Lecture 25 Neurons Flashcards
Rundown of nervous system
● The nervous system transmits information between specific locations
● The information conveyed depends on a
signal’s pathway, not the type of signal
● Nerve signal transmission is very fast
Neuron structure
The neuron is a cell that exemplifies the close fit between form and function 7
● Cell body - Most of organelles
● Dendrites, highly branched
extensions that receive signals from other neurons
● The axon is often a much longer
extension that transmits signals to other cells at synapses
● The cone-shaped base of an axon
is called the axon hillock
Structural diversity of neurons
sensory neurons, interneurons, motor neurons
Sensory neurons
transmit information about external stimuli such as light, touch, or smell
Interneurons
integrate (analyze and interpret) the
information
Motor neurons
transmit signals to muscle cells,
causing them to contract
Synapse
is a junction between an axon and another cell
Synapse passes info through neurotransmitters (short distance: chemical)
neurotransmitters
The synaptic terminal of one axon passes
information across the synapse in the form of chemical messengers
Transmitting a signal (long distance)
silde 10
Complex nervous system: Organisation
● Central nervous system (CNS),
where integration takes place;
○ brain or simpler clusters called
ganglia
○ spinal cord
● Peripheral nervous system (PNS),
which carries information into and out
of the CNS
● Neurons of both the CNS and PNS
require supporting cells called glial
cells, or glia
Central nervous system (CNS)
where integration takes place;
○ brain or simpler clusters called ganglia
○ spinal cord
Peripheral nervous system (PNS)
which carries information into and out
of the CNS
● Neurons of both the CNS and PNS
require supporting cells called glial
cells, or glia
Membrane potential
Every cell has a voltage (difference in electrical charge) across its plasma
membrane
Changes in membrane potential can be graded or action potentials
resting potential
is the membrane potential of a
neuron not sending signals
Changes in membrane potential can be graded or action potentials
The plasma membrane is decorated with
proteins
slide 13
3 major ways to transport molecules across membranes
- Facilitated diffusion
- Active transport
- Bulk transport (exocytosis and endocytosis)
Facilitated diffusion: channels
Facilitated diffusion does not require energy
A channel, or a carrier protein, allows passive diffusion to occur through the protein
Active transport
● But cells aren’t simply at equilibrium with their environment!
● E.g. different concentrations of Na+
and K+ ions inside and outside the cell.
● These “electrochemical gradients”are necessary for the transmission of nerve impulses.
Active transport: the Na+/K+ pump
● Transport “up” the concentration gradient for each ion.
● Requires energy (hydrolysis of ATP)
● Phosphorylation / dephosphorylation drive conformational change
how the electrochemical gradient is maintained
What is Active transport
moves substances against their concentration gradients
Active transport requires energy, usually in the form of ATP.
Active transport allows cells to maintain concentration gradients that differ from their surroundings.
Membrane potential
slide 19
Formation of the Resting Potential
● In most neurons, the concentration of K+ is higher inside the cell, while the concentration of Na+ is higher outside the cell.
● Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane.
● These concentration gradients represent chemical potential energy.
Sodium-potassium pump
- Cytoplasmic Na+binds to the sodium- potassium pump. The affinity for Na+
is high when the protein has this
shape.
2.Na+ binding stimulates phosphorylation by ATP.
3.Phosphorylation leads to a change in
protein shape, reducing its affinity for Na+, which is released outside.
4.The new shape has a high affinity for K+
, which binds on the extracellular side and triggers release of the phosphate group.
5.Loss of the phosphate group restores the protein’s original shape, which has
a lower affinity for K+
6.K 6 + is released; affinity for Na+
is high again, and the cycle
repeats
(slide 21 and 22 and 23)
cycle on slide 24
IMPORTANT sodium potassium pump
slide 25
Facilitated diffusion: channels
Facilitated diffusion does not require energy
A channel, or a carrier protein, allows passive diffusion to occur through the protein
The diffusion of solutes across a membrane
The condition in which no net ionic flux
occurs across a membrane because
concentration gradient is in exact balance.
(slide 28)
The diffusion of ions across a membrane
The condition in which no net ionic flux
occurs across a membrane because
concentration gradients and opposing
transmembrane potentials are in exact
balance.
(slide 29)
Membrane selectively permeable to K+
slide 30 and 31
Ion channels
slide 32
Ion Channel diversity
Nongated channels, voltage-gated channels, chemically-gated channels
Nongated channels
Nongated channels are responsible for the resting membrane potential
Voltage-gated channels
Voltage-gated channels are responsible for generation and propagation of the action potential, the outgoing signal from the neuron
Chemically-gated Channels
Chemically-gated channels are responsible for synaptic potentials, the incoming signals to the neuron
Ion channels
slide 35 (nongated channels)
Ion channels with a K+ channel
slide 36 (nongated channels)
Ion channels; Resting potential dominated by K+
slide 37 (nongated channels)
Neuronal resting potential: -70
slide 38 (nongated channels)
The cell changes its potential
slide 39 (nongated channels)
Resting potential
slide 40 (nongated channels)
Voltage-gated ion channel
slide 42
gate closed: no ions flow across membrane
gate open: ions flow through channel
What triggers Depolarization
opening other types of ions channels
(voltage-gated channels)
Depolarizatoin
a reduction in the magnitude of the membrane potential
For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell
(voltage-gated channels)
Graded potentials
are changes in polarization where the magnitude of the change varies with the strength of the stimulus
(voltage-gated channels)
Action potential
If a depolarization shifts the membrane
potential sufficiently, it results in a
massive change in membrane voltage
(voltage-gated channels)
Threshold
Action potentials have a constant
magnitude, are all-or-none, and transmit
signals over long distances.
They arise because some ion channels
are voltage-gated, opening or closing
when the membrane potential passes a
certain level
(voltage-gated channels)
Voltage-gated ion channel
slide 47 and 48
(voltage-gated channels)
Resting Stage
slide 49
(voltage-gated channels)
Stimulation
slide 50
(voltage-gated channels)
Depolarisation
slide 51
(voltage-gated channels)
Repolarisation
slide 52
(voltage-gated channels)
Hyperpolarisation
slide 53
(voltage-gated channels)
Hyperpolarization
When gated K+ channels open, K+
diffuses out, making the inside of the cell more negative
an increasing in magnitude of the membrane potential
(voltage-gated channels)
Action potentials “travel” along the axon
slide 57
(voltage-gated channels)
Conduction of Action Potentials
● At the site where the action potential is generated (usually the axon hillock), an electrical current depolarizes the neighboring region of the axon
membrane.
● Action potentials travel in only one direction: toward the synaptic terminals.
● Inactivated Na+
channels behind the zone of
depolarization prevent the action potential from traveling backwards.
(voltage-gated channels)
Action potentials “travel” along the axon
slide 59
(voltage-gated channels)
Conduction of an action potential
slide 60
(voltage-gated channels)
Conduction of an action potential
slide 61
(voltage-gated channels)
Conduction of an action potential
slide 62
(voltage-gated channels)
Conduction of an action potential
slide 63
(voltage-gated channels)
Myelinated axons
● The electrical insulation that surrounds vertebrate axons is called a myelin
sheath
● Produced by glia: oligodendrocytes in the CNS and Schwann cells in the
PNS.
● During development, these specialized glia wrap axons in many layers of
membrane.
● The membranes forming these layers are mostly lipid, which is a poor
conductor of electrical current and thus a good insulator
(voltage-gated channels)
Myelin sheath
the electrical insulation that surrounds vertebrate axons
(voltage-gated channels)
What is produced by glia in the CNS
oligodendrocytes
(voltage-gated channels)
What is produced by glia in the PNS
Schwann cells
(voltage-gated channels)
Transmitting a signal: Synapse
slide 65
(voltage-gated channels)
Transmitting a signal: Synapse
electrical synapses and chemical synapses
(voltage-gated channels)
electrical synapses
the electrical current flows from one neuron to another through gap junctions
(voltage-gated channels)
chemical synapses
a chemical neurotransmitter carries information between neurons.
● Most synapses are chemical synapses
(voltage-gated channels)
A chemical synapse: neurotransmitter release
slide 67 and 68
(voltage-gated channels)
Action potential causes the release of the neurotransmitter
● The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal.
● The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell.
(voltage-gated channels)
single neurotransmitter
may bind specifically to more than a dozen different receptors.
A single neurotransmitter could excite postsynaptic cells expressing one receptor and inhibit postsynaptic cells expressing a different receptor.
(voltage-gated channels)
Generation of Postsynaptic Potentials
● Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell.
● Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential.
(chemically-gated channels)
A chemical synapse: communication!
slide 73
(chemically-gated channels)
Metabotropic receptor
● In some synapses, a neurotransmitter binds to a receptor that is metabotropic
● In this case, movement of ions through a channel depends on one or more metabolic steps
(chemically-gated channels)
Postsynaptic potentials
Excitatory postsynaptic potentials (EPSPs) and Inhibitory postsynaptic potentials (IPSPs)
(chemically-gated channels)
Excitatory postsynaptic potentials (EPSPs)
are depolarizations that bring the membrane potential toward threshold
(chemically-gated channels)
Inhibitory postsynaptic potentials (IPSPs)
are hyperpolarizations that move the membrane potential farther from threshold
(chemically-gated channels)
Temporal Summation
slide 76
(chemically-gated channels)
Spatial Summation
slide 77
(chemically-gated channels)
Termination of Neurotransmitter Signaling
After a response is triggered, the chemical synapse returns to its resting state.
The neurotransmitter molecules are cleared from the synaptic cleft.
Two mechanisms of terminating
neurotransmission
(chemically-gated channels)
Nerve gas sarin
Blocking this process can have severe effects. The nerve gas sarin triggers paralysis and death due to inhibition of the enzyme that breaks down the neurotransmitter controlling skeletal
muscles.
(chemically-gated channels)
The connection among neurons make neuronal circuits
slide 80
(chemically-gated channels)
Flow of information
slide 81
(chemically-gated channels)
