Exam 2: Neurons, Action Potentials, Conduction, and Velocity

What is a neuron?

specialized cell that can alter its membrane potential and send a signal impulse, called an action potential, to different parts of the body

What is the cell body (aka soma) of a neuron?

contains the typical cellular machinery: nucleus, endoplasmic reticulum, mitochondria,

What are the dendrites of a neuron?

projections of the cell that receive input

What is the axon of a neuron?

a long projection from the cell body that transmits the action potential through the cell; usually the axon has branches, called collateral axons

What is the axon terminal of a neuron?

the ending of an axon that contains synaptic vesicles filled with a chemical signal called a neurotransmitter; the synaptic vesicles are triggered to release their contents by action potentials

What is the Spike initiation zone (SIZ)?

the start of the axon, which is where each action potential is initiated

An action potential is…

a brief switching of the membrane’s polarity (negative to positive, and back to negative) that travels down the axon

What is the purpose of the threshold potential?

the membrane potential at which a voltage- gated channel’s gates change position and allow for action potential to take place

What is the threshold potential?

-55

Na+ voltage-gated channels typically have…

both an activation and inactivation gate

The activation gate of a Na+ voltage-gated channel typically lets…

Na+ into the cell via simple diffusion from high to low gradient

The inactivation gate of a Na+ voltage-gated channel has a…

1 millisecond (ms) delay to position itself to block the channel’s influx of Na+

Which types of gates have a 1ms delay?

Na+’s inactivation gate, K+ activation gate, and Ca2+ activation gate

Which section of a neuron has chemically gated channels?

the dendrites and cell body

Which section of a neuron has voltage-gated channels?

the axon and axon terminal

Which specific channels are found in the axon?

Na+ and K+ voltage-gated channels

Which specific channels are found in the axon terminal?

Ca2+ voltage-gated channels

What is a graded potential?

a variable (i.e., adjustable) change in the membrane potential that can occur in the

dendrites and cell body; are generated when the dendrites of a neuron receive input

from another neuron

What triggers an action potential to start?

graded potentials

What types of chemically gated channels are found in dendrites of neurons?

Na+, K+, and Ca2+

Chemically gated Na+ channels typically

let Na+ in, causing a depolarization

Chemically gated K+ channels typically

let K+ out, causing a hyperpolarization

Chemically gated Cl- channels typically

let Cl - in, causing a hyperpolarization

What is summation?

graded potentials caused by presynaptic neurons can overlap if they are close enough together

What is temporal summation?

when two or more EPSPs from the same presynaptic neuron overlap

What is spatial summation?

when two or more EPSPs from the different presynaptic neurons overlap

If a presynaptic neuron causes the postsynaptic neuron to depolarize, the connection is an

excitatory connection

If a presynaptic neuron causes the postsynaptic neuron to depolarize, the resulting graded potential is called an

excitatory post-synaptic potential (EPSP)

If a presynaptic neuron causes the postsynaptic neuron to hyperpolarize, the connection is an

inhibitory connection

If a presynaptic neuron causes the postsynaptic neuron to hyperpolarize, the resulting graded potential is called an

inhibitory post-synaptic potential (IPSP)

If the SIZ reaches -55 mV, the neuron

fires an action potential

If the SIZ fails to reach -55 mV, the neuron

won’t fire an action potential

When the action potential reaches the axon terminal, it depolarizes it to the threshold potential of -55

Ca2+ voltage-gated channels open and rush into the ICF

Higher levels of calcium in the ICF triggers

synaptic vesicles to move toward the cell membrane and fuse with it

Once fused with the cell membrane, the synaptic vesicles are

opened up and exocytosis of their contents (neurotransmitter) occurs

neurotransmitter diffuses out of the cell into the

synaptic cleft

conduction velocity of a neuron is how

fast an action potential can be moved down a given length of the axon

Fastest neurons travel at

120 meters/second (motor neurons and proprioceptors (muscle sensors)

Slowest neurons travel at

0.5 meters/second (pain, itch, temperature sensory neurons)

Conduction velocity is affected by what two main factors?

axon diameter and myelination

How does axon diameter affect conduction velocity?

Larger diameter —> faster speed

How does myelination affect conduction velocity?

more myelination (myelin sheath) secures ions from leaking out of the cell → 50x faster speed than non-myelinated

Gaps between glial cells care called

nodes of Ranvier

It is only in the gaps between glial cells (nodes of Ranvier)

that there are voltage-gated channels, leak channels, and ion pumps

What is frequency coding?

the mechanism by which neurons convey/encode the magnitude or strength of the signal they are sending

The amplitude of a neuron’s action potentials is

always the same size (usually reaching around +30 mV)

If a neuron’s action potential is always the same size, how does it create different magnitude with its signals?

a neuron will increase the rate of its firing (more action potential per second) to relay a larger signal

How does myelination work?

glial cells (small neuronal “helper” cells) wrap themselves around and around the neuron’s axon