synapse all
refer to different synapse structures and therefore different functions in cells
textbook Fig 4.27 pg 182
NEUROPHYSIOLOGY: Diversity in neural signaling
Variation in synaptic transmission
• each of these types of synapse can also be found in different locations along
the neuron
LOCATIONS OF SYNAPSE
•
dendritic synapse, axoaxonic synapse, axosomatic synapse and
axodendritic synapse all refer to different synapse locations
textbook Fig 4.27 pg 182
NEUROPHYSIOLOGY: Diversity in neural signaling
Neurons are Unique to Animals
• neurons are only found in animals, though plants do have sensory receptors, ion
channels, action potentials, and can process information.
• the Venus Flytrap catches prey when sensory hairs are stimulated
• triggers an action potential which spreads through cells via PLASMODESMATA
very similar to GAP JUNCTIONS, causing the trap to shut
LECTURE SUMMARY
• increases in axon diameter and axon myelination increase the speed of
conduction
• squid use giant axons to ensure mantle tissue contracts simultaneously
• large axons take up more space and require more energy to maintain
• vertebrates use myelination to increase conduction speed an innovation that
helped to develop complex nervous systems
• neural signal are also varied by differences in synaptic transmission
• animals can alter the types of synaptic transmission (electrical or chemical), the
structure of the synapse, the location of the synapse and the type of
neurotransmitters
• this diversity in neural signaling is necessary for the complex nervous systems
found in vertebrates
NEXT LECTURE:
Cellular Movement and Muscles
(Chapter 5)