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Neurotransmission
ISAT 351, Spring, 2004
College of Integrated Science and Technology
James Madison University
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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
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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
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Neurons
Signal Signal Propagation Relay to next
Reception (electrical) cell (chemical)
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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
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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
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Structures are Similar
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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
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Nerve Signals
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Neurons Form Circuits
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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
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Electrical Signal
Myelin sheath insulates nerve Prevents signal
attenuation Promotes signal
propagation and amplification
Multiple sclerosis involves demyelination
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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
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+++++++++
-
-
-
-
Propagation of Action Potential
Resting V1 V2
Baseline Membrane Action Potential Propagation :Potential -60mV -40mV Depolarization Wave
+---++++
+++---
++++---++
---+++-
Recovery
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So,
Depolarizing membrane by about 20 mV triggers action potential
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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?
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Three Conformational States
Channel inactivated until K+ ions
repolarize membrane; speeds recovery
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The Action Potential
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Voltage-Gated Channel
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Measurement of Potential
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Propagation Measurement
1 electrode inside, other outside
Stimulate & measure as a function of time
V1, V2, V3 have identical amplitudes
Shape & intensity of potential maintained
Zero attenuation as signal propagated
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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
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Synapses Communicate Between Neurons
10-100 BILLION neurons in human brain 10-100 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
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Electrical to Chemical Signal Conversion at Synapse
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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
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Conversion Back to Electrical Signal
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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
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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
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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
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Use it or lose it!
Mental activity over lifetime reinforces synaptic junctions
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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
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Mult iple sclerosis (MS) is a diseasethat destroys myelin, an insulatingmaterial that coats nerve fi bers and isnecessary for normal electricalconduction in the nervous system. Thisbreakdown of the myelin, calleddemyelination, results in impairment ofthe function of the nerve.
Mult iple sclerosis (MS) is a diseasethat destroys myelin, an insulatingmaterial that coats nerve fi bers and isnecessary for normal electricalconduction in the nervous system. Thisbreakdown of the myelin, calleddemyelination, results in impairment ofthe function of the nerve.
I n MS, repeated incidents ofinfl ammation cause scarring(sclerosis) and permanentabnormal function.
I n MS, repeated incidents ofinfl ammation cause scarring(sclerosis) and permanentabnormal function.
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The name is derived f rom this process- multiple (many) since it occurs in a numberof places within the nervous system andsclerosis (scars) which means the hardenedtissue that replaces damaged myelin.
The name is derived f rom this process- multiple (many) since it occurs in a numberof places within the nervous system andsclerosis (scars) which means the hardenedtissue that replaces damaged myelin.
During an MS attack, myelin becomesinfl amed, causing symptoms such as lack ofcoordination, weakness, tingling, impairedsensation, double vision or bladder problems.I f the infl ammation is severe, the myelin mayactually be damaged, however, regrowth ofmyelin may occur naturally during periods ofremission.
During an MS attack, myelin becomesinfl amed, causing symptoms such as lack ofcoordination, weakness, tingling, impairedsensation, double vision or bladder problems.I f the infl ammation is severe, the myelin mayactually be damaged, however, regrowth ofmyelin may occur naturally during periods ofremission.
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Mult iple sclerosis is anautoimmune disease. Somethingcauses the body to becomeallergic to its own myelin.
Mult iple sclerosis is anautoimmune disease. Somethingcauses the body to becomeallergic to its own myelin.
MS aff ects about 250,000Americans and is about twice ascommon in women as in men.
MS aff ects about 250,000Americans and is about twice ascommon in women as in men.
There may be a genetic component--a predisposition--to susceptibility to MS. People with particular types ofhistocompatibility antigens (HLA antigens) develop MSmore of ten than those who have other HLA antigens.HLA antigens of ten associated with MS are A3, B7, andDW2.
There may be a genetic component--a predisposition--to susceptibility to MS. People with particular types ofhistocompatibility antigens (HLA antigens) develop MSmore of ten than those who have other HLA antigens.HLA antigens of ten associated with MS are A3, B7, andDW2.
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Common symptoms include:
Loss of vision in one eye or double vision.
Loss of coordination and trembling of ahand.
I nstability in walking and spasticity.
Loss of bladder control.
Peculiar, spontaneous, nerve sensationssuch as a pins-and-needles feeling overpart of the body (paresthesias).
Common symptoms include:
Loss of vision in one eye or double vision.
Loss of coordination and trembling of ahand.
I nstability in walking and spasticity.
Loss of bladder control.
Peculiar, spontaneous, nerve sensationssuch as a pins-and-needles feeling overpart of the body (paresthesias).
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MS is notoriously hard to diagnose.Current diagnostic tests include:
using evoked potentials (EP) to measure the rateof nerve conduction in various parts of thecentral nervous system.
Computer-assisted tomography (CT) may be usedto scan the central nervous system using an x-raytechnique which can detect areas ofdemyelination.
Magnetic resonance imaging (MRI ) may also beused to detect areas of demylenation, butwithout the use of x-rays.