Download last lecture neurophysiology - Evans Laboratory: Environmental

ANIMAL
PHYSIOLOGY
BIOL 3151:
Principles of Animal
Physiology
Dr. Tyler Evans
Email: [email protected]
Phone: 510-885-3475
Office Hours: M 8:30-11:30 or appointment
Website: http://evanslabcsueb.weebly.com/
LAST LECTURE
NEUROPHYSIOLOGY
• each neuron has specialized regions that perform specific tasks:
• these tasks are: reception, integration, conduction or transmission
• e.g. VERTEBRATE MOTOR NEURON (Fig 4.2 pg. 145)
LAST LECTURE
NEUROPHYSIOLOGY
WHAT TYPES OF ELECTRICAL SIGNALS ARE SENT BY NEURONS?
1.
GRADED POTENTIALS: weak signals that occur in the soma and decrease in
strength they get further away from the opened channel
e.g. Graded potential created by ligand gated sodium channel
several properties of the neuron
influence why a graded potential
decreases as it travels:
• leakage of ions across membrane
• electrical resistance of cytoplasm
• electrical properties of
membrane
textbook Fig 4.6 pg 151
LAST LECTURE
NEUROPHYSIOLOGY
WHAT TYPES OF ELECTRICAL SIGNALS ARE SENT BY NEURONS?
1.
GRADED POTENTIALS: weak signals that occur in the soma and decrease in
strength they get further away from the opened channel
e.g. Graded potential created by ligand gated sodium channel
• more sodium outside of
cells than inside.
• when ligand-gated Na
channels open, sodium
enters cells and intracellular
regions become more
positively charged.
• at +60mV there is no longer
a gradient driving sodium
inward.
textbook Fig 4.4 pg 148
LAST LECTURE
NEUROPHYSIOLOGY
WHAT TYPES OF ELECTRICAL SIGNALS ARE SENT BY NEURONS?
2. ACTION POTENTIALS: are stronger signals used to transmit information over
longer distances without degrading
textbook
Fig 4.10
pg 155
LAST LECTURE
NEUROPHYSIOLOGY
WHAT TYPES OF ELECTRICAL SIGNALS ARE SENT BY NEURONS?
2. ACTION POTENTIALS: are stronger signals used to transmit information over
longer distances without degrading
• the three phases of an action potential are driven by the opening and closing of
ion channels
textbook Fig 4.10 pg 155
• Na+ channels open during
depolarization and Na+ enters cell
• K+ channels open during
repolarization and K+ exits cells,
making interior more negatively
charged than exterior
• K+ channels close slowly which
causes the hyperpolarization
response
• membrane returns to resting
potential.
NEUROPHYSIOLOGY
TRANSMISSION ACROSS THE SYNAPSE
• once the action potential reaches the AXON TERMINAL, the neuron must
transmit the signal across the SYNPASE to the target cell
• the cell that transmits the signal is called the PRE-SYNAPTIC CELL and the cell
that receives the signal is referred to as the POST-SYNAPTIC CELL
• the space between the pre-synaptic and post-synaptic cell is the SYNAPTIC
CLEFT.
• collectively, these three components make up the SYNPASE
textbook Fig 4.2 pg 145
NEUROPHYSIOLOGY
TRANSMISSION ACROSS THE SYNAPSE
• in the example we have been describing so far, the vertebrate motor neuron,
the synapse forms at a NEUROMUSCULAR JUNCTION
NEUROPHYSIOLOGY
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
NEUROPHYSIOLOGY
TRANSMISSION AT THE NEUROMUSCULAR JUNCTION
• the main neurotransmitter released at vertebrate neuromuscular junctions is
ACETYLCHOLINE
• acetylcholine is released from synaptic vesicles and binds to specific cell
surface receptors in the membranes of post-synaptic cells
• acetylcholine binds to receptors to induce muscle contraction
• the enzyme ACETYLCHOLINESTERASE removes acetylcholine from its receptor
to terminate the signal.
Textbook
Fig 4.17
pg 163
NEUROPHYSIOLOGY
PATHOLOGIES THE NEUROMUSCULAR JUNCTION
• strength of contraction is determined by two factors:
1. amount of neurotransmitter released
2. number of receptors on target cells
• if the amount of neurotransmitter or density of receptors is high a strong
muscle contraction will result. In contrast, a weak muscle contraction will
result when amount of neurotransmitter or density of receptors is low
• disease called MYASTHENIA GRAVIS occurs when muscles contain a reduced
number of acetylcholine receptors
• experience muscle weakness and muscle fatigue
• weakened eye muscles can cause
a drooping eyelid or PTOSIS, a
common symptom
NEUROPHYSIOLOGY
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
NEUROPHYSIOLOGY
Diversity in Neurophysiology
Brain, Sensory or Muscle?
NEUROPHYSIOLOGY
Diversity in Neurophysiology
1. SENSORY NEURONS
dendrites
receptors
• sensory neurons are found in animal
senses: sight, hearing, touch, taste,
smell
• at one end of the neuron is a receptor
that is associated with that particular
sense
• for example, olfactory receptors
involved in smell are activated by
airborne chemicals
• at the other end are lots of dendrites
that allow sensory neuron to connect
to the brain for processing
NEUORPHYSIOLOGY
Diversity in Neurophysiology
2. BRAIN NEURONS
• this type of neuron is called an
INTERNEURON
• have large numbers of dendrites
on both ends to maximize
connections with other neurons
• often lack an obvious axon
because are only transmitting
signals over short distances
between other neurons densely
packed in the brain
NEUROPHYSIOLOGY
Diversity in Neurophysiology
3. VERTEBRATE MOTOR NEURONS
• have long axons covered in
MYELIN SHEATH that allows
signals to travel long distances
• one end branches into
NEUROMUSCULAR JUNCTIONS
• other end has lots of dendrites
for connecting muscle to brain or
spinal cord
NEUROPHYSIOLOGY
Diversity in Neurophysiology
• diversity in neural signaling can also be achieved by varying cells associated with
each neuron. These accessory cells are called GLIAL CELLS
• in vertebrates, there are five types of glial cells:
1.
2.
3.
4.
5.
SCHWANN CELLS: form MYELIN SHEATHS and are associated with
neurons with long axons. Increase the conduction speed and prevent
the decay of action potentials.
OLIGODENDROCYTES: form myelin sheaths in the central nervous
system.
ASTROCYTES: found in central nervous system and have a number of
functions including transport of nutrients and neuron development.
MICROGLIA: are the smallest glial cells and are involved in neuron
maintenance (e.g. removes debris and dead cells)
EPENDYMAL CELLS: found in fluid-filled cavities of central nervous
system. They are often CILIATED (tiny-hairs) and circulate spinal fluid.
NEUROPHYSIOLOGY
Diversity in Neurophysiology
1.
SCHWANN CELLS: form MYELIN SHEATHS and are associated with neurons
with long axons.
• myelin increases the conduction speed and prevent the decay of action
potentials.
• Schwann cells wrap
around an axon many
times to form the
myelin sheath
Fig 4.14 pg. 160
NEUROPHYSIOLOGY
Diversity in Neurophysiology
• invertebrates lack a true myelin sheath, but are instead
wrapped in membranes of glial cells called GLIOCYTES
THE EVOLUTION OF MYELIN SHEATHS
textbook Box 4.2 pg 170
• certain invertebrate neurons are wrapped in multiple layers of cell
membrane similar in appearance to vertebrate myelin sheaths
• includes nerve fibers in the ventral nerve cords of shrimp, crabs and
earthworms.
NEUROPHYSIOLOGY
THE EVOLUTION OF MYELIN SHEATHS
• protein complexes called SEPTATE JUNCTIONS hold cells in place as they wrap
around invertebrate axons.
• this structure suggests invertebrate wrappings play a similar role to vertebrate
myelin sheaths, but evolved independently
INVERTEBRATE
VERTEBRATE
NEUROPHYSIOLOGY
THE EVOLUTION OF MYELIN SHEATHS
Invertebrate vs. Vertebrate Axon Wrappings
• the layers of membrane in the invertebrate wrappings are not as closely stacked
as vertebrate layers of myelin sheath
• protein composition of myelin sheath is different
• proteins critically important forming myelin sheath are largely missing from
invertebrate axon wrappings
• wrappings likely evolved separately by CONVERGENT EVOLUTION
e.g. wings of birds,
reptiles and mammals are
all used for flying, but
evolved independently
and thus have different
structures
textbook Box 4.2 pg 170
NEUROPHYSIOLOGY
Diversity in Neurophysiology
• neurons also differ in the speed at which signals are transmitted
• axons conduct action potentials at different speeds: some quickly, some slowly
• speed of action potentials are influenced by two variables:
1. PRESENCE OF MYELIN
2. AXON DIAMETER
textbook Table 4.3 pg. 172
NEUROPHYSIOLOGY
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
NEUROPHYSIOLOGY
Diversity in Neurophysiology
• resistance (the force opposing conduction) is applied by different
components of the cell
textbook
Fig 4.20
pg. 173
each component
is has a different
resistance value,
which will
reduce the
strength of the
signal over space
NEUROPHYSIOLOGY
Diversity in Neurophysiology
• MYELIN SHEATH prevents the signal from traveling through these
areas of high resistance
• as a result, current or conduction speed increases
NEUROPHYSIOLOGY
Diversity in Neurophysiology
• the same can be said for large diameter axons
• less of the axon surface area is exposed to the membrane and cytoplasm that
slows down the signal, so resistance is low
• as a result, current or conduction speed increases
LECTURE SUMMARY
• once the action potential reaches the AXON TERMINAL, the neuron must
transmit the signal across the SYNPASE to the target cell
• in muscles involves the flow of calcium and the neurotransmitter
ACETYLCHOLINE
• all neurons have DENDRITES, a CELL BODY (SOMA) and an AXON, but details of
each structure are variable
• interneurons in the brain have very short axons and many dendrites
• sensory neurons have a receptor on one end and dendrites on the other
• motor neurons have neuromuscular junctions
• SCHWANN CELLS form MYELIN SHEATHS that prevent the decay of action
potentials when traveling down the axon
• OHM’S LAW describes the speed of an action potential traveling down an axon
• signals travel fastest down myelinated and large diameter axons because
resistance is lowered