Graded Potentials and Circuits

pseudounipolar neuron

type of sensory neuron with 1 short axon from soma that divides into 2 branches

unipolar neuron

single extension from soma that acts as both dendrites and axon

bipolar neuron

2 extensions on either side of soma = 1 dendrite end and 1 axon end

anaxonic neuron

no apparent axon -looks like blob

multipolar neuron

1 single axon and 2+ dendrites - stereotypical neuron structure

types of neural circuits

1. diverging

2. converging

3. reverberating

diverging circuit

1 presynaptic neuron stimulates increasing large number of postsynaptic neurons

• looks like tree diagram

converging circuit

many presynaptic neurons stimulate decreasing amount of postsynaptic neurons

• looks like upside down tree diagram

reverberating circuit

1st neuron stimulates 2nd neuron which stimulates 3rd etc

• looks like chain

axon hillock

region of soma that connects to axon and acts as trigger zone = graded potentials accumulate and potentially reach threshold to trigger AP

type of graded potentials

1. excitatory postsynaptic potentials (EPSP)

2. inhibitory postsynaptic potentials (IPSP)

3. end plate potentials

excitatory post-synaptic potential

causes depolarisation

inhibitory postsynaptic potential

causes hyperpolarisation

end plate potential

temp depolarisation of postsynaptic membrane of muscle fibre at neuromuscular junction

factors affecting if graded potentials reach threshold

• distance - closer to hillock = more likely bc of decremental conduction

• strength of stimulus

• spatial summation

• temporal summation

types of stimulus strength

1. subthreshold stimulus - weak = doesn’t reach threshold = no AP

2. threshold stimulus - just strong enough = reaches threshold = AP

3. suprathreshold - stronger = passes threshold = AP

what does suprathreshold stimulus effect

increases frequency of APs but doesn’t change AP amplitude (same size as threshold stimulus)

presynaptic inhibition

inhibitory neuron forms synapse directly to axon terminal of another neuron = blocks release of neurotransmitters at that specific terminal = inhibits only 1 targer cell

postsynaptic inhibition

inhibitory neuron forms synapse with dendrite/soma of another neuron = reduces excitability = if below threshold no AP = inhibits all target cells equally

If the afferent neuron is excitatory, and the interneuron is inhibitory, increasing the firing frequency of the afferent neuron would _______ the firing frequency of the interneuron

increases

If the afferent neuron is excitatory, and the interneuron is inhibitory, increasing the firing frequency of the afferent neuron would _______ the firing frequency of the efferent neuron

decreases

will smaller or larger neuron generate AP first if receives same amount of stimulation

smaller bc has smaller surface area and fewer ion channels = higher electrical resistance = smaller input creates larger change in membrane = reches threshold quicker

Henneman’s size principle

when wanting to move a muscle brain reqcruits smallest motor neurons first bc fires with little input. larger motor neurons have lower resistance and larger volume so needs stronger input = only recruited when need powerful, fast or heavy muscle

graded vs action potentials

• g = at dendrites and soma vs a = axon

• g = ligand or mechanically-gated channels vs a = voltage-gated

• g = decrementa so short distance vs a = propogates so longer distance

• g = size depends on stimulus vs a = always same size

• g = summation vs a = no summation bc refractory period