Membrane Potential and Synaptic Transmission

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Ion Movements and Electrical Signals

-all plasma (cell) membranes produce electrical signals by ion movements
-transmembrane potential is particularly important to neurons

Five Main Membrane Processes in Neural Activities

resting potential - transmembrane portential of resting cell
graded potential - temporary, localized change in resting potential
action potential - electrical impulse; produces by graded potential; propagates along surface of axon to synapse
synaptic activity - releases neurotransmitters at presynaptic membrane; produces graded potentials in postsynaptic membrane
information processing - response (integration of stimuli) of postsynaptic cell

The Transmembrane Potential

1. Very different ion composition inside and outside the cell
Cells have selectively permeable membranes, with passive and active transport
Inner surface of cell is more negative than outside the cell

Membrane permeability varies by ion
Due to the presence of passive (leak) channels

Passive Forces acting Across the Plasma Membrane

Chemical gradients
Concentration gradients (chemical gradient) of ions (Na+, K+)
Electrical gradients
Separate charges of positive and negative ions
Result in potential difference

Electrical Currents

Movement of charges to eliminate potential difference (opposite charges attract)

Resistance

The amount of current a membrane restricts

Electrochemical Gradient

The diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ion across a membrane and its tendency to move relative to the membrane potential.

Active Forces across the Membrane

Sodium-potassium ATPase (exchange pump)
Powered by ATP, it carries 3 Na+ out and 2 K+ in
Balances passive forces of diffusion(through passive channels)
Maintains resting potential (-70 mV)

The Resting Potential

Because the plasma membrane is highly permeable to potassium ions:
The electrochemical gradient for sodium ions is very large, but the membrane's permeability to these ions is very low
The sodium-potassium exchange pump ejects 3 Na+ ions for every 2 K+ ions that it brings into the cell
It serves to stabilize the resting potential when the ratio of Na+ entry to K+ loss through passive channels is 3:2

The Resting Potential

At the normal resting potential, these passive and active mechanisms are in balance
The resting potential varies widely with the type of cell
A typical neuron has a resting potential of approximately -70 mV

Changes in the Transmembrane Potential

Transmembrane potential rises or falls
In response to temporary changes in membrane permeability from opening or closing specific membrane channels

Sodium and Potassium Channels

Membrane permeability to Na+ and K+ determines transmembrane potential
These channels are either passive or active

Passive channels

Are always open, permeability changes with conditions

Active channels

Open and close in response to stimuli

Three States of Gated Channels

Closed, but capable of opening
Open (activated)
Closed, not capable of opening (inactivated

Three Classes of Gated Channels

Chemically gated channels
Voltage-gated channels
Mechanically gated channels

Chemically Gated Channels

Open in presence of specific chemicals (e.g., ACh) at a binding site
Found on neuron cell body and dendrites

Voltage-gated Channels

Respond to changes in transmembrane potential
Have activation gates (open) and inactivation gates (close)
Characteristic of excitable membrane
Found in neural axons, skeletal muscle sarcolemma, cardiac muscle

Mechanically Gated Channels

Respond to membrane distortion
Found in sensory receptors (touch, pressure, vibration

Transmembrane Potential Exists Across Plasma Membrane

Because:
Cytosol and extracellular fluid have different chemical/ionic balance
The plasma membrane is selectively permeable

Transmembrane Potential

Changes with plasma membrane permeability in response to chemical or physical stimuli

Graded Potentials (local potentials)

Are changes in transmembrane potential that cannot spread far from site of stimulation
Includes any stimulus that opens a gated channel which produces a graded potential

Graded Potentials
The resting state

Opening sodium channel produces graded potential. Events include:
Resting membrane exposed to chemical
Sodium channel opens
Sodium ions enter the cell
Transmembrane potential rises
Depolarization occurs

Depolarization

- A shift in transmembrane potential toward 0 mV. Events include:
Movement of Na+ through channel
Produces local current
Depolarizes nearby plasma membrane (graded potential)
Note: change in potential is proportional to stimulus

Depolarizing or Hyperpolarizing, share four basic characteristics

The transmembrane potential is most changed at the site of stimulation, and the effect decreases with distance
The effect spreads passively, due to local currents

The graded change in transmembrane potential may involve either depolarization or hyperpolarization
The properties and distribution of the membrane channels involved determine the nature of the change
For example, in a resting membrane, the opening of sodium channels causes depolarization, whereas the opening of potassium channels causes hyperpolarization
The stronger the stimulus, the greater the change in the transmembrane potential and the larger the area affected

Repolarization

- occurs when the stimulus is removed, the transmembrane potential returns to normal

Hyperpolarization

- caused by increasing the negativity of the resting potential
Result of opening a potassium channel
Opposite effect of opening a sodium channel
Positive ions move out, not into cell

Effects of graded potentials

At cell dendrites or cell bodies
Trigger specific cell functions
For example, exocytosis of glandular secretions
At motor end plate
Release ACh into synaptic cleft

Action Potentials

A rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered, selective opening and closing of voltage-sensitive gates in sodium and potassium ion channels.

Initiating Action Potential

Initial stimulus
A graded depolarization of axon hillock large enough (10 to 15 mV) to change resting potential (-70 mV) to threshold level of voltage-gated sodium channels (-60 to -55 mV)

All-or-none

principle - An action potential is either triggered, or not

If a stimulus exceeds threshold amount, an action potential is generated.
The action potential is the same no matter how large the stimulus

Four Steps in the Generation of Action Potentials

Step 1: Depolarization to threshold
Step 2: Activation of Na channels
Step 3: Inactivation of Na channels and activation of K channels
Step 4: Return to normal permeability

Step 1: Depolarization to threshold

Graded potential reaches axon hillock to depolarize it to threshold

Step 2: Activation of Na channels

Voltage gated Na+ channels cause rapid depolarization
Na+ ions rush into cytoplasm
Inner membrane changes from negative to positive

Step 3: Inactivation of Na channels and activation of K channels

At +30 mV
Inactivation gates close (Na channel inactivation)
K channels open
Repolarization begins

Step 4: Return to normal permeability

K+ channels begin to close when membrane reaches normal resting potential (-70 mV)
K+ channels finish closing when membrane is hyperpolarized to -90 mV
Transmembrane potential returns to resting level
Action potential is over

The Refractory Period

The time period from beginning of action potential to return to resting state during which membrane will not respond normally to additional stimuli

Absolute Refractory Period

Sodium channels are open or inactivated
No action potential possible

Relative Refractory Period

Membrane potential almost normal
Very large stimulus can initiate action potential

Powering the Sodium-Potassium Exchange Pump

Remember, the pump is required to maintain concentration gradients of Na+ and K+ over time
Uses energy (1 ATP for each 2 K+/3 Na+ exchange)
Without ATP, Na+ and K+ concentration gradient would disappear
Neurons stop functioning

Propagation

Moves action potentials generated in axon hillock along entire length of axon to the axon terminals
Two methods of propagating action potentials

Continuous propagation

unmyelinated axons)

Saltatory propagation

(myelinated axons)

Continuous Propagation

Of action potentials along an unmyelinated axon
Affects one segment of axon at a time
Steps in propagation
Step 1: Action potential in segment 1
Depolarizes membrane to +30 mV
Local current
Step 2: Depolarizes second segment to threshold
Second segment develops action potential
Step 3: First segment enters refractory period
Step 4: Local current depolarizes next segment
Cycle repeats
Action potential travels in one direction (1 m/sec) -

Saltatory Propagation

Action potential along myelinated axon
Faster and uses less energy than continuous propagation
Myelin insulates axon, prevents continuous propagation
Local current "jumps" from node to node
Depolarization occurs only at nodes

Axon Diameter and Propagation Speed

Ion movement is related to cytoplasm concentration
Axon diameter affects action potential speed
The larger the diameter, the lower the resistance

Three Groups of Axons

Type A fibers
Type B fibers
Type C fibers
These groups are classified by:
Diameter
Myelination
Speed of action potentials

Type A Fibers

Myelinated
Large diameter
High speed (140 m/sec)
Carry rapid information to/from CNS
For example, position, balance, touch, and motor impulses

Type B Fibers

Myelinated
Medium diameter
Medium speed (18 m/sec)
Carry intermediate signals
For example, sensory information, peripheral effectors

Type C Fibers

Unmyelinated
Small diameter
Slow speed (1 m/sec)
Carry slower information
For example, involuntary muscle, gland controls

Information

"Information" travels within the nervous system as propagated electrical signals (action potentials)
The most important information (vision, balance, motor commands)
Carried by large-diameter, myelinated axons

Synaptic Activity

Action potentials (nerve impulses)
Are transmitted from presynaptic neuron to postsynaptic neuron (or other postsynaptic cell) across a synapse

Two Types of Synapses

Electrical synapses - Direct physical contact between cells
Chemical synapses - Signal transmitted across a gap by chemical neurotransmitters

Electrical Synapses

Are locked together at gap junctions (connexons)
Allow ions to pass between cells
Produce continuous local current and action potential propagation
Are found in areas of brain, eye, ciliary ganglia

Chemical Synapses

Are found in most synapses between neurons and all synapses between neurons and other cells
Cells not in direct contact
Action potential may or may not be propagated to postsynaptic cell, depending on:
Amount of neurotransmitter released
Sensitivity of postsynaptic cell

Two Classes of Neurotransmitters

Excitatory neurotransmitters
Cause depolarization of postsynaptic membranes
Inhibitory neurotransmitters
Cause hyperpolarization of postsynaptic membranes

The Effect of a Neurotransmitter

Depends on the receptor, NOT on the neurotransmitter
For example, acetylcholine (ACh)
Usually promotes action potentials

Cholinergic Synapses

Any synapse that releases Ach. Located at:
All neuromuscular junctions with skeletal muscle fibers
Many synapses in CNS
All neuron-to-neuron synapses in PNS
All neuromuscular and neuroglandular junctions of ANS parasympathetic division

Events at a Cholinergic Synapse

Action potential arrives, depolarizes synaptic terminal
Calcium ions enter synaptic terminal, trigger exocytosis of ACh
ACh binds to receptors, depolarizes postsynaptic membrane
ACh removed by AChE
AChE breaks ACh into acetate and choline

Synaptic Delay

A synaptic delay of 0.2-0.5 msec occurs between:
Arrival of action potential at synaptic terminal
And effect on postsynaptic membrane
Fewer synapses mean faster response
Reflexes may involve only one synapse

Synaptic Fatigue

Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli
Synapse inactive until ACh is replenished

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