ns chapter

All cells possess

a membrane potential related to the non-uniform distribution of Na+ and K+.

Nerve and muscle are

excitable tissue that use this potential by undergoing controlled, transient, rapid changes in membrane potential. Such fluctuations in membrane potential serve as electrical signals.

Two kinds of such electrical signals

Action potentials are long distance signals. Graded potentials are short distance signals.

3 factors create the membrane potential

Action of sodium/potassium pumps (3Na+ out and 2K+ in). Permeability differences between Na+ and K+. Presence of fixed anions (-) unable to leave the cell.

A voltmeter

Is shown measuring the difference in charge between the inner cell membrane and outer cell membrane.

Sodium-potassium (SP) pump-protein complex

continually pumps 3 Na+ ions out of cells while drawing 2 K+ ions into cell, which helps to maintain the electrical gradient.

The membrane is

selectively permeable, allowing some ions to pass more freely than others. Sodium, potassium, calcium, and chloride pass through channels in the membrane.

When the membrane is at rest

Sodium channels are closed. Potassium channels are partially closed allowing the slow passage of K+ (moves slowly back and forth to maintain -70mV).

Stimulus to presynaptic cell

produces graded potential.

If the graded potential is large enough

it produces action potential (AP) in the axon membrane.

AP is propagated and

the impulse travels along the axon to one or more synapses.

Graded potentials in the postsynaptic cell

are then produced due to synaptic activity.

Resting potential (RP) of a neuron

refers to the steady state of the neuron prior to the sending of a nerve impulse. (This equals roughly -70mV.)

RP remains stable

until the neuron is stimulated.

Hyperpolarization

increasing the polarization or the difference between the electrical charge of two places. (Driving the potential more negative = harder to stimulate neuron).

Depolarization

decreasing the polarization towards zero (causes AP if reaches threshold which fires neurons).

Threshold of excitement

refers to any stimulation beyond a certain level and results in a massive depolarization (-55 mV).

In order to create an action potential

the much smaller graded potentials must create enough charge to move the voltage from -70mV (resting potential) to -55mV (threshold).

Depolarization that meets or exceeds

55mV generates AP.

Voltage gated channels

ion channels that only open at a certain voltage.

When the voltage meets threshold

(-55mV), it opens voltage gated Na+ gates first.

Na+ floods into cell until

the cell reaches +30mV.

+30mV is the trigger voltage to

close the voltage gated Na+ channels and also open voltage gated K+ channels. Stop influx of Na+. Lets K+ rush out to restore balance.

After an action potential occurs

sodium channels are quickly closed.

The neuron is returned to its resting state by

the opening of the potassium channels.

Potassium ions flow out

due to the concentration gradient and take with them their positive charge.

The sodium-potassium pump later

restores the original distribution of ions, so the membrane potential returns to -70 mV.

Follows an All-or-None Law (Action potential)

either stimulus produces AP or doesn't.

After an action potential a neuron has a

refractory period during which time the neuron resists another action potential and is trying to achieve Resting Potential.

The absolute refractory period

is the first part of the period in which the membrane can not produce an action potential.

The relative refractory period

is the second part in which it takes a stronger than usual stimulus to trigger an action potential.

Action Potentials can be blocked

Local anesthetics (example-Novocain) block sodium channels, therefore preventing action potentials from occurring.

Propagation

the action potential moves down the axon toward the axon terminals.

The voltage change in one area

triggers voltage gated channels in nearby areas.

Unmyelinated axons

propagate the action potential slowly.

Myelinated axons

propagate the action potential much more quickly due to saltatory conduction.

The AP jumps from

one Node of Ranvier to the next instead of opening every ion channel in between.

CNS DEMYELINATING CONDITIONS

Multiple sclerosis.

Multiple Sclerosis

an autoimmune reaction destroys myelin in random patches within the brain and/or spinal cord.

PNS DEMYELINATING CONDITIONS

Guillain-Barré Syndrome, Charcot-Marie-Tooth Syndrome.

Guillain-Barré Syndrome

an autoimmune reaction, often following viral illness, attacks myelin in the peripheral nerves. Progresses rapidly, but most people recover.

Charcot-Marie-Tooth Syndrome

Inherited genetic disorder that causes issues with myelin and other parts of neurons, often seen with neuropathy and foot deformities.

Synapse

junction between two neurons (or other effector).

#1 Synapse step

Neuron synthesizes chemicals - neurotransmitters.

#2 Synapse step

Store neurotransmitters in axon terminals or transport them there.

#3 Synapse step

AP triggers release of neurotransmitters into the synaptic cleft.

#4 Synapse step

Neurotransmitters travel across cleft & attach to receptors on postsynaptic neuron.

#5 Synapse step

Neurotransmitters separate from receptors.

#6 Synapse step

Neurotransmitters are then either taken back into presynaptic neuron, diffuse away, or inactivated by enzymes.

#7 Synapse step

Postsynaptic cell may send negative feedback to slow the release of further neurotransmitters.

Neurotransmitters released into synapse

do not remain & are subject to either inactivation or reuptake.

Reuptake

when the presynaptic neuron takes up and reuses most of intact neurotransmitter molecules. Example- Serotonin is taken back up into the presynaptic terminal. SSRI - selective serotonin reuptake inhibitor (SSRI), leaves more serotonin available.

Inactivation and reuptake of components

Example- Acetylcholine broken down by acetylcholinesterase into acetate & choline.

Research has begun to

investigate the role of events at the synapse and their effects on personality.

Research suggests that

some dopamine receptors may be related to "pleasure-seeking" and "thrill-seeking" behaviors.

How do graded potentials "add up" to enough voltage to trigger an action potential?

ANS, Summation.

ANS

summation ("sum").

Temporal Summation

Repeated stimuli over short period of time produced stronger response. Can have a cumulative effect. Can produce a nerve impulse when a single stimuli is too weak.

Spatial Summation

Several small stimuli on a similar location produced a reflex when a single stimuli did not. Synaptic input from several locations can have a cumulative effect and trigger a nerve impulse.