3. Inside the Neuron Flashcards
Nerve Impulse/Action Potential
The electrical impulse that travels between neurons to share messages
A massive momentary reversal of a neuron’s membrane potential from about -70 mV to about +50 mV
Action Potential Process
Resting Potential
Depolarization
Repolarization
Hyperpolarization
- Resting Potential
- High in tension, enables neuron to react quickly to stimulation
- Begins at resting -70mV
- Depolarization
- The charges flip, positive inside the neuron, negative outside (after threshold is reached)
- When the right amount of stimulation is achieved (-55mV), the sodium ion channels open and Na+ ions flood the cell
Caused by the electrostatic pressure and and pressure from the concentration gradient
- Repolarization
- At the peak of the sodium ion influx (+30), the sodium channels close while the potassium channels stay open (potassium ions leave)
- Membrane potential decreases
- Hyperpolarization
- As the ions are flooding out and moving around, the neuron becomes hyperpolarized
- Further decrease in membrane potential
- Opposites, sodium is inside, potassium is outside
Refractory period
- After the action potential
- Sodium channels need to close, potassium is flowing at a faster rate
- Resists another action potential
- Explains why the action potential can only go in one direction
Absolute refractory period
Membrane cannot produce an action potential
Relative refractory period
A very strong stimulus is required to generate another action potential
All-or-none Law
the action potential always has the same strength, despite any changes in strength of the stimulation
Rate law
stronger stimulation leads to more frequent action potential
Cause of action potential
The electrical impulse is caused by ions (which have an electrical charge) moving across the cell membrane
Membrane potential
difference in electrical charge inside and outside of the cell
- Caused by the unequal distributions of ions inside and outside
Sodium (Na+) and Potassium (K+) ions
- More sodium ions outside, more potassium inside (neuron is polarized)
Resting membrane potential
-70mV
3 causes of uneven ion distribution
- Sodium potassium pump
- Electrostatic pressure
- Pressure from concentration gradient
Sodium potassium pump
- Takes 3 Na+ ions out, lets 2 K+ ions in
- This active ion transport requires energy
- Working during resting potential stage - maintains membrane potential
Electrostatic pressure
- Inside of the neuron is negative, outside positive
- Na+ ions (outside) are drawn to the negative inside, opposites attract
Pressure from concentration gradient
- Pressure as a result of the concentration gradient
- ions don’t want to be so polarized, Na+ wants in, K+ wants out
- Concentration gradient - concentration of particles is very high in one area and not another. Particles try to restore equilibrium
Potassium ion channels
- Mostly closed during resting stage, only a few can get through
- Leave the cell
Sodium ion channels
Closed during resting stage
Domino effect
- The action potential depolarizes nearby areas, causing more sodium channels to open
- Each area is stimulated by the depolarization in the area near it
Myelin sheaths
- Increase the distance/reach of the action potential
- Action potential only has to be generated at the ends of the Myelin sheaths, aka the nodes
Nodes of Ranvier
Gaps between the myelin sheaths
Saltatory conduction
action potential leaps between the gap
- More than 10x faster than normal conduction
- Less energy is required as fewer ions need to be transported through the membrane
Neurotransmitters (by size)
Amino acids
Monoamines
Neuropeptides
Amino acids
GABA - inhibitory
Glutamate - excitatory
Monoamines
Dopamine
Serotonin
Norepinephrine
Neuropeptides
Endorphin
Additional neurotransmitter
Acetylcholine
Production & Release (amino acids & monoamines)
Amino acids & monoamines are produced at the presynaptic terminal
- Both are stored in vesicles
- These vesicles can then move freely
Production & Release (neuropeptides)
Also stored in vesicles but have to be transported to the site of release
Synthesis
Production of chemical compounds through reactions with simpler materials
Ionotropic receptors
- Similar to ion channels - neurotransmitter binds, ion-channel opens and lets ions into the cell
On the side of an ion channel - Changes postsynaptic potential very fast
- Used for fast events
Excitatory Postsynaptic Potentials (EPSPs)
A stimulating, excitatory change
Increases the chance that an action potential will occur
Inhibitory Postsynaptic Potentials (IPSPs)
An inhibitory change
Decreases the chance that an action potential will occur
Metabotropic Receptors
aka G-protein receptor
Neurotransmitters bind, metabolic reactions are activated
- Uses a G-protein & second messenger system
Slower than ionotropic receptors
G-protein
- Activates a ‘second messenger’ which can
- alter a metabolic pathway
- change gene expression
- open or close an ion channel
- increase production of a certain protein
Endogenous chemicals
Originate outside the body e.g. neurotransmitters
Exogenous chemicals
Originate outside the body but can influence neurotransmitters once inside e.g. psychoactive drugs
Reflex arcs
stimulus - sensory neuron - muscle
Goes through the spinal cord, not the brain
Sherrington’s 3 Properties of Reflexes
- A reflex isn’t as fast as a message travelling along an axon
- Several weak stimuli close together in position or time produce a stronger effect than one stimulus
- One set of muscles will relax when another is excited
Summation
Spacial or temporal
Determines the start of an action potential
Temporal summation
EPSPs combine in one location (or stimulate several times) to exceed the threshold and kickstart another action potential
Spacial summation
EPSPs in different locations (e.g. different dendrites) combine to exceed the threshold and kickstart another action potential
Synapse
The gap between one neuron & the next
Receptors
- Located in the cell membrane of the neuron
- Neurotransmitters can bind here
- Each receptor is ‘sensitive’ to specific neurotransmitters
- Key-lock principle
- Sub-types of receptors for each neurotransmitter
Presynaptic
Sends information from the end of the neuron’s axon
Postsynaptic
Receives information form dendrites
Synaptic transmission
- Neurotransmitters are held in vesicles at the axon terminal, waiting for stimulation to be released
- Action potential causes the release of the neurotransmitters into the synapse (exocytosis)
- Neurotransmitters bind to receptor sites, causing a change in potential
- Reuptake - after binding and changing potential, the neurotransmitter is reabsorbed into the first neuron through uptake pumps
Exocytosis
Bursts of releases of neurotransmitters (or anything else in the body) from the presynaptic neuron
Vesicles fuse with the membrane after releasing the neurotransmitters
More vesicles are created again from the membrane (pinched off)
Reuptake
- Feedback mechanism
- Autoreceptors on the presynaptic neuron monitor the release of neurotransmitters
- Neurotransmitters bind to the autoreceptors on their original neuron as part of reuptake & a signal is released to send less of the neurotransmitter into the synaptic gap
