Lecture 2: Neurons Flashcards
The processing of perceptions, thoughts and actions in the brain is accomplished by networks of small cells called neurons.
The essential activities of all neurons, including those involved in sensory processes, are chemical and electrochemical.
The Structure of Neurons
Neurons communicate with each other through neurotransmitters, molecules that cross the synapse from the axon of one neuron to the dendrite of the next.
Nerve impulses are electrochemical; voltages change along the axon as electrically charged ions (sodium and potassium) pass in and out of the membranes of nerve cells.
Dendrites
Receive information from other neurons
Axon
Conducts that information to other parts of the brain, sometimes over great distances
The Electric Neuron
Molecules in neural tissue are “charged”: + or -
Charged Ions are potassium K+, sodium Na+, chlorine Cl-, and Proteins-
In the absence of stimulation, the inside of the neuron is slightly more negative than the outside (-70 mv). This is called the resting potential.
The Electric Neuron
Molecules in neural tissue are “charged”: + or -
Charged Ions are potassium K+, sodium Na+, chlorine Cl-, and Proteins-
In the absence of stimulation, the inside of the neuron is slightly more negative than the outside (-70 mv). This is called the resting potential.
resting potential
-70 mv
In the absence of stimulation, the inside of the neuron is slightly more negative than the outside.
Ion channels in a segment of neuronal membrane and measuring resting membrane potential.
Idealized neuron shown with intracellular recording electrode penetrating the neuron.
The electrode measures the difference between the voltage inside versus outside the neuron, and this difference is amplified and displayed on an oscilloscope screen.
The oscilloscope screen shows voltage over time, and shows that prior to the electrode entering the neuron, voltage between the electrode and the extracellular reference electrode is zero, but when the electrode is pushed into the neuron, the difference becomes –70 mV, which is the resting membrane potential.
The resting membrane potential arises from the asymmetric distribution of ions of sodium (Na+), potassium (K+), and chloride (Cl–), as well as of charged protein molecules (A–), across the neuron’s cell membrane.
Ion channels pump ions across the membrane.
The Na+–K+ pump preserves the cell’s resting potential by maintaining a larger concentration of K+ inside the cell and Na+ outside the cell. The pump uses ATP as energy.
synapse
The concentration of ions inside and outside of a neuron can be affected by neurotransmitters that are released into the synapse.
Within the axon terminals, the relative charge between the inside and outside of a neuron is called the ______ .
presynaptic potential.
Within the dendrites and soma, the relative charge between the inside and outside of a neuron is called the ____ .
postsynaptic potential.
Neural Transmission
When neurotransmitters are absorbed by the dendrites or soma, they can alter the postsynaptic potential.
Excitatory neurotransmitters cause the charge to become depolarized (more positive). This change is called an excitatory postsynaptic potential (EPSP).
Inhibitory neurotransmitters cause the charge to become hyperpolarized (more negative). This change is called an inhibitory postsynaptic potential (IPSP).
Both EPSPs and IPSPs are produced by altering the relative concentration of ions between the inside and outside of the cell membrane.
Excitatory Neural Transmission
Excitatory neurotransmitters cause the charge to become depolarized (more positive).
This change is called an excitatory postsynaptic potential (EPSP).
Inhibitory Neural Transmission
Inhibitory neurotransmitters cause the charge to become hyperpolarized (more negative).
This change is called an inhibitory postsynaptic potential (IPSP).
hyperpolarized
Inhibitory neurotransmitters cause the charge to become hyperpolarized (more negative).
depolarized
Excitatory neurotransmitters cause the charge to become depolarized (more positive).
Gated Ion Channels, EPSPs
Gated Na+ Channel
Causes depolarization of Post-synaptic neuron
Gated Ion Channels, IPSPs
Gated K+ Channel
K+ leaves, Cl- enters
Causes hyperpolarization of Post-synaptic neuron
IPSPs and EPSPs are Graded Potentials
They vary in magnitude based on the quantity of neurotransmitters with which they are stimulated.
They travel passively and are attenuated (weakened) with distance.
axon hillock
the base of the axon
At the axon hillock, a cell’s potential is determined by the sum of all the excitatory & inhibitory inputs in the dendrites & soma.
action potential.
If the voltage at the axon hillock reaches a threshold of -50 mv, then it will increase rapidly to 50 mv, and then quickly rebound to -70 mv.
This sequence of voltage changes is called an action potential.
absolute refractory
During the absolute refractory period it is impossible to generate a new action potential.
relative refractory
During the relative refractory period a new action potential can be generated, but it requires a stimulus that is much stronger than usual.
The Toilet Metaphor
When you push the handle, water floods the bowl.
This event takes a couple of seconds and you cannot stop it in the middle.
Once the bowl empties, the flush is complete. Now the upper tank is empty.
If you try pushing the handle at this point, nothing happens (absolute refractory period).
Wait for the upper tank to begin refilling. You can now flush again, but the intensity of the flush increases as the upper tank refills (relative refractory period)
An action potential (firing) of a neuron
Na+ entry locally depolarizes axon, which sufficiently depolarizes the adjacent region of the axon to open more of the voltage-gated Na+channels, re-creating the action potential there.
This process continues down the length of the axon.
The action potential is non-decremental.
That is to say, its magnitude does not change as it travels along the axon.
synapse
When an action potential reaches an axon terminal, it causes neurotransmitters to be released into the synapse.
graded potentials
Note that this is quite different from graded potentials (EPSPs or IPSPs).
Their magnitudes become diminished as they travel within the dendrites and soma, and are therefore referred to as decremental.
Primary Sensory & Motor Cortex and Surrounding Association Cortex.
The blue regions show the primary cortical receiving areas of the ascending sensory pathways and the primary output region to the spinal cord.
The secondary sensory and motor areas are colored pink.
The remainder is considered association cortex.
Excitation and Inhibition
Any neuron can excite or inhibit another neuron based on the type of neurotransmitter it releases from its axon terminals.
The stimulation of a neuron modifies the frequency at which it generates action potentials.
