send or receive electrical impulses. 3 types based on function: sensory, motor and interneurons
are a diverse group of cells specialized for the detection of various types of stimuli. Communication stream of information from various receptors to the brain about the body and environment
process signals received from other neurons and relay on information to other parts of the nervous system
form the insulating myelin sheath around neurons of the CNS and those of the peripheral nerves., produces myelin sheath in CNS
a layer of fatty tissue segmentally encasing the fibers of many neurons; enables vastly greater transmission speed of neural impulses as the impulse hops from one node to the next
largest, most numerous glial cells; maintain blood-brain barrier to isolate CNS from general circulation; provide structural support for CNS; regulate ion and nutrient concentrations; perform repairs to stabilize tissue and prevent further injury
The giant squid axon has a diameter of .5-.1 mm allowing easy inset ion of micro electrodes to measure and control electrical potentials and ionic currents across potential
includes a nucleus and many components of the endomembrane system, the part of a neuron that produces the energy needed for the activity of the cell
the bushy, branching extensions of a neuron that receive messages and conduct impulses toward the cell body. Function is to receive and combine signals received form other neurons
a part of a neuron that carries impulses away from the cell body. Carrying of signals can be over long distances
nodes of Ranvier
Gaps in the myelin sheath of the axons of peripheral neruons. Action potentials can 'hump' from node to node, thus increasing the speed of conduction (saltatory conduction).Ensures depolarization renewed periodically down the axon. Meylination reduces the membrane capacitance allowing sodium current entering at one point of the membrane to spread much further along the membrane..
ONLY PLACE ON MYELINATION AXON WHERE ACTION POTENTIAL CAN BE GENERATED BECUASE CURRENT FLOWS THROUGH THE MEMBRANE IS RESTRICTED ELSEWHERE
IN NODES ITSELF VOLTAGE SENSITIVE SODIUM CHANNELS ARE HIGHLY CONCENTRATED
aka synaptic boutons- responsible for transmitting the signal to the new cell which may be another neuron or gland cell. ( termination of branch)
resting membrane potential
An electrical potential established across the plasma membrane of all cells by the Na+/K+ ATPase and the K+ leak channels. IN most cells, the resting membrane potential is approximately -70 mV with respect to the outside of the cell. denoted as Vm. Can be measured by placing one micro electrode inside the cell and another out side the cell;its value in given in millivolts.
Junction. usually occur between axon and dendrite. Typically neurons make synapse with many other neurons, The cant do so not only at the ends of their axons but at other points along their length as well
the ability to respond to certain stimuli by producing electrical signals such as action potentials
a neural impulse; a brief electrical charge that travels down an axon. the action potential is generated by the movement of positively charged atoms in and out of channels in the axon's membrane
when ions are in solution they are always present in pairs, one positive and one negative so there is no net charge imbalance.
current ( in amperes A)
When negative or positive ions are actually moving, one towards other then the current is flowing and measured in amperes (A)
electrical potential ( voltage)
the tendency of oppositely charged ions to flow back towards each other after local separation
An equation predicting the voltage needed to just counterbalance the diffusion force pushing an ion across a semipermeable membrane from the side with a high concentration to the side with a low concentration., Ecell= E°cell - (RT/nF)(lnQ); where Q=reaction quotient=products/reactants, with stoichiometric coefficients as exponents;
The condition in which no net ionic flux occurs across a membrane because ion concentration gradients and opposing transmembrane potentials are in exact balance., When net flux = 0 and net current = 0
Example. Inside cell is -90mV. High concentration of K+. Concentration pushes K+ out and electrical concentration pulls it back into the cell. Equilibrium is when these two forces are equal.
although the plasma membrane is relatively impermeable to sodium ions, there is always a small amount of leakage. To compensate for this leakage the pump continually pumps sodium out of the cell while carrying potassium inward. 3 Na for eat K ion
equilibrium membrane potential
Every ion conctration gradient across a membrane can be counterbalanced by an electrical potential, equilibrium potential, Ex, for an ion X.
A change in a cell's membrane potential such that the inside of the membrane becomes more negative relative to the outside. Hyperpolarization reduces the chance that a neuron will transmit a nerve impulse. Increasing permeability of cells to Cl has two effects one being hyper polarization. The net entry of Cl ions without matching cation. The membrane potential becomes more negative than usual .
voltage-gated ion channels
potassium channels are multimeric proteins
Na are monomeric proteins
A specialized ion channel that opens or closes in response to changes in membrane potential.
Chloride ions tend to diffuse into the cell, which should in principle make the membrane more negative, however Cl ions are also replied by the negative membrane so that chloride ions enter in association with positively charged ions ssh as Na. The paired movement nullifies the depolarizing effect of sodium entry.
defines this relationship and allows one to calculate the membrane potential from the known constants of the Nernst potentials and the relative permeability for all ions at any instant in time - not just rest, Determines MEMBRANE POTENTIAL (Vm) using 1) distribution of ions across plasma membrane (concentrations) 2) relative permeability of membrane to these ions 3) charges of these ions
Vm=RT/F ln Pk(k out) P na(na) out P cl (cl)in/ out out in, Vm = 61 log (P [K] outside + P [Na] outside / P [K] inside+ P [Na] inside)
is an indirect measure of permeability of a channel when a specify voltage is applied across the membrane. In chemical terms is inverse of resistance
closing of a membrane ion channel in such a way that it can reopen immediately in response to an appropriate stimulus.gates are never semi open. Have the ability to open rapidly in response to some stimulus and then close again
Refractory period when channel cannot open after an action potential to ensure that action potentials are separated. Acts like placing a padlock on a closed gate.
During inactivation, a particle inserts into the opening of the channel. For the channel to reactivate and open in response to stimulus the inactivating particle must move away from the pore.
The minimum potential shift at which an action potential is initiated (around -50mV usually). Changes of around +20 will drop back to normal resting levels
Once an action potential is initiated in one region of the membrane, it will travel along the membrane away from the site of origin, The spread of the action potential down an axon, caused by successive changes in electrical charge along the length of the axon's membrane.
absolute refractory period
For a few milliseconds, after an action potential it is impossible to trigger a new action potential. Sodium channels are inactivated and cannot be opened by depolarization. During hyperpolarization it is possible to re open but difficult. This is because both the leak channels and gated channels are open to potassium
relative refractory period
During hyperpolarization the membrane potential is very negative and far from triggering another round of channel opening. , a period after firing when a neuron is returning to its normal polarized state and will fire again only if the incoming message is much stronger than usual
passive spread of depolarization
depolarization at one point on the membrane spreads to adjacent regions. As the wave of depolarization spreads passively away from the site of origin in also decreases in magnitude
region where action potential are indicated most easily. Because of sodium channel. a given amount of depolarization will produce the greatest amount of sodium entry at sites where channels are abundant
the electrical discharge that travels along a nerve fiber. Can move only away from the initial site of depolarization because the sodium channels that have just be depolarized are in the inactive state ( refractory period
propagated action potential
It is the method of conduction in "Unmyelinated" Nerve Fibers reversing the polarity of one segment of the membrane, then the next adjacent segment and so on.
..., The type of propagation by which action potentials are transmitted along myelinated nerve fibers, Jumping from node of ranvier to next node of ranvier, greatly increasing the speed of conduction and conserving energy
electrical insulation for segmentes of the axon it envelops and is formed in CNS by oligodendrocytes and in PNS by schwann cells.
in a synapse, of or pertaining to the neuron that bears receptors for neurotransmitter released into the synaptic cleft by the presynaptic neuron.
ligand gated channel in which activation directly affects the cells and receptions that function as ion channels. chemical gated
The presynaptic plasma membrane is separated from the postsynaptic plasma membrane by a small space
must elicit the appropriate response
must occur naturally in pre synaptic neuron
must be released at the right time when stimulated
Dopamine and the hormones norepinephrine and epinephrine derivatives of tyrosine
most common at neuromuscular junctions
increases permeability of post synaptic membrane to sodium within 1msec of binding to receptor
Excitatory to vertebrates skeletal muscles
synapses that use norepinephrine or epinephrine
receptors that respond to norepinephrine are called adrenergic receptors
short chains of amino acids formed by proteolytic cleavage of precursor proteins.
can excite and inhibit or modify
differ from neurotransmitters they act on groups of neurons and have long lasting effects
voltage gated ca channels
secretion of neurotransmitter by the pre synaptic is directly controlled by concentration of ca ions I synaptic bouton
action potential arrives, depolarization causes ca concentration to increase due to the opening of these channels
ca is large out side cell, inside cytosol is low
10 ⁻³ inside 10⁻⁷ inside
involves pumping neurtransmitters back into the presynaptic axon terminals or nearby support cells. The rate of some can be rapid, for some the synapse may be clreared of stray neurotransmitter within as little as a millisecond.
poisoned arrows snake bites and nerve gas
acetylcholine is such an important neurotransmitter any substance that interferes with its function is lethal. They bind covalently to the acetylcholine and blocks depolarization of the postsynaptic membrane and are antagonist of systems.
measuring membrane potentials
Because the inside of a cell typical has an excess of negative hare, electrodes compare ratio of negative to positive charge inside and out. Always need 2 electrodes, one inserted inside the axon which is recording and one placed in the fluid surrounding the cell known as reference electrode.
Measurement of action potential requires four electrodes 1 axon, 1 recording, 2 more for reference
ionic concentrations of axons and neurons
substances tend to diffuse from an area where they are more highly concentrated. Cell normally have high concentration of K ions inside and fewer outside. Negatively charged macromolecules can't pass through the membrane and remain inside the cell
makes it possible to study the behavior of individual ion channels in a small patch of membrane . when each channel opens the amount of current that flows through is always the same . following the burst of channel opening a quiescent period occurs due to channel inactivation
Used to show that when a soudim channel opens it conducts the same amount of current ( either must be opened or closed.
steady state ion movements
The compartment on the left represents cytosol of the cell, right extracellular fluid. The membrane is permeable to potassium ions but not relatively charge macromolecules . The result is accumulation of anions on the left side and cations on the right side.
relative concentrations of K NA and CL ions across membrane of neuron
a) Potassium ions- K+ are more concentrated in the cytosol and have a tendency to move out of the cell leaving behind trapped anions. Membrane potential become more negative
b) NA ions- Na are much more concentrated outside the cell than inside and tend to enter the cell. As Na ions enter they neutralize some excess negative charge in the cytosol and membrane potential becomes more positive.
c) Cl ions- Cl ions usually cross the membrane with a permeable cation ( normally K+) as Cl- ions enter the tell it tends to make the membrane potential more negative.
General structure of Voltage gated ion
Domain structure: voltage-gated channels for Na K and Ca ions all share the same basic structure themes. The channel is essentially a rectangular tube whose four walls are formed from with four subunits or four domains of a single polypeptide
Ion selective of changes, the transmembrane region of a potassium channel in the closed position. Hydrated K ions enter why channel where they give up their water and bind of oxygen atoms, precisely psotion in the amino acids lining the selectivity filter.
Function of a voltage gated ion channel
changes in membrane voltage cause rapid opening of channel by affecting its gate domain. Channel inactivation occurs when an inactivating particle blocks the channel opening.
changes in ion channels and currents in the membrane of squid during action potential
The change in membrane potential caused by movement of NA and K through their voltage gated channels which are shown at each step of the action potential. The absolute refractory period is caused by sodium channel inactivation. Notice that at the peak of action potential the membrane potential approaches the E na value of + 55 mV similarly the potential undershoots nearly to the EK values of about -75 mv. The change in membrane conductance. The depolarized membrane initially becomes very permeable to Na ions, facilitating a large inward rush of sodium, Thereafter as permeability to sodium declines the permeability of the membrane to potassium increases transiently causing it to hyper polarize.
Resting state. The voltage-dependent Na and K channels are closed, Because of the leakiness to K ions the cell is more than 100 times more permeable to K . When the region of a nerve cell is slightly depolarized a frantic of the Na channels respond. Voltage is around -60 Vm
Once the threshold potential is reached, a significant number of sodium channels begin activating. At this point the membrane potential shoots up rapidly . When the rate of sodium entry slightly exceeds the maximum rate of potassium exit, an action potential is triggered. When the potential peaks at +40 it approaches equilibrium
Repolarizing phase and Hyperpolarizing phase
One risen to its peak the membrane quickly depolarizes. this is due to a combination of the inactivation of sodium channels and the opening of voltage potassium channels. Cell repolarizes as potassium leaves the cell.
The lower dip in membrane potential. At the end of an action potential most neurons show a undershot in which the membrane potential briefly becomes even more negative than normal at rest.
Conductance measure of permeability
as permeability of sodium declines, the permeability of K increases causing hyper polarization
Passive spread of depolarization and propagated action potentials.
The transmission of a nerve impulse along a neuron depends on both the passive spread of delpoarization and the propagation of action potentials . A neuron is stimulated when its dendrites receives a depolarizing stimulated from other neurons. A depolarization starting at a dendrite spreads passively over the cell body to the axon hillock where an action potential forms. The action potential is then propagated down the axon
in electrical synapses the presynaptic and pst synaptic neurons are coupled by gap junctions which allow small molecules and ions to pass freely from cytosol from one cell to the next. When an action potential arrives at the presynaptic side of the electrical synapse the depolarization spreads passively across the gap junction. The gap junction is a set of channels.
the presynaptic and post synaptic newborn are not connected by gap junctions, although they are connected by cell adhesion proteins. The membranes are connected by synaptic cleft.
Electrical signals can not pass, so it must be converted b the presynaptic newborn and be carried by a neurotranmitter molecule.
different kinds of receptors that act as chemical synapses.
direct neurontransmitter action-ion otero pic as ion channels
when they bond a neurotransmitter they undergo conformational change and ions pass.
indirect action done by met
transmission of a signal across synapse
an action potential arrives at the bouton resulting in transient depolarization. Then depolarization opens Ca voltage gate allowing calciums into the terminal
Secretion of neurosecretory vesicles
action potential arrives in boutons resulting in transient depolarization. Ca channels open Ca enters terminal
increasing Ca releases vesicles
Increasing Ca induces the secretion of some neuorsecretory vesicles. Prolonged stimulation additional reserve vesicles. Then neurotransmitter diffuses across the synaptic cleft to the receptors on the postsynaptic cell
Binding alters properties
channels open letting ions flow into the postsynaptic cell, leads to either depolarization or depolarization, if sufficient depolarization occurs and action potential will result is the post cell
Docking of synaptic Vesicles with plasma membrane in presynaptic
In response to local elevation of Ca in presynaptic neuron, some vesicles become tightly assoicated within the membrane. when Ca channels open, they fuse with the plasma membrane releasing their contents
Integration of inputs
Neurons, particularly those in the CNS, receive inputs from thousands of synapses some excitatory and other inhibitory. An action potential may be generated in such a neuron at the axon hillock if the combined effects of membrane potentials induced results in depolarization above the threshold potential. Both temporal and spatial summation contribute to the likleyhood that AP will be initiated.
a neuron performs the same chemical action at all its synaptic connections to other cells, regardless of identity of target cell
receptor and can produce an influx of chloride into the postsynaptic neuron causing it to hyperpolarize. Valium enhances effects of receptor and produces tranquilizing effects
outside of axon
At the start the membrane is completely polarized. Stimulation of the membrane and sudden rush of sodium ions rush into the axon at that location
passive depolarization spreads
Membrane polarity is temporarily reversed at this point , depolarization then spreads to an adjacent point.
Sodium in the cell
The depolariztion at this adjacent point is strong enough to bring it above the threshold potential triggering the inward rush of sodium.
By this time the original region of the membrane has become highly permeable to potassium ions. When K rushes out of cell the depolarization spreads forward triggering an action potential
depolarization spreads forward repeating the process
Meanwhile the depolarization has spread to a new region, initating the same sequences of events.
Occurs when a single synapse generates EPSPs so quickly that each is generated before the previous decays. This allows the EPSPs to add up to reach a threshold voltage that triggers an action potential.