Ch11B-Fundamentals of Nervous System/Tissue

Group A:

*largest diameter *myelinated somatic sensory and moter fibers of skin, skeletal muscles, and joints *transmit at 150m/s (~300 mph)

When gated channels are open:

- ions diffuse quickly: -along chemical concentration gradients from higher concentration to lower concentration -along electrical gradients toward opposite electrical charge

After repolarization, Na+/K+ pumps

-(thousands of them in an axon) restore ionic conditions.

2. Depolarization: Na+ channels open

--depolarizing local currents open voltage-gated Na+ channels, and Na+ rushes into cell -Na+ activation and inactivation gates open -na+ influx causes more depolarization, which opens more Na+ channels *as a result, ICF becomes less negative -at threshold (-55mV to -50mV), positive feedback causes opening of all Na+ channels *results in large action potential spike *membrane polarity jumps to +30mV

What is the resting potential of a neuron?

-70mV *the cytoplasmic side of membrane is negatively charged relative to the outside *the actual voltage differences varies from -40mV to -90mV *the membrane is said to be polarized

Conduction velocity:

-APs occur only in axons, not other cell areas -AP conduction velocities in axons vary widely

Difference in Ionic Composition

-ECF has higher concentration of Na+ then ICF *balanced chiefly by chloride ions (Cl+) -ICF has higher concentration of K+ than ECF *balanced by negatively charged proteins -K+ plays most important role in membrane potential

3. Repolarization: Na+ channels are inactivating, and K+ channels open

-Na+ channel inactivation gates close *membrane permeability to Na+ declines to resting state *AP spike stops rising -voltage-gated K+ channels open *K+ exit ells down its electrochemical gradient -repolarization: membrane returns to resting membrane potential

What measures potential (charge) difference across membrane of resting cell?

-a voltmeter

Not all depolarization events produce

-action potentials.

All action potentials are:

-alike and are independent of stimulus intensity

Propagation:

-allows action potential to be transmitted from origin down entire axon length toward terminals -Na+ influx through voltage gates in one membrane area cause local currents that cause opening of Na+ voltage gates in adjacent membrane area *leads to depolarization of that area, which in turn causes depolarization in next area

All-or-None response:

-an AP either happens completely, or does not happen at all

Leakage (nongated) channels

-are always open

Activation gates:

-closed at rest; open with depolarization, allowing Na+ to enter cell

Membrane potential changes when:

-concentrations of ions across membrane change -membrane permeability to ions changes

Two terms describing membrane potential changes:

-depolarization: decrease in membrane potential (moves toward zero and above) *inside membrane becomes less negative than RMP *probability of producing impulse increases -hyperpolarization: increase in membrane potential (away from zero) *inside becomes less more negative than RMP *probability of producing impulse decreases

In conduction velocity, nerve fibers are classified according to:

-diameter -degree of myelination -speed of conduction

RMP is generated by:

-differences in ionic composition of ICF and ECF -differences in plasma membrane permeability

Current flows but ______ quickly and decays.

-dissipates

Repolarization resets _____ conditions and not ionic conditions.

-electrical

Electrochemical gradient:

-electrical and chemical gradients combined -ion flow creates an electrical current, and voltage changes across membrane *expressed by rearranged Ohm's law equation: V=IR

Current:

-flow of electrical charge (ions) between two points *can be used to do work *flow is dependent on voltage and resistance

CNS tells difference between a weak stimulus and a strong one by:

-frequency or impulses.

Ohm's Law:

-gives relationship of voltage, current, and resistance Current (I)=voltage (V)/resistance (R) -current is directly proportional to voltage *greater the voltage (potential difference). greater the current *no net current flow between points with same potential -current is inversely proportional to resistance *the greater the resistance, the smaller the current

Changes produce two types of signals:

-graded potentials: incoming signals operating over short distances -action potentials: long-distance signals of axons -changes in membrane potential are used as signals to receive, integrate, and send info

Fall into 3 groups:

-group A -group B -group C

Resistance:

-hindrance to charge flow *insulator: substance with high electrical resistance *conductor: substance with low electrical resistance

Differences in plasma membrane permeability

-impermeable large anionic proteins -slightly permeable to Na+ (through leakage channels) *sodium diffuses into cell down concentration gradient -25 times more permeable to K+ than sodium (more leakage channels) -potassium diffuses out of cell down concentration gradient -quite permeable to Cl-

Group B:

-intermediate diameter -lightly myelinated fibers -transmit at 15 m/s (~30 mph) *B/C groups include ANS visceral motor and sensory fibers that serve visceral organs

Role of membrane ion channels:

-large proteins serve as selective membrane ion channels *K+ ion channel allows only K+ to pass through

Axon diameter

-larger-diameter fibers have less resistance to local current flow, so have faster impulse conduction

Voltage:

-measure of potential energy generated by separated charge *measured between two points in volts (V) or millivolts (mV) *called potential difference or potential ~charge difference across plasma membrane results in potential *greater charge difference between points=higher voltage

At threshold:

-membrane is depolarized by 15 to 20 mV -Na+ permeability increases -Na+ influx exceeds the K+ efflux -the positive feedback cycle begins

Differences in plasma membrane permeability (2)

-more potassium diffuses out than sodium diffuses in *as result, inside of cell is more negative *establishes RMP -sodium-potassium pump [Na+/K ATPase] stabilizes RMP *maintains concentration gradients for Na+ and K+ *3 Na+ are pumped out while 2 K+ pumped in

Membrane potential and neurons

-neurons have a resting membrane -neurons rapidly change RMP -neurons are highly excitable

Propagation (2):

-once initiated, an AP is self-propagating *nonmyelinated axons, each successive segment of membrane depolarizes, then repolarizes *propagation of myelinated axons differs -since Na+ channels closer to the AP origin are still inactivated, no new AP is generated there *AP occurs only in a forward direction

1. Resting state

-only leakage channels for Na+ and K+ are open *maintains the resting membrane potential -each Na+ channel has two voltage-sensitive gates: activation gates and inactivation gates

Voltage-gated channels:

-open and close in response to changes in membrane potential

Mechanically gated channels:

-open and close in response to physical deformation of receptors, as in sensory receptors

Inactivation gates:

-open at rest; block channel once it is open to prevent more Na+ from entering cell

Chemically gated (ligand-gated) channels:

-open only with binding of a specific chemical (ex: neurotransmitter)

Basic principles of electricity

-opposite charges attract each other -energy is required to separate opposite charges across a membrane -energy is liberated when the charges move toward one another -when opposite charges are separated, the system has potential energy

Gated channels

-part of protein changes shape to open/close channel

Action potential (AP):

-principal way neurons send signals *means of long-distance neural communication -occur only in muscle cells and axons of neurons -brief reversal of membrane potential with a change in voltage of ~100mV -AP do not decay over distance like graded potentials -in neurons, also referred to as a nerve impukse -involves opening of specific voltage-gated channels

Graded potentials are signals only over ____ distances.

-short

Graded potentials are:

-short-lived, localized changes in membrane potential *stronger the stimulus, more voltage changes and farther current flows -triggered by stimulus that opens gated ion channels *results in depolarization and sometimes hyperpolarization -named according to location and function

4. Hyperpolarization: Some K+ channels remain open, and Na+ channels reset

-some K+ channels remain open, allowing excessive K+ efflux *inside membrane becomes more negative than in resting state -this causes hyperpolarization of the membrane (slight dip below resting voltage) -Na+ channels begin to rest

For an axon to "fire", depolarization must reach

-threshold voltage to trigger AP.

Two main types of ion channels:

1. Leakage (nongated) channels 2. Gated channels

4 main steps of generating an action potential:

1. Resting state:all gated Na+ and K+ channels are closed 2. depolarization: Na+ channels open 3. repolarization: Na+ channels are inactivating, and K+ channels open 4. hyperpolarization: some K+ channels remain open, and Na+ channels reset

Rate of propagation depends on two factors:

1. axon diameter 2. degree of myelination

Three types of gated channels:

1. chemically gated 2. voltage gated 3. mechanically gated

Two types of graded potentials:

1.receptor potential: graded potentials in receptors of sensory neurons 2. postsynaptic potential: neuron graded potential

Relative refractory period:

:follows absolute refractory period *most Na+ channels have returned to their resting state *some K+ channels still open *repolarization is occurring -threshold for AP generation is elevated -only exceptionally strong stimulus could stimulate an AP

Frequency is:

:numbers of impulses (APs) received per second -higher frequencies mean stronger impulses

Saltatory conduction:

:occurs only in myelinated axons and is about 30 times faster -myelin sheaths insulate and prevent leakage of charge -Na+ channels are located at myelin sheath gaps -electrical signal appears to jump rapidly from gap to gap

Continuous conduction:

:slow conduction that occurs in nonmyelinated axons

Absolute refractory period:

:time from opening of Na+ channels until resetting of the channels -ensures that each AP is an all-or-none event -enforces one-way transmission of nerve impulses

Refractory period:

:time in which neuron cannot trigger another AP * voltage-gated Na+ channels are open, so neuron cannot respond to another stimulus

Each K+ channel has one voltage-sensitive gate

closed at rest; opens slowly with depolarization

Once gated ions open,

depolarization spreads from one area of membrane to next

Degree of myelination

two types of conduction depending on presence or absence of myelin: -continuous conduction -saltatory conduction

Group C:

Smallest diameter -unmyelinated ANS fibers -transmit at 1 m/s (~2mph) *B/C groups include ANS visceral motor and sensory fibers that serve visceral organs

Two types of refractory periods are?

absolute refractory period and relative refractory period