Neurophysiology

What are the functions of the nervous system?

1. detection of information from internal and external environments

2. integrating this information to determine what action, if any, should be done

3. coordinating a response

How is information conducted throughout the body?

electrical impulses

What are the specilized cells that generate these impulses?

neurons

What is the body of the neuron?

soma

What are the two processes that extend from the soma?

dendrites and axons

What is the function of dendrites?

receive and carry information toward the soma

What is the axon’s function?

generates and conducts nerve impulses away from the soma

Where does the axon originate?

axon hillock

where does the axon terminate?

axonal terminal

Where are neurotransmitters?

axonal terminal and contained in synaptic vesicles

Myelin sheath and axons?

axons may or not be contained

What does myelin provide?

the presence of myelin increases the conduction velocity of a nerve impulse

What happens when a nerve impulse reaches the terminal?

neurotransmitters are released and diffuse across the synaptic cleft (gap) to bind to the receptors of another neuron.

presynaptic neurons

the neurons that conduct impulses toward the synapse

postsynaptic neurons

the neurons that carry impulses away from the synapse

what can the negative membrane potential be attributed to?

an asymmetrical distribution of ions across the membrane, which is created by charged proteins and the Na/K-ATPase pump

Why is the inside of the cell so negative?

many amino acids are negatively charged, therefore, the proteins they produce are also negatively charged

what do the negatively charged proteins attract?

draw positively charged potassium ions into the cell by an electrical gradient

What happens as potassium enters the cell?

the concentration within the cell increases creating a concentration gradient

What are the two forces acting on potassium ions?

• an electrical gradient pulls them into the cell and a concentration gradient pushes them out.

What do these gradients look like at equilibrium?

• the electrical and concentration gradients would be equal and opposite so no net ion flow occurs in either direction.

How is the flow of potassium ions out of the cell counterbalanced?

by the NA/K-ATPase pump which transports potassium ions back into the cell.

sodium flow into cell?

• flow is slower because permeability of the cell membrane is much lower permeability to potassium.

What is the ratio of potassium and sodium removed or pumped in by the Na/K-ATPase pump?

• it pumps three sodium ions out of the cell for every two potassium ions it pumps in

Concentration of sodium inside and outside of cell?

• there is a lower concentration of sodium inside the cell compared to the outside due to the fact sodium that enters the cell is removed

Electrical and concentration gradient flow for sodium?

both the electrical and concentration gradients push sodium into the cell

concentration of potassium inside and outside of cell?

• higher concentration in the inside of the cell than outside

What is the resting membrane potential?

-70 mV

When are action potentials triggered?

when the membrane is depolarized (made less negative) beyond a certain threshold (-55mV)

Threshold value?

-55 mV

excitatory postsynaptic potentials?

small membrane depolarizations that are usually not sufficient to bring the cell to threshold and produce an action potential.

Inhibitory postsynaptic potentials?

create a small hyper-polarization, moving the membrane voltage further from threshold.

What can produce a larger depolarization?

if many presynaptic cells have action potentials at the same time that result in EPSPs , their EPSPs can piggy-back on one another to produce a larger depolarization.

What does the response of the postsynaptic neuron depend on>

• the net input of EPSPs and IPSPs

spatial summation?

the phenomenon where the synchronous activity of many motor neurons combines to produce a response.

What happens when the membrane is depolarized?

voltage-gated sodium channels open allowing the sodium to freely cross the membrane resulting in a rapid upstroke of the action potential.

What happens during repolarization?

voltage-gated potassium channels open more slowly, allow potassium to move out of the cell, repolarizing the membrane back towards resting levels. Na channels close

What does the presence of these voltage-gated ion channels allow?

enables excitable cells to generate action potentials and conduct them along their cell membranes

What happens if many presynaptic cells have action potentials at the same time?

results in EPSPs, and their EPSPs can piggy-back on one another to produce a larger depolarization.

What happens when the membrane is depolarized during a response?

voltage-gated sodium channels open sodium to freely cross the membrane resulting in the rapid upstroke of the action potential. Rises from -55 millivolts to +30 mV

what happens at the neuromuscular junction?

an action potential in the motor neuron (the presynaptic cell) may produce an action potential in the muscle (the postsynaptic cell) and a contraction.

what does action potential formation usually rely on?

summation of these exciatory postsynaptic potentials or EPSPs.

relationship between shock intensity and axon recruitment

as shock intensity increases, more axons (nerve fibers) are recruited.

difference between compound action potential and an action potential?

Action potential: the electrical signal generated by one neuron or axon.

Compound action potential: The summed electrical response from many axons in a nerve bundle.

Does the compound action potential follow the all-or-none rule?

no, as it is the summation of many axons electrical response that have various thresholds.

Do action potentials follow the all-or-nothing rule?

• Yes

• If the stimulus reaches threshold, the neuron fires an action potential of full size.

• If not, no action potential occurs at all.

resting membrane charge distribution of a resting neuron?

• greater negative charge in the inside of the cell

• greater positive charge in the outside of cell

what do the pumps maintain?

the resting membrane potential

What happens at +30mV ?

Voltage-gated Na+ channels close and voltage-gated K+ channels open

What happens during repolarization?

potassium ions diffuse out of the axon, the membrane potential begins to be negative again and goes even beyond the resting state.

What happens during hyperpolarization?

voltage-gated potassium channels close and all voltage-gated Na+ channels are released from inactivation and ions move through leak channels only. Membrane potential is below resting membrane potential of -70 but then balanced out back to resting.

what is the space between the first and second neuron called

synaptic cleft or gap

presynaptic cell

cell that comes before the synaptic cleft

postsynaptic cell

cell that comes after synaptic cleft

what happens when the electrical impulse gets to the axon terminal?

its going to active calcium voltage-gated channels causing an influx of calcium into the cell and it will tell the the axon to excise the products that are stored within the vesicles.

• the neurotransmitter with go from high to low concentration and bind to receptors on the post-synaptic cell

continuous propagation

each segment needs to elicit an action potential to send an electrical impulse down

saltatory propagation?

• involves myelin that makes the action potential move faster down the axon

what maintains our resting membrane potential?

Na/K-ATPase pump

what is the threshold?

-55 mV

hyper polarization brings out membrane potential down to?

-90 mV

temporal summation?

one neuron receives multiple signals in quick succession from the same presynaptic neuron and those signals add up over time to trigger an action potential and reach threshold

what are the types of refractory periods?

Absolute- no matter how strong stimulus is, you cannot elicit another action potential. Period is during depolarization.

Relative- we can have another action potential but it requires a stimulus stronger than normal. Period is during hyperpolarization