1. CHAPTER 9 REVIEW
Nervous System

2. NEURONS
Structural and functional units of the nervous system.
React to physical and chemical changes in their
surroundings.
Transmit information in the form of electrochemical
changes called nerve impules.

3. NEURONS

4. NERVES
 Are bundles of axons.

5. WHITE MATTER VS GRAY MATTER
 Myelinated axons appear white and masses of
these axons form the white matter in the CNS.
 Unmyelinated axons and neuron cell bodies form
gray matter in the CNS.

7. TYPES OF NEURONS
 Sensory Neurons (afferent)- carry nerve impulses
from peripheral body part into the brain or spinal
cord.
 Interneurons (association) – lie within the brain or
spinal cord and link other neurons. Transmit
impulses from one part of the brian or spinal cord to
another.
 Motor Neurons (efferent) – carry nerve impulses out
of the brain or spinal cord to effectors. Muscles
contract, glands to release secretions.

8. NEUROGLIAL CELLS
 Provide physical support, insulation, and nutrients
for neurons.
 Before birth, neuroglial cells release and relay
signals that guide the differentiation of neurons.
 Oligodendrocytes – provide myelin sheath around
axons in CNS
 Astrocytes – found between neurons and blood
vessels – provide nutrients and ions
 Microglial cells – scattered throughout CNS and are
the “garbage trucks” phagocytizing bacterial cells
and cellular debris

9. NEUROGLIAL CELLS

10. CNS VS PNS
 CNS – brain and spinal cord.
 PNS – composed of peripheral nerves that connect
the CNS to the other body parts.
 Together the CNS and PNS provide 3 functions:
sensory, integrative, and motor.
 Motor Division of the PNS is somatic sending info to
the skeletal muscles and autonomic sending info to
the smooth and cardiac muscles.

11. SENSORY FUNCTION OF NS
 Sensory receptors are a the ends of peripheral
neurons.
 Gather info by detecting changes inside and
outside the body.
 Sensory receptors convert environmental info into
nerve impulses.
 Nerve impulses are transmitted over P nerves to
the CNS.

12. INTEGRATIVE FUNCTION
 Signals are brought together creating sensations,
adding to memory, help produce thoughts that
translate sensations in perceptions.
 As a result of integration we make conscious or
subconscious decisions.
 The we use MOTOR functions to act on them.

13. MOTOR FUNCTIONS
 Use P neurons to carry impulses from the DNS to
responsive structures called effectors.
 Effectors are OUTSIDE the NS and include
muscles and glands.

14. NERVE IMPULSES
 Travel along complex nerve pathways.
 Synapse is the junction between any two
communicating neurons.
 Neurons at a synapse ARE NOT in direct physical
contact, they are separated by a gap.
 The gap is called a synaptic cleft.
 Communication must cross the gap.

16. SYNAPTIC TRANSMISSION
 One way process carried out by neurotransmitters.
 Distal ends of axons have one or more synaptic
knobs with can sacs called synaptic vesicles.
 Nerve impulse reaches synaptic knob some
synaptic vesicles release neurotransmitters.
 Neurotransmitter diffuses across the synaptic cleft
and reacts with specific receptors on the
postsynaptic neuron membrane.

17. ACTION OF NEUROTRANSMITTER
 Postsynaptic neuron is either excited (turned on)
OR inhibited (turned off).

18. CELL MEMBRANE POTENTIAL
 The surface of a cell membrane is usually
electrically charged = polarized (in respect to the
inside of the cell)
 Polarized because of unequal distribution of
positive and negative ions between sides of the
membrane.
 Important to the conduction of muscle and nerve
impulses!!

19. NERVE IMPULSE
 Is formed by a change in the neuron membrane
polarization and return to the resting state.

20. DISTRIBUTION OF IONS
 Potassium ions pass through cell membrane more
easily than sodium ions.
 Pumps in cell membrane work to pump sodium ions
out of cell and potassium ions into cell to create a
concentration gradient.
 Sodium is pumped out of cell, potassium is pumped
into cell.

21. RESTING POTENTIAL
 A resting cell membrane is more permeable to
potassium ions than to sodium ions.
 Potassium ions diffuse out more quickly than
sodium ions.
 Outside of cell membrane gains a slight surplus of
positive charges and inside is left with slight
negative charge.
 Difference in charge between 2 regions is called
potential difference.

22. RESTING POTENTIAL -70MV
 The difference in electrical charge between the
inside and the outside of an undisturbed nerve cell
membrane.

23. POTENTIAL CHANGES
 Changes (stimuli) affect the resting potential in a
particular region of a nerve cell membrane.
 If the membrane’s resting potential decreases
(inside of membrane becomes less negative when
compared to the outside) it is DEPOLARIZED.
 The greater the stimulus the greater the
depolarization.

24. THRESHOLD POTENTIAL -55MV
 If neurons are depolarized sufficiently, the
membrane reaches a level called the threshold
potential.
 At the TP permeability suddenly changes at the
trigger zone.
 Channels open and allow sodium ions to diffuse
freely INWARD.
 Membrane loses it negative electrical charge and
becomes depolarized.

25. HANG IN THERE, I KNOW THIS STUFF ISN’T
EASY TO UNDERSTAND.
 Membrane channels open that allow potassium
ions to pass through and as these positive ions
diffuse OUTWARD.
 The inside of the membrane becomes negatively
charged once more.
 Membrane potential may briefly come overly
negative (hyperpolarization).
 Membrane quickly returns to resting potential
(repolarization)
 Remains in this state until stimulated again.

26. RAPID SEQUENCE OF DEPOLARIZATION AND
REPOLARIZATION IS THE ACTION POTENTIAL
 If threshold is reached, an action potential results.
 Action Potential is the basis for the nerve impulse.
 Only a fraction of sodium and potassium ions move
through the membrane during an AP.
 Many AP can occur and RP’s be reestablished.

28. NERVE IMPULSES
 AP occurs in one region of a nerve cell membrane
and causes a bioelectric current to flow to adjacent
portions of the membrane.
 Local current stimulates the adjacent membrane to
its threshold level and triggers another AP.
 This stimulates the net adjacent region.
 A wave of Ap’s moves down the axon to the end.
 This propagation of AP’s along a nerve axon
constitutes the nerve impulse.

29. EVENTS LEADING TO THE CONDUCTION OF A
NERVE IMPULSE (TABLE 9.1. PG224)
 1. Neuron membrane maintains RP
 2. Threshold stimulus is reached
 3. Sodium channels in trigger zone open
 4. Sodium ions diffuse inward, depolarization
 5. Potassium channels open
 6. Potassium ions diffuse outward, repolarization
 7. AP causes current, stimulates adjacent portions
 8. Wave of AP’s travel length of axon as nerve
impulse.

30. IMPULSE CONDUCTION
 Speed of nerve impulse conduction is proportional
to the diameter of the axon – the greater the
diameter the faster the impulse.
 Unmyelinated axons have smaller diameter and
conduct impulse over its entire surface.
 Myelinated axons have larger diameter (think about
thickness of Schwann cells) and conduct impulses
faster.

31. SALTATORY IMPULSE CONDUCTION
 Myelin insulates and prevents almost all ion flow
through the membrane it encloses.
 Nodes of Ranvier between Schwann cells interrupt
the myelin sheath.
 AP’s occur at the nodes (exposed axon has sodium
and potassium channels)
 A nerve impulse traveling along myelinated axon
appears to jump from node to node (saltatory)
 Many times faster than conduction on an
unmyelinated axon.

32. ALL-OR-NONE RESPONSE
 Nerve impulse conduction is all or none
 If a neuron responds at all it responds completely
 A nerve impulse is conducted whenever a stimulus
of threshold intensity or above is applied to an
axon.
 Higher intensity = more rapid impulses per second.
 ALL impulses carried on an axon are of the same
strength.

33. REFRACTORY PERIOD
 For a very short time after a nerve impulse, a
threshold stimulus will not trigger another impulse
on an axon.
 This limits the frequency of impulses in a neuron.
 Also ensures impulse proceeds in only one
direction -> down the axon.
 Frequency of 700 impulses per second possible but
100 per second is more common