Voltage (V) – potential energy generated by separated charges

Current (I) – flow of charges between points Resistance (R) – hindrance to charge flow Insulator –high electrical resistance Conductor –low electrical resistance

Electricity Definitions

Flow of ions rather than electrons Generated by different [Na+], [ K+], [ Cl], [anionic

proteins] and charged phospholipids Ion gradients

Differential permeability to Na+ and K+

Sodium-potassium pump

Biological Currents & Resting Potential (Vr)

Ca2+

1.8 mM

150 mM

5 mM

Electrical current created & voltage across the membrane changes when channels open

Ions flow down their chemical gradient from high [] to low []

Ions flow down their electrical gradient toward opposite charge

Electrochemical gradient The combined potentials of the electrical and chemical gradients

taken together

Electrochemical Gradient

[Hi]

[Lo]+

-

+

Electrochemical Gradients & Nernst Equation Potential established by equilibrium of ion flow

down concentration gradient balanced by repulsion of charges

Vr is established when rate of K+ moving out = K+ moving in Nernst equation relates chemical equilibrium to electrical potential EK = [2.3RT/zF](log[Ko]/[Ki]) = 0.061V[log(.005M/.150M)] = -90mV

[Hi] -

[Lo] +

K+

Passive channels always slightly open

Ligand gated channels opened/closed by a specific ligand

Voltage-gated channels opened/closed by change in membrane polarity

Mechanically-gated channels opened/closed by physical deformation

Ion Channels

Operation of a Ligand Gated Channel

Operation of a Voltage-Gated Na+ Channel

Depolarization – the inside of the membrane becomes less negative

Repolarization – the membrane returns to its resting membrane potential

Hyperpolarization – the inside of the membrane becomes more negative than the resting potential

Changes in Membrane Potential

Short-lived, local changes in membrane potential Intensity decreases with distance Magnitude varies directly with the strength of stimulus If sufficiently strong enough can initiate action potentials

Graded Potentials

A brief reversal of membrane potential with a total amplitude of ~100 mV

Only generated by muscle cells & neurons Propagated by voltage-gated channels Don’t decrease in strength over distance

Action Potentials (APs)

Na+ & K+ channels closed Some leakage of Na+ & K+

Each Na+ channel has two voltage-regulated gates Activation gates – closed in the resting state Inactivation gates – open in the resting state

Action Potential: Resting State

Figure 11.12.1

Na+ permeability increases; Vr reverses Na+ gates opened; K+ gates closed Threshold – critical level of depolarization (-55mV) At threshold, depolarization becomes self-generating

Action Potential: Depolarization Phase

Change in polarity closes Na inactivation gates As Na gates close, voltage-sensitive K+ gates open K+ leaves & Vr is restored

Action Potential: Repolarization Phase

Action Potential: Hyperpolarization

K gates remain open, allowing excessive efflux of K+ causes hyperpolarization neuron refractory while hyperpolarized

Phases of the Action Potential

1 – resting state 2 – depolarization phase 3 – repolarization phase 4 – hyperpolarization

Na+ influx depolarizes patch of axonal membrane Positive ions in axoplasm move toward negative region of the

membrane

Action Potential Propagation (T = 0ms)

+ Extracellular ions diffuse to the area of greatest - charge Creates current that depolarizes adjacent membrane in forward

direction Impulse propagates away from its point of origin

Action Potential Propagation (Time = 1ms)

refractory refractory

Refractory Periods

Absolute - from opening to closing of Na+ activation gates Relative – after closing Na activation gates till K gates are

closed

Threshold ~ 20 mV depolarization

Graded potentials subthreshold stimuli that don’t transit to AP threshold stimuli are relayed into AP

All-or-none phenomenon – AP either happens completely, or not at all

Graded potentials occur along receptive zones of neurons due to presence of only ligand-gated channels

AP begins at axon hillock due to presence of voltage-gated channels

Threshold and Action Potentials

Conduction velocities vary widely among neurons Rate of impulse propagation is determined by:

Axon diameter – the larger the diameter, the faster the impulse

Presence of a myelin sheath – myelination dramatically increases impulse speed

Conduction Velocities of Axons

Voltage-gated Na+ channels are located at the nodes of Ranvier Action potentials occur at the nodes and jump from one node to

the next because that is only place current can flow through the axonal membrane

Much faster than conduction along unmyelinated axons

Saltatory Conduction

Junction for information transfer from one neuron to another neuron or effector cell

Presynaptic neuron – conducts impulses toward the synapse

Postsynaptic neuron – transmits impulses away from the synapse

Synapses

Synapses

Morphological Types Axodendritic –axon to dendrite Axosomatic –axon to soma Axoaxonic (axon to axon)

Chemical : release and reception of neurotransmitters

presynaptic membrane with synaptic vesicles postsynaptic membrane with receptors

Electrical : less common gap junctions important in CNS for:

Control of mental arousal Emotions and memory Ion and water homeostasis

Conductance Synapses Types

Synapse Structure

Figure 11.19

Synaptic cleft Space between pre- and postsynaptic neurons Halts action potential Transmission of signal occurs by neurotransmitter

APs reach terminal of presynaptic neuron & open Ca2+ channels

Neurotransmitter released into synaptic cleft Neurotransmitter crosses cleft & binds receptors on

postsynaptic membrane Postsynaptic membrane permeability changes, causing

an excitatory or inhibitory effect

Synaptic Events

>50 identified Classified chemically and functionally

Acetylcholine (ACh) Biogenic amines Amino acids Peptides Dissolved gases NO and CO

Neurotransmitters

1st neurotransmitter identified Released at neuromuscular junctions Synthesized and enclosed in synaptic vesicles Degraded by enzyme acetylcholinesterase (AChE) Released by:

All neurons that stimulate skeletal muscle Some neurons in the autonomic nervous system

Neurotransmitters: Acetylcholine

Broadly distributed in the brain Behaviors and circadian rythyms

Catecholamines – dopamine, norepinephrine (NE), and epinephrine

Indolamines – serotonin and histamine

Neurotransmitters: Biogenic Amines

Synthesis of Catecholamines

Enzymes present in the cell determine length of biosynthetic pathway

Norepinephrine and dopamine are synthesized in axonal terminals

Epinephrine is released by the adrenal medulla

Figure 11.22

Found only in CNS Include:

GABA – Gamma ()-aminobutyric acid Glycine Aspartate Glutamate

Neurotransmitters: Amino Acids

Tachykinin & substance P – mediator of pain signals

-endorphin, dynorphin, & enkephalins – natural opiates that block pain

somatostatin & cholecystokinin – communicate between gut and CNS

Neurotransmitters: Peptides

Nitric oxide (NO) Activates the intracellular receptor guanylyl cyclase Involved in learning and memory Vascular smooth muscle

Neurotransmitters: Gases

Excitatory neurotransmitters cause depolarization (e.g., glutamate)

Inhibitory neurotransmitters cause hyperpolarization (e.g., GABA and glycine)

Some can be either Determined by receptor on postsynaptic neuron i.e. acetylcholine

Excitatory at skeletal neuromuscular junctions Inhibitory in cardiac muscle

Functional Classification of Neurotransmitters

Direct: Directly activate (open) ion channels Promote rapid responses Examples: ACh and amino acids

Indirect: Bind receptors and act through second messengers Promote long-lasting effects Examples: biogenic amines, peptides, and dissolved gases

Neurotransmitter Receptor Mechanisms

Degradation by enzymes (acetylcholinesterase) Absorption by astrocytes or the presynaptic terminals Diffusion from the synaptic cleft

Termination of Neurotransmitter Effects

Neurotransmitter receptors mediate changes in membrane potential according to: # of receptors activated the amount of neurotransmitter

released The length of time the receptors are stimulated

The two types of postsynaptic potentials are: EPSP – excitatory postsynaptic potentials IPSP – inhibitory postsynaptic potentials

Postsynaptic Potentials

Graded potentials that initiate action potentials Use only ligand gated

channels Na+ and K+ flow in

opposite directions at the same time

Excitatory Postsynaptic Potentials (EPSPs)

Receptor activation increases permeability to K+ and Cl-

Makes charge on the inner surface more negative

Reduces postsynaptic neuron’s ability to produce an action potential

Inhibitory Synapses and IPSPs

Summation

EPSPs summate to induce an action potential Summation of IPSPs and EPSPs cancel each other out

Functional groups of neurons that: Integrate incoming information Forward the processed information to its appropriate

destination

Neural Integration: Neuronal Pools

Divergent – one incoming fiber stimulates ever increasing number of fibers, often amplifying circuits

Types of Circuits in Neuronal Pools

Figure 11.25a, b

Convergent – resulting in either strong stimulation or inhibition

Types of Circuits in Neuronal Pools

Figure 11.25c, d

Reverberating – chain of neurons containing collateral synapses with previous neurons in the chain

Types of Circuits in Neuronal Pools

Figure 11.25e

Parallel after-discharge – incoming neurons stimulate several neurons in parallel arrays

Types of Circuits in Neuronal Pools

Figure 11.25f

Serial Processing Input travels along one pathway to a specific destination Works in an all-or-none manner Example: spinal reflexes

Patterns of Neural Processing

Parallel Processing Input travels along several pathways Pathways are integrated in different CNS systems One stimulus promotes numerous responses

Example: a smell may remind one of the odor and associated experiences

Patterns of Neural Processing