PSYCH 2 - Week 3 Membrane
integration
Adding or combining a number of individual signals into one overall signal.
Pg 81 Q 15: Unlike postsynaptic potentials,which are graded, action potentials are _________________ responses.
All-or-none
The wave of excitation triggered by the generation fan action potential near the axon hillcock always spreads passively back through the the cell body and dendrites of the neuron.
Although little is yet known about the functions of these backward action potentials, they are currently the subject of intensive investigation.
Pg 92 Q5: nondecremental
Axonal conduction of action potentials
Myelination increases the speed of axonal conduction.
Because the conduction along the myelinated segments of the axon is passive, it occurs instantly, and the signal thus "jumps" along the axon from node to node.
Pg 81 Q 11: Neurotransmitters typically have one of two effects on postsynaptic neurons: They either depolarize them or ______________ them.
Hyperpolarize
sodium-potassium pump
It was subsequently discovered that the transport of Na+ ions out of neurons and the transport of K+ ions into the are not independent processes. Such ion transport is performed by emergency-consuming mechanisms in the cell membrane that continually exchange three Na+ ions inside the neuron for two K+ ions outside. These transporters are commonly referred to as sodium-potassium pumps. Mechanisms in the membrane of a cell that actively transportions or molecules across the membrane.
The conduction of EPSPs and IPSPs is passive, whereas the axonal conduction of action potentials is largely active.
Once an action potential has been generated, it travels passively along the axonal membrane to the adjacent voltage-activated sodium channels, which have yet to open.
Four kinds of ions contribute significantly to the resting potential:
Sodium ions (Na+) Potassium ions (K+) Chloride ions (Cl-) And various negatively charged protein ions.
Pg 81 Q 10: From their calculations, Hodgkin and Huxley inferred the existence of ____________ in neural membranes.
Sodium-potassium pumps
Pg 92 Q8: synaptic vesicles
Store neurotransmitters
temporal summation
Summation by a postsynaptic cell of input (EPSPs or IPSPs) from a single source over time. It shows how postsynaptic potentials produced in rapid succession at the same synapse sum to form a greater signal.
nondecremental
The conduction of action potentials along an axon. action potentials do not get weaker with distance as they travel along the atonal membrane.
Resting potential
The difference in electric charge between the inside and outside of a neuron's cell membrane. The steady membrane potential of about -70mV. In its resting state, with the -70mV charge built up across its membrane, a neuron is said to be polarized.
When stimulated by action potentials, these channels open, and Ca++ ions enter the button.
The entry of the Ca++ ions causes synaptic vesicles to fuse with the presynaptic cleft. At many—but not all—synapses, one action potential causes the release of neurotransmitter molecules from one vesicle.
In Myelinated axons, ions can pass through the axonal membrane only at the nodes of Ranvier:
The gaps between adjacent myelin segments.
4 factors for unequal distribution:
Two factors that act to distribute ions equally throughout the intracellular and extracellular fluids of the nervous system and two features of the neural membrane that counteract these homogenizing effects.
When a neuron is at rest, synaptic vesicles that contain small-molecule neurotransmitters tend to congregate near sections of the presynaptic membrane that are particularly rich in...
Voltage-activated calcium channels
iontropic receptors
fast-open an ion channel when the transmitter molecule binds. Those receptors that are associated with ligand-activated ion channels.
Random motion
first of the two homogenizing factors. the ions in the neural tissue are in constant random motion, and particles in random motion tend to become evenly distributed because they are more likely to move down their concentration gradients than up them; that is, they are more likely to move from areas of high concentration to areas of low concentration than vice versa.
voltage-activated ion channels
ion channels that open or close in response to changes in the level of the membrane potential.
Graded responses
the amplitudes of EPSPs and IPSPs are proportional to the intensity of the signals that elicit them: Weak signals elicit small postsynaptic potentials, and strong signals elicit large ones.
decremental
the flow of charge decreases as the distance from the site of origin of the graded potential increases. - the transmission of EPSPs and IPSPs is decremental: EPSPs and IPSPs decrease in amplitude as they travel through the neuron, just as a sound wave loses amplitude (the sound grows fainter) as it travels through air.
threshold
the level of stimulation required to trigger a neural impulse. If the sum of the depolarizations and hyperpolarizations reaching the section of the axon adjacent to the axon hillcock at any time is sufficient to depolarize the membrane to a level referred to as its threshold of excitation. Usually about -65 mV— an action potential is generated near the axon hillock.
electrostatic pressure
the propensity of charged molecules or ions to move toward areas with the opposite charge. the second homogenizing factor that promotes the even distribution of ions. Any accumulation of charges, positive or negative, in one area tends to be dispersed by the repulsion among the like charges in the vicinity and the attraction of opposite charges concentrated elsewhere.
The exocytosis of small-molecule neurotransmitters differs from the exocytosis of neuropeptides. Small-molecule neurotransmitters are typically released in a pulse each time an action potential triggers a momentary influx of Ca++ ions through the presynaptic membrane.
In contrast, neuropeptides are typically released gradually in response to general increases in the level of intracellular Ca++ ions, such as might occur during a general increase in the rate of neuron firing.
spatial summation
Integration by a postsynaptic neuron of inputs (EPSPs and IPSPs) from multiple sources. It shows simultaneously no different parts of the receptive membrane sum to form a greater EPSP, how simultaneous IPSPs sum to form a greater IPSP, and how simultaneous EPSPs and IPSPs sum to cancel each other out.
Pg 81 Q 14: An action potential is elicited when the depolarization of the neuron reaches the ________________.
Threshold of excitation
ion channels
channel proteins that transport ions Ions pass through the neural membrane at specialized pores. Each type of which is specialized for he passage of particular ions.
pharmacological
pertaining to the scientific study of drugs
EPSPs and IPSPs
- the transmission of post synaptic potentials is rapid, so rapid that it can be assumed to be instantaneous for most purposes. - all postsynaptic potentials, whether brief or enduring, are transmitted at great speed. - the transmission of EPSPs and IPSPs is decremental: EPSPs and IPSPs decrease in amplitude as they travel through the neuron, just as a sound wave loses amplitude (the sound grows fainter) as it travels through air. - Most EPSPs and IPSPs do not travel more than a couple of millimeters from their site of generation before they fade out; thus, they never travel very far along an axon. EPSPs (depolarizations) occur because the neurotransmitter opens sodium channels, thereby increasing the flow of Na+ ions into the neuron. IPSPs (hyperpolarizations) often occur becasue the neurotransmitter opens potassium channels or chloride channels, thereby increasing the floe of K+ ions out of the neuron or the flow of CL- ions into it, respectively.
To study neurotransmitters and behavior, researchers administer to human or nonhuman subjects drugs that have particular effects on particular neurotransmitters and then assess the effects of the drugs on behavior.
Drugs either facilitate (agonists) or inhibit (antagonists) synaptic transmission.
Pg 92 Q10: acetylcholinesterase
Enzymatic degradation
Why do the intracellular and extracellular concentrations of Na+ and K+ remain constant in resting neurons?
There are active mechanisms in the cell membrane to counteract the influx (inflow) of Na=ions by pumping Na+ ions out as rapidly as they pass in and to counteract the effluent (outflow) of K+ions by pumping K+ions in as rapidly as they pass out.
action potential
a neural impulse; a brief electrical charge that travels down an axon. is a massive but momentary—lasting for 1 millisecond—reversal of the membrane potential from about -70 to about +50 mV. Are not graded responses; their magnitude is not related in any way to the intensity of the stimuli that elicit them. They are all-or-nothing responses; that is,they either occur to their full extent or do not occur at all. The means by which axons conduct all-or-none signals nondecrementally over relatively long distances.
excitatory postsynaptic potential (EPSP)
a slight depolarization of a postsynaptic cell, bringing the membrane potential of that cell closer to the threshold for an action potential. Postsynaptic depolarizations, because they increase the likelihood that the neuron will fire.
inhibitory postsynaptic potential (IPSP)
an inhibitory hyperpolarization of the postsynaptic membrane of a synapse caused by the liberation of a neurotransmitter by the terminal button. Postsynaptic hyperpolarizations because they decrease the likelihood that the neuron will fire.
Receptors
parts of the cell membrane that receive the neurotransmitter and initiate or prevent a new electric signal. Neurotransmitter molecules produce signals in ostsynaptic neurons by binding to receptors in the postsynaptic membrane. Each receptor is a protein that contains binding sites for only particular neurotransmitters; thus, a neurotransmitter can influence only those cells that have receptors for it.
Ions
positively and negatively charged atoms or particles.
Vesicles
small membrane sacs that specialize in moving products into, out of, and within a cell.
