Download Ch 48: Nervous System – part 1

CH 48 NOTES – Neurons, Synapses, and Signaling
 The nervous system has three overlapping functions:
1) SENSORY INPUT: gather information from sensory receptors; detect changes in the external or internal
environment
2) INTEGRATION: information from sensory receptors is interpreted and associated with appropriate responses
(sensation, memory, perceptions, decisions)
3) MOTOR OUTPUT: conduction of signals from the integration center to effector cells (
)
or
*CENTRAL NERVOUS SYSTEM (CNS)


*PERIPHERAL NERVOUS SYSTEM (PNS)

(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
 transmit signals from one location to another
 made up of:
 many axons are enclosed by an insulating layer called the
 include:
2) GLIAL CELLS (“GLIA”) - SUPPORTING CELLS
 10 to 50 times more numerous than neurons
 provide structure; protect, insulate, assist neurons
 example:
and
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
….
***the #10 term!!! 
 example:
form connections between neurons; responsible for blood-brain barrier
ACTION POTENTIALS and NERVE IMPULSES:
 all cells have an electrical charge difference across their plasma membranes; that is, they are
 this voltage is called the
.
(usually –50 to –100 mV)
 arises from differences in ionic concentrations inside and outside cell
How is this potential maintained?...
 the
uses ATP to maintain the ionic gradients across the membrane (3 Na+ out; 2 K+ in)
 the “resting potential” of a nerve cell is approximately
 neurons have special 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
; sound waves/vibrations  hair cells in
(e.g.:
inner ear )
 HYPERPOLARIZATION: memb. 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
is triggered.
, an
ACTION POTENTIALS (APs):
 the nerve impulse

independent of the strength of the stimulus
; magnitude is
4 Phases of an A.P.:
1)
2)
3)
4)
5)
*(see chart on back of notes - shows ion gate conditions during 4 phases)
**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
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…
 SYNAPSE: junction between a neuron and another cell; found between:
-
-
-
-
 Presynaptic cell =
 Postsynaptic 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
: cells are separated by a synaptic cleft; a
series of events converts:
HOW…???
NEUROTRANSMITTERS:
vesicles fuse with presynaptic membrane
; released into synaptic cleft when synaptic
 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
**see diagram on last page of notes!
 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 (
IPSP (Inhibitory Postsynaptic Potential): membrane potential is
)
(more neg.)
 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
epin. & norep. also function as
hormones; “fight or flight response”
norepinephrine
dopamine
dop. & ser. both affect sleep, mood,
attention, learning; LSD &
mescaline bind to ser. & dop.
receptors
serotonin
endorphins
Neurotransmitters: Acetylcholine (Ach)
 ACETYLCHOLINE: triggers skeletal muscle fibers to contract…
 so, how does a muscle contraction stop???
 a muscle contraction ceases when the acetylcholine in the synapse of the neuromuscular junction is broken down
by the enzyme…..

wait for it………………….
= the enzyme the breaks down the neurotransmitter acetylcholine.
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