1. NOTES: CH 48 Neurons, Synapses, and Signaling

3.  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)

5. *CENTRAL NERVOUS SYSTEM (CNS)
 integration center
 brain and spinal cord

6. *PERIPHERAL NERVOUS SYSTEM (PNS) (ropelike bundles of neurons)
 made up of nerves
(ropelike bundles
of neurons)
 nerves communicate
motor and sensory
signals to and from CNS
and rest of body

8. 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

13. 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

16. MYELIN SHEATH:
 produced by Schwann cells in the peripheral nervous system;
 gaps between successive Schwann cells are called NODES OF RANVIER….
***the #10 term!!! 

17. 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

18. 2) GLIA (SUPPORTING CELLS)
 example: astrocytes: responsible for blood-brain barrier

19. Astrocyte
Nerve
cells

20. 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

22. **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

23. How is this potential maintained?
 the sodium-potassium pump uses ATP to maintain the ionic gradients across the membrane
(3 Na+ out; 2 K+ in)

25. of a nerve cell is approx. –70 mV
 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)

27.  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)

28.  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.

29. ACTION POTENTIALS (APs):
 the nerve impulse
 all-or-none event; magnitude is independent of the strength of the stimulus

30. 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

32. State of Voltage-Gated Sodium (Na+) Channel
Phase of A.P.
State of Voltage-Gated Sodium (Na+) Channel
State of Voltage-Gated Potassium (K+) channel
Activation gate
Inact. Gate
Entire channel
1) Resting
closed
open
2 & 3) Depolari-zation
4) Repolar-ization
5) Undershoot

39. **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

43. 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…

49. “Saltatory Conduction”

50. -motor neuron & muscle cell -neuron & gland cell
 SYNAPSE: junction between a neuron and another cell; found between:
-2 neurons
-sensory receptor
& sensory neuron
-motor neuron & muscle cell
-neuron & gland cell

51. Motor Neuron and Muscle Cell

54.  Presynaptic cell = transmitting cell
 Postsynaptic cell = receiving cell

56. 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???...

57. 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

61.  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)

63.  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

66. Examples of neurotransmitters:
**acetylcholine
 Neuromuscular junction; can be inhibitory or excitatory
epinephrine
norepinephrine
dopamine
serotonin
endorphins
 Decrease perception of pain by CNS; (heroin & morphine mimic endorphins)
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

67. Neurotransmitters: Ach
 ACETYLCHOLINE: triggers skeletal muscle fibers to contract…
 so, how does a muscle contraction stop???

68. 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………………….

69. ACETYLCHOLINESTERASE!!
It’s term #4!!!!!
 ACETYLCHOLINESTERASE = the enzyme the breaks down the neurotransmitter acetylcholine.

70. 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