Lecture 3: Graded Potentials And Action Potentials Flashcards
Anatomy of a neuron (8)
Big circle: cell body
Branching bits: dendrites
Long straight bit: Axon
Blocks along axon: myelin
Gaps between Myelin: node of ranvier
Wide boy at the end of an axon: presynaptic terminal
Dots on the presynaptic terminal: synaptic vesicles
Gap between presynaptic terminal and the postsynaptic cell: synaptic cleft
Types of neurons (4)
One axon with dendrites on the end: uniporlar neuron
One axon with dendrites on cell body and end of axon: multipolar neuron
Two axons coming from cell body: bipolar neuron
Single axon extending then splitting : pseudounipolar neuron
What are Glia (AKA glial cells) (3)
-They are the support cells for neurons
-In mammals and other vertebrates, there are several different types, depending on their locaiton
-in invertebrates there are less types
General function of Glia (5)
- Make neural stem cells
-make neuro-immune cells
-take up excess water and K+
-contribute to the blood brain barrier
-myelinate axons
Characteristics of Graded potentials (8)
-Signals communicated from one neuron to the next are graded potentials (postsynaptic potentials)
-Small “sub-threshold” charges in membrane potentials are graded potentials. Often have amplitudes of 0.1mV to 5mV
-can be depolarizing or hyperpolarizing
-Amplitude is proportional to the stimulus that caused them
-Caused by the flow of ions through Ion channels
-gradually dissipate as they travel through the cell
-Passively propagated
-Can be effected by summation
Why do graded potentials dissipate as they travel through the cell?
-Graded potentials travels like a ripple in a pond, where it gets weaker as it gets farther from the source
-it degrades because the neuron membrane is leaky and because there is a high electrical resistance in the surrounding cytoplasm
characteristics of Action Potentials (7)
- All or nothing! They either reach threshold and fire, or don’t and do nothing
- Large amplitude, about 100mV from RMP to peak
- ALWYS depolarizing
- There is a refractory period
- Cannot be effected by summation
- In neurons, site of AP generation is the axon hillock
- Actively propagated by voltage gated ion channels
Ionic basis of Action potential
1.cell is at Resting membrane potential
- Cell is depolarized by graded potential
- Membrane depolarizes to threshold. Voltage-gated Na+ channels open quickly, Na+ enters cell. Voltage-gated K+ channels begin to open slowly.
- Rapid Na+ entry depolarizes cell. Rising Phase
- Na+ channels begin to inactivate and slower K+ channels fully open.
- K+ leaves cell. Falling phase, repolarization phase.
- K+ channels remain open and additional K+ leaves cell, hyperpolarizing it. (Afterhyperpolarization)
- Voltage-gated K+ channels closing, less K+ moving out of the cell, returning to RMP.
- Cell returns to resting ion permeability and resting membrane potential.
Propagation of AP along unmyelinated axon
-There is a high density of channel proteins along the axon hillock (aka the TRIGGER ZONE) and a uniform distribution of Na+ and K+ along the axon.
-During an AP at the trigger zone, Na+ rushes in causing a depolarization
-Some Na+ is attracted to the nearby axon, causing the axon to have its own depolarization
-More Na+ is pulled to the next section of a axon, causing another depolarization further down the
-This cycle repeats along the entire axon, resulting in a “depolarization wave”
Propagation of AP in a myelinated axon
-Recall: Glia myelinate axons
-The myelin sheaths form insulating segments along the axon
-Na+ and K+ channels are localized between the myelin segments (called nodes of ranvier)
-instead of traveling as a wave, the action potential jumps from node to node
-This makes the whole process about 100x faster than in an unmyelenated axon
Increasing conduction velocity
-Myelination: All other areas being equal, a Myelinated axon conducts AP’s faster than an non-myelinated axon
-increase Diameter: All other areas being equal, a larger diameter axon conducts AP’s faster than a smaller diameter one
-Large axons can speed up AP’s but myelinated ones are even faster - and take up less space!
