LECTURE 8: SYNAPTIC TRANSMISSION OVERVIEW AND NMJREQUIRED READING: Kandel text, Chapters 10,11

In humans, 1011 neurons, each receiving average of 100-1000 synaptic inputs

= 1014 synapses !!!

Multiple synapses allows a neuron to integrate information from many sources.

Purkinje cells are the output cells of cerebellum, each receiving 105 inputs,enabling the cerebellum to monitor complex body movement and provide continual

corrective signals (a corrective guidance system)

Synapses undergo short-term and long-term functional adaptation.Adaptation is synapse-specific, not affecting other synapses on same cell.

Short-term adaptation allows an organism to ignore certain repetitiveor prolonged sensory inputs.

Long-term adaptation is a cellular mechanism of learning and memory.

ELECTRICAL VS. CHEMICAL SYNAPSES

FIGURE 10-1

TABLE 10-1

NEAR-INSTANTANEOUS TRANSMISSION ACROSS ELECTRICAL SYNAPSES

FIGURE 10-3Synaptic transmission isgraded, occuring even

in the absence ofaction potential

Transmission can beBI-DIRECTIONAL

ELECTRICAL SYNAPSES ARE COMPOSED OF GAP-JUNCTION CHANNELS

ELECTRICAL SYNAPSES ALLOW SYNCHRONOUS FIRING OF CONNECTED CELLS

STEPS OF TRANSMISSION AT CHEMICAL SYNAPSES

Chemical synaptic transmission has a 0.3 - 5 msec delay, depending upon type of post-synaptic receptor

Chemical synaptic transmission can depolarize or inhibit depolarization,depending upon type of post-synaptic receptor

Sub-threshold presynaptic depolarizations are not transmitted

DIRECT VS. INDIRECT GATING OF TRANSMISSION AT CHEMICAL SYNAPSES

DIRECTLY GATED MONOSYNAPTIC TRANSMISSIONAT THE NEUROMUSCULAR JUNCTION

Large size of vertebrate neuromuscular junction has

facilitated electrophysiologicaland morphological analysis of

this synapse

Each muscle fiber receives input from a single motor neuron axon

Synaptic transmission employs a single neurotransmitter,

acetylcholine (Ach) and a single species of directly-gated

neurotransmitter receptor (nicotinic AChR)

THE EXCITATORY POST-SYNAPTIC POTENTIAL

When recording Vm nearendplate, stimulation of

motor axon generates complexdepolarization in muscle fiber,

consisting of an excitatorypostsynaptic potential (EPSP)

and a followingaction potential (AP).

Partial inhibition of AChRsWith curare reduces EPSP

Below firing thresholdFor AP.

EPSP CURRENT APPEARS RAPIDLY AND DECAYS MORE SLOWLY

EPSP CURRENT OBEYS OHM’S LAW

IEPSP = gEPSP x (VM - EEPSP)

SMOOTH EPSP CURRENT IS SUM OF CURRENTS FROM OPEN ON/OFF CHANNELS

SINGLE-CHANNEL PATCH VOLTAGE CLAMP RECORDING OF ACETYLCHOLINE-INDUCED CURRENT

REVERSAL POTENTIAL OF SINGLE CHANNEL SAME AS FOR OVERALL EPSP

STEPS OF TRANSMISSION AT NEUROMUSCULAR JUNCTION

MODEL OF NICOTINIC ACETYLCHOLINE RECEPTOR

ACH-GATED CURRENT AND PASSIVE MEMBRANE PROPERTIESDETERMINE DYNAMICS OF THE EPSP

NEXT LECTURE: Synaptic Integration

READING: KANDEL text, Chapter 12