Neural Conduction. biologyaspoetry.com

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1 Neural Conduction biologyaspoetry.com

Resting Membrane Potential -70mV A cell s membrane potential is the difference in the electrical potential ( charge) between the inside and outside of the cell.

2 Resting Membrane Potential -70mV A cell s membrane potential is the difference in the electrical potential ( charge) between the inside and outside of the cell. The inside of the neuron is negative with respect to the outside. Resting membrane potential is about 70mV. In its resting state, a neuron is said to be polarized.

3 Ionic Basis of the Resting Potential outside Na Na + + Cl - X - Na + X- Cl - Cl - X- Na + Na + Na + Cl - X- X - Na + Na + X - more sodium (Na + ) and chloride (Cl - ) outside than inside anions maintain overall charge neutrality on both sides of the membrane inside K Na + A - + A- A - Na + Cl - A - A - A - A - A - A - cell membrane more potassium ( ) inside than outside

Distribution of Ions 1 Factors contributing to even distribution of ions Random motion: particles tend to move down their concentration gradient Electrostatic pressure: like charges repel like;

4 Distribution of Ions 1 Factors contributing to even distribution of ions Random motion: particles tend to move down their concentration gradient Electrostatic pressure: like charges repel like; opposites attract Equilibrium potential The membrane potential at which there is no net flow of an ion (concentration and electrical gradients are equal and opposite) concentration gradient electrostatic pressure

5 Factors contributing to uneven distribution of ions Selective membrane permeability to certain ions Sodium potassium pumps Distribution of Ions 2 Na + Na + biomhs.com

6 Membrane Permeable Only to Potassium outside leaves the cell due to the concentration gradient channels inside + - Equilibrium Potential: -100mV enters the cell due to the electrical gradient

7 Membrane Permeable Only to Sodium outside Na + Na+ Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + leaves the cell due to the electrical gradient Na + channels - + Equilibrium Potential: +40mV inside Na + Na + enters the cell due to the concentration gradient

8 Membrane Permeable Only to Chloride outside Cl - Cl- Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - Cl - leaves the cell due to the electrical gradient Cl - channels + - Equilibrium Potential: -70mV inside Cl - Cl - enters the cell due to the concentration gradient

The Neuron at Rest: Tug-of-War At rest, the cell membrane is permeable to, Na + and Cl - but is most permeable to Thus, the resting membrane potential is drawn towards the equilibrium potential and

9 The Neuron at Rest: Tug-of-War At rest, the cell membrane is permeable to, Na + and Cl - but is most permeable to Thus, the resting membrane potential is drawn towards the equilibrium potential and ends up at -70mV At rest, leaks out, Na + leaks in and Cl - is at equilibrium. Na+/K+ pumps transport 2 in for every 3 Na + they transport out to maintain the respective concentration gradients. academic.uprm.edu

Postsynaptic Potentials (PSPs) 1 Neurotransmitters bind at postsynaptic receptors. Chemical messengers make the postsynaptic membrane more or less permeable to specific ions.

10 Postsynaptic Potentials (PSPs) 1 Neurotransmitters bind at postsynaptic receptors. Chemical messengers make the postsynaptic membrane more or less permeable to specific ions. Depolarizations (excitatory postsynaptic potentials, EPSPs) make the membrane potential more positive Hyperpolarizations (inhibitory postsynaptic potentials, IPSPs) make the membrane potential more negative 0 5

11 membrane potential (millivolts) Postsynaptic Potentials (PSPs) 2 PSPs are graded, rapid and decremental Graded: amplitude proportional to intensity of stimulus (stronger stimuli = bigger PSPs) Rapid: EPSPs and IPSPs travel quickly from their site of generation (receptor) to the soma Decremental: they get smaller as they travel towards the soma receptor soma 0 time (ms) 10

12 PSPs and Neural Threshold AP -60 threshold cnx.org rest In order to generate an action potential (AP; or to make a neuron fire ), the threshold of activation (about -60 mv) must be reached near the axon hillock. One EPSP typically will not suffice summation is needed.

13 Types of Summation of PSPs Spatial summation Temporal summation integration of events happening at different places integration of events happening at different times

The Action Potential Action potential (AP): a shortlasting event (~1 millisecond in duration) in which the membrane potential of a cell rapidly rises (from about -70 mv to around +20 mv) and falls.

14 The Action Potential Action potential (AP): a shortlasting event (~1 millisecond in duration) in which the membrane potential of a cell rapidly rises (from about -70 mv to around +20 mv) and falls. action potential All-or-None: APs are not graded responses; their size (and velocity) is not related in any way to the intensity of the stimuli that elicit them. They occur to their full extent or do not occur at all (larger stimuli may elicit more APs than smaller stimuli).

Molecular Basis of the Action Potential APs are generated by special types of voltage-gated ion channels in the membrane of the axon (beginning at hillock).

15 Molecular Basis of the Action Potential APs are generated by special types of voltage-gated ion channels in the membrane of the axon (beginning at hillock). action potential When threshold is reached, voltage-gated sodium channels open allowing an influx of sodium; as the AP nears its peak, these channels close and voltage-gated potassium channels open allowing potassium to flow out. These channels close more slowly resulting in a return overshoot.

16 Dynamics of Voltage-Gated Channels The time-course of an AP can be divided into three phases: rising, falling and undershoot. The duration of these phases is dependent upon the opening and closing of the voltagegated sodium and potassium channels.

17 Refractory Periods Absolute RF action potential Relative RF Refractory Period: Time during which the cell resists generating new APs. There are two periods: Absolute (1 ms): impossible to initiate an AP because voltagegated Na + channels closed Relative (2-4ms): an AP is possible for a stronger than normal stimulus (to overcome fact that voltagegated K+ channels still open) Refractory periods prevent the backwards movement of APs and limit the rate of firing.

Conduction of Action Potentials The conduction of APs along an axon differs from that of PSPs in three ways: Not graded: APs are all-or-none Slow: channels open and

18 Conduction of Action Potentials The conduction of APs along an axon differs from that of PSPs in three ways: Not graded: APs are all-or-none Slow: channels open and close Nondecremental: always same size Orthodromic conduction: in the natural direction, from cell body to terminal buttons Antidromic conduction: towards the cell body

19 Conduction of APs voltage-dependent Na + channel (closed weakly) voltage-dependent channel (closed) Axon Soma

20 1. depolarization from dendrites Conduction of APs Axon Soma 2. threshold is crossed

21 Conduction of APs 1. depolarization from dendrites 4. sodium flows in 3. sodium channels open Na Axon Soma 2. threshold is crossed

22 Conduction of APs 4. sodium flows in Na Axon Soma 5. massive local depolarization 6. rapid passive propagation of depolarization (bidirectional)

23 Conduction of APs 7. sodium channels close strongly 8. potassium channels open and potassium flows out Na Axon Soma 5. massive local depolarization

24 Conduction of APs 8. potassium channels open 10. potassium and potassium channels close flows out Na Axon Soma 9. massive local repolarization

25 Conduction of APs 11. threshold crossed Na Axon Soma 6. rapid passive propagation of depolarization (bidirectional)

Action Potential Propagation APs do not travel continuously down the axon, but are triggered anew at each portion of the membrane that has voltage-gated

26 Action Potential Propagation APs do not travel continuously down the axon, but are triggered anew at each portion of the membrane that has voltage-gated channels. Saltatory conduction: in myelinated axons, APs hop from one node of Ranvier to the next. Speeds velocity from about 1-10 meters/second to up to 150 m/s.

Sample toxins: Na + channel: tetrodotoxin from puffer fish K+ channel:

27 Action Potential Blockers The propagation of APs can be blocked or reduced by toxins or demyelinating diseases. Sample toxins: Na + channel: tetrodotoxin from puffer fish K+ channel: noxiustoxin from scorpions Demyelination diseases: CNS: Multiple Sclerosis PNS: Guillian-Barre syndrome

Control and Integration. Nervous System Organization: Bilateral Symmetric Animals. Nervous System Organization: Radial Symmetric Animals

Control and Integration. Nervous System Organization: Bilateral Symmetric Animals. Nervous System Organization: Radial Symmetric Animals Control and Integration Neurophysiology Chapters 10-12 Nervous system composed of nervous tissue cells designed to conduct electrical impulses rapid communication to specific cells or groups of cells Endocrine