Presentation on theme: "Martini Chapter 12 Bio 103 Part 2"— Presentation transcript:
1 Martini Chapter 12 Bio 103 Part 2 Nervous TissueMartini Chapter 12Bio 103Part 2
2 How do neurons communicate? Electrically: Action PotentialsAll or NothingALWAYS EXCITATORY!
3 How do neurons communicate? Electrically: Action PotentialsAll or NothingALWAYS EXCITATORY!Chemically: Neurotransmittersvarious typescan stimulate or depress electrical activitycan have long impact on post synaptic cellular function
4 Electricity Terminology Voltagepotential energy generated by separated chargeCurrentthe flow of electrical charge from one point to anotherResistancehindrance to the currentinsulators = high resistanceconductors = low resistance
5 Electricity Terminology Voltagepotential energy generated by separated chargeCurrentthe flow of electrical charge from one point to anotherResistancehindrance to the currentinsulators = high resistanceconductors = low resistanceOhm’s LawCurrent (I) = voltage (V) / resistance (R)ORV = I x R
6 The Transmembrane Potential All cells have a difference in charge across their membranes resulting in potential energy.measured in voltagemVolts
7 An AnalogyMembrane Potential is like a damned up lake, except, instead of water trying to get through, its ions.
8 The Transmembrane Potential All cells have a difference in charge across their membranes resulting in potential energy.GENERALLY:extracellular fluid ishigh in Na+high in Cl-
9 The Transmembrane Potential All cells have a difference in charge across their membranes resulting in potential energy.GENERALLY:extracellular fluid ishigh in Na+high in Cl-intracellular fluid ishigh in K+high in proteins (A-)
10 The Transmembrane Potential All cells have a difference in charge across their membranes resulting in potential energy.THUS,the intracellular environment is relatively more negativeneurons usually mV at rest
11 The Resting Membrane Potential The voltage across the membrane when the cell is at rest.RMP for neurons usually around -70 mV
12 How is the Transmembrane Potential Created and Maintained? If the cell membrane were freely permeable, diffusion would eventually distribute the ions and proteins evenly across the membrane.
13 How is the Transmembrane Potential Maintained? If the cell membrane were freely permeable, diffusion would eventually distribute the ions and proteins evenly across the membrane.BUT:Ions must pass through ion channels or be transported by an active (ATP requiring) mechanismthe large, mostly negative proteins inside the cell cannot cross the selectively permeable membrane
14 How is the Transmembrane Potential Created? Passive forces create voltage across the membrane, which the cell uses as potential energy:Chemical Gradientconcentrations of a molecule differ across the membranemolecules diffuse to the areas of lower concentrationElectrical Gradientcharge differs across the membraneopposites (+/-) attract, molecules diffuse towards opposite chargeElectrochemical Gradientthe sum effect of electrical and chemical forces
15 How is the Transmembrane Potential Created? The electrochemical gradients of Na+ and K+ are the primary factor determining the transmembrane potential in neurons
16 How is the Transmembrane Potential Created? The electrochemical gradients of Na+ and K+ are the primary factor determining the transmembrane potential in neuronsNa+ and K+ must cross the cell membrane through channels, or via active transport
17 Ion Channels leaky (passive) ion channels ions can always pass through these protein channelsboth Na+ and K+ have leak channels in neurons, but K+ has significantly more
18 Ion Channels gated ion channels these channels open and let ions pass only under specific circumstances
19 Ion Channels chemical-gated channels regulated by chemical signals that bind to the channel
20 Ion Channels voltage-gated channels regulated by changes in voltage across the membrane
21 Ion Channels mechanical-gated channels regulated by changes in pressure
22 Ion Channels and Chemical Gradients Na+Na+Na+Na+K+Which direction will Na+ or K+ move through leak or gated channel?Na+K+K+NEURONK+K+K+Na+K+K+K+Na+Na+Na+Na+Na+
23 The membrane is more permeable to K+ relative to Na+ NEURONK+K+K+Na+K+K+K+Na+Na+Na+Na+Na+
24 The membrane is more permeable to K+ relative to Na+ Potassium leaves faster than sodium enters, and the cell become more negative.Na+K+K+NEURONK+K+K+Na+K+K+K+Na+Na+Na+Na+Na+
25 The membrane is more permeable to K+ relative to Na+ Potassium leaves faster than sodium enters, and the cell become more negative.This is one factor that contributes to the transmembrane potential.Na+K+K+NEURONK+K+K+Na+K+K+K+Na+Na+Na+Na+Na+
26 Electrochemical Gradient of K+ chemically K+ wants OUT A LOTelectrically K+ want IN A Littletogether, the net effect is to move out of the cell
27 Electrochemical Gradient of Na+ chemically Na+ wants IN A LOTelectrically Na+ want IN A Littletogether, the net effect is to move into the cell
28 Equilibrium Potential The transmembrane potential at which there is no net movement for a specific ionthe voltage at which the gradient for an ion is eliminatedpotassium = -90 mVclose to resting membrane potentialsodium = +66 mVfar from resting membrane potential
29 Maintaining the Resting Membrane Potential AT REST:cell is highly permeable to K+large positive charge leaves the cellcell has little permeability for Na+only slight positive charge enters cellHOWEVER,eventually enough Na+ will leak across to eliminate the resting membrane potential (-70mV)
30 Maintaining the Resting Membrane Potential In order to maintain the electrochemical gradients of Na+ and K+, they must be actively transported across the membrane
34 Overview of Resting Potential membrane is highly permeable to K+, so the RPM is close to K+ equilibrium potential
35 Overview of Resting Potential membrane is not very permeable to NA+, so RMP is not close to NA+’s equilibrium potential
36 Overview of Resting Potential the sodium-potassium exchange pump maintains the RMP3 NA+ out2 K+ in
37 Overview of Resting Potential at rest, the passive and active transport mechanisms are in balance and the RMP is stableneuron usually -70 mV
38 When a neuron is excited It’s membrane potential changes3 states of membrane potentialresting potentialat rest (-70 mV)graded potentialsome excitation (-69 to -61 mV)action potentialexcitation above threshold (-60 to -55 mV)
39 TERMINOLOGY EXCITATION INHIBITION when potential is made more positive (from -70mV to a more positive #) it is called depolarizationwhen resting potential (-70mV) is restored after depolarization it is called repolarizationINHIBITIONif potential is made more negative (from -70mV to a more negative #) it is called hyperpolarization.
40 How Do Changes in Membrane Potential Occur? Gated Ion Channels open and close
41 Graded Potentialsexcitation or depolarization of the transmembrane potential that doesn’t spread far from the site of stimulationThe stimulation was not strong enough to cause an action potential
42 An example of Graded Potential The neuron begins at rest
43 An example of Graded Potential a chemical (e.g., Acetylcholine), binds to its receptor on a chemically-gated Na+ channel
44 An example of Graded Potential Na+ rushes into the opened channels, causing a local current, depolarizing portions of the membrane
45 4 Characteristics of the Graded Potential membrane potential is most impacted at the site of stimulationchange in charge only spreads locally (local current)change in voltage can be:depolarizinge.g., if Na+ channels openhyperpolarizinge.g., if K+ channels open
46 Graded Potentials: actual terminology Excitatory Postsynaptic Potential (EPSP)Inhibitory Postsynaptic Potential (IPSP)At any point in time a neuron may be receiving numerous EPSP’s and IPSP’sIt is the summation of these individual inputs that determines if a neuron will send a message in the form of an action potential
47 Action Potential propagated excitation of the transmembrane potential chain reaction of depolarizing eventsneurons receives enough stimulation (graded potentials) to cross a threshold of voltageIf the threshold is exceeded voltage-gated Na+ channels openNa+ rushes into the cell setting of the chain reaction that is an action potential
48 Action Potential is ALL-OR-NONE An action potential only happens if enough excitatory stimulation occurs to bring the transmembrane potential above a thresholdMembrane Potential (mV)Time (ms)-70-5530thresholdThreshold typically -60 to -55 mVThreshold is the voltage that opens the voltage-gated Na+ channels
49 Action Potential is ALL-OR-NONE Once the threshold voltage is exceeded an action potential will take placeAction potentials have only 1 level of strengthThe amount that the threshold is exceeded will not affect the strength or speed of an action potentialMembrane Potential (mV)Time (ms)-70-5530thresholdaction potential
50 Generation of an Action Potential stimulation ABOVE thresholdincreased Na+ permeability causes depolarizationdecreased Na+ AND increased K+ permeability cause repolarizationprolonged increase in K+ permeability causes undershoot (hyperpolarization)return to normal membrane permeability and RMPMembrane Potential (mV)Time (ms)-70-5530threshold
51 Step 1 Stimulation of a Resting Neuron via excitatory chemicals binding to receptors on the post-synaptic neuronpost-synapticpre-synaptic
52 Step 2: Voltage-Gated Na+ Channels Open At rest, the voltage-gated Na+ channels are closed.
53 Step 2: Voltage-Gated Na+ Channels Open DEPOLARIZTION PHASE:When a neuron is stimulated above threshold (-60 to -55 mV) voltage-gated Na+ channels open and Na+ rushes INTO the cell making it even more positive.
54 Step 3: Na+ channels are inactivated and K+ channels open REPOLARIZATION PHASEAt the peak of the action potential curve (~30 mV), the inactivation gate (ball) on the Na+ channel snaps shut, stopping the rush of Na+ into the cell.As long as this gate is shut, the neuron cannot fire another action potential. It is in a REFRACTORY PERIOD
55 Step 3: Na+ channels are inactivated and K+ channels open REPOLARIZATION PHASEAt the same time, voltage-gated K+ channels are opening, allowing K+ to rush out of the cell.at 30 mV, both electrical and chemical gradients favor K+ movement out of the cell.
56 Step 4: Return to Normal Permeability HYPERPOLARIZATION PHASEa.k.a. undershootK+ channels remain open beyond the point of reaching -70 mV causing an undershoot of the RMP to about -90 mV (equilibrium of K+).When the membrane potential reaches -70mV, the inactivation gate on the Na+ channel (ball) snaps back open
57 Step 4: Return to Normal Permeability Eventually the voltage-gated K+ channels close and the membrane is again at the RMP of -70 mV.
58 The Refractory PeriodThere is a period after an action potential when a neuron cannot (1), or is unlikely (2) to produce another action potential.
59 The Refractory PeriodAbsolute Refractory Period: As long as the Na+ channel is deactivated, the neuron cannot fire another action potential.
60 The Refractory PeriodRelative Refractory Period: At this point the deactivation gate on the sodium channels has reopened, and the neuron can technically produce another action potential.HOWEVER:it has to overcome the membrane hyperpolarization!Threshold = -60 mVRMP = -70 mVHyperpolarized membrane = -90 mV
61 Na+/K+ exchange pump and Action Potentials 1 action potential does not change ionic concentrations enough to require the pump to reset the RMPHowever, a neuron can fire as many as 1000/secondThus, the exchange pump is necessary to maintain RMP
63 Comparing Graded Potentials and Action Potentials
64 Action Potential Propagation Unlike graded potentials, action potential spread across the entire neuron from soma to synapse.continuous propagation (axon is unmyelinated)saltatory propagation (axon has myelin – see picture below)
65 Continuous Propagation (unmyelinated axon) action potential in initial segmentlocal current depolarizes adjacent membrane
66 Continuous Propagation (unmyelinated axon) another action potential fires in this region, while the initial segment enters a refractory period
67 Continuous Propagation (unmyelinated axon) a local current depolarizes the adjacent portion of the membrane…and so on…..
68 Saltatory Propagation (myelinated axon) An action potential fires in the initial segment of the neuron.
69 Saltatory (leaping) Propagation (myelinated axon) The adjacent node is depolarized, skipping the myelinated segment.
70 Saltatory (leaping) Propagation (myelinated axon) an action potential fires in node 1the process, with action potentials being fired at each nodeBecause saltatory propagation leaps across internodes that may be 1-2 mm apart, propagation is much faster and consumes less energy (i.e., fewer Na ions have to be pumped back out).
72 Axon Diameter and Propagation Speed Larger diameter = faster propagationless membrane/surface area to impede flow of currentSquids have GIANT axons (100 X’s diameter of humans), to help them propel away from predators.
73 Axons are classified by diameter, myelin, and speed Type A fibersdiameter of 4-20 mmmyelinated300 mph propagation speedType B fibersdiameter of 2-4 mm40 mph propagation speedType C fibersdiameter of 2 mm or lessunmyelinated2 mph propagation speed
74 Type A Fibers What do they do? Carry sensory information to CNS position, balance, delicate touch and pressureCarry motor commands to the skeletal muscles
75 Type B and C Fibers What do they do? Carry sensory information to the CNS:temperature, pain, general touch and pressureCarry motor output information to smooth and cardiac muscles:
76 Why aren’t all fibers Type A? Compromise between speed and sizeonly 1/3 of fibers are myelinatedCritical information is carried over Type A