Neuron Physiology

Electrophysiology

cellular mechanisms for producing electrical potentials and currents

Basis for neural communication and muscle contraction

Pumps

-membrane proteins that maintain a concentration gradient by moving substances against their concentration gradient

-require cellular energy

Sodium-potassium

What pumps can be found in neuron membranes

Channels

Protein pores in the membrane that allow ions to move down their concentration gradients (into or out of the cell)

When open they allow a specific type of ion to diffuse

Leak channels

Channels always open for continuous diffusion

Chemically gated channels

Channels normally closed, but open when a neurotransmitter binds

voltage gated channels

Channels usually closed, but open when membrane charge changes

resting, activation, inactivation

3 states of voltage-gated Na+ channels

Resting stage

Activation gate closed; inactivation gate open

Entry of Na+ prevented

Activation state

Activation gate open (due to voltage change); inactivation gate open

Na+ moves through channel

inactivation state

Activation gate open; inactivation gate closed

Entry of Na+ prevented

This state lasts a short time—the channel quickly resets to resting state

Modality gated channels

Channels usually closed but open in response to a stimulus other than a chemical or a voltage change located at dendrite ends

Membranes of sensory neurons

Where are modality gated channels found

Receptive segment

Functional segment of a neuron containing dendrite and cell body

chemically gated channels (Na, K)

What kind of gates are found in the receptive segment

Initial segment

Functional segment of a neuron containing axon hillock

Conductive segment

Functional segment of a neuron containing axon, its branches

voltage gated channels (Na, K)

What kind of gates are located in the initial and conductive segments

Transmissive segment

Functional segment of a neuron containing synaptic knobs

voltage gated channels (Ca)

What kind of channels are located in the transmissive segment

Cytosol verses interstitial fluid

At rest, where can a higher concentration of K be found

Interstitial fluid rather than cytosol

At rest, where can a higher concentration Na, Cl, and Ca be found

Cytosol

Is the cytosol or IF relativiely negative compared to the other

K diffusion

Most important factor in setting RMP

Out of cell

Does K diffuse into or out of the cell

Na

What else affects RMP

Into cell

Does Na diffuse into or out of the cell

-70 mv

Typical neuron RMP

Graded potentials

small, short-lived changes in the RMP

Receptive segment

Where are graded potentials

Hyperpolarization

Change in the membrane potential to a value more negative than the resting potential

excitatory postsynaptic potential (EPSP)

Postsynaptic potential resulting in depolarization

inhibitory postsynaptic potential (IPSP)

postsynaptic potential resulting in hyperpolarization

Na faster in than K out (makes inside of neuron more positive)

In the generation of an EPSP, what flows faster into or out of the neuron faster than the other element

Inside neuron becomes more negative in IPSP while opposite is true in EPSP

Difference between EPSP and IPSP

1. Neurotransmitter is released

2. Chemically gated channels open

3. EPSP is established

4. EPSP moves toward initial segment (coming from receptive)

Steps to the generation of an EPSP or IPSP

In EPSP, cationic channels open while in IPSP it's either K+ or Cl-

In generating an EPSP or IPSP, what is different about the second step (chemically gated channels opening)

Axon hillock

Where does the summation of EPSPs and IPSPs occur

Threshold

The minimum voltage change required

Spatial summation

When multiple pre synaptic neurons release neurotransmitter at various locations

Temporal summation

When a single presynaptic neuron repeatedly released neurotransmitter to produce either multiple EPSP or IPSPs in the postsynaptic neuron

All or none law

If threshold is reached, action potential is generated and propagated down axon

If not, channels stay closed and no action potential

What law is this?

Action potential

In the conductive segment, what involves depolarization and depolarization

Depolarization

Gain of positive charge as Na enters

Repolarization

Return to negative potential as K exits

Hyperpolarization

When K channels stay open longer than needed and the membrane potential is more negative than the resting membrane potential

1. Threshold reached

2. Depolarization

3. Repolarization

4. Hyperpolarization

5. RMP is reestablished

What are the steps for the events of an action potential in the conductive segment

Refractory period

Period of time after start of action potential when it is impossible or difficult to fire another action potential

Absolutely refractory period

No stimulus can initiate another action potential

Relative refractory period

Another action potential is possible (Na+ channels have reset) but the minimum stimulus strength is now greater

Some K+ channels are still open; cell is slightly hyperpolarized and further from threshold

unmyelinated axons

Where does continuous conduction occur

Continuous conduction

Conduction occurring in unmyelinated axons and involves the opening of voltage-gated channels along the entire length of the axon

Myelinated axons

Where does Saltatory conduction occur

saltatory conduction

Conduction who's current becomes weaker with distance and its action potential only occurs at neurofibril nodes

Saltatory conduction

Which conduction type is faster

1. Nerve signal reaches knob

2. Voltage-gated Ca channels open

3. Neurotransmitter is released

4. Neurotransmitter attaches to ligand receptors

Steps involved when a nerve signal reaches synaptic knob in the transmissive segment:

Ligand receptors of another neuron or an effector organ

What does the neurotransmitter attach to when released from the synaptic knob

Action potentials

Potentials that occur on neurons conductive region and involve depolarization then repolarization

Graded potentials

Potentials that occur in neurons receptive region and can be positive or negative changes in charge

Axon diameter and myelination

What does conduction speed depend on?

Thick better then thin

What type of fiber conducts faster between thick and thin

Myelinated

Do myelinated or unmyelinated conduct faster

Nerve fiber

an axon and its myelin sheath

Group A

Group of nerve fiber with a large diameter, myelinated fibers

Group B

Group of nerve fiber with a small diameter and or unmyelinated

Acetylcholine

Chemical class of neurotransmitter whose structure differed substantially from other transmitters

Biogenic amines

Chemical class of neurotransmitter that is an amino acid slightly modified to synthesize the transmitter

Catecholamines

Biogenic amines made from tyrosine

-dopamine

Indolamines

Biogenic amines made from histidine or tryptophan

-seratonin

Amino acids

Chemical class of neurotransmitter that include common transmitters glutamate, glycine, GABA

Neuropeptides

Chemical class of neurotransmitter containing chains of amino acids

Excitatory transmitters

Functional classification of neurotransmitter causing EPSPs

Inhibitory transmitters

Functional classification of neurotransmitter causing IPSPs

Direct

Action classification of neurotransmitter that bind to receptors that are chemically gated channels

Indirect

Action classification of neurotransmitter that bind to receptors that involve G-proteins and secondary messengers

Acetylcholine

Best characterized neurotransmitter that is released into cleft from action potential

Acetylcholinesterase

What clears ACh from the synaptic cleft?

Stimulate skeletal muscle

What is ACh used for in PNS

Increases arousal

What is ACh used for in CNS

Neuronodulators

Chemicals that alter responses of local neurons