Neurons, Synapses, & Signaling
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Transcript of Neurons, Synapses, & Signaling
Neurons, Synapses, & Signaling
Campbell and ReeceChapter 48
Neurons
nerve cells that transmit information within the body
communication between neurons consists of:◦long distance electrical signals◦short distance chemical signals
Neurons
use pulses of electrical current toreceivetransmitregulate
the flow of information over long distances w/in the body
Neuron Organization
Nervous System
Types of Neurons
Sensory Neurons◦transmit information (senses) from body brain
◦are afferent◦specialized dendrites that initiate action potential when stimulated
Types of Neurons
2. Motor Neurons transmit signals to muscle fibers
& glandsare efferent
Types of Neurons
3. Interneurons majority of neurons in brain
◦ form local circuits connecting neurons
Synapse
junction between axon terminal & next cell (another neuron, muscle fiber, gland cell)
neurotransmitters are chemical messengers released @ most synapses that pass action potential to receiving cell
Synapse
presynaptic cell: cell releasing neurotransmitter & passing on action potential
postsynaptic cell: receiving neurotransmitter
synaptic cleft: physical space between the 2; neurotransmitter released into this space & diffuses across it attaching to receptors on postsynaptic cell
Synapse
Glial Cells
cells that support neuronsGreek: glueaka neuroglianourish neuronsinsulate axonsregulate ECF surrounding neurons
Ion Pumps
ions unequally distributed across plasma membrane
inside of cell slightly (-) compared to outside cell
source of potential nrgcalled the membrane potentialresting potential: the membrane
potential of neuron @ rest = -60 to –80 mV
Resting Potential
Formation of Resting Potential
Na+/K+ pump generates & maintains the ionic gradients of membrane potential
1 turn of pump◦1 ATP◦3 Na+ out◦2 K+ in
Membrane Potential
Ion Channels
pores that span the membrane allowing ions to diffuse across (in or out)
membranes are selectively permeable and variations in how easily any particular ion can cross a membrane depends on the # of channels & how often they are open
Types of Ion Channels
Action Potentials
neurons have gated ion channels that open or close in response to stimuli◦open/close changes permeability for that ion
neurons have K+ channels◦when open K+ diffuses out of cell ◦changes resting potential from: -60 mV to -90 mV
K+ Ion Channels in Neurons
Resting & Action Potentials
http://bcs.whfreeman.com/thelifewire/content/chp44/4401s.swf
http://www.dnatube.com/video/1105/Understanding-Action-Potential-and-Nerve-Impulses
Hyperpolarization
when K+ channels open & resting potential decreases to -90 mV inside of cell becoming more (-) than normal resting potential called:
hyperpolarization
Depolarization
when Na+ ion channels open Na+ diffuse into cell making inside less (-) compared to outside cell
membrane potential shifts toward (+) mv
this reduction in magnitude of membrane potential called
depolarization
Graded Potentials
any shift in membrane potentialmagnitude of shift varies with
strength of stimulusinduce a small electrical current that
flows along the membrane leaking out of the cell
so only lasts short distance from source
Action Potential
electrical signal that propagates along the membrane of a neuron as a nongraded (all or nothing) depolarization
have a constant magnitude & can regenerate in adjacent regions of the membrane
travel long distances
Voltage-Gated Ion Channels
ion channels that open/close based on membrane potential passing a particular level
Na+ channels in neurons are voltage gated: open when depolarization occurs Na+ diffuses into cell becomes more depolarized more Na+ channels open (+ feedback)
http://outreach.mcb.harvard.edu/animations/actionpotential_short.swf
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html
Interactive site to try at home:
Threshold
Action potentials occur when a depolarization increases the membrane voltage to a particular value (the threshold)
for mammals the threshold is a membrane potential ~ -55mV
once started the action potential has a magnitude independent of the strength of triggering stimulus
+ feedback loop of depolarization & channel opening triggers an action potential whenever the membrane potential reached the threshold
membrane depolarization opens both Na+ & K+ channels but Na+ opens faster initiating the action potential
Na+ channels become inactivated as action potential proceeds (gates close) & remain so until after membrane returns to resting potential
Refractory Period
(-) membrane potential restored by inactivation of Na+ channels, which increases K+ outflow
This is followed by a refractory period:◦no matter how strong the stimulus to initiate next action potential is cannot initiate one during refractory period
Conduction of Action Potentials
Myelin Sheaths
glial cells oligodendrocytes (CNS) and Schwann cells (PNS) form layers of electrical insulation along length of axons
Saltatory Conduction
Neurotransmitters
>100 neurotransmitters belonging to 5 groups:
1. Acetylcholine2. Amino Acids3. Biogenic Amines4. Neuropeptides5. Gases