1. Ch. 12 – Nerve Cells

2. Organization of the Nervous System (3 Ways to Organize):
1. CNS vs. PNS
Central Nervous System = ONLY cells that begin AND end in the brain and spinal cord
Peripheral Nervous System = Nerve tissue of the outer regions
-originating in brain =
cranial nerves
-originating in spine=
spinal nerves

3. 2. Afferent Division vs. Efferent Division
afferent division= all “incoming” sensory pathways
efferent division= all “outgoing” motor pathways

4. 3. Somatic (SNS) vs. Autonomic (ANS)
coordinates movement of skeletal muscle
all voluntary movements are SNS (and some involuntary)

5. Autonomic: coordinates smooth muscle, cardiac muscle, glands
Involuntary control
Efferent ANS:
Sympathetic responds to immediate threat (“fight or flight”)
Parasympathetic coordinates normal resting activity (“rest & repair”)
Afferent ANS: called visceral sensory division (viscera=internal organs)

6. Reflex Arc
sensory receptor senses stimulus  afferent neuron carries signal to CNS 
 interneuron in CNS (integrator) transmits signal to efferent neuron 
 efferent neuron stimulates the effector (muscle, gland, organ)  effector performs the necessary response
DUMB!
Many reflexes DO NOT REQUIRE THE BRAIN. (you become aware afterwards)

7. Nerve cells:
1. Neurons- excitable and conduct impulses (see handout for structure)
2. Glia- support the neurons – make up most of N.S.

8. Oligodendrocytes- holds nerve fibers together, produces fatty myelin sheath in CNS.
Astrocytes- star shape, cover capillaries in brain to form “blood/brain barrier”
Ependymal cells- produce cerebrospinal fluid - ciliated
Schwann cells- wrap around long nerve fiber in the PNS to form myelin sheath
Microglia- destroy microorganisms and debris (“immune cells”)

9. Myelin – fatty substance that insulates “wiring” (axons) to conduct efficiently.

10. White matter- nerve tissue containing fatty myelin (axons)
Gray matter- nerve tissue without myelin (cell bodies)

11. Classification of Neurons
classified by number of extensions from the cell body:
multipolar – many extensions – (motor neuron)
bipolar – two extensions – (interneuron)
unipolar – one extension – (sensory neuron)
pyramidal- pyramid shape cell body (brain neuron)

12. Nerve Repair: -most glial cells can replace/repair themselves
-Neuron repair is EXTREMELY limited (none if cell body is damaged)
-stem cell therapy for paralysis is hopeful! (been done in rats)

13. Nerve Signal Transmission
Nerve signals travel by impulses called action potentials – (waves of electrical excitation)
requires electrical ­membrane resting potential – charge difference between inside/outside of cell membrane (like a battery +/-)
this is a polarized cell

14. Electrical resting potential is created by the Na+/K+ pump.
pumps Na+ OUT and K+ IN against concentration gradient (requires energy in the form of ATP)
KEY!: only 2 K+ come in for every 3 Na+ out
This creates cell interior that is negative compared to exterior
Cell interior is -70 millivolts compared to outside

15. Nerve Transmission-
1.The process starts when a neurotransmitter molecule crosses a synapse between neuron- neuron OR when a sensory neuron is stimulated.
2. This signal opens membrane gates allowing Na+ to surge in depolarizing the neuron (making it more positive)
3. if depolarization is strong enough, more Na+ gates open and impulse travels down axon as an action potential (peaks at +30 milivolt)
4. signal travels from dendrite  cell body  axon
5. K+ channels then open behind the action potential allowing K+ out to re-polarize the cell
6. action potential reaches end of axon stimulating neurotransmitter to be released to next neuron OR an effector (muscle, gland, etc.)

17. Saltatory Conduction-
The action potential can “leap” down the axon to the nodes of Ranvier between the Schwann cell thereby allowing it to travel much faster.

18. Neurotransmitters-
Necessary for communication across the gap between neurons and continuation the signal.
There are many neurotransmitters depending on location and job in the NS. (see table 12-2)
Some inhibit nerve cells rather than excite them
Imbalances of these cause problems (i.e. serotonin= depression, Dopamine=motor control, psychosis etc.)