Week five Flashcards
Signaling Amongst Neurons
Whereas signaling within a neuron is primarily an electrical phenomenon…
Neurons communicate with one another by chemical signaling (neurotransmission)
Many different neurotransmitters have been discovered…
Neurotransmission
The transmitting neuron almost touches the receiving neuron at a juxtaposed tiny region of their cell membranes – called a synapse. The sending neuron’s membrane at this location is called the presynaptic membrane, whereas the receiving neuron’s membrane at this location is called the postsynaptic membrane. The pre and post synaptic membranes are separated by a tiny gap called a synaptic cleft. The synapse consists of these 3 parts.
NTs are produced within the signaling neuron and stored in vesicles near the neuron’s presynaptic membrane.
When an action potential reaches the synaptic bouton it causes fusion of these vesicles with the presynaptic membrane, resulting in release of NT into the synaptic cleft.
The NT diffuses across the synaptic cleft and binds to receptors in the post-synaptic membrane which alters the receiver membrane’s ionic permeability, resulting in EPSPs or IPSPs.
The NT remaining in the synaptic cleft is inactivated through dilution, reuptake and enzymatic decomposition.
Effects of Neurotransmitters
Neurotransmitters “bind” to
receptor sites on the post-
synaptic membrane.
The binding process alters
specific ionic permeability of the
post-synaptic membrane at the receptor site, resulting in a local graded potential (EPSP or IPSP)
Information can be transmitted
between two neurons when:
Neurotransmission results in EPSPs and
IPSPs in the receiving neuron’s dendrites
If this does NOT happen, no information can
be transmitted between two neurons…
Activation of receptors
Ionotropic receptor
A receptor that contains a binding site for a neurotransmitter and an ion channel that opens when a molecule of the neurotransmitter attaches to the binding site.
Activation of receptors
Metabotropic receptor
A receptor that contains a binding site for a neurotransmitter which activates an enzyme that begins a series of events, opening an ion channel elsewhere in the neuron’s membrane when a molecule of the neurotransmitter attaches to the binding site.
Metabotropic Activation
G protein
A protein coupled to a metabotropic receptor; conveys messages to other molecules when a ligand binds with and activates the receptor.
Second messenger
A chemical produced when a G protein activates an enzyme; carries a signal that results in the opening of the ion channel or causes other events to occur in the cell.
Neural circuits process information…
Circuits that produce cyclical signaling activity are called “reverbatory” circuits
A single cortical neuron can make synaptic connections with up to 10,000 other neurons so reverbatory circuits can range from simple to very complex (and the circuit cycle time may also vary)
Example: Spinal cord circuits can be quite simple, whereas cortical circuits can be very complex…
Some Limits to the Ability of Neurons to Transmit Information
Information is signaled by the DISTRIBUTION of Action Potentials over time within a neuron…
No one spike is ‘informative’ because neurons have high metabolic rates which makes them noisy…
Most neurons elicit a small number of “spontaneous” Action Potentials every second (e.g., typical ‘resting level’ spike rates vary over time between about 5-10 spikes/sec)
Neural signaling requires significant energy resources (ATP) so spontaneous spikes are wasteful…
Neurons code signals to create information (neural coding).
Done through INTEGRATION of EPSPs and IPSPs
To save energy, it would make sense for coding to be SPARSE (i.e., a code involving few cortical neurons rather than a code involving many neurons)
Neurons must code signals to create information in the nervous system.
For Example: The DYNAMIC range of a neuron is limited by the neuron’s ability to create action potentials.
Example: A sensory neuron has a maximum Action Potential production rate of ~100 spikes/sec and a resting spike production level of 5 spikes/sec. Its dynamic range is, therefore: 10log10(100/5) = ~13 deciBels (dB).
But… we can sense differences in stimuli (e.g., light, sound) over MUCH greater dynamic ranges than 13 dB! For example, you can hear over a dynamic range of well over 100 db! How can this be achieved if any one neuron’s dynamic range is so small?
Answer: Neural coding!
Neurons evolved to have specializations beyond a linear “current to spike rate” process associated with AP production.
“Just as anatomists have long classified neurons by the shapes of their dendritic trees, so cellular neurophysiologists may come to recognize classes of neuronal computational styles.”
– Calvin and Graubard, 1979
Effects of postsynaptic potentials
Neural integration
The process by which inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs) summate and control the rate of firing of a neuron.
Only within a given time frame and spatial area…
