Week 5

Action potential

brief but large changes in neuronal polarization that arise in the initial segment of the axon, just after the axon hillock, and then move rapidly down the axon

Equilibrium potential

the point at which the movement of ions across the cell membrane is balances, as the electrostatic pressure pulling ions in the direction is offset but the diffusion force pushing them in the opposite direction

Axon hillock

the cone-shaped area on the cell body from which the axon originates

Hyperpolarization

an increase in membrane potential (the interior of the neuron becomes even more negative)

Depolarization

a decrease in membrane potential (the interior of the neuron becomes less negative)

Graded responses

the greater the stimulus, the greater the response

Local potential

an electrical potential that is initiated by stimulation at a specific site, is a graded response that spreads passively across the cell membrane, and decreases in strength with time and distance

Threshold

the stimulus intensity that is just adequate to trigger an action potential in an axon

Action potential

rapid reversal of the membrane potential that momentarily makes the inside of a neuron positive with respect to the outside

All-or none property

the condition that the size (amplitude) of the action potential is independent of the size of the stimulus

Number of action potentials

used to encode information by the neuron

Afterpotential

the positive or negative change in membrane potential that may follow an action potential

Voltage-gated sodium channel

a sodium selective channel that opens or closes in response to changes in the voltage of the local membrane potential; It mediates the action potential

Refractory

temporarily unresponsive or inactivated

Absolute refractory phase

a brief period of complete insensitivity to stimuli

Relative refractory phase

a period of reduced sensitivity during which only strong stimulation produces an action potential

Conduction velocity

the speed at which an action potential is propagated along the length of an axon

myelin

The fatty insulation around an axon, formed by glial cells. The myelin sheath boosts the speed at which action potentials are conducted.

node of Ranvier

A gap between successive segments of the myelin sheath where the axon membrane is exposed.

saltatory conduction

The form of conduction that is characteristic of myelinated axons, in which the action potential jumps from one node of Ranvier to the next.

multiple sclerosis (MS)

Literally "many scars." A disorder characterized by the widespread degeneration of myelin.

neurotransmitter

Also called simply transmitter, synaptic transmitter, or chemical transmitter. The chemical released from the presynaptic axon terminal that serves as the basis of communication between neurons.

presynaptic

Located on the "transmitting" side of a synapse.

postsynaptic

Referring to the region of a synapse that receives and responds to neurotransmitter.

postsynaptic potential

A local change in the membrane potential that is initiated by stimulation at a synapse, can vary in amplitude, and spreads passively across the cell membrane, decreasing in strength with time and distance.

Excitatory Postsynaptic Potential (EPSP)

A depolarizing potential in a neuron that is normally caused by synaptic excitation. EPSPs increase the probability that the postsynaptic neuron will fire an action potential.

Inhibitory Postsynaptic Potential (IPSP)

A hyperpolarizing potential in a neuron. IPSPs decrease the probability that the postsynaptic neuron will fire an action potential.

Chloride Ion (Cl⁻)

A chlorine atom that carries a negative charge.

spatial summation

The summation of postsynaptic potentials that reach the axon hillock from different locations across the cell body. If this summation reaches threshold, an action potential is triggered.

temporal summation

The summation of postsynaptic potentials that reach the axon hillock at different times. The closer in time the potentials occur, the greater the summation.