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