Neurons, Synapses, &
Signaling
Campbell and Reece
Chapter 48
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 to
receive
transmit
regulate
the flow of information over long
distances w/in the body
Neurons
Neuron Organization
Nervous System

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 &
glands
 are efferent
Types of Neurons
3. Interneurons

◦
majority of neurons in brain
form local circuits connecting neurons
Types of Neurons
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
Synapse






cells that support neurons
Greek: glue
aka neuroglia
nourish neurons
insulate axons
regulate ECF surrounding neurons
Glial Cells





ions unequally distributed across plasma
membrane
inside of cell slightly (-) compared to
outside cell
source of potential nrg
called the membrane potential
resting potential: the membrane
potential of neuron @ rest =
 -60 to –80 mV
Ion Pumps
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
Formation of Resting Potential
Membrane Potential
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

Ion Channels
Types of Ion Channels

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
Action Potentials
K+ Ion Channels in Neurons

http://bcs.whfreeman.com/thelifewire/conten
t/chp44/4401s.swf

http://www.dnatube.com/video/1105/Unders
tanding-Action-Potential-and-Nerve-Impulses
Resting & Action Potentials

when K+ channels open & resting
potential decreases to -90 mV inside of
cell becoming more (-) than normal
resting potential called:
hyperpolarization
Hyperpolarization
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

Depolarization
any shift in membrane potential
 magnitude of shift varies with strength
of stimulus
 induce a small electrical current that
flows along the membrane leaking out of
the cell
 so only lasts short distance from source

Graded Potentials
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

Action Potential
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)

Voltage-Gated Ion Channels

http://highered.mcgrawhill.com/sites/0072495855/student_vie
w0/chapter14/animation__the_nerve_im
pulse.html

Interactive site to try at home:
http://outreach.mcb.harvard.edu/an
imations/actionpotential_short.swf
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

Threshold
+ 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

(-) 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
Refractory Period
Conduction of Action Potentials

glial cells oligodendrocytes (CNS) and
Schwann cells (PNS) form layers of
electrical insulation along length of
axons
Myelin Sheaths
Saltatory Conduction
>100 neurotransmitters belonging to 5
groups:
1. Acetylcholine
2. Amino Acids
3. Biogenic Amines
4. Neuropeptides
5. Gases

Neurotransmitters