1.5 Generation and Propagation of Action Potentials Flashcards
What are the ways neurons are stimulated
- a neurotransmitter being released from an adjacent neuron
-an external chemical
-physical pressure or touch
What does the stimulation of a neuron cause
It causes Na (and other) ion channels to open which helps to generate a nerve impulse
What are the 2 types of potentials and over what distances do they act
graded potentials act as short distance signals and action potentials act as long distance signals
Where do graded potentials and action potentials occur
graded potentials occur in the soma or dendrites, whereas action potentials are only in the axon
What is a post synaptic potential and how is it generated
A graded potential in the dendrites of a neuron that has just received a signal. To generate it, a ligand (e.g a neurotransmitter) binds to ligand gated Na+ channels on the post synaptic membrane. This causes the channels to open and Na+ enters the cell. The ligand is then broken down.
What happens if a post synaptic membrane then depolarises
It is an excitatory PSP and the membrane potential can move towards the threshold for an action potential.
What happens if a post synaptic potential hyperpolarises
It is an inhibitory PSP and the membrane potential moves away from the threshold for an action potential
Describe the flow of graded potentials
They occur in the active area of the membrane and the magnitude of the potential corresponds with the magnitude of the stimulus. The GPs spread decrementally by local current flow, from the active area to the inactive areas. The GPs die over a short period of time
Describe the 2 types of summation
temporal summation - several impulses from one neuron over time
spatial summation - impulses from several neurons at the same time
Describe the threshold potential
If enough EPSPs add together, the threshold potential (-55mv) aka suprathreshold is reached and the depolarisation “runs away”, peaking at +35mv. This is the action potential
Describe the stages of how an action potential is generated across a membrane.
once ligand gated Na+ channels have allowed Na+ in, the membrane depolarises to the suprathreshold level, this then opens voltage gate Na+ channels so even more Na+ enters (flowing down the electrochemical gradient). This is the runaway effect and an example of positive feedback. The Na+ channels then reset to inactivate and close and the voltage K+ channels then open. So Na is leaving and K is coming in (both flowing down the electrochemical gradient) , this then repolarises the membrane, and it goes all the way down to hyperpolarising, below resting level of -90mV (”undershooting”) - movement of ions down the electrochemical gradient coming from further away quickly corrects this. Ion channels are then inactivated Before the voltage gated K+ channels are inactivated and it returns to threshold potential.
Describe the gates in gated channels when the channels are activated and when they are inactivated
When the gated channels are activated the activation gate is open and the inactivation gate is closed
When the gated channels are inactivated the activation gate is closed and the inactivation gate is open
Constant ion exchange would lead to an ionic imbalance overtime (eventually leading to failure) , however why is this not an issue.
Only 1/3000 ions are exchanged for every AP and also the Na+/K+ pump restores the balance
In each area of the axon, how is resting potential restored following the action potential
the Na+/K+ pump
What 2 factors is nerve impulse speed dependent on
Nerve impulse speed is dependent upon the axon diameter, the wider the axon, the faster the transmission speed
Nerve impulse speed is also dependent on if the axon is myelinated, myelinated axons are faster
Describe why myelinated axons are faster
In myelinated axons, the myelin sheath surrounds the axon other than little gaps called the nodes of ranvier. The nodes of ranvier are the only places where the voltage gated channels are exposed. Therefore APs can only take place at the nodes. This allows for saltatory conduction, which speeds the AP transmission up as it jumps from node to node. This propagates the AP 50 times faster
Describe the impulse speed, length and myelination of autonomic axons for smooth and cardiac muscle
-unmyelinated
-0.3-1.3 micrometers
-0.7-2.2 m/s
Describe the impulse speed, length and myelination of pain axons
-both myelinated and unmyelinated
-1-5 micrometers
-12-30 m/s
Describe the impulse speed, length and myelination of sensory muscle position axons
-myelinated
-12-22 micrometers
-70-120 m/s
