action potentials and synapses handouts
TRANSCRIPT
K+
Na+ outside
inside
K+
Na+
Plasma membrane
K+
Na+ outside
inside
K+
Na+
Plasma membrane
fluid outsideneuron
gated sodiumchannel
In a membrane at rest, the inside of the neuron is negative relative to the outside. An electrical disturbance (yellow arrow) spreads from an input zone to an adjacent trigger zone of the membrane, which has a large number of gated sodium channels.
fluid outsideneuron
gated sodiumchannel
In a membrane at rest, the inside of the neuron is negative relative to the outside. An electrical disturbance (yellow arrow) spreads from an input zone to an adjacent trigger zone of the membrane, which has a large number of gated sodium channels.
©2007 Thomson Higher Education
Na+
Na+
Na+
A strong disturbance initiates an action potential. Sodium gates open. Sodium flows in, reducing the negativity inside the neuron. The change causes more gates to open, and so on until threshold is reached and the voltage difference across the membrane reverses.
voltage reversed
©2007 Thomson Higher Education©2007 Thomson Higher Education
Na+
Na+
Na+
A strong disturbance initiates an action potential. Sodium gates open. Sodium flows in, reducing the negativity inside the neuron. The change causes more gates to open, and so on until threshold is reached and the voltage difference across the membrane reverses.
voltage reversed
BIO 201 ‐ Trainor Action Potentials and Synapses Neurons and Potentials Neurons are highly irritable Respond to adequate stimulus by generating an action
potential (nerve impulse) Two types of signals
Graded potentials Incoming short‐distance signals
Action potentials Long‐distance signals of axons
Potential difference across the membrane of a resting cell Approximately –70 mV in neurons (cytoplasmic
side of membrane is negatively charged relative to outside)
Generated by the differences in ionic makeup of intracellular fluid and the extracellular fluid
©2007 Thomson Higher Education
Na+
Na+
Na+
K+
K+
K+
At the next patch of membrane, another group of gated sodium channels open. In the previous patch, some K+ moves out through other gated channels. That region becomes negative again.
©2007 Thomson Higher Education
Na+Na+
K+ K+K+
Na+
K+
After each action potential, the sodium and potassium concentration gradients in a patch of membrane are not yet fully restored. Active transport at sodium–potassium pumps restores them.
Na+/K+ pump
propagating action potential
©2007 Thomson Higher Education©2007 Thomson Higher Education
Na+Na+
K+ K+K+
Na+
K+
After each action potential, the sodium and potassium concentration gradients in a patch of membrane are not yet fully restored. Active transport at sodium–potassium pumps restores them.
Na+/K+ pump
propagating action potential
Action Potential Propagation Action potentials spread by themselves.
The action potential is self‐propagating and moves away from the stimulation site Potentials can self‐propagate because the changes to the membrane potential don’t lose strength
A neuron can’t “fire” again until ion pumps restore its resting potential. By diffusion, some potassium ions will always leak out of the cell and some sodium will always leak in The sodium‐potassium pumps use ATP to actively pump potassium ions in and sodium ions out of the neuron Necessary to keep the concentration of sodium ions higher outside, ready for another action potential to form.
Action Potential Resting state
Only leakage channels for Na+ and K+ are open
All gated Na+ and K+ channels are closed Depolarization
A reduction in membrane potential (toward zero)
Inside of the membrane becomes less negative than the resting potential
Increases the probability of producing a nerve impulse
Hyperpolarization An increase in membrane potential (away
from zero) Inside of the membrane becomes more negative
than the resting potential Reduces the probability of producing a nerve
impulse Action Potential Propagation Threshold At threshold:
Membrane is depolarized by 15 to 20 mV Na+ permeability increases Na influx exceeds K+ efflux The positive feedback cycle begins
Threshold Subthreshold stimulus—weak local
depolarization that does not reach threshold Threshold stimulus—strong enough to push the
membrane potential toward and beyond threshold
Threshold All action potentials are alike and are
independent of stimulus intensity Strong stimuli can generate action potentials
more often than weaker stimuli The CNS determines stimulus intensity by the
frequency of impulses Action Potentials Action potentials spread by themselves.
The action potential is self‐propagating and moves away from the stimulation site. Potentials can self‐propagate because the changes to the membrane potential don’t lose strength
Occurs in muscle cells and axons of neurons Does not decrease in magnitude over distance Principal means of long‐distance neural communication An action potential is an all‐or‐none phenomenon—action potentials either happen completely, or not at all Graded Potentials Short‐lived, localized changes in membrane potential Graded potential spreads as local currents change the membrane potential of adjacent regions Occur when a stimulus causes gated ion channels to open
E.g., receptor potentials, generator potentials, postsynaptic potentials Magnitude varies directly (graded) with stimulus strength Decrease in magnitude with distance as ions flow and diffuse through leakage channels Short‐distance signals Graded Potential vs. Action Potential Conduction Velocity Conduction velocities of neurons vary
widely Effect of axon diameter
Larger diameter fibers have less resistance to local current flow and have faster impulse conduction
Effect of myelination Continuous conduction in
unmyelinated axons is slower than saltatory conduction in myelinated axons
Conduction Velocity Effects of myelination:
Myelin sheaths insulate and prevent leakage of charge Saltatory conduction in myelinated axons is about 30 times faster Voltage‐gated Na+ channels are located at the nodes APs appear to jump rapidly from node to node
Conduction Velocity Multiple Sclerosis An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence Myelin sheaths in the CNS become nonfunctional scleroses Shunting and short‐circuiting of nerve impulses occurs Impulse conduction slows and eventually ceases Action Potentials and Synapses ‐ Part II The Synapse A junction that mediates information transfer from one neuron:
To another neuron To an effector cell
Presynaptic neuron—conducts impulses toward the synapse Postsynaptic neuron—transmits impulses away from the synapse Synaptic Cleft Fluid‐filled space separating the presynaptic and postsynaptic
neurons Prevents nerve impulses from directly passing from one neuron to
the next Transmission across the synaptic cleft:
Is a chemical event (as opposed to an electrical one) Involves release, diffusion, and binding of neurotransmitters Ensures unidirectional communication between neurons
Types of Synapses Axodendritic—between the axon of one neuron and the
dendrite of another Axosomatic—between the axon of one neuron and the soma of
another Less common types:
Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrite to soma)
Synapse Neurotransmitter must be released, diffuse across the synapse,
and bind to receptors Within a few milliseconds, the neurotransmitter effect is
terminated Degradation by enzymes Reuptake by astrocytes or axon terminal Diffusion away from the synaptic cleft
Synaptic delay—time needed to do this (0.3–5.0 ms) Synaptic delay is the rate‐limiting step of neural
transmission Excitatory Synapse Neurotransmitter binds to and opens chemically
gated channels that allow simultaneous flow of Na+ and K+ in opposite directions Na+ influx is greater that K+ efflux, causing a net depolarization If impulse is of threshold strength voltage‐gated channels open and start new action potential Inhibitory Synapse
Neurotransmitter binds to and opens channels for K+ or Cl–
Causes a hyperpolarization (the inner surface of membrane becomes more negative)
Reduces the postsynaptic neuron’s ability to produce an action potential
Summation Temporal summation
One or more presynaptic neurons transmit impulses in rapid‐fire order Spatial summation
Postsynaptic neuron is stimulated by a large number of terminals at the same time
Neurotransmitters 50 or more neurotransmitters have been identified Classified by chemical structure and by function
Acetylcholine Biogenic Amines
Norepinephrine, Dopamine, Serotonin, Histamine Amino Acids
GABA, Glutamate, Glycine, Peptides Peptides
Endorphins, Somatostatin, Cholecystokinin (CCK) Purines
ATP, Adenosine Gases and Lipids
NO2, CO2, Endocannabinoids Chemical Classes of Neurotransmitters Acetylcholine (ACh)
Released at neuromuscular junctions and some ANS neurons Synthesized by enzyme choline acetyltransferase Degraded by the enzyme acetylcholinesterase (AChE)
Functional Classification of Neurotransmitters Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing) Determined by the receptor type of the postsynaptic neuron
GABA and glycine are usually inhibitory Glutamate is usually excitatory Acetylcholine
Excitatory at neuromuscular junctions in skeletal muscle Inhibitory in cardiac muscle