Neurotransmission ISAT 351, Spring, 2004 College of Integrated Science and Technology James Madison University

Neuron Function  Neurons (nerve cells) receive, conduct, and transmit signals  Neurons carry signals from sense organs to the central nervous system (brain and spinal cord) where they are processed  From the central nervous system, neurons convey signals to muscles and glands

Neuron Structure  The cell body contains the nucleus and receives signals from other neurons on branches called dendrites or directly on the cell body  The axon conducts signals away from the cell body and divides into many branches at the nerve terminal

Neurons Signal Signal Propagation Relay to next Reception (electrical) cell (chemical)

Electrical & Chemical Signal Propagation  Electrical Signal Signal propagation within neuron Branched axon terminus amplifies signal Terminus makes synapses with target cells  Chemical Signal Propagation between cells Neurotransmitters Relay electrical signal via exo- & endocytosis Targets: Another neuron Dendrite Muscle cell

Types of Neurons  Sensory neurons receive and convert stimuli from the environment into electrical signals  Interneurons receive signals from neurons and transmits signals to neurons  Motor neurons receive signals from interneurons and stimulate muscle or glands

Structures are Similar

Neuron Signals  Electric signals transmit information within a cell from the cell body to the axon terminus by an electric impulse called an action potential  Chemical signals transmit information from sensory cells, between neurons (synapses), and to specialized cells such as muscle or glands

Nerve Signals

Neurons Form Circuits

Electrical Signal  Nerve signals are changes in the electrical potential across the neuron’s plasma membrane (membrane potential)  The action potential or nerve impulse can carry a message without signal attenuation  Action potentials actively propagate signal via voltage-gated Na + channels  Explosion of activity propagated & amplified along membrane

Electrical Signal  Myelin sheath insulates nerve Prevents signal attenuation Promotes signal propagation and amplification Multiple sclerosis involves demyelination

Electrical Signal = Action Potential  Intra- & extracellular [ion] different [K + ] high internally [Na +, Cl - ] high externally  Consequences: Unequal distribution of cations and anions Baseline membrane potential changes when ion distribution changes

Propagation of Action Potential Resting V 1 V 2 Baseline Membrane Action Potential Propagation : Potential -60mV -40mV Depolarization Wave Recovery

So, Depolarizing membrane by about 20 mV triggers action potential

Voltage-Gated Channels Mediate Action Potential Depolarization causes channels to open and an influx of anions (Na+) causes further depolarization resulting in the action potential. How is the membrane repolarized?

Three Conformational States Channel inactivated until K + ions repolarize membrane; speeds recovery

The Action Potential

Voltage-Gated Channel

Measurement of Potential

Propagation Measurement  1 electrode inside, other outside  Stimulate & measure as a function of time  V 1, V 2, V 3 have identical amplitudes Shape & intensity of potential maintained Zero attenuation as signal propagated

Consequences  All-or-none; neurons are resting or conducting  Amplitude constant, so size of action potential not important  THE FREQUENCY OF ACTION POTENTIAL FIRINGS CARRY INFORMATION  RATE OF PROPAGATION FACILITATED BY MYELIN INSULATION

Synapses Communicate Between Neurons  BILLION neurons in human brain  TRILLION synapses  Human forebrain: ratio of synapses:neurons about 40,000:1  Elastic: improve connectivity by using neurons  Neurons communicate via neurotransmitters: Electrical-to-chemical-to electrical signal conversion

Electrical to Chemical Signal Conversion at Synapse

Synapses  The action potential opens voltage-gated Ca + channels at the nerve terminal  The increase in Ca + triggers the release of neurotransmitters into the synaptic cleft  The neurotransmitter diffuses across the synaptic cleft, binds to the target cell, and triggers an action potential

Conversion Back to Electrical Signal

Neurotransmitter Tidbits  Certain psychotic drugs (cocaine, morphine) & venoms mimic NT  Feel good with dopamine and serotonin Natural reward system appeared early in evolution; reinforce behaviors favorable to survival Prozac et al

Dopamine Malfunctions Parkinson’s disease Insufficient dopamine due to destruction of cells that synthesize dopamine Motor malfunctions appear after about 70% of neurons destroyed Schizophrenia hallucinations: excessive dopamine Tourette’s syndrome: supersensitive receptors

Dopamine and Addictions  Stimulate feel good effects of dopamine using alcohol, nicotine, marijuana, and amphetamines Amphetamines stimulate secretion Cocaine keeps [dopamine] high  Dopamine may be common end-point of addictions; different mechanisms  Addicts’ feedback mechanisms impaired  Consequence: dopamine deficit

Use it or lose it! Mental activity over lifetime reinforces synaptic junctions

Learning and Memory  Thousands of nerve terminals synapse on a neuron  Combination of synapses determines if action potential is initiated  Synaptic pathways provide a mechanism to store, analyze, and recall inputs