Transcript Ch 48: Nervous System – part 1

NOTES: CH 48
Neurons, Synapses, and
Signaling
 A nervous system has three overlapping
functions:
1) SENSORY INPUT: signals from sensory
receptors to integration centers
2) INTEGRATION: information from sensory
receptors is interpreted and associated
with appropriate responses
3) MOTOR OUTPUT: conduction of signals
from the integration center to effector cells
(muscle cells or gland cells)
*CENTRAL NERVOUS SYSTEM (CNS)
 integration center

brain and spinal cord
*PERIPHERAL NERVOUS
SYSTEM (PNS)
 made up of nerves
(ropelike bundles
of neurons)

nerves communicate
motor and sensory
signals to and from CNS
and rest of body
Two Main Classes of Cells:
1) NEURONS:
 functional unit of the nervous system
 transmits signals from one location to another
 made up of: cell body, dendrites, axon
 many axons are enclosed by an insulating
layer called the MYELIN SHEATH
 include:
sensory neurons,
interneurons,
motor neurons
2) GLIAL CELLS (“GLIA”) SUPPORTING CELLS
 10 to 50 times more numerous than
neurons
 provide structure; protect, insulate,
assist neurons
 example: Schwann cells and
oligodendrocytes form myelin sheaths
in the PNS and CNS, respectively
MYELIN SHEATH:
 produced by Schwann cells in the
peripheral nervous system;
 gaps between successive Schwann
cells are called NODES OF
RANVIER….
***the #10 term!!! 
NODES OF RANVIER!
***word #10 on my list!!!
1) Okazaki fragments
2) plasmodesmata
3) ???????
4) ???????
5) ???????
6) rubisco
7) oxaloacetate
8) islets of Langerhans
9) Batesian mimicry
10) nodes of Ranvier
2) GLIA (SUPPORTING CELLS)
 example: astrocytes: responsible for
blood-brain barrier
Astrocyte
Nerve
cells
ACTION POTENTIALS & NERVE
IMPULSES
 all cells have an electrical charge difference
across their plasma membranes; that is,
they are POLARIZED.
 this voltage is called the MEMBRANE
POTENTIAL (usually –50 to –100 mV)
 inside of cell is negative relative to outside
 arises from differences in ionic
concentrations inside and outside cell
**A- = group of anions
including proteins,
amino acids, sulfate,
phosphate, etc.; large
molecules that cannot
cross the membrane
and therefore provide
a pool of neg. charge
that remains in the
cell
How is this potential maintained?
 the sodiumpotassium pump
uses ATP to
maintain the ionic
gradients across the
membrane
(3 Na+ out; 2 K+ in)
the “resting potential”
of a nerve cell is approx.
–70 mV


neurons have special
ion channels (GATED
ION CHANNELS) that allow the cell
to change its membrane potential
(a.k.a. “excitable” cells)
 when a stimulus reaches a neuron, it
causes the opening of gated ion
channels
(e.g.: light  photoreceptors in the eye;
sound waves/vibrations  hair cells in
inner ear)
 HYPERPOLARIZATION: membrane
potential becomes more negative (K+
channel opens; increased outflow of K+)
 DEPOLARIZATION: membrane potential
becomes less negative
(Na+ channel opens; increased inflow
of Na+)
**If the level of depolarization reaches the
THRESHOLD POTENTIAL, an ACTION
POTENTIAL is triggered.
ACTION
POTENTIALS
(APs):
 the nerve impulse

all-or-none event;
magnitude is
independent of the
strength of the
stimulus
5 Phases of an A.P.:
1) Resting state
2) Depolarizing phase
3) Rising phase of A.P.
4) Falling phase of AP (repolarizing phase)
5) Undershoot
Phase of
A.P.
State of Voltage-Gated Sodium (Na+) Channel
State of
VoltageGated
Potassium
(K+) channel
Activation gate
Inact. Gate
Entire
channel
1) Resting
closed
open
closed
closed
2 & 3)
Depolarization
open
open
open
closed
4) Repolarization
open
closed
closed
open
5)
Undershoot
closed
closed
closed
open
**during the undershoot, both Na+ channel
gates are closed; if a second depolarizing
stimulus arrives during this time, the
neuron will NOT respond
(REFRACTORY PERIOD)
 strong stimuli result in greater frequency
of action potentials than weaker stimuli
How do action potentials “travel” along an
axon?
 the strong depolarization of one action
potential assures that the neighboring region
of the neuron will be depolarized above
threshold, triggering a new action potential,
and so on…
“Saltatory Conduction”

SYNAPSE: junction
between a neuron and
another cell; found
between:
-2 neurons
-sensory receptor
& sensory neuron
-motor neuron & muscle
cell
-neuron & gland cell
Motor Neuron and Muscle Cell
 Presynaptic cell = transmitting cell
 Postsynaptic cell = receiving cell
Electrical Synapses: allow action potentials to
spread directly from pre- to postsynaptic cell
*connected by gap junctions (intercellular
channels that allow local ion currents)
**Most synapses are…
Chemical Synapses: cells are separated by a
synaptic cleft, so cells are not electrically
coupled; a series of events converts:
elec. signal  chem.signal  elec.signal
HOW???...
NEUROTRANSMITTERS: intercellular
messengers; released into synaptic cleft when
synaptic vesicles fuse with presynaptic
membrane
 specific receptors for neurotransmitters project
from postsynaptic membrane; most receptors
are coupled with ion channels
 neurotransmitters are quickly broken down by
enzymes so that the stimulus ends
 the electrical charge caused by the binding of
neurotransmitter to the receptor can be:
EPSP (Excitatory Postsynaptic Potential):
membrane potential is moved closer to
threshold (depolarization)
IPSP (Inhibitory Postsynaptic Potential):
membrane potential is hyperpolarized (more
negative)

most single EPSPs are not strong
enough to generate an action potential
 when several EPSPs occur close
together or simultaneously, they have
an additive effect on the postsynaptic
potential: SUMMATION
-temporal vs. spatial
Examples of neurotransmitters:
**acetylcholine
epinephrine
norepinephrine
dopamine
serotonin
endorphins
Neuromuscular junction; can
be inhibitory or excitatory
epin. & norep. also function as
hormones; “fight or flight
response”
dop. & ser. both affect sleep, mood,
attention, learning; LSD &
mescaline bind to ser. & dop.
receptors
Decrease perception of pain by
CNS; (heroin & morphine
mimic endorphins)
Neurotransmitters: Ach
 ACETYLCHOLINE: triggers
skeletal muscle fibers to contract…
 so, how does a muscle contraction
stop???
Neurotransmitters: Ach
 a muscle contraction ceases when
the acetylcholine in the synapse of
the neuromuscular junction is
broken down by the enzyme…..
wait for it………………….
ACETYLCHOLINESTERASE!!
It’s term #4!!!!!
 ACETYLCHOLINESTERASE = the
enzyme the breaks down the
neurotransmitter acetylcholine.
ACETYLCHOLINESTERASE!
***word #4 on my list!!!
1) Okazaki fragments
2) plasmodesmata
3) ????????
4) acetylcholinesterase
5) ????????
6) rubisco
7) oxaloacetate
8) islets of Langerhans
9) Batesian mimicry
10) nodes of Ranvier