Chapter 4 Neural Conduction and Synaptic Transmission How Neurons Send and Receive Signals This multimedia product and its contents are protected under copyright law. The following are prohibited by law: • any public performance or display, including transmission of any image over a network; • preparation of any derivative work, including the extraction, in whole or in part, of any images; • any rental, lease, or lending of
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Chapter 4Neural Conduction and Synaptic TransmissionHow Neurons Send and
Receive Signals
This multimedia product and its contents are protected under copyright law. The following are prohibited by law:• any public performance or display, including transmission of any image over a network;• preparation of any derivative work, including the extraction, in whole or in part, of any images; • any rental, lease, or lending of the program.
The Neuron’s Resting Membrane Potential
Inside of the neuron is negative with respect to the outside
Resting membrane potential is about -70mV
Membrane is polarized, it carries a charge
Why?
Ionic Basis of the Resting Potential
Ions, charged particles, are unevenly distributed
Factors influencing ion distribution• Homogenizing
• Factors contributing to uneven distribution
Ionic Basis of the Resting Potential
Homogenizing• Random motion – particles tend to move down
their concentration gradient
• Electrostatic pressure – like repels like, opposites attract
Factors contributing to uneven distribution• Membrane is selectively permeable
PSPs (IPSPs) EPSPs make it more likely a neuron will fire,
IPSPs make it less likely PSPs are graded potentials – their size varies
EPSPs and IPSPs
Travel passively from their site of origination Decremental – they get smaller as they travel 1 EPSP typically will not suffice to cause a
neuron to “fire” and release neurotransmitter – summation is needed
Integration of PSPs and Generation of Action Potentials (APs)
In order to generate an AP (or “fire”), the threshold of activation must be reached at the axon hillock
Integration of IPSPs and EPSPs must result in a potential of about -65mV in order to generate an AP
Integration
Adding or combining a number of individual signals into one overall signal
Temporal summation – integration of events happening at different times
Spatial - integration of events happening at different places
What type of summation occurs when:
One neuron fires rapidly? Multiple neurons fire at the same time? Several neurons fire repeatedly? Both temporal and spatial summation
occur simultaneously
The Action Potential
All-or-none, when threshold is reached the neuron “fires” and the action potential either occurs or it does not.
When threshold is reached, voltage-activated ion channels are opened.
The Ionic Basis of Action Potentials
When summation at the axon hillock results in the threshold of excitation (-65mV) being reached, voltage-activated Na+ channels open and sodium rushes in.
Remember, all forces were acting to move Na+ into the cell.
Membrane potential moves from -70 to +50mV.
The Ionic Basis of Action Potentials
Rising phase: Na+ moves membrane potential from -70 to +50mV.
End of rising phase: After about 1 millisec, Na+ channels close.
Change in membrane potential opens voltage-activated K+ channels.
Repolarization: Concentration gradient and change in charge leads to efflux of K+.
Hyperpolaization: Channels close slowly - K+ efflux leads to membrane potential <-70mV.
Refractory Periods
Absolute – impossible to initiate another action potential
Relative – harder to initiate another action potential
Prevent the backwards movement of APs and limit the rate of firing
Released NT produces signals in postsynaptic neurons by binding to receptors.
Receptors are specific for a given NT. Ligand – a molecule that binds to another. A NT is a ligand of its receptor.
Receptors
There are multiple receptor types for a given NT.
Ionotropic receptors – associated with ligand-activated ion channels.
Metabotropic receptors – associated with signal proteins and G proteins.
Ionotropic Receptors
NT binds and an associated ion channel opens or closes, causing a PSP.
If Na+ channels are opened, for example, an EPSP occurs.
If K+ channels are opened, for example, an IPSP occurs.
Metabotropic Receptors
Effects are slower, longer-lasting, more diffuse, and more varied.
NT (1st messenger) binds > G protein subunit breaks away > ion channel opened/closed OR a 2nd messenger is synthesized > 2nd messengers may have a wide variety of effects
Reuptake, Enzymatic Degradation, and Recycling
As long as NT is in the synapse, it is active – activity must somehow be turned off.
Reuptake – scoop up and recycle NT.
Enzymatic degradation – a NT is broken down by enzymes.
Small-molecule Neurotransmitters
Amino acids – the building blocks of proteins
Monoamines – all synthesized from a single amino acid