A&P - 12.5 Communication Between Neurons (graded potentials & synapses)

graded potential

change in the membrane potential that varies in size, depending on the size of the stimulus that elicits it

• graded potentials can be of two sorts, either they are depolarizing or hyperpolarizing

• graded potentials can summate and determine whether the neuron reaches its threshold

• for a membrane at the resting potential, a graded potential represents a change in that voltage either above -70 mV or below -70 mV

depolarizing graded potentials

typically the result of Na+ entering the cell (or Ca++ entering)

hyperpolarizing graded potentials

can be caused by K+ leaving the cell (or Cl- entering the cell), making inside the cell MORE NEGATIVE … threshold is that much further away

generator potential

graded potential from dendrites of a unipolar cell which generates the action potential in the initial segment of the cell’s axon

• stimulus received by unipolar neurons (such as sensory receptors of the skin) will will generate an action potential within the initial segment of the axon

receptor potential

graded potential in a specialized sensory cell that directly causes the release of neurotransmitter without an intervening action potential

• found in specialized sensory receptors for taste, vision, and hearing, results in the release of neurotransmitters at the synapse

postsynaptic potential (PSP)

graded potentials in the postsynaptic membrane caused by the binding of neurotransmitter to protein receptors

• graded potential in the dendrites of a neuron that is receiving synapses from other cells

• can be depolarizing or hyperpolarizing

excitatory postsynaptic potential (EPSP)

graded potential in the postsynaptic membrane that is the result of depolarization and makes an action potential more likely to occur

• moves graded potential toward the threshold

• postsynaptic neuron receives a depolarization signal meaning the graded potential is moving in a less negative direction (-70mV toward zero)

inhibitory postsynaptic potentials (IPSP)

graded potentials in the postsynaptic membrane that is the result of hyperpolarization and makes an action potential less likely to occur

• moves the membrane potential away from threshold

• postsynaptic neuron receives a hyperpolarization signal meaning the graded potential is moving in a more negative direction (-70mV toward -90mV)

summation

overall change in the membrane potential is called summation

• summation is the combined effects of all of the postsynaptic potentials

• to add together, as in the cumulative change in postsynaptic potentials toward reaching threshold in the membrane, either across a span of the membrane or over a certain amount of time

• spatial and temporal summation can act together, as well

• the process of converting electrical signals to chemical signals and back requires subtle changes that can result in transient increases or decreases in membrane voltage, to cause a lasting change in the target cell, multiple signals are usually added together, or summated

spatial summation

combination of graded potentials across the neuronal cell membrane caused by signals from separate presynaptic elements that add up to initiate an action potential

• related to the activity of multiple inputs to a neuron at the same time but in different locations

• multiple presynaptic neurons provide action potentials to a single neuron

temporal summation

combination of graded potentials at the same location on a neuron resulting in a strong signal from one input

• accumulation of multiple, repeated, and frequent inputs

• multiple action potentials from a single presynaptic neuron resulting in significant change in the membrane potential

synapses

narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell

• serve as the site where the first neuron (called the pre-synaptic neuron) is communicating with a second neuron (called the post-synaptic neuron)

• many electrical impulses must travel from neuron to neuron or from neuron to muscles or glands, when the message has to be sent from one cell to another, the cells are connected by a synapse

• presynaptic and postsynaptic neurons communicate via either an electrical synapse or a chemical synapse

presynaptic neuron

cell conducting the impulse towards the synapse

postsynaptic neuron

the cell transmitting the impulse away from the synapse 

types of synapses

• chemical synapses - use neurotransmitters

• electrical synapses - use gap junction

chemical synapses

a chemical signal—namely, a neurotransmitter—is released from one cell and it affects the other cell

• connection between two neuron, or between a neuron and its target, where a neurotransmitter diffuses across a very short distance

• where a synaptic cleft is formed between the two neurons and the presynaptic neuron releases neurotransmitters that stimulate an action potential in the postsynaptic neuron

• example: 

  • chemical synapse in the neuromuscular junction (NMJ)

electrical synapses

there is a direct connection between the two cells so that ions can pass directly from one cell to the next

• connection between two neurons, or any two electrically active cells, where ions flow directly through channels spanning their adjacent cell membranes

• if one cell is depolarized in an electrical synapse, the joined cell also depolarizes because the ions pass between the cells

• cell membranes of the two neurons are connected by gap junctions that allow ions from the presynaptic neuron to flow into the postsynaptic neuron so that the propagation of the action potential is continued in the next neuron

• this type of synapse can be found in both the CNS and PNS but are relatively rare