Chapter 4 - Neural Transmission and Synapatic Conduction

Membrane Potential

Difference in electrical charge between the inside and the outside of a cell

-Resting Potential - Steady membrane potential of -70mV, neuron is polarized (membrane potential is not 0)

-Potential inside the resting neuron is 70mV less than that outside the neuron

Recording Potential

Using microelectrodes, tips are in the extracellular fluid and in the neuron, recording the potential difference between the inside and outside of a cell

Ions

Charged molecules that can be positive or negative

-Sodium ions (Na+), potassium ions (K+), etc

Ionic Basis of Potential

During resting potential, there are more Na+ ions outside than inside, and more K+ ions inside than outside

-Unequal distribution maintained using Ion Channels

Substantial pressure on Na+ ions to enter the resting neurons, pressure caused by:

1. electrostatic pressure, positive charge of ion attracted to -70mV charge inside the neuron

2. Random motion for Na+ ions wanting to move down their concentration gradient

-Sodium ion channels are closed to prevent Na+ from entering

Sodium-Potassium Pump

Ion transport to maintain the concentrations of Na+ and K+ inside and outside the cell

-Continually exchanges three Na+ ions to be pumped out for two K+ to be pumped in

Postsynaptic Potentials (PSPs)

Potentials that move the postsynaptic cell’s membrane potential away from the resting state

-When neurons fire, they release neurotransmitters which diffuse into the synaptic cleft and interact with the neighboring neuron

Transmission of PSPs has two characteristics:

1. Rapid (instantaneously transmitted)

2. Transmission is decremental (decrease in amplitude as they travel through a neuron)

Neural acitvity is the sum of PSPs (EPSPs and ISPs)

Depolarization and Hyperpolarization

-When neurotransmitters bind to postsynaptic receptors, they may depolarize (decrease resting membrane potential, from -70mV to -60mV) or hyperpolarize (increase resting membrane potential, from -70 to -72mV)

Excitatory and Inhibitory Postsynaptic Potentials

Excitatory Postsynaptic Potentials - Graded postsynaptic depolarizations, which increase the likelihood that an action potential will be generated

Inhibitory Postsynaptic Potentials - Graded postsynaptic hyperpolarization, which decrease likelihood that an action potential will be generated

Graded Potentials

Amplitudes of PSPs are proportional to the intensity of the signals that elicit them:

(Weak signals elicit small PSPs, strong signals elicit large ones)

Axon Initial Segment

Site where action potentials are generated, located in the adjacent section of the axon

-Summation of the PSPs received at the Axon Initial Segment determines whether an AP is produced (if the Threshold of Excitation is reached)

Threshold of Excitation and Action Potential

Threshold of Excitation - Level of depolarizations necessary to generate an action potential, usually -65mV

-Based on the sum of EPSPs and IPSPs at the receptive sites of a neuron’s membrane

Action Potential - Momentary reversal of the membrane potential from -70 to +50mV

-Caused by a sufficiently large EPSP

-”All or None Response” - Either the neuron fires or it does not fire (no in-between)

Spatial Summation

Integration of signals that originate at different sites on the neuron’s membrane, three combinations

1. Two simultaneous EPSPs sum produce a greater EPSP

2. Two simultaneous IPSPs sum to produce greater IPSP

3. Or simultaneous IPSP and EPSP cancel each other out

Temporal Summation

Integration of neural signals that occur at different times at the same synapse, PSPs produced in rapid succession at the same synapse sum to form a greater signal

1. Two EPSPs elicited in rapid succession sum to produce a larger EPSP

2. Two IPSPs elicited in rapid succession sum to produce a larger IPSP

Voltage-Gated Ion Channels

Ion channels that open or close in response to changes in membrane potentials

-Once threshold of excitation is reached, voltage-gated sodium channels open allow Na+ ions to enter, reversing membrane potential (-70mv → +50mV)

-This triggers the opening of voltage-gated potassium channels, k+ ions are driven out of the cell

-After 1 millisecond, sodium channels close marking the ending of the rising phase of the AP and beginning repolarization (continued efflux of K+ ions)

-Potassium channels close gradually, resulting in too many K+ ions leaving (hyperpolarization)

Absolute and Relative Refractory Period

Absolute Refractory Period - Brief period (1-2 millisecond) after initiation of an AP during which it is impossible to elicit a second AP

Relative Refractory Period - Period during which it is possible to fire the neuron again ONLY by applying higher-than-normal levels of stimulation

-Refractory periods ensure APs travel along axons in one direction and for ensuring the rate of neural firing is related to the intensity of the stimulation

Conduction of AP along Axons

Conduction of AP on an axon differs from conduction of PSPs:

1. Conduction of APs is typically nondecremental (APs do not grow weaker as they travel along axonal membranes)

2. APs are conducted more slowly than PSPs

-Differences are a result of the voltage-gated sodium channels in AP conduction, AP is continually regenerated at each sodium channel along the length of the axon until a full-blown AP is triggered as the axon terminal buttons

Antidromic and Orthodromic Conduction

If electrical stimulation of sufficient intensity is applied to the midpoint of an axon, two APs are generated:

1. Antidromic Conduction - One AP will ravel along the axon back to the cell body

2. Orthodromic Conduction - Second AP will travel along the axon towards the terminal buttons

-Generation of the AP at the Axon Initial Segment also spreads back through the cell body and dendrites as a large graded potential, this antidromic potential play a role in synaptic plasticity

Saltatory Conduction

Conduction of an AP in a myelinated axon, where the AP jumps from one node of Ranvier to the next

-Contributes to the greater conduction speed in myelinated axons

Velocity of Axonal Conduction

Conduction is faster in large-diameter axons, in myelinated axons

-Motor neurons are large and myelinated (can conduct at speeds up to 60 meters per second)

Conduction in Neurons Without Axons

Conduction in these interneurons are done only through graded potentials

Hodgkin-Huxley Model

Based on the study of motor neurons (simple, large and readily accessible in the PNS)

-Provided the current account of neural conduction, but it cannot fully account for the variety, complexity and plasticity of neurons in the mammalian brain

-Used squid motor neurons as they’re larger (easier to study) but made it difficult to apply the model to the mammalian brain